Wall conditioning with lithium evaporation

Abstract

Thin lithium layers are deposited on metal or graphite walls by evaporation in vacuum. Clean lithium surfaces have wide chemical activities on such gases as O 2, CO and CH 4. In particular, a strong gettering effect on oxygen is observed; the maximum number of O atoms gettered by the lithium layer is about one half the total number of Li atoms deposited on the wall, thus suggesting the formation of Li 2O. On the other hand, H 2 gas hardly reacts with the lithium surface which, however, displays a large pumping effect in a hydrogen glow discharge. The maximum number of H atoms pumped by the lithium layer below 180°C is approximately equal to the number of Li atoms in the vessel, probably due to the formation of LiH. Helium glow conditioning of the used lithium layer allows the partial recovery of lithium from losses to hydrogen pumping or oxygen gettering. This lithium evaporation method has been applied to JIPP T-IIU. 100–250 mg of lithium was deposited onto a limited vessel area of ∼ 1 m 2. The lithium coating leads to 20–50% reduction in oxygen and carbon impurities with less hydrogen recycling in ohmic and NBI discharges.