User:A Passing Physicist/Dilution fridge

A dilution refrigerator or commonly a dilution fridge is a device used to cool objects to very low temperatures, usually for research pourposes. It uses the cooling effect provided by mixing two isotopes of helium to reach temperatures as low as 5mK.

Principle of Opperation
If a liquid mixture of two isotopes of helium, helium-4 (4He) and helium-3 (3He) is cooled below about 870mK, it separates into two phases. One phase is mostly 3He and is refered to as the rich phase, and the other is mostly 4He and is refered to as the dilute phase, the rich phase floats on top. If 3He can be forced to move from the rich layer to the dilute phase, then the mixing process cools the liquid.

The rich phase is a superfluid and the dilute phase is not. In a superfluid particles of 3He move without friction ar drag, much as though they are in a vacuum. This leads to a common analogy with the cooling system in a conventional cryostat which works by evaporating a liquid into a gas.

Cooling can be acheived by bringing pure 3He into contact with pure 4He and alowing them to mix, but the system will soon reach equilibrium and stop cooling. To provide continious cooling, which is required in the majority of applications, 3He must be removed from the dilute phase to allow more to flow in. This is done by distillation, the gaseous 3He can then be reliquified and put back into the rich phase to be used again.