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 x 10(27) 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 similar to 0.6 ms is demonstrated. The reflectivity recovers to the initial value when the laser pulse energy exceeds similar to 1.2 MJ m(-2). (C) 2009 Elsevier B.V. All rights reserved.