A Chapman-Enskog-like (CEL) Kinetic Closure Approach in NIMROD

When simulating the macroscopic dynamics of high temperature magnetized plasmas, a fluid description has limitations due to approximations commonly employed in the closure problem. Rigorous closure methods exist in higher collisionality regimes, but many processes become intrinsically kinetic in lower collisionality regimes. While heuristic forms have proven useful for mimicking some of this kinetic closure physics, this effort seeks to close the plasma fluid equations self-consistently in all collisionality regimes.

In particular, a Chapman-Enskog-like (CEL) kinetic closure approach [1] has recently been added to the plasma fluid code NIMROD [2,3]. This CEL implementation allows for a rigorous kinetic closure of NIMROD’s fluid model by evolving a kinetic equation in the presence of a self-consistent background Maxwellian defined by temporally and spatially evolving density, temperature and fluid velocity (evolved using the fluid equations).

We employ this new closure scheme in a resonant field error penetration problem in tokamak geometry. A kinetic ion response is used to accurately describe the ion flow dynamics in toroidal geometry. Numerical stability considerations relevant to the coupled CEL-fluid evolution approach in NIMROD are discussed. [1] J. J. Ramos, Phys. Plasmas extbf{17}, 082502 (2010). [2] C. Sovinec, et. al. , Nonlinear magnetohydrodynamics with high-order finite elements, J. Comp. Phys. extbf{195} (2004) 355 [3] J. R. Jepson, et. al. , Phys. Plasmas extbf{28} (2021) 082503.