L–H threshold studies in NSTX

Abstract

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.