Thermodynamic Design of a Cryostat for a Radiofrequency Cavity Detector in BabyIAXO in Search of Dark Matter

Despite compelling evidence, dark matter (DM) has still not been directly detected yet. The promising DM candidate, the axion particle, is searched for by the International Axion Observatory (IAXO). BabyIAXO is the demonstrator of IAXO and will be built in the near future. The Relic Axion Detector Exploratory Setup (RADES) plans on installing a 10 m long resonant cavity detector at BabyIAXO. This serves as an additional experiment in BabyIAXO, which probes unexplored axion parameter space and for which low temperatures are stringent for the success of the operation. Thus, the need for a cryostat arises that fulfills the requirements of a dry cryostat, the operation in a low-temperature and magnetic field environment, and the low-interference with the sensitive measurements. This work develops a first design of the cryostat which is based on the cryocooler remote cooling via a closed circulation loop of helium. The heat loads of the components are estimated and a thermodynamic model of the cooling system is compiled. An experiment imitating the present system is conducted at the European Organization for Nuclear Research (CERN) as proof-of-concept and compared with the model. The operation of the experiment reveals a substantial background heat load and a susceptibility for thermal acoustic oscillations (TAOs), which is why the envisioned low temperature regime is not reached. Nevertheless, the remaining data suggest a satisfactory operation of the cooling system and an acceptable agreement of the temperature levels of the experimental setup and the thermodynamic model. Especially, the intermediate temperatures and the cryogenic circulator can be portrayed sufficiently. Thus, for now, the predictions of the thermodynamic model are cautiously trusted. The latter enables the optimization of the layout and operating parameters of the cooling system. The findings of the optimization are discussed in the present thesis. The present cooling system, consisting of two cryocoolers, a cryogenic circulator, a counter-flow heat exchanger (CFHX), and connecting cooling pipes, can reach cavity and low noise amplifier (LNA) temperatures of about 4.6 K. The results demonstrate the feasibility of the application of the resonant cavity detector in BabyIAXO. A more detailed design of the resonant cavity, the tuning mechanism, and the support structure inside the BabyIAXO bore seem exciting and are needed to progress in the cryostat design and to increase the accuracy of the model. Additionally, an electromagnetic and mechanical analysis must assess the influence of a quench of the BabyIAXO magnet on the present system.