Recent Results from Real Time Active Control of MHD Modes in RFX-mod

The paper focuses on the recent results achieved using the technology developed for the RFX-mod real time control system of MHD modes, which provides features unique in the worldwide fusion experimental program. The paper highlights the successful technological aspects that have led to the outstanding scientific achievements obtained in the recent machine operation, including a dramatic increase in plasma duration and performance and a significant decrease in magnetic perturbations. RFX-mod addresses the MHD mode control by acting on the radial magnetic field configuration at the boundary. The approach is not limited to the stabilization of Resistive Wall Modes in RFX-mod, but it also aims at acquiring experimental knowledge on MHD mode control that can be relevant for high β discharges in tokamaks. To the purpose, an ad-hoc field correction coil system has been installed, consisting of 192 saddle coils covering the whole torus surface arranged in 48 poloidal arrays, each made up of 4 equispaced coils. Each coil is fed independently. By means of this actuating system, magnetic field poloidal and toroidal harmonic components (m, n) can be generated with m=0 n=0÷24 and m = 1 n=-23÷24. Two basic control strategies have been developed: the Selective Virtual Shell and the Mode Control. The former cancels all modes (Virtual Shell concept) except for selected preset ones (Selectivity concept). The Selectivity concept allows operating without interfering with the axi-symmetric equilibrium field (m=1, n=0) and studying the growth of selected unstable RW modes. The Mode Control interacts with single or multiple modes, taking into account the mode penetration into the passive structures.