Characterization of disruption mitigation via massive gas injection on MAST

Abstract

Disruptions are of significant concern to future devices, due to the large amount of energy released during the rapid quenching of the plasma. Disruption mitigation has been performed on MAST, to study the effect on heat loads and disruption time scales in a spherical tokamak. Massive gas injection is performed using a disruption mitigation valve capable of injecting between 0.6 and 1.5 × 1022 particles, corresponding to 10–150 times the plasma inventory. Noble gases are used for mitigation studies, specifically helium, argon, neon and a mixture of 10% argon and helium. The effect of mitigation is studied using a comparison between mitigated discharges and reference unmitigated disruptions.Mitigation has been seen to be effective at reducing peak divertor power loads, with reductions of up to 60% in the divertor power load being observed. The decrease in divertor power load is reflected by an increase in radiated power, mainly as a result of line radiation from the injected impurities. The largest reduction in the divertor power loads are seen for the impurities with Z > 4. The energy load to the divertor has been reduced by between 30–40% of the total stored plasma energy in ohmic, L mode and H mode discharges. Mitigation also affects the current quench times, accelerating the quench of the plasma with increasing injection quantity, with saturation observed at the highest injected quantities.