High-Efficiency Continuous Cooling for Cryogenic Instruments and sub-Kelvin Detectors

Although large, diffraction-limited telescopes are approaching a size limited by available launch vehicles, there is still an enormous “discovery space” open to astrophysics through the use of advanced low temperature instruments and deep sub-Kelvin detectors. These devices offer the potential for orders of magnitude improvement in sensitivity and spectral resolution. In the past, cryogenic instruments have been large, expensive, and power hungry, consisting of complex cooling chains with multiple coolers using different technologies. High cost and complexity have been the major impediment to the selection of missions using these advanced capabilities. We propose to develop a compact cooling system that will span more than a factor of 200 in temperature, lifting heat continuously at temperature below 50 mK and rejecting it at over 10 K, simplifying the overall cryogenic system. The device, based on Adiabatic Demagnetization Refrigerators (ADRs), will have high thermodynamic efficiency. The prototype system will exceed the requirements of all currently conceived cryogenic detector arrays, including those for flagship missions such as the Far-IR Surveyor, Inflation Probe, X-ray Surveyor, and possibly HabEx and LUVOIR. In particular, it will have more than 5 times the cooling power at 50 mK than previous sub-Kelvin coolers, greatly relaxing the requirements on the heat generation in large detector arrays, and simplifying the thermal design of the focal plane assemblies. ADRs by themselves have no moving parts and produce no measurable vibration, however upper-stage mechanical coolers have been linear piston devices that export significant vibration. Ameliorating the problems due to upper-stage cooler vibrations has contributed to increased costs on recent astrophysics missions such as JWST and Astro-H. By raising the heat reject temperature to 10 K, the proposed sub-Kelvin cooler becomes compatible with recently-demonstrated extremely low vibration mechanical coolers, eliminating this problem for future missions. Furthermore, a complete cooling chain with extremely low vibration will enable the use of advanced sub-Kelvin detectors on missions with tight pointing requirements. Multi-stage ADRs offer great flexibility. In addition to continuous cooling at the lowest temperatures, the prototype will have a stage that provides enough power at 4 K to cool a modest sized telescope. This stage could be scaled up to cool a large (multi-meter) telescope. Continuous stages could be added at other temperatures to provide cooling to, for example, Superconducting Quantum Interference Device (SQUID) 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 recently completed the end-to-end design, build, and on-orbit qualification of the ASTRO-H ADR. Launched successfully in February 2016, the ADR now provides a stable 50 mK on-orbit detector array temperature. In September 2015, our team demonstrated a laboratory ADR that provided cooling at 4 K and rejected heat to 10 K. We are presently building continuous ADRs with heat rejection at 4.5 K that have much higher cooling power per unit mass than the traditional, ASTRO-H style “single-shot” ADRs. In short, the team is now ready to make this technology mission-selectable by 2020. At the conclusion of this work, NASA will have a TRL-6 magnetic cooling system ready for missions in the coming decades. More »