Effect of gas fuelling location on H-mode access in NSTX

The dependence of H-mode access on the poloidal location of the gas injection source has been investigated in NSTX. We find that gas fuelling from the centre stack midplane area produces the most reproducible H-mode access, with generally the lowest L–H threshold power in the lower single-null configuration. The edge toroidal rotation velocity of C2+ is largest (in the direction of the plasma current) just before the L–H transition with centre stack midplane fuelling and then reverses direction after the L–H transition. Simulation of these results with a guiding centre Monte Carlo neoclassical transport code (XGC) is qualitatively consistent with the trends in the measured velocities. Double-null discharges exhibit H-mode access with gas fuelling from either the centre stack midplane or centre stack top locations, indicating a reduced sensitivity of H-mode access to fuelling location in that shape.