Achieving ultra high vacua: pumps and pumping systems

Abstract

During the several decades of ultra high vacuum history many changes have occurred both in technology and users' needs, and even the meaning of ‘ultra high vacuum’ has somehow evolved. The focus has moved from achieving the lowest pressure, when the only gas load is wall outgassing (static vacuum), to obtaining low contamination and high purity with a flow of process gas (dynamic vacuum). The dream of a universal pump seems now less realistic, as the use of the different vacuum pumping techniques is more clearly defined for each application. This is valid also for the ion getter pumps, the traditional ultra high vacuum pump, which in most cases is used in combination with other pumps in the design of systems. Many attempts have been made in the past to improve the performance of the ion getter pumps at higher pressure, with limited success. Relatively little attention has been paid to the performance improvement at lower pressure. Some of the factors which cause pumping speed to decay at low pressure are analysed, and a possible configuration is proposed. Experimental results are then illustrated.