Calculation of thermal radiation input via funneling through a duct shield with baffles for KAGRA

Many studies worldwide are currently developing interferometric methods for detecting gravitational waves and one of the challenges in such methods has been to sufficiently cool the mirror in interferometric detectors in order to reduce thermal noise. Although the mirror is surrounded by a radiation shield, a hole in the shield is necessary to allow the laser beam to pass. To reduce the thermal radiation caused by the presence of the hole, we will install a duct shield with baffles. To calculate the heat input through the duct shield, we applied a ray trace model whose results were consistent with those of an experiment without baffles. As an application of our model, the heat input in the case of KAGRA (a Japanese cryogenic gravitational wave detector project) was calculated. Our analysis suggests that baffles in the duct shield can considerably reduce the heat input in KAGRA.