Studies on the cooling systems of the Einstein Telescope cryogenic payloads

Einstein Telescope (ET) is a third-generation gravitational wave detector that will cover a wide spectrum of frequencies measurable on the Earth's surface, paying specific attention to low frequencies from a few Hz up to 10 Hz (ET-LF). At these frequencies, to achieve a sensitivity at least ten times better than that of current detectors, ET will have to develop innovative technologies that will make the reduction of noise sources possible, especially those of a thermal nature. Therefore, it will be necessary to cool the ET main optics and their suspensions (payload) at cryogenic temperature (10-20 K). This will require the development of new cooling techniques, low vibration noise of the cryogenic system and the study of high thermal conductivity and low mechanical dissipation materials. It is extremely important to not only ensure an efficient thermal connection between the payload and the cooling system, but also preserve the mechanical isolation and, therefore, avoid introducing excessive vibrations to the entire system. Here some preliminary studies carried on how to test cryogenic payloads using a test cryostat cooled by means of pulse tube refrigerators and how to cool the payload using superfluid helium-4 are presented.