Vibration Analysis of KAGRA Cryostat at Cryogenic Temperature

TL;DR

This study analyzes the vibration impact of KAGRA's cryogenic cooling system on detector sensitivity, finding that cryocooler vibrations do not significantly limit the detector's performance at cryogenic temperatures.

Contribution

It provides detailed vibration measurements at cryogenic temperatures and assesses their effect on the KAGRA gravitational wave detector's sensitivity.

Findings

Shield vibration below 1 Hz is unaffected by cryocooler operation.

Noise floor in 1-100 Hz is 2-3 orders of magnitude above seismic motion.

Cryocooler operation does not significantly alter the noise floor.

Abstract

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 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 measurement show shield vibration below 1 Hz is not impacted by cryocooler operation or structural resonances and follows ground motion. The noise floor of the shield in 1-100 Hz was observed to be 2-3 order of magnitude larger than seismic motion even without cryocooler operation. The operation of cryocoolers does not change the noise floor, but 2.0 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.