Studies of Li sputtering and particle recycling in glow discharges and in TJ-II

Lithium sputtering is studied at TJ-II with lithiated, boron coated walls. As reported previously, the sputtering yield measured in H plasmas was found to be significantly lower than expected, based on laboratory experiments and TRIM code calculations. The edge temperature dependence of the yield could be consistent with either an energy threshold far exceeding the corresponding pure lithium threshold, or with a strong perturbation of the impinging ion energies. The material mixing effect, one of the candidates for explaining the observed behavior, was studied by depositing boron on the Li layer. Also, particle retention and release was investigated in H/He plasmas in this very low recycling scenario. The release of either species in the opposite plasma was studied in different plasma conditions as well as in glow discharge (GD) plasmas. In He GD plasmas, the I/V characteristics of lithiated stainless steel electrodes were found to be anomalous, in spite of the fact that the dependence of sputtering on the incident particle energy agreed reasonably with expectations. The possible implications of these phenomena for the interaction of reactor plasmas with lithium elements are addressed.

► The sputtering and gas desorption characteristics of Li/H and Li/He systems have been investigated in TJ-II and laboratory experiments at Ciemat.
► Anomalously low values of the Li sputtering yield were found, indicating a much higher threshold than that deduced form solid lithium properties.
► Very similar energy dependence was found for the release of H and Li sputtering by He ions in TJ-II plasmas.
► Laboratory experiments on coated SS probes support the possibility of material mixing effects as the main factor affecting threshold energies for the sputtering processes.
► An anomalous secondary electron emission of Li surfaces exposed to He plasmas has been recorded, leading to a significant distortion of the I/V probe characteristics.