Divertor Power Exhaust with Impurity Powders in DIII-D

DIII-D experiments in upper-single-null ELMy H-mode plasmas demonstrate that injection of low-Z materials in particulate form can effectively enhance dissipation in a closed divertor configuration. A rapid reduction of the downstream electron temperature, ion flux and heat flux was measured in the small-angle slot divertor as a result of local injection of lithium, boron, and boron nitride powder, in 2-s intervals at constant rates of 1-50 mg/s. BN injection led to a substantial increase of near-target neutral pressure and a transition to strike-point detachment with only a 2-15\% degradation in global energy confinement. 3D modeling with EMC3-EIRENE supported by spectral divertor imaging measurements is used to analyze the radiative losses within the divertor plasma. While Li radiation is concentrated near the separatrix, N radiative losses occur in the main scrape-off layer, and the B peak emission front is located in the far SOL. B and N are about an order of magnitude more efficient radiators than Li.