Feedback control of resistive wall modes by saddle coils in RFX-mod

Active control of resistive wall modes (RWMs) is a key issue for high-beta operation of future tokamaks, including ITER. RFX has been recently modified to specifically address this issue by means of a new saddle coil system, which is the most powerful and flexible among all fusion devices, including 192 coils covering the whole torus. Unstable RWMs have been experimentally observed during the recent machine operation, showing poloidal and toroidal components m = 1, 2 < |n| < 7 and growth times of some tens of ms, consistent with theoretical predictions. RFX-mod addresses the control of RWMs by acting on the magnetic field configuration at the plasma boundary. The system aims not only at RWM stabilization, but also at interaction with tearing modes. By means of the system, successful experimental campaigns have been executed demonstrating the feasibility of RWM active control. The paper discusses on the system engineering and technological features and presents the results achieved.