Reversed-Field Pinch Contributions to Resistive Wall Mode Physics and Control

Optimal feedback control of resistive-wall modes (RWM) is of common interest for toroidal fusion concepts that use conducting walls for stabilization of ideal MHD modes. From the RWM control point of view, the RFP situation is in many respects similar to the advanced tokamak situation in the presence of very low plasma rotation, where the most effective stabilizing mechanism is the feedback action of a set of active coils. Results from EXTRAP T2R (Sweden) and RFX-mod (Italy) RFP experiments have shown that full feedback control of multiple RWMs is possible and their deleterious effects can be completely suppressed. However it is now important to optimize the RWM control systems both for the RFP and tokamak configuration for future implementation. Important aspects of optimization are effective mode identification and tracking capability, avoidance of the harmful effects of sideband modes (aliasing) in the control spectrum, minimized power requirements and robust controller stability. The paper describes collaborative work carried out on the two RFP experiments. Controller models based on the mode harmonic control concept and on a state-space multipleinput multiple-output intelligent shell concept are studied. Progress in development of optimal control schemes are presented both through experimental studies and simulations.