Gravitational-wave detectors cooled with superfluid helium – GRAVITHELIUM

The Einstein Telescope (ET) is a third generation gravitational-wave detector planned in Europe, combining a low-frequency (LF) and a high-frequency (HF) laser interferometer. Cryogenic operation of ET-LF in the temperature range of 10 K to 20 K is essential to suppress the fundamental suspension thermal noise (STN) that dominates the detection sensitivity at frequencies below 10 Hz down to the limitations of Newtonian noise. This requires a new key technology development in ultra-low noise cryogenic cooling.
Motivated by both the quietness and the heat conductivity of superfluid helium, a promising solution relies on a thin-wall titanium tube in the super-attenuator suspension chain of the core optics, filled with static He-II. The theoretical feasibility regarding the ET total noise curve has been demonstrated already. Open questions remain, however, regarding the noise of the thermal transport and the dissipation of vibrational energy at the liquid-tube interface or in the bulk phase of the superfluid.
The ERC project GRAVITHELIUM addresses these questions experimentally, using mechanical Q-factor measurements to analyze the dissipative mechanisms. ... mehrQ-factors are measured by the ring-down method, exciting suspension samples to resonant vibrations and analyzing the decay time. The new test facility enables full-size studies with various suspension materials and geometries, including silicon and sapphire suspensions as well as empty and He-II-filled titanium suspension tubes. The test facility also includes the development of a lab-scale low-noise He-II supply unit that includes an attenuation system and provides 400 mW of cooling capacity at 1.8 K.
Starting from the application requirements in the Einstein Telescope, the presentation describes the objectives, the methods and the status of the ERC project GRAVITHELIUM.