Incorporation and Verification of a Fluid Model for Runaway Electrons in NIMROD.

A reduced fluid model for runaway electrons (REs) is incorporated into the NIMROD code. REs are treated as a distinct fluid species that flows with a velocity consisting of a large prescribed parallel component and a perpendicular component arising from E cross B drift. There is both a volumetric source density for the runaway species given by the local background density and parallel electric field via the Dreicer mechanism and a volumetric source density representing the avalanche runaway generation. As in the model presented in Bandaru, et al. [PRE 99, 063317(2019)], the RE density evolution couples to the MHD equations via Ohm's law and the momentum evolution in accordance with the assumption that the RE species does not contribute to the resistive electric field. Comparison of numerical results to analytic expressions for simple cases is presented. Comparisons of NIMROD results from a test case in which REs are generated via a thermal quench (TQ) to results obtained in Bandaru, et al. serve as a benchmark for the fully coupled nonlinear system. A second benchmark addresses RE production for a simulated TQ of an equilibrium reconstructed from experiment and includes comparison with M3D-C1.