The Lithium Vapor Cave: Simplifying the Lithium Vapor Box With SOLPS*

Unmitigated heat flux to the divertor of advanced reactors has been predicted to surpass the capabilities of any solid surface. The lithium vapor box functions by evaporating lithium near the target while the plasma flows and divertor geometry work to contain the lithium below the X-point. The lithium vapor box is predicted to succeed in reducing target heat fluxes to below 10 MW/m² while limiting (n_Li/n_e)^LCFS to 0.05 in near-term experiments such as NSTX-U, even in cases with unmitigated target heat flux of 90 MW/m² [1]. Evaporation from the Private Flux Region (PFR) side is found to efficiently ionize lithium at the separatrix, where the largest power dissipation is required. Comparatively, evaporation from the Common Flux Region (CFR) side is found to be ineffective at target heat flux dissipation. Self-consistent evaporation from the target, via a capillary porous system with fast flowing liquid metal [2], is seen to successfully maintain a safe target operating temperature. By removing the CFR side baffle of the vapor box, but keeping the PFR side baffle, the configuration resembles Plato’s cave (complete with fire outside) instead of a box. Thus, effective lithium evaporation is not sacrificed while achieving reduced design complexity. The "vapor cave" is found to have comparable performance to a lithium vapor box making it a viable design simplification.

[1] E.D. Emdee and R.J. Goldston Nuclear Fusion (2023) DOI 10.1088/1741-4326/ace6be

[2] A. Khodak and R. Maingi, Nucl. Mater. Energy 26 (2021) 100935