Continuous Adiabatic Demagnetization Refrigerator for Large Arrays of Superconducting Bolometers

The next generation of cosmic microwave background experiments aim to provide definitive measurements of the early universe by employing large focal plane arrays with ~100,000 superconducting detectors operating at low temperatures ~0.1 K. This creates a requirement for cryogenic cooling systems that are reliable and cryogen-free, and that offer high cooling power, multiple thermal intercepts, and the capability for continuous-cycle operation. Our project is developing a novel cryogenic cooling system that meets these requirements by employing “continuous adiabatic demagnetization refrigeration.” This technology offers continuous cooling to 0.1K and below with extreme reliability, as there are no moving parts. It is intrinsically modular and extensible and thus well-suited to create high power and thermal intercepts, and it can be operated in any orientation. In Phase I we successfully performed development of two key components of the cooler: the “passive gas-gap heat switch,” and the growth of crystals of a low-temperature magnetic refrigerant. We also developed software tools to understand the design and optimization of the magnetic refrigerant capsules, the electromagnets, and their shielding. By combining those modeling results with calculations of thermal resistances, we developed software tools for end-to-end performance estimation of the whole cooling system. In Phase II we will develop the prototype cooling system. We will develop two additional heat switch variants. We will synthesize and test a novel new magnetic refrigerant offering even larger cooling power at low temperatures. The software tools developed in Phase I will be used to optimize the overall design and to design the individual refrigerant capsules, electromagnets, magnetic shields, and heat switches. These parts will be fabricated and integrated into a dedicated cryostat to create the prototype cooler. Commercial applications and other benefits: In addition to the cosmic microwave background focal planes, there are numerous other scientific and commercial applications requiring high-reliability cooling to temperatures of 0.1K and below. One emerging high-profile application with considerable commercial potential is quantum computing. More immediately, the proposed cooler will improve the capabilities of high-resolution X-ray spectrometers based on low-temperature particle detectors. These systems are improving capabilities in materials and bio-materials analysis, and in nuclear safeguards and nuclear forensics. These instruments are in demand by semiconductor manufacturers and researchers at synchrotron facilities, with a potential annual market of some tens of millions of dollars.