PPPL

Frontiers Colloquia

The Frontiers of Plasma Physics colloquium is jointly sponsored by PPPL and the Journal of Plasma Physics.

A full list of upcoming colloquia can be found on the JPP Frontiers of Plasma Physics Colloquium Upcoming Talks webpage.

Upcoming

  • The thermo-Alfvénic instability: from toy model to torus (abstract)
    Toby Adkins, University of Otago, New Zealand , Google Scholar
    #s1798, Thursday, 30 May 2024, 11:00am
    A comprehensive understanding of electromagnetic effects on the microinstability properties of tokamak plasmas is becoming increasingly important as experimental values of the plasma beta and, therefore, electromagnetic fluctuations will be higher in reactor-relevant tokamak scenarios. Despite significant numerical progress in understanding the behaviour of instabilities such as the micro-tearing mode (MTM) or kinetic ballooning mode (KBM), there is still a lack of clarity about the fundamental physical processes that are responsible for them, owing to the complexity of full toroidal geometry. Constructing simplified models offers a path towards distilling the fundamental physical ingredients behind electromagnetic destabilisation. This talk focuses on electromagnetic instabilities driven by the electron-temperature gradient (ETG) in a local 'toy' model of a tokamak-like plasma. The model has constant equilibrium gradients (including magnetic drifts, but no magnetic shear) and is derived in a low-beta asymptotic limit of gyrokinetics. A new instability is shown to exist in the electromagnetic regime, the so-called 'thermo-Alfvénic instability' (TAI), whose physical mechanism hinges on a competition between diamagnetic drifts (due to the ETG) and rapid parallel streaming along perturbed field lines. Using linear gyrokinetic simulations, the TAI's presence is confirmed in slab geometry, validated against solutions to an analytical dispersion relation. The mapping of the TAI onto a more realistic Miller equilibrium is considered, demonstrating that it survives aspects of the transition to toroidicity. A comparison is then drawn with the properties of the MTM and KBM, contextualising the TAI within the wider 'zoo' of electromagnetic instabilities commonly observed in tokamak simulations.
  • Title: TBC (abstract)
    Wonho Choe, Korea Advanced Institute of Science and Technology, Korea , Google Scholar
    #s1797, Thursday, 23 May 2024, 11:00am
    Abstract: TBC
  • Gyrokinetic studies on momentum transport and energy exchange (abstract)
    Hideo Sugama, National institute for fusion science, Japan , Google Scholar
    #s1795, Thursday, 16 May 2024, 11:00am
    Governing equations of electromagnetic turbulent plasma systems are derived from the variational principle for the action integral of the Lagrangian based on gyrokinetics. It is shown in the first part of this colloquium that the invariance of the Lagrangian of the system under an arbitrary spatial coordinate transformation can be used to derive the local momentum balance equation. The symmetric pressure tensor representing the local momentum transport is directly derived from the variational derivative of the Lagrangian with respect to the metric tensor. The derived symmetric pressure tensor is useful to treat momentum conservation in symmetric magnetic configurations. In the second part, gyrokinetic simulation results of turbulent energy exchange between electrons and ions in ITG turbulence are presented. In low collisional plasmas, turbulent effects dominate over energy exchange. Energy transfer from ions to electrons occurs due to the ITG turbulence even when electrons are hotter than ions. In addition, quasilinear modeling of the turbulent energy exchange is discussed based on comparison between linear and nonlinear simulation results.
  • Turbulence in collisionless, high-beta plasmas (abstract)
    Stephen Majeski, Princeton University, USA , Google Scholar
    #s1794, Thursday, 09 May 2024, 11:00am
    Owing to their simplicity, the MHD equations have enjoyed a long tenure as the go-to model for describing turbulence in astrophysical environments ranging from the solar wind to the intracluster medium of galaxy clusters (ICM). Unfortunately, the vast majority of these astrophysical plasmas can only be considered weakly collisional at best, meaning significant deviations from the MHD assumption of local thermodynamic equilibrium are not only possible but common. Additionally, recent studies have increasingly found that the various nonequilibrium- and micro-physics of the weakly collisional regime can fundamentally transform the global evolution of such astro-plasmas, especially when the plasma beta is of order unity or larger. In this talk, I will explore the consequences of micro-instabilities and anisotropic pressure-stress on collisionless, high-beta turbulence to distinguish what essential physics of the MHD model remains, and what must be done away with. This will be accomplished through analytical and numerical means, beginning with an examination of the individual waves that form the foundation of collisionless plasma turbulence, and ending with the collective, self-organization effect known as ‘magneto-immutability’. Some of the novel findings that will be covered include modifications to the interaction of hydromagnetic waves, a surprisingly low but highly intermittent volume filling fraction of turbulence-driven microinstabilities, and inertial-range suppression of turbulent perturbations to the magnetic-field strength. The application of these results to black hole accretion flows and the ICM will be discussed, with special attention given to observations of suppressed viscosity in ICM turbulence.
  • Fueling the emerging fusion industry: fusion fuel cycle modeling and tritium breeder blanket technology development at the MIT PSFC (abstract)
    Sara Ferry, Massachusetts Institute of Technology, USA , Google Scholar
    #s1793, Thursday, 02 May 2024, 11:00am
    Decades of global work on fusion technology and plasma physics, coupled with a burgeoning private fusion industry, have led to a renewed optimism in the potential of magnetically confined, deuterium-tritium (DT) fueled fusion power plants (FPP). However, to unlock this potential, we must rapidly accelerate development of tritium breeding blanket technology. Due to the scarcity of tritium, it is imperative that DT-fueled FPPs breed their own tritium in lithium-containing blankets that surround the plasma. The first part of the talk will discuss the broad challenges associated with developing breeder blankets and fuel cycle systems capable of attaining tritium self-sufficiency in an FPP, as identified by the recently culminated U.S. Fusion Fuel Cycles and Breeding Blankets workshop and roadmapping process. These challenges are heightened by the fact that experimental data regarding tritium production and extraction in breeder materials is very limited, but the development timelines of private companies are quite accelerated. The second part of the talk will focus on recent results from the tritium breeding research program at the MIT Plasma Science and Fusion Center. First, I will discuss our efforts to build improved models for understanding the tritium needs of a given FPP design, including updated definitions of tritium burn efficiency and more accurate methods of incorporating tritium loss mechanisms. These models help us understand key performance targets for an FPP, predict on-site inventory, and determine whether a given design is likely to be able to sustain sufficient levels of tritium production to ensure its continued operation and the eventual startup of new plants. Second, I will provide an overview of the MIT LIBRA project, which aims to demonstrate global measurements of tritium breeding ratios approaching 1 in molten FLiBe salt exposed to a 14 MeV neutron environment. I will present the key advantages and drawbacks of a molten-salt liquid immersion breeding blanket, present results from our most recent tritium breeding campaigns, and discuss the practical experimental challenges - both past and ongoing - that the LIBRA project team is tackling as we carry out this work.
  • L-H transition studies at JET, presented by E.R. Solano, for the JET L-H Team (abstract)
    Emilia Solano, Centre for Research on Energy, Environment and Technology, Spain , Google Scholar
    #s1792, Thursday, 25 Apr 2024, 11:00am
    One of the most beautiful phenomena we observe in tokamaks is the transition between L and H-mode, a clear phase transition. Here we present and discuss results from dedicated L-H transition experiments at JET-ILW [ER Solano et al, Nuclear Fusion 63 (11), 112011, 2023]. Uniquely, we studied the power threshold in plasmas composed of pure Tritium, to be compared with Hydrogen and Deuterium plasmas, D+T mixtures, and H+T mixtures [G. Birkenmeier et al, PPCF 65 (5), 054001, 2023]. We show that critical pressure profiles are required for the L-H transition to occur, and such critical profiles are independent of plasma content, but the power threshold itself depends strongly on isotopic content. PLH is in fact determined by plasma transport characteristics in L-mode. Additionally, in Deuterium plasmas, an analysis of Doppler reflectometer measurements of the edge perpendicular velocity in D and He plasmas shows that there is no critical radial electric field value or critical vExB rotation before the transition [C. Silva et al, Nuclear Fusion 61 (12), 126006, 2021]. Instead, there appears to a vExB profile that is characteristic of the L-mode. The diamagnetic velocity, proportional to ∇p, is a better indicator of proximity to the L-H transition. Together, these results enable us to challenge the widely accepted model of the L-H transition being associated to the stabilisation of electrostatic turbulence by sufficient vExB shear. An alternate model of the L-H transition is presented, based on a magnetisation transition of the plasma.

