Effect of the presence of helium in tungsten on deuterium retention

Tungsten (W) samples were pre-irradiated with low-energy helium (He) ions with an energy of 80 eV, flux of 1021 He/m2s, at a sample temperature of 1200–1250 K in a inductively coupled plasma (ICP) source facility. Such irradiation conditions led to a formation of a nano-structured surface layer of tungsten, called ‘fuzz’. These samples were then irradiated with D3+ ions with an energy of 2 keV and small doses of 1019 D/m2 at room temperature and in-situ thermal desorption analysis (TDS) was performed. It was found that the main factor determining the deuterium (D) retention in W samples pre-irradiated with helium is the concentration of helium below the surface, namely, the D retention was increased with decreasing the He concentration until the nanostructured W ‘fuzz’ was removed. The effect of nano-structured tungsten ‘fuzz’ is only in a decrease of the reflection coefficient of deuterium ions compared to a smoother surface, resulting in the increase of the D influx into W and, consequently, increase of the D retention. However, this increase has a minor effect on the D retention compared to the He concentration in the subsurface layer of W.