Chapter 2.7 - Pumps for Ultra-High Vacuum Applications

Abstract

This chapter discusses several factors that should be considered when selecting pumps for ultrahigh vacuum (UHV) applications. A cryopump provides a nonselective, intrinsically clean, method for UHV pumping. Very large pumping speeds are readily available, and the simplicity of the device permits reliable operation. The capacity for pumping helium, hydrogen, and neon is very limited, compared to that for all other gases, but this is rarely of concern in a UHV system. A turbomolecular pump provides a nonselective, clean method for UHV pumping, combining simplicity of operation and high reliability. Current pumps are limited to a maximum speed of 10,000 L/s, but this is not a limitation for most UHV applications. Although turbomolecular pumps are effective for all gases, a limitation is observed in the pumping of hydrogen, one of the principal residual gases in a UHV system. The compression ratio for this gas is always less than that for all other gases, frequently being as low as 600 in a standard turbomolecular pump, resulting in a limit to the minimum residual hydrogen pressure that can be achieved.