The influence of displacement damage on helium uptake and retention in tungsten

The influence of pre-existing displacement damage on the early stages of helium (He) interaction with tungsten (W) and the resulting defect creation was investigated experimentally. Samples were irradiated with 20.3 MeV W ions to different displacement-damage levels of 0.005, 0.01 and 0.1 displacements per atom (dpa). Displacement-damaged samples were exposed to a He plasma at room temperature at a He flux in the range of 10¹⁸ He/m²s to fluences of up to 10²² He/m² together with pristine W samples. He ion energy of 100 eV was used to remain well below the threshold for displacement damage creation in the bulk. Elastic recoil detection analysis (ERDA) shows that the He retention in the damaged samples is one order of magnitude larger than in the undamaged sample. Detailed He depth distributions were derived by stepwise removal of near-surface layers (via anodic oxidation and dissolution of the oxide) and subsequent ERDA measurements of the remaining He content. Pre-damaged samples show a significantly faster decrease in He concentration with depth than the undamaged sample, indicating that He is efficiently stopped from diffusing into deeper regions beyond 30 nm by pre-existing defects. The undamaged sample exhibits a lower He concentration in the near surface region and a flatter distribution of He up to a depth of 100 nm. From the clear influence of the initial displacement damage level on the He uptake and retention we conclude that He self-trapping mechanisms do not yet have a strong effect on the He diffusion depth in W at low fluxes used in this study.