Deuterium retention in tungsten and tungsten–tantalum alloys exposed to high-flux deuterium plasmas

A direct comparison of deuterium retention in samples of tungsten and two grades of tungsten–tantalum alloys—W–1% Ta and W–5% Ta, exposed to deuterium plasmas (ion flux ∼1024 m−2 s−1, ion energy at the biased target ∼50 eV) at the plasma generator Pilot-PSI was performed using thermal desorption spectroscopy (TDS). No systematic difference in terms of total retention in tungsten and tungsten–tantalum was identified. The measured retention value for each grade did not deviate by more than 24% from the value averaged over the three grades exposed to the same conditions. No additional desorption peaks appeared in the TDS spectra of the W–Ta samples as compared with the W target, indicating that no additional kinds of traps are introduced by the alloying of W with Ta. In the course of the experiment the same samples were exposed to the same plasma conditions several times, and it is demonstrated that samples with the history of prior exposures yield an increase in deuterium retention of up to 130% under the investigated conditions compared with the samples that were not exposed before. We consider this as evidence that exposure of the considered materials to ions with energy below the displacement threshold generates additional traps for deuterium. The positions of the release peaks caused by these traps are similar for W and W–Ta, which indicates that the corresponding traps are of the same kind.