1) The condensing pump is aspirated to condense gas on a low temperature surface for the purpose of pumping. When there is a cold surface in the vacuum chamber, condensation will occur on it, which can play the role of air extraction. This is the working principle of the cryogenic condensation pump and cryogenic cold trap. The saturated vapor pressure is closely related to the temperature, and when the temperature goes down, the saturated vapor pressure goes down.
Table 1 shows the standard composition of the atmosphere. Figure 1 shows the temperature relationship curve of saturated vapor pressure of various gases. As can be seen from figure 1, when the temperature drops to 20K, except for neon, hydrogen, helium and other difficult-to-condense gases, the saturated vapor pressure of all other components in the air is lower than 10-10 torr. Thus, when the airtight container filled with air is cooled to 20K, the vacuum can reach 1.2E-3 torr. At 4.2k, only helium is left, and the vacuum is up to 5e-5torr. The cold surface of the condensing pump can be generally selected at about 14-20k, and the method of flushing with easy condensing gas or vacuuming with other pumps is adopted to greatly reduce the amount of hydrogen, helium and other difficult condensing gas in the pumped system. A small amount of difficult-condensing gas that leaks into the system during operation can also be removed by other pumps.
[table 1] standard atmospheric composition
[FIG. 1] temperature relationship of saturated vapor pressure of various gases
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