Miniature Dilution Refrigerator

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Almost all of the emerging applications for quantum technologies require low temperatures. This is fundamental to maintaining quantum coherence so while technologies such as on-chip cooling may lead to less stringent requirements for device platforms, it is clear that the need for sub-Kelvin systems is going to keep growing. Two-stage cryocoolers are well developed and typically provide cooling power at both 40K and 4K and temperatures down to about 1K may be achieved by pumping on liquid Helium-4. However, the only really practical to provide continuous cooling at lower temperatures is to use Helium-3: either by pumping on its surface (to about 250mK) or by dilution refrigeration (to a few mK). The most compact system available today needs three-phase power, cooling water, a separate rack to house gas handling equipment, a dewar of liquid nitrogen to cool a charcoal trap and stands above head-height to allow demountability. Furthermore most machines use between 10 and 35 litres of Helium-3. Currently, Helium-3 is only produced as a by-product of the manufacture and purification of tritium for nuclear weapons so it is a rare and politically volatile resource. As the requirement for low-temperature devices increases, it is essential that they get smaller, lower power, easier to use and use less Helium-3. These improvements will lead to lower environmental impact, wider access to the technology and lower costs. Compact Cryogenics is a new venture to take advantage of this market opportunity to make a desktop size, low power dilution refrigerator with no external Helium-3 circulation and a total Helium 3 requirement of 2-3 litres. The key aspect of this proposal is the innovative circulation system for helium-3/helium-4 mixture. Most attempts to date to commercialise a dilution refrigerator without external pumps have attemped to use a cycle in which a pair of independent Helium-3 refrigerator stages take it in turns to provide cooling power at 300mK, and this 300mK cold stage is used to run a 'cryogenic -cycle' dilution stage, in which Helium-3 evaporating from the still is condensed at a 300mK plate and so returned to the mixing chamber. This cycle was first developed by researchers in the USSR in the 1980s. These refrigerators had the disadvantage of very low cooling power at the mixing chamber as the mixture circulation could only be very small. A two-sorption pump, circulating design with 1K pot was developed in 1974 and a prototype made in the 1990s. There appear to be no current patents relating to this. We propose a radically different design in which the helium mixture itself is continuously circulated by a cold pump consisting of four cryopump sections and is cooled to the point of condensation by a Joule-Thompson expansion. In this configuration, the mixture circulation rate (and so cooling power) is limited only by the available 4K cooling power of the cryocooler. This is made possible by our introduction of greatly improved cryogenic valves, which can be actively driven in order to open and close each cryopump section.