Comparison of strategies and regulator design for active control of MHD modes

A system of evenly spaced poloidal arrays of saddle coils was recently installed on the reversed field pinch device EXTRAP T2R to perform experiments on the active control of MHD modes. The implementation of different control strategies, such as “intelligent shell” and “mode control”, was made possible by a flexible digital control system. After giving some results on the performances of the innermost coil current control loop, two versions of “mode control” recently tested on the machine are presented. In the “wise shell” approach, equilibrium related modes are ruled out and a systematic increase of the pulse length is obtained. In a second, more model based, approach, a mode estimator/controller is designed aiming at a full state feedback by including modes, which are not directly measurable due to the limited number of available real-time signals.