Very-Low-Temperature Cooling Systems

Abstract

Very low temperatures can take on a variety of different meanings, depending on one’s low-temperature experience. Those unfamiliar with cryogenics would usually refer to liquid nitrogen as being at very low temperatures. The adverb “very” is generally used for something out of the ordinary, and for those more familiar with cryogenics, the temperature range below 1 K might better fit this category. Such a choice merely reflects the present status of our refrigeration efforts and could therefore change with time. For this book, however, we arbitrarily define very low temperatures as those below about 1 K. Other terms, such as ultralow temperatures or millikelvin temperatures, have also been used in the past by others to describe the same region. The temperature of 1 K seems to be a good dividing line since the refrigeration techniques used below 1 K are usually quite different from those used above 1 K. The vapor pressure of He⁴ becomes too small below about 1 K to be of any use in a gas refrigerator. The isotope He³ retains a reasonable vapor pressure down to about 0.3 K, but for lower temperatures liquid-vapor equilibrium cannot be used for refrigeration. Not only are the refrigeration techniques below 1 K usually different from those above 1 K, but the temperatures are expressed in terms of millikelvin, microkelvin, or even nanokelvin.