Energy balance, radiation and stability during rapid plasma termination via impurity pellet injections on DIII-D

Injections of impurity {open_quotes}killer{close_quotes} pellets on DIII-D have demonstrated partial mitigation of undesirable disruption phenomena; namely reducing the convected heat loss to the wall, and the halo current`s magnitude and toroidal asymmetry. However, the appearance of a runaway electron population and large magnetic fluctuations (B/B{sub T} {approx} 1%) is coincident with the measured rapid loss of the plasma`s thermal energy ({approx}1 MJ in 1 ms) due to impurity radiation. A numerical code is developed to simulate the impurity radiation and predict the rapid plasma cooling observed. The simulation predicts two mechanisms for the generation of runaway electrons: the {open_quotes}slideaway{close_quotes} of hot tail electrons due to rapid cooling or the transport of hot electrons into the thermally collapsed plasma due to instabilities. Pressure gradients caused by the rapid non-adiabatic cooling of the impurity are identified as the probable source of these instabilities which also lead to convective heat losses. Results of a modeling effort to optimize pellet content, impurity species and cooling time for the avoidance of instabilities and runaway electrons are shown