Tungsten evolution under helium irradiation: Shape of bubbles by TEM and in-situ GISAXS analysis

International audienceThe evolution of the tungsten microstructure and properties under helium irradiation could cause problems for both operational and safety reasons in fusion reactors. In particular, the presence of helium bubbles, formed mainly in the subsurface area, modifies the mechanical and retention properties of radioactive elements. In that context we have investigated the shape of helium bubbles in tungsten induced by helium irradiation under welldefined experimental conditions. We have used single crystals with controlled bombardment energy, flux, fluence and post-irradiation annealing at high temperature under ultra-high vacuum in order to get closer to the thermodynamic equilibrium of the helium bubbles. The bubble shape has been characterized by a combination of TEM and 3D reciprocal space map by in-situ GISAXS. We have shown the presence of facetted bubbles exposing {110} and {100} facets of similar size. Using Wulff theorem and experimental results from both techniques, we derive a surface energy ratio of $\frac{γ 100} {γ 110}$ = 1.03 ± 0.03