WALL CONDITIONING WITH LITHIUM EVAPORATION

Thin lithium layers are deposited on metal or graphite walls by evapor ation in vacuum. Clean lithium surfaces have wide chemical activities on such gases as O-2, CO and CH4. In particular, a strong gettering ef fect 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 depos ited on the wall, thus suggesting the formation of Li2O. On the other hand, H-2 gas hardly reacts with the lithium surface which, however, d isplays a large pumping effect in a hydrogen glow discharge. The maxim um number of H atoms pumped by the lithium layer below 180 degrees C i s approximately equal to the number of Li atoms in the vessel, probabl y due to the formation of LiH. Helium glow conditioning of the used li thium layer allows the partial recovery of lithium from losses to hydr ogen 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 similar to 1 m(2). The lithium coating leads t o 20-50% reduction in oxygen and carbon impurities with less hydrogen recycling in ohmic and NBI discharges.