Design of a new controller of MHD modes in RFX-mod

The RFX-mod machine is equipped with a powerful system for the active control of MHD modes. It consists of 192 active saddle coils and corresponding amplifiers, 192 × 3 magnetic field sensors and a number of passive structures. A control system designed following SISO techniques allowed to achieve a good quality of the static spatial spectrum, but its dynamic behavior was not completely satisfactory in meeting the requirements of tearing mode control. Thus, a new MIMO design procedure was started on the basis of an available and already validated MIMO full electromagnetic model, whose inputs and outputs are the coil currents and the radial field fluxes measured by the sensors, respectively. A dynamic compensator derived from the pseudo-inverse of the system transfer function matrix was designed and a decentralized controller was then implemented. Simulations of mode tracking and mode rejection at different frequencies were run to compare the performances of a SISO-based controller and the newly designed one. Satisfactory results were obtained showing the possibility of designing a controller able to equalise the spatial spectral dynamic response. These results are presented and commented in the paper.