Effects of finite feedback loop gain and bandwidth on stabilization of magnetohydrodynamic instabilities by an open-quotes intelligent shellclose quotes

The open-quotes intelligent shellclose quotes [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 modes of the plasma which are unstable when the plasma is surrounded by a resistive wall, but stable if the wall were perfectly conducting. Several concepts of magnetic confinement, such as reversed field pinches, spheromaks, and tokamaks may benefit from an intelligent shell. The paper addresses the question of the dependency of the stabilizing effect on the gain and bandwidth of the feedback circuits (assumed linear). A simple model for the phenomena involved is made and solved numerically for certain parameter values. A characteristic time of the model is a resistive time τ of the wall; the calculations suggest that an upper cutoff frequency of ∼50/τ and sufficient gain provides a stabilization similar to that of ideal circuits with infinite bandwidth and gain. Under laboratory circumstances with τ∼10-3 ampersand hthinsp;s it is thus practical to obtain mass produced components which make the circuits as effective as ideal circuits. copyright 1999 American Institute of Physics