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Abstract The Einstein Telescope (ET), the planned gravitational-wave observatory for Europe, will increase the sensitivity and expand the observation band to lower frequencies. The main optics of the ET-Low Frequency (LF) interferometer will be cooled to cryogenic temperatures below 20 K and the whole system, consisting of the beam pipes, the suspension towers and the cryostat containing the mirror, requires high to ultra-high vacuum conditions. To fulfill the vacuum related requirements the use of tailor-made in-situ cryopumps is envisaged. Simulations at KIT, performed with the in-house Test Particle Monte Carlo code ProVac3D and a simplified model of the system, considered the three main gas sources from the neighbouring systems and the sinks by pumping stations distributed along the pipe arms, the cryogenic pump sections close to the mirror and the cryogenic mirror environment. These simulations showed different needs for the pumping of hydrogen (to lower the residual pressures) or heavier species like water (to lower the frost formation on the cryogenic mirror surface) – depending on the position related to the mirror. With these findings, the development of a pump arrangement concept was worked out. In parallel, the outgassing rates of the inner walls of the beam pipe tubes, influencing strongly the vacuum pumping demands, have been varied to investigate the potential use of mild steel as a beam pipe material of reduced costs. This paper describes the developed pump arrangement concept, utilizing cryogenic pumps integrated into the beam pipe tubes of 1 m diameter, with their individual objectives regarding pumped species and needed temperatures. Furthermore, the key parameter of frost formation on the cryogenic mirror, important for long operational phases without pause and maintenance, being also a design driving demand, is derived from the predicted residual pressure of sticky gases like water.
Hanke et al. (2024) studied this question.