Japanese large-scale cryogenic gravitational wave telescope, KAGRA, aims to detect gravitational waves using an interferometer having 3 km in length of arms. The notable feature of KAGRA is four main mirrors of the interferometer are cooled down below 20 K in order to reduce thermal noise. With the design of the cryogenic system for KAGRA, a cryo-payload consisting of the mirror and its suspension system is connected with two very-low-vibration cryo-cooler units as cooling devices, and is surrounded by the radiation shield (2.5m^3 in volume) below 20K in a cryostat.
Each cooling device consists of a pulse-tube type cryocooler (0.9W at 4K), a specifically designed mechanical frame and the conduction cooling links made of 6N class high purity aluminum.
Fabrication of four cryostats and sixteen very-low-vibration cryocooler units was started at 2011, and completed on the end of March 2013. All of the cryocooler units confirmed their expected cooling performance, as 2.5 W at 9 K, and the overall vibration characteristic less than 10^-7 m/ Hz^0.5 at the connection edge of conductive cooling passage. KAGRA collaborators carried out total performance tests including cooling characteristic with a half size of dummy cryo-payload and measurements of vibration of the radiation shield.
Detail of thermal and mechanical design of the cryogenic system for KAGRA and its performance was reported in the conference.