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%0 Thesis %A Kuhlbusch, Tim Johannes %T Cryogenic sensing and actuation techniques for the Einstein Telescope %I RWTH Aachen University %V Dissertation %C Aachen %M RWTH-2025-09249 %P 1 Online-Ressource : Illustrationen %D 2025 %Z Veröffentlicht auf dem Publikationsserver der RWTH Aachen University %Z Dissertation, RWTH Aachen University, 2025 %X Improving the sensitivity of gravitational wave detectors is essential to increase the observable fraction of our universe and to enable new observation possibilities. The low frequency region below 10 Hz is essential for many science cases like multi-messenger astronomy. Therefore, future gravitational wave detectors like the Einstein Telescope will require cryogenic mirrors and cryogenic suspension systems to decrease thermal noise. Many components of the mirror suspension systems must be improved to work across the temperature range from 10 to 300 K. Commercial availability of components specified for these low temperatures is scarce. This work explores the challenges of selecting components and evaluates possible solutions. Optical displacement sensors are required to monitor the movement of the suspension system, characterize mechanical properties, and derive feedback signals for the control systems. LEDs are used in many sensors of the current generation of gravitational wave detectors. They were successfully operated at cryogenic temperatures as part of this work but have limitations regarding efficiency, optical beam quality, and noise. Another option is to use optical fibers and collimators to make light from stabilized light sources accessible in the cryogenic environment. Photodiodes are the most common solution for reading out optical sensors. The challenges in selecting appropriate photodiodes are outlined in this thesis, and the selection of a photodiode for 1550nm is discussed. Presented measurements were used to select the optical components for sensors of a cryogenic suspension prototype developed in the E-TEST project. To control the movement of the suspension structures and steer the optical components, feedback forces must be applied. Voice-coil actuators are employed in all current detectors. Cryogenic temperatures add new requirements for these actuators, such as compatibility with cryogenic temperatures and low waste heat. Superconductors are a promising option to eliminate resistive heating in actuator coils, as the cryogenic temperatures required for superconductivity are already present. A process for additive manufacturing of the superconductor yttrium barium copper oxide in a powder bed fusion process and the characterization of produced samples are outlined. Additionally, techniques for noise prediction and mitigation in interferometric detectors are discussed and evaluated on a bench-top Michelson interferometer. %F PUB:(DE-HGF)11 %9 Dissertation / PhD Thesis %R 10.18154/RWTH-2025-09249 %U https://publications.rwth-aachen.de/record/1020812