The structure and principle of the diffusion pump are shown in Figure 1. The bottom is the evaporator, in which the diffusion pump oil is stored. The upper part is the air inlet, the right side pipe is the air outlet, and the mechanical pump provides the pre vacuum at the air outlet during operation. When the oil is heated by the electric furnace, the oil vapor generated is guided along the
The flow tube is ejected downward through the umbrella nozzle. Because there is vacuum (10-2 ~ 10-3 Torr) provided by mechanical pump outside the nozzle, the oil steam flow can be ejected for a long distance and hooked into a jet moving towards the air outlet. When the jet finally touches the wall cooled by cooling water, it will be given a liquid rest and flow back to the evaporator for reuse.
Once the gas molecules entering the pump from the air inlet fall into the steam flow, they will obtain the downward movement and fly down. Because the jet has high speed (about 200m / s), high vapor density (several to dozens of Torr of vapor pressure), and the diffusion pump oil has high molecular weight (350-500), it can effectively take away gas molecules. Therefore, in the jet interface, gas molecules can not stay for a long time, and its concentration is very small. In other words, on both sides of the jet interface, the extracted gas has a large concentration difference. It is this concentration difference that causes the energy source of the extracted gas to continuously cross the interface, diffuse into the jet and be taken to the outlet. There, they are taken away by mechanical pumps.
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