Divertor heat flux reduction and detachment experiments in NSTX

We report the first successful experiments at achieving significant outer divertor peak heat flux reduction, from 4-7 MW/m² to 1-2 MW/m², in 4 MW NBI-heated H-mode plasmas in NSTX. Steady-state divertor deuterium puffing at Γ ≤ 1.1 × 10²² s^-1 resulted in a high-recycling radiative divertor regime with a 50-80% reduction of the outer strike point (OSP) peak heat flux, without a loss of H-mode confinement or degradation of core plasma parameters. At higher D₂ injection rate, Γ = 1.12-2.80 × 10²² s^-1 up to 80% reduction in the peak heat flux and spectroscopic signatures of volume recombination have been observed, suggesting the OSP partial detachment. Radiative mantle plasmas were obtained by neon injection. The OSP peak heat flux decreased by 50-75% as the scrape-off layer power was reduced through the main plasma radiation which was about 30% of the input power. The results suggest that while an H-mode compatible radiative divertor can be obtained in NSTX, the realization of a partially detached divertor regime may be hindered by divertor properties specific to a spherical torus and NSTX.