  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.
