Effects of helium irradiation on degradation of optical properties of single and polycrystalline Mo mirrors for plasma diagnostics

Deterioration in the reflectivity of single and polycrystalline Mo mirrors irradiated with 1–5 keV helium ions has been studied by in situ reflectivity measurement, SEM, AFM and spectroscopic ellipsometry. It has been shown that the reflectivity of these mirrors at a wavelength of 670 nm deteriorates with an increase in the fluence, depending on the energy of the irradiated ions, and eventually decreases to approximately 15% in the lowest case at a fluence of 1.6×10 23 ions m −2 . Dependence of reflectivity on the wavelength in the range of 190–2500 nm has also been shown. Single crystal Mo specimens have been shown to exhibit comparatively higher reflectivity than polycrystalline Mo specimens at all the energies of helium ions examined. It has been demonstrated that optical constants such as the effective dielectric constant, refractive index, and extinction coefficient decrease due to the formation of damages such as helium bubbles in the subsurface region, which causes deterioration in the reflectivity.