MHD activity in PBX-M

Magnetic activity leading to disruptions was found to be the β-limiting mechanism in low-q (2≤q≤4.5) discharges in the PBX tokamak. This activity was characterized by low frequency (few to 25 kHz) m=3/n=1 oscillations growing over the course of tens of μsec to two msec, and culminating in a plasma disruptions or a large sawtooth-like event followed several msec later by a disruption. Both theoretical analyses and experimental data indicated that the observed growing mode could be explained to zeroth order by destabilization of the ideal n=1 external kink mode. Theory further showed that a conducting wall in close proximity (a few cm) to the plasma could effectively stabilize these external kinks for higher-<βt> operation. (author) 5 refs., 4 figs