The technology domains

The construction and operation of ET will require many advances in research and development and new technological developments. Many of the innovations needed will build on research already conducted at LIGO (United States), VIRGO (Italy), KAGRA (Japan) and other similar facilities. 

For the actual construction of the Einstein Telescope (in particular the instrumentation), five technology domains have been defined that will be further developed through research and innovation. The technology domains are:

  1. Vibration-Free Cooling: development of vibration-free cooling of mirrors to a temperature of 10-20 K.
  2. Vacuum technology: vacuum system cost savings and design of production facility and installation scenario.
  3. Vibration damping: development of optimal combination of passive and active vibration damping.
  4. Optics: development of large Si mirrors and coating for application at 10-20 K temperature.
  5. Thermal deformations: development of technology to monitor and compensate for thermally induced deformations.


Vibration-free cooling

Development of vibration-free cooling of mirrors to temperature 10-20 K

Observation of gravitational waves is only possible if the acceleration of mirrors is reduced billions of times compared to the quietest research laboratories.

Although this excellent performance has been achieved with current detectors at room temperature, operation under cryogenic temperatures presents new challenges and control of introduced vibrations is critical. The planned strategy for mirror cooling is to combine ultra-low vibration cryo-coolers, active vibration isolation of the cold head of the cryo-cooler, and low stiffness cold transmissions to connect it to the cryogenic payload.

A mono-crystalline silicon fiber was chosen for the suspension of the ET core optics because this material provides the best performance in terms of high-efficiency heat extraction from the mirror at 10K and the lowest possible mechanical damping, a property crucial to achieving the scientific goals of the project.


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Vacuum technology

Vacuum system cost savings and design of production facility and installation scenario


More specifically, this is about the arm vacuum system that includes about 120 km of 1 m diameter UHV pipes. The system must function for at least 50 years and require low maintenance.

In order not to disturb the laser beams repeatedly reflecting between the test masses (mirrors used to detect the passage of a gravitational wave), the laser beams must be in a UHV (Ultra High Vacuum) system.

Current detectors use standard stainless steel (AISl304L) and 3-4 km long, 0.7-0.9 m diameter and 3-4 mm wall thickness long UHV tubes for the arms of the interferometer (= 'arm-vacuum system') with a volume of several 1000 m3, set at 1.E-8 mbar.


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If we apply this technology to the Einstein Telescope (ET) - which requires about a hundred times larger volume and a much lower residual vacuum pressure of 1.E-10 mbar - it will result in an expensive arm vacuum system.

In view of these potentially high costs and also in an effort to avoid or at least simplify in-situ burnout operations, alternative - particularly more cost-effective - concepts for ET's arm vacuum system should be studied.

Given the huge system, an on-site production plant, also minimizing logistics, seems desirable. Similarly, the production and underground installation scheme should be optimized together.


Vibration damping

Development of optimal combination of passive and active vibration damping

Ground vibrations are a major source of noise in laser interferometer gravity wave detectors located at or in the Earth's surface. Seismic environmental noise, caused by natural micro-seismicity and human activities, is thought to cause displacements of the interferometer optics ten orders of magnitude greater than the effect expected from gravitational wave signals. For this reason, seismic isolation systems are used as a mechanical interface between the environment and the detector optics.

Seismic isolators are Ultra-High Vacuum (UHV)-compatible complex mechatronic systems that perform important functions.

At the Einstein Telescope, extending the observing band of gravitational waves to 3 Hz poses serious technological challenges by requiring a performance improvement of several orders of magnitude over state-of-the-art low-frequency vibration isolation techniques.


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Development and demonstration of large Si mirrors and coatings for application at 10-20K

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One of the unique features of the Einstein Telescope is the use of cryogenically cooled mirrors to significantly improve the detection rate for low gravitational wave frequencies (below 20 Hz) by reducing thermal noise in the mirror coatings and suspensions in particular. Crystalline silicon, a well-known material from the semiconductor industry, is ideal for this purpose.

Regarding the crystalline silicon substrate or block, the bulk absorption of the laser light heating the mirrors must be kept below about 5 ppm/cm in order to maintain the mirrors, especially at 20 K operation, at a stable cryogenic temperature. 


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The polishing specifications of the silicon will be similar to the very demanding requirements currently met by fused silica for the LIGO and Virgo observatories.

Polishing processes that can yield the same specifications on silicon substrates will need to be investigated and their quality proven by appropriate metrology, including parameters such as transmitted wavefront distortion and absorption. Given the different wavelengths and given the operation at cryogenic temperature, entirely new multilayer coatings must be developed. This is probably one of the most challenging issues related to mirrors and a current area of research in the global gravitational wave research community.


Thermal deformations

Development of technology to monitor and compensate for thermally induced deformations

Gravity wave detectors use powerful laser beams to measure the kilometer-long distance between super-fine polished mirrors. The interferometric process compares two beams for a differential measurement. For the interferometer to achieve the extreme sensitivity expected for ET, optical losses in the system must reach levels of tens of ppm, and the wavefront of the main laser must remain undistorted to achieve contrast defects at a similar level.

Parts of the optical system in ET will maintain several MW (Megawatts) of continuous laser power. Residual absorption causes the optical test masses to heat up and mechanically deform, which in turn leads to wavefront distortions and increased optical loss due to scattering. To reduce this effect, we provide a closed-loop feedback system, which continuously measures the quality of the optical beam and uses non-contact activation to correct the distortion of the test masses.


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