Study of a cryogenic payload for the Einstein Telescope Low Frequency

The advent of Gravitational Wave (GW) astronomy has opened unprecedented opportunities for exploring fundamental physics, necessitating the development of next-generation detectors with enhanced sensitivity and extended frequency coverage. Among these, the Einstein Telescope (ET) represents Europe’s effort toward a third-generation ground-based GW observatory, featuring a hybrid design optimized for both high-frequency and low-frequency detection, with the latter operating at cryogenic temperatures down to 2Hz. This thesis presents the research and development activities conducted at the ARC-ETCRYO laboratory in Rome, which serves as a testbed for the cryogenic payload of the ET Low Frequency (ET-LF). The work focuses on the thermal characterization of the prototype payload and its cryostat, combining theoretical modeling, experimental measurements, and numerical simulations to evaluate the thermal behavior, cooling performance, and associated thermal noise. Material properties were experimentally determined as the basis of the thermal models, and the performance of innovative design features, such as suspended thermal connections, high-emissivity surface coatings, and Rigid Multi-Layer (RML), was assessed. Steady-state and transient simulations of the facility reveal cooling dynamics, equilibrium temperatures, and the influence of design parameters on the thermal noise budget. The insights gained from ARC-ETCRYO were directly applied to the baseline ETLF cryogenic payload design, allowing evaluation of the trade-offs between thermal performance, mechanical isolation, and cooling time. The study also addresses residual vibrations by developing a cryogenic geophone capable of reliable low-temperature operation and calibrated across a broad temperature range, making it suitable for monitoring environmental motion relevant to GW detection. Overall, this work contributes to the technological readiness of the ET-LF cryogenic infrastructure by providing validated thermal models, experimental characterizations, and design optimizations, ready for upcoming experimental validations.