Evaluation of radiation damages on the first-wall surface in LHD exposed to charge-exchanged helium particles

In order to clarify the radiation damages caused by charge-exchanged helium particles, metal specimens were inserted into the first wall position by using material probe system in LHD, and then, exposed to the neutral beam heated helium plasmas (87 s in total). After the exposure, dense helium bubbles with size of 1–2 nm and dislocation loops were confirmed in all specimens from transmission electron microscope 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). Majority of such incidence helium particles are energetic neutrals created by charge-exchange collisions [R.J. Goldston, P.H. Rutherford, Introduction to Plasma Physics, 1996, p. 156]. It was tried to estimate the flux and energy of CX-helium to the first walls by comparing with systematic helium ion irradiation experiments [H. Iwakiri et al., J. Nucl. Mater. 283–287 (2000) 1134], [M. Tokitani et al., J. Nucl. Mater. 329–333 (2004) 761]. The estimated flux and energy from size and density of the defects is the order of ∼1019 He/m2 s, and about 1–2 keV, respectively.