Theory Weekly Highlights for May 2020

May 22, 2020

A previous study of large-radius internal transport barriers in DIII-D high beta poloidal discharges demonstrated that while the large local Shafranov shift stabilized local ballooning modes, it destabilized microtearing modes (MTMs) [X. Jian et al, Phys. Rev. Lett 123, 225002 (2019)]. These MTMs were shown via nonlinear CGYRO simulations to be capable of reproducing observed electron thermal transport levels in the ITB region. Continued analysis of these conditions demonstrates that the MTMs have a strong slab-like character and peak in the high-field-side region of the plasma, rather than the low-field-side region where ballooning modes typically peak. Consideration of transport in an analogous scenario in ITER suggests a similar transition from low-field-side ballooning modes to high-field-side slab like modes will occur at large normalized local pressure gradients. However, due to the lower collisionality expected in ITER, the modes are predicted to be predominantly electrostatic rather than MTM-like. For both the DIII-D and ITER-like conditions, the transition in mode peaking location can be correlated and understood in terms of a change in sign of the eigenfunction-averaged magnetic drift-frequency.

May 08, 2020

Recent work on the anti-symmetric representation of plasma fluid models appears on the April 2020 cover of the Physics of Plasmas, where it is also featured as an Editor's Highlight. The paper, (F.D. Halpern, Physics of Plasmas 27, 042303 (2020); develops a new paradigm for interpreting the magnetohydrodynamic (MHD) model where time evolution results from tiny pieces of plasma rotating about space. The new approach breaks with traditional lore, where plasma “packets” are either followed through space as the move, or counted inside boxes as they pass through. The anti-symmetric model has the property that some fundamental physical properties can be, for the first time, closely mimicked in computer codes. For instance, since rotations are reversible, then MHD simulations using this approach should be time reversible as well. To demonstrate this, the paper features simulations where the classic Orszag-Tang vortex problem is advanced in time, and then accurately rewound back to its initial point. The use of anti-symmetric fluid equations has the potential to increase the fidelity and reliability of numerical applications in fusion plasmas, astrophysics, engineering, and other fields.

May 01, 2020

We have engaged with collaborators at JET and KSTAR to identify the plasma and shatter-pellet-injection (SPI) configurations needed to initialize modeling with M3D-C1 and NIMROD. A high-quality equilibrium for a JET “Scenario 1” plasma, reconstructed from shot 95707, was obtained along with necessary kinetic profiles. Initial 2D and 3D modeling of a single, solid-neon pellet injected into this equilibrium using the realistic JET geometry have begun in M3D-C1, thus far showing a radiation-driven thermal quench with relatively little magnetohydrodynamic activity. Higher resolution simulations with more-localized impurity sources and a shattered-pellet plume are planned to validate with JET experiments properly.

These highlights are reports of research work in progress and are accordingly subject to change or modification