Divertor Heat Flux Mitigation in High-Performance H-mode Plasmas in NSTX

Divertor heat flux mitigation scenarios based on the radiative divertor and high poloidal magnetic flux expansion divertor geometry are studied in highly-shaped 1.0 - 1.2 MA, 6 MW NBI-heated H-mode discharges in NSTX. Radiative divertor performance was optimized by varying the additional divertor D$_2$ injection rate and therefore, the divertor radiated power (due to intrinsic carbon radiation) and ion momentum sink. Significant steady-state divertor peak heat flux reduction, from 8-12 MW/m$^2$ to 2- 4 MW/m$^2$ was obtained in a partially detached divertor regime with minimal core confinement degradation. In a separate experiment, the dependence of high flux expansion divertor parameters, including heat and particle fluxes, recombination rate, neutral pressure, and radiated power, on flux/area expansion factors was systematically measured by varying the X-point height and outer strike point radius. Implications of the divertor geometry for scrape-off layer power and momentum balance will be discussed using estimates from analytic 1D transport and impurity radiation modeling. Supported by the U.S. DOE under Contracts DE-AC52- 07NA27344, DE-AC02-76CH03073, DE-AC05-00OR22725, and W-7405-ENG-36.