Speaker
Description
Ab-initio modeling of helium (He) behavior in tungsten (W) predicts clustering mechanisms, self-trapping and trap mutation, but to date there is little experimental evidence for isolated He self-trapping in a defect-free lattice. In order to investigate these mechanisms and the influence of intrinsic defects in W on them, the effects of He implantation into polycrystalline (poly-W) and single-crystalline (sc-W) W samples were studied in a set of exposures, where several parameters were systematically varied: He energies ranged between 50 and 200 eV, the flux was varied between $10^{17}$ and $10^{19}\,\, \mathrm{He/m^2s}$ and the fluence ranged from $10^{19}$ to $10^{23}\,\, \mathrm{He/m^2}$. Before exposure, the samples were polished carefully and probed for open-volume defects by positron annihilation spectroscopy, and then exposed to low-temperature He plasma at 292 K surface temperature. The He energies 100 eV and 50 eV were chosen for most He exposures including sc-W samples to avoid kinetic defect introduction. Elastic recoil detection analysis (ERDA) was used to measure absolute He areal densities after exposure. To derive high-resolution He concentration depth profiles, ERDA measurements were performed successively in combination with a stripping method using anodic oxidation.
The sc-W data show a flux-dependent threshold behavior: While there is no measurable He signal at lower fluxes, significant He retention is observed at a flux of $1\times10^{19}\,\, \mathrm{He/m^2s}$. This flux-dependence indicates that self-trapping dominates the He retention. Exposed with the same parameters, the poly-W retention data exhibit the threshold-behavior between the fluxes $1\times 10^{17}$ and $1\times10^{18}\,\, \mathrm{He/m^2s}$. A comparison of He retention after low-flux ($10^{17}\,\, \mathrm{He/m^2s}$) exposure of electron-beam irradiated sc-W with undamaged sc-W and poly-W eliminates pre-existing open-volume defects as the cause for the differences in He interaction between the undamaged materials. Impurities in the material emerge as the most plausible promoters for He clustering mechanisms and He trapping in poly-W. The significant influence of impurity defects in He clustering and trapping is not only supported by the differences in total He retention between poly-W and sc-W samples at medium-to-low fluxes but also by a difference in He trapping beyond the implantation depth found in He depth profiles of poly-W and sc-W.