PPPL

External Seminars

The following “external seminars” have been given by Theory Staff.

Past

  • U. Maryland
    Onset and nonlinear relaxation of coherent current-carrying edge filaments during transient events in tokamaks
    Fatima Ebrahimi, abstract, slides
    [#s726, 09 May 2018]
    The onset and nonlinear evolution of coherent current-carrying filaments are examined using global nonlinear three-dimensional resistive MHD simulations in a spherical tokamak (ST). We show that physical current sheets/layers develop near the tokamak edge under different circumstances, in particular as a peeling component of ELMs (due to bootstrap currents), and during vertical displacement events (associated with the scrape-off layer currents). In all these cases, edge current sheets can become unstable to nonaxisymmetric 3-D current-sheet instabilities and nonlinearly form edge coherent current-carrying filaments. Time-evolving edge current sheets in ST configurations are identified in our nonlinear simulations. [F. Ebrahimi, Phys. Plasmas 23, 120705 (2016); F. Ebrahimi, Phys. Plasmas 24, 056119 (2017)] In the case of peeling-like edge localized modes, the longstanding problem of quasiperiodic ELM cycles is explained through the relaxation of the edge current source through direct numerical calculations of reconnecting local bi-directional fluctuation-induced electromotive force (emf) terms. Second, we examine the stability and formation of reconnecting edge peeling-driven filaments during induced vertical displacement events (VDEs) simulations. Similar to fast reconnection due to axisymmetric plasmoids, [F. Ebrahimi & R. Raman, Phys. Rev. Lett. 114, 205003 (2015)] we find that the growth rate of these edge filamentary structures becomes independent of Lundquist number. As well as edge reconnection physics in tokamaks, the 3-D coherent current-carrying fi lament structures and their nonlinear dynamics due to the dynamo effect presented here are also relevant to flares, which also exhibit ejection of field-aligned filamentary structures into the surrounding space.
  • Swarthmore
    Plasma science - From laboratory-fusion to astrophysical plasmas
    Fatima Ebrahimi, abstract, slides
    [#s686, 20 Apr 2018]
    Our universe is immersed in magnetized plasma, electrically conducting ionized gas. Some of the most fundamental and long-standing astrophysical problems, such as the magnetization of the universe, collimation of astrophysical jets, the accretion process and transport in astrophysical disks (surrounding e.g. black holes) and their coronas can only be explored through plasma physics. Our sun as a natural laboratory for plasma physics provides inspiring as well as challenging problems, including its dynamo cycles, heating, and the replication of its core reaction, fusion energy, on earth in a lab. There is an abundance of observational/experimental data emerging from natural phenomena of space and astrophysical plasmas, as well as laboratory plasma experiments, for plasma physicists to explore. I will review some of these topics, in particular magnetic reconnection, the rearrangement of the magnetic field topology of plasmas, which energizes many processes in nature and has been shown to also be critical in the nonlinear dynamics of many processes in toroidal fusion plasmas. Using global simulations, I will demonstrate the instrumental role of magnetic reconnection, which enables an innovative technique for producing current in fusion plasmas.
  • Brookhaven National Laboratory
    Plasma science - From laboratory-fusion to astrophysical plasmas
    Fatima Ebrahimi, abstract, slides
    [#s685, 17 Apr 2018]
    Our universe is immersed in magnetized plasma, electrically conducting ionized gas. Some of the most fundamental and long-standing astrophysical problems, such as the magnetization of the universe, collimation of astrophysical jets, the accretion process and transport in astrophysical disks (surrounding e.g. black holes) and their coronas can only be explored through plasma physics. Our sun as a natural laboratory for plasma physics provides inspiring as well as challenging problems, including its dynamo cycles, heating, and the replication of its core reaction, fusion energy, on earth in a lab. There is an abundance of observational/experimental data emerging from natural phenomena of space and astrophysical plasmas, as well as laboratory plasma experiments, for plasma physicists to explore. I will review some of these topics, in particular magnetic reconnection, the rearrangement of the magnetic field topology of plasmas, which energizes many processes in nature and has been shown to also be critical in the nonlinear dynamics of many processes in toroidal fusion plasmas. Using global simulations, I will demonstrate the instrumental role of magnetic reconnection, which enables an innovative technique for producing current in fusion plasmas.