Past

  • Forced Vlasov-Poisson turbulence
    Michael Nastac, University of Oxford, UK , Website, abstract
    [#s1791, 18 Apr 2024]
    A theory is proposed, and confirmed by numerical simulations, of Vlasov-Poisson kinetic plasma turbulence at Debye and sub-Debye scales. This theory describes what is likely to be the “final cascade”—a universal regime to be encountered at the extreme small-scale end of any turbulent cascade in a nearly collisionless plasma. The cascaded invariant is the generalized (negative) entropy C_2, which is the quadratic Casimir invariant of the distribution function. C_2 cascades to small scales in both position and velocity space via both linear and nonlinear phase mixing, in such a way that the time scales associated with the two processes are “critically balanced” at every scale. We derive the scalings of the wavenumber spectra of C_2 and the electric field. The electric-field spectrum is sufficiently steep for the mixing to be controlled by the largest scales of the electric field, and so the C_2 cascade resembles the Batchelor cascade of a passive scalar field, albeit in the kinetic phase space. Our theory’s conclusions are corroborated by direct numerical simulations of a forced 1D-1V plasma. The phase-space cascade enables the irreversibility of particle heating to be achieved on collisionless time scales. The mean distribution function arising from this heating process is shown to be derivable from quasilinear considerations and is not a Maxwellian.
  • Cosmic-ray transport and feedback in star-forming environments
    Lucia Armillotta, Princeton University, USA , Google Scholar, abstract
    [#s1790, 11 Apr 2024]
    In recent years, there has been a growing interest in understanding the role of cosmic rays (CRs) in shaping galaxy evolution. MHD galaxy simulations including CRs have demonstrated that CRs can significantly contribute to the dynamics of the interstellar medium (ISM), aiding in the internal support against gravity and helping to launch galactic outflows. However, the degree to which CRs influence these phenomena is highly contingent upon the chosen CR transport prescription within the model. To address this issue, we have recently developed a transport prescription that is grounded in physical principles rather than empirical considerations, aiming for a more realistic representation of CR transport in ISM simulations. In my talk, I will outline the novelty of our model and discuss the implications of adopting this more realistic formulation for CR transport. Furthermore, I will present the application of our novel scheme in MHD simulations of galactic outflows. These simulations reveal that CRs efficiently accelerate extra-planar material if the latter is mostly warm gas, while they have very little effect on fast, hot outflows.
  • Neoclassical transport in strong gradient regions in tokamaks
    Silvia Trinczek, Princeton University, New Jersey, USA , Website, abstract
    [#s1732, 04 Apr 2024]
    Theoretical descriptions of neoclassical transport in tokamaks are usually restricted to the core where the profiles of density, temperature, and radial electric field are relatively flat, compared to those in transport barriers. For example, in the pedestal, density and temperature can fall off on very short length scales and as a result a deep well forms in the profile of the radial electric field. In transport barriers, the gradient scale lengths can be of the order of the ion poloidal gyro radius. If standard, ”weak gradient”, neoclassical theory is applied to understand the properties of transport barriers, important strong gradient modifications to transport and other neoclassical quantities such as the bootstrap current are missed. Using a large aspect ratio and low collisionality expansion, we can capture gradient length scales as small as the ion poloidal gyro radius and calculate these strong gradient effects. One such effect is the poloidal variation in the electric potential, which contributes to trapping and causes transport modifications. We present a model describing the neoclassical transport of particles, momentum and energy for ions and electrons including strong gradients in the banana regime. We study the changes in the transport and the bootstrap current for some example pedestal profiles. The modifications show nonlinear dependence on the gradient strength and can enhance or decrease transport in comparison to the weak gradient neoclassical predictions.
  • Dusty plasmas under weightlessness (video)
    Andre Melzer, University of Greifswald, Germany , Website Scholar, abstract
    [#s1767, 28 Mar 2024]
    Dusty (complex) plasmas provide a fascinating system to study fundamental processes in many-particle systems since the (dust) particles can be imaged and followed on the kinetic individual-particle level. Gravity is often a major problem when experimenting with dusty plasmas since, on Earth, larger dust clouds are compressed to the lower edge of the plasma. We have performed experiments on parabolic flights without the influence of the disturbing force of gravity where we have measured the dust motion in three dimensions by stereoscopic techniques. This makes new properties of the dust-plasma system accessible for measurements, such as self-excited waves, forces at the dust boundary or the dynamics of phase separation. Moreover, I will present the planned multi-user facility for complex plasma experiments COMPACT on the ISS.
  • Reflection-driven turbulence in the super-Alfv\'enic solar wind
    Romain Meyrand, University of Otago, New Zealand , Google Scholar, abstract
    [#s1766, 21 Mar 2024]
    In magnetized, stratified environments such as the Sun's corona and solar wind, Alfv\'enic fluctuations ``reflect'' from background gradients, enabling nonlinear interactions that allow their energy to dissipate into heat. This process, termed ``reflection-driven turbulence,'' likely plays a key role in coronal heating and solar-wind acceleration, explaining a range of detailed observational correlations and constraints. During this talk, I will present the basic physics of reflection-driven turbulence using reduced magnetohydrodynamics in an expanding box---the simplest model that can capture local turbulent plasma dynamics in the super-Alfv\'enic solar wind. Although idealized, our high-resolution simulations and simple theory reveal a rich phenomenology that is consistent with a diverse range of observations. Outwards-propagating fluctuations, which initially have high imbalance (high cross helicity), decay nonlinearly to heat the plasma, becoming more balanced and magnetically dominated. Despite the high imbalance, the turbulence is strong because Els\"asser collisions are suppressed by reflection, leading to ``anomalous coherence'' between the two Els\"asser fields. This coherence, together with linear effects, causes the growth of ``anastrophy'' (squared magnetic potential) as the turbulence decays, forcing the energy to rush to larger scales and forming a ``$1/f$-range'' energy spectrum in the process. Eventually, expansion overcomes the nonlinear and Alfv\'enic physics, forming isolated, magnetically dominated ``Alfv\'en vortices'' with minimal nonlinear dissipation. I will discuss, how these results can plausibly explain the observed radial and wind-speed dependence of turbulence imbalance (cross helicity), residual energy, fluctuation amplitudes, plasma heating, and fluctuation spectra, as well as making a variety of testable predictions for future observations.
  • Turbulent heating of the solar corona and acceleration of the solar wind: Insights, theory, and modeling from NASA Parker Solar Probe and ESA Solar Orbiter missions (video)
    Gary Zank, University of Alabama, Huntsville, USA , Google Scholar, abstract
    [#s1764, 14 Mar 2024]
    The problem is easily stated and yet has endured for longer than the space age: the surface temperature of the Sun is a mere 6,500 degrees Kelvin yet the temperature of the solar corona exceeds 1 million degrees, hot enough to create a supersonically expanding wind that fills interplanetary space. What heats the solar corona? Sixty five years after the discovery of the solar wind, this remains one of the most outstanding unanswered question in space physics. The launches of ESA’s Solar Orbiter and NASA’s Parker Solar Probe spacecraft were designed to answer this question, venturing closer to the surface of the Sun than any previous missions. Parker Solar Probe has now made multiple excursions below the Alfven surface, during encounters 8 – 14, which for the first time has allowed us to observe the properties of the sub-Alfvenic solar wind that in principle is in direct contact with the surface of the Sun. The Parker Solar Probe and Solar Orbiter missions have motivated considerable development in theory and modeling to explain the heating of the solar corona. An emerging consensus is that the solar corona is heated via the transport and dissipation of low-frequency magnetohydrodynamic turbulence. Recent measurements from Parker Solar Probe and observations by the Metis instrument on the Solar Orbiter spacecraft suggest that interchange reconnection of magnetic field lines in the lower solar corona generates turbulence that can yield sufficient energy, likely in the form of advected nonlinear magnetic structures, to heat the solar corona and thus drive the solar wind. Current solar turbulence theory and modeling will be presented in the context of relevant Parker Solar Probe and Solar Orbiter observations to suggest that we may have a solution to the 65-year-old question of the origin of the supersonic solar wind.
  • Fundamental Physics Opportunities with Multi-PetaWatt- and Multi-MegaJoule-class Facilities(video)
    Peter Norreys, University of Oxford, UK , Google Scholar, abstract
    [#s1763, 07 Mar 2024]
    In my talk, I will present some recent highlights from my research group in the Clarendon Laboratory, obtained working in partnership with many outstanding international collaborators. These fall under the three broad themes. The first is novel laser-plasma interactions, which include those resulting from twisted light, ionisation injection for laser- and beam-driven wakefield accelerators, along with frequency domain holographic and compressive sensing diagnostic developments relevant to future colliders. The second theme is that of extreme field physics using multi-petawatt laser facilities. These include first principles ionisation processes, absorption processes and ultra-bright attosecond X-rays from high harmonic generation, photon-photon scattering in the non-linear QED regime and, most recently, gravitational wave generation using petaWatt- and/or megaJoule-class facilities. We are now extending these studies to those of gravity-wave detection, with promising preliminary results. The third theme is that of inertial fusion studies. All of these studies indicate that an international, dual-use, 20-MJ ICF/IFE facility, with the first 2-MJ at high repetition rate supplying single-shot high energy amplifiers, will open many new exciting avenues for both fundamental physics and high energy density science in the decades ahead.
  • From planetary interiors to harnessing star power: exploring the frontier matter at extreme conditions. (video)
    Arianna Gleason, Stanford University, USA , Google Scholar, abstract
    [#s1762, 29 Feb 2024]
    The study of matter under extreme conditions is a highly interdisciplinary subject with broad applications to materials science, plasma physics, geophysics and astrophysics. Understanding the processes which dictate physical properties in warm dense plasmas and condensed matter, requires studies at the relevant length-scales (e.g., interatomic spacing) and time-scales (e.g., phonon period). Experiments performed at XFEL lightsources across the world, combined with dynamic compression, provide ever-improving spatial- and temporal-fidelity to push the frontier. This talk will cover a very broad range of conditions, intended to present an overview of important recent developments in how we generate extreme environments and then how we characterize and probe matter at extremes conditions– providing an atom-eye view of transformations and the fundamental physics dictating plasma and materials properties. Examples of case-studies closely related to planetary sciences and inertial fusion energy, as enabled by ultrafast X-ray imaging, diffraction and spectroscopy, will be discussed.
  • Formulation of a self-consistent reduced transport theory for discrete modes-with applications to resonant dynamics in plasmas and galaxies (video)
    Vinícius Duarte, Princeton Plasma Physics Laboratory, USA , Google Scholar, abstract
    [#s1752, 15 Feb 2024]
    Resonances, though delicate fine structures in phase space, can mediate massive transport of particles in plasmas. In this colloquium, it will be shown that a quasilinear plasma transport theory that incorporates Fokker-Planck dynamical friction (drag) and pitch angle scattering self-consistently emerges from first principles for an isolated, marginally unstable mode resonating with an energetic minority species. It is found that drag fundamentally changes the structure of the wave-particle resonance, breaking its symmetry and leading to the shifting and splitting of resonance lines. In contrast, scattering broadens the resonance in a symmetric fashion. Comparison with fully nonlinear simulations shows that the emergent quasilinear system preserves the exact instability saturation amplitude and the corresponding particle redistribution of the fully nonlinear theory. Even in situations in which drag leads to a relatively small resonance shift, it still underpins major changes in the redistribution of resonant particles. This novel influence of drag is equally important in plasmas and self-gravitating systems. In fusion plasmas, the effects are especially pronounced for fast-ion-driven instabilities in tokamaks with low aspect ratio or negative triangularity, as evidenced by past observations. The same theory directly maps to the resonant dynamics of the rotating galactic bar and massive bodies in its orbit, providing new techniques for analyzing galactic dynamics.
  • Burning plasma physics studies on JET , Video
    Michael Fitzgerald, United Kingdom Atomic Energy Authority, UK , Google Scholar , abstract
    [#s1751, 08 Feb 2024]
    The era of burning magnetically confined plasmas is on the horizon. The optimization of plasma scenarios for stability and performance will have to be re-learned when a large population of alpha particles drives kinetic instabilities and produces the majority of the heating. A primary mission of the JET tokamak was to successfully confine alpha particles to allow the study of their behaviour, with JET being one of only two tokamaks to have ever operated with deuterium-tritium (DT) fuelling. In this talk, we discuss recently published results from JET experiments that give glimpses into future alpha particle effects, such as destabilization of Alfven eigenmodes, electron heating, and losses.
  • Mirror diffusion of cosmic rays in MHD turbulence , Video
    Siyao Xu, University of Florida, USA , Google Scholar, abstract
    [#s1748, 01 Feb 2024]
    Diffusion of cosmic rays (CRs) in magnetized and turbulent plasmas is a long-standing and challenging problem. The recent explosive interest in understanding CR diffusion arises from its broad impacts on plasma physics, space physics, and astrophysics. However, the existing CR diffusion theories in general face severe difficulties when explaining new-generation CR observations, making it more urgent to reexamine the CR diffusion mechanisms. Very recently, the progresses in understanding and modeling MHD turbulence lead to intense research effort toward a paradigm shift of CR diffusion. In this talk, I will focus on one of the new diffusion mechanisms, the mirror diffusion, which is induced by large-scale magnetic compressions, i.e., magnetic mirrors, in MHD turbulence. I will present its basic physical picture, our theoretical predictions on the diffusion behavior, and our numerical testing with test particle simulations. I will also briefly discuss its implications on astrophysical observations and the IBEX Ribbon.
  • Physics-preserving AI-Accelerated simulations of plasma turbulence , Video
    Robin Greif, University of Oxford, UK , Website, abstract
    [#s1747, 25 Jan 2024]
    Turbulence in fluids, gases, and plasmas remains an open problem of both practical and fundamental importance. Its irreducible complexity usually cannot be tackled computationally in a brute-force style. Combining Large Eddy Simulation (LES) techniques with Machine Learning (ML) allows us to only retain the largest dynamics explicitly, while small-scale dynamics are described by an ML-based sub-grid-scale model. Applying this novel approach to self-driven plasma turbulence allows us to remove large parts of the inertial range, reducing the computational effort by about three orders of magnitude, all while retaining the full statistical description of the turbulent systems' physical properties.
  • Dynamic and transport properties in warm dense matter , Video
    Thomas White, University of Nevada, Reno, USA , Google Scholar, abstract
    [#s1731, 18 Jan 2024]
    The concept of warm dense matter (WDM) emerged over two decades ago, delineating a state of matter existing under extreme conditions—typically at pressures exceeding 100 GPa and concurrently at temperatures ranging between 104–106 K. WDM lies at the intersection of solids and plasmas, and displays a hybrid set of characteristics from both states. In this unique region of phase space, matter attains a state of partial degeneracy and ionization, fostering intricate correlations among charged particles that also exhibit considerable quantum mechanical behaviors. Its intricate nature challenges conventional perturbative techniques, resulting in considerable disparities between theoretical predictions and computational models. In particular, ion transport properties, which play a crucial important for planetary physics, show some of the largest variation with few experimental results. Scattering experiments, utilizing high-intensity beams of penetrating radiation to probe micro-structure and dynamics, represent a pivotal tool for investigating particle dynamics in this regime. Our team recently showcased a groundbreaking inelastic X-ray scattering (IXS) technique for use at X-ray free electron lasers, offering energy resolutions down to 50 meV. This high-resolution technique enables unprecedented access to ion dynamics in plasmas, where the ions typically exhibit much smaller energy shifts than the electrons. In the collective regime, we have measured acoustic waves in warm dense methane, directly determining the sound speed at planetary interior conditions. In the single-particle regime, we have pioneered a direct measurement of ion temperatures in dense plasmas, unveiling new insights into electron-ion temperature exchange. In this presentation, I will provide an overview of our current understanding of transport and dynamics in WDM, spotlighting recent experimental results and discussing future directions.
  • Breaking the Alfvén wave: harder than it looks , Video
    Alfred Mallet,Space Sciences Laboratory, University of California, Berkeley, USA , Google Scholar, abstract
    [#s1724, 11 Jan 2024]
    The Alfvén wave is ubiquitous in the solar wind. In particular, NASA’s Parker Solar Probe, currently exploring the outer edges of the corona, has revealed frequent patches of extremely large-amplitude waves (∆B/B ~1), often even locally reversing the direction of the background magnetic field - these structures have therefore been dubbed “switchbacks”. Often, switchbacks also exhibit extremely sharp boundaries, with large magnetic field rotations over only a few ion inertial lengths. I will attempt to answer several questions suggested by these new observations: How do these waves attain such large amplitudes while maintaining their Alfvénic character? How do they develop such sharp discontinuities? And how are these discontinuities maintained? I will show that the Alfvén wave is surprisingly resilient, and is able to survive unscathed at very large amplitude and even at relatively small scales. These results have interesting implications for the heating of the corona and solar wind by imbalanced Alfvénic turbulence.
  • Electromagnetic QED cascades: showers, avalanches, electron-positron pair plasmas and future experiments , Video
    Mickael Grech, Ecole Polytechnique, Paris, France , Google Scholar
    [#s1737, 21 Dec 2023]
  • Structure formation in magnetohydrodynamic turbulence as a modulational instability , Video
    Suying Jin, Princeton University, USA, USA , Google Scholar, abstract
    [#s1730, 14 Dec 2023]
    Structure formation in magnetohydrodynamic (MHD) turbulence is a problem that has long been of interest, for example, in the context of turbulent dynamo and other types of self-organization of astrophysical and laboratory plasmas. Its onset can be approached as an instability of a statistical equilibrium with respect to a small perturbation. Such modulational instabilities (MIs) can be described much like familiar linear plasma instabilities in kinetic theory, except the role of mean fields there is played by coherent structures (such as an average magnetic field), and the role of plasma particles is played by the turbulent background fluctuations. Although local closures for the electromotive force are the current standard in mean-field dynamo theories, scale separation is far from guaranteed and quantumlike wave-kinetic theory, generally known as Wigner-Moyal kinetics, is needed instead. In the past, this framework has proven fruitful for studies of drift-wave and other types of turbulence, even within the quasilinear approximation (QLA), or collisionless-wave approximation. We show that although quasilinear Wigner-Moyal kinetics yields quantitatively accurate predictions for MHD in some regimes, remarkably, in adjacent regimes QLA can be inaccurate even qualitatively. This raises the question: what is the cause of such dramatic variations? We explore this question within two-dimensional incompressible MHD assuming simple nonlinear backgrounds that allow for a tractable model without truncating the modulational spectrum. We find that, although MI modes are typically assumed to be exponentially localized in the spectral space, i.e. have the form of evanescent spectral waves, this assumption is violated for some parameters. Then, MI modes become propagating spectral waves (PSWs) instead. PSWs have a constant amplitude and thus provide efficient ballistic energy transport down the modulational spectrum, something that is not captured by a typical wave-wave collision operator. Once transients have dissipated, PSWs are self-maintained as global modes with specific real frequencies and drain energy from the primary structure at a constant rate until the primary structure is depleted. The presence of PSWs thereby introduces dissipation to almost any nonlinear MHD wave even at arbitrarily small viscosity; this suppresses MIs in regimes where QLA would predict instabilities. Although PSWs cannot be captured by naive truncations, it may be possible to accommodate them with a proper closure. Finally, we find that introducing dispersion or dissipation constrains the scaling of modulational modes, suppressing PSWs and reinstating the accuracy of QLA. In this sense, ideal MHD is a "singular" system that is particularly sensitive to the accuracy of the closure within mean-field models.
  • New Strategies for Dramatically Increasing Confinement in Magnetic Fusion Energy , Video
    David Hatch, Institute for Fusion Studies, University of Texas at Austin, USA , Google Scholar, abstract
    [#s1735, 07 Dec 2023]
    The projected size and cost of a fusion power plant is strongly dependent on plasma confinement. In most magnetic fusion devices, the confinement time is determined by small scale turbulent transport of heat and particles. This turbulence can be suppressed under certain conditions, resulting in enhanced confinement regimes with enormous gradients of temperature and density. I will present a fundamental picture of the physical mechanisms by which such extreme conditions are thermodynamically viable. This fundamental understanding points to strategies by which transport barriers can be controlled and optimized. I will also describe numerical investigations interpreting wide-ranging experimental observations of transport barriers in tokamaks. One of the most pressing challenges for a fusion power plant is the conflicting demands of heat exhaust and confinement. I will introduce a new solution to this conundrum: a divertor concept that works in synergy with transport barriers to unlock even greater gains in confinement. If this can be achieved, it would enable much smaller and cheaper fusion devices than typically envisioned.
  • Electromagnetic Instabilities in Spherical Tokamaks , Video
    Daniel Kennedy, United Kingdom Atomic Energy Authority, UK , Google Scholar, abstract
    [#s1723, 30 Nov 2023]
    Electromagnetic microinstabilities are likely to dominate transport in high β next generation spherical tokamaks (STs) such as STEP. While gyrokinetic (GK) simulations have thus far proven to be a very accurate tool in modelling turbulent transport in predominantly electrostatic regimes at low β, obtaining saturated nonlinear simulations of plasmas with unstable kinetic ballooning modes (KBMs) and microtearing modes (MTMs) has proven computationally and conceptually challenging. However, recent investigations that retain only MTMs and exclude KBMs (by neglecting compressional perturbations) reveal strong sensitivity of the heat flux to parallel dissipation and velocity space resolution. With sufficient resolution or dissipation to avoid numerical instabilities, recent MTM-only simulations saturate cleanly at much reduced electron heat flux. We find that including compressibility can unleash a KBM-like instability which drives very large heat fluxes (orders of magnitude greater than the available heating power). The saturated fluxes are sensitive to equilibrium flow shear, and at mid-radius diamagnetic levels of flow shear can reduce the fluxes to much lower values.
  • Plasma Physics at the higher energy and QED frontier , Google Scholar
    Thomas Grismayer, Higher Technical Institute, Lisbon , Video, abstract
    [#s1722, 16 Nov 2023]
    Plasma physics generally involves well-known physics of electromagnetism, special relativity and statistical mechanics. Various descriptions of the plasma state (kinetic, fluid, MHD) have been developed and each of them have a specific scope that depends on time/length scales, chemical composition, density, and temperature. Whereas these descriptions are well established, we could wonder when quantum electrodynamics (QED) comes into play. The answer lies in the threshold of QED cross sections which become non negligible for certain critical lengths, energies and fields. The presence of ultra-strong electromagnetic field and relativistic temperatures modifies the underlying basic physics to such a great extent that relying on classical plasma physics is often not justified. The zoo of QED processes implies emission of hard photons, photon-lepton collisions, non-constant number of particles (creation and annihilation), stochastic particle orbits, which have stimulated the community for constructing a QED-based plasma physics. While the state of development of QED-based plasma physics theory is still far from being as mature as that of the classical plasma theory, lots of progresses in this area has been achieved in the past few decades. These advances have implications in many astrophysical and laboratory scenarios (associated to new PetaWatt class lasers), that share common underlying microphysics and collective plasma effects associated with intense fields. This new physics is highly nonlinear and multi scale, and require a combination of theory and large scale numerical simulations. The main challenges in this area with be presented and the recent progresses triggered by large scale simulations of compact objects and of multi dimensional laser/beam-plasma interactions in the presence of fields close to the critical Schwinger field will be discussed, emphasising the interplay between collective plasma dynamics and QED processes.
  • Kinetic and Two-Temperature Physics of Black Hole Accretion Disks and X-ray Coronae , Video
    Lia Hankla, University of Colorado, Boulder , Google Scholar, abstract
    [#s1721, 09 Nov 2023]
    Understanding the plasma physics of accretion disks and coronae around black holes is crucial for interpreting the radiation observed from these systems. However, these plasmas span several different physical regimes. They can be highly collisional and well-described by a single temperature, or collisionless with nonthermal particles that have been accelerated to high energies. My work brings small-scale kinetic and two-temperature physics into the global setting of the accretion disk/corona system. I first use particle-in-cell (PIC) simulations to understand turbulence and particle acceleration in a collisionless, magnetized plasma. Next, I build an analytic model using prescriptions from PIC simulations to demonstrate an observable power-law from within the plunging region of a black hole. Finally, I use general relativistic magnetohydrodynamic (GRMHD) simulations to examine the impact of two-temperature physics on the radial structure of the full accretion disk.
  • Cosmic ray self-confinement around their sources Video
    Pasquale Blasi, Gran Sasso Science Institute, Italy , Google Scholar, abstract
    [#s1720, 26 Oct 2023]
    The large density and density gradients that cosmic rays have around their sources suggest that they may have a dynamical role in such regions and may be able to excite instabilities capable of self-confining such particles in the circus-source regions. I will discuss the important implications that this phenomenon has in the surroundings of supernovae and pulsar wind nebulae, where we can potentially observe the effects of self-confinement, in terms of suppressed diffusivity. I will also discuss self-confinement in the context of Galactic Cosmic Rays escaping our Galaxy and ultra high energy cosmic rays escaping their sources.
  • Shear dynamics at the edge of magnetised fusion plasmas Video
    Guilhem Dif-Pradalier, CEA, Cadarache, France , Google Scholar, abstract
    [#s1718, 19 Oct 2023]
    Transport bifurcations are essential to fusion performance and mostly originate in a narrow, peripheral region of the confined plasma, the ‘edge’. This region however is especially challenging to model and to understand. The plasma edge is home to large, concentrated shear and to steep gradients. As turbulent scales become comparable to free energy gradient scales, oft-assumed scale separations in turbulence models break down, promoting the use of more computationally-intensive ‘flux-driven’ frameworks. Sources and boundary conditions play an important role as the plasma edge sits at the confluence of conflicting dynamics, notably outgoing heat and particle fluxes from the confined core and incoming particle or momentum fluxes from the unconfined Scrape-Off Layer (SOL). Through magnetic connection to the material boundaries, sheath physics also strongly influences SOL dynamics, which in turn constrains electric field (and thus shear) dynamics in the plasma edge. We propose to review some of the key features above, some recent results and passage points that have been obtained in view of a consistent flux-driven description of the plasma edge. In particular, we will discuss evidence that magnetic connection with the material boundaries deeply modifies the turbulence drive in the edge, leading to a global reorganisation of fluctuations and the onset of a modest yet stable transport barrier close to the magnetic separatrix. We will briefly touch upon the mechanisms and causality behind electric field and shear (vorticity) production in the edge, highlighting the special importance of diamagnetic fluctuations. We will question the electric field dependence on plasma current at the edge and confront calculations to experimental measurements. The last part of the talk will be concerned with describing sheath physics within our kinetic framework and steps that have been undertaken in view of incorporating this physics into a comprehensive (gyro)kinetic turbulence model.
  • Theoretical studies towards a negative triangularity tokamak power plant , Video
    Justin Ball, EPFL, Switzerland , Google Scholar, abstract
    [#s1717, 12 Oct 2023]
    Experimental observations show that negative triangularity plasma shaping can significantly improve the energy confinement time of tokamaks. Moreover, unlike the standard positive triangularity shape, negative triangularity plasmas typically cannot access H-mode. Together these two facts may enable an attractive power plant design – the plasma can be heated to reactor-relevant conditions while remaining in L-mode to avoid the material survivability concerns associated with ELMs, yet still achieve sufficiently good confinement for high fusion gain. This potential has motivated the creation of EUROfusion’s Theory, Simulation, Verification, and Validation (TSVV) project on negative triangularity, which is investigating the feasibility of a negative triangularity power plant. In this talk, we will synthesize the most important results including the physical reasons behind the confinement time improvement, how performance scales to new parameter regimes (like spherical tokamaks), insights from reduced transport modeling, the scrape-off layer width, and more. We will connect these results to recent experiments and comment on the prospects for a negative triangularity power plant.
  • The role of large-scale plasma turbulence in influencing particle transport and energization in astrophysical contexts , Video
    Oreste Pezzi, Institute for Plasma Science and Technology, Italy , Google Scholar, abstract
    [#s1716, 05 Oct 2023]
    Populations of energetic particles, ranging from solar energetic particles to incredibly high-energy cosmic rays, are ubiquitous in space and astrophysical plasmas. Several intertwined phenomena, including shocks, magnetic reconnection, and turbulence, are responsible for the efficient acceleration of particles and for determining their transport properties for efficient particle energization. Plasma turbulence produces patchy coherent structures, such as reconnecting current sheets, plasmoids, and vortices across a vast range of spatial scales. Under some circumstances, these structures can entrap particles, thus providing fast energization through, for example, drift acceleration. In this talk, I will review some of these mechanisms and outline recent numerical efforts aimed at investigating how the large-scale complexity of the turbulent magnetic field influences particle transport and how large-scale coherent structures impact particle energization. I will also comment on the applicability of these results in space and astrophysical contexts.
  • Flying focus beams as a tool to investigate strong-field physics , Video , Google Scholar
    Antonino Di Piazza, University of Rochester, USA , abstract
    [#s1707, 28 Sep 2023]
    In a flying focus beam (FFB) the velocity of the focus can be “programmed” and it is independent of the group and the phase velocity of the beam itself. Recent experiments have demonstrated a moving focus over centimeter lengths, i.e., much longer than the Rayleigh length. Scaling this technology to higher power laser pulses would allow one to employ FFBs as a tool for fundamen[1]tal high-field physics, especially to investigate effects that accumulate with the interaction length. Specifically, by considering an ultrarelativistic electron beam counterpropagating with respect to a FFB, whose focus copropagates with the electrons at the speed of light, we show that the effects of the so-called transverse formation length of radiation on the radiation itself can be enhanced as compared to the case of a conventional Gaussian beam. Analogously, radiation-reaction and vacuum-polarization effects can be rendered measurable at much lower intensities than conventionally required in a similar setup. Finally, we show how FF beams with angular momentum can be an efficient tool to transport ultrarelativistic electron beams over macroscopic distances without significantly spreading on the transverse plane.
