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The characteristics and performance of a new class of 3He refrigerators are discussed. We introduce a 3He refrigerator, which allows a temperature of 296 m K to be reached with a starting point of 4.2 K, without pumping on the main 4He bath. The operating principle is based on the single-expansion helium liquefier: gas cooling by isothermal compression and adiabatic expansion.

We introduce a mini 3 He refrigerator, operating at ∼300 mK starting from 4.2 K without pumping on the main 4 He bath. The innovative idea is that the present one is suitable for a very fast operation; for its use, it is sufficient a storage 4 He Dewar. In this way we drastically reduce the time required to cool it down, because there is no need for a classic cryostat. This prototype is particularly aimed for all those operations in which it is necessary to test a large number of samples that do not require long duration measurements at low temperature.

The sensitivity increase in bolometers obtained by operating temperatures of 1 K (helium-4) and 0.3 K (helium-3), with respect to standard bolometers cooled at pumped 4He temperature, represents in infrared and submillimetric astronomy a remarkable step forward in applications both on large ground telescopes and on stratospheric balloons. The characteristics are introduced of a double stage refrigerator (4He/3He) which allows a temperature of nearly 300 mK to be reached, starting from a temperature of 4.2 K. No moving parts and external links are involved, except for the electric leads, so as to exploit only the condensation and cryosorption cycles of 4He and 3He, without pumping on the main 4He bath.

The new generation of miniature 3He refrigerators, exploiting only the condensation and cryosorption cycles of 3He without moving parts and external links, represents the most common adopted solution in low temperature physics for cooling detectors such as bolometers at 0.3 K. The characteristics and performance of an improved 3He refrigerator working at a temperature of nearly 0.3 K are discussed. We introduce a new class of refrigerator (patented) which allows one to reach the operating temperature in a much shorter time than with standard 3He refrigerators, without using any thermal switch. The system has the potential to be very useful in all applications where several modifications are required.