Surface morphology in tungsten and RAFM steel exposed to helium plasma in PSI-2

Impact of the helium plasma exposure on the surface modification in tungsten and reduced activation ferritic/martensitic (RAFM) steel have been investigated on the linear plasma device PSI-2 assuming the condition of DEMO first wall. In tungsten, a nanoscale undulating surface structure, which has a periodic arrangement, is formed under low temperature conditions below fuzz nanostructure formation threshold similar to 1000 K. Interval and direction of the undulation shows dependence on the crystal orientation. A large variation in surface level up to 200 nm has been observed among grains at a fluence of 3 x 10(26) He m(-2) showing dependence of the surface erosion rate on the crystal orientation. The (100) plane in which the undulating surface structure is not formed shows the highest erosion rate. This significant erosion is due to the multistage sputtering through impurity. In RAFM steel, sponge-like nanostructure is developed and it grows with increasing helium fluence beyond 1 #mu#m. In the sponge-like nanostructure, a composition change from the base material is observed in which the tungsten ratio increases while the iron ratio decreases showing differences in sputtering ratio depending on the atomic mass. (authors)