Simulations of plasma flow evolution of an axisymmetric tokamak using a Chapman-Enskog-like (CEL) kinetic closure approach in NIMROD

In an effort to incorporate kinetic ion physics in extended MHD simulations, a Chapman-Enskog-like (CEL) closure approach [1] is used in NIMROD simulations. This formulation has been successfully benchmarked against the predictions of neoclassical theory for poloidal ion flows in axisymmetric tokamak geometry [2]. The CEL implementation allows for a rigorous kinetic closure of NIMROD’s fluid model as the kinetic equation is solved for in the presence of a self-consistent background Maxwellian defined by temporally and spatially evolving density, temperature and fluid velocity. Herein, we model the axisymmetric fluid evolution from an EFIT equilibrium based on DIII-D Iter-Baseline-Scenario (IBS) discharge 174446 at 3390 ms. For testing purposes, we initialize an MHD equilibrium in the absence of equilibrium fluid flows. The evolution of ion fluid flows and kinetic closures is simulated for an initially static axisymmetric equilibrium, until a self-consistent equilibrium state is developed. The stabilizing effects of a new “upwinding-like” artificial diffusion operator in the kinetic equation are also investigated. [1] J. J. Ramos, Phys. Plasmas \textbf{17}, 082502 (2010). [2] J. R. Jepson, C. C. Hegna, E. D. Held, J. A. Spencer, and B. C. Lyons, Phys. Plasmas \textbf{28}, 082503 (2021)