MHD modes control in fusion devices

The control of the 3D deformations of the plasma column due to MHD modes is becoming a crucial issue in modern fusion experiments since it is a prerequisite for operation at high beta and is essential for the advanced configurations. Aim of the paper is to present some issues related to the design of the mode control system which are of general interest for experimental reactors. The work presented mainly originates from the studies performed on the RFX-mod machine which is equipped with the most flexible and complete saddle coils system available, at present, for experimenting the control of MHD plasma modes. The paper focuses in particular on the multivariable linear model developed for the design of the controller and on the different strategies envisaged for the stabilization of the modes. Some results obtained in RFX-mod with the designed algorithms are also proposed along with some issues specific to configurations with a limited number of saddle coils available as actuators