Hydrogen isotope retention in plasma-facing materials: Review of recent experimental results

Recent data on deuterium retention in carbon fibre composites and tungsten both irradiated with D ions and exposed to D plasmas are presented. Deuterium depth profiles measured up to depths of 7–14 μm allow understanding of the mechanism which is responsible for the hydrogen isotope trapping in these materials. In the CFC materials the amount of retained deuterium increases with the ion fluence at all irradiation temperatures in the range from 323 to 723 K. No saturation is reached as observed in pyrolytic graphite. Depth profiles show that saturation occurs only within a near surface layer corresponding to the ion range. The increase in total retention at near-room temperature is accompanied by an increasing of the long profile tail extending beyond 14 μm with the D concentration of about 10−1 at.% at a depth of 10 μm for fluences above 1024 D m−2. The depth at which deuterium is retained in tungsten (W) can be divided into three zones: (i) the near-surface layer (up to a depth of 0.2–0.5 μm depending on ion energy), (ii) the sub-surface layer (from ~0.5 to ~2 μm), and (iii) the bulk (>5 μm). Low-energy D ion irradiation modifies the W structure to depths of up to about 5 μm, both for W single crystals and polycrystalline W. The high D concentration (0.1–0.3 at.%) at depths of 1–3 μm relates to accumulation of D2 molecules in vacancy clusters and voids. These defects are supposed to be generated due to plastic deformation of the W surface caused by deuterium supersaturation within the near-surface layer.