Surface modifications on unirradiated and ion-irradiated tungsten after exposed to deuterium plasma at LHD divertor-leg position

Abstract In order to clarify the divertor plasma-induced tungsten (W) surface modifications as well as the irradiation defects effect, two kinds of ITER grade W were exposed to the large helical device (LHD) deuterium (D) plasma at the divertor-leg position. One was the iron (Fe) ion irradiated W to produce irradiation defects, and the other was the unirradiated W. The distributions of divertor plasma-induced surface modifications on these two kinds of W were clarified by scanning electron microscope (SEM) and transmission electron microscopy (TEM). A co-deposition layer which was mainly made up of carbon (C) and Fe was formed at the private flux region (>21 mm). No significant surface change was observed at the strike point region (14 – 16 mm). The oxygen-enriched amorphous W structures (OEAWs) caused by plasma surface interactions were observed at the 2- 14 mm (scrape-off layer region (SOL) region) and 20 mm. A the 2-14 mm and 20 mm, the OEAWs density on the pre-irradiated W sample is lower than that on the unirradiated W sample. On the other hand, the OEAWs size on the pre-irradiated W sample is larger than that on the unirradiated W sample at the 8 – 12 mm and 20 mm. While, the OEAWs size on the pre-irradiated W sample is smaller than that on the unirradiated W sample at the 2 – 6 mm. This study implies the possibility of forming OEAWs on the surface of W divertor. And the irradiation defects affect the density and size of OEAWs.