Abstract
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 beta(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 kappa = 2.2-2.4 and triangularity delta = 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 D-2 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, CIII emission rates were measured.