Development of flash lithium evaporators for NSTX-U wall conditioning

We present the design basis for a flash lithium evaporator (f-LITER) for
wall conditioning in the National Spherical Torus Experiment Upgrade
(NSTX-U). The evaporator is intended to deliver fresh lithium to plasma-
facing components (PFCs) in the upper-divertor and center-column regions
for double-null operation, while reducing lithium loss, impurity pickup,
and service complexity relative to the legacy downward-facing NSTX lithium
evaporators that target the lower divertor. The design follows from
evaporator development on the Lithium Tokamak Experiment (LTX) and LTX-β .
Early LTX coating methods produced broad lithium cov- erage, but required
hour-scale pumpout or shell-heating cycles that exposed fresh lithium to
residual gases. Low-thermal-mass Mark-I evaporators reduced the
evaporation cycle to minutes, but quartz crystal microbal- ance
(QCM)-constrained coverage modeling showed that geometric shadowing left
part of the high-field-side limiter undercoated. Mark-II and Mark-III
evaporators improved coverage, increased lithium inventory, and retained
molten lithium with a sintered felt-metal liner, but still required manual
solid-lithium loading. Run- day analysis on LTX-β showed that plasma
current, density, and particle pumping evolved over the first shots after
evaporation, consistent with lithium surface oxidation on comparable
timescales. These results motivate an evaporator that can refresh lithium
without venting and with reduced impurity exposure. A Mark-III evapo-
rator modified for in-vacuo loading using a liquid-lithium dropper was
tested on LTX-β and in the Deposition Analysis and Reliability Testbed
(DART). QCM measurements on LTX-β showed 100-nm-class evaporation on a
minutes timescale, and temperature-programmed desorption (TPD) showed
reduced impurity release af- ter liquid-lithium loading relative to solid
loading. The NSTX-U f-LITER design uses a tantalum mesh basket,
liquid-lithium dropper for in-vacuo loading, and a detachable evaporator
head for serviceable, remotely actuated lithium delivery.