Robust feedback systems for resistive wall modes

The feedback stabilization of magnetohydrodynamic instabilities that grow on the resistive time scale of the chamber walls is an important issue for steady-state tokamaks. Constraints are derived on the inductance coefficients of the feedback system that will ensure that fixed amplification coefficients can be used to provide stability over the entire region in which feedback is in principle possible. Such feedback systems are called robust. Successful feedback is possible for systems that do not satisfy these constraints, but then the signs of the feedback coefficients will depend on the level of plasma instability. (C) 2004 American Institute of Physics.