Achieving High Stability and Efficiency in the Next Generation of Adiabatic Demagnetization Refrigerators

Large cryogenic detector arrays drive astrophysical science capabilities from millimeter to X-ray wavelengths. The continuous adiabatic demagnetization refrigerator (CADR) is ideal to support these space-borne arrays because it has no moving parts or cryogens, it achieves temperatures <0.1 K, and it has low mass and very high efficiency. We will implement and test two technologies that improve the next generation of CADRs by boosting the overall efficiency and stability.

Ordinary refrigeration uses mechanical compression and expansion to interact with kinetic degrees of freedom in a gas. The magnetic refrigerators investigated here use magnetic fields to interact with spin degrees of freedom in a crystal. The magnets are cryogenic and superconducting, and the crystal is a material with large magnetocaloric effects like gadolinium gallium garnet. We cool the system using liquid helium, but mechanical cryocoolers would be used in space applications. Continuous cooling can be achieved by cycling pairs of magnetic coolers. In high-sensitivity bolometer systems, stability ~10^-5 is required, presenting a challenge for the control of cycling. We are investigating materials and algorithms that permit more stable and efficient operation of these systems.