Thermal testing of the first mirror unit mock-up for H-alpha and visible spectroscopy in ITER

•Sandwich-like design was proposed for single-crystal molybdenum mirror.•High optical quality was provided for aspheric molybdenum mirror by diamond turning.•First mirror unit made of molybdenum was thermally tested in vacuum chamber.•Observations during the tests showed no degradation of image quality. H-alpha and visible spectroscopy (HA&VS) diagnostic in ITER is aimed at the measurements of the intensity of hydrogen isotopes and of impurity lines in the SOL emission. It is based on the endoscope scheme with the first mirror unit (FMU) containing two mirrors (flat and aspheric) located behind a small entrance pupil and a discharge cleaning system. A primary candidate material for the FMU mirror is a single-crystal molybdenum (SC-Mo). A sandwich-like design is proposed, in which few small-sized SC-Mo plates are bound by a diffusive welding to a large-size polycrystalline Mo substrate. An acceptable welding regime is presented. A recent progress is discussed on the polishing of SC-Mo focusing mirrors up to 150mm diameter. Thermal tests of the molybdenum FMU mock-up had been performed. The FMU was heated up to 200°C whereas the mirror temperature was up to 300°C under the pressure of 8–9Pa. A technique for the quantitative assessment of the image quality (contrast and resolution) at the elevated temperatures has been suggested. As a result, a sub-mm shift of an image plane along the optical axis had been observed with no significant degradation of the image quality.