Results from a lithium vapor box test stand at PPPL and B2.5-Eunomia simulations for a proposed Magnum-PSI lithium vapor box experiment*

The lithium vapor box is a concept for a divertor designed to handle the extreme heat fluxes generated in future fusion reactors. Within a slot lined with capillary-porous materials, Li vapor induces plasma detachment by cooling the divertor plasma until it volumetrically recombines. Strong differential pumping via condensation localizes the Li vapor. Two linear-geometry experiments study elements of the physics involved. A test stand at PPPL studies the evaporation, flow, and condensation of Li vapor in three 5\,cm long, 5\,cm diameter cylindrical boxes without plasma, as would occur between shots in near-term experiments. By using a Direct Simulation Monte Carlo code we were able to reproduce the measured value of mass transferred during the experiments to within~$\pm$15\%, and demonstrate the expected differential pumping. A second, proposed for the linear plasma divertor simulator Magnum-PSI, studies the interaction of a 4e20\,m$^{-3}$, 1.5\,eV, 1\,cm radius plasma beam with a 16\,cm long Li vapor cloud. In simulations with B2.5-Eunomia, a 12\,Pa vapor cloud from a 625$\,^\circ$C liquid Li surface reduces the plasma pressure at the target by a factor of 15 and the heat flux there from 3.7\,MW\,m$^{-2}$ to 0.13\,MW\,m$^{-2}$; the power is dissipated within the vapor box.