Cryogenic solution to providing continuous operation for THz cameras

太赫兹（THz）波长介于红外光和无线电波之间，像X射线一样，可以用来穿透不透明的物体。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，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下都是如此。因此，天文学家们开展了大量工作，以开发越来越灵敏的THz探测器。与X射线不同，所有物体都在这些波长下“发光”，因此可以在不使用太赫兹光源的情况下拍摄照片，而不像X射线图像那样，X射线必须穿过（或照射）物体。太赫兹成像有许多应用，包括大气科学、聚变能研究、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THz波的困难在于灵敏和快速的探测器需要冷却到低温（-272.8 C，仅比绝对零度高0.35 C，这是可能存在的最低温度）。在这些温度下的探测器比在室温下工作的探测器灵敏一千倍以上。我们目前正在开发一种太赫兹相机，它可以快速反应，拍摄视频图像，但前提是要足够冷却。现在可以建造一个“冰箱”，它可以冷却到这些温度，就像家用冰箱一样，只需要电力就可以运行（传统上，需要不断提供非常冷的液氦，这使得这种系统很难操作）。提供最终冷却（从绝对零度以上3 ℃）的冷却器通过蒸发稀有元素（氦-3）工作。当它全部蒸发时，冷却必须停止一段时间，因为气体又变成液体。需要连续操作的时间越长，需要的气体就越多。氦-3最近变得昂贵得多（从每升大约200美元到3000美元），使得这种冷却器非常昂贵。我们建议开发一种新型冷却器，它可以提供连续冷却。这将通过具有两个冷却阶段来实现，其中一个冷却阶段正在运行，而另一个冷却阶段正在重置。除了不用每隔10个小时就暂停几个小时的操作而重新设置冰箱的明显优势（这对于机场的安全扫描仪来说并不理想）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。虽然非常适合我们的THz相机等敏感探测器阵列，但这种冷却器在冷却THz探测器、其他波长的探测器和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.