Temperature dependent study of helium retention in tungsten fuzz surfaces

A temperature dependent study was performed on the helium retention in tungsten fuzz layers, in an attempt to provide further insight into the fuzz formation process and its relationship to the amount of trapped helium. Fuzz layers with thicknesses of 1.0 or 1.7 mu m were initially formed with pure 4He plasma at 1100 K, then sequentially exposed to a mixed 3He-4He plasma at a range of sample temperatures (i.e., 316-1053 K). 3He retained in the fuzzy layer and in the bulk substrate after plasma exposure was measured by nuclear reaction analysis, and the total (3He+4He) retention due to the sequential exposure was inferred from the 3He plasma concentration. Helium retention in the fuzz layer was found to be approximately independent of sample temperature. Below 1053 K, helium retention in the bulk remained at 1-10x1019 m-2, similar to saturated values previously measured in smooth tungsten, suggesting sufficient helium reaching the bulk. At 1053 K, a more than an order of magnitude increase in helium retention in the bulk was observed. Its occurrence near the fuzz formation temperature threshold suggests that fuzz growth is correlated with the enhanced helium concentration in the bulk.