An Intelligent Shell for the Toroidal Pinch*

1. I N T R O D U C T I O N LARGE SCALE STABILITY in a reversed field pinch is generally achieved by surrounding the plasma with a close-fitting highly conducting shell. Macroscopic instabilities must drive flux through the shell and this sets up eddy currents which generate an opposing field distribution. On short time scales the flux through the shell remains fixed and growth of the unstable mode is inhibited. However, on longer time scales Ohmic dissipation damps the eddy currents and allows flux to diffuse through the shell. Theoretical models (HENDER et al., 1986 ; GIMBLETT, 1986 and references therein) have shown that instabilities are only suppressed on time scales which are short compared to the long time constant of the shell (i.e. the characteristic time constant for penetration of vertical field). Kecent experiments on HBTXlC (ALPER et al., 1989) and OHTE (TAYLOR et al., 1988) using thin shells with short time constants have confirmed these results and suggest that the plasma cannot be sustained for times much longer than some multiple of the shell time. Lowering the resistance of the shell increases the shell time constant so that, with a thick shell, resistive diffusion times of a few hundred milliseconds may be achieved. If the system is to be used as a reactor, however, much longer time scales, of the order of many seconds, will be required. To achieve this we propose a novel method of active flux freezing which creates, within certain limits, the effects of a perfectly conducting shell.