Linear and Nonlinear Simulations of Tokamak Discharges with Fluid Runaway Electrons in NIMROD.

A reduced fluid model for runaway electrons (REs) is incorporated in the time advance of 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 given by the E cross B drift. There is a volumetric source density for REs given by the background density and parallel electric field via the Dreicer mechanism [Connor and Hastie, NF 15, 415], and a source representing the avalanche RE generation [Rosenbluth and Putvinski, NF 37, 1355]. Like the JOREK model presented in Ref. [Bandaru, et al., PRE 99, 063317] and the M3D-C1 model in Ref. [Liu et al., PoP 27, 092507], the RE density evolution couples to the MHD equations via Ohm's law in under the assumption that the RE current does not contribute to the resistive electric field. The NIMROD code has the capability to perform both linear and nonlinear calculations with the RE model. Linear calculations of an m=2, n=1 tearing mode in both cylindrical and toroidal geometry confirms prior findings that the presence of RE current introduces rotation of the eigenmode structure in the poloidal plane [Liu]. We also present 3D, nonlinear simulations of MST tokamak discharges with REs.