Cryogenic solution to providing continuous operation for THz cameras

太赫兹(THz)波长位于红外光和无线电波之间，就像X射线一样，可以用来看到不透明的物体。这对天文学非常有用，因为它使我们能够透过太空中的尘埃，而这些尘埃在正常照片中是完全不透明的，实际上在大多数其他波长都是如此。因此，天文学家们开展了大量工作，以开发越来越灵敏的太赫兹探测器。与X射线不同的是，所有物体都在这些波长“发光”，所以不使用太赫兹光源就可以拍照，这与X射线图像不同，X射线必须透过(或对准)物体发光。太赫兹成像有许多应用，包括大气科学、聚变能源研究、安全扫描甚至艺术品保护。一个例子是探测衣服下的金属以寻找隐藏的武器。探测太赫兹波的一个困难是敏感而快速的探测器需要冷却到较低的温度(-272.8摄氏度，仅比绝对零度高0.35摄氏度，绝对零度是存在的最低温度)。这种温度下的探测器比常温下工作的探测器灵敏度高一千多倍。我们目前正在开发一种太赫兹相机，它可以做出足够快的反应来拍摄视频图像，但前提是要有足够的冷却。现在可以建造一种冷却到这样的温度的“冰箱”，就像家用冰箱一样，只需要电力就可以运行(传统上，需要继续供应非常冷的液氦，这使得这样的系统很难操作)。冷却器提供最终冷却(从绝对零度以上3摄氏度)，其工作原理是蒸发稀有元素(氦-3)。当它全部蒸发后，冷却必须停止一段时间，因为气体又变成了液体。持续运行的时间越长，需要的气体就越多。最近，氦-3的价格上涨了很多(从每升200美元到3000美元)，这使得这种冷却器非常昂贵。我们正在建议开发一种新型的冷却器，可以提供持续的冷却。这将通过两个冷却阶段来实现，其中一个阶段正在运行，而另一个阶段正在重置。除了在冰箱重置时不必每隔10小时暂停操作几个小时(这对于机场的安检扫描仪来说并不理想)的明显优势之外，它还减少了所需的昂贵的氦-3的数量，因为这两个冰箱都不需要特别长的操作时间。虽然这类制冷器非常适合像我们的太赫兹相机这样敏感的探测器阵列，但在冷却太赫兹探测器、其他波长的探测器以及其他应用中也有许多其他用途。

Terahertz (THz) wavelengths lie between infra-red light and radio waves, and - like X-rays - can be used to see through opaque objects. This is very useful for astronomy, as it enables us to see through dust in space which is completely opaque in normal photographs and indeed at most other wavelengths. Much work has therefore been carried out by astronomers to develop increasingly sensitive THz detectors. Unlike X-rays, all objects "shine" at these wavelengths, so it is possible to take pictures without using a THz source, unlike an X-ray image where X-rays have to be shone through (or at) the object. There are many applications for THz imaging, including atmospheric science, research into fusion energy, security scanning and even art conservation. One example is to detect metal under clothing to search for concealed weapons.A difficulty in detecting THz waves is that sensitive and fast detectors need to be cooled to low temperatures (-272.8 C, which is only 0.35 C above absolute zero, the lowest temperature which can exist). Detectors at these temperatures are more than a thousand times more sensitive than detectors which can operate at room temperature. We are currently developing a THz camera which can react quickly enough to take video images, but only if cooled sufficiently.It is now possible to build a "fridge" which cools to these temperatures, and like a domestic fridge only requires electricity to run (traditionally, it was necessary to keep supplying very cold liquid helium, making such systems difficult to operate). The cooler which provides the final cooling (from 3 C above absolute zero) works by evaporating a rare element (helium-3). When it has all evaporated, cooling must stop for a while as the gas is turned back into a liquid. The longer that continuous operation is required, the more gas is needed. Helium-3 has recently become much more expensive (from around $200 to $3000 a litre), making such coolers very expensive.We are proposing to develop a new type of cooler which provides continuous cooling. This will be achieved by having two cooling stages, one of which is operating while the other one is being reset. Besides the obvious advantage of not having to pause operations every 10 hours for a few hours while the fridge is reset (not ideal for, say, a security scanner in an airport), it reduces the quantity of expensive helium-3 required, since neither of the two fridges needs to have a particularly long operating time. While ideal for sensitive arrays of detectors such as our THz camera, such coolers will also have many other uses in cooling THz detectors, detectors for other wavelengths, and in other applications.