Crystal orientation dependence of surface modification in molybdenum mirror irradiated with helium ions

Abstract

The crystal orientation dependence on a surface damage in molybdenum mirror has been investigated under the irradiation with 3keV helium ions to a fluence of 1022ions/m2. A strong correlation between the surface roughening and the grain orientation was confirmed from SEM–EBSD analysis. It was found that significant surface roughening including crater-like depressions is formed on the grain surfaces with crystalline planes close to (100), while relatively smooth surface remains on grains with the other directions. In contrast, the reflectivity measurement of the single crystal molybdenum with a spectroscopic ellipsometry showed smaller reduction in (100) sample than in (110) and (111) samples. The smallest helium desorption in (100) sample was also detected with thermal desorption spectroscopy in comparison with them in (110) and (111) samples, which indicates (110) and (111) samples include the large amount of defects acting as effective trapping site for He atoms. A cross-sectional TEM observation actually showed a relatively large bubbles at sub-surface regions of (110) sample. These results indicate that the optical reflectivity should be affected by not only a top surface profile but also internal radiation damages such as high density of helium bubbles.