Strongly temperature-dependent, anomalous secondary electron emission of liquid lithium surfaces exposed to a plasma

Abstract

• First time reported SEE of a CPS wetted with liquid lithium exposed to a plasma. • Formation and dissolution of a strongly temperature dependent surface oxide film. • SEE values from ~0.5 for clean surfaces to ~2 for heavily oxidized samples. • Exposure to 3 × 10 −7 Torr during one hour increases the SEE to values of about 2. • Annealing at increasing temperatures decreases the SEE to lower values. Electron-induced secondary electron emission (SEE) on a liquid lithium surface confined in a capillary porous system (CPS) and exposed to a plasma is reported for the first time. The liquid lithium surface is exposed to the bombardment of a suprathermal electron flux with energies up 150 eV created in a low pressure He Dc-Glow discharge. Various methods of surface oxidation have been used to reproduce realistic conditions in fusion plasma experiments. Exposure to very low residual gas pressures (around 3 × 10 −7 Torr) is enough to increase the maximum of the SEE to values of about 2. The formation and dissolution of a surface oxide film is strongly temperature dependent and plays an important role in the resulting SEE yield and its time evolution: values have been found to range from ~0.8 for clean surfaces to ~2 for samples that have been heavily oxidized. In the case of O 2 molecule exposure, a clear difference with temperature was observed. The molecular exposure at 330 °C had a much stronger effect on the increase in SEE than the molecular exposure at 220 °C. These results have a direct impact on the development of lithium-based divertor targets in fusion as well as in the understanding of the SEE characteristics of contaminated liquid surfaces, which has never been reported before.