TEM investigation of the effect of He-seeding on plasma induced defects in tungsten

A series of W samples in the recrystallized state and plastically deformed condition were exposed to a mixed He-D plasma, with He-to-D ratios varying from 80/20 to 40/60 and the effect on the microstructure was investigated with transmission electron microscopy. The microstructure of the exposed samples was studied at the surface (100 nm layer) and in a sub-surface region located at a depth of 1-5 µm from the exposed surface. A strong increase of the dislocation density in the surface layer was detected in all the inspected samples and cavities were found irrespective of the D-to-He ratio or plastic deformation applied prior the exposure. In the sub-surface area, the initial microstructure of the recrystallized or plastically deformed sample was nearly recovered and no cavities were observed anymore. He seeding strongly promotes the formation of cavities at the surface layer, but its impact does not spread further in the sub-surface layer.