  • NASA Goddard Space Flight Center
    Laboratory Studies of Magnetized Collisionless Shocks
    Derek Schaeffer, abstract
    [#s682, 13 Apr 2018]
    Collisionless shocks are ubiquitous in space and astrophysical systems, and the class of supercritical shocks is of particular importance due to their role in accelerating particles to high energies. While these shocks have been traditionally studied by spacecraft and remote sensing observations, laboratory experiments can provide reproducible and multi-dimensional datasets that provide complementary understanding of the underlying microphysics. We present experiments undertaken on the OMEGA and OMEGA EP laser facilities that show the formation and evolution of high-Mach number collisionless shocks created through the interaction of a laser-driven magnetic piston and magnetized ambient plasma [1,2]. Through time-resolved, 2-D imaging we observe large density and magnetic compressions that propagate at an Alfvénic Mach number MA ~ 15 and that occur over ion kinetic length scales. Additional shock structure and electron and ion heating are observed with optical Thomson scattering, which is also used to characterize the initial ambient plasma. Particle-in-cell simulations constrained by experimental data further detail the shock formation and separate dynamics of the multi-ion-species ambient plasma. The results show that the shocks form on timescales as fast as one gyroperiod, aided by the efficient coupling of energy, and the generation of a magnetic barrier, between the piston and ambient ions. The development of this experimental platform opens the way for controlled laboratory investigations of high-Mach-number collisionless shocks, including mechanisms of shock heating and particle acceleration.
    [1] D. B. Schaffer, W. Fox et al., Phys. Rev. Lett. 119, 025001 (2017)
    [2] D. B. Schaffer, W. Fox et al., Phys. Plasmas 24, 122702 (2017)
  • MIT Plasma Science Fusion Center
    Effect of velocity-space anisotropy on waves, turbulence, and transport in high-beta astrophysical plasmas
    Matthew Kunz, abstract
    [#s600, 15 Dec 2017]
    Many space and astrophysical plasmas are so hot and diffuse 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. Currently, the lack of a rigorous theory for the plasma microphysics in these systems is a formidable obstacle to answering several fundamental questions in space and astrophysics. Much of the difficulty arises from the freedom in a weakly collisional plasma for the particle distribution function to be anisotropic in velocity space, as well as from the shaping of such temperature anisotropy by magnetic fields. In this talk, I will present hybrid-kinetic simulations of waves, turbulence, and magnetorotationally driven transport in collisionless, high-beta plasmas, which address how Larmor-scale kinetic instabilities are driven by and ultimately regulate temperature anisotropy, and how this regulation in turn feeds back on the macroscale dynamics. Gyrokinetic theory will also be used to explore how temperature anisotropy affects Alfvénic turbulence and ion versus electron heating in high-beta plasmas.
  • Los Alamos National Laboratory (LANL)
    Metriplectic dynamics
    Eero Hirvijoki, slides
    [#s603, 14 Dec 2017]
  • University of Maryland
    A simple stellarator coil-design algorithm
    Stuart Hudson, slides
    [#s583, 13 Dec 2017]
  • Magneto-Fluid Dynamics Seminar, Courant Institute for Mathematical Sciences, New York U.
    Does a photon have a linear polarizability and why does it matter?
