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We report the mirror suspension design for large-scale cryogenic gravitational wave telescope, KAGRA, during bKAGRA phase 1. Mirror thermal noise is one of the fundamental noises for room-temperature gravitational-wave detectors such as Advanced LIGO and Advanced Virgo. Thus, reduction of thermal noise is required for further improvement of their sensitivity. One effective approach for reducing thermal noise is to cool the mirrors. There are many technical challenges that must be overcome to cool the mirrors, such as cryocooler induced vibrations, thermal drift in suspensions, and reduction in duty cycling due to the increased number of potential failure mechanisms. Our mirror suspension has a black coating that makes radiative cooling more efficient. For conduction cooling, we developed ultra high purity aluminum heat links, which yield high thermal conductivity while keeping the spring constant sufficiently small. A unique inclination adjustment system, called moving mass, is used for aligning the mirror orientation in pitch. Photo-reflective displacement sensors, which have a large range, are installed for damping control on marionette recoil mass and intermediate recoil mass. Samarium cobalt magnets are used for coil-magnet actuators to prevent significant change of magnetism between room temperature and cryogenic temperature. In this paper, the design of our first cryogenic payload and its performance during bKAGRA phase 1 are discussed. [ABSTRACT FROM AUTHOR]

We are trying to make a hardware logic-circuit for pipelines of fast Fourier transformation (FFT) with a field programmable gate array (FPGA) for data analyses of an interferometric gravitational-wave detector. That FFT processor is connected to a personal computer (PC) through PCI bus and will increase the calculation speed of FFT which is the most time-consuming step for typical gravitational-wave analyses. [ABSTRACT FROM AUTHOR]

Thermal conductance and light scattering were measured for two bonded sapphire samples. A sample with adhesion free bonding (AFB) showed the same thermal conductivity as that of the bulk at cryogenic temperatures. In contrast, a finite thermal resistance was observed in the sample with hydroxide-catalysis bonding (HCB). Light scattering at λ = 488 nm from bonded region was about 10% larger than the bulk part and was about 800 ppm of an incident laser power for AFB sample. [ABSTRACT FROM AUTHOR]

The thermal fluctuation of mirror surfaces is the fundamental limitation for interferometric gravitational wave (GW) detectors. Here, we experimentally demonstrate for the first time a reduction in a mirror's thermal fluctuation in a GW detector with sapphire mirrors from the Cryogenic Laser Interferometer Observatory at 17 and 18 K. The detector sensitivity, which was limited by the mirror's thermal fluctuation at room temperature, was improved in the frequency range of 90 to 240 Hz by cooling the mirrors. The improved sensitivity reached a maximum of 2.2 × 10-19 m/√Hz at 165 Hz. [ABSTRACT FROM AUTHOR]