Investigation of Pedestal Bifurcations in Hybrid Plasmas on DIII-D

Experiments with double null divertor (DND) on DIII-D have achieved hybrid plasmas with bifurcated pedestals yielding moderate pedestal pressures around 7 kPa or high pressures up to 18 kPa [F. Turco APS 2020, C. Petty IAEA 2020]. The plasmas have a safety factor range q₉₅ = 5-6, with combined neutral beam and electron cyclotron heating powers up to 15 MW. Due to a pedestal bifurcation between a lower state and a higher state, correlated with a 2 MW heating power change, the confinement factors and normalized beta have a wide range H₉₈ = 1.1–1.8 and β_N = 2.5–4.1 [T. Petrie NF 57 (2017) 086004]. The cause of the confinement transition and pedestal bifurcation is investigated by analyzing pedestal stability and inspecting the role of ion diamagnetic stabilization, ExB shear contributions and ratio of separatrix to pedestal density. In some discharges, the higher pedestal state is found to be consistent with the entry region to a Super H-mode channel. The pedestal contribution to βN and E in the high confinement regime is about 10 %. Since the hybrid is a leading candidate for advanced tokamak operational regimes, strategies using plasma current ramps based on Super H-mode experience are developed to extend the hybrid parameter space and raise performance via pedestal optimization.