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A numerical investigation of the effects of impurity penetration depth on disruption mitigation by massive high-pressure gas jet

The injection of a high pressure jet of noble gas is one possible approach to mitigating potential disruption damage to tokamaks. The efficacy of this approach given only modest penetration of the gas-jet into the plasma is of particular concern in a high absolute pressure device, like Alcator C-Mod (Hutchinson I.H. et al 1994 Phys. Plasmas 1 1511) or ITER. To investigate the effectiveness of gas-jet penetration to various depths, a simple radiation model based on a corona equilibrium assumption is employed in resistive magnetohydrodynamic (MHD) simulations of an Alcator C-Mod plasma. Destabilization of an m = 2/n = 1 mode is key in each case to the onset of stochasticity producing rapid parallel thermal transport to the impurity cooled region just inside the equilibrium boundary (separatrix). Results indicate that a thermal quench is triggered even at very shallow penetration, but penetration of the impurities to the q = 2 surface affords a significant advantage for hastening the quench onset.