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

The paper focuses on the recent results achieved using at best the technology developed for the RFX-mod real time control system of MHD modes, which provides control features unique in the worldwide fusion experimental program. The paper aims at highlighting the successful technological aspects that have led to the outstanding scientific achievements obtained in the machine recent operation, including a dramatic increase in plasma duration and a significant decrease in magnetic perturbations. RFX-mod addresses the MHD mode control by acting on the magnetic field configuration at the boundary. The approach is not limited to the stabilization of Resistive Wall Modes (RWMs) in RFX-mod, but it also aims at acquiring an experimental knowledge on the MHD mode control that can be relevant for high beta 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. The coils are 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 to 24 and m=1, n=-23 to 24. The control system is based on a modular hardware and software infrastructure, capable to fulfill the tight system requirements in terms of input/output channels (>700/>250), real time data flow (>2 Mbyte/s), computation capability (>1 GFLOP/s), and real time constraints. The control system, which has been extensively and routinely used in the operation of RFX-mod since the commissioning, provides a very powerful environment for interaction with MHD modes and for their control. Two basic control strategies have been developed: the Selective Virtual Shell (SVS) and the Mode Control (MC). The former aims at canceling all modes (Virtual Shell concept) except for selected preset ones (Selectivity concept). The Selectivity concept allows operating without interfering with the vertical equilibrium field (m=1, n=0), allows studying the growth of selected unstable RWMs, and facilitates producing Quasi-Single Helicity plasmas. The MC scheme aims at interacting with single or multiple modes. In this strategy, the regulators act directly on the modes by means of complex gains that can account for mode rotation.