Impact of lithium-coated walls on plasma performance in the TJ-II stellarator

Abstract

Lithiumization in the TJ-II heliac has been recently performed by vacuum evaporation of metallic Li. Compared to previous wall coating scenarios, an enhanced density control by external puffing and discharge reproducibility, leading to the drastic enlargement of the operational window, has been obtained for hydrogen and helium plasmas. Strong changes in recycling, impurity composition, edge radiation profiles and global energy confinement have been recorded. The long-lasting effect of the new coating extends to H-wall inventories beyond the expected saturation of the Li layer by particle implantation. Sputtering yields for Li much lower than bibliographic values were measured. These changes had also impact on the plasma profiles under NBI heating scenarios, with increased peaking of the electron density profiles and fuelling rates corresponding to the nominal beam current. Strong ELM-type activity was seen, associated to significant changes in edge fluctuation levels and the presence of steep gradients in edge parameters.