1. The principle of air extraction
Inside the cryopump is a cold plate cooled to a very low temperature by liquid helium or a refrigerator. It condenses the gas and keeps the vapor pressure of the condensate below the ultimate pressure of the pump, so as to achieve the pumping effect. The main functions of low temperature pumping are low temperature condensation, low temperature adsorption and low temperature capture.
① Low temperature condensation: gas molecules condense on the surface of the cold plate or on the condensed gas layer, and the equilibrium pressure is basically equal to the vapor pressure of the condensate. When pumping air, the temperature of the cold plate must be lower than 25K; when pumping hydrogen, the temperature of the cold plate is lower. The thickness of the low-temperature condensation and extraction condensation layer can reach about 10 mm.
②Low temperature adsorption: The gas molecules are adsorbed on the surface of the adsorbent coated on the cold plate with a thickness of a monomolecular layer (in the order of centimeters). The equilibrium pressure for adsorption is much lower than the vapor pressure at the same temperature. If the vapor pressure of hydrogen is equal to atmospheric pressure at 20K, the equilibrium pressure of adsorption is lower than Pa when absorbing hydrogen with 20K activated carbon. This makes it possible to perform pumping by cryogenic adsorption at higher temperatures.
③Cryogenic trapping: Gas molecules that cannot be condensed at the extraction temperature are buried and adsorbed by the growing layer of condensable gas.
Generally speaking, the ultimate pressure of the pump is the vapor pressure of the condensed gas at the temperature of the cold plate. When the temperature is 120K, the vapor pressure of water is already lower than Pa. At a temperature of 20K, except for helium, neon and hydrogen, the vapor pressures of other gases are also lower than Pa.
However, due to the different temperatures of the pumped container and the cryogenic cold plate, the ultimate pressure of the pump is higher than the vapor pressure of the condensate. For a vessel at room temperature, with a cryopanel of 20K, the ultimate pressure of the pump is about 4 times the vapor pressure of the condensate.
2. Type
Cryopumps are divided into two types: injection-type liquid helium cryopumps and closed-circuit gas helium refrigerator cryopumps.
Injection liquid helium cryopump
It is mainly composed of a liquid helium container, a pump body and a liquid nitrogen cavity connected to a baffle. In order to reduce the consumption of liquid helium, the outer wall of the liquid helium container adopts a double-layer thermal insulation wall and is evacuated in between.
When the pump is pre-pumped to Pa pressure, liquid nitrogen and liquid helium are injected, and the gas condenses on the working cold plate of 4.2K. After pre-pumping, the partial pressure of helium and hydrogen is in the order of Pa, so the pump can obtain the ultimate pressure below Pa. If the liquid helium container is evacuated and decompressed to 6650 Pa, the temperature of the liquid helium can be reduced to 2.3K, and a lower limit pressure can be obtained.
Closed cycle gas helium refrigerator cryopump
It is a new type of cryopump that appeared in the 1970s. This pump does not consume helium, is easy to operate, easy to maintain, and is increasingly used. The refrigeration medium of the refrigerator is gas helium, the temperature of the first-stage cold plate is 50-100K, and the typical application is 65K, which is used to condense water vapor and pre-cool other gases; the temperature of the second-stage cold plate is 10-20K, which is used to condense nitrogen, gases such as oxygen and argon.
The inner surface of the secondary cold plate is coated with activated carbon. Activated carbon has strong adsorption capacity for helium, neon and hydrogen at low temperature. The cold plate is made of oxygen-free copper, and the surface is polished to a mirror level to reduce the emissivity.
The ultimate pressure of the pump is Pa, the working pressure range is Pa, and the pre-pumping pressure is required to be 1 Pa. The pumping rate of the finished product has reached 60,000 liters/second. In addition, according to the characteristics of the process, the air extraction cold plate can be arranged in the pumped container, and the air extraction rate can reach more than 106 liters/second.