Core impurity reduction using divertor D$_2$ injection in lithium-conditioned H-mode discharges in NSTX

The application of lithium evaporative coatings in NSTX resulted in improved confinement and MHD stability. However, the routine achievement of ELM-free regimes caused core impurity accumulation with $Z_{eff}$ (due to carbon) increasing up to 3-4 and core $P_{rad}$ (due to metals) ramping up to several MW. Strategies for impurity reduction are thus essential for NSTX ELM-free discharges. In a dedicated experiment, small divertor D$_2$ injections (3-7 Torr-l) were used in high triangularity, NBI heated, H-mode discharges resulting in the reduction in core $n_C$ and $Z_{eff}$ by up to 30$\%$ without confinement degradation. The purpose of this analysis is to identify the contributions from underlying effects such as the reduction of divertor carbon and metal sputtering, changes in the SOL transport, improved impurity compression, or neoclassical edge convection due to increased neutral pressure. Supported by the U.S. DOE under Contracts DE-AC52-07NA27344, DE-AC02-09CH11466, DE-AC05-00OR22725, DE-FG02-08ER54989, and W-7405-ENG-36.