High beta stability research on KSTAR at high non-inductive fraction*

KSTAR plasmas have exceeded the ideal n = 1 no-wall MHD stability limit and reached β_N > 4. Analysis of advanced operation regimes using TRANSP indicate that the non-inductive current fraction has reached 75 percent. Regions of weak safety factor q shear can form in different parts of the plasma dependent upon the broadness of the bootstrap current profile. Kinetic equilibrium reconstructions with MSE data to constrain the local magnetic field pitch angle are used as input to evaluate ideal and resistive stability using DCON. At present, high β_N operation is limited by tearing instabilities rather than resistive wall modes (RWM) that are computed to be stabilized by kinetic MHD effects. TRANSP code predictive capability is used to examine the effects of the second (off-axis) NBI system installed for the 2018 run determining plasma parameters important for stability. Values of the global energy confinement quality (H98y2) and the Greenwald density fraction are set to match past performance for reliable extrapolation. Predictive analyses are used to design experiments yielding solutions with β_N~4.5 and 100% non-inductive current drive. Active RWM feedback with both DC and AC field compensation has been implemented for mode control in 2018.