Development of a Vibration-Free Sorption-based J-T Cryochain for the Gravitational Wave Detectors of ETpathfinder

Future gravitational-wave observatories such as the Einstein Telescope require cryogenic operation in the 10 to 20 K range with ultra-low vibration. This thesis develops a sorption-driven Joule-Thomson cooling chain for the pilot ETpathfinder and advances the underlying sorption-compressor technology while establishing quantitative methods to assess fluid-induced vibrations in JT cold-stage components.
At the system level, a three-stage sorption JT cryochain concept is put forward with neon at 40 K, hydrogen at 15 K, and helium at 8 K. Operating conditions are selected by thermodynamic optimization to maximize the coefficient of performance. A one-dimensional finite-volume sorption-compressor model is used to size the compressor unit, yielding a baseline near 364 W input power with twenty-six sorption cells that meet the ETpathfinder cooling targets.
At the component level, a multi-physics finite-element framework is developed for sorption compressor cells. The modeling framework couples the modified Dubinin-Astakhov adsorption model with Brinkman porous flow and the conservation of mass and energy. Three spatial representations of a legacy central-heater cell design are implemented and show satisfactory agreement with experimental data. The framework extends naturally to accommodate multi-heater cell concept. To manage the more involved operation of multi-heater cells and their design, a global optimization workflow that combines a Genetic Algorithm with a Random Forest regressor is developed to explore the design space efficiently. The resulting cylindrical multi-heater designs improve performance and increase design flexibility. Flatter cell geometries are examined as well. A comparative analysis between an obround multi-heater cell and the baseline cylindrical design shows size reductions up to forty percent while maintaining performance.
A prototype cylindrical neon multi-heater sorption-compressor cell is fabricated and characterized near 70 K. Measured pressures, temperatures, and mass-flow-derived figures of merit confirm the predicted improvements over a single-heater design across relevant operating settings.
Fluid-induced vibrations are quantified through complementary numerical and experimental pathways. Fluid-structure-interaction simulations are conducted and successfully predict displacement spectra for representative JT components. Room-temperature with an atomic force microscope are performed, to measure nanoscale vibrations on a U-shaped tube and on two JT valve types, a porous plug and an orifice plate.
Overall, this work establishes a coherent framework for ultra-low-vibration cryogenic cooling that links system design, advanced modeling with optimization, and quantitative vibration metrology. The results provide validated tools and practical guidelines for deploying sorption-based JT coolers in precision experiments.