Dilution Refrigeration of Multi-Ton Cold Masses

Dilution refrigeration is the only means to provide continuous cooling at temperatures below 250 mK. Future experiments featuring multi-ton cold masses require a new generation of dilution refrigeration systems, capable of providing a heat sink below 10 mK at cooling powers which exceed the performance of present systems considerably. This thesis presents some advances towards dilution refrigeration of multi-ton masses in this temperature range. A new method using numerical simulation to predict the cooling power of a dilution refrigerator of a given design has been developed in the framework of this thesis project. This method does not only allow to take into account the differences between an actual and an ideal continuous heat exchanger, but also to quantify the impact of an additional heat load on an intermediate section of the dilute stream. In addition, transient behavior can be simulated. The numerical model has been experimentally verified with a dilution refrigeration system which has been designed, built and tested at CERN in the framework of this doctoral thesis project. Furthermore, the determination of the residual heat load to the cold mass of a very low temperature (VLT) experiment is addressed. The results of this analysis can be used to predict the required cooling power for large future experiments. Based on the experience gained with the dilution refrigeration system at CERN, design rules for large dilution refrigerators have been derived. They are consequently used in a design and feasibility study of a refrigeration system, which is tailored for the future cryogenic dark matter search EURECA and meets the requirements of low background experiments and operation in remote underground laboratories. This doctoral thesis project has been funded CERN and by the Austrian Federal Ministry for Education, Science and Culture.