Cryogenics

After the initial cryostat cooling, the operational temperature of the payload can only be maintained in two ways. The first option is via infrared radiation. The second option is conduction via thermal wires (heat links) with very low stiffness. The low stiffness is needed to ensure that the telescope performance is not affected by vibrations from the cryogenic system. In this scenario, it is essential to use ultra- low noise cryocoolers and active vibration isolators that can continue to operate at the low temperature and under the high vacuum.

The low temperature mirrors will be suspended by means of monocrystalline silicon wires that are about one meter long and only a few millimeters thick. The production technology of such wires is still in its early stages and requires a strong research and development program. The support of the industry is needed to make this development successful.

The observation of gravitational waves is only possible if the vibrations in the mirrors are reduced by billions of times compared to the quietest research laboratories in the world.

These low values have already been achieved at room temperature in current gravity detectors. The need to work at cryogenic temperatures brings new challenges. The machines that have to provide cooling in a closed system add extra vibration noise to the environment. Proper control of introduced vibrations is crucial.

The envisioned strategy for cooling the mirrors is to combine ultra-low vibration cryocoolers , active vibration isolation of the cooled mirrors, and the use of low-stiffness thermal wires from which the cryogenic mirrors are suspended.

The suspension of the ET core optics must take place at 10K. At these temperatures, the suspension wires must transport heat from the mirrors to the environment with high efficiency. They must also provide the best possible mechanical damping to achieve the scientific goals of the project. The chosen monocrystalline silicon still has the necessary strength and is able to dissipate heat at these cryogenic temperatures.

The KAGRA gravitational wave detector in Japan also operates at cryogenic temperatures. The measurements made at the KAGRA detector are therefore a good basis for dimensioning the cryogenic instruments of the Einstein Telescope.

KAGRA uses sapphire wires for the suspension of the mirrors, but for the ET these are not ideal. The thermal conductivity of sapphire drops drastically at temperatures below 20K (-253°C). Silicon appears to be a better material in terms of thermal properties and mechanical damping. The biggest problem then is to be able to manufacture the silicon wires.

The conclusion is that the machines that have to provide the cryogenic temperatures induce noise in the suspension wires. This noise is then visible in the measurements. The noise level measured in the KAGRA detector is too high for the Einstein Telescope . This means that new types of cryocoolers have to be designed, which cause less noise. Furthermore, better vibration isolation will be necessary for all connections from the cooling installations to the ET.

The cryogenic payload of the ET can weigh up to several hundred kg. That is up to a factor of ten more than for KAGRA, so the cooling system to bring the environment to 10K must also be heavier. The technologies that are needed to guarantee the cooling in the shortest possible time will be extremely important. Think of special coatings that must provide the infrared heat radiation at cryogenic temperatures, heat exchangers with very high efficiency, etc. The requirement is that a very high uptime must be guaranteed for a long period.

The commonly used Multi- Layer Insulation (MLI) materials for cryogenic environments (so-called superinsulators) use layers of metal film and plastic. These are not suitable for the Einstein Telescope because of the risk of contamination in the ultra-high vacuum. Plastics can release substances under vacuum that can deposit on the mirrors. This means that alternative designs and materials must be found.

Experience has shown that water can still leak from the stainless steel under the ultra-high vacuum. These are extremely small quantities, but there is still a serious risk that the water molecules will form a nanometer-thick layer of ice on the mirrors. Even that layer of a few molecules can seriously disrupt the performance of the Einstein Telescope . New methods must be found to prevent the formation of ice, and possibly also to regularly clear the surface of the mirrors.

There is increasing interest in performing cooling using superfluid helium. The first research activities have already started.

ET represents the most extreme testbed for advanced cryogenics where ultra-low noise is necessary. Innovations made for the Einstein Telescope can be used for example in the domain of quantum computers , where excessive vibrations of the cooling system can cause decoherence of qubits and disrupt ongoing processes.

In addition, the development of production methods for high-performance monocrystalline silicon fibers may be attractive for fiber-based optical communications at THz wavelengths, where silicon has been identified as the ideal core material due to its unique low attenuation properties.