Detector configuration of KAGRA - the Japanese cryogenic gravitational-wave detector

TL;DR

KAGRA advances gravitational-wave detection by combining underground operation, cryogenic sapphire optics at 20 K, and quantum non-demolition techniques to suppress both thermal and quantum noises. The paper presents a detailed noise budget, heat-management design, and a parametric optimization of a variable resonant-sideband-extraction system to maximize the NS-binary inspiral range. It analyzes trade-offs between laser power, suspension design, and coating losses, and outlines a phased deployment (iKAGRA and bKAGRA) with achievable sensitivities. This configuration promises timely contributions to global GW astronomy and informs future underground, cryogenic detectors like the Einstein Telescope.

Abstract

Construction of the Japanese second-generation gravitational-wave detector KAGRA has been started. In the next 6 \sim 7 years, we will be able to observe the space-time ripple from faraway galaxies. KAGRA is equipped with the latest advanced technologies. The entire 3-km long detector is located in the underground to be isolated from the seismic motion, the core optics are cooled down to 20 K to reduce thermal fluctuations, and quantum non-demolition techniques are used to decrease quantum noise. In this paper, we introduce the detector configuration of KAGRA; its design, strategy, and downselection of parameters.

Figures (7)

• Figure 1: Schematic view of KAGRA. The test masses are installed in the cryostats and are isolated from room-temperature optics by more than 20 m. The mirrors for iKAGRA will be installed in vacuum chambers next to the cryostats for the smooth transition to bKAGRA. Here MC/OMC stand for mode-cleaner/output mode-cleaner.
• Figure 2: The estimated noise budget of KAGRA.
• Figure 3: Left: Schematic view of the suspension systems. The type-A is for the test masses and the type-B is for the beamsplitter and the recycling mirrors. The table underneath the suspended mass in type-B is for auxiliary optics. Right: Seismic noise and gravity gradient noise (GGN) spectra.
• Figure 4: Schematic view of the cryostat. The heat absorbed from the laser light is transferred from the test mass to the upper stages and then extracted to the pulse-tube cryo-cooler (PTC). The radiation shield is extended along the vacuum duct over 20 m in order to reduce the heat from the 300 K radiation.
• Figure 5: Left: Temperature dependence of mirror thermal noise. Right: Dissipation and temperature profiles of a suspension fiber.