John Panek

A compact, high-performance continuous magnetic refrigerator for space missions

Abstract We present test results of the first adiabatic demagnetization refrigerator (ADR) that produce true continuous cooling at sub-kelvin temperatures. This system uses multiple stages that operate in sequence to cascade heat from a “continuous” stage up to a heat sink. Continuous operation avoids the usual constraints of long hold times and short recycle times that lead to the generally large mass of single-shot ADRs, and allows us to achieve much higher cooling power per unit mass. Our design goal is 10 μW of cooling at 50 mK while rejecting heat to a 6–10 K heat sink. The total cold mass is estimated to be less than 10 kg, including magnetic shielding of each stage. These parameters envelop the requirements for currently planned astronomy missions. The relatively high temperature 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. At present, we have assembled a three-stage ADR that operates with a superfluid helium bath. Additional work is underway to develop magnetocaloric materials that can extend its heat rejection capability up to 10 K. Design, operation and performance of the ADR are discussed.

Passive gas-gap heat switches for use in adiabatic demagnetization refrigerators

We have designed, built, and tested a gas-gap heat switch that turns on and off passively, without the need for a separate, thermally activated getter. This switch uses 3He condensed as a thin film on alternating plates of copper. The switch is thermally conductive at temperatures above about 0.2 K, and is insulating if either end of the switch cools below about 0.15 K. The “on” conductance (7 mW/K at 0.25 K) is limited by the surface area and gap between the copper leaves, the saturated vapor pressure of the 3He, and the Kapitza boundary resistance between the 3He and the copper. The “off” conductance is determined by the helium containment shell which physically supports the two conductive ends. We have also designed and are building passive gas-gap heat switches that will passively turn off near 1 K and near 4 K. For these switches we rely on the strong temperature dependence of the vapor pressure of 4He adsorbed onto neon or copper substrates, respectively, when the coverage is less than one monolayer...

A Magnetoresistive Heat Switch for the Continuous ADR

In compensated elemental metals at low temperature, a several Tesla field can suppress electronic heat conduction so thoroughly that heat is effectively carried by phonons alone. In approximately one mm diameter single crystal samples with impurity concentrations low enough that electron conduction is limited by surface scattering, the ratio of zero-field to high-field thermal conductivity can exceed ten thousand. We have used this phenomenon to build a compact, solid-state heat switch with no moving parts and no enclosed fluids. The switching time is limited by time scale for charging the magnet that supplies the controlling field. Our design and fabrication techniques overcome the difficulties associated with manufacturing and assembling parts from single crystal tungsten. A clear disadvantage of the magnetoresistive switch is the mass and complexity of the magnet system for the controlling field. We have discovered a technique of minimizing this mass and complexity, applicable to the continuous adiabatic demagnetization refrigerator.

Rare-earth garnets and perovskites for space-based ADR cooling at high T and low H

Future NASA satellite detector systems must be cooled to the 0.1 K temperature range to meet the stringent energy resolution and sensitivity requirements demanded by mid-term astronomy missions. The development of adiabatic demagnetization refrigeration (ADR) materials that can efficiently cool from the passive radiative cooling limit of ∼30 K down to sub-Kelvin under low magnetic fields (H⩽3 T) would represent a significant improvement in space-based cooling technology. Governed by these engineering goals, our efforts have focused on quantifying the change in magnetic entropy of rare-earth garnets and perovskites. Various compositions within the gadolinium gallium iron garnet solid solution series (GGIG, Gd3Ga5−XFeXO12, 0.00⩽X⩽5.00) and gadolinium aluminum perovskite (GAP, GdAlO3) have been synthesized via an organometallic complex approach and confirmed with powder x-ray diffraction. The magnetization of the GGIG and GAP materials has been measured as a function of composition (0.00⩽X⩽5.00), temperature...

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 μW 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 μK rms or better over the entire cycle, and the cooling power is 2.5 μW at 60 mK rising to 10 μW at 100 mK.

A Rapid Turnaround Two‐Stage Adiabatic Demagnetization Refrigerator for Cooling to 50 mK

Many research and development programs require rapid access to very low temperatures (∼50 mK). For detector development, relatively large experiment volumes are also needed for tests involving integrated detectors and readout amplifiers (which may need to be stationed at a different temperature). To provide this capability in a versatile, fast turnaround system, we have constructed a two‐stage adiabatic demagnetization refrigerator (ADR) that is operated in a simple bucket‐style dewar. The ADR/cryostat is separated into two concentric inserts; the outer insert supports two 3.3 T magnets, magnet leads and magnetic shielding, and the inner one consists of a hermetic experiment volume coupled to the ADR’s salt pills and heat switches. The magnet insert remains in the dewar at all times, while the ADR insert may be inserted and removed even when the dewar is cold. The cooldown from room temperature takes less than 1.5 hours, and cycling of the ADR as little as 30 minutes. Future tests will investigate the use...

Thermal Model and Experimental Validation of the Re-Design of High Temperature Superconducting Leads for X-Ray Spectrometer Applications

The thermal modeling for the redesign of an X-Ray Spectrometer (XRS) High Tc Superconducting Current (HTSC) lead assembly was the objective of this research work. In order to achieve a 2.5-year lifetime for the XRS, low thermal conductance leads were redesign to supply electric current to the Adiabatic Demagnetization Refrigerator (ADR) magnet and the cryostat valve motors with a minimal heat load. This research work consisted of the development of a mockup of the HTSC lead assembly and a computer model to simulate the thermal behavior of the system. Experimental data of the mockup was used to validate the thermal model, which was employed in the optimization of the design to minimize the heat load. The thermal model development and validation of the new HTSC lead design is discussed, with emphasis on thermal test results.Copyright