Dilution refrigerator

A 3He/4He dilution refrigerator is a cryogenic device that provides continuous cooling to temperatures as low as 2 mK, with no moving parts in the low-temperature region.

The dilution refrigerator was first proposed by Heinz London in the early 1950s, and was experimentally realized in 1964 in the Kamerlingh Onnes Laboratorium at Leiden University.

Finally, the pure 3He enters the mixing chamber, the coldest area of the device.

[4] On its way up, the cold, dilute 3He cools the downward flowing concentrated 3He via the heat exchangers and enters the still.

The pumps compress the 3He to a pressure of a few hundred millibar and feed it back into the cryostat, completing the cycle.

[5] No external supply of cryogenic liquids is needed in these "dry cryostats" and operation can be highly automated.

However, dry cryostats have high energy requirements and are subject to mechanical vibrations, such as those produced by pulse tube refrigerators.

The first experimental machines were built in the 1990s, when (commercial) cryocoolers became available, capable of reaching a temperature lower than that of liquid helium and having sufficient cooling power (on the order of 1 Watt at 4.2 K).

[6] Pulse tube coolers are commonly used cryocoolers in dry dilution refrigerators.

The other design is more demanding to realize, requiring heat switches that are necessary for precooling, but no inner vacuum can is needed, greatly reducing the complexity of the experimental wiring.

Inside of a mixing chamber there is negligible thermal resistance between the pure and dilute phases,

In dilution refrigerators, Ti is reduced by using heat exchangers as shown in the schematic diagram of the low-temperature region above.

Phase diagram of liquid 3 He– 4 He mixtures showing the phase separation.
Schematic diagram of a wet 3 He/ 4 He dilution refrigerator without the outer vacuum shield. (vector graphic)
Schematic diagram of a standard, or wet, dilution refrigerator.
Schematic diagram of the low-temperature part of a dilution refrigerator.
The inside of a wet Oxford Instruments helium dilution refrigerator, with the vacuum cans removed.
Gas control system for a helium dilution refrigerator.
Schematic diagram of a cryogen-free, or dry, dilution refrigerator precooled by a two-stage pulse tube refrigerator , indicated by the dotted rectangle.