Ten years after the plan of the Large-scale Cryogenic Gravitational wave Telescope (LCGT) project, its construction started in June, 2010. The scientific objective of the project is to detect gravitational waves that are produced by astrophysical events in the Universe. Since the events are so rare that the detection capability of the detector should be enhanced to the extent where its sensitivity is limited not by technical reasons but by ultimate physics of instrument such as thermal noise, quantum noise, and so on. LCGT adopts cryogenic mirror to reduce thermal noise, which is meaningful only after ultimate sensitivity is technically attained. Cryogenic mirror is still adventurous technique after ten years of development. I present here the overview of LCGT project.
The thermal noise of mirrors in laser interferometric gravitational wave (GW) detectors is inevitable, fundamental noise and is crucial to whether a sufficiently high sensitivity to detect GWs is reached. The cryogenic mirror technique is one method to reduce the thermal noise. Japanese GW collaboration started in a study of the cryogenic mirror technique in 1997 and continues now through the Cryogenic Laser Interferometer Observatory (CLIO) project, which started in 2002. The CLIO interferometer was completed in an underground site of the Kamioka mine in 2006 as the first cryogenic interferometer for GW detection. We investigated and reduced noise of the CLIO interferometer. The cryogenic technique will be adopted for the Large-scale Cryogenic Gravitational Wave Telescope (LCGT) project, which started in 2010. This article describes the development of the CLIO interferometer and a plan for macroscopic quantum measurement using the CLIO interferometer.
The vibration-free cryocooler remains one of the key components in the Large-scale Cryogenic Gravitational wave Telescope (LCGT). Accordingly, reviewing the design and results of the vibration-free cryocooler for the Cryogenic Laser Interferometer Observatory (CLIO) (prototype of the LCGT) would be useful for development of the LCGT. At the same time, the development of the pulse-tube cryocooler is now progressing to a stage where it is more powerful with less vibration. In this paper, the past development of the vibration-free cryocooler system for CLIO, and new technologies of the vibration-free cryocooler are presented.
The Large-scale Cryogenic Gravitational wave Telescope (LCGT) requires ultrahigh vacuum tubes, through which laser beams passed. Two 3-km vacuum tubes are kept at a vacuum pressure of ~10-7 Pa so as to reduce the scattering effect caused by residual gas molecules. The mirrors of the main interferometer are cooled to 20 K to reduce thermal noise. The evacuated multi-layer insulation (MLI) consists of organic films with aluminum coatings, which are generally used as thermal insulation for cryostats. We found a lightweight MLI film that has a low out-gassing rate comparable to that of the buffed surface of stainless steel.
Pure metals have extremely large thermal conductivity at cryogenic temperatures, and are therefore key materials for conduction cooling in cryogenic devices. Aluminum is a typical example of such a material, and as of recently, it can be highly purified and provided in commercial amounts. In this paper, the heat-transfer performance of 6N pure aluminum is reported, followed by a brief review of conduction theory on pure metals. It was confirmed that thermal conductivity of 6N pure aluminum fine wires reached 40, 000 W/(m K) at 6 K. This study also describes vibration transfer through a heat link made of pure aluminum, which is a critical issue for precise measurements, such as those using the cryogenic interferometric gravitational wave telescope. It is shown that use of stranded cable made of thin wires is effective for this purpose when the size effect of thermal conductivity does not dominate.
Pure metals have extremely large thermal conductivity at cryogenic temperatures, and are therefore key materials for conduction cooling of cryogenic devices. Copper is a typical example of such as material (as is aluminum): it can be highly purified up to the seven-nine level (7N) and is commercially available owing to recent technical developments. In this paper, the heat conductivity of 7N pure copper wire is analogized using the Wiedemann-Franz law from the resistivity measurement at cryogenic temperature. The measured RRR of the 7N copper was high, and the same as that of high-purity aluminum. Accordingly, high-purity copper can be considered as a good heat-conducting material.
The "cryogenic mirror" is one of the most important key issues for the Large-scale Cryogenic Gravitational wave Telescope (LCGT), which is the Japanese interferometric gravitational wave detector project. In this article, three main advantages of cryogenic mirrors are introduced: reduction in thermal noise, decrease of the thermal lens effect, and suppression of the parametric instability. A future project planned in Europe, the Einstein Telescope (ET), which includes the use of cryogenic interferometers, is also introduced.
A very small displacement (∼ 10.20 m/√Hz at 100 Hz) needs to be measured to directly detect gravitational waves (GW), which have been predicted by Einstein’s theory of general relativity. It is possible to detect such small displacement in the mirrors of the Large-scale Cryogenic Gravitational wave Telescope (LCGT) using interferometry. To reduce the noise level caused by thermal oscillation of the mirrors, LCGT mirrors will be cooled down to cryogenic temperature (∼20 K). For that purpose, the thermal radiation generated at room temperature has to be reduced, and this can be done using metal shields with low emissivity. To study the emissivity of some metals at low temperature, reflectivity at cryogenic temperatures has been measured at a wavelength of 10 μm, where black body radiation of 300 K has the largest intensity. As a result, the three kinds of samples measured satisfied the requirements for the LCGT with a safety factor of more than 2. In addition, the incident heat through the duct shields of the LCGT was calculated using the results of these measurements, and it was concluded that 5 baffles in the duct shield can reduce the incident heat to a sufficient level.