Ballistic helium transport during helium-induced fuzz growth in tungsten

Helium transport in tungsten during fuzz growth was investigated to provide insight into the helium-induced nanostructuring process which adversely affects the near-surface properties of plasma-facing tungsten components. Fuzz layers with thicknesses of 1.0 or 1.7 µm 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 were measured by nuclear reaction analysis. An increase in ³He retention was observed at 1053 K in the bulk, but no comparable decrease was observed the fuzz layer, suggesting more efficient trapping of helium in the bulk at this temperature, and no diffusion of helium from the fuzz layer to the bulk. An analytical model of helium transport in vacuum within the fuzz layer was developed for comparison. Model results were found to compare well with experimental results of retention in the bulk when considering helium reflection from the fibers sides and when considering that only ~10^-3% of helium incident on bulk tungsten is retained. Using model results for helium implantation in the bulk, the helium fluence-fuzz thickness relation was derived, which matched well with experimental data. Hence it was determined that fuzz growth is governed by helium content in the bulk, which occurs by implantation and not diffusion.