Disruption mitigation modeling for the SPARC tokamak

Two primary systems planned for disruption mitigation in the SPARC tokamak, massive gas injection (MGI), and the runaway electron mitigation coil (REMC), are modeled with the 3D MHD code NIMROD. MGI is modeled in four configurations, and each of the 6-valve and 4-valve configurations considered is predicted to produce a high radiation fraction (>95%) and low toroidal peaking factor [<1.5 during the thermal quench (TQ)]. The MGI-induced TQ is also modeled in conjunction with the REMC in two different scenarios: one in which the REMC is assumed to be a closed-circuit, and another in which it can carry current only during the CQ. The closed-circuit coil has some effect on the timing of the TQ onset but not on the overall TQ dynamics or subsequent REMC effectiveness. The REMC is able to maintain stochastic fields in the plasma so long as the safety factor at the magnetic axis remains below 2 (q0<2), but the safety factor evolution, in turn, depends on a number of factors, including the treatment of the region outside of the SPARC limiter surrounding the plasma.