Cryogenic developments towards the Einstein Telescope (ET)

The Einstein Telescope (ET) is a 3rd generation gravitational-wave detector planned in triangular shape with 10 km arm lengths in an underground installation at a depth of 200 m to 300 m. While 2nd generation detectors and the ET high-frequency interferometer are operated at room-temperature, the ET low-frequency (LF) interferometer shall be operated at cryogenic mirror temperatures of 10 K to 20 K, extending the frequency range of ET down to 3 Hz and opening a new window to precision astronomy. The cryogenic operation is required to reduce the Brownian motion of atoms in the mirrors and suspensions that limits the achievable sensitivity. The mirror cooling, however, needs to be performed in an ultra-quiet manner in order to ensure a mirror stability on the level of $1\times10^{-20}\,\mathrm{m/\sqrt{Hz}}$. Beside the challenging mirror cooling, cryogenic developments are required for cryopumping in the cryostats, in particular for the mitigation and the regeneration of frost formation on the mirror surfaces, and for the cooling of large cryo-traps between the 10 km arm pipe ends and the cryostats. Starting with an overview on the ET objectives, design and project schedule, this talk summarises the cryogenic requirements of this large-scale future experiment. Present developments for the ET-LF mirror cooling and the cryo-vacuum technology are being presented