Variable-spectrum mode control of high poloidal beta discharges

DIII-D experiments show that the changing structure of the least-stable plasma kink mode with q₉₅ can be tracked using a specially designed control strategy. In the new setup, DIII-D’s two rows of internal coils are configured as separate feedback loops with dedicated sensors that minimize the local n=1 perturbed magnetic field. The loops independently adjust amplitudes and toroidal phases of their control fields, bringing about changes in the poloidal spectrum of the total applied field. In the experiments, q₉₅ was varied between 11 and 6 while the pressure exceeded the ideal MHD no-wall stability limit by 40%. The relative phase-shift of the coil rows changed by approximately 50 degrees over this q₉₅ range, and approached the value typically used in traditional hard-wired coil setups at q₉₅=6. The dependence of the coil phase-difference on q₉₅ is qualitatively compatible with VALEN simulations of the coil couplings to the least-stable plasma mode. This new strategy provides a spectral optimization for kink mode control that works over large variations in plasma q.