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

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 × 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 × 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 T_i and the E_r × B flow-shearing rate as the contribution of divertor recycling to the overall neutral fuelling rate increases.