The design and performance of the LEP vacuum system at CERN

Abstract

The design of the 27 km long ultra-high vacuum system of LEP has been based largely on past experience with high energy electron and positron storage rings. However, stimulated by the large size of the machine and by the large number of individual components, new concepts had to be found in many areas of mechanical design such as flange connections between vacuum chambers and improved reliability with respect to vacuum leaks. The intense synchrotron radiation emitted by the electron and positron beams produces strong outgassing from the vacuum chamber walls during operation. This gas load exceeds by many orders of magnitude the static pressure and must be absorbed by a large, distributed pumping system around the whole machine circumference. LEP is the first large accelerator vacuum system which uses a combination of non-evaporable getter strip (NEG) and small, lumped ion pumps as its main pumping system. The operation of LEP for high energy physics requires a beam lifetime of many hours and thus an average vacuum pressure below 3 × 10 −7 Pa in the presence of circulating electron and positron beams. The performance and the operational experience with this new system are described.