Hydrogenic retention in irradiated tungsten exposed to high-flux plasma

Two sets of identical tungsten (W) targets are irradiated at 300 K with 12.3 MeV W4+ ions to peak damage levels ranging from 0.5 to 10 displacements per atom (dpa). This results in a damage profile that is peaked at ~0.8 µm and extends to a depth of ~1.5 µm. Both sets of targets are exposed to high-density (ne,center = 3 × 1020 m−3), low-temperature (Te,center = 1.6 eV) deuterium (D) plasma in Pilot-PSI. One set of irradiated targets is exposed at high surface temperatures (TW = 950–680 K) and the other at low surface temperatures (TW = 480–340 K). The surface temperature is determined by the local plasma conditions. Nuclear reaction analysis (NRA) is used to determine the D depth profiles at specific radial locations, thus giving a surface temperature scan of the D retention in the damaged W. Global retention is determined by thermal desorption spectroscopy, which yields total D retained in the target and also gives information of the different types of lattice defects that are trapping the D in the W lattice. The main results are that there is no measurable difference between the different dpa levels, implying a saturation of the retention enhancement at a level ≤0.5 dpa. For both irradiated and unirradiated tungsten, a peak in the retention is seen at TW = 480 K; however, the W4+ irradiation clearly enhances the retention. This enhancement is also temperature dependent and increases with increasing surface temperature up to an enhancement by a factor of 15–23 at TW = 950 K. At the lowest surface temperatures, a fluence dependence appears since the implanted deuterium is diffusion limited to only a small fraction of the irradiated zone. TDS spectra show an enhancement of both low-energy trap sites and high-energy trap sites. For these conditions, diffusion-limited, low fill fraction trapping determines the hydrogenic retention of the W.