Masaki Ando

Mirror suspension system for the TAMA SAS

Several R&D programmes are ongoing to develop the next generation of interferometric gravitational wave detectors providing the superior sensitivity desired for refined astronomical observations. In order to obtain a wide observation band at low frequencies, the optics need to be isolated from the seismic noise. The TAMA SAS (seismic attenuation system) has been developed within an international collaboration between TAMA, LIGO, and some European institutes, with the main objective of achieving sufficient low-frequency seismic attenuation (−180 dB at 10 HZ). The system suppresses seismic noise well below the other noise levels starting at very low frequencies above 10 Hz. It also includes an active inertial damping system to decrease the residual motion of the optics enough to allow a stable operation of the interferometer. The TAMA SAS also comprises a sophisticated mirror suspension subsystem (SUS). The SUS provides support for the optics and vibration isolation complementing the SAS performance. The SUS is equipped with a totally passive magnetic damper to suppress internal resonances without degrading the thermal noise performance. In this paper we discuss the SUS details and present prototype results.

Japanese large-scale interferometers

The objective of the TAMA 300 interferometer was to develop advanced technologies for kilometre scale interferometers and to observe gravitational wave events in nearby galaxies. It was designed as a power-recycled Fabry–Perot–Michelson interferometer and was intended as a step towards a final interferometer in Japan. The present successful status of TAMA is presented. TAMA forms a basis for LCGT (large-scale cryogenic gravitational wave telescope), a 3 km scale cryogenic interferometer to be built in the Kamioka mine in Japan, implementing cryogenic mirror techniques. The plan of LCGT is schematically described along with its associated R&D.

Measurement of the mechanical loss of crystalline samples using a nodal support

Abstract We measured the mechanical loss of crystalline anisotropic samples (silicon and sapphire) using a nodal support. The measured quality factor of the silicon sample reached 1.0×10 8 at room temperature. The sapphire sample showed lower quality factors, 6.4×10 7 at most, which were analytically confirmed to be dominated by surface loss.

Measurement of the intrinsic mechanical loss of low-loss samples using a nodal support

Abstract We measured the mechanical loss of cylindrical samples made of fused silica. The loss due to the support was excluded by using a nodal support technique. We evaluated separately the intrinsic loss of the material and the loss due to the surface roughness by systematically analyzing the measured losses.

Status of the CLIO project

The CLIO project involves the Cryogenic Laser Interferometer Observatory (CLIO) detector complex for gravitational wave detection and the Kamioka Laser Interferometric Strainmeter for the acquisition of geophysical data. CLIO has been constructed to demonstrate the feasibility of a future project, the Large-scale Cryogenic Gravitational wave Telescope (LCGT). It will utilize the low seismic and stable environment of the Kamioka mine as well as sapphire mirrors and suspension fibres at low temperature to reduce thermal noise. We designed CLIO to have a noise level limited by the thermal noise of sapphire mirrors and sapphire suspension fibres, which vary from 3 × 10−19 m Hz−1/2 at 300 K to 2 × 10−20 m Hz−1/2 at 20 K around 100 Hz. The strainmeter has already succeeded in monitoring the Earths tidal motion with a strain sensitivity of 2 × 10−12. The seismic noise veto between these same-scale interferometers is expected to provide an effective means of data selection for the gravitational wave signal analysis, and the ground motion data obtained by the strainmeter will help to maintain the stable operation of CLIO.

Experimental study of thermal noise caused by an inhomogeneously distributed loss

Abstract The thermal fluctuation of a mechanical system with an inhomogeneously distributed loss was measured to prove the invalidity of the normal-mode expansion method which is commonly used to calculate the thermal noise of mechanical oscillators. The measured spectrum is inconsistent with the modal expansion model, while they agree well with an evaluation obtained by applying the fluctuation-dissipation theorem to the measured mechanical response. These are the first experimental results which show that mode expansion is invalid.

Current status of the CLIO project

CLIO (Cryogenic Laser Interferometer Observatory) is a Japanese gravitational wave detector project. One of the main purposes of CLIO is to demonstrate thermal-noise suppression by cooling mirrors for a future Japanese project, LCGT (Large-scale Cryogenic Gravitational Telescope). The CLIO site is in Kamioka mine, as is LCGT. The progress of CLIO between 2005 and 2007 (room- and cryogenic-temperature experiments) is introduced in this article. In a room-temperature experiment, we made efforts to improve the sensitivity. The current best sensitivity at 300 K is about 6 × 10-21/√Hz around 400 Hz. Below 20 Hz, the strain (not displacement) sensitivity is comparable to that of LIGO, although the baselines of CLIO are 40-times shorter (CLIO: 100m, LIGO: 4km). This is because seismic noise is extremely small in Kamioka mine. We operated the interferometer at room temperature for gravitational wave observations. We obtained 86 hours of data. In the cryogenic experiment, it was confirmed that the mirrors were sufficiently cooled (14 K). However, we found that the radiation shield ducts transferred 300K radiation into the cryostat more effectively than we had expected. We observed that noise caused by pure aluminum wires to suspend a mirror was suppressed by cooling the mirror.

Vacuum-compatible vibration isolation stack for an interferometric gravitational wave detector TAMA300

Interferometric gravitational wave detectors require a large degree of vibration isolation. For this purpose, a multilayer stack constructed of rubber and metal blocks is suitable, because it provides isolation in all degrees of freedom at once. In TAMA300, a 300 m interferometer in Japan, long-term dimensional stability and compatibility with an ultrahigh vacuum environment of about 10−6 Pa are also required. To keep the interferometer at its operating point despite ground strain and thermal drift of the isolation system, a thermal actuator was introduced. To prevent the high outgassing rate of the rubber from spoiling the vacuum, the rubber blocks were enclosed by gas-tight bellows. Using these techniques, we have successfully developed a three-layer stack which has a vibration isolation ratio of more than 103 at 300 Hz with control of drift and enough vacuum compatibility.

Intrinsic losses in various kinds of fused silica

We systematically measured the intrinsic mechanical quality factors of 13 kinds of bulk fused silica from four companies using a nodal support system. Some of them have actually been adopted as mirror substrates in interferometric gravitational wave detectors. The measured quality factors widely ranged from 7 × 105 to 4 × 107. They turned out to be independent of one specific property, such as the amount of OH content, suggesting that the loss mechanism has several origins. We found that many of the samples showed smaller losses at lower frequency. From the viewpoint of the mirror thermal noise, it would be fortunate if the loss continued to decrease with decreasing frequency. We also found that an annealing process increases their quality factors.

Demonstration of power recycling on a Fabry-Perot-type prototype gravitational wave detector

Abstract We describe preliminary results of a power-recycling experiment on a 3-m prototype interferometric gravitational wave detector. We locked the interferometer and operated it stably for over one hour. This is the first experimental demonstration of power recycling on a Fabry-Perot-Michelson interferometer with suspended mirrors.