  • Geometric numerical methods , Google Scholar , Video
    Eric Sonnendrucker, IPP Garching, Germany , abstract
    [#s1706, 21 Sep 2023]
    Neglecting collisions and other dissipative effects, many models of plasma physics including kinetic, fluid, MHD and hybrid models have been shown to possess a noncanonical hamiltonian structure. This comes with some important properties in particular conservation of energy and some Casimir invariants, typically div B = 0 and also Gauss’ law for the Vlasov-Maxwell equations. Adequate conservation of these quantities has been proven to be essential for well behaving numerical solutions. Many numerical methods have been devised to this aim. Geometric numerical methods achieve this by discretizing the Hamiltonian structure, i.e. Poisson bracket and Hamiltonian, rather than the resulting PDEs. This approach approximates the infinite dimensional original hamiltonian system by a Finite Dimensional hamiltonian system and in this way guarantees the conservation of the appropriate discretized invariants. After introducing the concept of Geometric discretization, we will illustrate it on a hamiltonian formulation of the Vlasov-Maxwell model and hybrid fluid-kinetic models. We will also extend geometric numerical methods to dissipative physical systems by adding to the hamiltonian part of the model a dissipative part as for example the Landau collision operator to the Vlasov-Maxwell model in the form of a dissipative symmetric bracket yielding altogether a so-called metriplectic formulation.
  • Towards a unified picture of energy partition in unmagnetized collisionless shock waves , Video , Google Scholar
    Arno Vanthieghem, Princeton University, USA , abstract
    [#s1705, 14 Sep 2023]
    Collisionless shock waves shape the nonthermal emission in a wide range of environments, including modern laboratory experiments and astrophysical outflows. In weakly magnetized plasma flows, self-generated nonlinear electromagnetic plasma processes are inferred to heat and accelerate electrons and ions. Understanding the mechanisms that underpin the energy transfer between plasma species and the downstream temperature ratio between electrons and ions constitutes a fundamental challenge in modeling such blast waves. In this talk, I will outline recent theoretical efforts to model the transport of electrons in Weibel-mediated shocks. I will introduce a new model accounting for electron heating in an ambipolar-type process through the interplay between pitch-angle scattering in the microturbulence and the coherent electrostatic field induced by the difference in inertia between species. Via analytical kinetic and fluid estimates, a semi-analytical Monte Carlo-Poisson method, and large-scale ab-initio Particle-In-Cell simulations, I will discuss the electron-ion energy partition in the downstream of high Alfvén Mach numbers shocks relevant to supernova remnants and laboratory experiments. I will then present the extension of this model to the relativistic regime to demonstrate equipartition inferred from the afterglow emission of gamma-ray bursts. Finally, I will explore the implications of this model on electron injection in nonthermal distributions.
  • ICRF heating and ICRF-accelerated fast ions in tokamaks: Modelling and theory-to-experiment comparisons Google Scholar
    Mervi Mantsinen, Barcelona Supercomputing Center, Spain , abstract
    [#s1638, 07 Sep 2023]
    Heating with waves in the ion cyclotron range of frequencies (ICRF) is a well-established method on present-day tokamaks and is envisaged as one of the main heating techniques for ITER. The reference ICRF heating scheme for ITER deuterium-tritium plasmas at the full magnetic field of 5.3 T is the second harmonic heating of tritium. Its heating mechanism is a finite Larmor radius effect, i.e. it takes place when the Larmor radius of the resonant tritons are finite with respect to the wave length of the launched wave. The physics of such a scheme can be studied on present-day fusion devices using second or higher-harmonic ICRF heating scenarios together with available diagnostics to compare experimental results with the predictions of ICRF modelling codes. In addition to providing challenging benchmarks between experiments and modelling, such schemes can also be used for fast ion physics studies and for testing alpha particle diagnostics in preparation of ITER. Representative examples from present-day experiments and the underlying physics are reviewed.
  • Electron acceleration by a two stage laser-plasma interaction , Video , Google Scholar
    Meirielen Caetano De Sousa, École Polytechnique, France , abstract
    [#s1703, 31 Aug 2023]
    Ultraintense, short pulsed lasers with high contrast allow the exploration of new mechanisms for particle acceleration. In particular, when such lasers irradiate an overdense plasma, electromagnetic waves appear at the vacuum-plasma interface, enabling local field confinement and enhancement. Since these mechanisms involve an overdense plasma, high values of total electron beam charge (~nC) can be achieved, which is important for applications in the fields of plasma-based accelerators, electron sources, among others. Recent Particle-in-Cell (PIC) simulations analyzed a laser pulse interacting with the wedge of an overdense plasma, and showed that it produces a diffracted electromagnetic wave with an intense longitudinal electric field that efficiently accelerates collimated electron bunches with high charge (~nC) and relativistic energies up to ~130 MeV. In this talk, I will present a new scheme which couples electron acceleration in an overdense plasma with the wakefield produced by the laser propagating through an underdense plasma. Highly charged electron bunches are emitted from the overdense plasma, and trapped by the wakefield in which they are accelerated even further. Such a scheme enables us to obtain highly charged and highly energetic electron bunches at the end of the second acceleration stage.
  • Physics of the Tokamak Pedestal, and Implications for Magnetic Fusion Energy , Google Scholar
    Philip Snyder, Oak Ridge National Laboratory, USA , abstract
    [#s1702, 24 Aug 2023]
    Fusion, the process that powers the stars, offers the potential to provide plentiful clean energy here on earth. Magnetic confinement of high temperature plasmas in toroidal devices known as tokamaks is a promising approach to fusion, one that has successfully produced millions of watts of fusion power. High performance in tokamaks is achieved via the spontaneous formation of a transport barrier in the outer few percent of the confined plasma. This narrow insulating layer, referred to as a “pedestal,” typically results in a >30x increase in pressure across a 0.4-5cm layer. The overlap of drift orbit, turbulence, and equilibrium scales across this narrow layer leads to rich and complex physics, and challenges traditional analytic and computational approaches. Development of high resolution diagnostics, and coordinated experiments on several tokamaks, have validated understanding of important aspects of the physics, while highlighting open issues. A predictive model (EPED) has proven capable of predicting the pedestal height and width to ~20-25% accuracy in large statistical studies. This model was used to predict a new, high pedestal “Super H-Mode” regime, which was subsequently discovered on DIII-D, leading to high fusion performance, and motivated experiments on Alcator C-Mod which achieved world record, reactor relevant pedestal pressure. Coupling this new understanding of pedestal physics to models of the core plasma and open field line region enables extensive optimization of global fusion performance in concert with desired exhaust conditions, including development of attractive scenarios for a fusion pilot plant.
  • 1/f noise in the solar wind: connections to dynamo, inverse cascade and causality , Video , Google Scholar
    William Matthaeus, University of Delaware, Delaware, USA , abstract
    [#s1700, 17 Aug 2023]
    I review the observations of 1/f noise in the heliosphere and discuss the theoretical background of generic 1/f noise models as they may be relevant to the solar wind. The planning for Parker Solar Probe emphasized that understanding 1/f “noise” in the solar wind may inform central problems in heliospheric physics such as the solar dynamo, coronal heating, the origin of the solar wind, and the nature of interplanetary turbulence [1]. It is of significance that 1/f noise can occur in a great variety of physical systems ranging from Johnson noise in vacuum tubes and semiconductors, fluctuations in heart rates, brain waves sandpiles [2], and solids [3], the spectrum of music and economic indicators, and many other examples. An appealing generic mechanism that can explain many of these phenomena is the superposition principle [4,5] in which a composite signal is formed from a collection of individual signals that are characterized by scale-invariant correlation times. In the solar wind context, it was proposed [4] that such a superposition might occur in the corona due to scale-invariant reconnections, thus explaining the observed 1/f signal in the interplanetary magnetic field that spans the approximate frequency range 2 x 10^-6 Hz to several times 10^-4 Hz. at 1 au. Subsequent observational analyses showed that signals in the photosphere and the corona [5,6] exhibit 1/f signatures at frequency ranges compatible with the 1 au observations. This suggests an even lower altitude origin in the dynamo itself and this possibility is supported by both dynamo experiments and simulations [7]. There is also empirical evidence that inverse cascade systems (including dynamo) generically exhibit 1/f noise, possibly due to nonlocality of interactions at the largest scales. Recent interest in employing Parker Solar Probe observations to address whether interplanetary 1/f noise can be generated by local dynamics [as proposed e.g., in 8, 9,10] is also reviewed. Here we present theoretical arguments that due to causality issues, interplanetary 1/f noise extending to very low frequencies is not explained by the recent observations.
  • Inertial range of magnetorotational turbulence , Video , Google Scholar
    Yohei Kawazura, Tohoku University, Japan , abstract
    [#s1699, 10 Aug 2023]
    Accretion of matter onto a compact object is triggered by turbulence driven by magnetorotational instability (MRI). While there have been numerous theoretical and computational studies on MRI turbulence in the past few decades, the inertial range of MRI turbulence remains poorly understood. For instance, both the magnetic and kinetic energy spectra deviate significantly from those predicted by theories of MHD turbulence and those observed in simulations of externally driven MHD turbulence. This discrepancy arises due to the fact that the energy injection via MRI happens broadly in a Fourier space, posing challenges for numerically resolving the inertial range. In this colloquium, I will present the numerically resolved inertial range of MRI turbulence using Rotational Reduced MHD (RRMHD) model. This model assumes the presence of a near-azimuthal mean magnetic field and considers small amplitude turbulent fluctuations elongated along the mean magnetic field. By making these assumptions, RRMHD allows us to reach the inertial range with minimal computational resources. Our findings include: (1) both magnetic and kinetic energy exhibit -3/2 spectra, (2) Alfvenic fluctuations and slow-mode-like compressive fluctuations are decoupled at small scales, and (3) compressive fluctuations are approximately twice as strong as Alfvenic fluctuations. Furthermore, we will show our ultra-high resolution simulation of MRI turbulence using standard MHD and demonstrate that the small-scale spectra closely resemble those of RRMHD, thereby validating the RRMHD approach.
  • Testing cosmic ray acceleration in the laboratory , Video , Google Scholar
    Subir Sarkar, University of Oxford, UK , abstract
    [#s1698, 03 Aug 2023]
    Petawatt lasers can be used to recreate interesting astrophysical processes, e.g. amplification of magnetic fields by plasma turbulence generated in colliding laser-produced plasma flows. MHD instabilities in such turbulent, magnetized plasmas are seen to energise electrons above the thermal background, thus demonstrating an injection mechanism for cosmic ray acceleration. Ongoing experiments are studying whether stochastic acceleration by the Fermi process indeed occurs in such astrophysical environments, e.g. supernova remnants or the Galactic Fermi bubbles or radio galaxies.
  • PIC simulations of particle acceleration at relativistic magnetized shocks , Video , Website
    Virginia Bresci, Leibniz Institute for Astrophysics Potsdam, Germany , abstract
    [#s1697, 27 Jul 2023]
    The efficiency of particle acceleration at shock waves in relativistic, magnetized astrophysical outflows is a debated topic with far-reaching implications. Most of previous numerical studies based on fully PIC simulations consider laminar in-flow conditions, i.e., nonturbulent, homogeneous background plasma of uniform magnetization. In this talk, based on a recent study, I will show how the presence of a well-developed turbulence upstream of a fast shock may change the picture. In particular, we carried out PIC simulations of a mildly relativistic magnetized pair shock (Lorentz factor γsh ≃ 2.7, magnetization level σ ≃ 0.01), and we found that strong turbulence can revive particle acceleration in a superluminal configuration that otherwise prohibits it. By the analysis of tracked particles we investigated the acceleration process and could conclude that, depending on the initial plasma temperature and magnetization, shock-drift or diffusive-type acceleration governs particle energization, producing power-law spectra dN/dγ ∝ γ^−s with s ≈ 2.5–3.5. At larger magnetization levels, stochastic acceleration within the preshock turbulence becomes competitive and can even take over shock acceleration.
  • Tearing and secondary instabilities in collisionless plasmas based on gyrofluid modeling , Video , Google Scholar
    Camille Granier, Observatoire De La Cote D’azur, France , abstract
    [#s1696, 20 Jul 2023]
    This talk presents the application of a Hamiltonian gyrofluid approach allowing to account for finite Larmor radius corrections, parallel magnetic field fluctuations, and electron inertia, in the study of magnetic reconnection and related secondary instabilities. Non-collisional current sheets that form during the nonlinear development of spontaneous magnetic reconnection are characterized by a small thickness comparable to the electron skin depth and can become unstable, leading to the formation of plasmoids and facilitating high reconnection rates. In the context of cold ions with negligible parallel velocity, we investigate the marginal stability conditions for plasmoid formation in collisionless plasma, considering the impact of finite but moderate βe, a regime that has seen little investigation before. These results are tested against gyrokinetic simulations, showing a very good agreement. The gyrofluid approach also enables the study of the transition from the cold to the hot-ion regime. By considering a regime with small βe and hot ions, we present an analysis of the turbulent regime arising from Kelvin-Helmholtz-like secondary instabilities outside magnetic islands, resulting in the formation of magnetic vortices. Turbulence develops at scales smaller than the electron skin depth, with a magnetic energy spectrum exponent close to -11/3 as predicted by strong-turbulence phenomenology.
  • Radiative cooling-driven (thermal) instabilities in weakly magnetized, diffuse, hot plasmas , Website , Video
    Prakriti Pal Choudhury, University of Cambridge, UK , abstract
    [#s1695, 13 Jul 2023]
    Astrophysical environments are diverse in characteristic density, temperature, and magnetic field. For example, intracluster and circumgalactic medium (ICM/CGM) are large-scale, mildly magnetized, weakly collisional, hot, and diffuse environments. On the other hand, solar and planetary coronae have strong magnetic fields in hot and relatively tighter gravitationally bound atmospheres. A common feature in many of these distinctly different environments is the multiphase nature of the medium, or in other words, the local coexistence of a range of temperatures and the lack of monolithic cold or hot phase. This feature suggests (i) maintenance of an approximate thermal equilibrium in a time-averaged sense and (ii) a mechanism to produce multiphase condensation despite an equilibrium. In this talk, I will discuss the physical mechanisms responsible for (i) and (ii) in the ICM/CGM using analytic models and numerical experiments. I will include a discussion on the permitted length scales in a multiphase medium. Further, I will highlight the role of the weak magnetic field in condensation (specifically, large-scale mode along the field) and maintenance of the hot background medium (energy transport problem). Some of these ideas can also be relevant to speculate on such instabilities and interpret solar prominence, clumpy AGN winds, and condensation in planetary coronae.
  • Plasma Observatory ESA M7 candidate mission: science and numerical support , Google Scholar , Video
    Francesco Valentini, University of Calabria, Italy , abstract
    [#s1694, 06 Jul 2023]
    Plasma Observatory (PO) multi‐scale space mission is one of the five ESA M7 candidates for a launch in 2037 and is currently undergoing a competitive Phase 0 at ESA, for further downselection to Phase A at the end of 2023. The mission concept is tailored to study plasma energization and energy transport in the Earth's Magnetospheric System, simultaneously at both fluid and ion scales, at which the largest amount of electromagnetic energy is converted into energized particles and energy is transported. PO targets the two ESA Voyage 2050 themes for ESA-led M Mission: Magnetospheric Systems and Plasma Cross-scale Coupling. Baseline mission includes one mothercraft and six identical smallsat daughtercraft, covering all the key regions of the Magnetospheric System. Plasma energization and energy transport are central in space plasma physics research, with important implications for space weather science as well as for the understanding of distant astrophysical plasmas, and are strictly interconnected with important processes such as shocks, magnetic reconnection, turbulence and waves, plasma jets and their combination. The Earth's Magnetospheric System is the complex and highly dynamic plasma environment where the strongest particle energization and energy transport occur in the near‐Earth space. PO Numerical Simulation Working Group (NSWG) provides support to the development of the mission concept during all phases of mission design, not only from the scientific point of view but also for the definition of the payload, as well as for the optimization of the configuration of the spacecraft constellation. Here, we discuss the concept of the PO space mission and the role of the NSWG, focusing specifically on the complex interaction between shocks and plasma turbulence and presenting a novel combination of magnetohydrodynamic (MHD) and small‐scale, hybrid kinetic simulations, where a shock is propagating in a turbulent medium. Finally, we put our modelling effort in the context of spacecraft observations, elucidating the role of cross‐scale, multi‐spacecraft simultaneous measurements in resolving shock front irregularities at different scales.
  • Title: High-energy cosmic-ray propagation and interaction in magnetized plasmas , Website
    Julia Tjus, Ruhr Universitaet Bochum, Germany , abstract
    [#s1673, 29 Jun 2023]
    Already detected more than 100 years ago, cosmic rays and their origins are still among the biggest riddles in astrophysics and physics. The reason is that they are transported in a highly magnetized plasma, thus interacting with the magnetic field and even with the ionized plasma background. While the back-reaction with the plasma is of highest relevance at low energies (below 10-100GeV), the highest energy cosmic rays can typically be considered to be transported in static magnetic field configurations. Here the biggest challenge is to properly describe the interaction with the magnetic field. In this talk, I will discuss this high-energy cosmic-ray regime, including the challenges in the modeling of the particle-wave interactions and compare the theoretical description and propagation models with data. I will discuss the possibility of contributions of magnetic mirrors to the propagation as well as the transition to the quasi-ballistic propagation regime and how it is imprinted in the measured particle spectra. I show that even cosmic-ray secondaries iike gamma-rays and neutrinos, that are produced in inelastic, hadronic interactions are useful diagnostic tools to understand cosmic-ray propagation in the magnetized plasma. Gamma-ray data will be used to test the models and an outlook will be given on what we will be able to learn from neutrino astronomy once it has come of age.
  • Towards a new theory of cosmic ray transport , Semantic Scholar
    Philipp Kempski, Princeton University , UK , abstract
    [#s1672, 22 Jun 2023]
    Detailed energy spectra of relativistic cosmic rays (CRs) measured close to Earth provide powerful constraints on the physics of their transport in the Milky Way (MW). According to these measurements, the escape time of CRs from the MW is orders of magnitude larger than the MW’s light-crossing time and has an approximately power-law dependence on CR energy. In the standard paradigm of CR transport, their escape rate is set by resonant interactions with volume-filling, small-amplitude magnetic-field fluctuations that permeate the galactic volume. In this talk, I will argue that these traditional propagation models remain theoretically uncertain and are generally not in good agreement with observations. This suggests that the microphysical theory of CR propagation needs to be revisited. I will discuss possible new models of CR transport that may bridge the gap between theory and observations. In particular, I will argue that CR propagation may depend on the geometry of small-scale magnetic field reversals and scattering in intermittent regions of resonant field-line curvature.
  • Shear flow generation in magnetised plasmas , Video , Google Scholar
    Xavier Garbet, CEA and NTU, Singapore , abstract
    [#s1671, 15 Jun 2023]
    Mean shear flows are essential to control confinement in magnetised plasmas for fusion. In absence of external torque, and atomic physics processes, two types of processes are known to drive or damp flows: collisional drag and turbulent flow generation. While the theory of flow generation is well documented in axisymmetric magnetic configurations like tokamaks, it is much less understood when 3D effects are accounted for. In tokamaks, 3D effects may come from magnetic ripple due to the finite number of coils, or magnetic perturbations produced with external coils. In this talk, the physics at play, collisional or turbulent, will first be clarified in a unified framework. Analytical theory is compared to a set of comprehensive simulations based on drift-kinetic or gyrokinetic approaches able to treat both collisions and turbulence on an equal footing, thus allowing an assessment of competing processes. This methodology has recently been applied to understand the improvement of confinement with plasma current. When 3D effects are accounted for, it appears that collisional drag prevails over turbulence beyond a threshold in ripple amplitude that is expected to be reached in ITER. The talk will end with some considerations on the edge-core interplay regarding flow generation.
  • Separatrix parameters and core performances across WEST L-mode database video
    Clarisse Bourdelle, CEA, France , abstract
    [#s1668, 08 Jun 2023]
    WEST database analysis shows a correlation of the recycled neutral source around the separatrix with core performances. This observation questions the causality chain between particle source and turbulent transport up to the core in L-mode, high recycling plasmas, an unavoidable phase of all scenarios. The best core performances correlate with the lowest values of the density at the separatrix n_sep, similarly to AUG and JET in H-mode [G. Verdoolaege et al 2021 Nucl. Fusion 61 076006]. Reflectometry in the midplane provides n_sep, while T_sep is inferred by the ‘two-point model’ using Langmuir Probe data on divertor targets. Lower separatrix resistivity does not correlate with better core performances, unlike H-mode observations [T. Eich et al 2020 Nucl. Fusion 60 056016]. As expected in presence of an efficient neutral source due to recycling fluxes, n_sep correlates with the D recycled particle flux at the divertor measured by visible spectroscopy. Coherently, at a given controlled central line integrated density n ̅, lower n_sep correlates with larger density gradient around the separatrix as well as larger global density peaking, n ̅/〈n〉 measured by interferometry. The later correlates as well with lower collisionality in the core, similarly to JET and AUG H-modes [C. Angioni et al 2007 Nucl. Fusion 47 1326]. The correlations reported allow phrasing the subsequent causality question: What is the interplay chain between low neutral recycling at the divertor plates, low density at the separatrix, high density peaking at the separatrix, high global density peaking, higher central temperature, better core energy confinement quality? The causality chain understanding is essential to prepare ITER operation and design DEMO scenarios where the ratio of the divertor leg to the ionization length will be larger and where the pumping efficiency with respect to the vacuum vessel volume will be weaker with respect to actual operating tokamaks.
  • Plasma instabilities and turbulence through a stellarator lens video , Google Scholar
    Alessandro Zocco, IPP Greifswald, Germany , abstract
    [#s1667, 01 Jun 2023]
    Having non-planar magnetic axis and rotating highly-shaped poloidal cross sections, stellarators are the most complex devices in the family of toroidal chambers used for magnetic confinement fusion. Their complexity, if not dealt with care, can lead to a string of undesirable transport properties such as unsustainable collisional heat losses, hollow density profiles, impurities accumulation, and lack of confinement of trapped and energetic particles. The discovery of stellarator quasi-symmetries, however, opened the way to the solution of the many problems stellarators faced for decades. Today, the stellarator is a viable reactor concept whose potential is constantly reinvigorated by the results of the Wendelstein 7-X (W7-X) experiment, based at Greifswald, Germany. In this talk, we will examine some questions that a modern experiment like W7-X is compelling us to reconsider, and propose the answers to them. Are heat losses turbulent? Which turbulence? How does complex geometry change turbulence? What suppresses or exacerbates this turbulence? How does it saturate? Do electromagnetic fluctuations affect the turbulence saturation process? Do impurities accumulate? Do we observe current-driven magnetic reconnection in a stellarator, even if it was designed to be virtually "current free"? We will close our discussion by summarising the key steps needed to be taken to address the problem of turbulence and stability in high-performance stellarator reactors.
  • Subion-scale turbulence driven by Magnetic Reconnection , Video
    Davide Manzini, École Polytechnique, France , abstract
    [#s1666, 25 May 2023]
    The interplay between plasma turbulence and magnetic reconnection remains an unsettled question in astrophysical and laboratory plasmas. Here we report the first observational evidence that magnetic reconnection drives subion-scale turbulence in magnetospheric plasmas by transferring energy to small scales. We employ a spatial coarse-grained model of Hall magnetohydrodynamics, enabling us to measure the nonlinear energy transfer rate across scale ℓ at position x. Its application to Magnetospheric Multiscale mission data shows that magnetic reconnection drives intense energy transfer to subion-scales. This observational evidence is remarkably supported by the results from Hybrid Vlasov-Maxwell simulations of turbulence to which the coarse-grained model is also applied. These results open new pathways to investigate the interplay between turbulence, reconnection and energy dissipation in collisionless magnetized plasmas and can potentially explain spacecraft observations in various planetary magnetosheaths that showed the ubiquity of turbulent fluctuations at sub-ion scales even when no energy cascade emerges from the larger (MHD) scales.
  • A Kinetic Theory Generalization of the First Law of Thermodynamics Capturing Non-Equilibrium Effects , Video , Google Scholar
    Paul Cassak, West Virginia University, USA , abstract
    [#s1665, 18 May 2023]
    Plasmas in many settings are out of local thermodynamic equilibrium (LTE) because the time scales for collisions to achieve LTE are longer than dynamical time scales in the plasma. Examples abound in naturally occurring and laboratory plasmas - the solar corona, interplanetary space, magnetospheres of Earth and other planets, interstellar space, compact astrophysical objects, magnetically confined fusion plasmas, high energy density plasmas, and low temperature plasmas. How energy is converted in plasmas that are not in LTE is a forefront research area across the plasma sciences, and is only well-understood for small departures from LTE. However, many plasmas can be far from LTE, and therefore a non-perturbative approach is needed. The standard approach to studying energy conversion in plasmas that can be far from LTE is to investigate changes in thermal energy and the work being done that changes the density, which are both captured by the first law of thermodynamics. In this talk, we argue that this approach omits energy conversion associated with any other higher order moments of the phase space density f, and that energy conversion associated with these moments can be important for systems far from LTE. We perform a first-principles, non-perturbative, analytic theory of the energy associated with all higher order moments of f. It is based on the concept of entropy in kinetic theory and is derived from the Boltzmann equation. We use the result to calculate energy conversion in particle-in-cell simulations of collisionless two-dimensional magnetic reconnection, which reveal that energy conversion associated with higher order moments of f can be locally significant. The theoretical results may be useful in a wide array of plasma settings.
  • Intermittency of density fluctuations and zonal flow generation in MAST edge plasmas , Google Scholar
    Alsu Sladkomedova, Tokamak Energy Ltd, UK , abstract
    [#s1664, 11 May 2023]
    Edge-plasma fluctuations in a tokamak represent a complex system exhibiting intermittent behaviour due to the presence of coherent structures. These structures are long-lived plasma filaments with large-amplitude positive (blobs) and negative (holes) density fluctuations. It is crucial to understand edge fluctuations because of the link between the pedestal pressure and core confinement. Coherent structures may also impact on the scrape-off-layer width, which is important for predicting heat loads on plasma-facing components. In this presentation, we analyse the edge ion-scale density fluctuations using the Beam Emission Spectroscopy diagnostic on the spherical tokamak MAST. We provide evidence of high-amplitude coherent structures, which are blobs and density holes, existing amongst ambient turbulent fluctuations. Our measurements of radial velocity and skewness of the density fluctuations indicate that density holes propagate radially inwards, with the skewness profile peaking at 7−10 cm inside the separatrix. Our research reveals that the bursts in the density-fluctuation power, originating from turbulence and radially moving density holes, are followed by an increase in the power of a Geodesic Acoustic Mode. We utilise bispectral techniques to illustrate non-linear interactions between zonal flows and density oscillations.
  • UKAEA’s fusion energy strategy , Video , Website
    Sir Ian Chapman (CEO, UKAEA), UK Atomic Energy Authority, UK , abstract
    [#s1644, 04 May 2023]
    This talk will begin by explaining the UK government strategy to delivering fusion and setting this in the context of other major fusion programmes internationally. We will explore the new facilities and programmes being pursued at UKAEA, including an overview of the progress made on the prototype powerplant STEP – the Spherical Tokamak for Energy Production. Finally, the talk will explore the relationships between the public and private sector that will be important in the delivery of fusion power.
  • Cosmic ray plasma physics and magnetic dynamos in galaxy formation video , Website
    Christoph Pfrommer, Leibnitz Institute for Astrophysics, Germany , abstract
    [#s1643, 27 Apr 2023]
    Understanding the physics of galaxy formation is an outstanding problem in modern astrophysics. Recent cosmological simulations have demonstrated that feedback by star formation, supernovae and active galactic nuclei appears to be critical in obtaining realistic disk galaxies and to slow down star formation to the small observed rates. However the particular physical processes underlying these feedback processes still remain elusive. In particular, many of these simulations neglected magnetic fields and relativistic particle populations (so-called cosmic rays). Those are known to provide a pressure support comparable to the thermal gas in our Galaxy and couple dynamically and thermally to the gas, which seriously questions their neglect. Early attempts to include magnetic fields and cosmic rays put the plasma kinetics of wave-particle interactions into the limelight for explaining the power of galactic winds and for regulating star formation in and cosmic accretion onto galaxies. After introducing the underlying physical concepts, I will present our recent efforts to model kinetic plasma physics in macroscopic hydrodynamical models of galaxy formation. In particular, I will explain how cosmic rays interact with and propagate through the magnetized plasma in the interstellar and circumgalactic media. I will elucidate the role of plasma instabilities in regulating the momentum transfer from cosmic rays to the background plasma and how we can observationally test these theoretical considerations using new high-sensitivity MeerKAT observations. I will then demonstrate that cosmic rays play a decisive role in the formation and evolution of spiral galaxies by providing feedback that regulates star formation and drives gas out in galactic winds. Comparing cosmic ray spectra of electrons and protons to observational data and studying the correlation of the far-infrared emission with the gamma-ray and radio emission from galaxies enables us to test the cosmic ray feedback and dynamo models for the growth of galactic magnetic fields. This argues that a complete understanding of galaxy formation necessarily includes plasma astrophysics.
  • Energy partition in collisionless shocks: a microphysical perspective video Google Scholar
