Continuous Adiabatic Demagnetization Refrigerator for Large Arrays of Superconducting Bolometers

A recent report of the 25-member Particle Physics Project Prioritization Panel (P5) recommends funding cosmic microwave background (CMB) experiments under all budget scenarios, specifically, the next- generation ground-based CMB experimental program whose aim is to provide definitive measurements of the early universe. For this program, large focal plane arrays with ~100,000 superconducting detectors operating at ~0.1 K temperatures will be required, along with reliable, cryogen-free cryogenic refrigeration systems with high cooling power, multiple thermal intercepts, and the capability for continuous-cycle operation to operate these focal plane arrays. We propose to develop a continuously-cycling adiabatic demagnetization refrigerator (CADR) suitable for continuous 10-70mK cooling of future large arrays of superconducting bolometers for CMB experiments. CADRs are more robust and more reliable than dilution refrigerators, the only other viable competing technology for this application. Our development effort will leverage existing expertise and infrastructure that have been developed for the production of commercial single-shot ADRs, which the company has been producing for several years. During Phase I, we will develop a passive gas-gap heat switch and characterize its thermal properties, complete numerical optimizations of the combined superconducting electromagnet/magnetic shield assemblies, develop a CMN-based refrigerant assembly, and develop a straw-man engineering model of the CADR that will be finalized and built in Phase II.In addition to the CMB instrumentation application requiring high-performance and high-reliability cooling for focal plane arrays, the CADR will have direct applicability to other commercial products and market applications STAR Cryoelectronics currently serves. In particular, we anticipate that the proposed CADR will enable the company to improve two types of X-ray spectrometers for materials analysis and basic research that the company currently offers. 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.