The Effect of Radial Particle Diffusivity and Deuterium Recycling on Lithium Vapor Cave Performance in NSTX-U SOLPS-ITER Predictions

The lithium vapor cave, a detached divertor concept, is planned for NSTX-U to mitigate >90 MW/m² heat fluxes. Lithium vapor is produced by an external evaporator and confined by a baffled structure in the private flux region (PFR). The system aims to provide strong power exhaust while keeping lithium concentration low at the last closed flux surface. Prior SOLPS-ITER modeling showed PFR deuterium injection can screen lithium from the core while maintaining power handling, but uncertainties remained due to the assumption of cross-field diffusivities. New modeling quantifies this uncertainty. SOLPS results show that varying radial heat diffusivity to yield power widths of 1–3 mm changes the upstream lithium concentration needed to maintain <10 MW/m² heat flux from 1–3% at fixed gas puff rate. Similarly, varying radial particle diffusivity gives concentrations of 1–4%. High and low target deuterium recycling regimes are also analyzed (R^D=1.0 vs R^D=0.80). The Greenwald limit may restrict PFR deuterium injection at high recycling, resulting in upstream lithium concentrations 5-7%. At low recycling, a wider range of allowable deuterium injection rates improves the achievable upstream concentration of lithium to <3%. Low upstream lithium concentrations and effective power exhaust are found to be achievable across a range of conditions.