Thermal tests of broadband dielectric mirrors

The paper describes the thermal tests done to broadband dielectric mirrors which are considered to be used as secondary mirrors in ITER optical diagnostics. The tests were performed in a vacuum camera accompanied with measurements of mirrors reflectivity during heating. The main goal was to investigate possible variation of reflectivity in ITER relevant conditions. The samples of TiO2/SiO2 and Nb2O5/SiO2 coatings on KU-1 substrate showed degradation of reflectivity for 2–3% in the wave range 400–500 nm being heated to 150 °C in vacuum. This degradation was found to be constant at the stable temperature and reversible after cooling. Insufficient heat contact between quartz mirror and its copper base was observed that draws attention to the problem of providing proper efficiency of heat transfer from secondary mirror by its mounting to ITER cooling system.