    Frederico Fiuza, SLAC, Stanford, USA , abstract
    [#s1642, 20 Apr 2023]
    Astrophysical shock waves are among the most powerful particle accelerators in the Universe. Generated by violent interactions of supersonic plasma flows with the interstellar or intergalactic medium, shocks are inferred to heat the plasma, amplify magnetic fields, and accelerate electrons and protons to highly relativistic speeds. However, the exact mechanisms that control energy partition in shocks remain a mystery. This is particularly challenging for high Mach number shocks, such as those associated with supernova remnants, where spacecraft data in the relevant regime is scarce and the shock structure cannot be directly resolved from observations. I will review recent progress in using the combination of fully kinetic simulations and laser-driven laboratory experiments to study energy partition in high-Mach number collisionless shocks. In particular, I will present results on magnetic field amplification, plasma heating and particle acceleration, and discuss how experimental measurements are helping benchmark models of the shock microphysics.
  • Advancing the concept of the quasi-isodynamic stellarator Video, Google Scholar
    Gabe Plunk, IPP Greifswald, Germany
    [#s1641, 13 Apr 2023]
  • Advances in Stellarator Design in Nuclear Fusion Research: The Direct Optimisation Architecture Google Scholar ,Video
    Rogerio Jorge, Instituto Superior Técnico, Portugal , abstract
    [#s1640, 06 Apr 2023]
    In pursuit of magnetic confinement nuclear fusion, magnetic fields of premier quality are imperative. These fields must sustain high-heat plasmas for an adequate amount of time while also enabling control of plasma density, fast particle management, and turbulence regulation. The standard design of stellarator machines, as seen in experiments such as HSX and W7-X, optimises magnetic fields and coils separately, resulting in limited engineering tolerances and often neglect turbulent transport in the optimisation process. Furthermore, such process is highly dependent on the initial conditions used and often needs multiple restarts with relaxed requirements, making the process inefficient and potentially compromising the optimal balance between alpha particles, neoclassical transport, and turbulence. Recent breakthroughs in the optimisation of stellarator devices, such as direct near-axis designs, integrated plasma-coil optimisation algorithms, precise quasisymmetric and quasi-isodynamic fields, and direct turbulence optimisation, are revolutionising the way in which these machines are designed. The main outcomes of these advancements, as well as the prospects for a more efficient fusion device, will be discussed in this presentation.
  • Design of the first fusion laboratory experiment to achieve target gain > 1 Video, Google Scholar
    Annie Kritcher, Lawrence Livermore National Laboratory, USA , abstract
    [#s1639, 30 Mar 2023]
    The inertial fusion community have been working towards ignition for decades, since the idea of inertial confinement fusion (ICF) was first proposed by Nuckolls, et al., in 1972. On August 8, 2021 and Dec 5th 2022, the Lawson criterion for ignition was met and more fusion energy was created than laser energy incident on the target at the National Ignition Facility (NIF) in Northern California. The first experiment produced a fusion yield of 1.35 MJ from 1.9 MJ of laser energy and appears to have crossed the tipping-point of thermodynamic instability according to several ignition metrics. Building on this result, improvements were made to increase the fusion energy output to >3MJ from 2.05 MJ of laser energy on target, resulting in target gain exceeding unity for the first time in the laboratory. This result is important in that it proves that there is nothing fundamentally limiting controlled fusion energy gain in the laboratory.
  • The role of structures and kinetic effects in astrophysical plasma turbulence , Video , Google Scholar , Website
    Sergio Servidio, University of Calabria, Italy , abstract
    [#s1650, 23 Mar 2023]
    Astrophysical plasmas exhibit complex behavior similar to the erratic motion of turbulent viscous fluids. Despite the undeniable differences between collisionless plasmas and classical collisional fluids, there are a few (but remarkable) similarities: turbulent plasmas, like ordinary fluids, are characterized by the presence of a sea of coherent, long-living structures at which dissipation might occur. Local magnetic reconnection events, flux tubes, velocity shears, small-scale compressive layers, and so on are examples of such structures. In this talk, we will start our journey in the heliosphere by reviewing some recent (and less recent) results and focusing on the role that the above fauna of coherent patterns plays in turbulence. Since these intermittent spikes are highly correlated with local non-Maxwellian (kinetic) patterns, they could be the "hot spots" for energy conversion mechanisms in poorly-collisional systems. The end result of this interaction between coherent fields and particles is a "phase space" turbulent cascade, which has been invoked by theory, supported by simulations, and confirmed by observations. We investigate the locality of the phase space dynamics both in homogeneous turbulence as well as in the unique interaction between astrophysical shocks and turbulence, by presenting a new coarse-graining transport model. Finally, taking advantage of this experience with the heliospheric wind, we will end our trip nearby highly turbulent, supermassive black holes, demonstrating the need to model plasma accretion onto compact objects by microscopic, kinetic plasma models.
  • Spectral properties and energy transfer at kinetic scales in collisionless plasma turbulence , Video
    Giuseppe Arrò, Katholieke Universiteit Leuven, Belgium , abstract
    [#s1649, 16 Mar 2023]
    Turbulence in collisionless magnetized plasmas is a complex process involving the transfer of energy from large magnetohydrodynamic (MHD) scales to small ion and electron kinetic scales. This multi-scale nature of plasma turbulence is reflected in the properties of the turbulent spectra that have different shapes in different ranges of scales. Satellite observations show that at MHD scales the magnetic field spectrum follows a power-law that breaks at ion scales, where a steeper power-law develops. A second break and steepening is observed also towards electron scales but the shape of the spectrum in this range is still under debate and it is not clear if it follows a new power-law or if an exponential falloff develops. By means of a fully kinetic simulation of freely decaying plasma turbulence, we study the spectral properties and the energy transfer characterizing the turbulent cascade from large to kinetic scales. We find that both the magnetic field and electron velocity spectra have an exponential decay at scales of the order of the electron gyroradius. On the other hand, the ion velocity spectrum shows a steep power law behaviour at sub-ion scales, without reaching far into electron scales. The dynamics responsible for the development of such spectral features is investigated by analysing the scale-filtered energy balance. Our study reveals the presence of an indirect electron-driven mechanism that channels the electromagnetic (e.m.) energy from large to sub-ion scales more efficiently than the direct nonlinear scale-to-scale transfer of e.m. energy. This mechanism consists of three steps: in the first step the e.m. energy is converted into electron fluid flow energy at large scales; in the second step the electron fluid flow energy is nonlinearly transferred toward sub-ion scales; in the final step the electron fluid flow energy is converted back into e.m. energy at sub-ion scales.
  • Ian Abel, University of Maryland, USA
    [#s1633, 09 Mar 2023]
  • Multiscale Nature of Turbulence in the Tokamak Pedestal: Exploiting Extreme-Scale Computing with Gyrokinetic Simulations Google Scholar
    Emily Belli, General Atomics, USA , abstract
    [#s1632, 02 Mar 2023]
    Plasma confinement in tokamaks is limited by slow particle and energy losses due to turbulence, driven by unstable drift waves triggered by plasma inhomogeneities. Understanding the mechanisms that drive turbulence in burning plasmas is necessary to design next-generation fusion reactors like ITER with optimal confinement properties and fusion performance. While ion-scale turbulence in the tokamak core has been modeled extensively, gyrokinetic simulations of turbulent transport in the edge pedestal of high-confinement mode (H-mode) regimes are more difficult due to the large magnetic shear, strong flux-surface shaping, and proximity to ballooning stability limits. In addition, in the H-mode pedestal, large gradients in density and temperature can drive multiple drift-wave instabilities across a broad range of scales, namely ion and electron scales that differ in size by nearly two orders of magnitude. Simulating this so-called "multiscale turbulence", which captures the complex interaction between the slow, large scale motion of the hydrogenic ions and the fast, small-scale motion of the electrons, requires leadership-scale computing resources and advanced numerical algorithms. In this talk, we will present a first analysis of the spectral transition of multiscale turbulence in pedestal-like regimes. The ensemble of multiscale simulations required over 250k node-hours on the 200-petaflop OLCF Summit supercomputer. The simulations show that the experiments can lie in an interesting bifurcation region between ion-scale and multiscale-dominated turbulence regimes. The mechanisms by which electron-scale transport can be reduced by nonlinear mixing with ion-scale fluctuations and ion-scale driven zonal flows will be reviewed. These simulations were enabled using recents advances in numerical algorithms for gyrokinetic simulations that use spectral/pseudospectral algorithms to target scalability on next-generation, exascale HPC systems that use multicore and GPU-accelerated hardware. Preparations for scaling-up to Frontier, the first supercomputer to achieve exaflop performance, will also be discussed.
  • On the Kinetics of Spinning Stellar Systems or, The Galactic Tokamak video, Google Scholar
    Chris Hamilton, IAS, Princeton, USA , abstract
    [#s1618, 23 Feb 2023]
    Many galaxies, including our own galaxy the Milky Way, have a ‘bar’ structure at their center— an elongated collection of millions of stars, that spins as if it were a solid body. Galaxies are also embedded in massive dark matter haloes. When the rate at which the bar rotates resonates with a dark matter particle’s orbital frequency, the dark matter can suck angular momentum out of the bar, causing it to slow down. Previous theories of the bar-halo interaction calculated this ‘dynamical friction’ on the bar in a manner directly analogous to Landau’s calculation of the collisionless damping of an electric wave (and subsequently to O’Neil’s nonlinear generalisation thereof). This is no coincidence — bar-halo interactions are just one of a plethora of gravitational dynamics problems that have direct plasma-kinetic analogues. In this talk I will introduce the astrophysical context of galactic bars and their host dark matter haloes, and describe some of the aforementioned classic studies of the bar-halo interaction. However, those studies routinely ignored the fact that dark matter particles also experience random ‘diffusive’ kicks from other passing dark matter clumps, gas clouds, and so on. I will describe recent work done in collaboration with Princeton plasma theorists on quantifying the impact of diffusion on bar-halo friction, a problem which turns out to be mathematically identical to that of understanding particle energization in tokamaks. More broadly, I will argue that galactic dynamics has, over the last several decades, largely failed to learn from its more well-developed cousin and therefore has a lot of catching up to do. But I will also argue that in return, we stellar dynamicists can provide you plasma theorists with fresh contexts in which to ply your trade, and an opportunity to work on something both fantastically beautiful and totally useless.
  • Marta Fajardo, Instituto Superior Técnico, Portugal , abstract
    [#s1617, 16 Feb 2023]
    Solid density plasmas are present in Inertial Confinement Fusion, astrophysical objects and in material processing with lasers. With moderate temperatures (ranging from 0.1 to 100 eV) and roughly solid densities (0.1 to 10 times nsolid), the Warm Dense Matter state sits in the transition between solid, liquid gas and plasma. Warm dense plasmas have comparable thermal and Fermi energies, and sufficiently high densities for the Coulomb potential to exceed ion kinetic energy. Challenging to describe accurately with reduced theoretical frameworks, this last frontier of plasma physics is now being explored thanks to recent developments in numerical and experimental capabilities. Our group has been pursuing the diagnostic of warm, solid density plasmas for the past decade. Such states can be isolated by ultrafast isochoric heating, via X-ray laser heating of bulk solid targets, or IR heating of skin-depth thin foils. Probes capable of penetrating solid densities and with femtosecond resolution are required to capture their properties. Our team has led the use of High Harmonic Generation in the EUV to probe these elusive states of matter. In parallel with bulk heating experiments at XFELs, we have built a platform for creating and probing Warm Dense Matter by femtosecond heating of free-standing nano-foils in Lisbon. Here, we give an overview of the insights gained from XFEL and IR laser experiments on solid density plasmas, and propose novel opportunities for WDM studies from small-scale to large-scale facilities. All-optical Time-and-space resolved measurements of electron-ion coupling in solid density titanium plasma. GO Williams, S. Antunes, M Fajardo – to be published Collisional ionization and recombination in degenerate plasmas beyond the free-electron-gas approximation. GO Williams, M Fajardo - Physical Review E, 2020 Impact of free electron degeneracy on collisional rates in plasmas. GO Williams et al- Physical Review Research, 2019
  • PIC simulations of the magnetorotational instability (MRI) in stratified, collisionless accretion disks Google Scholar video
    Mario Riquelme, University of Chile, Chile , abstract
    [#s1615, 09 Feb 2023]
    Plasma accretion onto astrophysical compact objects, such as black holes and neutron stars, is often considered to occur in the collisionless regime. This implies that kinetic plasma processes may play a crucial role in the physics of accretion in these systems, giving rise to important phenomena like non-thermal particle acceleration, temperature anisotropies and different temperatures between ions and electrons. Kinetic simulations, beyond MHD modeling, are thus needed in order to acquire a fully self-consistent picture of the processes that participate in disk accretion and to predict their observational signatures. In this talk, we will first present our initial results of 2D and 3D fully kinetic, particle-in-cell (PIC) plasma simulations of the collisionless magnetorotational instability (MRI), an MHD-scale instability that is crucial to produce outward transport of angular momentum in accretion disks. Our simulations are local and stratified, which means that we use the shearing box approximation and self-consistently include the vertical stratification of the disk. We find that the MRI saturation and the transport of angular momentum in our stratified simulations are more efficient than in the case where disk stratification is not included. Particle acceleration in our runs is efficient and mainly driven by magnetic reconnection, and is also more significant when the simulations include stratification. In the final part of the presentation, we will also focus on the non-linear evolution of (microscopic) temperature anisotropy instabilities, which play a crucial role in the MRI evolution by providing an effective collisionality to the plasma. We will show that these instabilities can also contribute to non-thermal particle acceleration in collisionless accretion disks. Our simulation results mainly concentrate on the sub-relativistic plasma regime, relevant for accretion onto black hole systems like Sgr A* and M87.
  • On the origin of magnetic fluctuations in low magnetic Prandtl number plasmas Google Scholar
    Maarit-Korpi-Lagg, Aalto University, Finland , abstract
    [#s1616, 02 Feb 2023]
    Magnetic fields on small scales are ubiquitous in the universe. For example, the fluctuating magnetic fields in star-forming regions of galaxies are more than twice the strength of the magnetic fields coherent over large scales. On the solar surface, magnetic fields are mostly concentrated in medium and small-scale structures, while the proportion comprising the mean field strength is even lower than in galaxies. The generation mechanisms of the fluctuating magnetic fields are not fully understood. One possibility is the so-called small-scale dynamo (SSD), the other is tangling of the large-scale field structures through turbulence acting on them. In the interstellar medium of galaxies, the resistivity η is much lower than the viscosity ν, such that magnetic instabilities are easier to excite relative to the turbulence. SSD in such high magnetic Prandtl number (Pm= ν / η) conditions has therefore been predicted to be easily excited. In the Sun and cool stars, Pm is much lower, namely in the range of 10-6 - 10-3. Both theoretically and especially numerically, SSD is more difficult to excite at such very low magnetic Prandtl numbers. Indeed, some recent numerical studies had indicated that the threshold for SSD excitation should systematically increase with decreasing Pm, concluding that SSD would be impossible in the Sun and cool stars. Continuing increases in CPU resources, for now, have permitted us to perform even higher-resolution simulations employing the lowest Pm values to date to mimic solar conditions. Contrary to earlier findings, the SSD turns out not only to be possible for Pm down to 0.0031, but even to become increasingly easy to excite for Pm below ≃ 0.05. We relate this behaviour to the known hydrodynamic phenomenon, referred to as the bottleneck effect. Extrapolating our results to solar values of Pm indicates that an SSD would be possible under such conditions. We have recently developed a GPU-accelerated version of our solver, capable of bringing us to the solar parameter regime in terms of Pm in the near future, rendering the extrapolations into real findings in the solar regime.
  • Electron scale kinetic instabilities in magnetized plasmas via radiation reaction and laser ionization , Google Scholar
    Luis Oliveira e Silva, Instituto Superior Técnico, Universidade de Lisboa, Portugal , abstract
    [#s1613, 26 Jan 2023]
    Anisotropic electron distribution functions in a strong magnetic field can be unstable to the electron cyclotron/synchrotron maser instability (ECMI), leading to the generation of coherent radiation. We explore laboratory and astrophysical scenarios where this instability can be present and/or generated due to collective processes and/or by carefully tuning the experimental conditions. We show that radiation reaction, in the classical and quantum regimes, depicts a general property that can lead to ring-like electron populations that are unstable to the ECME. A theoretical model is developed and compared with QED PIC simulations [1]. Laboratory scenarios where ring-like distributions can be generated via radiation reaction are also explored and demonstrated. We generalize previous works on anisotropic distributions driven by laser-ionized gases to demonstrate that similar distribution functions and the ensuing instability are naturally driven via a careful combination of laser parameters (intensity and polarization), plasma density, and external magnetic fields [2]. Fully self-consistent simulations demonstrate the key signatures of these processes, with an excellent agreement on the instability's features and the emitted radiation properties. Our results open two closely connected new directions, showing that radiation reaction naturally leads to strongly anisotropic ring-like distribution functions, prone to collective plasma instabilities and that these distributions and conditions can be mimicked in the laboratory allowing for the detailed exploration of maser radiation with the state-of-the-art laser technology. [1] P. Bilbao, L. O.Silva, arXiv:2212.12271 [2] T. Silva, P. Bilbao, L. O. Silva, in preparation (2023)
  • New insights on plasma turbulence in the boundary region of tokamaks , Google Scholar , Video
    Paolo Ricci, EPFL, Switzerland University, Switzerland , abstract
    [#s1612, 19 Jan 2023]
    One of the greatest uncertainties in the success of the fusion program is related to the turbulent dynamics of the fusion fuel in the boundary region. The plasma behavior in the boundary governs the overall confinement properties of the device, regulates the impurity dynamics and the level of fusion ashes, and determines the heat load to the vessel walls – a showstopper for the whole fusion program if material requirements cannot be met. With the goal of improving our understanding of the boundary dynamics, the GBS code was developed during the past years. By solving the drift-reduced Braginskii equations self-consistently with the kinetic neutral atom dynamics, GBS simulates the plasma turbulence in the boundary of tokamaks as it results from external sources, recycling, turbulent transport and flows, and plasma losses at the vessel. GBS simulations have allowed us to advance the basic understanding of boundary turbulence. This incudes progress in estimating the SOL width, a crucial quantity to determine the heat flux on the vessel walls, and the identification of a turbulent regime characterized by a catastrophically large turbulent transport, which has been associated with the crossing of the density limit. We will present an overview of our simulation and theoretical results, as well as their comparison with experimental measurements from several tokamaks worldwide. Predictions for ITER and other future reactors will be discussed.
  • 6-dimensional hybrid-Vlasov modelling of the near-Earth space: First Vlasiator results , Google Scholar
    Minna Palmroth, University of Helsinki, Finland , abstract
    [#s1609, 12 Jan 2023]
    Numerical simulations are key in modern space physics, as they can be used as 1) context to data, 2) predict future behaviour of the system, 3) understand the system using unforeseen boundary conditions, and increasingly also in 4) discovering new phenomena that are hard to be observed using point-wise satellite measurements. Especially, the discovery of new phenomena pertains to global systems, where phenomena of interest may be initiated far away from the point of observations. The most typical method of simulating the global solar wind - magnetosphere - ionosphere system is based on magnetohydrodynamics (MHD), which is however not representing the actual plasma behaviour in locations where kinetic physics becomes important. Such regions are e.g., the foreshock - magnetosheath interaction, reconnection, and the inner magnetosphere. Vlasiator is the world’s first and so far the only global simulation based on the hybrid-Vlasov approach that simulates the ion distributions accurately without noise. Earlier Vlasiator results have shown without a doubt that ion-kinetic effects cannot be neglected from the large scales, as small-scale phenomena affect large scales and vice versa. This scale coupling leads to phenomena that are not predicted using local simulations without proper boundary conditions, or with spacecraft measurements lacking the global context. Here, we present the world’s first global 6-dimensional ion-kinetic global magnetospheric simulation run, accurate both locally and globally. The simulation box extends from the dayside to the nightside, and includes global dynamics and both dayside and nightside reconnection regions. We show the newest runs showing a magnetotail eruption, and discusses the physics leading to this phenomenon that has puzzled space scientists for decades.
  • Magnetospheric waves and bursts from magnetars , Video
    Andrei Beloborodov , Columbia University, USA
    [#s1577, 15 Dec 2022]
  • Fundamental Scaling of Adiabatic Compression of Field Reversed Configuration Thermonuclear Fusion Plasmas , Video
    David Kirtley, Helion Energy, USA
    [#s1576, 08 Dec 2022]
  • Fully kinetic simulations of plasma accretion in a three-dimensional shearing box , Google Scholar , Video
    Fabio Bacchini, Katholieke Universiteit, Leuven, Belgium , abstract
    [#s1575, 01 Dec 2022]
    One of the main uncertainties in the physics of plasma accretion onto compact objects (black holes and neutron stars) is represented by the dynamics of microscopic processes at kinetic scales. Although often expected to be collisionless, accretion flows around black holes are customarily studied with magnetohydrodynamic (MHD) models; these models however do not include out-of-equilibrium physics and cannot describe nonthermal particle acceleration and radiation, particle scattering and diffusion, and the transport of angular momentum through astrophysical accretion disks. To study these processes self-consistently, fully kinetic simulations are necessary; but the prohibitive costs typically associated to such numerical experiments have so far inhibited significant progress in this direction. In this talk, we will present the first mesoscale (i.e. attaining global MHD-like behavior) simulations of plasma accretion carried out with a fully kinetic approach in three dimensions. Our work is based on the shearing-box paradigm, which allows the first-principles study of a localized sector of an accretion disk at affordable costs. Our 3D kinetic simulations are large enough to reach convergence (with respect to physical parameters and box size), allowing us to analyze the detail of particle acceleration and angular-momentum transport in the turbulent plasma dynamics developing in a typical collisionless accretion flow.
  • No seminar today due to Thanksgiving Holiday
    [#s1574, 24 Nov 2022]
  • Accelerating the rate of discovery: Toward high-repetition-rate laser experiments for High-Energy Density (HED) Science and Inertial Fusion Energy (IFE) , Google Scholar , Website , Video
    Tammy Ma, Lawrence Livermore National Laboratory, USA , abstract
    [#s1547, 17 Nov 2022]
    As high-intensity short-pulse lasers that can operate at high-repetition-rate (HRR) (>10 Hz) come online around the world, the high-energy-density (HED) science they enable will experience a radical paradigm shift. The >10^3 increase in shot rate over today’s shot-per-hour drivers translates into dramatically faster data acquisition and more experiments, and thus the potential to significantly accelerate the advancement of HED science. However, to fully realize the potential benefits of HRR facilities requires a fundamental shift in how they are operated, and in fact, how the experiments done on them are designed and executed. Current energetic driver facilities depend on the ability to manually tune the lasers, the targets, the diagnostics settings, and more, between single shots or sets of shots through a manual feedback loop of data collection, data analysis, and optimization largely driven by experience and intuition. At 10 Hz, this paradigm is no longer sustainable as more complex data is collected more quickly than is possible to analyze manually. Simultaneously, on-the-fly optimization of experiments will become ever more crucial as higher repetition rates will lead to more deliberate inter-shot variations and the improved operational range to allow exploration over larger regions of phase space. Consequently, it is likely that the next generation of laser facilities will be limited not by their hardware but by our ability to use it effectively. We will present the vision and ongoing work to realize a HRR framework for rapidly delivered optimal experiments coupled to cognitive simulation to provide new insights in HED science, and lay the groundwork for Inertial Fusion Energy (IFE) rep rate operations.
  • The root cause of 2,1 tearing instabilities in DIII-D H-mode plasmas , Google Scholar
    László Bardóczi, General Atomics, USA , abstract
    [#s1511, 10 Nov 2022]
    Tearing modes open magnetic islands on rational surfaces of tokamaks, which ruin the confinement and can lead to plasma termination. The Neoclassical Tearing Mode is the leading single root cause of disruptions of high performance tokamak experiments. In order to develop and project stable operating solutions to future machines, a first principles based and experimentally validated model is necessary. I will present the analysis of an ensemble of over 16,000 DIII-D H-mode discharges exploring the root cause of 2,1 tearing onsets and its parameter dependency. The onset time distribution is exponential and the instability onset rate is constant in intermediate and high edge safety factor plasma scenarios, in accordance with Poisson-point processes as for example the radioactive decay. This supports that the instability has a threshold for growth and it is triggered by random events, in line with the Rutherford-theory of neoclassical tearing modes (NTMs). In scenarios characterized by low edge safety factor, such as the ITER baseline scenario, the onset rate increases over the course of the βN flattop, indicating that this group of plasmas evolve toward more unstable conditions. The onset statistics are well replicated by Poisson point process models, in contrast to models of classical stability index evolution. At a low edge safety factor, the increasing rate parameter is correlated with the drop of differential rotation between the q=1 and q=2 surfaces to near zero. Explanation is offered by the reduction of stabilizing polarization currents acting on sawtooth driven 2,1 seed islands when the differential rotation is approaching zero. The rate parameter and onset relative frequency analyses show strong correlation with βN , in agreement with NTMs. The magnetic perturbation amplitude of these islands grows linearly in time, as expected from NTMs, and it is unaffected by the current profile relaxation, suggesting that classical effects are weak. Recent experiments testing the effect of the current profile shape at q=2 on the 2,1 tearing instability found no causality between the island onset and the shape of the current profile. Overall, the agreement of various aspects of the data with the Rutherford theory supports that the 2,1 islands are NTMs triggered by random events in DIII-D at all edge safety factor values. This database analysis also suggests that the plasma stability can be improved by reducing the elongation, triangularity, density, impurity density, increasing the internal inductance and by running the plasmas in upper single null configuration.
  • Evolution of current end vorticity sheets in collisionless plasma turbulence , Video , Google Scholar
    Daniela Grasso, Lawrence Livermore National Laboratory, USA , abstract
    [#s1546, 03 Nov 2022]
    The evolution of current and vorticity sheets in collisionless plasmas, where magnetic reconnection within turbulence may take place driven by the electron inertia, is studied. We start from a linear analysis of magnetic vs. fluid instability, that might affect these layers, to understand the complex situation that generates in a turbulent plasma. Here, due to the presence of strong velocity shears, the typical plasmoids formation, observed to influence the energy cascade in the resistive magnetohydrodynamic context, has to coexist with the Kelvin-Helmholtz instability. We find that the current density layers may undergo the plasmoid or the Kelvin-Helmholtz instability depending on the local values of the magnetic and velocity fields. The competition among these instabilities affects not only the evolution of the current sheets, that may generate plasmoid chains or Kelvin-Helmholtz-driven vortices, but also the energy cascade, that is different for the magnetic and kinetic spectra.
  • ST40: Advancing the Physics Basis of Spherical Tokamak Reactors , Video , Google Scholar , Website
    Michele Romanelli, Tokamak Energy Ltd, UK
    [#s1545, 27 Oct 2022]
  • Collective modes in QED plasma , Website
    Mikhail Medvedev, University of Kansas, USA , abstract
    [#s1544, 13 Oct 2022]
    Ultra-magnetized plasmas where the magnetic field strength exceeds the Schwinger (critical) field become of great scientific interest, thanks to the astrophysical observations of magnetar emission and current advances in laser-plasma experiments. These advances demand better understanding of how quantum electrodynamics (QED) effects that are present in ultra-strong-field environments affect plasma dynamics. Interestingly, magnetars -- neutron stars with magnetic fields of ~1e15 Gauss or greater -- do exist and QED effects on their magnetospheric plasma cannot be ignored. In particular, Maxwell's equations become nonlinear in the strong-QED regime. This effect has not been systematically considered in theoretical studies. Here we discuss how "textbook" linear plasma modes are modified in an arbitrarily strong magnetic field. These results can be important for understanding of a magnetospheric pair plasma of a magnetar and for future laser-plasma experiments.
  • Advances in the understanding of ultrarelativistic beam-plasma instabilities , Video
    Laurent Gremillet, CEA-DAM-DIF, France, abstract
    [#s1542, 06 Oct 2022]
    Relativistic beam-plasma instabilities are thought to arise in many astrophysical systems. One of their major effects is to dissipate into heat, suprathermal particles and radiation the kinetic energy of fast outflows from powerful objects. This occurs via self-induced, kinetic-scale electromagnetic fields that scatter and decelerate particles to the point of forming collisionless shocks waves or triggering bright synchrotron-type emissions. In initially unmagnetized, relativistic beam-plasma systems, two main instability classes are known to prevail: the essentially electrostatic, oblique two-stream modes (OTSI) and the essentially magnetic, transverse current filamentation (CFI) modes. These can also operate, usually detrimentally, in laboratory schemes involving the interpenetration of relativistic electron streams and plasmas, as in plasma-wakefield accelerators or ultraintense laser-solid interactions. My talk will cover some recent theoretical and particle-in-cell simulation results on relativistic beam-plasma instabilities. Most of this work is connected with the E 305 project underway at the SLAC/FACET-II accelerator, which aims to probe the instabilities excited by a 10 GeV electron beam through gas or solid materials. First, I will show that the OTSI dynamics in the case of a finite-size beam impinging onto a collisionless, semi-infinite plasma deviates from the standard picture of a uniform, infinite system. I will present a model for the spatiotemporal effects caused by a sharp beam front and discuss the competition between the OTSI and the self-focusing of a finite-width beam, an issue of prime relevance for experiments. Then, I will consider the case of solid-density target and demonstrate that the hierarchy between the OTSI and CFI depends on the target collisionality and the beam density. The simulation results will be interpreted in light of a general kinetic linear theory of the OTSI and CFI modes in the presence of collisional background electrons. Finally, I will report our latest analysis of the saturation mechanisms of the CFI in asymmetric plasma flows, under conditions representative of relativistic collisionless shocks.
  • Relativistic Magnetospheres: Current sheets, Reconnection, Particle Acceleration , Website , Video
