CONTROL OF NON-AXISYMMETRIC MAGNETIC FIELDS FOR PLASMA ENHANCED PERFORMANCES: THE RFX CONTRIBUTION

Active control of non-axisymmetric magnetic fields has been developed to increase the plasma performances in Tokamaks and RFPs. In the past at RFX, the effect of non-axisymmetric fields, static and rotating, with m = 0, n > 0, was studied. Through a plasma non linear coupling, the internal tearing modes coupled with the external m = 0 fields are dragged into rotation. The results have encouraged to realize a dedicated system for the generation of non-axisymmetric magnetic field by using ad hoc saddle coil arrays. One hundred and Ninety-two uniformly distributed saddle coils, each fed through switching power supply have been installed for the generation of m = 0 and 1 magnetic fields in a range n = 1-24. Each coil will be operated in a frequency range from dc to 300 Hz and will produce a maximum field intensity of 50 mT dc at the plasma surface. The resulting configuration of coils, power supplies, measurements and real time control equipment gives nowadays the most versatile system for studies of non-axisymmetric field effect on plasma. The experiments will be focused to increase the present understanding in the control of Resistive Wall Modes (RWMs) and to analyse the resonant and non-resonant MHD mode interaction with external magnetic fields, common issues for present Tokamaks and RFPs.The paper reviews the state-of-art of MHD control performed with non-axisymmetric external magnetic fields on Tokamaks and RFPs, with reference to the RWMs and field errors. The RFX contribution to the MHD mode control by external fields is discussed.