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.
Structural features
Vertical structure: suitable for burying in the ground, it will not be affected by the hot air of the motor, air temperature and sunshine, etc., and it is easy to keep cold; it can reduce the suction height of the pump, increase the filling head, and improve the effective NPSH; the vertical structure also It is beneficial to exhaust air, so that the suspension shaft has sufficient height to facilitate the arrangement of the shaft sealing device, so the pump is also called a vertical barrel pump.
Symmetrical structure: The overall structure of the pump strives to be symmetrically arranged so that it can deform uniformly in the cold state. The cold head generally adopts a two-stage mechanism, the first stage reaches 80K, and the second stage reaches a temperature of 10K.
Double-layer shell: In order to keep cold and facilitate the transportation of dangerous media, it is usually made into a double-layer shell. The inner shell is subjected to pump pressure, and the suction pressure is between the inner shell and the outer shell, which can appropriately reduce the strength requirements of the inner shell. The outer shell needs to be made into a vacuum interlayer, which has achieved the effect of heat insulation.