    Benoit Cerutti, University of Grenoble, France, abstract
    [#s1540, 29 Sep 2022]
    Spinning neutron stars and black holes are the central engines of some of the most extreme astrophysical phenomena such as gamma-ray bursts, pulsars, X-ray binaries, binary mergers, or active galactic nuclei. The activity of these compact objects is often associated with the creation and the launching of a relativistic magnetized plasma within their magnetospheres. Similarly to their non-relativistic analog in the Solar System, these magnetospheres are highly dynamical. Large-scale current sheets form and reconnect, leading to efficient particle acceleration, pair production and non-thermal radiation. However, the interplay between general relativity, quantum electrodynamics and plasma physics makes this problem not easily tractable. In this talk, I will review current efforts to model pulsar and black hole magnetospheres from first principles by means of global particle-in-cell simulations. Results will be discussed in the context of gamma-ray pulsars, and recent horizon-scale observations of the weakly accreting supermassive black holes M87* and SgrA*.
  • A private path to laser-fusion energy, Focused Energy, a new startup in the fusion community , Video , Website
    Markus Roth, Focused Energy Inc., Germany , abstract
    [#s1539, 22 Sep 2022]
    As clean and safe energy is needed more than ever new developments have led to the rise of startup companies around the globe taking advantage of the science developed of the years and combining the results of the past with the technology of the 21st century to make fusion energy a reality. Focused Energy is a US/German startup supported by the TU Darmstadt and deeply embedded in the international science community. We are focusing on the concept of direct-drive laser-based inertial confinement fusion and fast ignition. In our approach, a small pellet containing a milligram of DT is directly irradiated by intense laser light and compressed to roughly 1000 times solid density. At the moment of maximum density, a burst of energetic, laser-driven ion beams is focused into a small part of the compressed fuel to rapidly rise the temperature above ignition temperature and start a bootstrap fusion reaction, which results in a supersonic burn wave consuming the fuel. More than two decades of research have led to this path, which has recently been quoted the most promising approach in inertial fusion energy by international leaders in the field. We have started assembling the best scientist in the field and will move on a fast track to get fusion demonstrated in a decade time frame. Thus, we build a first facility, T-STAR and UT in Austin, while we are working on laser and target development in Darmstadt. Focused Energy finally plans to develop a demonstration facility within this decade to demonstrate ignition, burn and gain sufficient for attractive energy production based on the unique combination of high-energy and high-power lasers.
  • Quantum computing for plasma physics: An overview of recent progress , Video , Google Scholar , Website
    Yuan Shi, LLNL, USA , abstract
    [#s1538, 15 Sep 2022]
    Quantum computing (QC) promises to bring game changing capabilities. However, the potential for quantum speedup of plasma problems remains unclear. This colloquium surveys recent progress on applying QC to plasma related problems. To connect plasma physics with quantum hardware, both top-down and bottom-up approaches have yielded interesting results. The top-down approach starts from future ideal computers and reconceptualizes plasma problems to fit into the QC framework. The bottom-up approach starts from near-term noisy devices and builds up toy problems towards more realistic applications. The research field is still in its early stage, and wide gaps remain to be bridged before QC will become useful for plasma physics.
  • Plasma physics of Fast Radio Bursts , Video , Google Scholar , Website
    Maxim Lyutikov, Purdue University, USA , abstract
    [#s1537, 08 Sep 2022]
    Fast Radio Bursts are millisecond long events of coherent radio emission coming from half way across the Universe. The inferred plasma and radiation conditions are extreme: radio waves carry macroscopic amount of energy, the laser non-linearity parameter $a_0$ can be as large as millions, the guiding magnetic field may reach critical quantum values. I will highlight plasma challenges of producing high brightness coherent emission in astrophysical surrounding, new regimes of plasma-laser interaction, and discuss how the concept of Free Electron Lasers may help us to understand these mysterious events.
  • Plasma wave topology and topological plasma waves , Google Scholar
    Hong-Qin, PPPL, USA , abstract
    [#s1523, 01 Sep 2022]
    The hairy ball theorem is well-known to plasma physicists because it implies that the surface of the magnetic bottle for confining fusion plasmas should not look like a 2D sphere. Chern’s celebrated method of 1944 to prove the hairy ball (Gauss-Bonnet-Poincare) theorem can be used to prove that the cyclotron wave must vanish somewhere on a 2D sphere enclosing the Weyl point of Langmuir-cyclotron resonance in the parameter space. A physical consequence of this fact is that there must exist a topological surface excitation called Topological Langmuir-Cyclotron Wave (TLCW) in magnetized plasmas, which can propagate along complex phase transition interfaces in a unidirectional manner and without scattering. Due to this topologically protected robustness, the TLCW could be explored as an effective mechanism to drive current and accelerate particles in plasmas. The topological methods that we recently proposed for plasma waves bear similarities to those used in condensed matter physics in the past four decades. But there are significant differences. In condensed matters, periodic lattices lead to nontrivial topology in momentum space, but we show that in continuous media such as plasmas, nontrivial topology only exists in phase space because of the contractibility of momentum space. Using the algebraic topological concepts and tools that we recently developed, such as the boundary isomorphism theorem, and an Atiyah-Patodi-Singer type of index theorem formulated by Faure, the existence of TLCW as a spectral flow across the band gap is rigorously proven. We also show that the TLCW can be faithfully represented by a tilted Dirac cone. The entire spectrum of a generic tilted Dirac cone in phase space, including its spectral flow, is found analytically. [This research was supported by the U.S. Department of Energy (DE-AC02-09CH11466).] References: 1) Topological Langmuir-cyclotron wave. 2) The dispersion and propagation of topological Langmuir-cyclotron waves in cold magnetized plasmas. 3) Topological phases and bulk-edge correspondence of magnetized cold plasmas.
  • Investigating radiatively driven, magnetized plasmas with a university scale pulsed-power generator , Video , Website
    Jack Halliday, Imperial College London, UK , abstract
    [#s1522, 25 Aug 2022]
    We describe results from a novel experimental platform that is able to access physics relevant to topics including indirect-drive magnetized inertial confinement fusion, laser energy deposition, various topics in atomic physics, and laboratory astrophysics (for example, the penetration of B-fields into high energy density plasmas). This platform uses the x-rays from a wire array Z-pinch to irradiate a silicon target, producing an outflow of ablated plasma. The ablated plasma expands into ambient, dynamically significant B-fields (∼5 T), which are supported by the current flowing through the Z-pinch. The outflows have a well-defined (quasi-1D) morphology, enabling the study of fundamental processes typically only available in more complex, integrated schemes. Experiments were fielded on the MAGPIE pulsed-power generator (1.4 MA, 240 ns rise time). On this machine, a wire array Z-pinch produces an x-ray pulse carrying a total energy of ∼15 kJ over ∼30 ns. This equates to an average brightness temperature of around 10 eV on-target.
  • Exploring Novel Properties of Strongly Magnetized Plasmas , Google Scholar , Website
    Scott Baalrud, University of Michigan, USA, abstract
    [#s1521, 18 Aug 2022]
    An often-underappreciated aspect of plasma theory is that it assumes weak magnetization, applying only when the electron gyroradius is much larger than the Debye length. This allows one to ignore the magnetic field at the scale of Coulomb interactions. Here, we use first-principles molecular dynamics simulations and new theoretical methods to explore the properties of strongly magnetized plasmas. A few novel behaviors have been uncovered. One is that the friction force on a test ion moving through a strongly magnetized plasma shifts to obtain components that act perpendicular to its velocity. These components cause qualitative changes to the average trajectory of the ion, such as changing its gyroradius and gyrofrequency in non-intuitive ways. They also translate to qualitative changes in macroscopic material properties of the plasma, such as the electrical conductivity, viscosity, and energy relaxation rates. Although strongly magnetized plasmas are not the norm, they do arise in several contexts, including non-neutral plasmas, antimatter traps, high magnetic field approaches to fusion energy, and in dense astrophysical objects such as magnetars. These results suggest that unexpected behaviors arise in these systems, and it motivates potential applications that make use of these novel properties.
  • Turbulence and transport research beyond the burning plasma era , Google Scholar , Website
    Anne White, MIT, USA, abstract
    [#s1520, 11 Aug 2022]
    The prospect of near-term fusion electricity opens new doors for university-based plasma physics research. Even after the grand societal challenge of putting fusion on the grid is achieved, research addressing grand intellectual challenges in plasma transport will remain vibrant. University groups will engage with sponsors and collaborators including not only governments and national labs around the world, but also private companies and utilities. In this talk I present side-by-side examples of recent research results on turbulence and transport measurements, as well as predictive simulation and modeling, carried out by researchers at MIT in support of both the nascent fusion industry and the established fission industry. I will share my perspective, as an academic department head, on the future of fusion research in universities as we move through and beyond the era of burning plasmas.
  • Highlights of T and DT results from JET-ILW experiments , Google Scholar
    Jon Hillesheim, UK Atomic Energy Authority, UK - Chaired by: Bill Dorland, Editor, JPP , abstract
    [#s1519, 04 Aug 2022]
    JET has recently completed the first DT campaign in a tokamak since 1997, and for the first time ever with ITER-like W/Be divertor and first wall materials. Scientific results have been achieved in all goals of the campaign including demonstration of high (average Pfus>10 MW ) sustained fusion power for 5 s, demonstration of an integrated radiative scenario, demonstration of clear alpha particle effects, exploration of isotope and mixed plasma species effects on energy and particle transport, addressing plasma-wall interaction in DT plasmas, and demonstration of RF schemes relevant to ITER DT operation. Highlights from the DT campaign, and from the preparatory isotope campaigns, will be presented.
  • Compressed Current Sheets in the Magnetotail: Importance of the Ambipolar Electric Field video , Google Scholar
    Ami Dubois, Naval Research Laboratory, USA , abstract
    [#s1510, 28 Jul 2022]
    Micro-scale features are now being resolved by NASA’s Magnetospheric Multi-Scale (MMS) mission, which means for the first time, we are able to investigate thin and non-ideal current sheets (i.e. current sheets that cannot be explained by the Harris equilibrium model) in detail and assess their role in magnetic reconnection. We use MMS satellite data to analyze kinetic-scale structures and dynamics associated with compressed current sheets. Our analysis shows that a transverse ambipolar electric field is localized to the region of lower hybrid fluctuations and the pressure gradient in this region is comparatively small, leading to the interpretation that compression of the current sheet and the resulting velocity shear is the underlying fluctuation driving mechanism. Our kinetic equilibrium model shows that as a large scale Harris current sheet is compressed, an ambipolar electric field forms and produces velocity shear near the magnetic null, indicating that velocity shear-driven waves can arise in the center of compressed current sheets. The presence and location of shear-driven waves at the center of current sheets is notable for a couple of reasons. First, because laboratory experiments and PIC simulations have shown that shear-driven lower hybrid fluctuations are capable of producing significant anomalous cross-field transport and resistivity, which can trigger magnetic reconnection. Second, using MMS wave data we can calculate the anomalous resistivity directly and show that the resistivity is significant, particularly at the magnetic null. Finally, we show that the electron distribution function is non-gyrotropic, which theoretical arguments suggest is an indicator of the possibility for magnetic reconnection to occur. Our kinetic equilibrium shows that such non-gyrotropic distribution functions can be generated by a quasi-static electric field and does not necessarily arise from wave induced effects.
  • Particle acceleration in turbulent plasmas: colourizing the Fermi picture video , Website
    Martin Lemoine, Institut d'Astrophysique de Paris, France
    [#s1509, 21 Jul 2022]
  • Validation of low-Z impurity transport theory using boron perturbation experiments in ASDEX Upgrade video , Google Scholar
    Rachael McDermott, IPP, Garching, Germany , abstract
    [#s1508, 14 Jul 2022]
    Impurities are unavoidable in fusion plasmas and potentially problematic as they result in fuel dilution and radiative energy losses. Therefore, it is important to have accurate predictions of impurity behavior in future fusion devices, which requires a validated theoretical description of impurity transport. With this goal, an experimental technique was developed at ASDEX Upgrade (AUG) to separately identify the diffusive and convective components of the boron particle flux [1-2]. Using this technique, a database of B transport coefficients covering a wide range of plasma parameters has been assembled and can now be used to validate theoretical predictions of low-Z impurity transport [2]. This database shows that the normalized ion temperature gradient (R/LTi) is the strongest organizing parameter for both the B diffusion and convection and strong R/LTi (>6) is a necessary ingredient to obtain hollow B density profiles in AUG. This database also shows that large changes in the applied neutral beam injection (NBI) have a relatively small impact on impurity transport compared to similar changes in electron cyclotron resonance heating (ECRH). Even low levels of ECRH power dramatically increase both the diffusive and convective fluxes and lead to peaking of the impurity density profile. Comparisons to a combination of neoclassical and quasi-linear gyrokinetic simulations show good agreement in the measured and predicted diffusion coefficients. The outward convection measured in NBI dominated plasmas, however, is not well captured by the simulations, despite the inclusion of fast ions [3]. In contrast, the convection is reasonably well reproduced for plasmas with flat or peaked boron density profiles. This dataset provides an excellent experimental validation of the non-monotonic, predicted, convective-particle-flux created by the combination of pure-pinch, thermodiffusion, and roto-diffusion. In addition, this dataset demonstrates a non-monotonic dependence of the experimental particle diffusivity to ion heat conductivity (D/χi) in qualitative agreement with theoretical predictions. [1] C. Bruhn et al Plasma. Phys. Control. Fusion 60 (2018) 085011 [2] Corrigendum Bruhn C. et al. Plasma Phys. Control Fusion 62 (2020) 049501 [2] R. M. McDermott et al. 2022 Nucl. Fusion 62 (2022) 026006 [3] P. Manas et al Nucl. Fusion (60) 2020 056005
  • Plasma Physics of Pulsar Magnetospheres , Website
    Alice Harding, NASA Goddard Space Flight Center, USA , abstract
    [#s1507, 07 Jul 2022]
    Pulsars turn out to be much more complex than a seemingly simple rotating dipole field. The relativistic plasma required for current closure in their magnetospheres requires two signs of charge, so production of electron-positron pairs is required. A great deal of progress has been made over the last 15-20 years in understanding the structure of fields and currents in the pulsar magnetosphere. However, the source of the plasma and how the microphysics of its production is self-consistently coupled with the global magnetosphere are still not resolved. While we have determined that the main site of particle acceleration and high-energy radiation is in the current sheet outside the light cylinder, the details of the mechanisms involved are also not resolved. I will review the theoretical progress to date, from force-free MHD global models to particle-in-cell simulations. I will also review the recent ideas for generating the pulsar emission from radio to Very-High-Energy wavelengths that is ultimately needed to connect with observations.
  • Collisionless Shockwaves in Magnetized High-Energy-Density Laboratory Plasmas , Video , Google Scholar , Website
    Derek Schaeffer, Princeton University, USA , abstract
    [#s1506, 30 Jun 2022]
    As a fundamental process for converting kinetic to thermal energy, collisionless shocks are ubiquitous throughout the heliosphere and astrophysical systems, from Earth's magnetosphere to supernova remnants. While these shocks have been studied for decades by spacecraft, telescopes, and numerical simulations, there remain key open questions in the fundamental physics of collisionless shocks, such as how do shocks accelerate particles to extremely high energies? Or, how is energy partitioned between particles across a shock? In this talk, I will discuss results from high-energy-density experiments and simulations on the formation of supercritical collisionless shocks created through the interaction of a supersonic laser-driven magnetic piston and magnetized ambient plasma. Through proton and refractive imaging, we observe for the first time a magnetized collisionless shock, comparable to some of the strongest shocks in the heliosphere. By probing particle velocity distributions with Thomson scattering, we directly measure the coupling interactions between the piston and ambient plasmas that are critical steps in the formation of magnetized collisionless shocks. Particle-in-cell simulations constrained by experimental data further detail the shock formation process and predict key signatures that are observed in experiments. I will also discuss how the development of this experimental platform can complement, and in some cases overcome, the limitations of similar measurements undertaken by spacecraft missions and can allow novel investigations of energy partitioning and particle acceleration in shocks.
  • Plasma Physics of Pulsar Magnetospheres video
    Piero Martin, University of Padova, Italy , abstract
    [#s1450, 23 Jun 2022]
    This talk illustrates the physics basis which supports the main engineering choices in the Divertor Test Tokamak facility (DTT) under construction in Frascati, Italy. DTT is a superconducting tokamak with 6 T on-axis maximum toroidal magnetic field, carrying plasma current up to 5.5 MA in pulses with total length up to 100 s. The D-shaped device has a major radius R=2.19 m, minor radius a=0.70 m, with an average triangularity 0.3. The auxiliary heating power coupled to the plasma at maximum performance is 45 MW, which allows matching the PSEP/R values with those of ITER and DEMO, where PSEP is the power flowing through the last closed magnetic surface. The primary mission of DTT is the study of the plasma exhaust and of tokamak divertor performance in conditions relevant to ITER and DEMO and in regimes where plasma core and edge behaviors are integrated. In addition to that DTT will provide a facility for high performance tokamak physics and to address core confinement and stability issues in a variety of plasma configurations, including negative triangularity scenarios and the management of transient events like disruptions and ELMs.
  • Turbulence and thermodynamics in expanding, collisionless, magnetised plasma , Google Scholar
    Archie Bott, Princeton University, USA , abstract
    [#s1505, 16 Jun 2022]
    The magnetised plasma composing many different astrophysical systems of interest – from the solar wind to the intracluster medium of galaxy clusters – is often weakly collisional or collisionless, with the Larmor radii of the constituent particles being many orders of magnitude below their Coulomb mean free paths. This feature results in a complex interplay between a plasma's macrophysical evolution (e.g., due to expansion, compression, or large-scale shear) and its microphysical response (e.g., triggering of kinetic instabilities). In this talk, we will elucidate this phenomenon aided by the results of several hybrid-kinetic expanding-box simulations. We will show how the nonlinear dynamics of strong Alfvénic turbulence in a collisionless plasma efficiently adapts to changes in fundamental wave physics that are induced by the effect of macroscopic expansion on microscopic particle motions. This adaptation holds irrespective of a qualitative transformation to the plasma’s thermodynamics caused by pressure-anisotropy-driven kinetic instabilities. We will also demonstrate that different rates of expansion can lead to two qualitatively distinct thermodynamic states: in one state, Alfvén waves are supported; in the other, they are suppressed. These states will be characterised in detail, including the firehose-induced effective collisionality. Our results may help to disentangle the signatures of kinetic instabilities and strong Alfvénic turbulence in key observables in the near-Earth solar wind, such as magnetic power spectra and ion velocity distribution functions.
  • Prospects for real-time, first-principles transport simulations and stellarator optimization including turbulence , Video , Google Scholar
    Bill Dorland,University of Maryland, Princeton Plasma Physics, USA , abstract
    [#s1513, 09 Jun 2022]
    The open-source Trinity code solves for the time-dependent radial profiles of density, temperature, etc, using turbulent fluxes obtained from any radially local gyrokinetic turbulence code, neoclassical fluxes obtained from any drift kinetic solver, external sources, and edge boundary conditions supplied by the user. While originally developed for tokamak applications, the multiscale approach of Trinity is easily generalized for stellarator applications, as long as the equilibrium is assumed to consist of nested flux surfaces without islands. We present results using the original Trinity code together as well as a new Python version that will enable broader, easier uptake by the community. In 2018, we embedded Trinity into an optimization framework and demonstrated the ability to optimize tokamak shaping to maximize fusion power using first-principles estimates for turbulence-induced fluxes. Here, we will present our approach to embedding these gyrokinetic tools into the SIMSOPT framework. GX is an open-source, radially-local, GPU-native, gyrokinetic turbulence code that uses pseudo-spectral methods and native CUDA libraries to calculate turbulence-induced fluxes and critical gradients. At high resolution, GX is simply yet another GK code, but it can be run successfully at low resolution, in lieu of uncontrolled approximations and reduced models. With these tools, we demonstrate the ability to solve for the time-dependent evolution of core fusion reactor profiles in approximately real time, without resorting to reduced models. We also demonstrate the ability to find a shape, size, etc, that maximizes fusion performance by minimizing turbulence-induced losses “inside the optimization loop” for families of tokamak and stellarator reactor concepts, using equilibrium information calculated by VMEC and/or a near-axis expansion approximation, and we present machine-learned, sub-grid techniques that could further accelerate these calculations. I will show linear and nonlinear benchmarks against standard codes from the community, for both tokamak and stellarator configurations. Finally, we introduce the concept of specific computational intensity and use it to demonstrate how one can decide when to retire a given reduced model and rely instead on a higher-fidelity approach. There are many, many reduced models available for modeling fusion plasmas and without some kind of easy-to-use, objective method to distinguish the appropriateness of one approach from another, modelers and designers are often left to work out how to proceed more or less randomly. This leads to severe combinatoric complexity in design and interpretation efforts, which we hope to help bring under control.
  • The road to pedestal tailoring at ASDEX Upgrade , Website , Video
    Elizabeth Wolfrum, IPP Garching, Germany , abstract
    [#s1504, 02 Jun 2022]
    In the narrow edge region of a tokamak transport can be reduced by suppression of turbulence. The core plasma confinement is then elevated and consequently, the region with reduced turbulence is called ‘pedestal’. This work gives an overview of recent investigations at ASDEX Upgrade that show our current understanding of the transport mechanisms in the pedestal and how transport and stability in this narrow region can be influenced. For electron heat transport a constant temperature gradient length hints towards a local small-scale turbulent transport mechanism. The ion heat transport is close to neoclassical values, however in some cases this only holds in the central part of the pedestal with deviations at the pedestal top and foot. The shape and position of the edge density profile are key to both stability and transport and remains the parameter which can be most varied in the pedestal. In our search for a scenario without large edge localised modes, ballooning modes can be driven unstable at the pedestal foot. Careful balance of the drive and stabilising terms allows the pedestal to be tailored such that the global peeling-ballooning stability limit is not breached. Another globally stable regime is achieved with strong nitrogen seeding, leading to the formation of an X-point radiator. These two ELM-free regimes are important research topics for the extrapolation to larger devices.
  • Cross-scale interactions between ion and electron-scale turbulence in magnetized plasmas , Video , Google Scholar
    Shinya Maeyama, Nagoya University, Japan , abstract
    [#s1501, 26 May 2022]
    Recent gyrokinetic simulations have revealed the existence of cross-scale interactions between disparete turbulence at ion and electron gyroradius scales. I would like to start my talk by reviewing recent studies of multi-scale gyrokinetic simulations and discussing problems to be solved in future. For addressing one of these issues, we examine the extrapolation of cross-scale interactions toward high electron temperature burning plasmas, and demonstrate a possibility of reduction of turbulent transport by cross-scale interactions. In the latter part of this talk, I also would like to discuss the methodology for extracting and modeling cross-scale interactions between disparate-scale turbulence. To this end, we have developed a statistical analysis technique based on Mori-Zwanzig projection operator method, which decomposes time evolution of variable of interests into correlated/uncorrelated terms with regard to the explanatory variables. We discuss validity/applicability of the method to multi-scale turbulence problem based on the results of application example to simple plasma turbulence problem.
  • Overview of plasma transport processes relevant to Inertial Confinement Fusion at the National Ignition Facility , Google Scholar
    Mark Sherlock, Lawrence Livermore National Laboratory, USA , abstract
    [#s1500, 19 May 2022]
    A predictive simulation capability is a long term goal of the Inertial Confinement Fusion research program at the Nation Ignition Facility laser. Achieving this goal requires us to understand a number of plasma transport processes in detail in order to assess their overall impact on achieving a sufficiently efficient and symmetric energy transfer from the laser “drive” to the fusion fuel. This talk will give an overview of the processes currently being explored and the associated computational and theoretical techniques. Topics include: modeling electron thermal transport in the kinetic regime with Vlasov-Fokker-Planck codes; generation and transport of magnetic field by lasers; generation of ion turbulence by strong heat flow; ion kinetic effects inside the fuel capsule; transport instabilities involving magnetic field including the thermomagnetic, collisional Weibel, electrothermal and magnetothermal instabilities; the effect of laser speckles on transport; and the theory of laser absorption in non-thermal plasmas.
  • Quantum-inspired methods for solving the Vlasov-Poisson equation ,video
    Erika Ye, MIT, USA , abstract
    [#s1453, 12 May 2022]
    Kinetic simulations of collisionless (or near-collisionless) plasmas using the Vlasov equation are often infeasible due to high resolution requirements and the exponential scaling of computational cost with respect to dimension. Recently, it has been proposed that matrix product state (MPS) methods, a quantum-inspired but classical algorithm, can be used to approximately solve partial differential equations with exponential speed up, provided that the solution can be compressed and efficiently represented as an MPS within some tolerable error threshold. In this work, we explore the practicality of MPS methods for solving the Vlasov-Poisson equations in 1D1V, and find that important features of linear and nonlinear dynamics, such as damping rates and saturation energies, can still be captured while compressing the solution by at least a factor of 8. Furthermore, by comparing the performance of different mappings of the distribution functions onto the MPS, we generate some intuition of the MPS representation and its behavior, which will be useful for extending these methods to higher dimensional problems.
  • Michael Barnes, University of Oxford, UK , Google Scholar
    [#s1452, 28 Apr 2022]
  • Vladimir Zhdankin, Flatiron Institute, USA , Google Scholar
    [#s1451, 21 Apr 2022]
  • Particle acceleration in collisionless shocks: connecting micro and macro scales
    Anatoly Spitkovsky, Princeton University, USA , Google Scholar , abstract
    [#s1473, 14 Apr 2022]
    Sudden deceleration of supersonic flows results in shock waves, which in the conditions of low density plasmas are mediated by collisionless processes. Such colliisionless shocks in astrophysical environments are thought to be responsible for the generation of nonthermal particles that span many decades in energy. These particles produce synchrotron radiation from astrophysical sources, such as supernova remnants and relativistic jets, or are observed directly as energetic cosmic rays. The main acceleration mechanism for these particles is known as "diffusive shock acceleration" and involves particle scattering and diffusion around a shock wave. In the nonlinear stage, shock acceleration couples together the internal structure of the shock with magnetic turbulence generated by accelerated particles, and presents a fascinating self-propagating nonlinear system with multiscale feedbacks. With the development of ab-initio numerical simulations of collisionless shocks, many details of the shock acceleration mechanism can now be studied directly. In this talk I will review the progress in kinetic (PIC) simulations of shock structure and particle acceleration in various regimes, and focus on processes that lead to electron acceleration in non-relativistic shocks, including field amplification, electron heating, and nonlinear regulation of shock injection. The lessons learned from microscopic PIC simulations suggest pathways to larger simulations that use augmented MHD techniques to study shock acceleration on the scales of astrophysical objects. I will discuss such MHD-PIC approaches and applications of current results to morphologies and spectra of nonthermal emission from supernova remnants and galaxy clusters.
  • Laser-matter interactions at ultra-high intensity: how do we simulate them and what can experiments tell us?
    Tom Blackburn, University of Gothenburg, Sweden , Google Scholar , abstract
    [#s1472, 07 Apr 2022]
    As the intensity frontier pushes past 1023 W/cm-2, experiments with high-intensity lasers interacting with matter, whether plasma or relativistic particle beams, enter a new regime. Here the dynamics arise from the interplay between relativistic plasma physics and strong-field, nonperturbative, quantum electrodynamics (QED). Understanding these processes is essential for developing our knowledge of extreme astrophysical environments, such as pulsars, magnetars and black-hole magnetospheres. In this talk I will present an overview of the progress that has been made in investigating the strong-field regime, from the simulation models we use, to the experiments that are possible with today's high-power lasers.
  • Improvement of confinement in tokamaks by a weakening the poloidal magnetic field at the boundary, invariants, and attractors ,video
    Vladimir Yankov, Ergophos LLC, USA , Google Scholar , abstract
    [#s1464, 31 Mar 2022]
    Density profiles of tokamaks are enigmatically peaked and can be described as a turbulent attractor defined by a conservation law, namely, the plasma is frozen in the poloidal magnetic field. The profiles aka Turbulent EquiPartition are accurately described by a simple formula nv=const where v is the specific poloidal volume. The formula predicts that density and temperature at the border will decrease if the v is increased. This can be done in many ways and was observed experimentally before any theory emerged. The first way observed was current rampdown and the latest way was negative triangularity. Since almost all results in tokamaks were obtained experimentally, the theory will be presented briefly as well as several new ways to improve confinement. The theory will include the origin of many plasma and tokamak invariants from the Poincare invariant.
  • Primordial magnetic fields ,video
    Axel Brandenburg, NORDITA, Sweden , WebSite , Google Scholar
    [#s1449, 24 Mar 2022]
  • Electron holes in collisionless plasmas: how long do these common nonlinear structures last? ,video
    Ian Hutchinson, MIT, USA , abstract
    [#s1448, 10 Mar 2022]
    Electron phase-space holes are now widely observed in space plasmas. They consist of a solitary positive potential peak with depleted electron population on trapped orbits that sustains the potential; and so they are intrinsically kinetic: governed by the Vlasov equation. Important new details about their speed and structure are now emerging from multi-satellite measurements. This talk will introduce the principles, observations, and simulations of electron holes; explain the ways that they behave like composite objects possessing lumped momentum, negative mass, and kinematic properties; and show how these concepts determine how and when they break up by instabilities. Instability probably determines the lifetime of a hole when collisions are negligible.
  • Exploring edge turbulence in the low and improved confinement regimes at the ASDEX Upgrade tokamak ,video
    Rachel Bielajew, MIT , WebSite

