Effects of finite feedback loop gain and bandwidth on stabilization of magnetohydrodynamic instabilities by an "intelligent shell"

The "intelligent shell" [C. M. Bishop, Plasma Phys. Contr. Fusion 31, 1179 (1989)] utilizes a feedback system intended to make a resistive wall appear perfectly conducting to a plasma. It can thus be used for stabilizing mode s of the plasma which are unstable when the plasma is surrounded by a resis tive wall, but stable if the wall were perfectly conducting. Several concep ts of magnetic confinement, such as reversed field pinches, spheromaks, and tokamaks may benefit from an intelligent shell. The paper addresses the qu estion of the dependency of the stabilizing effect on the gain and bandwidt h of the feedback circuits (assumed linear). A simple model for the phenome na involved is made and solved numerically for certain parameter values. A characteristic time of the model is a resistive time tau of the wall; the c alculations suggest that an upper cutoff frequency of similar to 50/tau and sufficient gain provides a stabilization similar to that of ideal circuits with infinite bandwidth and gain. Under laboratory circumstances with tau similar to 10(-3) s it is thus practical to obtain mass produced components which make the circuits as effective as ideal circuits. (C) 1999 American Institute of Physics. [S1070-664X(99)01207-0].