Variable-spectrum mode control of high poloidal beta discharges

DIII-D experiments show that broad current profile equilibria can be accessed at high poloidal beta β_p by optimizing the MHD mode control poloidal spectrum. β_p of 3% was attained coincident with a bootstrap current fraction f_BS=70% and normalized beta β_N above the ideal MHD n=1 no-wall stability limit. A novel, variable spectrum magnetic feedback scheme implemented using internal non-axisymmetric coils (I-coils) helped avoid beta collapses caused by marginally unstable resistive wall mode (RWM) activity. The lower and upper I-coil rows were configured in independent feedback loops, allowing the poloidal spectrum to vary and track plasma mode structure changes as the edge safety factor q₉₅ varied slowly from 11 to 6 during the discharges. The q₉₅ dependence of the observed lower/upper I-coil phase-shift is qualitatively compatible with ideal MHD simulations. This approach helps improve resilience to mode structure variations as plasma parameters change, facilitating an extension of the operating space to lower internal inductance that is compatible with high f_BS operation.