Dilution refrigerator
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[[Image:Helium dilution refrigerator.jpg|325px|thumb|Helium Dilution Refrigerator]]
A '''dilution refrigerator''' is a [[cryogenics|cryogenic]] device first proposed by [[Heinz London]]. Its refrigeration process uses a mixture of two [[isotope]]s of [[helium]]: [[helium-3]] and [[helium-4]]. When cooled below ~ [[Orders of magnitude (temperature)|870]] [[Kelvin|millikelvins]], the mixture undergoes spontaneous [[phase separation]] to form a <sup>3</sup>He-rich phase and a <sup>3</sup>He-poor phase.
As with [[evaporative cooling]], energy is required to transport <sup>3</sup>He atoms from the <sup>3</sup>He-rich phase into the <sup>3</sup>He-poor phase. If the atoms can
be made to continuously cross this boundary, they effectively cool the mixture. Because the <sup>3</sup>He-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 <sup>3</sup>He-poor phase. Once the <sup>3</sup>He is all in the <sup>3</sup>He-poor phase, the refrigerator cannot continue to operate.
More commonly, dilution refrigerators run in a continuous cycle. The <sup>3</sup>He / <sup>4</sup>He mixture is liquified in a [[condenser]], which is connected through an impedance to the <sup>3</sup>He-rich area of the mixing chamber. Atoms of <sup>3</sup>He migrate across into the <sup>3</sup>He-poor phase, providing cooling power, and then into a [[still]] where the liquid <sup>3</sup>He 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.
==See also==
* [[Adiabatic demagnetization]] that can be used over the temperature range [[Orders of magnitude (temperature)|4 kelvins down to microkelvins]] and the related room temperature application of [[Magnetic refrigeration]].
* [[Dry dilution refrigerator]]
* [[Helium-3 refrigerator]]
==External links==
*[http://cdms.berkeley.edu/UCB/75fridge/inxsrc/dilution/ 3He-4He Dilution Explanation]
*[http://www.lancs.ac.uk/depts/physics/research/condmatt/ult/tech.html Lancaster University, Ultra Low Temperature Physics] - Description of dilution refrigeration.
*[http://marcuslab.harvard.edu/how_to/Fridge.pdf Harvard University, Marcus Lab] - Hitchhiker’s Guide to the Dilution Refrigerator.
[[Category:Cryogenics]]
[[Category:Cooling technology]]
[[de:³He-⁴He-Mischungskühlung]]
[[it:Refrigeratore a diluizione]]
[[ja:3He-4He希釈冷凍法]]
[[ru:Рефрижератор растворения]]