Techniques for Reducing Radiation Heat Transfer between 77 and 4.2 K

Abstract

The present-day applied superconductivity is a liquid-helium-based technology. The efficiency of a cryogenic or superconducting device is determined by its rate of consumption of the cryogen. Liquid helium has an extremely small heat of vaporization; thus, its storage almost always requires a state-of-the-art insulating method. Radiation heat leak becomes significantly more important as larger superconducting devices are built, such as huge high-energy physics analysis magnets, fusion reactor systems, and energy storage facilities. For large superconducting magnets,[^1–3] it is common practice to surround the liquid helium vessel with a nitrogen shield (at 77 K) and wrap multilayer insulation around both the liquid helium vessel and the nitrogen shield in an attempt to further reduce the heat leaks (Fig. 1a). Multilayer insulation is inexpensive and generally effective; yet, it is a rather difficult material to apply because its performance depends on a few hard-to-control parameters such as the layer density, the compressive loading, and the lateral heat transfer effect. The effective insulation capability obtained in practice is at least a factor of 2 worse than carefully measured laboratory values (or those claimed by manufacturers). Careless and/or inexperienced application can easily generate heat leak values a few times higher than the predicted value, especially when dealing with peculiarly shaped cryostats.

Work sponsored by the U. S. Department of Energy.