MHD analysis of high (β_t) disruptions in PBX

Princeton Beta Experiment (PBX) discharges run at the lowest q and highest (β_t) always terminated in a hard disruption. The discharges, with (β_t) values of up to 5.5% and q-values down to 2.2, were obtained by employing large current ramps and large gas feed rates during neutral beam injection. Previous work has indicated that the achieved (β_t) values were consistent with the limit imposed by the n = l ideal external kink with a conducting wall at b/a = 2. The authors of the paper investigate further the validity of ideal MHD theory in explaining the low q disruptions. In particular, the characteristics of the pre-disruption MHD activity in these low-q discharges, specifically the time-scale of growth and internal and external mode structures, are compared with those determined from theoretical calculations. The results of these comparisons indicate that non-ideal effects must be considered in order to obtain detailed agreement between theory and experiment.