The study of the cold Universe allows us to go back to the origins of our solar system with the observation of star embryos and the origins of the Universe with the measurement of the polarization of the Cosmic Microwave Background. The astronomical instruments in the sub-millimetre and millimetre domains require the use of superconducting detectors cooled down to 100mK. This makes these experiments particularly complex to implement, especially at the system level. A simplification of cryogenic systems would be a major asset for future instruments.
The helium adsorption refrigerators (0.8K in 4He, 0.3K in 3He) used for cooling cryogenic detectors have the advantage of simplicity of operation, the control and piloting of the refrigerators being carried out using thermometers and heating elements. This simplicity is counterbalanced by the minimum achievable temperature on the one hand (at best 0.27K with a 3He refrigerator) and the need for regeneration cycles on the other hand during which the temperature of the detectors rises to 4K. To compensate for the need for regeneration, it is possible to combine two sets of refrigerators with alternating operating and regeneration cycles, which makes it possible to obtain quasi-continuous cooling of the detectors. Another possibility is to combine the use of a dilution refrigerator cycle with adsorption pumps replacing the usual 4He and 3He circulation pumps.
We propose to design a 100mK refrigerator for a 10µW load based on a dilution refrigerator cycle combined with adsorption pumps to replace the room temperature helium circulation pumps, the basic cold sources (40K/40W and 4K/1W) are provided by a Pulse Tube type cryocooler.
Initially, the study will focus on the definition of the cryogenic configuration enabling a dilution refrigeration cycle with a set of adsorption pumps to be carried out. It will be based on work already carried out at the University of Manchester. For this first stage, the refrigerator will be designed for intermittent operation linked to the regeneration phases of the adsorption pumps. The conceptual study will be followed by a phase of modeling, dimensioning, design and implementation of a mock-up. After the characterization and optimization of the refrigerator mock-up cycle, the study will focus on the evolution of the initial refrigerator design from intermittent to continuous refrigeration mode. In this new step, the constraints related to the integration of a refrigerator in an instrument will be taken into account. These studies will be carried out with the low temperature modeling and characterization means available at APC (cryostats with 3He/4He adsorption refrigerators, dilution cryostat, cryostat with adiabatic demagnetization system, ...). This work will be carried out in collaboration with the future Orsay Valley Laboratory, the University of Manchester and the INFN teams.