The cryogenic pump is provided with a cold plate which is cooled to extremely low temperature by liquid helium or refrigerator. It causes the gas to condense and keeps the vapor pressure of the condensate below the pump's ultimate pressure, thus achieving the pumping action. The main functions of cryogenic pumping are cryogenic condensation, cryogenic adsorption and cryogenic capture. Cold condensation: the equilibrium pressure of a gas molecule condensing on the surface of the cold plate or on the condensed gas layer is basically equal to the vapor pressure of the condensate. The temperature of cold plate must be lower than 25K when air is pumped. When hydrogen is pumped, the cold plate is colder. The thickness of the cryogenic condensate is about 10 mm. Cryogenic adsorption: gas molecules are adsorbed to the surface of a adsorbent coated on a cold plate by a single molecular layer thickness (order of centimetres). The equilibrium pressure of the adsorption is much lower than the vapor pressure at the same temperature. If the vapor pressure of hydrogen is equal to the atmospheric pressure at 20K, the adsorption equilibrium pressure is lower than that at 20K activated carbon. This makes it possible to pump at higher temperatures by adsorbing at lower temperatures. Temperature trap: gas molecules that cannot be condensed at the pumping temperature are buried and absorbed by a growing layer of condensable gas.
In general, the ultimate pressure of the pump is the vapor pressure of the condensed gas at the cold plate temperature. At a temperature of 120K, the vapor pressure of water is lower than that of pa. At a temperature of 20K, the vapor pressure of all gases, except helium, neon and hydrogen, is lower than that of the pa. However, due to the temperature difference between the pump and the cold plate, the pump's ultimate pressure is higher than the vapor pressure of the condensate. For a container at room temperature and a low-temperature plate at 20K, the pump's ultimate pressure is about four times that of the condensate vapor pressure.