Vibration analysis of KAGRA cryostat at cryogenic temperature

KAGRA uses cryogenics to cool its sapphire test masses down to 20 K to reduce the thermal noise. However, cryocooler vibration and structural resonances of the cryostat couple to the test mass and can contaminate the detector sensitivity. We performed vibration analysis of the cooling system at cryogenic temperature to study its impact on detector sensitivity. Our measurements show shield vibration below 1 Hz is not impacted by cryocooler operation or structural resonances and is dominated by ground motion. The noise floor of the shield in the range 1-100 Hz was observed to be 2-3 orders of magnitude larger than seismic motion even without cryocooler operation. The operation of cryocoolers does not change the noise floor, but 2 Hz peaks and their harmonics were observed over the entire spectrum (1-100 Hz). These results were used to calculate the coupling of cooling system vibration to the test mass. We conclude that vibration from the cooling system does not limit KAGRA design sensitivity.