First evidence of helium in the tungsten divertor of the WEST tokamak

ITER will operate with plasma-facing materials made of tungsten, a material whose surface morphology changes when exposed to helium plasmas at sufficient high energy, fluence and surface temperature. To predict any potential changes to tungsten in future fusion reactors, a dedicated helium campaign was carried out in WEST in 2019. The plasma scenarios developed during this campaign were aimed in particular at forming tungsten fuzz on the surface of tungsten-coated graphite divertor components. A large-scale post-mortem study reveals for the first time the radial and toroidal distributions of helium content in the components determined post-mortem by ion beam analysis at the divertor scale. The results show that helium is mainly found in tungsten at the strike point areas, with concentrations reaching up to 10 at.% near the surface to an analysed depth of 0.2 µm. This is accompanied by the presence of nanobubbles in the microstructure up to 10 nm below the surface observed by electron microscopy. The same observation was made in bulk tungsten ITER-like PFUs. However, no tungsten fuzz was observed.