Dependence of the L-H transition on X-point geometry and divertor recycling on NSTX

Abstract

The edge electron (T-e) and ion temperature (T-i) at the time of the L-H transition increase when the X-point radius (R-X) is reduced to a high-triangularity shape while maintaining constant edge density. Consequently the L-H power threshold (P-LH) is larger for the high-triangularity shape. This supports the prediction that a single-particle loss hole, whose properties are strongly linked to R-X and T-i, influences the edge radial electric field (E-r) and E-r x B flow-shearing rate available for turbulence suppression. Simulations using XGC0, a full-f drift-kinetic neoclassical code, indicate that maintaining a constant E-r x B flow-shearing rate does require a larger heat flux and edge T-i as R-X decreases. NSTX also observes a decrease in P-LH when the divertor recycling is decreased using lithium coatings. However, the edge T-e and T-i at the L-H transition appear independent of the divertor recycling for a constant shape. XGC0 calculations demonstrate that more heat flux is needed to maintain the edge Ti and the E-r x B flow-shearing rate as the contribution of divertor recycling to the overall neutral fuelling rate increases.