Introduction-Cryogenic detectors have higher sensitivity and better energy resolution than alternative sensors, making them an attractive option for space exploration and essential for observing low energy photons in the near- or far-IR, X-ray and submillimeter ranges.[1] Since NASA’s first cryogenic missions in the early 1980s, increasingly complex space detectors have required continuous advancement in cryogenic technology.[2]

The development of a sub-Kelvin Active Magnetic Regenerative Refrigerator (AMRR) is discussed in this work. This novel AMRR addresses many of the deficiencies of current cooling solutions, as it can provide continuous and distributed sub-Kelvin cooling via circulation of a 3He-4He mixture using a non-moving Superfluid Magnetic Pump (SMP).[3] The resulting system eliminates the requirement for heat switches, extends the allowable distance between the detector and magnets and enables a variety of thermal integration options. It is a reliable, no-vibration, low mass and scalable sub-Kelvin cooling solution for space instrumentation. An SMP, two regenerators, one cold heat exchanger (CHX), and two hot heat exchangers (HHXs) comprise the AMRR system shown in Figure 1.

Background
The AMRR system takes advantage of a few key characteristics of both the paramagnetic refrigerant and the helium working fluid mixture to provide distributed sub-Kelvin cooling. Paramagnetic materials have magnetic properties that are related to unfilled electron shells in some of the ions which create magnetic moments.[4] This results in a coupling between the applied magnetic field and material temperature through the entropy. Manipulation of this coupling allows for the heating or cooling of paramagnetic material through the magnetization or demagnetization of an applied field, respectively, and is the primary motivation behind using these materials in refrigeration. The paramagnetic refrigerant used in this research is Gadolinium Gallium Garnet (GGG).

At near and sub-Kelvin temperatures, each of the two stable helium isotopes exhibits unique properties. There is a significant reduction in heat capacity of 4He at 1 K, as 99% of the 4He has transitioned into its superfluid state, which cannot carry energy.[5] In contrast, 3He has a large magnetic spin entropy that gives rise to a large heat capacity. Therefore, 3He is required to carry energy throughout the system, and 4He is responsible for driving flow in the system through the thermomechanical fountain effect.[5,6] The unique combination of the GGG and 3He-4He mixture properties enables the AMRR refrigeration cycle using the oscillatory flow provided with the SMP.

Each magnetic regenerator consists of a canister of crushed GGG suspended inside the bore of a superconducting solenoid. As fluid is displaced out of the SMP, it moves down through the demagnetizing regenerator, which cools the mixture so that it exits the regenerator bed at the desired outlet temperature. The mixture then moves through the CHX, providing sub-Kelvin cooling to the load (e.g., one or more detectors). The mixture flows back through the opposite, magnetizing regenerator, which rejects more heat into the fluid. Finally, this heat is rejected to the precooling stage at a temperature of approximately 1.6 K. The system can then be reversed, sending flow back in the opposite direction. The fact that this AMRR system produces a flow is one of its most important advantages relative to other cryogenic refrigeration solutions.
AMRR System Design and Construction

The primary objective of this research is to provide measurable cooling at the CHX with a proof-of-concept prototype of this novel AMRR. The AMRR components were designed to be compatible with an existing SMP to ensure that there is enough paramagnetic material and sufficient magnetic field to both force circulation and lift heat at the CHX. A simple 1D transient design model was developed that focused on one regenerator canister during a flow process in the AMRR system in order to assist in the design process. Using an energy balance on the regenerator canister, the canister dimensions required to maintain an outlet temperature of 750 mK during the flow process were determined. These dimensions, along with an assumed field swing, were used to constrain the regenerator magnet design. The final regenerator design, shown in Figure 2, includes the canister pieces, magnet mandrel and winding specifications and suspension design.

The regenerator canister consists of one thin-walled tube and two endcaps, all machined out of stainless steel. We packed the two canisters with crushed GGG particles to a porosity of 0.38. To achieve a target GGG particle size of roughly ≤1 mm in diameter, two sieves with 1.2 mm and 0.2 mm openings were used to create a go/no-go gage.

Thick-walled 6061 aluminum tubing was used to create the magnet mandrels, polishing the surfaces prior to winding. During the winding process, CTD-A521 magnet epoxy was continuously brushed onto the coil to improve the thermal contact between the wires and mandrel. To finish the magnets, two back-to-back 1N4001 diodes were soldered across the leads to redirect the current in the event of a quench.

Several of the regenerator components have been installed, and thus, the remaining work to finish the AMRR system assembly consists primarily of integrating the components with a circulation loop.

Conclusion and Future Work
Near-term future work includes finishing the assembly and experimental validation of the AMRR system. Once optimized, this system can be used to provide precooling to even lower temperature stages or for distributed cooling over large areas, offering an improvement over current systems and making new types of cryogenic refrigeration configurations possible.
This article is a shortened version of a paper published at the International Cryocooler Conference, where it was the recipient of the ICC21 Best Student Paper Award.

References:
[1] Rando, N., Lumb, D., Bavdaz, M., Martin, D. and Peacock, T., “Space science applications of cryogenic detectors,” Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Vol. 522, No. 1-2 (2004), pp. 62-68.
[2] Rando, N., “Cryogenics in Space,” Observing Photons in Space, ISSI Scientific Report Series, Vol. 9, Springer, New York (2013), pp. 639-655.
[3] Jahromi, A.E., “Development of a Proof of Concept Low Temperature Superfluid Magnetic Pump with Applications,” PhD dissertation at University of Wisconsin-Madison (2015).
[4] Wikus, P., Canavan, E., Heine, S.T., Matsumoto, K. and Numazawa, T., “Magnetocaloric Materials and the Optimization of Cooling Power Density,” Cryogenics, Vol. 62, (2014), pp. 150-162.
[5] Sciver, S. V., Helium Cryogenics, Springer, New York (2012).
[6] Papoular, D.J., Ferrari, G., Pitaevskii, L.P. and Stringari, S., “Increasing Quantum Degeneracy by Heating a Superfluid,” Physical Review Letters, Vol. 109, No. 8-24 (2012).

Image: Figure 1. A diagram of the AMRR system. Credit: University of Wisconsin-Madison