Vacuum

The future Einstein Telescope is a third-generation gravitational-wave observatory that relies on the measurement of minute relative length differences between the ten kilometre-scale long arms of a laser interferometer. An essential system herein is the huge UHV (about 10–10 mbar pressure) vacuum system: in the order of 130 kilometres of about 1 meter diameter UHV vacuum pipes to allow laser light beams to travel undisturbed between mirrors suspended from multiple coupled pendula to damp even the smallest external vibrations and housed in 10-20 meter high, 3-5 meter diameter UHV vacuum towers.

The installation of the UHV vacuum system for the Einstein Telescope will start around 2029 and be completed in 2032-2033. The estimated cost of the complete UHV vacuum system is 400-600 M€.

Present gravitational-wave observatories like LIGO (USA) and Virgo (Italy) use stainless steel (AISI304L) for their 3-4 kilometers long UHV vacuum tubes (3-4 mm wall thickness, 0.7-0.9 meter diameter) and their up to 12 meters high towers (10 mm wall thickness, few meters diameter). The vacuum tubes are welded from 10-20 meter long segments in situ with bellows to allow for (thermal) deformations during bake-out, with large vacuum valves to facilitate compartmentalized venting/access operations and with pumping stations typically every 500 meters. Near the vacuum towers, large cryotraps –to freeze out water vapor– are installed to improve the vacuum pressure in the long vacuum tubes to avoid disturbances of the laser beams. Prior to installation most components are baked-out at temperatures as high as 450^oC for one week to reduce notably hydrogen and hydro-carbons contaminations while in-situ the systems can in principle be baked-out to reduce residual water at temperatures up to 150 ^oC for ten days (to avoid damaging Viton O-rings and pumps). These systems and notably the long vacuum tubes that have proven to operate reliably at 10^-9 mbar pressure for extensive periods.