Optical properties of nanostructured tungsten in near infrared range

Abstract

In nuclear fusion devices, infrared (IR) imaging diagnostics have been widely used for the temperature measurement of in-vessel surfaces. For the diagnostics, variations of the optical emittance and reflectance of wall materials will be an essential issue to measure the temperature correctly. In this study, variations in the optical properties in near IR range of tungsten exposed to helium plasmas were experimentally investigated in detail by observing the radiation spectrum from a heated sample. By the exposure to the helium plasma, nanostructures were grown from the surface, while the nanostructures were annealed out when the surface temperature was raised without helium ion irradiation. The optical emittance was measured from the radiation spectrum in the wavelength range 1400–4000 nm; it was revealed that the emittance significantly altered for a short time period in the formation and annealing processes of nanostructures. The influence of the variation in the optical properties on the IR diagnostics is discussed.