Cryogenic suspension design for a kilometer-scale gravitational-wave detector

TL;DR

This paper presents the design and performance of a cryogenic mirror suspension system for the KAGRA gravitational-wave detector, addressing thermal noise reduction and technical challenges of cooling mirrors in a large-scale setup.

Contribution

It introduces a novel cryogenic suspension system with efficient radiative and conduction cooling, including a unique inclination adjustment and displacement sensors, for gravitational-wave detectors.

Findings

Successful implementation of cryogenic suspension during bKAGRA Phase 1

Effective thermal noise reduction techniques demonstrated

Design solutions for vibration and thermal drift challenges

Abstract

We report the mirror suspension design for Large-scale Cryogenic Gravitational wave Telescope, KAGRA, during bKAGRA Phase 1. Mirror thermal noise is one of the fundamental noises for room-temperature gravitational-wave detectors such as Advanced LIGO and Advanced Virgo. Thus, reduction of thermal noise is required for further improvement of their sensitivity. One effective approach for reducing thermal noise is to cool the mirrors. There are many technical challenges that must be overcome to cool the mirrors, such as cryocooler induced vibrations, thermal drift in suspensions, and reduction in duty cycling due to the increased number of potential failure mechanisms. Our mirror suspension has a black coating that makes radiative cooling more efficient. For conduction cooling, we developed ultra high purity aluminum heat links, which yield high thermal conductivity while keeping the spring constant sufficiently small. A unique inclination adjustment system, called moving mass, is used for aligning the mirror orientation in pitch. Photo-reflective displacement sensors, which have a large range, are installed for damping control on marionette recoil mass and intermediate recoil mass. Samarium cobalt magnets are used for coil-magnet actuators to prevent significant change of magnetism between room temperature and cryogenic temperature. In this paper, the design of our first cryogenic payload and its performance during bKAGRA Phase 1 are discussed.