Development of a 30 mK ultra-low temperature Continuous Adiabatic Demagnetization Refrigerator with a continuous 700 mK intermediate stage for heat intercept (CADR)

Several past, and many future, astronomical instruments require cooling to sub-Kelvin temperature to obtain high sensitivity. In many cases, large arrays of cryogenic detectors will be required. The cooling systems associated with these detectors will require performance surpassing the limits of present flight sub-Kelvin coolers. We have begun to address this technology gap with an SAT-funded refrigeration system that will come to a conclusion in FY 2022. We propose to extend that development of a compact cooling system to provide significant cooling at even lower temperature, lifting significant heat continuously at temperatures down to 30 mK and reject the heat to a cryocooler at 4.5 K, simplifying the overall cryogenic system. We will also develop a new continuous cooling stage at 0.7 K to intercept heat, which is crucial for low temperature detector harnesses, optics, as well as the lower stage coolers (30 mK) of our proposed system. Our proposed system will exceed the requirements of all currently conceived cryogenic detector arrays, including flagship and Probe missions recommended by the 2020 Astrophysics Decadal Committee: the Far-IR Probe, Inflation Probe, X-ray Probe, and possibly the HabEx/LUVOIR flagship with far-IR sub-Kelvin detectors. Multi-stage ADRs offer great flexibility. The same refrigerator can be operated at any temperature within the range of 30 to 300 mK. The 0.7 K heat intercept stage could be regulated at other temperatures between 300 mK and 1 K to suit the cooling requirements of a detector system. Continuous stages could be added at other temperatures to provide cooling to, for example, Superconducting Quantum Interference Device arrays for Transition Edge Sensors (TES) or High Electron Mobility Transistors (HEMT) for Microwave Kinetic Inductance Detectors (MKID). Now is the time to pursue this effort. Our team completed the end-to-end design, build, and flight qualification of the Hitomi ADR. Launched successfully in February 2016, the ADR provided a stable 50 mK on-orbit detector array temperature for over one month until the untimely demise of the spacecraft. We have delivered a similar 3 stage ADR for the replacement mission, XRISM. The SAT awarded to our team in 2017 has resulted in a fully assembled unit about to be tested and vibration qualified at the system level. This proposal builds on that successful effort utilizing the same experienced team. In short, the team is ready now to make this technology mission-selectable by 2025. We will set new standards in sub-Kelvin spaceflight cooling. At the conclusion of this work, NASA will have a TRL-6 magnetic cooling system ready for missions in the coming decades.

The Strategic Astrophysics Technology program (SAT) supports focused development efforts for key technologies to the point at which they are ready to feed into major missions in the three science themes of the Astrophysics Division: Exoplanet Exploration, Cosmic Origins, and the Physics of the Cosmos. This program is specifically designed to address middle technology readiness level (TRL) "gaps" between levels 3 and 6: the maturation of technologies that have been established as feasible, but which are not yet sufficiently mature to incorporate into flight missions without introducing an unacceptable level of risk. NASA does not require a data management plan for proposals to SAT.

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