Impact of material migration and radiation damage on the reflectivity of molybdenum mirrors: laboratory test for DEMO

Molybdenum mirrors were irradiated with high-Z (Mo) and low-Z (He) ions to simulate the effect of plasma exposure on diagnostic mirrors to be applied in D-T fusion devices. Selected mirrors before irradiation were additionally coated with tungsten thin films of various thicknesses to simulate the real conditions in the reactor: tungsten erosion and migration. Irradiations were followed by reflectivity measurements and detailed microscopy observations. The threshold dose for the degradation of optical performance by 4He+ irradiation is below 0.1 × 1017 cm−2. The presence of tungsten films (4 and 10 nm in thickness) decreases total reflectivity by 30%-40% in the short wavelength and 5% (4 nm-thick film) and 12% (10 nm-thick film) in the infrared. Ion irradiation further worsens reflectivity by 5%-10%. The size of blisters visible on the surface of mirrors after irradiation depends on the thickness of tungsten films. Helium content after irradiation with helium ions of a dose 6 × 1017 cm−2 decreased during two years by 70% from the value measured directly after the implantation.