Understanding helium (He) plasma-induced tungsten (W) surface modifications and the effect of irradiation defects on He plasma-induced W surface modifications under a real divertor environment are important for the operation of fusion reactors. In this study, two iron (Fe) ions pre-irradiated in W samples with dislocation loops and voids, and two unirradiated W samples were exposed to He plasma at the divertor leg position of the Large Helical Device. The gross erosion rate is 1.0 × 1020 atoms m−2 s−1, 1.0 × 1020 atoms m−2 s−1, 9.3 × 1019 atoms m−2 s−1 and 7.4 × 1019 atoms m−2 s−1 for W9, W10, W11 and W12, respectively. The surface of each sample after the exposure was different at the strike point and the two regions on either side of the strike point, the scrape-off layer (SOL) region and the private region. The typical He plasma-induced structures in the SOL region are stripe structure, sawtooth structure and non-undulating structure, which are collectively called He-structures in the present study. At the strike point, the typical He plasma-induced structures are dense W protrusions. In the private flux region, the typical He plasma-induced structures are semi-formed He-structures. The formation of these structures is dependent on the grain orientation. Pinholes were observed in these structures. No significant difference in He plasma-induced structures is found between the pre-irradiated W and the unirradiated W. The formation of non-undulating structure and stripe structure is discussed based on the observed semi-formed He-structures.
This paper investigates the effects of helium plasma exposure on the surface of tungsten, both with and without prior ion irradiation. The study was conducted at the divertor leg position of the Large Helical Device (LHD) fusion reactor. The results show that the surface structures formed by the helium plasma, such as stripes, sawtooth patterns, and protrusions, depend on the grain orientation and location on the sample. The study is important for understanding the behavior of tungsten, a key material for fusion reactor components, under fusion-relevant conditions.