Overview of cryopumps

    The low-temperature surface placed in the vacuum container has the function of pumping air. This kind of pump that uses the low-temperature surface to condense and adsorb gas to achieve the purpose of pumping is called a cryopump. Generally, people call a device that uses a cryoplate with a general cold temperature (above 150K) to capture water vapor and organic solvents as a condenser; the liquid nitrogen and liquid air temperature (below 80K) placed between the pump and the container are called condensers. Cryogenic plate, a device used to capture the reflux vapor of the pump, is called a cold trap or cryogenic baffle; only the pumping device whose temperature of the cryogenic plate is below 20K is called a cryopump.

    The earliest people used liquid nitrogen temperature low-temperature baffle to trap the oil (mercury) vapor of the diffusion pump. This method can increase the ultimate pressure of the diffusion pump by 1 to 2 orders of magnitude. However, the use of cryogenic panels to remove common gases (nitrogen, oxygen, carbon monoxide, etc.) in the vacuum system requires a low temperature below the liquid hydrogen temperature (20.4K), and a low temperature below 4K to remove hydrogen, neon and other gases. Among the common gases, helium has the highest saturated vapor pressure, and the saturation pressure at 4.2K is 1atm (latm=101325Pa). It is very difficult to extract helium only by cryogenic plates.

    Modern liquid helium refrigerators have reached a fairly high level, with high liquefaction efficiency and automatic long-term continuous operation, especially the appearance of miniature refrigerators, which has laid a good foundation for the development of cryogenic vacuum pumps for refrigerators. In 1957, Lasarew and Baily first applied extremely low temperatures to cryogenic pumps. In order to extract all gases in the temperature range of 10-20K, modern cryopumps are coated with getters such as activated carbon and molecular sieve on the cryogenic plate, which greatly improves the pumping performance of the cryopump.

    The basic principle of cryogenic pumping is the condensation, adsorption and cryogenic capture of gas on the surface of the cryogenic plate. Low-temperature condensation refers to the continuous condensation of gas on the low-temperature plate when the pressure of the gas being pumped into the space is higher than the corresponding saturated vapor pressure of the condensation plate. The limitation of cryogenic condensation is that once the gas pressure reaches its saturated vapor pressure, the pumping stops. In order to obtain a lower pressure, a cryogenic adsorption pumping method is required. If the low-temperature surface can keep the coverage of the monomolecular layer small, the saturation pressure of the surface can be reduced to 10^-1-10^-12  times of its own condensate vapor pressure. 

    The saturated adsorption capacity at different temperatures varies greatly. At low pressure, the adsorption capacity decreases linearly with the pressure, which means that more and more adsorption sites are occupied, and the pressure continues to rise. After a few monolayers are deposited, the adsorption capacity reaches the limit. If the gas pressure at this time Above the saturated vapor pressure, condensation begins to occur. The thickness of the adsorption layer increases with the pumping time. The maximum thickness of the deposited layer is limited by the surface temperature of the deposited layer and the heat radiation of the surrounding high-temperature surface.

    Low temperature trapping refers to the adsorption of gas molecules on the surface or inside of the deposited loose frost layer. Under ultra-high vacuum conditions, the thickness of the deposited layer and the deposition rate are very small, so it is not the main mechanism of cryopump pumping. This phenomenon may exist, but the effect is limited.

    The biggest advantage of cryopumps is that they can obtain a clean vacuum (or oil-free vacuum). Its working medium is completely enclosed in the Dewar or refrigerator without being exposed to vacuum, and there is no pollution problem of grease, mercury and other impurity gases.

    Secondly, the pumping speed of the cryopump can be from 10²L/s to 10⁵L/s. The pumping speed of cryopumps used in large-scale space ring die equipment requires hundreds of thousands to millions of liters, and such a large pumping speed is difficult to achieve with other vacuum pumps.

    The ultimate pressure of the cryopump is low. The stainless steel test cover can obtain an ultimate pressure of 10^-7 Pa without baking; after baking at 300 ℃, the ultimate pressure can be increased by 1 to 2 orders of magnitude.

    Compared with diffusion pumps and turbomolecular pumps, cryopumps have a strong selective pumping effect. The pumping speed, maximum pumping volume and ultimate pressure of different gases vary greatly. When selecting cryogenic pumps, the influence of the components of the pumped gas on the pumping performance of cryogenic pumps should be considered. In addition, the starting pressure, maximum working pressure, and maximum pumping capacity of cryopumps are also limited by the maximum cooling capacity of the refrigerator. Similar to sputter ion pumps, cryopumps are only suitable for high vacuum and ultra-high vacuum, but not for low vacuum.

    Because cryopumps have the advantages of no oil pollution, high efficiency, and easy operation, they are used in the fields of vacuum coating, atomic energy accelerator, space ring mold equipment, controllable thermonuclear reaction, electron microscope, mass spectrometer, high-frequency crystal calibration and scientific research. All have been widely used.

(Huiputech)