Micro/nano scale modification of plasma facing components in LHD and its impact on the metal dust generations

First walls and divertor plates of large helical device (LHD) are made of stainless steels and isotropic graphite, respectively. They are bombarded by energetic plasma particles, and are caused the radiation damages. For understanding of long term irradiation effects of metallic first walls, probe specimens of Mo were mounted on the special sample stage of outer-port of the LHD torus which is located at about 2500 mm from LCFS (last closed flux surface) of heliotron plasma, and then, exposed to plasma discharges through the single experimental campaign. Majority of discharge gas was hydrogen or helium. Amorphous re-deposition layer of about few nm thick and large sized blisters with few micrometer were formed at the specimen surface. Formation of dense gas bubbles (1-2 nm), dislocation loops and surface roughening were simultaneously occurred in the sub-surface layer about 30 nm thick. Such nano scale damages may be mainly caused by helium particle bombardment.