Ellipsometric and electron spectroscopic study of degradation of optical properties in Mo mirror irradiated with deuterium and/or helium ions

Abstract Degradation of optical properties in Mo mirrors irradiated with helium and/or deuterium ions has been studied by ellipsometric spectroscopy, electron energy loss spectroscopy, and electron microscopy. It was found that values of the optical constants (reflectivity, refractive index, and extinction coefficient) of the specimens decreased in the order of deuterium, deuterium and helium dual-beam, and helium irradiations. The penetration depth of the laser light in the specimen, estimated from the observed extinction coefficient, increased in the same order. Electron energy loss spectroscopic study on an individual helium bubble formed in a specimen without surface roughness clearly revealed that the reflectivity decreased owing to a change in the di-electric constants caused by the formation of the bubble, and it was associated with a decrease in the optical constants. These results and the microstructure observation indicate that the degradation of reflectivity originating from the helium bubble is the primary issue affecting a diagnostic mirror irradiated with charge-exchange neutral particles.