Introducing LCGT - the powerful next generation interferometer from Japan
The Large-scale Cryogenic Gravitational-wave Telescope, LCGT, is the next generation interferometer now under proposal in Japan. This powerful new instrument has a sensitivity comparable to the Advanced LIGO design (Figure 1) and a far-reaching sky coverage complementary to that of other gravitational-wave detectors (Figure 2).
The basic interferometer configuration, shown in Figure 3 below, is similar to that of Advanced LIGO, with the base line arm length of 3km. But LCGT incorporates several unique design features to further improve its performance.
These designs are based on small R&D prototypes at the University of Tokyo at the National Astronomical Observatory of Japan (NAOJ), and at the Institute for Cosmic Ray Research (ICRR). NAOJ's first generation large scale interferometer with 300m arms, known as TAMA, along with a few small R&D prototypes, have specifically validated the new technologies now being planned for LCGT.
In 1994, a Memorandum of Understanding to support the construction of the TAMA detector was signed by the directors of three major institutes: NAOJ, ICRR and the National Laboratory for High Energy Physics (KEK, which reorganized as the High Energy Accelerator Research Organization in 1997). TAMA operations began with much success in 1999, soon achieving world's-best sensitivity and the longest accumulated data set during its runs in 2000 and 2001. With the start of the LIGO science run in 2002, TAMA aggressively joined the effort, helping to forge the international collaboration of coincidence analysis by scheduling operation time and sharing analysis methods and data.
The host organization of LCGT is the ICRR, and the Kamioka mine in which this 3km interferometer is planned to be built is the current location of the Super-Kamiokande neutrino detector. The seismic motion at this venue is quieter by one order of magnitude than that of the Hanford site. During TAMA's science data taking period, a 20m interferometer (originally built at NAOJ for the power recycled Michelson experiment) was placed at this site and collected data for coincidence measurement. This test demonstrated superior performance thanks to the quiet environment.
The unique design of LCGT is its use of a cryogenic system to suppress thermal noise. Sapphire was chosen for the substrate material because of its optical thermal conductivity and measured high Q values at low temperatures. Further research is underway with the collaboration of ICRR, KEK and industrial companies. After completing the basic research using laboratory scale R&D, an interferometer with 100m arms and sapphire test masses, the Cryogenic Laser Interferometer Observatory (CLIO), seen in Figure 4 below, was constructed at the Kamioka site to test the technology in a realistic environment.
The CLIO project could successfully operate the 100m interferometer with all mirrors cooled down to 20K degrees. Unexpected heat flow had been found, but the cause was discovered and the system adjusted to reduce the heat flow. CLIO is presently operating at "room temperature" to improve the performance of the interferometer. When the sensitivity reaches the level at which it becomes limited by the room temperature thermal noise, the cryogenic system will be turned on again. Ensconced in its quiet location, CLIO shows superb performance, excelling even LIGO and Virgo in the very low frequency region (Figure 5).
Adoption of the cryogenic mirror allows a design such that LCGT sensitivity is limited only by quantum noises, shot noise and radiation pressure noise.
The cryogenic mirror is suspended by sapphire fibers connected to an auxiliary mass, which is cooled down to 10K degrees. This auxiliary mass is part of a suspension point interferometer and serves to suppress the noise induced by the vibration of the cryogenic system. This suspension system is installed in a cryostat, and a seismic attenuation system is mounted in the room-temperature vacuum chamber over the cryostat.
The optical configuration chosen uses the resonant sideband extraction (RSE) method, with the power recycling gain of 10 and the arm finesse of 1250. With this design, the power going through the input test mass can be kept low, and the current design of the cryogenic system can cool down test masses with an input power of 150W. The signal bandwidth is regulated to be 230Hz. With this configuration, the gravitational-wave source of coalescence of a 1.4M solar neutron-star binary can be detected as far as 257Mpc.
The first LCGT budget request of 15 billion ¥ (about $130 million) for FY2006, and the second one for FY2007, failed to meet approval, in part due to the large restructuring of the Japanese government organization involving fundamental research. In December 2006, a letter was sent to GWIC representatives from the Japan Gravitational Wave Committee (JGWC). This committee is composed of Japanese scientists studying gravitational waves. Their letter served to clarify the commitment of JGWC members toward the importance of the Large-scale Cryogenic Gravitational-wave Telescope. The authors believe that LCGT would play a significant role as part of the international GW detector network. The importance of the international collaboration and commitment from foreign research groups were likewise stressed in the letter. Eager support from the LIGO Laboratory was also highly encouraging. About LCGT, LIGO Chief Scientist Stan Whitcomb said,
"LCGT will expand the international network of gravitational-wave detectors and greatly increase the usefulness of gravitational-wave data for astrophysics. Its sensitivity and location will provide an important addition to planned detectors in the US and Europe. LIGO has had a long and productive collaboration with the gravitational-wave community in Japan, including exchanges of students and scientists, joint searches for gravitational waves between LIGO and TAMA, and joint experimental programs in seismic isolation and sapphire optics. We look forward to continuing this collaboration for many years to come."
On February 28, 2007, a new Memorandum of Understanding was exchanged by the original three institutes--ICRR, NAOJ and KEK--to affirm their commitment to promote research of gravitational waves, and endorsing the early realization of LCGT.
The importance of international collaboration is emphasized in the new proposal of LCGT for FY2008. The earlier Japanese TAMA project had played a sizeable role as part of the international GW detection network. It is believed that LCGT, with the high-performance expected from the next generation of GW detectors, will have an even greater role by providing a vital complement to other detectors in sky detection areas and operational periods. The technology of future interferometers is growing more and more sophisticated and LCGT would be a valuable source of collaborative R&D for these innovations. One such venture has already begun. Joint research into sapphire mass is now underway with a team from the LIGO Laboratory. Finally there is the great benefit that results from the mutual cooperation among allied foreign partnerships. The Large-scale Cryogenic Gravitational-wave Telescope will offer an invaluable resource to perform fresh explorations in the pioneering technologies of tomorrow.
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LIGO is supported by the National Science Foundation
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