    , abstract

    [#s1447, 03 Mar 2022]
    Future tokamak fusion reactors will need to operate in a regime of high energy confinement while particle confinement remains low enough that impurities can be exhausted. The low confinement operating regime L-mode has no edge transport barrier and lacks high energy confinement. The high confinement operating regime H-mode has been a target for high confinement operation, however its steep pedestal gradients lead to the edge instability Edge Localized Modes (ELMs). ELMs exhaust impurities and allow for steady-state high confinement operation, but they also release substantial energy which can damage material surfaces. The “improved” confinement regime I-mode is a promising operational scenario for future fusion reactors because it features an edge energy transport barrier without a particle transport barrier and it is naturally ELM-free. The mechanism that leads to this separation of transport channels in I-mode is an open question. The nature of the edge and pedestal turbulence in I-mode plasmas, and its role in determining transport, is still under investigation. In this work we explore edge fluctuations in the L-mode and I-mode edge at the ASDEX Upgrade tokamak through detailed study with turbulence diagnostics. In conjunction, linear gyrokinetic studies probe the nature of the turbulence from the outer core to the pedestal top. We find that the pedestal Weakly Coherent Mode (WCM) remains similar in nature in L-mode and I-mode and that ion-scale fluctuations in the outer core and pedestal top also undergo little change between L-mode and I-mode. The electron scale is a potential candidate for the suppression of heat flux in I-mode, separated from possible particle flux mechanisms. Cross-scale coupling is seen to be important in the I-mode outer core and pedestal.
  • Tokamak turbulence stabilization by electromagnetic effects and fast ions ,video
    Paola Mantica, IFP-CNR, Milan , Google Scholar

    , abstract

    [#s1438, 24 Feb 2022]
    A decade ago the experimental discovery on JET that ion temperature profile stiffness (due to the strength of turbulence reaction to changes in ion temperature normalized gradient) is reduced by increased Neutral Beam and/or Ion Cylotron Resonant Heating power triggered an intense work to understand and reproduce/expand these results. The JET results were explained by means of gyrokinetic simulations as due to non-linear electromagnetic stabilization of ion turbulence associated with pressure gradients (including thermal and suprathermal components). In both JET and ASDEX-Upgrade evidence has been found that these mechanisms are at the basis of improved ion confinement and ion temperature peaking in high power Hybrid scenarios. On DIII-D a similar stabilizing effect was found. An additional mechanism linked to a purely fast ion driven resonant linear electrostatic stabilization has been found in JET high ICRH power discharges and very recently used in ASDEX-Upgrade to design pulses with improved ion temperature peaking. Significant progress has been achieved in the theoretical understanding of these stabilizing effects and work is still in progress to better understand the extrapolability to ITER conditions, especially in presence of highly energetic α particles. This talk will present an overview of the experimental and theoretical work on this topic and will discuss its impact on our predictive capabilities of tokamak scenarios.
  • Fast and furious: reconnection and turbulence in magnetically-dominated astrophysical plasmas ,video
    Lorenzo Sironi, Columbia University, USA , Google Scholar , Website

    , abstract

    [#s1437, 17 Feb 2022]
    In the most powerful astrophysical sources, reconnection and turbulence operate in the “relativistic” regime, where the magnetic field energy exceeds even the rest mass energy of the plasma. Here, reconnection and turbulence can lead to fast dissipation rates and efficient particle acceleration, thus being prime candidates for powering the observed fast and bright flares of high-energy non-thermal emission. With fully-kinetic particle-in-cell (PIC) simulations and analytical theory, we investigate the physics of relativistic reconnection and turbulence, and demonstrate that they can be the “engines” behind: (1) high-energy flares in blazar jets; and (2) the hard-state spectra of black hole X-ray binaries and Active Galactic Nuclei.
  • New horizons for stellarator optimization via fast 3D MHD equilibrium and stability calculations with islands and chaos ,video
    Joaqium Loizu, EPFL, Switzerland , Google Scholar , Website

    [#s1436, 10 Feb 2022]
  • Turbulence and transport in the Large Plasma Device: shear suppression, nonlinear instability and electromagnetic turbulence video
    Troy Carter, UCLA, USA , Google Scholar , Website

    [#s1399, 27 Jan 2022]
  • Anomalous Electron Diffusion in Magnetic Islands and Stochastic Magnetic Fields , Video
    Eva Kostadinova, Auburn University, USA , Google Scholar , Website

    , abstract

    [#s1398, 20 Jan 2022]
    Magnetic islands and regions of stochastic magnetic fields originate from the dynamical processes of magnetic reconnection and turbulence in plasma. These structures are ubiquitous in both laboratory settings (e.g., tokamaks and stellarators) and space environment (e.g., solar wind plasma and Earth’s magnetosphere). An interesting feature of magnetic islands and stochastic regions in plasmas is their connection to plasma particle acceleration, often resulting in anomalous diffusion. An important question is what universal principles relate the properties of energetic particles as a function of the underlying magnetic field topology in both lab and space. The answer to this question requires the development of universal transport models. This talk will introduce a Fractional Laplacian Spectral (FLS) approach to anomalous diffusion in plasmas with magnetic islands and stochastic magnetic fields. The FLS is a novel technique which computes the probability for particle transport as a function of nonlocal interactions and stochasticity in the examined field. The inputs for the model are informed from DIII-D experiments where energetic electrons (exhibiting anomalous diffusion) were observed in the presence of resonant magnetic perturbation (RMP) of the magnetic field and from simulations of the corresponding B-field topology. The perturbation on the B-field results in two characteristic structures: magnetic islands (leading to nonlocal transport) and stochastic regions (leading to chaotic transport). We show how the interplay between typical island scale and the magnitude of stochasticity determine the resulting electron diffusion.
  • Dusty plasma experiments: strong coupling, shocks, and testing theories of statistical physics,, Video
    John Goree, University of Iowa, USA , Google Scholar , Website

    , abstract

    [#s1397, 13 Jan 2022]
    Dusty plasmas contain small solid particles, which gain large electric charges. Typically they are micron size polymer spheres. Unlike the electron and ion components, the dust particle component tends to behave like a strongly coupled plasma, with Coulomb collisions dominating to such a degree that particles arrange themselves like atoms in a liquid or a solid. Due to their large size, the dust particles can be imaged individually in video recordings. This video imaging allows experimenters to track individual particles, which is an enormously powerful diagnostic that is unavailable in traditional plasma physics experiments, where electrons and ions cannot be imaged individually. In this talk I present, as two example research topics: shocks and tests of theories of statistical physics. While the shock topic is a traditional one for plasma physics, the topic of testing statistical physics takes the discipline of plasma physics in a new direction, by exploiting particle tracking.
  • Relativistic plasma physics and high field phenomena using intense lasers video
    Karl Krushelnick, University of Michigan Ann Arbor, USA , Google Scholar

    , abstract

    [#s1396, 16 Dec 2021]
    The past two decades have witnessed the development of revolutionary light sources having the unprecedented ability to probe new physical regimes and control matter with atomic scale precision. The ongoing development of multi-Petawatt lasers around the world will allow exploration of fundamental yet unanswered questions regarding non-linear Quantum Electrodynamics in relativistic plasmas, including non-perturbative quantum radiation reaction and electron-positron pair production mechanisms. Further experiments enabled by such lasers will include pump-probe experiments using femtosecond x-rays as a probe of material dynamics on ultra-short timescales, the production of GeV ion beams, the generation of instabilities in electron-positron jets, the exploration of vacuum polarization effects, relativistic shocks and the production of “exotic” particles such as pions and muons. I will review recent advances in this field and also describe the new NSF funded ZEUS facility under construction at the Center for Ultrafast Optical Science (CUOS) at the University of Michigan. ZEUS will be a dual-beamline 3 PetaWatt laser system that will provide unique capabilities for research. This will be a new high power laser user facility for US scientists as well as for the wider international research community, and will have an open and transparent external review panel for facility access and 30 weeks per year dedicated to external user experiments. After completion in 2023, the ZEUS laser system will be the highest-power laser system in the US.
  • Maximal Energy Release and the Rules of Rearrangement video
    Elijah Kolmes, Princeton University, USA , Google Scholar

    , abstract

    [#s1390, 02 Dec 2021]
    Throughout plasma physics, we are often interested in processes through which kinetic energy is transferred out of a distribution of particles. Examples of these processes include wave-particle interactions (for instance, the amplification of a wave) as well as the growth of turbulent internal modes. Some particle distributions are more prone to these energy transfers than others. Of interest is the maximal possible energy that can be liberated from a distribution function by wave-particle interactions with constraints on the nature of the interaction. These constraints might be that the waves can only rearrange the 6D phase space, or that they must conserve adiabatic invariants, or that instead they can only act to diffuse particles. This talk will trace the development of these ideas, starting in the 1960s. Among the developments that we will cover is the recent and surprising result that, with enough fine-tuning, the energy recoverable from diffusive processes can reach the energy recoverable from entropy-conserving processes.
  • Laser plasma accelerators: First results from the HIGGINS 2x100 TW laser
    Victor Malka, Weizmann Institute of Science, Israel , Google Scholar

    , abstract

    [#s1391, 24 Nov 2021]
    Laser Plasma Accelerators (LPA) are changing the scientific and societal landscape. Opening new hopes for high energy physics, offering alternative to synchrotron light sources with the recent demonstration with LPA’s based Free Electron Radiation, and delivering particle and radiation beams for medical and security applications, they are among the most innovative tools of modern sciences. The laser plasma accelerators are a perfect illustration of what cross-domain fertilization with a zest of imagination can produce. In this talk I’ll explain the main involved concepts, and why these wonderful machines rely on our ability to control finely the electrons motion with intense laser pulses. I’ll show how the electrons collective manipulation permits to produce giant electric fields of value in the TV/m exceeding by 3 orders of magnitude or more the ones used in current machines. These collective motions when controlled permits also to modify and to shape the longitudinal and radial components of the plasma fields for either accelerating efficiently electrons or for producing energetic photons by wiggling electron during their acceleration. This control is crucial for electrons injection that is essential for delivering stable ultra-short and ultra-bright energetic particle or radiation beams. To illustrate the beauty of laser plasma accelerators I will show some concepts we recently demonstrated that allow these controls for beams improvements. Finally, I will show the commissioning of the HIGGINS dual laser system of the Weizmann Institute of Science, together with a set of first experimental results showing new insights of the relativistic plasma fields and a new approach for producing plasma refractive optics for relativistic beam manipulation.
  • The helicity barrier: how low-frequency turbulence triggers high-frequency heating of the solar wind ,video
    Jonathan Squire, University of Otago, New Zealand , Google Scholar , Webpage

    , abstract

    [#s1385, 18 Nov 2021]
    Weakly magnetized, relativistic collisionless shocks have been studied extensively over the past couple of decades using electron-ion and pair plasma compositions, whereas the broader landscape of electron-ion-positron mixtures has been left unexplored. The more general case is of astrophysical relevance for the early afterglow phase of gamma-ray bursts (GRBs), where the prompt radiation loads the external medium ahead of the shock with electron-positron pairs. In this talk, I will address the microphysics of external, pair-loaded GRB shocks using a set of first-principles kinetic simulations. I will show that even a small number of electron-positron pairs per ion significantly changes the shock structure. In particular, I will demonstrate that a pair-loaded shock is mediated by the Larmor gyration of ions in the compressed mean magnetic field even when this field is extremely weak, and therefore, pair-loaded shocks accelerate ions only in the limit of vanishing external magnetization. Electrons, on the other hand, can form distinctively non-thermal distributions even when the ions are essentially thermal. Although the shock structure significantly changes with respect to the plasma composition, the energy fraction carried by the pairs downstream of the shock is nearly independent of the pair-loading factor. Finally, I will comment on the implications of the results for the early afterglow emission of GRBs.
  • Daniel Groselj, Colombia University, USA , Google Scholar

    , abstract

    [#s1384, 04 Nov 2021]
    Weakly magnetized, relativistic collisionless shocks have been studied extensively over the past couple of decades using electron-ion and pair plasma compositions, whereas the broader landscape of electron-ion-positron mixtures has been left unexplored. The more general case is of astrophysical relevance for the early afterglow phase of gamma-ray bursts (GRBs), where the prompt radiation loads the external medium ahead of the shock with electron-positron pairs. In this talk, I will address the microphysics of external, pair-loaded GRB shocks using a set of first-principles kinetic simulations. I will show that even a small number of electron-positron pairs per ion significantly changes the shock structure. In particular, I will demonstrate that a pair-loaded shock is mediated by the Larmor gyration of ions in the compressed mean magnetic field even when this field is extremely weak, and therefore, pair-loaded shocks accelerate ions only in the limit of vanishing external magnetization. Electrons, on the other hand, can form distinctively non-thermal distributions even when the ions are essentially thermal. Although the shock structure significantly changes with respect to the plasma composition, the energy fraction carried by the pairs downstream of the shock is nearly independent of the pair-loading factor. Finally, I will comment on the implications of the results for the early afterglow emission of GRBs.
  • Ilya Dodin, Princeton Plasma Physics Laboratory USA , Webpage

    , abstract

    [#s1362, 28 Oct 2021]
    Modern geometrical optics is a powerful framework that allows modeling wave processes more efficiently than just via solving wave equations like any other PDEs. I will overview some of the recent applications of this theory to reduced linear modeling of radiofrequency waves, including mode conversion, cutoffs, and caustics, which is usually assumed to require the full-wave approach. I will also show how modern wave theory helps fundamentally improve quasilinear theory and understand inhomogeneous turbulence. The presentation is mainly targeted at curious theorists, but the applications addressed are also of practical importance for fusion science and beyond.
  • The magnetised plasma sheath and its role in the boundary of magnetic fusion devices video
    Alessandro Geraldini, EPFL, Switzerland , Google Scholar , Webpage

    , abstract

    [#s1361, 21 Oct 2021]
    Sheaths form wherever a plasma is in contact with a solid target. They are characterised by a spatial variation of the electrostatic potential and density over very small length scales normal to the surface. Within a few Debye lengths of the target, the electric field is so strong that the plasma is non-neutral. Debye sheaths exist in order to repel electrons, which are lighter and more mobile than ions. When a magnetic field is present in the plasma at an oblique angle with the target, the sheath develops a two-scale structure, with a part of the electrostatic potential variation occurring in a larger quasineutral magnetic presheath (or Chodura sheath) a few ion gyro radii from the target. When simulating turbulence in the open field line region (Scrape-Off Layer) of a fusion device, it is numerically prohibitive to resolve the small timescales and length scales of the magnetised sheath, which comprises the magnetic presheath and Debye sheath. Instead, an iterative method can be used to directly obtain numerical solutions for the electrostatic potential in the magnetised sheath in steady state. This is demonstrated using a fully kinetic model exploiting the grazing magnetic field angle (typical of fusion devices) to approximate the ion and electron trajectories. Numerical solutions include the ion distribution function reaching the target, important for sputtering predictions. Analytical calculations show that the kinetic Chodura condition must be satisfied by the ion distribution function reaching the magnetised sheath from the rest of the plasma. The implications of this for boundary conditions to gyrokinetic codes of the open field line region are discussed.
  • Scott Hsu, ARPA-E, USA , Google Scholar , Webpage

