In situ reflectivity of tungsten mirrors under helium plasma exposure

The formation of micron-sized helium bubbles/holes on tungsten surface due to helium ion irradiation has been investigated in the linear divertor plasma simulator NAGDIS-II [1]. In order to investigate the surface variation during the helium plasma exposure, the optical reflectivity is measured by using a He–Ne laser and a photodiode. When the incident helium ion energy is 15 eV, a great amount of bubbles and holes were formed by the exposure with the ion fluence of 1.8 × 1027 m−2. The degradation of the reflectivity is enhanced when increasing the incident ion energy. When the incident ion energy is 30–50 eV, the surface is covered with fiberform nanostructured tungsten. The reflectivity recovery effect by a ruby laser with a long pulse duration of ∼0.6 ms is demonstrated. The reflectivity recovers to the initial value when the laser pulse energy exceeds ∼1.2 MJ m−2.