Divertor heat flux mitigation in high-performance H-mode discharges in the National Spherical Torus Experiment

Experiments conducted in high-performance 1.0 and 1.2 MA 6 MW NBI-heated H-mode discharges with a high magnetic 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 β_t = 15-25%, a high bootstrap current fraction f_BS = 45-50%, longer plasma pulses and an H-mode regime with smaller ELMs has been achieved in the strongly shaped lower single null configuration with elongation κ = 2.2-2.4 and triangularity δ = 0.7-0.8. Divertor peak heat fluxes were reduced from 6-12 to 0.5-2 MW m^-2 in ELMy H-mode discharges using the inherently high magnetic flux expansion f_m = 15-25 and the partial detachment of the outer strike point at several D₂ injection rates. A good core confinement and pedestal characteristics were maintained, while the core carbon concentration and the associated Z_eff were reduced. The partially detached divertor regime was characterized by an increase in divertor radiated power, a reduction in ion flux to the plate and a large neutral compression ratio. Spectroscopic measurements indicated the formation of a high-density, low-temperature region adjacent to the outer strike point, where substantial increases in the volume recombination rate and C II, C III emission rates were measured.