Brief Description of Purification of Ultra-high Vacuum System

The main pump of the ultra-high vacuum system consists of eight external cryocoolers and a built-in jaw cryopump. 8 sets of zDB-1320 chiller cryogenic pumps are symmetrically arranged in the upper part of the soil container. The nominal pumping speed of each pump is sxlO4L/S, which ensures that the container pressure of 6.5x10-IPa can be obtained when the heat sink is at room temperature. In order to increase the pumping speed and increase the degree of vacuum, a built-in cryogenic pump is installed in the container. It is pumped with cold helium below ZOK under liquid nitrogen shield, and the pumping speed is about 2x106L/s. In the space ring mold equipment, the cryogenic pump meets the requirements for obtaining a large vacuum pumping speed. However, it has low pumping speeds for gases such as helium and hydrogen, and it is prone to saturation. Therefore, when there are more such gases in the test, such as the leakage of the helium plate and the release of hydrogen gas from the test piece, the vacuum degree will be destroyed. It requires an auxiliary pump with a large pumping speed for gases such as helium and hydrogen. Due to the large volume of the vacuum vessel, the gas composition of different spacecraft specimens is also different. Therefore, the analysis and calculation of the residual gas must be used to establish the combined pumping system. After investigation and calculation, we have equipped three TPH2200 turbomolecular pumps. The nominal pumping speed of this pump is 2200L/s, and the nominal pumping speed for hydrogen is 320L/S. The combined system not only completes the pumping of the high vacuum stage, but also completes the leak detection task of the cryopump vacuum pumping system.

The container wall of the commissioning vacuum vessel when the external cryopump is evacuated at room temperature is made of stainless steel. In order to adapt to the ring die test of the spacecraft, the vessel is equipped with a liquid nitrogen heat sink. Therefore, we must complete the debugging and leak detection of the vacuum vessel before installing the heat sink. First of all, the pre-extraction unit is used to rough the vacuum of the container. At this time, not only the pre-extraction work needs to be completed, but also the stability test of the container subjected to external pressure is completed. A large-scale clean ultra-high vacuum was used to obtain a high vacuum in the container, and a leak was detected at the sealed part of the container. A total of four times of vacuum and leak detection were performed. At room temperature vacuum curve, in the debugging process, only the cryopump was used at first, and no molecular pump was used, so only the 4-pole mass spectrometer could be used to detect leakage. This process can also be at the beginning, but as time goes on, the background of the eel gradually rises and even the vacuum is destroyed. In one of them, the vacuum degree reached 10Pa shortly after power on. However, in the standard leak hole, due to the large amount of helium being charged, the cryogenic pump was poisoned, the vacuum degree decreased rapidly, and finally only shutdown. After using 3 TPH2200 molecular pumps, not only does this phenomenon not occur again, but also can use the molecular pump alone to complete the leak detection of the container. After the preliminary commissioning of the vacuum vessel was completed by the ultra-high vacuum commissioning, joint debugging was performed on the 3200m, space subsystem sub-systems. For vacuum debugging, the heat sink in the container is mainly used to pass liquid nitrogen, making its temperature lower than 10K, and then using internal and external cryogenic pumps for vacuum debugging, so that the vacuum degree reaches 4.5x10-6Pa, and the test vacuum test curve is in the test process. We generally only use a cryogenic pump to maintain the vacuum, and the vacuum is basically stable within the allowable range. However, sometimes the degree of vacuum suddenly deteriorates, and G point reflects the sudden change in the degree of vacuum. This is due to a change in the gas composition of the test piece (usually hydrogen or helium). In the space environment simulation test, we will often encounter some phenomena, some mechanisms are clearer, and some are new problems. For large space ring mold equipment, cryogenic pump is a clean high vacuum obtaining equipment with suitable pumping speed due to the need for large pumping rate. However, due to the strong selectivity of cryogenic pumping, molecular pumps must be used to make up for them, and at the same time meet the system's need for leak detection tasks. Therefore, in order to meet the vacuum requirements in the ring-mode test equipment, a cryogenic pump and a molecular pump must be scientifically and reasonably configured according to the gas composition to form a unique composite high vacuum pumping system.

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