GRAVITHELIUM · Gravitational wave detectors cooled with superfluid helium

GRAVITHELIUM addresses the central technology challenge in third generation gravitational wave (3G GW) detectors, proposing to cool the core optics of cryogenic detectors with superfluid helium. The Einstein Telescope (ET) will be the future European 3G GW observatory, designed to observe the whole Universe. It has entered the European ESFRI Roadmap in 2021 and is supported by the major international roadmaps. ET includes new technological challenges on the verge of feasibility, particularly in the cryogenic low-frequency interferometer (ET-LF) that is crucial to exploit the full scientific potential. Cryogenic operation of the ET-LF payloads at 10 K to 20 K is indispensable to suppress the fundamental suspension thermal noise (STN) to the level of Newtonian noise, requiring new key technology developments in ultra-low noise cryogenic cooling, cryopumping and thermal shielding.GRAVITHELIUM aims for the experimental proof of a pioneering concept that proposes cryogenic payload suspensions filled with superfluid helium, as the quantum fluid He-II is the thermal reservoir that absorbs and conducts heat in the quietest possible manner. Motivated by the theoretical proof of concept, experiments on dissipative mechanisms and their STN contributions in He-II filled payload suspensions will be conducted. For this purpose, a new test facility for cryogenic Q-measurements will be build, which will also deliver data on dissipation in full-size solid-state suspensions at low temperature, needed in the GW detector community for the development of ET-LF payloads. The project further focuses on a new technology development for the attenuated and force-free supply of helium to the cryogenic payloads, cooperating with a world-leading industry partner. GRAVITHELIUM will thus achieve significant advancements in one of the key technologies to enable future frontier science with ET, providing also essential physical data for the modelling and engineering design of ET-LF payloads.