Saturation of deuterium retention in self-damaged tungsten exposed to high-flux plasmas

Polycrystalline, annealed tungsten targets were bombarded with 12.3 MeV W ⁴⁺ ions to various damage levels. Deuterium was implanted by high-flux plasmas in Pilot-PSI (>10 ²⁴ m ^-2 s ^-1) at a surface temperature below 525 K. Deuterium retention has been studied by nuclear reaction analysis and by thermal desorption spectroscopy. We found that deuterium retention is strongly enhanced by the tungsten bombardment and that saturation occurs at a W ⁴⁺ fluence of about 3 × 10 ¹⁷ m ^-2. The maximum deuterium concentration in the damaged region was measured to be 1.4 at.%. This is in accordance with other experiments that were carried out at much lower fluxes. We therefore conclude that the saturation behaviour and the maximum retention are not affected by the high fluxes used in our experiments. A simple geometric model is presented that assumes that the saturation solely originates in the tungsten irradiation and that explains it in terms of overlapping saturated volumes. The saturated volume per incident MeV ion amounts to 3 × 10 ⁴ nm ³. From our results, we are able to obtain an approximate value for the average occupation number of the vacancies.