Applications of Helium to Very-Low-Temperature Refrigeration
For many years the temperature region below 1°K was accessible only by the method of adiabatic demagnetization of magnetic salts. This method, though powerful, has some intrinsic technical disadvantages. Substantial magnetic fields are required while the low temperature is not maintained continuously. Both of these disadvantages are overcome in the He3 evaporation refrigerator and the continuously operating He3-He4 dilution refrigerator . In the latter, temperatures below 10 m°K are routinely obtained and maintained continuously for days at a time [2,3]. Dilution refrigerators operating in the “single-cycle” mode  achieve temperatures transiently of 4 to 5 m°K, where they are limited by intrinsic factors. It is also possible to obtain substantial refrigeration by compression of a two-phase mixture of liquid and solid He3. Devices based on compressional cooling reach temperatures of about 2 m°K .
KeywordsDilution Refrigerator Molar Entropy Thermal Boundary Resistance Adiabatic Demagnetization Porous Vycor Glass
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