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Dilution RefrigeratorA 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 700 mK, 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 helium-3 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 helium-3 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 helium-3 migrate across into the 3He-poor phase, providing cooling power, and then into a still where the liquid helium-3 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 mK can be achieved with the best systems. External link - http://cdms.berkeley.edu/UCB/75fridge/inxsrc/dilution/
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