Simulation of DIII-D Plasma Shutdown by Deuterium Dilution Cooling

To mitigate ITER disruptions and avoid large numbers of runaway electrons, a significant increase in the total (free + bound) electron inventory is likely required. The Rosenbluth criterion --- $E_c\approx 0.12\,n_{e,20}$ --- determines the critical electric field (in V/m) at which exponential runaway avalanching will occur. Here we consider instantaneous dilution cooling of a DIII-D plasma by the injection of 100 times the initial deuterium density to simulate rapid core penetration of a D$_2$ pellet train or liquid jet. The 3D NIMROD MHD simulation is initialized with an equilibrium pressure profile, but a 100$\times$ density increase and a corresponding 100$\times$ temperature reduction. The plasma is assumed to have in situ carbon fraction of 1\% of the pre-dilution density, which produces strong edge radiation at the dilution cooled temperatures. A cooling front propagates inward and ultimately triggers a central 1/1 MHD event. The central current density transiently increases by more than a factor of 2. The 3D simulation is compared with a 1D simulation which incorporates a Kadomtsev mixing model for the 1/1 MHD event.