Cryogenic Pumps

Abstract

Cryogenic pumping is the capture of molecules on a cooled surface by weak van der Waals or dispersion forces. In principle, any gas can be pumped, provided that the surface temperature is low enough for arriving molecules to remain on the surface after losing kinetic energy. Cryogenic pumping is a clean form of pumping. The only gas or vapor contaminants are those not pumped, or released from pumped deposits. Unlike the ion pump, the cryopump does not retain condensed and physically adsorbed gases after the pumping surfaces have been warmed. Proper precautions must be taken to vent the pumped gas load.Cryopumps are used in a wide range of applications and in many forms. Liquid nitrogen-cooled molecular sieve pumps are used as roughing pumps. Liquid nitrogen traps are used between diffusion pumps and chambers to pump oil and water vapor. Liquid nitrogen or liquid helium "cold fingers" are used in high vacuum chambers to augment other pumps. Liquid cryogens or closed-cycle helium gas refrigerators are used to cool high-and ultra high vacuum cryopumps. Turbomolecular, TSP, or NEG pumps are sometimes appended to cryopumps to improve their pumping speed for hydrogen and deuterium. Cryopumping is the only form of pumping by which extremely large pump speeds (lo7 L/s) can be realized.Cryogenic pumping is not a new technique. The theory and techniques of pumping on cooled surfaces have been a part of vacuum technology much longer than the helium gas refrigerator; however, knowledge of both is necessary to understand the operation of a He-gas cryopump. Several reviews of cryopumping have been published. Hands [ 11 reviewed small refrigerator-cooled cryopumps and very large pumps used for fusion experiments. Bentley [2] explained the operation of the Gifford-McMahon refrigerator, and Haefer [3] has discussed the mechanisms of cryogenic pumping, given system calculations, and examples of pumps and applications. Welch [4] has written a comprehensive treatise on capture pumping technology. In this chapter we review the mechanisms of cryocondensation and cryosorption on which all cryogenic pumping is 263 A User's Guide to Vacuum Technology, 3rd Edition. John F. O'Hanlon