Some improvement work on ConFlat joints and their limit of reliability in a large-size ultrahigh vacuum system

The 2-km ultrahigh vacuum system of the CERN Intersecting Storage Rings (ISR) was operated in the 10-12 Torr range. During operation with circulating proton beams, it was found that the lifetime and intensity of the beams were deteriorated by a pressure rise effect. The measures to cope with this phenomenon included the installation of additional pumping speed, surface conditioning of the vacuum chambers by glow discharge, and longer bakeouts at 3000C. The latter put particular stress on the demountable all-metal [ConFlat (CF)] joints. This paper describes the summary of investigations and measurements on the CF joints of the ISR and the subsequent changes in design and material. A recording of vacuum leaks during a period of almost 12 years on the tubular UHV system with more than 6000 CF joints has enabled a statistical study of their reliability to be made. An analysis of these leaks is presented and failure details are illustrated using modern laboratory equipment such as scanning electron microscope and x-ray analysis