A plasma-material interaction design basis for NSTX-U flash lithium evaporators
Plasma-material interaction (PMI) at lithium conditioned plasma facing components (PFCs) depends not only on lithium inventory, but also on coating freshness, chemical state, and spatial coverage. We use LTX-$\beta$ measurements and lithium evaporator development to translate these constraints into a design basis for the NSTX-U flash lithium evaporator (f-LITER). During an LTX-$\beta$ run day with evaporations at the beginning and midpoint of the run day, discharges immediately after each evaporation exhibited higher plasma current, lower line averaged density, and shorter effective particle confinement times than later discharges with identical programmed fueling and coil currents, consistent with stronger particle pumping by the freshly conditioned wall. The subsequent relaxation is interpreted in the context of published in vacuo x-ray photoelectron spectroscopy observations of Li$_2$O growth and molecular dynamics calculations showing that deuterium reflection and retention depend on oxide thickness. To refresh this evolving interface while limiting contamination, a low thermal mass evaporator was coupled to an in vacuo liquid lithium dropper and tested on LTX-$\beta$ and in a dedicated test chamber. Quartz crystal microbalance measurements gave a 401 nm deposition in 17.4 min, equivalent to 100 nm in approximately 4.8 min of active evaporation. A qualitative temperature programmed desorption comparison showed weaker impurity desorption from a coating deposited by the evaporator loaded using the liquid dropper than from one deposited by the evaporator loaded with solid lithium. These results establish the PMI requirements for an NSTX-U f-LITER architecture based on the LTX-$\beta$ low thermal mass evaporators. The system is intended to make lithium delivery a reproducible PMI actuator for controlling recycling, impurity uptake, and PFC conditioning in double null NSTX-U operation.