Feedback control model of the m = 2, n = 1 resistive wall mode in a circular plasma

Feedback stabilization of the m = 2, n = 1 resistive wall mode in q(a) < 2 plasmas is obtained in the RFX-mod device operated as a circular tokamak. The feedback scheme implements radial sensors within the clean mode control (CMC) algorithm, a technique based on the de-aliasing of the measurements from the high periodicity sidebands produced by the coils. A cylindrical zero-pressure MHD model, which includes a realistic discrete-time description of the feedback, is able to explain the main features of the experimental observation, including the necessity of the removal of sidebands when using radial sensors. The model is also used to show that the radial field CMC presents some advantages in comparison with poloidal field based control schemes, allowing more freedom in the arrangements of the sensors.