Comparing lithium vapor box designs in a high heat flux scenario using SOLPS-ITER

Abstract

SOLPS calculations of lithium vapor box divertor designs on NSTX-U are presented. Predictive high power simulations (Pheat=10 MW, qpeakunmitigated∼65 MW/m2) are used to compare and contrast two divertor designs. Specifically a baffled “box” divertor, where a region of neutral density is allowed to build up, is compared to a more typical slot divertor geometry. It is found that significant differences in lithium containment lead to profoundly different viability of the two designs. These differences are seen to be due to far SOL flow patterns that change based on the presence of baffling as well differences in efficiency of the lithium evaporator. Outer-midplane (OMP) separatrix lithium content is found to be strongly detrimental to upstream temperature when nLi/ne>0.1 is reached. This regime of high upstream contamination is avoided via baffling. The reduction in upstream lithium allows access to low heat flux solutions below 5 MW/m2 with very little reduction to upstream temperature from the unmitigated, 65 MW/m2 solution. The slot is able to reach sub-10 MW/m2 heat fluxes though raising the evaporation rate much further reduces the upstream temperature, such that the range of stable evaporation rates with low heat flux to the target is small. Higher performance solutions (low heat flux and low upstream lithium content) are accessible by controlling recycling coefficients of deuterium on the walls above the box.