Damage evolution and particle retention in metals bombarded by neutral helium particles at the first wall positions in LHD

Specimens of W, Mo and stainless steel were inserted into the first wall equivalent position by using movable material probe system in LHD, and then, exposed to the several sequential helium discharges. After the exposure, very dense helium bubbles and dislocation loops were confirmed in all specimens from transmission electron microscopy (TEM) observation. It means that the energy of incidence helium particles is sufficiently higher than that of the minimum energy for creating the knock-on damage (Emin) in all specimens (e.g. W; Emin=0.53keV). Majority of such incidence helium particles to the specimens are energetic neutrals created by charge-exchange (CX) collisions [1]. It is known that helium atoms once injected into metals cause more intensive radiation damages than hydrogen atoms, because they have a strong interaction with lattice defects such as vacancies or dislocation loops. Understanding of the effects of charge exchanged helium particles (CX-helium) to the first wall is important for not only the elucidation of materials degradation but also realization of the high performance plasma operations in future fusion devices.