Deterioration of the reflectivity in first-mirror materials under irradiation with low-energy helium ions

To evaluate the optical reflectivity deterioration of first mirrors for plasma diagnostics, in situ measurements of the laser reflectivity have been performed under the irradiation with low-energy helium ions in stainless steel, Mo and W. Surface morphology and radiation damage in sub-surface region have also been examined using scanning electron microscope, atomic force microscope and transmission electron microscope. It is found that the deterioration of the reflectivity under the irradiation with helium ions is very severe and depends on the ion energy, fluence and wavelength. Although fine roughness on the specimen surface was observed in irradiated materials, the size is too tiny to explain the present degradation of the reflectivity by using a Bennett's equation. On the other hand, from the present observation a strong correlation between the deterioration of the reflectivity and the formation of radiation damages in the surface and sub-surface regions of the laser penetration depth is suggested.