Speaker
Description
The lithium vapor cave is a detached divertor concept that uses a single Private Flux Region (PFR) baffle to confine a dense lithium vapor cloud, reducing target heat flux while minimizing contamination of the main plasma. Plasma flows are driven by deuterium puffing to control lithium transport from the divertor to the main plasma. Previous SOLPS-ITER modeling demonstrated sufficient outer target heat flux reduction, reducing an unmitigated 90 MW/m$^2$ to less than 10 MW/m$^2$. This was achieved with acceptable lithium concentrations at the last closed flux surface (n$_{Li}$/n$_e$<0.05) across variations in divertor geometry, target recycling, upstream plasma conditions, lithium evaporation, and fueling locations.
Within the SOLPS-ITER code, both cross-field anomalous transport and recycling are set by user-defined, spatially varying coefficients. This work systematically examines the impact of varying both cross-field transport and recycling on lithium contamination predictions. The role of deuterium recycling is considered since deuterium recycling is known to decrease with lithium injection and can strongly influence poloidal divertor flows that determine contamination levels. Simulations indicate that decreasing the radial particle transport by a factor of four increases the prediction for upstream lithium density by a factor of 2.1, when adjusting the external gas puff to maintain a similar outer midplane separatrix electron density. Varying the recycling coefficient over the potentially accessible range from high to low recycling, results in a factor 2.4 variation in upstream lithium concentration, at fixed upstream density, with lower recycling corresponding to lower concentration. Across all recycling regimes and cross field transport assumptions, the lithium vapor cave was successful at target heat flux reduction with sufficient lithium evaporation. The solutions were also always found to improve with greater deuterium gas puffing which resulted in a more favorable combination of impurity forces (mostly friction and thermal forces) acting on the lithium in the SOL.
These results demonstrate that both cross-field transport and recycling assumptions introduce significant uncertainty in lithium concentration SOLPS-ITER predictions for the lithium vapor cave. Bounding these uncertainties provides guidance for divertor design optimization, helping ensure sufficient heat flux reduction while limiting lithium contamination.
This work is sponsored by DOE Contracts No. DE-AC02-09CH11466