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Description
The lithium vapor cave is a detached divertor design that aims to mitigate divertor heat flux via near-target lithium evaporation with a private flux region (PFR) baffle to contain dense lithium vapor [1]. The future implementation of a lithium vapor cave in NSTX-U is planned to be a staged process where the early iterations consist of a single, unbaffled tile insert comprising a 15$^\circ$ toroidal sector, designed to serve as a test bed for in-situ lithium evaporator technology, diagnostics, and to provide preliminary physics results. Such a system will produce a toroidally localized lithium vapor source in the divertor region. In preparation for these experiments, multiple elements of the evaporator technology are being developed [2], including the embedded capillary porous system for evaporation and liquid lithium reservoir level sensors. Furthermore, fluid edge modeling is being performed to predict the toroidal extent of divertor heat flux reduction in the single tile tests, to help determine the degree of toroidal coverage required.
The multi-fluid plasma edge code SOLEDGE3X is being used to examine the induced toroidal asymmetries. Preliminary 2D simulations of NSTX-U have been performed to benchmark SOLEDGE3X results with SOLPS-ITER simulations. A reference simulation without lithium injection has been developed where the unmitigated peak heat flux at the low field side (LFS) target is predicted to be ~32 MW/m$^2$ and at the high field side (HFS) target ~16 MW/m$^2$. A lithium source in the PFR evaporating at a rate of $10^{23}$ s$^{-1}$ reduces the peak heat flux to both the LFS and HFS target by about a factor of 2. These initial results indicate that early NSTX-U experiments will need to rely on gas puffing to retain lithium in the divertor, where there will not be baffling to restrict the particle flows. Comparisons with equivalent SOLPS-ITER modeling of the same NSTX-U discharge are ongoing work.
These simulations are being extended into 3D to examine the toroidal extent of the heat flux reduction, and the effect of deuterium gas puffing will be examined as was done in previous SOLPS-ITER simulations, to contain lithium in the lower divertor region [3]. Understanding these SOLEDGE3X simulation results for a toroidally localized lithium vapor source are crucial for understanding initial stages of lithium vapor divertor experiments, as well as informing subsequent installation stages.
[1] Emdee and Goldston NME 41 (2024)
[2] Parsons et al. JFE 44 (2025)
[3] Emdee and Goldston NME 34 (2023)