Key challenges in a nutshell
- Mirror cooled at cryogenic temperature almost in absence of mechanical contact: mirror suspended in UHV environment via four thin and long fibers
- Large suppression of cryogenic plant vibration noise
- Reducing cooldown time for higher duty cycle of the telescope
- Manufacturing silicon monocrystalline fibers
- Management of ice film formation on the mirror surface
Short description of the technology
The Einstein Telescope (ET) will feature three detectors with core optics operating at 10K. For this reason, 12 large volume (10m3) UHV cryostats will be realized, each of them surrounding a cryogenic payload consisting of a mirror (200kg mass, 500-mm diameter monocrystalline silicon substrate) and its suspension and steering mechanics.
After the initial cooldown, the operating temperature of the payload must be maintained only via radiation and conduction through very low stiffness thermal straps (heat links), to ensure that the telescope performance is not affected by vibrations from the cryogenic plant. In this scenario, availability of ultralow noise cryocoolers and active vibration isolators compatible with the UHV low temperature environment is essential.
The low temperature mirrors will be suspended by means of monocrystalline silicon fibers 1-m long and a few mm thick. Manufacturing technology of such those fibers is at very early stage and requires a strong R&D program and support from industry to reach the readiness needed for the success of the project.
State of the Art: technology in existing gravitational wave detectors / TRL
KAGRA, the Japanese gravitational waves telescope is now in commissioning phase and it is pioneering, on a smaller scale and with relaxed requirements, the use of cryogenic optics. In KAGRA, the four main mirrors of the detector, 22kg each made out of single crystal sapphire, operate steadily around 20K, with the 4W heat load extracted by means of the monocrystalline sapphire thin rods that suspend them. Cooling is provided by pulse-tube cryo-coolers, weakly linked to the mirror suspension mechanics by means of braids of ultra-pure aluminum thin wires to reduce coupling of vibrations.
Intended use in the frame of the Einstein Telescope
Observation of gravitational waves is only possible if the acceleration of the mirrors is reduced by billions of times compared to the quietest research laboratories.
Though this outstanding figures have been achieved at room temperature in the current detectors, cryogenic operation brings in new challenges and the control/management of introduced vibrations is crucial. The foreseen strategy for the refrigeration of the mirrors is to combine ultra low vibration cryocoolers, active vibration isolation of the cryocooler cold head and low stiffness heat links to connect it to the cryogenic payload.
Single-crystal silicon fibers have been chosen for the ET core optics suspension since 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 that is crucial to reach the scientific goals of the project.
Improvements needed: Technological challenge for the Einstein Telescope
- sapphire fibers used in KAGRA are not ideal for ET since the thermal conductivity of the material drops significantly below 20K. Silicon is better material in terms of thermal properties and mechanical damping figures. Manufacturing is at the moment the main challenge towards a practical implementation.
- cryogenic plant induced noise figures achieved in KAGRA are not compatible with the Einstein Telescope requirements; quieter cryocoolers and better performing vibration isolation for the heat links are needed.
- the ET cryogenic payload will add up to a few hundred kg mass (nearly a factor of 10 larger than in KAGRA) to be cooled down to 10K. Methods and technologies (high emissivity coatings, high efficiency radiation heat exchangers, etc.) for minimizing the cooldown time are of paramount importance for ensuring the telescope a high duty cycle.
- standard super-insulation (MLI or similar) is not compatible with the mirror environment due to contamination requirements. Alternative design solution must be explored.
- ice thin layer formation (nanometer level) on the mirror surface is expected to degrade the telescope performance. Methods and strategies for minimizing the ice formation and for periodical regeneration of the surface must be developed.
- interest is growing around an alternative solution, more projected towards the future, in which the cooling power is provided by means of superfluid He. R&D activities are now starting in this direction.
Economic perspectives of participation beyond the ET applications
ET represents the most extreme test bed for advanced ultra low noise cryogenics.
ET-proof innovation in this field can be transferred, among others, to the realm of quantum computers in which excess vibrations from the cooling system can cause decoherence of qubits and disrupt ongoing processes.
On the other side, the development of manufacturing methods of high quality silicon monocrystalline fibers can be attractive for fiber-optic communication at THz wavelengths where silicon has been identified as the ideal core material for its unique low attenuation properties.
Related projects and labs
ETpathfinder: INTERREG Vlaanderen/NL funded project. Location: Maastricht NL. Permament R&D large infrastructure for gravitational waves observatories. Principal investigator: Prof. Stefan Hild (University of Maastricht).
E-TEST: INTERREG EMR funded project. Location: CSL Liège BE. Principal investigator: Prof. Christophe Collette (University of Liège).
DBHC: project funded by Dutch NWA. In the framework of the project a cryogenic test facility to investigate coating thermal noise of silicon optics will be realized. Contact person: Dr. Alessandro Bertolini (Nikhef).
Ongoing and future procurements
Not at the moment. Collaboration and support from companies in the following areas is highly desirable:
- active vibration isolation at cryogenic temperatures
- ultralow noise cryocoolers
- production of monocrystalline silicon fibers