    , abstract

    [#s1360, 14 Oct 2021]
    Since 2015 with the launch of the ALPHA program [1], the Advanced Research Projects Agency-Energy (ARPA-E) of the U.S. Department of Energy has supported over 50 R&D projects relating to fusion energy. ARPA-E’s fusion R&D portfolio is focused on high-risk, high-reward translational/applied R&D to enable timely commercially viable fusion energy, while incentivizing collaborations between privately and publicly funded fusion teams. This colloquium will be organized into two parts: (1) brief overview of the mission/approach of ARPA-E as an R&D funding agency, and how fusion-energy R&D is motivated/pursued within the overall context of the agency, and (2) overview of ARPA-E’s active fusion programs (BETHE, GAMOW, and Fusion Diagnostics), including its technology-to-market (T2M) approach, and technical research highlights from selected fusion projects.
  • Research on Complex/Dusty Plasmas in the Lab and in Space video
    Hubertus Thomas, DLR — German Aerospace Centre, Germany , Google Scholar

    , abstract

    [#s1357, 07 Oct 2021]
    Complex/dusty plasmas are plasmas containing small solid particles, which get charged by the collection of plasma electrons and ions. Due to their high charge in laboratory plasmas they start to interact strongly and can form liquid and solid structures, the latter is called plasma crystal. This can be seen as a classical condensed matter system where the main component – the solid particles – can be visualized and tracked dynamically. This allows investigations of fundamental processes in liquids and solids and their transitions. Solid particles of sizes of around a micrometer in diameter start to react strongly on gravity and levitating forces are mandatory. The sheath electric field of a rf-discharge allows the trapping of microparticles in the sheath and can be used to form2-dimensional (horizontal) or compressed 3-dimensional (with a small extend in the vertical direction) complex plasma systems. To study large 3-dimensionalcomplex plasmas in the bulk of a discharge microgravity experiments are necessary. PK-4 is the third plasma crystal facility on the International Space Station ISS continuing the successful research under microgravity conditions started in 2001 already. In this presentation I will give an overview on complex plasma research and will show recent results like active matter and electrorheological plasmas from ground based and ISS-based laboratories. This work was supported in part by DLR (BMWi), ESA, Roscosmos and NASA/NSF.
  • Exploring Stellar Nucleosynthesis and Basic Nuclear Science using High Energy Density plasmas at OMEGA and the NIF
    Maria Gatu Johnson, MIT, USA , Website

    Abstract: Thermonuclear reaction rates and nuclear processes have been explored traditionally by means of accelerator experiments, which are difficult to execute at conditions relevant to Stellar Nucleosynthesis. High-Energy-Density (HED) plasmas closely mimic astrophysical environments and are an excellent complement to accelerator experiments. This talk will focus on HED experiments to study the T+T reaction at the OMEGA laser facility, and the mirror 3He+3He reaction at OMEGA and at the National Ignition Facility (NIF). We present neutron spectra from the T(t,2n)α(TT) reaction measured in HED experiments at ion temperatures from 4 to 18 keV, corresponding to center-of-mass energies (Ec.m.) from 16 to 50 keV. A clear difference in the shape of the TT-neutron spectrum is observed between the two Ec.m., providing the first conclusive evidence of a variant TT-neutron spectrum in this Ec.m. range. Preliminary data from a recent discovery science experiment at the NIF exploring the solar 3He+3He reaction at Ec.m. from 60-120 keV will also be discussed. In addition, the talk will cover the potential of this new field of research, ongoing efforts, and future directions for studying nuclear astrophysics-relevant nuclear processes at OMEGA and the NIF. This work was supported in part by the U.S. DOE, the MIT/NNSA CoE, LLE and LLNL.

    [#s1356, 30 Sep 2021]
  • The role of turbulence in determining the density profile in magnetic confinement devices
    Saskia Mordijck, College of William & Mary, USA , Google Scholar , Web Page

    Abstract:

    Abstract: The fusion gain in a tokamak is directly linked to the density of the plasma. However, due to the high temperatures necessary for fusion, it is impossible to fuel the core of the plasma directly and directly influence the core density. Without any direct fueling in the core of a tokamak, the plasma density is fully controlled by transport perpendicular to the confining magnetic field surfaces. In this talk, I will show how cross-field transport of electrons is dominated by turbulence in the plasma core by comparing experiments with existing models. These models capture how various types of turbulence influence transport and thus the density profile. While the density profile in the core is fully determined by turbulent transport, at the plasma edge, the picture is more complicated. At the edge of the tokamak, turbulent transport effects intermingle directly with fueling through ionization of the surrounding gas. To better understand the impacts of turbulence on the particle flux, we perform a series of experiments on LAPD varying the neutral density and electron density gradient. While some trends follow linear predictions of resistive drift wave turbulence, other phenomena cannot be explained using linear predictions..

    [#s1341, 23 Sep 2021]
  • Extreme Plasma Astrophysics: a Shining New Frontier
    Dimitri Uzdensky, University of Colorado Boulder, USA , Google Scholar ,Video

    Abstract:

    While traditional plasma physics deals with plasmas made up of a fixed number of electrons and ions that are nonrelativistic and nonradiative, there exist in the Universe important plasma environments with physical conditions so extreme that additional “exotic physics” (from a plasma physicist’s point of view) processes come into play: special and general relativity, strong coupling between plasma particles and photons, and, in most extreme cases, QED effects like pair production and annihilation. These processes alter the plasma dynamics near compact relativistic astrophysical objects — neutron stars and black holes — arguably, the most enigmatic and fascinating objects in the Universe. Understanding how collective plasma processes (waves, instabilities, shocks, magnetic reconnection, turbulence, etc.) operate under these exotic conditions calls for the development of a new, richer physical framework, which forms the scope of Extreme Plasma Astrophysics. I will review the rapid progress that is being made now in exploring this exciting new frontier, stimulated by the exploding astrophysical motivation and enabled by concerted, vigorous theoretical efforts and recent computational breakthroughs such as the advent of novel relativistic kinetic plasma codes incorporating radiation reaction and pair creation. I will illustrate this progress with recent studies of radiative relativistic turbulence and magnetic reconnection with pair creation in the context of accreting black-hole coronae and neutron-star magnetospheres. I will end by outlining the future directions of the burgeoning field of Extreme Plasma Astrophysics.

    [#s1355, 16 Sep 2021]
  • Fast Magnetic Reconnection
    Allen Boozer, Columbia University, USA , Google Scholar , Web Page ,Video

    Abstract:

    When a magnetic field undergoes a near-ideal evolution that involves all three spatial coordinates, mathematics and Maxwell's equations give a characteristic time scale for the initiation of reconnection. This time is given by the ideal evolution multiplied by a factor that depends only logarithmically on the strength of the non-ideal effects. The critical mathematical concept is chaos, which means the streamlines of the ideal flow of the magnetic field lines can separate exponentially in time. The mathematics of vector representations in three dimensions together with Faraday's law define the ideal flow velocity of magnetic field lines as well as an electromotive-like constant on each line which gives the non-ideality. Maxwell's equations imply chaotic flows are energetically impossible in a two-dimensional evolution, which makes conventional two-dimensional reconnection theory an extremely specialized subject. The magnitude of the current density in a three-dimensional reconnection depends only logarithmically on the strength of the non-ideal effects instead of being inversely proportional as in two dimensions. The current density lies in numerous thin but wide ribbons along the magnetic field lines. The concepts that underlie three-dimensional reconnection theory are unfamiliar to the plasma physics community. The talk will both explain these concepts and give simple examples of their application. To ensure those who would like have time to assess unfamiliar concepts, the slides that will be used are attached here.

    [#s1336, 09 Sep 2021]
  • Modeling Complex Interactions in a Complex Plasma
    Lorin Swint Matthews, Baylor University, USA , Google Scholar , Web Page

    Abstract:

    A complex, or dusty, plasma consists of sub-micron to micron-sized grains immersed in a plasma environment. Micron-sized dust grains have been successfully employed as non-perturbative probes to measure variations in plasma conditions on small spatial scales, such as those found in plasma sheaths. Within a sheath, ions are accelerated from the bulk plasma towards the charged boundary. Ions flowing past a dust grain form a positively charged spatial region downstream of the grain, called the ion wake. The ion wake in turn modifies the interaction potential between charged grains and can contribute to the stability of the dust structures which are formed in a given plasma environment. Thus, although dust grains can be used as non-invasive probes on “small scales”, on even “smaller scales” the perturbations to the plasma flow are necessary to establish a stable dust configuration. Here we present a multi-scale N-body model of the dust-plasma interactions. Results are compared with ground-based lab experiments as well as microgravity experiments onboard the International Space Station to determine quantities such as the charge on individual grains, the electric field within the region, and the nature of the ion wakefield.

    [#s1335, 02 Sep 2021]
  • Reconnection-controlled decay of magnetohydrodynamic turbulence and the role of invariants
    David Hosking, University of Oxford, UK , Google Scholar , Web Page

    Abstract:

    In this talk, I will describe a new theoretical picture of magnetically dominated, decaying turbulence in the absence of a mean magnetic field. I shall argue that the energy-decay rate of such a system is governed by the reconnection of magnetic structures, and not necessarily by ideal dynamics, as has previously been assumed. I will explain how a prediction for the decay law of magnetic energy can be obtained by assuming reconnection-mediated dynamics that respects the conservation of certain integral invariants, which represent topological constraints satisfied by the reconnecting magnetic field. As is well known, the magnetic helicity is such an invariant for initially helical field configurations, but does not constrain non-helical decay, where the volume-averaged magnetic-helicity density vanishes. For such a decay, I shall propose a new integral invariant, analogous to the Loitsyansky and Saffman invariants of hydrodynamic turbulence, that expresses the conservation of the random [scaling as volume^(1/2)] magnetic helicity contained in any sufficiently large volume. The existence of this `Saffman helicity invariant’ leads to a natural explanation of the inverse-transfer phenomenon reported by previous numerical studies. Finally, I shall describe an application of these results to the decay of primordial magnetic fields in the early Universe.

    [#s1340, 26 Aug 2021]
  • Magnetospheric Multiscale Observations of Collisionless Plasma Turbulence in Earth’s Magnetosheath: Turbulent Electric Fields & Turbulence-Driven Magnetic Reconnection
    Julia Stawarz, Imperial College London, UK , Google Scholar , Web Page

    Abstract:

    Plasmas throughout the Universe undergo complex, highly nonlinear turbulent dynamics, which transfer energy from large to small-scale fluctuations and in the process generate a multitude of small-scale structures, such as current sheets. However, many space plasmas are nearly collisionless, making the question of how the turbulent fluctuations are dissipated a particularly challenging question. NASA’s Magnetospheric Mutiscale (MMS) mission is a formation of four Earth-orbiting satellites providing the high-resolution plasma measurements and inter-spacecraft separations necessary to examine plasma dynamics at scales approaching those of the electrons. In this presentation, I will discuss two recent studies that make use of the unique measurements from MMS in Earth’s magnetosheath to examine the small sub-proton scale dynamics of turbulent plasmas in greater detail than previously possible. In the first study, the behaviour of the turbulent electric field is examined by directly measuring the contributions from the terms in generalised Ohm’s law from fluid to electron-scales. In the second study, MMS observations are used to systematically identify magnetic reconnection events at the thin current sheets that are generated by the turbulent fluctuations. The large-scale properties of the turbulent fluctuations, in particular the correlation length, are found to influence the nature of the reconnection dynamics potentially leading to, so called electron-only reconnection, in which there is not enough space for ion to fully couple to the newly reconnected magnetic fields. Both of these studies provide insight into the nonlinear couplings, and potentially the dissipative dynamics, in collisionless plasmas.

    [#s1339, 19 Aug 2021]
  • Some basic principles of inertial confinement fusion and some recent “burning plasma” results*
    Omar Hurricane, Lawrence Livermore National Laboratory, USA ,Google Scholar

    Abstract: Inertial confinement fusion (ICF) has existed as a field of study since the 1970s, but the field was born out of the Cold War. In the decades since the 1970s, pioneering research developing the principles and technologies of ICF culminated in the creation of several major facilities that exist today. While the technology of ICF facilities themselves is fascinating, this talk concentrates upon a handful of basic physics principles of “indirect-drive” (x-ray driven) targets fielded on the National Ignition Facility (NIF) in Northern California and upon some key results from the last decade of research, including some recent experiments that appear to have broached the burning plasma regime [1,2,3].

    [1] A.B. Zylstra, O.A. Hurricane, D.A. Callahan, et al., in preparation (2021) [2] J.S. Ross, J.E. Ralph, A.B. Zylstra, et al., in preparation (2021) [3] A.L. Kritcher, C.V. Young, H.F. Robey, et al., in preparation (2021) *Work performed under the auspices of the U. S. Department of Energy by LLNL under contract DE-AC52-07NA27344

    [#s1330, 12 Aug 2021]
  • Progress and plans for Princeton Field-Reversed-Configuration Research
    Samuel Cohen, PPPL, USA ,Webpage

    Abstract: The Princeton Field Reversed Configuration-2 (PFRC-2) is a research device for studying innovative physics methods to enable small clean fusion reactors. PFRC novel physics regimes are characterized by J┴B and kinetic conditions. Based on the limited availability of one fuel component, 3He, such reactors would be limited to use in niche applications, such as for spacecraft propulsion or emergency terrestrial power generation. First experiments, motivated by single-particle simulations of plasma heating by rotating magnetic fields of odd-parity symmetry (RMFo), produced electron temperatures in excess of 100 eV. The present research program addresses three topics: ion heating by RMFo; confinement; and stability. To achieve bulk ion energies in excess of 100 eV, ARPA-E-supported upgrades are being made to machine hardware, modeling capabilities, and diagnostics. Two new diagnostics have been installed, two-photon laser-induced fluorescence (TALIF, PU-MAE) and Thomson scattering (TS, ORNL). The TALIF diagnostic has measured the H° density in quasi-state-state and puffed gas discharges, allowing evaluation of particle confinement time and energy loss by CX. TS is now being put into operation. Additional planned increased capabilities include reflectometry (UC-Davis), DFSS for internal fields (ORNL), an ion energy analyzer (PPPL), and a PIC simulation code (U Rochester). Benefits of and the requirements for scrape-off-layer modification are described.

    [#s1329, 05 Aug 2021]
  • Effects of distribution structure on predictions of plasma behavior in marginally unstable plasma
    Emily Lichko, University of Arizona, USA Google Scholar ,Webpage

    Abstract: Due to low collisionality in space and astrophysical plasmas, distributions of ions and electrons observed by spacecraft exist in a state far from thermodynamic equilibrium. The non-Maxwellian features in these distribution functions can trigger microinstabilities, which likely play a role in some of the largest open questions in solar physics, including coronal heating, heating of the bulk solar wind, and accounting for high-frequency waves observed alongside the Alfvenic turbulent cascade. While there is a tremendous amount of information in the structure of these distribution functions, they are typically only represented by a fit of one or two Maxwellian or bi-Maxwellian distributions. In this work, we examine how the fidelity of the model to the observed distribution function affects our predictions for the stability of the plasma, and how much of the information in the distribution function is needed to accurately predict the behavior of the plasma. To do this, we use marginally stable one-dimensional, electrostatic simulations of the electron two-stream instability. For these simulations, there is significantly better agreement between the behavior of the plasma and the predictions of linear theory when a higher-fidelity representation of the distribution function is used. Preliminary work on the extension of these electrostatic results to the electromagnetic regime and the comparison of the predictions of linear wave activity with measurements of waves from Parker Solar Probe will be presented as well.

    [#s1302, 29 Jul 2021]
  • Muni Zhou, Massachusetts Institute of Technology, USA Google Scholar, News
    [#s1301, 22 Jul 2021]
  • William Daughton, Los Alamos National Laboratory, USA Google Scholar
    [#s1300, 15 Jul 2021]
  • Benedikt Geiger, University of Wisconsin, Madison, USA Webpage
    [#s1299, 08 Jul 2021]
  • Solar Wind Turbulence: in-situ observations from magneto-fluid to kinetic plasma scales
    Olga Alexandrova, LESIA, Observatoire de Paris, France Google Scholar

    Abstract: Solar wind turbulence was mostly studied at MHD scales: there, magnetic fluctuations follow the Kolmogorov spectrum. The fluctuations are mostly incompressible and they have non-Gaussian statistics (intermittency), due to the presence of coherent structures in the form of current sheets, as it is widely accepted. Kinetic range of scales is less known and the subject of debates. We study the transition from Kolmogorov inertial range to small kinetic scales with a number of space missions. It becomes evident that if at ion scales (100-1000 km) turbulent spectra are variable, at smaller scales they follow a general shape. Thanks to Cluster/STAFF, the most sensitive instrument to measure magnetic fluctuations by today, we could resolve electron scales (1 km, at 1 AU) and smaller (up to 300 m) and show that the end of the electromagnetic turbulent cascade happens at electron Larmor radius scale, i.e., we could establish the dissipation scale in collisionless plasma. Furthermore, we show that intermittency is not only related to current sheets, but also to cylindrical magnetic vortices, which are present within the inertial range as well as in the kinetic range. This result is in conflict with the classical picture of turbulence at kinetic scales, consisting of a mixture of kinetic Alfven waves. The dissipation of these waves via Landau damping may explain the turbulent dissipation. How does this picture change if turbulence is not only a mixture of waves but also filled with coherent structures such as magnetic vortices? These vortices seem to be an important ingredient in other instances, such as astrophysical shocks: for example, they are observed downstream of Earth's and Saturn's bow-shocks. With the new data of Parker Solar Probe and Solar Orbiter we hope to study these vortices closer to the Sun to better understand their origin, stability and interaction with charged particles.

    [#s1274, 01 Jul 2021]
  • Tim Horbury, Imperial College London, UK ,Webpage
    [#s1310, 24 Jun 2021]
  • Christopher Reynolds, University of Cambridge, UK Webpage

    Abstract: The baryonic component of galaxy clusters is dominated by the intracluster medium (ICM), a hot and tenuous plasma atmosphere in an approximate state of hydrostatic equilibrium within the gravitational potential of the dark matter halo. The ICM is an important actor in many astrophysical processes within the cluster - the ram pressure of the ICM can strip cold gas out of orbiting galaxies, and radiative cooling can lead to significant galaxy building in the ICM core in a manner that is well-known to be regulated by feedback from the central supermassive black hole. However, all of these phenomena are influenced by transport processes within the weakly-collisional and high-beta ICM which are still poorly understood. In this talk I focus on the physics and astrophysical role of thermal conduction in the ICM. I summarize recent developments in understanding the role of whistler modes in the regulation of thermal heat transport and proceed to discuss some astrophysical implications of this new transport model. I end by discussing the future observational landscape of these ICM plasma studies.

    [#s1297, 17 Jun 2021]
  • Reduced turbulence in optimised maximum-J stellarators video
    Josefine Proll, Eindhoven University of Technology, Holland Google Scholar

    Abstract: Turbulence is one of the main obstacles to a working fusion reactor. Especially in stellarators, the large space of available magnetic field shapes allows for optimisation towards low levels of turbulence. A useful nonlinear measure of turbulence is that of available energy. Here I will show that the available energy calculated for trapped-electron-mode turbulence in different magnetic configurations can predict trends in the (simulated) heat flux of trapped-electron mode (TEM) turbulence in these configurations and could thus serve as a valuable proxy in future optimisation routines. Both, the available energy and the nonlinear simulations, support a previous linear prediction: that the class of optimised maximum-J stellarators, amongst them Wendelstein 7-X, particularly benefits from reduced turbulence. Previously, we had analytically shown that in these devices, the electron-driven TEM is absent. Here I will show that the stabilising property of the electrons also extends to ion-temperature gradient (ITG) modes and can thus explain the levels of low turbulence in the record-shots of Wendelstein 7-X at finite density gradient. Finally, I will present evidence that in the absence of TEMs, the universal instability can emerge and actually dominate the turbulence in optimised stellarators

    [#s1296, 10 Jun 2021]
  • Expanding frontiers for dusty plasmas: magnetic fields to microgravity
    Edward Thomas, Auburn University, USA Google Scholar

    Abstract: The presence of charged, solid, particulate matter in plasmas, i.e., “dust”, is ubiquitous. From stellar nurseries to planetary rings and from fusion experiments to plasma processing reactors, “dusty” plasmas are found in a wide variety of naturally occurring and human-made plasma systems. Therefore, understanding the physics of dusty plasmas can provide insights into a broad range of astrophysical and technological problems. This presentation will focus on how the small charge-to-mass (q/m) ratio of the charged microparticles gives rise to many of the unique spatio-temporal properties of dusty plasmas. Moreover, this small charge-to-mass ratio strongly influences how magnetic field and microgravity studies of dusty plasmas are performed, leading to new investigations of previously unexplored regimes of plasma parameters. This presentation will discuss results from our studies of dusty plasmas in high magnetic fields (B ≥ 1 T) using the Magnetized Dusty Plasma Experiment (MDPX) device at Auburn University and in microgravity experiments using the Plasmakristall-4 (PK-4) laboratory on the International Space Station. At the end, the presentation will discuss the prospects for the future of dusty plasma research., abstract
    [#s1295, 03 Jun 2021]
  • Zonally dominated dynamics and the transition to strong turbulence in cold-ion Z-pinch plasma
    Plamen Ivanov, University of Oxford, UK Google Scholar

    Abstract: Following the discovery of the Dimits shift (Dimits et al. 2000), the role of zonal flows (ZFs) for the transition to turbulence in tokamak plasmas has been an area of intense research. We attempt to shed some light on this problem by studying the transition to turbulence in a simplified cold-ion fluid model for ion-scale turbulence in Z-pinch magnetic geometry. Our equations are obtained in a highly collisional, cold-ion, asymptotic limit of the ion gyrokinetic equation and capture the two well-known ion-temperature-gradient (ITG) instabilities driven by either magnetic curvature or parallel compression. We find that this model has a well-defined Dimits (low-transport, ZF-dominated) state characterised by a staircase-like arrangement of ZFs and zonal temperature that suppresses turbulence. Viscous decay of the ZFs leads to occasional turbulent bursts that reconstitute the staircase by providing a negative zonal turbulent viscosity. In 2D, at sufficiently large equilibrium temperature gradients, the zonal turbulent viscosity switches sign, hence the turbulent bursts no longer reinforce the zonal staircase and the Dimits state is destroyed. In 3D, the Dimits state is much more resilient and can always be sustained provided sufficient parallel extent of the system. This is because the large-scale curvature-driven perturbations go unstable to small-scale "parasitic" 3D slab-ITG modes that give rise to a negative zonal turbulent viscosity and provide an effective thermal diffusion for the large-scale modes. If we restrict the parallel extent of the system, the Dimits state is destroyed, and a strongly turbulent, high-transport state is established. In this state, energy is injected into large-scale perturbations by the curvature-ITG instability, then transferred into the parasitic small-scale modes, and finally dissipated by the finite collisionality. Moreover, we find that sufficient parallel resolution is critical for the 3D Dimits state and failure to resolve the small parallel scales of the parasitic modes results in a non-physical transition to strong turbulence. This analysis is based on analytical calculations and numerical simulations of the cold-ion fluid model.

