Effect of exposure inside the LHD vessel on optical properties of stainless steel mirrors

Three stainless steel mirrors were exposed inside the LHD vacuum vessel during the 3-rd experimental campaign. Then the change of spectral reflectance, R(λ), at normal incidence for λ = 200/700 nm was measured. Mirrors in the divertor region and in the diagnostic port became coated with carbon-based films but mirror closest to the plasma confinement volume became cleaner than was initially. The characteristics of films were obtained by different techniques that are in practice for analyzing the near-surface layers. After that the films were slowly removed by series exposures of mirrors to low energy ions of deuterium plasma with measurements of R(λ) after every exposure. In the present paper the comparison is made on reflectance behavior during cleaning procedure with results of reflectance calculations in the framework of the model: the half-transparent film on the stainless steel substrate. The n(λ) and k(λ) data for carbon-based films were taken from previous measurements and from literature. (author)