    Ilya Dodin, abstract
    [#s537, 10 Oct 2017]
    A photon (phonon, plasmon, etc) has a linear polarizability. To see this and to understand why this matters, it helps to set aside Maxwell's equations and quantum mechanics per se and start with the following basic physics. Suppose a rapidly oscillating wave field in a weakly inhomogeneous linear medium. Assuming the dispersion operator for the wave field is known, a reduced operator can be defined that governs just the wave envelope. Using the Weyl calculus, an asymptotic approximation of the reduced operator can then be constructed to any power n in the geometrical-optics (GO) parameter. The corresponding truncations yield GO (n = 0), extended GO (n = 1), and quasioptics (n = 2). Notably, an accurate formulation of the latter for inhomogeneous media has only been given recently [unpublished]. But there is even more to this approach. For waves propagating in modulated media (i.e., interacting with other waves), a reduced operator can be derived similarly for Floquet envelopes. The modulation-dependent term in this operator serves as the ponderomotive Hamiltonian of a wave, and its derivative with respect to the (loosely speaking) modulation intensity serves as the wave polarizability [Phys. Rev. A 95, 032114 (2017)]. When applied to charged particles treated as quantum waves, this gives the conventional particle polarizability. Conversely, when applied to classical waves, this defines an effective linear polarizability of a photon (phonon, plasmon, etc). Using this concept, one can interpret modulational dynamics (MD) of nonlinear electromagnetic waves as linear dispersive dynamics of a polarizable photon gas. This significantly simplifies calculations of MD and makes them less error-prone than the standard Maxwell--Vlasov approach [J. Plasma Phys. 83, 905830201 (2017)]. Even more generally, quasilinear MD of all wave ensembles are governed by Wigner--Moyal-type equations that are identical up to a (generally non-Hermitian) Hamiltonian. Then, for example, the modulational instability in the nonlinear Schrodinger equation, the zonostrophic instability of drift-wave turbulence, and the standard two-stream collisionless-plasma instability formally appear as essentially the same effect. In a broader context, elaborating on this approach seems promising for studying inhomogeneous wave turbulence.
  • Magneto-Fluid Dynamics Seminar, Courant Institute for Mathematical Sciences, New York U.
    Metriplectic dynamics -- a framework for plasma kinetic theory and numerics
    Eero Hirvijoki, abstract
    [#s541, 03 Oct 2017]
    In dissipationless systems, Hamiltonian mechanics, culminating in a Poisson bracket and a Hamiltonian, provides a convenient framework for both theoretical and numerical studies. In systems that obey both the First and the Second Law of Thermodynamics, the dissipationless dynamics can often be extended with a symmetric bracket and an entropy to account for the dissipation. The resulting, so-called metriplectic framework captures many interesting models, including the Navier-Stokes equations, non-isothermal kinetic polymer models, and the Vlasov-Maxwell-Landau model used in plasma physics. In this talk, we review the basic principles of metriplectic dynamics and discuss some prominent methods for time discretization. We focus on the Vlasov-Maxwell-Landau model and, especially, on the Landau collision operator for which a genuine metriplectic integrator is demonstrated. The gyrokinetic version of the Vlasov-Maxwell-Landau system is briefly visited.
  • ITER Organization
    Gyrokinetic Simulation of Pedestal and SOL Transport in Present Experiments and ITER
    C-S. Chang, abstract
    [#s540, 25 Sep 2017]
    Abstract.
  • Culham Center for Fusion Energy
    Gyrokinetic simulation of a fast L-H like bifurcation dynamics in a realistic diverted tokamak edge geometry
    C-S. Chang, abstract
    [#s538, 25 May 2017]
    Abstract.
  • Culham Center for Fusion Energy
    Recent advances in the gyrokinetic heat-footprint simulation
    C-S. Chang, abstract
    [#s539, 25 May 2017]
    Abstract.
  • ASDEX-Upgrade seminar, Garching, Germany
    Energy exchange dynamics across L-H transitions in NSTX
    T. Stoltzfus-Dueck
    [#s677, 22 May 2017]
  • ASDEX-Upgrade seminar, Garching, Germany
    Parasitic momentum flux in the tokamak core
    T. Stoltzfus-Dueck, slides
    [#s676, 17 May 2017]
  • Magneto-Fluid Dynamics Seminar, Courant Institute for Mathematical Sciences, New York U.
    Variational Integration for Ideal Mhd and Formation of Current Singularity
    Yao Zhou
    [#s542, 14 Feb 2017]
  • Friday Science Meeting, General Atomics, San Diego
    Parasitic momentum flux in the tokamak core
    T. Stoltzfus-Dueck, slides
    [#s678, 16 Dec 2016]
  • Friday Science Meeting, General Atomics, San Diego, CA.
    Parallel electron force balance and the L-H transition
    T. Stoltzfus-Dueck, slides
    [#s679, 13 May 2016]