DIII-D Studies of Massive Gas Injection for Disruption Mitigation,

Experiments with massive ($\approx 3\times 10^{22}$ particles) argon injection in the DIII- D tokamak have shown that neutral delivery rate is the crucial jet parameter. Nozzle aiming is not crucial, as the neutrals are stopped at the plasma edge. This was demonstrated over a range of plasma thermal energies from $W_{th}\approx 1.0$ MJ down to $W_{th}\approx 0.02$ MJ. Calculations suggest that magnetic field pressure is contributing to the observed neutral jet stopping. The subsequent core radiative thermal collapse is greatly accelerated by the onset of low-order ($m=1,2/n=1$) MHD modes; this was demonstrated by shutting down target plasmas with different \mbox {$q$-profiles} and observing a delay in the collapse onset as the low order ($q=1$ and 2) rational surfaces were buried deeper in the target plasma. Experiments using a new large valve with a 10x higher flow rate will also be presented.