Hydrogen trapping in tungsten: impact of helium irradiation and thermal cycling

The impact of helium (He) plasma exposure with He fluxes relevant for ITER and WEST on the near-surface microstructure of polycrystalline tungsten (W) is investigated by coupling transmission electron microscopy (TEM) analysis and thermal desorption spectrometry (TDS) measurements. The samples were exposed in the PSI-2 linear plasma device to 75 eV He ions up to the fluence of 3 × 10²³ He m⁻² with the surface temperature in the range 1053–1073 K. The obtained He bubbles–enriched W samples are subsequently probed with sequences of low flux and low fluence 250 eV deuterium (D) ion implantations and TDS measurements in an ultra-high-vacuum setup to study the effects of the near-surface morphology changes due to the helium irradiation on fundamental mechanisms of deuterium retention. The results obtained for two different near-surface layer He bubbles morphologies revealed that the effects of He irradiation on D retention in W strongly depend on its subsequent thermal cycling. For annealing below 900 K, deuterium retention is similar to the one measured in pristine W. In contrast, for annealing above 1150 K, deuterium retention in the He bubbles-enriched W is increased 3–8 fold as compared to non-damaged W. Additionally, the deuterium desorption peak shifts from 540 to 450 K. This increase of D trapping in the He bubbles-enriched W annealed above 1150 K is presumably associated with a modification of the near-surface microstructure concurrent with an outgassing of He. (topical issue article)