Lithium vapor trapping at a high-temperature lithium PFC divertor target

Liquid lithium has been proposed as a novel plasma-facing material for NSTX-U and next-step fusion devices but questions remain on the ultimate temperature limits of such a PFC during plasma bombardment. Lithium targets were exposed to high-flux plasma bombardment in the Magnum-PSI experimental device resulting in a temperature ramp from room-temperature to above 1200$^{\circ}$C. A stable lithium vapor cloud was found to form directly in front of the target and persist to temperature above 1000$^{\circ}$C. Consideration of mass and momentum balance in the pre-sheath region of an attached plasma indicates an increase in the magnitude of the pre-sheath potential drop with the inclusion of ionization sources as well as the inclusion of momentum loss terms. The low energy of lithium emission from a surface measured in previous experiments ($<1$eV) is conducive to trapping within this modest potential well. The scale length derived from the ionizing pre-sheath model of 3mm is consistent with the observed neutral lithium emission found in the experiment. The strong trapping inferred from the lifetime of the coating indicates previously calculated temperature limits for lithium PFCs need to be re-evaluated.