Abstract
The Large-scale Cryogenic Gravitational wave Telescope (LCGT) requires ultrahigh vacuum tubes, through which laser beams passed. Two 3-km vacuum tubes are kept at a vacuum pressure of ~10-7 Pa so as to reduce the scattering effect caused by residual gas molecules. The mirrors of the main interferometer are cooled to 20 K to reduce thermal noise. The evacuated multi-layer insulation (MLI) consists of organic films with aluminum coatings, which are generally used as thermal insulation for cryostats. We found a lightweight MLI film that has a low out-gassing rate comparable to that of the buffed surface of stainless steel.
