Control noise reduction of cryogenic suspension in KAGRA

In gravitational wave detectors, the laser is used to observe how the distance between mirrors changes due to space distortions caused by gravitational waves. The displacement of the mirrors is minute, so the mirrors must be sufficiently isolated from ground vibrations to achieve the required sensitivity. Therefore, the main mirrors are suspended by nine-stage pendulums in KAGRA, the gravitational wave detector in Japan. In such a pendulum-based vibration isolator, the mirror oscillates significantly at the resonant frequency. Hence we need the control system to damp the resonances, but the noise from the sensors used in such a control system can be a problem. In fact, the sensitivity of KAGRA was limited by the noise from the cryogenic payload control system at 10-50 Hz in the previous observation. Therefore, a low-noise control filter was designed and implemented for use during the previous observing run. As a result, the control noise of the cryogenic payload at 10-100 Hz was reduced by 2-3 orders of magnitude, and the target sensitivity for the O4 observing run was achieved at low frequency (below 100 Hz).