Cooling techniques at subKelvin temperature

A major trend in the low-temperature sector is the elimination of cryogenic liquid tanks in favor of mechanical cryorefrigerators. This technological breakthrough saves space and weight, eliminates the need for cryogenic fluids in terms of consumables and consequently, especially for space applications, considerably extends mission life. For extremely cold temperatures, several systems need to be cascaded to cover the entire temperature range. Mechanical cryocoolers are then used as pre-cooling systems, bringing with them additional specificities and constraints: cooling capacities are limited, and some systems require thermal load management (power peaks). In addition, cold is generally produced locally on a copper interface, and means of distributing these "cold Joules" may be required. Finally, induced vibrations must be taken into account to avoid overheating at very low temperatures.

This trend is driving developments in all elements of the cryogenic chain. In addition, this approach applies to requirements at isolated sites, where the availability of cryogenic fluids such as liquid nitrogen and a fortiori liquid helium is difficult and/or extremely costly.

This article is limited to the subkelvin range, i.e. systems capable of cooling an object to temperatures below 1 K (– 272.15 °C). Joule Thomson helium-3 loop cooling, which can achieve temperatures well below the kelvin, is also deliberately omitted. Its principle is identical to that of a loop operating with the conventional helium 4 isotope, and the difference in performance stems from the particular physical characteristics of helium 3 (see the article "Liquefaction of helium and medium- and high-power helium refrigeration" [BE 9 816] ). Temperature excursions are also limited to the millikelvin range. Lower temperatures concern the ultra-low-temperature range, which is a priori limited to a few laboratories worldwide, and which would require an article of its own, given the complexity and difficulty of the techniques used.

In practice, three technologies or methods are emerging:

• evaporative cooling using helium isotopes;

• magnetic cooling (adiabatic demagnetization) ;

• dilution (property of the phase separation of the two helium isotopes).

The first two methods are so-called "one-shot" techniques, enabling...