Operational Envelope for Magnetic Refrigerators

Abstract

Magnetic refrigerators have been used for more than fifty years to attain millikelvin temperatures and provide microwatts of cooling power. In the last decade, numerous groups have begun development of magnetic refrigerators for applications above 1 K. The initial perception of many people was that magnetic devices would only be useful for small cooling loads of a watt or less at temperatures near liquid helium temperatures. However, calculations in the last several years indicate that magnetic refrigerators are attractive at much higher cooling powers at a variety of temperatures, e.g., 100W at 1.8 K or 50 kW at 270 K. Some areas where magnetic refrigeration does not apply are also being identified, e.g., providing a few watts of cooling at 80 K can be done much better with gas-cycle units than with magnetic devices because of the need for cooling of superconducting magnets to near 4K. In general, magnetic refrigerators do not miniaturize because of the high magnetic field requirements. In a desire to know where magnetic refrigerators best apply, detailed thermal models of magnetic refrigerators have been developed to predict cooling power, input power and efficiency as a function of temperature and frequency. These models show that different mechanisms limit the performance of magnetic devices at different design conditions. A combination of these limits produces a thermal operational envelope which is the primary topic of this paper.