Predictive modeling for successful disruption mitigation on SPARC

The high energy density and high total current in the SPARC tokamak will put disruption mitigation strategies to the test in reactor relevant regimes, and predictive MHD modeling has informed the design and will enable projection of SPARC's disruption mitigation tools to a fusion pilot plant. NIMROD extended-MHD modeling has predicted successful performance both for a planned six-valve massive gas injection (MGI) system for thermal quench (TQ) mitigation and for a novel runaway electron mitigation coil (REMC). The MGI system has three upper and three lower valves, with each set spaced 120 deg apart and clocked 60 deg from the other. This configuration is predicted by NIMROD to have a radiation toroidal peaking factor below 1.5 for all times except very early in the pre-TQ, even when the m=1/n=1 convected heat flux dominates the radiation pattern in the late-TQ. With a larger number of valves than on any previous MGI experiment, the radiated energy fraction is predicted to exceed 99%, with radiated power peaking at 65GW during the TQ. A comparison is presented with other 4- and 6-valve options. The REMC will be the first experimental test of the concept in a tokamak with an RE avalanche multiplication factor comparable to a reactor (~1e10 vs e.g. ~100 on DIII-D). NIMROD modeling combined with results from the ThinCurr, ASCOT5 and DREAM codes has predicted full suppression of RE beams on SPARC due to the 3D perturbations when current is induced in the coil by the current quench (CQ) loop voltage [R.A. Tinguely, et al, 2021 Nucl. Fusion 61, 124003]. Crucially, the n=1 coil remains resonant with the plasma only so long as the q0 remains below 2, where the q-profile evolution can be affected by the boundary conditions and by current redistribution during the TQ. For an integrated prediction of a SPARC mitigated disruptions, results combining the MGI model and the REMC model into a single NIMROD simulation are presented.