Origin of possible contamination introduced by a turbomolecular pumping system

Abstract

Turbomolecular pumping groups are widely used in accelerators for the pre-evacuation and during the bake-out of the vacuum system. A major requirement for these groups, apart from pumping speed considerations, is the cleanliness of the vacuum produced. In an attempt to clarify this question, a bakeable low-pressure vacuum system has been constructed to allow the direct comparison of the contamination introduced by a turbomolecular pump and by an ideally clean cryopump. This contamination has been checked by the quantitative analysis of the residual gas as well as of the gases desorbed from surfaces under electron bombardment. Contamination by the rotary pump oil is only apparent below 40% of the turbomolecular pump nominal rotation speed. When the pump is stopped, the system is contaminated by heavy hydrocarbons which can be eliminated by a 300°C vacuum bake out.