Design aspects of a differential vacuum pumping system

Abstract

A differential vacuum pumping system was designed as part of an experiment to study the neutralizing characteristics of different gases in the presence of a high-energy H− ion beam. The pumping system consisted of three vacuum chambers (stages) attached to each end of a 2-l neutralizer with each stage separated by a thin orifice plate. The test gases (nitrogen, argon, and xenon) were pressure fed into the neutralizer at a variety of flow rates to allow the experimenters to investigate neutralization at different gas densities. Working with fixed orifice sizes, dictated by the beam dimensions, the neutralizer exit orifices were extended in length to meet the density requirements in the neutralizer without exceeding the refrigeration capacity of the first-stage cryopumps. Because more than 90% of the gas expanding out of each end of the neutralizer into the first stages was pumped by the cryopumps, the gas loads into the second and third stages could be managed by lower pumping speed turbomolecular pumps. The double-ended differential pumping system met the requirements of neutralizer pressures as high as 1.1 Torr while, at the same time, maintaining <1×10−6 Torr in the third stage. This paper will describe the vacuum system and compare calculated data with measured data.