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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 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. |
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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. |
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Such a system can be run with only one sorption cooler, and is a "single-shot" use, until complete evaporation of helium inside the evaporator of the sorption cooler. In order to run our closed dilution cooler in a continuous cycle, we can use two sorption coolers, used successively and linked to the condenser with two heat switches.
| The sorption cooler is made with a sorption pump, a pipe linked to
the cold source (cold plate of the cryostat), and an evaporator. This
cooler is filled with helium, 3 or 4. The pump contains active charcoals which adsorb or desorb gas at their surface. The thermal link in the middle of the cooler condenses the gas desorbed by the charcoals. When the pump is heated (45 to 50 K) the charcoals desorb helium; when all the gas is desorbed, it is stocked into the evaporator. Then the pump charcoals are cooled; gas is then little by little adsorbed : we are pumping over the helium bath. Pression over helium decreases, which results in a temperature fall. |
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