Salt pill design and fabrication for adiabatic demagnetization refrigerators

• Salt pills are one of the key components of ADR systems, providing containment of the magnetic refrigerant.
• Salt pill design focuses on maximizing the cooling capacity of the refrigerant, as well as efficient heat transfer.
• Salt pill design and construction are highly dependent on the type of refrigerant and the operating temperature.
• Techniques for constructing salt pills for very low (<100 mK) and moderate (>0.5 K) temperature are discussed.

The performance of an adiabatic demagnetization refrigerator (ADR) is critically dependent on the design and construction of the salt pills that produce cooling. In most cases, the primary goal is to obtain the largest cooling capacity at the low temperature end of the operating range. The realizable cooling capacity depends on a number of factors, including refrigerant mass, and how efficiently it absorbs heat from the various instrument loads. The design and optimization of “salt pills” for ADR systems depend not only on the mechanical, chemical and thermal properties of the refrigerant, but also on the range of heat fluxes that the salt pill must accommodate. Despite the fairly wide variety of refrigerants available, those used at very low temperature tend to be hydrated salts that require a dedicated thermal bus and must be hermetically sealed, while those used at higher temperature – greater than about 0.5 K – tend to be single- or poly-crystals that have much simpler requirements for thermal and mechanical packaging. This paper presents a summary of strategies and techniques for designing, optimizing and fabricating salt pills for both low- and mid-temperature applications.