Photograph (through a fish-eye lens) of the interior of LTX during a vent, with labels for a number of the in-vessel components.
One of the LTX evaporator crucibles after initial use. Two evaporators, 180° apart toroidally, are installed. Each crucible is typically filled with 8 g of lithium. Typically, a total of 4 g of lithium is used per evaporation. The crucibles are ceramic yttria, and are not attacked by liquid lithium at temperatures up to 600 °C. The two wirelike projections into the white ceramic yttria crucible are thermocouples. The bandlike structure surrounding the crucible is a tantalum strip heater, with outer heat shields. The crucible is installed on a bellows-sealed linear motion feedthrough, which allows insertion of the evaporation crucible through a gate valve mounted on LTX, into the volume enclosed by the shells.
(a) Discharge current pre- (blue trace) and post- (green trace) 5 g of lithium wall coatings in LTX. The prefill was increased for the post-lithium discharge (see Fig. 3(b) ), but all other field programming was identical for the two discharges. (b) Time history of the neutral pressure before and after several discharges in LTX, with various wall conditions. The discharge start and end are denoted by the vertical red dotted lines. Note that the pressure gauge is connected to the main vessel by a duct, which significantly slows the time response of the system.
The fraction of the total number of injected hydrogen atoms which are pumped, per discharge, by the LTX wall, under various conditions. A negative fraction implies that the wall is a source of particles, rather than a sink.
Summary plot of fueling efficiency vs. fueling rate for the systems tested in LTX. The highest fueling efficiencies are obtained for the SGI and the MCI. The “top puffer” is a conventional wall-mounted piezoelectric gas valve. The “side puffer” is also a piezoelectric valve system, but it is more closely coupled to the vacuum chamber, and gas is ducted from the valve to the plasma edge through a short 2 cm diameter tube.
Photograph of the hot (300 °C) shells, immediately after coating with lithium. The photograph was taken through a glass viewport. The brownish coloration is indicative of a reacted (oxidized, hydroxided) lithium surface.
Residual gas analyzer trace taken during the first hot wall experiment. Hydrogen is the dominant background gas, but other components are present at the level of a few ×10−6 Torr.
Evolution of the plasma current with temperature. Discharges run during the shell heating cycle are in red; discharges run as the shell was cooling are indicated in blue. The heating experiment ran over a 3 day period. In the figure, a “.” denotes a discharge on the first day of the experiment, a “+” denotes a discharge on the second day of the heating cycle, a “◻” denotes a discharge on the third day of the heating cycle. Discharges during the shell cooldown all occurred on one day. Note that the temperatures are referenced to the outer surface of the shell, rather than the inner, plasma-facing surface. Discharges against walls which are heated above the melting point of lithium show a marked degradation compared to discharges run against walls just below the melting point. Two discharges are indicated in the plot—discharge “a” was run with the plasma-facing surface just below the melting point of lithium, and discharge “b” with the plasma-facing surface just above the melting point, when the temperature is corrected for the thermocouple location.
(a) EUV emission spectrum for a discharge with the wall just below the melting point of lithium (marked “a” in Figure 8 ), and (b) EUV emission spectrum for a discharge with the wall just above the melting point of lithium (marked “b” in Fig. 8 ). No emission lines are seen for a molten wall film.
Oxygen II emission, normalized to the plasma stored energy, for the two discharges discussed in connection with Figures 8–10 .
H-α emission, normalized to the plasma stored energy, for the two discharges discussed in connection with Figures 8–11 .
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