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Understanding how a diffusion pump works and operates, is fairly straight forward, however there are different parts that are essential for it to function properly. In this article, the main focus will be on what the main purpose of a cold trap is, and its maintenance process.  A cold trap is a device that looks like a lid that is attached to the inlet to the chamber, it is also known as a cryotrap in other types of pumps, but they both serve the same purpose.


The number one purpose of a cold trap is pretty self-explanatory, basically to trap the gas, utilizing the cold. Most of the time, a cold trap uses liquid nitrogen as its fuel to cool the gasses, and by doing so, the gasses will condense and no longer raise any further. The reason why such device is needed, is due to the gasses in the vacuum chamber often rise and can potentially escape through the inlet or even through the vacuum lines. The ideal function, is to keep all the gasses trapped, and cold trap devices are a major part of it. In addition to a cold trap device, manufacturers often use vacuum silicone grease to cover the o-rings and joints of the diffusion pump to make sure it’s completely airtight.


Since the cold trap is more or less an essential part of every diffusion pump, it rarely gets removed and when it is, it’s usually for cleaning purposes. The liquid nitrogen can easily be refilled without removing the actual device so the only time the cold trap needs to be removed, is when there has been a build-up of ice due to the extremely low temperatures, see Figure 1. Although the cold trap device itself is relatively easy to maintain, the whole process of diffusion pump maintenance can require some work and extra attention.


Ice build-up due to the extremely low temperatures caused by liquid nitrogen. Too much ice build-up can damage the pump.