Real-time control systems for plasma rotation in RFX

Real-time feedback control is a key issue in the RFX experiment, as it can play a significant role in solving the problem of the wall locking of the RFX localised helical deformation (LHD) that severely limits the machine performance. Two techniques have been developed at RFX to overcome the problem, both based on the application of external magnetic fields. The first one aims at distributing the energy of plasma-wall interaction on a wide wall surface and operates by dragging the LHD in rotation along the toroidal and poloidal directions. The second technique aims at cancelling the radial magnetic field errors in the magnetic field structure, which contribute to the LHD wall locking significantly. After a brief description of the systems used in RFX to induce the plasma column rotation and to correct the field errors, the paper presents the functional structure of the digital real-time system to control the process and discusses its design. The implementation of the resulting, integrated, distributed real-time control system is finally presented. This is based on a network of floating point DSPs, equipped with built-in communication ports, particularly designed for parallel processing.