Deuterium transport and retention in the bulk of tungsten containing helium: the effect of helium concentration and microstructure

The effect of helium (He) on deuterium (D) retention and transport in the bulk of tungsten (W) was investigated. For this purpose samples were irradiated by 500 keV He ions at 300 K to different fluences in order to obtain He maximum concentrations of 1 at.%, 3.4 at.%, and 6.8 at.% in 0.84 mu m depth. In order to discern the effect of irradiation damage caused by He implantation from the effect of the pure He presence on D retention, the W samples were irradiated at 300 K by 20 MeV W ions in advance to create displacement damage in the crystal lattice to a damage dose of 0.23 dpa. The samples were exposed to a D atom beam at 600 K with a flux of 3.5 x 10(18)D m(-2)s(-1)to populate all the created defects. The D depth profiles were measuredin situduring and at the end of exposure by nuclear reaction analysis to follow the dynamics of the D uptake. Thermal desorption spectra were collectedex situat the end of the exposure. We show that D retention increases with implanted He fluence linearly following a D/He ratio of 0.29. We obtained peaking of D concentration at the position of maximum He concentration, reaching for the 6.8 at.% He sample three times higher D concentration (1.1 at.%) than obtained on high dpa W ion irradiated samples (0.37 at.%) for the same loading conditions. D retention and transport was also studied on He-containing samples that were annealed to 1700 K. There was no reduction of D retention in the He zone but 80% reduction in the only W irradiated zone was observed, meaning displacement damage was almost completely removed. In the He zone the D concentration increased to 1.35 at.%, and we attribute this to trapping of D around He bubbles of 1.5 nm size created at the He peak maximum as obtained by transmission electron microscopy. Neither in the as He implanted nor the 1700 K annealed sample did He act as diffusion barrier. From this study we can conclude that in the main wall of a future fusion device the effect of He will not dominate D retention in W, but at high heat flux areas where displacement damage possibly anneals out He could accumulate in the material and it could eventually dominate over the effect of displacement damage.