Toshikazu Suzuki

Maximum heat transfer along a sapphire suspension fiber for a cryogenic interferometric gravitational wave detector

Abstract We have shown experimentally that the thermal conductivity of a sapphire fiber, which is used as a suspension and thermal conductor of mirrors in a cryogenic interferometric gravitational wave detector, is limited by boundary scattering of phonons in the temperature range below 40xa0K, and that maximum heat transfer along the fiber is proportional not to the square of the diameter of the fiber, but to the cubic. For the design of a sapphire suspension fiber for the LCGT project (1xa0mm in diameter, 25xa0cm in length and two loops), the maximum heat transfer along the fiber from the mirror at 20xa0K to the medium mass at 10xa0K is 340xa0mW.

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.

Thermal lensing in cryogenic sapphire substrates

We report the reduction of thermal lensing in cryogenic sapphire mirrors, which are planned to be used in the large scale cryogenic gravitational wave telescope project. We measured three key parameters of sapphire substrate for thermal lensing at cryogenic temperature. They are the optical absorption coefficient, thermal conductivity and temperature coefficient of refractive index at cryogenic temperature. On the basis of these measurements, we estimated the shot noise sensitivity of the interferometer with thermal lensing by using a wavefront tracing simulation. We found that thermal lensing in cryogenic sapphire mirrors is negligible.

Application of sapphire bonding for suspension of cryogenic mirrors

In order to design a suspension for large cryogenic mirror, we have measured thermal conductance and shear strength of sapphire bonding in comparison with direct bonding and hydroxide-catalysis bonding. Thermal conductance per unit area of 4 [W/K/mm2] for the direct bonding and 0.3 [W/K/mm2] for the hydroxide-catalysis bonding were obtained around 20K. Shear strength of 28[MPa] for the direct bonding and 6.5 [MPa] for the hydroxide-catalysis bonding were measured at 300K. Based on those values, an estimated area that support a weight of a mirror produces a temperature step of less than 1% of a difference of temperature in between the main mirror and a mirror of Suspension Point Interferometer.

Measurement of vibration of the top of the suspension in a cryogenic interferometer with operating cryocoolers

In the LCGT and CLIO projects for the interferometric gravitational wave detectors of Japan, the mirrors and a part of the suspension systems are cooled by cryocoolers to reduce the thermal noise. For the CLIO, extremely small vibration cryocoolers were specially developed by improving a commercial Gifford-McMahon type pulse tube cryocooler. We measured the vibration at the top of the suspension base in the CLIO interferometer while operating these cryocoolers. Although the seismic motion of 10−9(1 Hz/f)2 m/Hz½ at the site of the CLIO and LCGT in the Kamioka mine is 100-times smaller than that around Tokyo, these cryocoolers did not seriously increase the vibration. Consequently, a reduction of thermal noise by the cooled mirrors and suspension fibers using these cryocoolers is expected to be observed without any additional fluctuation disturbance due to the cryocoolers.

Study of optical dumpers used in high vacuum system of interferometric gravitational wave detectors

Outgassing velocities and optical absorption coefficients of black colored optical absorbers (UB-NiP, Phosblack II, Raydent, ECB+DLC and Alumite) were studied to reduce scattered and strayed light in interferometric gravitational wave detectors. The measured results showed that the UB-NiP had the largest optical absorption coefficient of 99.89% for 1064nm wavelength of light. For the outgassing velocity, the ECB+DLC had the lowest value, which was 1 × 10−8 Pam3/s/m2 at 20 hours. This value was about two orders of magnitude smaller than that of the UB-NiP, however, its optical absorption coefficient was only 65 %. Therefore, we concluded that the UB-NiP is suitable as an optical dumper with small area and high efficiency, and the ECB+DLC is suitable as large area coating such as a use for vacuum chamber walls.

Development of a cryocooler vibration-reduction system for a cryogenic interferometric gravitational wave detector

We have developed a pulse tube cryocooler vibration-reduction system for a cryogenic interferometric gravitational wave detector, based on an experimental vibration analysis for commercial 4 K cryocoolers. In a preliminary test, the vibration of the cold stage was reduced by two orders of magnitude using this vibration–reduction system.

Ultra‐low Vibration Pulse Tube Cryocooler with a New Vibration Cancellation Method

The vibration of the cold stage of model cryocoolers was measured for proving a method of vibration cancellation. A model cryocooler consists of multi‐pairs of pipes on the same cold stage. When a presented working gas is supplied with a proper phase shift to those pairs, the vibration of the cold stage can be cancelled. We made models of the PT cryocoolers with a single pipe, two pairs of pipes and three pairs of pipes. Compared with the single‐pipe model, the six‐pipe model shows about a 98% reduction of vibration without any additional structure for vibration isolation.

Force measurements of a superconducting-film actuator for a cryogenic interferometric gravitational-wave detector

We measured forces applied by an actuator with a YBCO film at near 77 K for the Large-scale Cryogenic Gravitational-wave Telescope (LCGT) project. An actuator consisting of both a YBCO film of 1.6 micrometers thickness and 0.81 square centimeters area and a solenoid coil exerted a force of up to 0.2 mN on a test mass. The presented actuator system can be used to displace the mirror of LCGT for fringe lock of the interferometer.Abstract We measured forces applied by an actuator with a YBa 2 Cu 3 O 7 (YBCO) film at near 77 K for the Large-scale Cryogenic Gravitational-wave Telescope (LCGT) project. An actuator consisting of both a YBCO film of 1.6 μm thickness and 0.81 cm 2 area and a solenoid coil exerted a force of up to 0.2 mN on a test mass. The presented actuator system can be used to displace the mirror of LCGT for fringe lock of the interferometer.