    [#s1294, 27 May 2021]
  • Vortex dynamics in non-neutral electron plasmas subject to externally imposed ExB flows
    Noah Hurst, University of Wisconsin, USA Webpage

    Abstract: A series of experiments is described in which magnetized non-neutral electron plasmas are subjected to strong applied electric fields in the plane perpendicular to the magnetic field. The resulting ExB drift dynamics are isomorphic to those of a two-dimensional ideal fluid described by the Euler equations. In this correspondence, the electron density is analogous to the fluid vorticity, and so the plasmas mimic the behavior of fluid vortices. The transverse electric fields act as externally imposed ExB strain flows which can deform and destroy the vortices. Details of the experimental procedure are given, as well as an overview of the experiments that have been carried out so far using this technique. Recent work is then discussed in greater detail, including studies of adiabatic behavior of elliptical electron vortices subject to slowly growing strain flows, and studies of spatial Landau damping of vortex oscillations due to a fluid-wave resonance near the vortex edge. The results are compared with a low-dimensional theoretical model of elliptical vortices, and with particle-in-cell simulations. Finally, the relationship of these results to other similar systems in geophysics, astrophysics, and plasma physics is discussed.

    [#s1290, 20 May 2021]
  • Turbulence in high-energy-density experiments: inference and generation
    Seth Davidovits, Lawrence Livermore National Laboratory (LLNL), USA

    Abstract: High-energy-density (HED) experiments pursuing fusion or X-ray generation can become turbulent. Facilities for HED experiments are also utilized for generating plasma turbulence for study, often with astrophysical applications in mind. The first part of this talk discusses the inference of turbulent flow in experiments without spatial (diagnostic) resolution of the flows; a need for such inference often arises in fusion or X-ray generation experiments, where the plasma is rapidly compressed to small size. Here I highlight examples from Z-pinch experiments optimized for X-ray production, and also briefly discuss recent work showing that turbulence in such two-dimensional compressions may exhibit stronger growth rates with decreasing volume than three-dimensional compressions. The second part of the talk discusses the turbulence generation principles underlying a new experimental design being developed for future laboratory studies of astrophysically-relevant turbulence.

    [#s1289, 13 May 2021]
  • X-ray view of the Coma galaxy cluster with SRG/eROSITA
    Eugene Churazov, Max Planck Institute for Astrophysics, Garching, Germany and Space Research Institute, Moscow, Russia Google Scholar, abstract
    [#s1288, 06 May 2021]
    Coma (Abell 1656) is a massive nearby galaxy cluster famous for being the first object where the presence of Dark Matter was noted by Fritz Zwicky back in 1933. In radio band, it became the first cluster where a “radio halo” and a “radio relic” were detected. In X-rays, which are emitted by hot plasma filling the cluster gravitational well, it is one of the three brightest clusters in the sky. Coma is also a spectacular case of cluster merger with a smaller galaxy group. All this makes Coma a testbed for studies of the phenomena ranging from collisionless dynamics of merging clusters to hydrodynamics, particle acceleration, and weakly collisional intracluster plasma on small scales. In X-rays, the only “trouble” is the large angular size (a few degrees) of the Coma cluster, which is difficult to map with telescopes having a small field of view. This difficulty was recently overcome with the SRG/eROSITA observations yielding a spectacular X-ray map of the entire cluster. Preliminary results of the analysis of these data will be discussed.
  • Direct laser acceleration (DLA) of leptons in plasma channels in radiation-dominated regime
    Marija Vranic, Istituto Superior Tecnico, Lisbon, Portugal Google Scholar, abstract
    [#s1283, 29 Apr 2021]
    DLA occurs in partially void plasma channels as a consequence of the simultaneous interaction of particles with the laser field and the plasma background. The particles perform betatron oscillations in the large-scale electric and magnetic field generated by displacing plasma electrons. In addition, they oscillate in the rapidly alternating laser field. By gaining momentum in the direction of laser propagation, the particles perceive a lower laser frequency, and the two types of oscillations can become resonant. The DLA electrons to ~500 MeV were obatianed in experiments using near-critical plasma densities and ps optical lasers. The principal advantage of DLA is that it generates relativistic electron beams with > 100 nC of charge. Using the next generation of lasers (~10 PW power), one could expect energies > 10 GeV, maintaining the high-charge content. In this regime, the interaction becomes dominated by the radiation losses, which counter-intuitively become favourable for acceleration. With a few modifications, DLA can be used for positron acceleration as well. I will address the underlying physics, the analytical model of the acceleration and the scaling laws predicting the asymptotic energy of the accelerated particles. The presented results are supported by particle-in-cell simulations.
  • Collisional transport in large aspect ratio stellarators
    Feliz I. Parra, University of Oxford, UK Google Scholar, abstract
    [#s1282, 22 Apr 2021]
    Collisional transport at the small collision frequencies characteristic of fusion reactors can be enormous in stellarators. In order to reduce this transport and the associated energy loss, the position and strength of the external magnets that produce the magnetic field in stellarators must be optimized. In this talk, I will revisit collisional transport in the particularly interesting limit of large aspect ratio stellarators. I will derive a new formulation to calculate collisional transport in the small collision frequency regime relevant to stellarator reactors. This new formulation has been implemented in KNOSOS (KiNetic Orbit-averaging SOlver for Stellarators), a very efficient, fast code that calculates collisional transport in a variety of regimes and can hence be used in stellarator optimization exercises. I will show both numerical and analytical results obtained using the new model that illustrate the nature of stellarator collisional transport at small collision frequencies.
  • Understanding the complex interaction between supra-thermal particles and turbulence in magnetic confinement devices
    Alessandro di Siena,University of Texas, Austin, USA Google Scholar, abstract
    [#s1281, 15 Apr 2021]
    The performance of magnetic confinement devices is strongly limited by turbulent transport inducing particle and energy losses and reducing plasma confinement. Among the different experimental actuators of turbulence, supra-thermal particles – generated via external heating schemes – are typically considered one of the most efficient in suppressing ion-temperaturegradient (ITG) driven turbulence in the core of fusion devices. In this talk, I will present some of the most recent insights into understanding the underlying physical mechanisms responsible for this turbulence regulation from first principle gyrokinetic simulations, theory and experiments. Finally, I will discuss the possible implications of this turbulence stabilization via energetic particles to existing and future tokamak and optimized stellarator devices.
  • The runaway electron landscape of cooling plasmas
    Tünde Fülöp, Chalmers University of Technology, Sweden Google Scholar, abstract
    [#s1273, 08 Apr 2021]
    The phenomena of runaway acceleration in plasmas has general importance in many fields of physics, for example it is a candidate mechanism for lightning initiation in thunderstorms and electron acceleration in solar flares. In fusion plasmas, understanding of runaways has a great practical importance, as the severity of runaway avalanches increases strongly with plasma current. Therefore, generation of runaways is expected to be a serious issue in ITER and other high-current reactor-scale fusion devices. We will discuss the characteristics and consequences of runaway generation, as well as possible mitigation strategies in fusion devices.
  • HelioSwarm: Leveraging Multi-point, Multi-scale Observations to Uncover the Nature of Turbulence in Space Plasmas
    Kristopher Klein, University of Arizona Webpage, abstract
    [#s1272, 01 Apr 2021]
    There are many fundamental questions about the temporal and spatial structure of turbulence in space plasmas. Answering these questions is complicated by the multi-scale nature of the turbulent transfer of mass, momentum, and energy, with characteristic scales spanning many orders of magnitude. The solar wind is an ideal environment in which to measure turbulence, but multi-point observations with spacecraft separations spanning these scales are needed to simultaneously characterize structure and cross-scale couplings. Recently selected for phase A study for NASA’s Heliophysics MidEx Announcement of Opportunity, the HelioSwarm Observatory proposes to transform our understanding of the physics of turbulence in space and astrophysical plasmas by deploying nine spacecraft to measure the local plasma and magnetic field conditions at many points, with separations between the spacecraft spanning MHD and ion scales. HelioSwarm resolves the transfer and dissipation of turbulent energy in weakly-collisional magnetized plasmas with a novel configuration of spacecraft in the solar wind. These simultaneous multi-point, multi-scale measurements of space plasmas allow us to reach closure on questions of how energy is distributed in typical solar wind conditions, as well as in extreme conditions relevant to astrophysical plasmas.
  • Magnetic reconnection andplasmoid formation in black hole accretion flows
    Bart Ripperda, Flatiron Institute and Princeton University Webpage, abstract
    [#s1271, 25 Mar 2021]
    Plasmoids, or hotspots, forming due to magnetic reconnection in current sheets, are conjecturedto power frequent X-ray and near-infrared flares from Sgr A*, the black hole inthe center of our Galaxy. It is unclear how, where, and when current sheetsform in black-hole accretion flows. We show extreme resolution 3Dgeneral-relativistic resistive magnetohydrodynamics and 2D general-relativisticparticle-in-cell simulations to model reconnection and plasmoid formation inblack hole magnetospheres. Plasmoids can form in thin current sheets In theinner 15 Schwarzschild radii from the event horizon, after which they canmerge, grow to macroscopic hot spots of the order of a few Schwarzschild radiiand escape the gravitational pull of the black hole. Large plasmoids areenergized to relativistic temperatures via magnetic reconnection near the eventhorizon and they significantly heat the jet, contributing to itslimb-brightening. We find that only hot plasmoids forming in magneticallydominated plasmas can potentially explain the energetics of Sgr A* flares. Theflare period is determined by the reconnection rate, which we find to beconsistent with studies of reconnection in isolated Harris-type current sheets.
  • Studies of plasma confinement in a gas-dynamic trap

    Alexander Ivanov, Budker Institute of Nuclear Physics, Novosibirsk, Russia

    The gas dynamic trap (GDT) was invented by Vladimir Mirnov and Dmitrii Ryutov in Novosibirsk in the late 1970s. It is basically a version of a magnetic mirror which is characterized by a long mirror-to-mirror distance exceeding the effective mean free path of ion scattering into a loss cone, a large mirror ratio (R ~ 100) and axial symmetry. Under these conditions the plasma confined in a GDT is isotropic and Maxwellian. The plasma loss rate out of the end mirrors is governed by a set of simple gas-dynamic equations, hence the device's name. Plasma magnetohydrodynamic stability in GDT can be achieved through a favorable averaged pressure-weighted curvature of the magnetic field lines, as was initially proposed, or, alternatively through a sheared plasma rotation at periphery induced by electrically biased electrodes at the end wall. A high flux volumetric neutron source based on a GDT is proposed, which benefits from the high β achievable in magnetic mirrors. Axial symmetry also makes the GDT neutron source more maintainable and reliable, and technically simpler. This review discusses the results of a conceptual design of the GDT-based neutron source which can be used for fusion materials development and as a driver of fission–fusion hybrids. The main physics issues related to plasma confinement and heating in a GDT are addressed by the experiments at the GDT device in Novosibirsk. The review concludes by updating the experimental results obtained, a discussion about the limiting factors in the current experiments and a brief description of the design of a future experimental device for more comprehensive modeling of the GDT-based neutron source. Conceivable approaches to improvement of plasma confinement in a GDT are also considered which would allow to consider the concept application in a fusion reactor.
    [#s1262, 18 Mar 2021]
  • Waves, Turbulence, and Transport in Weakly Collisional, High-Beta Plasmas

    Matt Kunz, Princeton University

    Many space and astrophysical plasmas are so hot and dilute that they cannot be rigorously described as fluids. These include the solar wind, low-luminosity black-hole accretion flows, and the intracluster medium of galaxy clusters. All of these plasmas are magnetized and weakly collisional, with plasma beta parameters of order unity or even much larger (“high-beta”). In this regime, deviations from local thermodynamic equilibrium ("pressure anisotropies") and the kinetic instabilities they excite can dramatically change the material properties of such plasmas and thereby influence the macroscopic evolution of their host systems. This talk outlines an ongoing programme of kinetic calculations aimed at elucidating from first principles the physics of waves, turbulence, and transport under these conditions. Three key results will be featured. (1) Shear-Alfvén waves “interrupt” themselves at sufficiently large amplitudes by adiabatically driving a field-biased pressure anisotropy that both nullifies the restoring tension force and excites a sea of ion-Larmor-scale instabilities. (2) Ion-acoustic waves in a collisionless, high-beta plasma similarly excite Larmor-scale instabilities, which ultimately aid the waves' propagation by rendering the plasma more fluid-like and, therefore, incapable of Landau damping. (3) Pressure anisotropy generated either by turbulent fluctuations or by global expansion (as in the solar wind) qualitatively change the properties of magnetized turbulence, affecting plasma heating and the so-called “critical balance”. Contact with observations of the near-Earth solar wind and the intracluster medium of galaxy clusters will be made.
    [#s1253, 11 Mar 2021]
  • Plasma Modelling in Support of the STEP Fusion Reactor Programme, video

    Howard Wilson, University of York, UK

    STEP – the Spherical Tokamak for Electricity Production – aims to deliver a prototype power plant by 2040 that will deliver net electricity at the 100MW level. This is a challenging timescale, that will require disruptive changes to how we design and regulate. The plasma scenario presents some of the biggest challenges, and this talk will discuss some of them. A spherical tokamak plasma has some advantages over one at conventional aspect ratio, allowing access to high elongation and beta (ratio of thermal plasma pressure to magnetic field pressure), in a compact geometry. Therefore, while much of the physics in the two designs is similar, there are also key differences. For example, the compact nature means there is little space for a solenoid, so non-inductive current drive is essential; the low magnetic field and high density require novel radio frequency methods for this current drive; the high beta affects the micro-instabilities that drive plasma turbulence and influence confinement; magnetohydrodynamic instabilities must be controlled to limit disruptions while achieving high fusion power and bootstrap current fraction; novel systems are required to manage the exhaust power loads, especially for the inner divertor leg. This talk will explore progress and challenges in modelling these key physics issues in support of STEP.
    [#s1235, 04 Mar 2021]
  • Progress and Challenges in TAE's Quest Towards an FRC-Based Fusion Reactor
    Artem Smirnov, TAE Technologies, Inc

    TAE Technologies, Inc. (TAE) is a privately funded company pursuing a novel approach to magnetic confinement fusion, which relies on Field-Reversed Configuration (FRC) plasmas composed of mostly energetic and stable particles. This advanced FRC-based system simplifies the reactor design and could offer a path forward to clean, safe, and economical aneutronic p-B11 fusion. To validate the science behind the FRC-based approach to fusion, an active experimental program is underway at TAE’s state-of-the-art plasma research facility in Orange County, California. The core of the facility is the world’s largest FRC device named Norman. In Norman, tangential injection of variable energy neutral beams (15 – 40 keV hydrogen, up to 20 MW total), coupled with plasma edge biasing, active plasma control, and advanced surface conditioning, led to production of steady-state, hot FRC plasmas dominated by fast ion pressure. High-performance, advanced beam-driven FRCs were produced,1-4 characterized by (1) macroscopic stability, (2) steady-state plasma sustainment, and (3) dramatically reduced transport rates (more than an order of magnitude improvement over conventional FRCs). Collectively, these accomplishments represent a strong argument validating the FRC-based approach to fusion power. This talk will provide a comprehensive overview of the TAE experimental program.

    [#s1243, 26 Feb 2021]
  • Astrophysical collisionless shock formation and nonthermal electron acceleration in laboratory experiments

    Hye-Sook Park, Lawrence Livermore National Laboratory, USA

    Collisionless shocks are ubiquitous in astrophysical environments such as in supernova remnants, jets in active galactic nuclei and gamma ray bursts and are known to be responsible for cosmic ray acceleration. While the theory of diffusive, or Fermi, shock acceleration (DSA) is well-established, the plasma microphysics responsible for the generation of the shocks, the nature of their resulting magnetic turbulence residue, and the injection of particles into DSA is not yet well understood. With the advent of high-power lasers, laboratory experiments with high-Mach number, collisionless plasma flows can provide critical information to help understand the mechanisms of shock formation by plasma instabilities and self-generated magnetic fields. A series of experiments were conducted on Omega and the National Ignition Facility to observe: the filamentary Weibel instability that seeds microscale magnetic fields [1, 2]; collisionless shock formation (seen by an abrupt ~4x increase in density and ~6x increase in temperature); and electron acceleration distributions that deviated from the thermal distributions [3]. Experimental results along with theoretical interpretations aided by particle-in-cell simulations will be discussed. [1] H.-S. Park et al., High Energy Density Phys. 8, 38 (2012); [2] C. Huntington et al., Nat. Phys. 11, 173 (2015); [3] F. Fiuza et al., Nat. Phys. 16, 916 (2020).

    [#s1236, 25 Feb 2021]
  • Thermonuclear Fusion in an Equilibrium Z Pinch

    Uri Shumlak, University of Washington, USA and Zap Energy, Inc.

    The equilibrium Z pinch is a novel approach to magnetic confinement fusion because it does not rely on external magnetic field coils. Equilibrium conditions are reached through the use of sheared plasma flows, which enhance stability and provide a path to thermonuclear fusion. Simple geometry and strong scaling of fusion gain with pinch current form the cornerstones of this compact fusion device. The sheared-flow-stabilized Z pinch has been developed through integrated computational and experimental investigations at the University of Washington in collaboration with Lawrence Livermore National Laboratory. Experimental results demonstrate plasma stabilization, sustained thermonuclear fusion, and agreement with theoretical and computational predictions. Building on these advances, Zap Energy Inc. is developing a low-cost fusion reactor core based on the equilibrium Z pinch.

    [#s1239, 18 Feb 2021]
  • Plasma Physics Challenges on the Road to a Tokamak DEMOnstration Fusion Power Plant
    Hartmut Zohm, Max Planck Institute for Plasma Physics, Germany

    In the EU Roadmap to Fusion Electricity, DEMO is the step between ITER and a commercial power plant. It is supposed to generate net electricity and have a self-sufficient fuel cycle. The pre-conceptual studies carried out for a tokamak-based DEMO show that the plasma scenario cannot be simply transferred from the ITER Q=10 scenario. We will discuss the physics issues encountered and possible solutions how to overcome them. The focus of the talk will be on the plasma physics aspects, not on reactor integration, and therefore meant to be exciting for plasma physicists from all fields.

    [#s1242, 11 Feb 2021]
  • Random flows and rotation in galactic coronae
    Anvar Shukurov, Newcastle University, UK

    After a review and summary of the observational evidence for random flows and rotation at large altitudes (1-10 kpc) above the discs of spiral galaxies, I discuss the physical parameters of the off-planar gas and the energy sources of the random motions. I argue that the effects of the turbulent viscosity on large-scale gas flows are significant and propose an exploratory model of the viscous coupling of the rotating galactic disc and the corona.

    [#s1244, 28 Jan 2021]
  • Understanding the Building Blocks of the Solar Wind and How They Fit Together: Heat Flux, Radiation, and Alfven-Wave Turbulence
    Benjamin Chandran, University of New Hampshire, USA

    A growing body of evidence suggests that the solar wind is powered to a large extent by an Alfven-wave energy flux that is generated by convective motions on the surface of the Sun. The solar wind and solar corona are also affected by the flux of heat, including conductive losses into the radiative lower solar atmosphere. Numerical simulations that account for the above physics are increasingly able to reproduce remote observations of the corona and solar wind. On the other hand, we still lack an analytic theory that provides formulas for key quantities such as the solar mass-loss rate. Analytic treatments are needed for several reasons. They deepen our understanding by distilling complex processes into their most essential elements, they show how different quantities scale with one another, and they encapsulate our understanding into a portable form that can be applied to other systems and used by anyone. In this talk, I will present a recently developed analytic theory of coronal heating and solar-wind acceleration that provides analytic formulas and intuitive explanations for the solar mass-loss rate, the solar-wind speed far from the Sun, the coronal temperature, the heat flux from the corona into the lower solar atmosphere, and the plasma density at the base of the corona.

    [#s1245, 21 Jan 2021]
  • Drift kinetic theory of alpha particle transport by tokamak perturbations
    Elizabeth Tolman, Institute for Advanced Study, USA

    Upcoming deuterium-tritium tokamak experiments are expected to have large energetic alpha particle populations. These experiments can be used to study the interaction between these alpha particles and perturbations to the tokamak’s electric and magnetic fields. In this talk, I will first describe why this behavior is important and interesting. Then, I will discuss a new drift kinetic theory to calculate the alpha heat flux resulting from a wide range of perturbation frequencies and periodicities. This theory suggests that the alpha heat flux caused by toroidal field ripple, one type of perturbation, is small. Applied to the toroidal Alfvén eigenmode (TAE), another type of perturbation, the theory suggests a significant alpha heat flux that scales with the square of the TAE amplitude. The TAE amplitude calculated from one saturation condition suggests that TAEs in SPARC, one upcoming deuterium-tritium experiment, will not cause significant alpha transport via the mechanisms in this theory. However, saturation above the level suggested by the simple condition, but within numerical and experimental experience, could cause significant transport.

    [#s1246, 14 Jan 2021]
  • Exploring the physics of turbulent collisionless shocks in conditions of laboratory experiments
    Anna Grassi, Sorbonne University, Paris, France

    Collisionless shocks are ubiquitous in astrophysical plasmas and play an important role in magnetic field generation/amplification and particle acceleration. While diffusive shock acceleration (DSA) is well established, the details of particle injection into DSA remain a long-standing puzzle, particularly for electrons. High-energy-density (HED) plasma experiments and kinetic plasma simulations offer a promising route to identify the dominant processes at play. Very recently experiments performed at the National Ignition Facility have observed for the first time the formation of high-Mach number collisionless shocks mediated by electromagnetic instabilities and nonthermal electron acceleration. I will discuss the physics behind shock formation and particle acceleration in these laboratory systems and how they can be connected to astrophysical models. Using large-scale, multi-dimensional particle-in-cell (PIC) simulations, we find that the inhomogeneous profiles of laser-ablated plasmas lead to shock formation that can be up to 10 times faster than previous models predicted. The shock front can also develop strong corrugations at the ion gyroradius scale, which can be controlled by changing the electron temperature of the flow. Finally, we show that electrons can be effectively accelerated to nonthermal energies and injected into DSA via a Fermi-like mechanism occurring within the finite, turbulent shock transition. These findings can help guide the development and interpretation for current experimental programs and open exciting prospects for studying the microphysics of turbulent collisionless shocks in the laboratory.
    [#s1247, 07 Jan 2021]
  • When will we be able to predict plasma confinement in fusion devices?
    Frank Jenko, IPP Garching, Germany

    It is a key goal of magnetic confinement fusion (MCF) research to develop and build devices that allow us to create a plasma at sufficiently high pressure and energy confinement time, so that Lawson's criterion for a burning plasma can be met. There was breathtaking progress along these lines between the 1970s and 1990s, largely based on a "trial-and-error" approach. With the preparation of ITER operation and attempts to design first versions of future fusion power plants, it became clear, however, that a more targeted "predict-first" approach is needed at this point to save significant amounts of time and resources in the further development of fusion energy. Fortunately, the power of High Performance Computing keeps growing at a remarkable speed, with exascale systems around the corner. These platforms open up new possibilities to solve the complex nonlinear equations underlying many observed phenomena in MCF plasmas, and to move from an interpretative to a truely predictive approach. In this context, computing, data analysis, and machine learning are increasingly intertwined to provide reliable predictions. So how and when will we be able to predict plasma confinement in fusion devices? This question will be at the heart of this presentation.
    [#s1258, 16 Dec 2020]
  • Some open questions in the plasma physics of cosmic rays
    Ellen Zweibel, University of Wisconsin, Madison, USA

    It's now widely recognized that cosmic rays have considerable influence on the dynamics and energy balance of thermal gas in and around galaxies. While it's their collisional interactions that render cosmic rays most directly visible, in many respects their collisionless interactions, mediated by kinetic scale plasma waves and instabilities, are more significant. These microscale interactions are now captured by fluid models and used in large scale simulations of galaxy formation and evolution, where they are revealing surprising behavior. After briefly reviewing the basics, I will discuss some open questions in the microphysics of cosmic ray - thermal gas coupling, and how the large scales react back on the small ones.
    [#s1259, 16 Dec 2020]
  • For a full list of past colloquia, see the JPP Frontiers of Plasma Physics Colloquium webpage.
    [#s1260, 01 Jan 2020]