Scientists of the Theory Department are frequently invited to give presentations at major physics and computing conferences.
The following presents a selection of the Invited Presentations given by Theory Staff and their collaborators.
Plasmoid-mediated reconnection is examined using global nonlinear three-dimensional resistive MHD simulations in a spherical tokamak for two cases: 1) generation of closed flux surfaces during helicity injection experiments for start-up current-drive (for solenoid-free tokamak design) and 2) nonlinear edge localized modes. An initial poloidal flux is created, in this case utilizing the helicity injection technique, in the presence of a toroidal guide field. A rare, classical example of plasmoid formation in a large-scale toroidal fusion plasma has been demonstrated during helicity injection, where the injected magnetic field lines are oppositely directed near the injection region and form elongated Sweet-Parker current sheets. At high Lundquist number a transition to plasmoid instability has been shown. This is the first observation of plasmoid instability in a laboratory device configuration predicted by realistic MHD NIMROD simulations and then supported by experimental camera images from NSTX.
Second, it is shown that the 3-D non-axisymmetric magnetic fluctuations could arise due to edge current-sheet instabilities. It is found that i) regardless of non-axisymmetric 3-D edge perturbations, large volume flux closure  is formed during start-up helicity injection, ii) 3-D magnetic fluctuations can cause local flux amplification to trigger axisymmetric reconnecting plasmoids formation at the reconnection site. We also show coherent current-carrying filament structures (sometimes referred to as 3-D plasmoids) wrapped around the torus that are nonlinearly formed due to nonaxisymmetric reconnecting current sheet instabilities, the so called peeling-like edge localized modes. These fast growing modes saturate by breaking axisymmetric current layers isolated near the plasma edge and go through repetitive relaxation cycles by expelling current radially outward and relaxing it back. The longstanding problem of quasiperiodic ELMs cycles is explained through the edge reconnection process.
 F. Ebrahimi, R. Raman, Phys. Rev. Lett. 114, 205003 (2015)
 F. Ebrahimi, R. Raman, Nucl. Fusion Lett. 56, 044002 (2016)
 F. Ebrahimi, Phys. Plasmas 23, 120705 (2016)
 F. Ebrahimi, Phys. Plasmas 24, 056119 (2017)
Photon, electron and ion bombardment of materials leads to the emission of electrons from the materials. This so-called secondary electron emission (SEE phenomenon is a common link between particle-surface interactions in plasmas, particle accelerators, light sources, and space environments. The plasma-surface interaction in the presence of a strong electron emission is omnipresent in numerous plasma applications such as, for example, cathodes, emissive probes, divertor plasma, surface discharges, dusty plasma, plasma thrusters and plasma processing. In a plasma system, electron and ion fluxes to the wall are determined by particles velocity distribution functions and by the sheath potential, which are consistent with the wall properties. Electrons with sufficient energy to overcome the wall sheath potential and ions accelerated by the sheath potential can impact the wall and produce secondary electrons. The secondary electron emission can then reduce the sheath potential, leading to an increased loss of plasma electrons to the wall, increased wall heating, and increased cooling of the bulk plasma.
Although the role of the secondary electron emission in the above processes and applications has been acknowledged, its effects are neither well characterized nor well understood and therefore, cannot be reliably predicted. For example, electron emission significantly changes the space-charge distribution around emissive probes, adding uncertainty to plasma potential measurements. This status quo is in a great part due to a complex synergistic nature of particle-surface interactions, which often involves a coupling between impinging particles and materials properties and surface geometry. This coupling is particularly strong for plasma-surface interactions. In this problem, plasma and materials sciences are not separable – the plasma and surface interact and evolve together. The plasma science challenges are i) to develop an understanding of SEE effects on plasma and plasma effects on SEE, including but not limited to heating and energy relaxation of emitted electrons in the plasma through collisions and collective effects, surface recombination, surface charging, and surface breakdown, ii) to characterize SEE properties and SEE effects directly in plasma rather than in vacuum as it is commonly done, and iii) to develop control of SEE effects. The materials and surfaces sciences challenges are to understand i) how surface evolves from interaction with plasma, ii) how these surface and materials modifications affect the SEE from these materials, and iii) how to control SEE properties of materials. For example, changing surface properties with various coatings or due to wall erosion, trapping of emitted particles in complex surfaces, nanoscale effects all can significantly alter the electron emission properties of plasma facing surfaces.
A major challenge for supercomputing today is to demonstrate how advances in HPC technology translate to accelerated progress in key application domains. This is the focus of an exciting new program in the US called the “National Strategic Computing Initiative (NSCI)” – announced by President Obama as an Executive Order on July 29, 2015 involving all research & development (R&D) programs in the country to “enhance strategic advantage in HPC for security, competitiveness, and discovery.” A strong focus in key application domains is to accelerate progress in advanced codes that model complex physical systems -- especially with respect to reduction in “time-to-solution” as well as “energy to solution.” It is understood that if properly validated against experimental measurements/observational data and verified with mathematical tests and computational benchmarks, these codes can greatly improve much-needed predictive capability in many strategically important areas of interest.
Computational advances in magnetic fusion energy research have produced global particle-in-cell (PIC) simulations of turbulent kinetic dynamics for which computer run-time and problem size scale very well with the number of processors on massively parallel many-core supercomputers. For example, the GTC-Princeton (GTC-P) code, which has been developed with a “co-design” focus, has demonstrated the effective usage of the full power of current leadership class computational platforms worldwide at the petascale and beyond to produce efficient nonlinear PIC simulations that have advanced progress in understanding the complex nature of plasma turbulence and confinement in fusion systems for the largest problem sizes. Instead of the familiar Fortran-90 language, this is a modern code written in C and deploying OpenMP/MPI, CUDA, and OpenACC programming strategies with a strong focus on performance optimization of key operational functions within particle-in-cell codes in general. This has produced significant advances in scalability, performance, and portability on path-to-exascale supercomputing systems worldwide. Results have provided strong encouragement for being able to include increasingly realistic dynamics in extreme-scale computing campaigns with the goal of enabling predictive simulations characterized by unprecedented physics resolution/realism needed to help accelerate progress in delivering fusion energy.
We describe the construction of stepped-pressure equilibria as extrema of a multi-region, relaxed magnetohydrodynamic (MHD) energy functional that combines elements of ideal MHD and Taylor relaxation, and which we call MRxMHD. The model is compatible with Hamiltonian chaos theory and allows the three-dimensional MHD equilibrium problem to be formulated in a well-posed manner suitable for computation, and numerical solutions are constructed using the stepped-pressure equilibrium code, SPEC. Highlights of recent calculations will be presented and discussed, including: that the self-organized single-helical-axis (SHAx) and double-axis (DAx) states in reversed field pinch experiments can be reproduced; that MRxMHD can recover ideal MHD; and the SPEC code is used to compute (for the first time) the singular current densities predicted in ideal MHD equilibria in three-dimensional geometry and a new class of solution to the ideal MHD equilibrium equation will be presented. Some ongoing developments of MRxMHD and SPEC will be discussed, including: vacuum verification calculations of W7-X equilibria; free-boundary, non-up-down symmetric DIIID calculations, and including a non-trivial flow into the energy principle.