Key challenges in a nutshell

  • Can a properly coated mild-steel solution be found as economic base material for the Einstein Telescope UHV vacuum tubes?
  • Are composites a cost-effective alternative for stainless steel UHV vacuum tubes?
  • Can a cost-effective procurement and installation scenario be found for the Einstein Telescope UHV vacuum tubes?
  • Can synergies be identified with other ‘large tube’ projects such as in areas such as: oil & gas, offshore, windmills, hyperloop, hydrogen fuel cells

Short description of the technology

The future Einstein Telescope is a gravitational-wave observatory that relies on the measurement of minute relative length differences between the two kilometre-scale long arms of a laser interferometer. An essential system herein is the huge UHV (about 1010 mbar pressure) vacuum system: many tens of kilometers 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€.

State of the Art: technology in existing gravitational wave detectors / TRL

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 400oC 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 170 oC (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.

Intended use in the frame of the Einstein Telescope

Improvements needed: Technological challenge for the Einstein Telescope

A straightforward extrapolation of the LIGO and Virgo vacuum system would be very expensive because of the base material (stainless steel) as well as the underground installation costs.

The main challenge here is to reduce costs. We are thinking of several ways to accomplish this:

  • Production of the UHV vacuum tube segments in a dedicated local production facility in or near the Einstein Telescope site. This will warrant overall operational performance an allow inevitable repair interventions.
  • Use alternatives for the ‘classic’ stainless steel tubes. For example mild steel, nested tubes or corrugated thin-walled stainless steel vacuum tubes. This technology may already be available in industry (oil & gas, offshore, windmill, hyperloop, hydrogen fuel cells). Institutes like CERN could also contribute to solutions. And corrugated thin-walled stainless steel tubes are already used by the GEO600 project in Hannover. A major challenge for the nested tubes concept will be its robustness in view of a leak in the outer shell risking the disastrous collapse of the thin (stainless steel) inner lining.
  • Reducing the costs of the UHV-vacuum related instrumentation (pumps, valves, bellows and diagnostic tools like residual gas analysers, all should operate with minimum vibration to maintain the Einstein Telescopes sensitivity).

Economic perspectives of participation beyond the ET applications

Related projects and labs

The UHV vacuum tubes can be tested and developed in the R&D-lab ETpathfinder in Maastricht. Nikhef as well as CERN-facilities can be used for characterization and diagnostic purposes.

Ongoing and future procurements

In the coming years, the ETpathfinder/E-TEST consortia will post many tenders, including tenders for the UHV vacuum system. In some areas, co-development will be crucial.

Technology contact

Jo van den Brandt
VU Universiteit Amsterdam / Nikhef –

Business Development contacts

Peter Gier

Dirk Kalinowski

Michel Stassart
Skywin BE –

Maxime Corvilain
POM Limburg BE –

René Kessen