Negative-lithium emission from a tungsten surface in a plasma

The authors report the observation of Li- from a surface in a He-Li plasma. The apparatus consists of an arc-discharge chamber with a small tungsten plate opposite a collimated beam line and momentum analyzer. The plate is biased negatively with respect to the plasma to accelerate positive ions from the plasma into, and negative ions away from, the surface. For this experiment, the arc was initiated with helium gas. The discharge heated a small crucible filled with lithium metal to introduce lithium vapor and form a composite plasma. The discharge was a bright rose color when the best Li- yield was obtained. A spectrum is shown with Li- peaks (composed of three parts due to instrumental constraints). The angular resolution (+/- 20) permitted observation of only that negative ion current emitted normal to the surface. The plasma sheath causes all very low-energy Li- ions to be accelerated in this direction, while backscattered, higher energy ions are emitted in a larger cone. 4 references, 2 figures