Vacuum systems
The future Einstein Telescope is a third-generation gravitational wave observatory that relies on measuring tiny relative length differences between the ten-kilometer-long arms of a laser interferometer. A key system here is the huge Ultra-High Vacuum (UHV) system with a pressure of the order of 10–10 mbar or less. Read more about the Vacuum Systems technology domain here.
About 130 kilometers of UHV vacuum tubes with a diameter of about 1 meter are needed to let laser light beams bounce back and forth undisturbed between the ET mirrors. The mirrors are suspended with multiple coupled vibration dampers to filter out even the smallest external vibrations. They will be housed in 10 to 20 meter high UHV vacuum towers with a diameter between 3 and 5 meters.
It is estimated that the installation of the UHV vacuum system for the Einstein Telescope will be completed in about four years. The estimated cost of the complete UHV vacuum system will then be €400 million to €600 million.
The existing gravitational wave observatories, such as LIGO (USA) and Virgo (Italy), use stainless steel (AISI304L) for their three to four kilometer long UHV vacuum tubes. The tubes have a wall thickness of 3 to 4 mm and a diameter of 0.7 to 0.9 meters. The vacuum towers that house the mirrors are twelve meters high and have a wall thickness of 10 mm and a diameter of several meters. The 10 to 20 meter long vacuum tubes are welded together on site. Bellows are placed between the segments to absorb (thermal) deformations during baking.
Large vacuum valves divide the detector into different compartments. These compartments simplify venting and access operations. Every 500 meters there are pumping installations to ensure the low pressure. The (approximately) last 100 meters of vacuum tube to each of the vacuum towers are cooled very deeply, even below the cryogenic temperature of the mirrors. They form large cryotraps in which possible water molecules can precipitate and thus serve to freeze out water vapor and improve the vacuum pressure in the long vacuum tubes. The cryotraps not only serve to prevent a layer of ice on the mirrors, but also prevent the laser beams from being disturbed by the minute water vapor.
Before being installed in the ET, the ultra-high vacuum components are baked out for a whole week at 450°C. In the baking process, residual hydrogen and hydrocarbons are removed as much as possible.
It cannot be avoided that water vapour enters the system again during and after installation. The mounted vacuum tubes must therefore be baked out again for ten days at 150°C, so that the Viton O-rings and pumps are not damaged. Experience shows that these systems, and certainly the long vacuum tubes, continue to operate reliably for long periods at a pressure of 10-9 mbar.
The logic would be to use a similar vacuum system and materials as LIGO or Virgo. However, the one-to-one extrapolation of the LIGO and Virgo vacuum system would be very expensive due to the base material (stainless steel) and the cost of the underground installation.
The main challenge is therefore to reduce costs. Several options are considered.
A first option is to produce the UHV vacuum tube segments locally in a dedicated local production facility on or near the Einstein Telescope site . This simplifies the operational process and ensures a better yield of the installations. It also simplifies foreseeable repair interventions in the future.
The use of alternative materials instead of the ‘classic’ stainless steel tubes. For example non-stainless steel, laminated tubes or the use of corrugated thin-walled stainless steel vacuum tubes. These technologies may already be available in the industry: oil & gas, offshore, wind turbines, hyperloop, hydrogen fuel cells …
Institutes such as CERN can also contribute to solutions. For example, corrugated thin-walled stainless steel tubes are already being used by the GEO600 project in Hannover. A major challenge for the laminated tube concept will be to ensure its robustness in the event of a leak in the outer shell, which could cause the catastrophic collapse of the thin (stainless steel) inner cladding.
Reduce the cost of UHV vacuum related instrumentation, such as pumps, valves, bellows and diagnostic tools such as residual gas analyzers. All of this equipment must operate with minimal vibration to maintain the sensitivity of the Einstein Telescope.
In-situ welding of the tubes where larger coils of metal are transported, folded and welded on site. Larger sections of vacuum tube are welded in one piece. This would avoid transportation costs for the many vacuum tubes and reduce the risk of errors when welding many shorter pieces of vacuum tube.
The new welding techniques are directly applicable in other areas where long pipes need to be installed. Think of mining, pipelines, offshore constructions, etc.
Using a single local welding station reduces the number of welds, so there is less risk of welding defects. It also allows longer sections of pipe to be created with minimal transportation.
The techniques developed to create UHV can find their way into industry where ultra-high vacuum or vacuum for large systems is required.