Recent experiments in the low aspect ratio National Spherical Torus Experiment (NSTX) have been run in support of the high priority ITER and ITPA issue of access to the H-mode. Specifically, a series of experiments showed reduced power threshold values for deuterium versus helium plasmas, and for plasmas with lower current, lower triangularity and with lithium conditioning. Application of n = 3 fields at the plasma edge resulted in higher power thresholds. To within the constraints of temporal and spatial resolutions, no systematic difference in Te, ne, pe, Ti, v or their derivatives was found in discharges that transitioned into the H-mode versus those at slightly lower power that did not. Finally, H98y,2 ∼ 1 confinement quality could be achieved for powers just above the threshold power in ELM-free conditions.
The torque dependence of the H-mode power threshold in hydrogen, deuterium and helium plasmas in DIII-D