Conceptual design of a sorption-based cryochain for the ETpathfinder

Abstract

Next-generation gravitational wave detectors, including the Einstein Telescope [1][2], aim to achieve amplitude-spectral-density strain sensitivities on the order 10−24m/Hz[3]. In the low-frequency band such sensitivities can only be obtained when thermal noise, mainly stemming from the mirror coating, is reduced by employing cryogenic cooling techniques for the mirrors. The optical surface of the mirror should not vibrate with strain noise amplitude spectral densities above 10−20m/Hz for the cryogenic mirrors in the Einstein Telescope [3]. The ETpathfinder research facility [4][5], aims to facilitate the development and testing of critical new technologies required for the design and operation of future gravitational wave detectors. A key enabling technology for the design and operation of such advanced interferometers is the cryogenic system that cools the main optics to a temperature of approximately 10 K. Given the stringent requirements on vibrational noise for these optics, the cryogenic cooling under continuous operation should be essentially vibration free. Joule-Thomson cryocoolers using sorption compressors are known to generate an absolute minimum of vibrational noise during operation. We propose a modular cryochain design comprised of a system of sorption compressors and Joule-Thomson cold stages fitting the ETpathfinder project requirements. In this paper, we present the conceptual design of the cooler chain that is based on a parallel cascade arrangement of a 40 K neon stage, a 15 K hydrogen stage and a 8 K helium stage. The operating parameters of the sorption-based cooler chain are selected via a hybrid modeling workflow, aiming to optimize performance and other design considerations within an envelope of acceptable design parameters.