A continuous low-temperature magnetic refrigerator

We report on recent progress in the development of a continuous adiabatic demagnetization refrigerator (ADR). Continuous operation avoids the constraints of long hold times and short recycle times that lead to the generally large mass of single-shot ADRs, allowing us to achieve an order of magnitude larger cooling power per unit mass. Our current design goal is 10 muW of cooling at 50 mK using a 6-10 K heat sink. The estimated mass is less than 10 kg, including magnetic shielding of each stage, The relatively high heat rejection capability allows it to operate with a mechanical cryocooler as part of a cryogen-free, low temperature cooling system. This has the advantages of long mission life and reduced complexity and cost. We have assembled a three-stage ADR and have demonstrated continuous cooling using a superfluid helium bath as the heat sink. The temperature stability is 8 muK rms or better over the entire cycle, and the cooling power is 2.5 muW at 60 mK rising to 10 muW at 100 mK.