Cryogenics and Einstein Telescope

Abstract

The dominant noises which limit the present sensitivity of the gravitational wave detectors are the thermal noise of the suspended mirrors and the shot noise. For the third generation of gravitational wave detectors as the Einstein Telescope (ET), the reduction of the shot noise implies to increase the power stored in the detector at 1 MW level and, at the same time, to compensate the huge optic distortion due to induced thermal lensing. At low temperature it is possible to reduce both these effects. However, lowering the temperature of the test masses without injecting vibration noise from the cooling system is a technological challenge. We review here the thermal noise impact on the ultimate ET sensitivity limit and we discuss possible cryogenic configurations to cool the mirror

Sensitivity studies for third-generation gravitational wave observatories

“…When operating the low-frequency interferometer at cryogenic temperatures the last stage suspension does not only need to be compliant with the thermalnoise requirements, but the actual design is also driven by the requirement to extract any heat (deposited by the laser beams in the test masses) via the suspension. Our model [34] assumes a mirror temperature of 10 K, silicon fibers of 2 m length and 3 mm diameter as well as a temperature of the penultimate mass of 2 K. The right-hand plot of figure 2 shows the simulated suspension thermal-noise contribution for the ET low-frequency detector [34] using a branched system of multiple oscillators consisting of the main mirror, the penultimate mass and the reaction mass [32,33].…”

Toward a third generation of gravitational wave observatories

“…New technologies are now available, for example based on cryo-cooling systems [37] that are actively damped to reduce the seismic vibration and that promise an easier achievement of low temperatures with less impact on the low frequency performance of the interferometer than previous cryo-coolers. Details of these technologies are reported in another article of this special issue edition [38], where the scheme of a vibration free cryostat is reported. A special design of the suspension system is needed to cool down the test mass without introducing additional vibration.…”

“…2.2.2 and [38]) will require to select a new material for the test masses. The physical properties of Silicon make it a good candidate.…”

Challenges in thermal noise for 3rd generation of gravitational wave detectors

“…Thus, the design of a 3rd generation detector is likely to need to incorporate new approaches to overcome the limitations by the currently dominating noise sources and technical limits. These include reconsideration of the optical layout of the detector, the operating temperature, the material selection for the test mass material and the coatings, and the investigation of alternative laser wavelengths if the detector needs to be operated at cryogenic temperatures [85].…”