Divertor Heat Flux Mitigation in High-Performance H-mode Plasmas in the National Spherical Torus Experiment

Experiments conducted in high-performance 1.0-1.2 MA 6 MW NBI-heated H-mode plasmas with a high flux expansion radiative divertor in NSTX demonstrate that significant divertor peak heat flux reduction and access to detachment may be facilitated naturally in a highly-shaped spherical torus (ST) configuration. Improved plasma performance with high βp = 15− 25 %, a high bootstrap current fraction fBS = 45− 50 %, longer plasma pulses, and an H-mode regime with smaller ELMs has been achieved in the lower single null configuration with higher-end elongation 2.2-2.4 and triangularity 0.6-0.8. Divertor peak heat fluxes were reduced from 6-12 MW/m2 to 0.5-2 MW/m2 in ELMy H-mode discharges using high magnetic flux expansion and partial detachment of the outer strike point at several D2 injection rates, while good core confinement and pedestal characteristics were maintained. The partially detached divertor regime was characterized by a 30-60 % increase in divertor plasma radiation, a peak heat flux reduction by up to 70 %, measured in a 10 cm radial zone, a five-fold increase in divertor neutral pressure, and a significant volume recombination rate increase.