Active toroidal field ripple and MHD feedback control coils in FAST

The Fusion Advanced Study Torus (FAST) has been proposed as a high magnetic field, compact size tokamak machine providing a flexible integrated environment to analyze plasma-wall interaction aspects in DEMO relevant conditions, to cope with ITER relevant operational issues and to study in an integrated way burning plasma physics and steady-state operation features. FAST has a quite large natural toroidal field ripple (around 2%) due to its compactness and to the number of large ports foreseen for external heating, diagnostics and remote handling purposes: this ripple must be lowered at acceptable level to allow safe operations and a good confinement of the fast particles produced by the external heating systems. An Active Ripple Control System (ARCS) has been designed, based on a set of coils placed between the plasma chamber and the Toroidal Field (TF) coils. These ARCS coils will be fed up with adjustable currents, opposite in direction respect to the TF currents, enough to lower the ripple up to zero and beyond. This will provide an unique tool to study plasma operations with fast particles at different ripple values. The CAD model of FAST including the ARCS coils has been completed and preliminary EM and thermal analyses in operating conditions have been carried out. Recently the chance to safely work in FAST at very high plasma current (up to 10 MA) with a low safety factor at the plasma edge has been studied and a new plasma scenario (10 MA, 8.5 T) has been developed. Moreover, large (up to few MJ) ELM events are expected in FAST due to core MHD activity in H-mode scenarios with good transport quality and a plasma edge very similar to ITER. The needs to allow safe low safety factor operation and to mitigate possibly large ELMs effects have been satisfied by envisaging in the plasma chamber a set of Feedback Active Control System (FACS) saddle coils. These FACS coils are located around the ports between the first wall and the plasma chamber (around 25 cm far from the plasma edge) and are designed to be accessible for remote maintenance. They will be fed up by a feedback system to control MHD modes: a first engineering assessment of the current requirements shows that 80 kAturn in each coil with AC frequency up to few kHz is enough to produce the necessary field.