Monte Carlo simulation of the thermal radiation heat load to the cryogenic mirror and vacuum system of the Einstein Telescope

Abstract A third-generation underground gravitational wave (GW) observatory, known as the Einstein Telescope (ET), will be developed by European countries together. It is designed as an equilateral triangle with 10 km long arms, 200 to 300 meters underneath the ground, and with detectors being located in each corner. Any two adjacent arms comprise two independent interferometers, of which one interferometer will detect low-frequency gravitational wave signals (ET-LF), while the other will be optimized for operation at higher frequencies (ET-HF). In order to reduce seismic noise, thermal noise and other systematic noise, the beamline pipes require ultra-high or high vacuum conditions. In addition, the mirrors of ET-LF will be cooled to cryogenic temperatures below 20 K. Obviously, the mirror at this temperature will adsorb gases as a frost layer, degrading its optical properties and increasing laser power absorption. In this study, systematic Monte Carlo simulations are used to assist the development of the cryopump concept to fulfil the requirements regarding to pressure and frost mitigation aspects of ET-LF, and obtain the radiation heat loads for the corresponding cryogenic supplies at different temperatures, which are important information for the planning of the cryogenic infrastructure.