Miniature Dilution Refrigerator

几乎所有新兴的量子技术应用都需要低温。这是保持量子相干性的基础，因此，尽管芯片上冷却等技术可能会降低对设备平台的严格要求，但很明显，对亚开尔文系统的需求将继续增长。两级低温冷却器发展得很好，通常可以提供40K和4K的冷却功率，通过泵送液氦-4可以实现温度降至1K左右。然而，在较低温度下提供持续冷却的唯一实际方法是使用氦-3：要么在其表面泵送（约250mK），要么稀释制冷（至几mK）。目前可用的最紧凑的系统需要三相电源、冷却水、一个单独的机架来容纳气体处理设备、一个装有液氮的杜瓦瓶来冷却木炭捕集器，并且要高于头部高度，以便拆卸。此外，大多数机器使用10到35升氦-3。目前，氦-3只是制造和净化核武器用氚的副产品，所以它是一种罕见的、政治上不稳定的资源。随着对低温设备需求的增加，它们变得更小、更低功耗、更容易使用和使用更少的氦-3是必不可少的。这些改进将降低对环境的影响，扩大技术的使用范围，降低成本。Compact Cryogenics是一项新的冒险，利用这个市场机会，制造桌面大小，低功率稀释冰箱，没有外部氦-3循环，总氦-3需求量为2-3升。这个方案的关键方面是氦-3/氦-4混合物的创新循环系统。迄今为止，大多数试图将没有外部泵的稀释冰箱商业化的尝试都试图使用一个循环，在这个循环中，一对独立的氦-3冰箱级轮流提供300mK的冷却功率，这个300mK的冷级用于运行“低温循环”稀释级，在这个循环中，从蒸馏器蒸发的氦-3在300mK的板上凝结，然后返回混合室。这个循环最早是由苏联的研究人员在20世纪80年代提出的。这种冰箱的缺点是混合室的冷却功率很低，因为混合循环只能很小。1974年研制出1K罐双吸式循环泵，90年代研制出样机。目前似乎没有与此相关的专利。我们提出了一个完全不同的设计，在这个设计中，氦混合物本身由一个由四个低温泵组成的冷泵连续循环，并通过焦耳-汤普森膨胀冷却到冷凝点。在这种配置中，混合物循环速率（以及冷却功率）仅受制冷机可用的4K冷却功率的限制。这是由于我们引进了大大改进的低温阀，它可以主动驱动，以便打开和关闭每个低温泵部分。

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.