Role of the Stabilizing shell in High-b/low-q disruptions in PBX-M
Author: M. Okabayashi, N. Pomphrey, J. Manickam, et al
The characteristics of high-b/low-q disruptions have been studied in PBX-M, a device with a nearby conducting shell. The coupling between the wall and the plasma was varied by choosing different plasma shapes, including nearly Circular plasmas, D-shaped plasmas and bean-shaped plasmas (indented on the midplane), and by increasing the effective coverage of the plasma by the shell. Disruption precursors were observed to have a strong dependence on the coupling between the plasma and the shell. Measured mode growth times vary from between several times the Alfvén time-scale (~100 ms) to the L/R times-scale of the wall (~20 ms). The behavior of observed disruption precursors is interpreted tin terms of the resistive wall mode theory of ideal plasmas, and a detailed calculation of the stability of a strongly coupled Bean configuration using the NOVA-W linear stability code is presented. The experimental observations are in good agreement with the theoretical predictions.