Masao Tokunari

The CLIO project

The CLIO project including a 100 m baseline cryogenic gravitational wave laser interferometer and a 100 m baseline geophysical strain meter was conducted in the Kamioka mine in Japan to investigate the technical feasibility of the large-scale cryogenic gravitational wave telescope (LCGT), which is planned to be constructed in the same Kamioka mine with 30 times longer baseline than CLIO, and to demonstrate the collaborative operation between these instruments about long-term continuous operation and gravitational wave signal veto analysis. About the cryogenic gravitational wave interferometer, the whole vacuum system and four cryostats, which house and cool sapphire mirrors, were constructed, and the required vacuum level of 10 −6 mbar and the temperature of 8 K at the inner radiation shield in the cryostat were achieved. About the geophysical strain meter, the obtained geophysical strain in the Kamioka mine was successfully simulated with a finite element model with a good agreement with less than 5% error. The strain meter also verified a permanent ground step change of micrometre order due to some earthquakes. We present the recent progress about the CLIO project.

Conduction Effect of Thermal Radiation in a Metal Shield Pipe in a Cryostat for a Cryogenic Interferometric Gravitational Wave Detector

A large heat load caused by thermal radiation through a metal shield pipe was observed in a cooling test of a cryostat for a prototype of a cryogenic interferometric gravitational wave detector. The heat load was approximately 1000 times larger than the value calculated by the Stefan-Boltzmann law. We studied this phenomenon by simulation and experiment and found that it was caused by the conduction of thermal radiation in a metal shield pipe.

Cryogenic systems of the Cryogenic Laser Interferometer Observatory

Cryogenic Laser Interferometer Observatory (CLIO) is a laser interferometric gravitational wave detector using cryogenic cooled mirrors. In order to cool the mirrors, cryogenic environment is necessary. We made four vacuum chambers with cryogenic cooled shields inside. The mirror is suspended by a mirror suspension system with a heat path for transferring heat from the mirror to the shield. Test cooling of the chambers and the mirror suspension system has been done. After one week cooling, the chambers was cooled from 8K to 10K and the mirror were cooled at 21K successfully.

Development of an automatic birefringence measuring device of mirror substrates for gravitational wave detectors

We developed an automatic measuring device of birefringence inhomogeneity in synthetic sapphire substrates to evaluate their crystal quality suitable for laser interferometric gravitational wave (GW) detectors. The orientation of the projection of the c-axis on a plane orthogonal to the beam was measured with an accuracy of 2 ? 10?2 rad. The phase retardation was measured with an accuracy of 7 ? 10?4 rad, which was equivalent to 4 ? 10?10 in terms of the fluctuation of the refractive index for samples of 150mm thickness. The reproducibility was 1% for both the orientation and phase retardation. The automatic measuring device meets the requirements of the LCGT project, which is a next-generation laser interferometer project for GW detection.

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.

Rayleigh scattering, absorption, and birefringence of large-size bulk single-crystal sapphire

While the thermomechanical properties of sapphire make it an excellent candidate of test mass for advanced laser interferometers, its optical quality is not well understood or well controlled. We have studied the results from high-resolution measurements of scattering, absorption, and birefringence in test-mass samples to better understand issues of quality. Samples show large-scale scattering structures clearly linked to the crystal-growth process. Samples characterized by the presence of point defects have significantly lower scattering (except at the point defects). In general on a large scale, high scattering also correlates with higher absorption and higher average birefringence inhomogeneity. However, on a smaller scale there is not a clear point-to-point correlation between scattering and absorption. Often a large-scale scattering structure is spatially displaced by tens of millimeters from a similar absorption structure, indicating that quite separate microscopic mechanisms give rise to scattering and absorption. The spatial displacements indicate that absorption centers and scattering centers are laid down during crystal growth at different distances from the solid-liquid interface. We suggest that absorption may be linked to F centers, while scattering may be linked to impurities such as iron.

Reduction of heat load of LCGT cryostat

An unexpected large heat load was observed in a cooling test of the cryostat for a prototype cryogenic interferometric gravitational wave detector. By conducting additional studies involving an experiment and a simulation, we found that the large heat load was caused by conduction of thermal radiation in a thermal radiation shield pipe, which was inserted in the beam duct to reduce solid angle from 300K to 4K. To achieve the design of LCGT cryogenic system, the heat load had to be reduced below a few percent. By introducing metal baffles in the shield pipe, this requirement was fulfilled.

DEVELOPMENT OF AN AUTOMATIC BIREFRINGENCE MEASURING DEVICE OF MIRROR SUBSTRATES FOR LCGT

We developed an automatic measuring device of birefringence inhomogeneity in synthetic sapphire substrates to evaluate their crystal quality suitable for laser interferometric gravitational wave (GW) detectors. The phase retardation was measured with an accuracy of 7×10−4 rad and the orientation of the fast axis with an accuracy of 2 × 10−2 rad. The automatic measuring device is useful to check the inhomogeneity of substrates for the LCGT project, which is a next-generation laser interferometer project for GW detection.