Mirrors for diagnostic and remote handling applications in ITER: Problems with specifications for commercial mirrors

With as-received commercial mirrors for diagnostic and remote handling applications in ITER, some problems have been encountered concerning reflectivity and overcoating. Mirrors provided as identical in the same batch by the manufacturer, have very different reflectivities in the UV range, analysis of the surface material has shown varying amounts of SiO and SiO2 for nominally SiO overcoating. In another case the specified coating (MgF2) for an UV enhanced mirror was observed to contain Hf, neither Mg nor F were found. Hf is commonly used to enhance the reflectivity in the UV spectral range, but Hf has a large low energy neutron capture cross-section and may cause problems due to transmutation. It is therefore very important to analyse the composition and measure reflectivity of all commercial mirrors before use in ITER, as commercial specifications are not completely reliable.