Interactions between the plasma and the stabilizing shell during disruptions in PBX-M

To stabilize vertical and external kink modes in the elongated, indented PBX-M plasma, a close-fitting stainless-steel-clad aluminum conducting shell surrounds over 85% of the plasma surface. During normal operation, the currents induced in the shell by the plasma are small, usually reflecting changes in plasma shape initiated by the shaping control system. However, voltage spikes of less than 2 kV were observed across gaps in the plates during both low-q and density limit disruptions. After the thermal quench, the plasma became unstable to the n = 0 vertical mode. The resulting rapid (20 to 5000 m/s, a resistive timescale) plasma motion induced large poloidal currents through segments of the shell to the vacuum vessel, despite the fact that each segment was electrically isolated except for a common single-point ground. Through impact loading, these large currents caused the failure of insulators and mechanical members in the shell support structure. These events have been numerically simulated with a resistive force-free halo plasma surrounding the main plasma, which provides a current path between the plates along open field lines. This model is consistent with measured plasma current decay times of 1-3 ms, characteristic of a resistive 10 eV plasma. Confirmation that the duration of the post-disruption plasma is about a resistive tearing time was provided from bolometers viewing both along the mid-plane and into the divertors. About 30-60% of the total energy and 30% of the magnetic energy was radiated during the thermal and current quench phases. This model led to improvements in the electrical configuration and insulator design which eliminated all failures. Disruptions have been prevented even when radiating over 100% of the input power during current profile control experiments with counter beam injection.