Simulations of a DIII-D plasma disruption with the NIMROD code

To investigate the dynamics of the disruption of DIII-D discharge number 87009, two types of initial-value simulations with the NIMROD code were performed to investigate different characteristics. In the first set of simulations, a conducting wall was placed on the last close flux surface, and an equilibrium that was close to the ideal-MHD marginal stability point was created. A heating source was added to the pressure equation to drive the plasma equilibrium through an ideal-MHD instability point. Excellent agreement with analytic theory was obtained. To investigate how the stored energy is deposited on the wall, free-boundary simulations were performed with an ideal-MHD unstable equilibria. The unstable modes grow until the magnetic islands overlap and the magnetic field is stochastic over a large part of the plasma domain. The rapid stochastization of the field allows the plasma to lose two thirds of its internal energy in approximately 200 microseconds in qualitative agreement with the experiment. The deposition of thermal energy on the wall is localized poloidally and toroidally on the wall due to helically-localized temperature gradients and the rapid parallel heat conduction which carries this heat flux to the wall. (author)