Designing the Lithium Vapor Cave for Near-Term Deployment on NSTX-U Using SOLPS-ITER*

The predicted divertor heat flux of reactor-scale fusion devices necessitates detached operation to ensure the safety of plasma facing components. One detachment design, the lithium vapor box divertor, plans to achieve detachment via lithium evaporation near the strike point. Recently, it has been discovered via predictive SOLPS modeling that evaporation from the private flux region (PFR) would be significantly more effective than from the common flux region in dissipating the plasma peak heat flux, allowing the far SOL baffles to be removed to form a lithium vapor “cave”. Here, SOLPS studies are shown which demonstrate there is PFC design flexibility with the lithium vapor cave, especially in regards to the cave aperture height and depth. Though, the PFR structure of the cave is seen to have a significant effect on the neutral leakage from the divertor. Altering the PFR structure shape and deuterium gas puff location results in a range of predicted concentration of lithium ions at the last closed flux surface, between 0.020-0.049, required to reduce the lower outer divertor heat flux to below 10 MW/m². Drifts are also turned on in these SOLPS simulations which demonstrate that full drift predictions have increased outer divertor heat flux, and a reduced inner divertor heat flux. This indicates that the amount of lithium may need to be increased or the location of the gas puff adjusted to detach both divertors.