ROLE OF THE STABILIZING SHELL IN HIGH-BETA, LOW-Q DISRUPTIONS IN PBX-M

The characteristics of high-beta, low-q disruptions have been studied in PBX-M, a device with a nearby conducting shell. The coupling betwee n the wall and the plasma was varied by choosing different plasma shap es, including nearly circular plasmas, D-shaped plasmas and bean-shape d plasmas (indented on the midplane), and by increasing the effective coverage of the plasma by the shell. Disruption precursors were observ ed to have a strong dependence on the coupling between the plasma and the shell. Measured mode growth times vary from between several times the Aifven time-scale (similar to 100 mu s) to the L/R time-scale of t he wail (similar to 20 ms). The behaviour of observed disruption precu rsors is interpreted in terms of the resistive wall mode theory of ide al plasmas, and a detailed calculation of the stability of a strongly coupled bean configuration using the NOVA-W linear stability code is p resented. The experimental observations are in good agreement with the theoretical predictions.