RFX: new tools for real-time MHD control

The RFX device (Padova, Italy) has evolved in time from the passive control of the magnetic field configuration to the active control of both the field errors and the m=0 field harmonics. Encouraging results were obtained in terms of reduction of localized plasma-wall interaction and of induced rotation of MHD modes: on this basis the RFX load assembly has been recently modified, reducing by a factor of ten the time constant of the stabilizing shell and covering the whole plasma surface by a system of 192 saddle coils, each of them independently fed by a fast amplifier. Moreover, the new toroidal field power supply allows to produce robust m=0 rotating field harmonics, to drag the locked modes by non-linear coupling among different modes. These features realize the most powerful control system of plasma dynamics in any fusion device, with the best space resolution and the fastes response time. Specific real-time control technologies and software tools have been developed with the aim of offering a variety of control scenarios: driven rotation of MHD modes, controlled formation of Quasi-Single Helicity states, feedback stabilization of resistive wall modes, combinations of the above schemes. The work is supported by theoretical simulations of plasma response to the various control actions performed by the digital regulators and the fast amplifiers. The scientific programme to be pursued by means of these real-time control tools is considered to be highly relevant not only for Reverse Field Pinches, but also for Tokamaks, in particular when operating under advanced confinement scenarios.