Dilution refrigerator

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Helium Dilution Refrigerator

A dilution refrigerator is a cryogenic device first proposed by Heinz London. Its refrigeration process uses a mixture of two isotopes of helium: helium-3 and helium-4. When cooled below ~ 870 millikelvins, the mixture undergoes spontaneous phase separation to form a ³He-rich phase and a ³He-poor phase.

As with evaporative cooling, energy is required to transport ³He atoms from the ³He-rich phase into the ³He-poor phase. If the atoms can be made to continuously cross this boundary, they effectively cool the mixture. Because the ³He-poor phase cannot have less than 6% helium-3 at equilibrium, even at absolute zero, dilution refrigeration can be effective at very low temperatures. The volume in which this takes place is known as the mixing chamber.

The simplest application is a "single-shot" dilution refrigerator. In single-shot mode, a large initial reservoir of helium-3 is gradually moved across the boundary into the ³He-poor phase. Once the ³He is all in the ³He-poor phase, the refrigerator cannot continue to operate.

More commonly, dilution refrigerators run in a continuous cycle. The ³He / ⁴He mixture is liquified in a condenser, which is connected through an impedance to the ³He-rich area of the mixing chamber. Atoms of ³He migrate across into the ³He-poor phase, providing cooling power, and then into a still where the liquid ³He evaporates. Outside the refrigerator, this gas is pumped up to a higher pressure and usually purified, and finally returns to the condenser to start the cycle again.

Continuous-cycle dilution refrigerators are commonly used for low-temperature physics experiments. Temperatures below 2 millikelvins can be achieved with the best systems.