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 . 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 x 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.