General Cryopumping Study
A great deal of enthusiasm has been generated in recent years over the attainment of ultrahigh vacuums by immobilizing or freezing gas molecules onto a cold surface in a closed system. This interest is mainly the result of the pumping requirements for space environmental simulation chambers of large capacity, that is, in the pressure range of 10-7 to 10-13 mmHg. However, the problem of attaining a high vacuum in a reasonably short time and of maintaining such a vacuum, if obtained, is a serious one in view of the outgassing characteristics of the materials inside the system and of the metal retainer walls, as well as the inevitable in-leakage to the system from the atmosphere. The volume of gases which must be pumped to maintain a vacuum of the magnitude of 10-7 to 10-13 mm Hg is, in many cases, very great, and the necessary size of the mechanical pumping facilities must be correspondingly enormous. (For example, the pump load of the forthcoming Mark I space simulation chamber to be built at the Arnold Center is estimated to be 100,000,000 liters/sec at 10-7 mm Hg .) In order to design and fabricate vacuum chambers of such size, the use of cryopumping or the cryogenic pumping technique has been considered and, in most cases, adopted as a general design requirement. This cryogenic technique for vacuum pumping has been proved to be more practical and economical than the mechanical pumping methods, according to several studies [2, 3, 4, 5].
KeywordsCold Trap Cold Surface Free Molecular Flow Capture Coefficient Trapping Phenomenon
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