An adiabatic demagnetization refrigerator capable of continuous cooling at 10 mK and below

Upcoming far-IR space missions are faced with a very challenging requirement: to make detectors with sufficient sensitivity to be limited by the photon noise present in the darkness of space. At long wavelengths (>100 μm), where the Zodiacal light is insignificant and the sky brightness corresponds to only ∼100 aW, detectors must have noise equivalent powers of <10–19 W/√Hz. For fundamental reasons, this can only be achieved with detectors operating at ∼20 mK, giving rise to the need for coolers operating as low as 10 mK. Only adiabatic demagnetization refrigerators (ADR) have demonstrated both zero-g operation and the capacity for cooling into this regime. We have recently developed a multi-stage ADR that can produce continuous cooling at temperatures of 35 mK or lower, and have begun developing additional stages that will enable 10 mK operation. We describe the design and development program for this ADR.