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He-3 cryostat & dilution refrigerator system


A helium-3 refrigerator is a device used in cryogenic physics. When immersed in liquid helium, the refrigerator can be cooled to 4.2K. To acquire lower temperatures, down to about 300mK, the system will evaporate helium-3 to cool the heat load further.

The refrigerator works in two steps. In the first step, the 3He gas is cryo-pumped by the cryogenic pump (sorb), is free to condense in the 1K plate region before accumulating in the 3He pot at around 1.5 K after approximately 20 minutes. In step 2, the sorb is cooled to below 10 K and begins to reduce the vapour pressure above the liquid 3He. The sample temperature cools to its base level. The refrigerator is able to keep its base temperature for tens of hours without a heat load.

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 approximately 870 mK, the mixture undergoes spontaneous phase separation to form a 3He-rich phase and a 3He-poor phase.

Since energy is required to transport 3He atoms from the 3He-rich phase into the 3He-poor phase, if the atoms can be forced to continuously cross this boundary, the mixture will be cooled. At equilibrium the 3He-poor phase cannot have less than 6% helium-3, even at absolute zero, so as a result a dilution refrigerator can be effective at very low temperatures. At 0.6K, 3He evaporates about 1000 times faster than 4He, so the circulated gas mainly consists of 3He. The volume in which this takes place is known as the mixing chamber. The 3He/4He mixture is liquified in a condenser, which is connected by an impedance tube to the 3He-rich area of the mixing chamber. Atoms of 3He migrate 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 to a higher pressure and is usually purified by cold traps, firstly at 77K, and then at 4.2K to remove any other gases, before being returned to the condenser to start the cycle again.

  •  Dilution refrigerator base temperature 0.01K
  •  He-3 cryostat base temperature 0.3K
  •  15T superconducting magnet