Secondary electron emission of tin and tin-lithium under low energy helium plasma exposure

Secondary electron emission (SEE) yields of tin (Sn) and tin-lithium (SnLi) eutectic (20 at.% Li) samples were measured in He-plasma at a mean incoming electron energy up to 120 eV. SnLi shows a maximum yield of about 1.45 at 110 eV electron energy while the yield of the Sn surface was measured to be maximally 1.05 at 120 eV. X-ray photoelectron spectroscopy (XPS) analysis demonstrated the segregation effect of Li to the surface of the eutectic, both after melting in the argon atmosphere and in molten state with simultaneous He-plasma exposure. At least the top 10 nm of the SnLi samples were heavily enriched with Li, and Sn/Li ratios varied in the range 0.8-5% depending on eutectic treatment conditions. After the plasma exposure Sn3d is detected predominantly in the oxidized state while after extended atmospheric oxidation there was still a significant amount of Sn3d detected in the metallic state. The liquid surface of SnLi indicated a possible decrease of SEE yield. All measurements gave values of SEE yield close to or above unity. Such values can lead to significant plasma sheath disturbances and subsequent additional heat flux from electrons on such a plasma-facing material, thus, should be accounted for in designing fusion reactors using these components.