The pressure and gas composition evolution during the operation of the LEP accelerator at 100 GeV

The operation of the electron-positron storage ring LEP at an energy of 100 GeV leads to the deposition of a high-power flux on the vacuum chamber walls. The high critical energy of the photons emitted as synchrotron light (740 keV), places the photon-stimulated desorption in this machine in a completely new energy range compared to the existing electron storage rings. At this high photon energy, the energy dissipation of the photons in aluminium is done predominantly via the Compton process. This absorption mechanism leads to a larger enhancement of the gas desorption with the photon energy compared to the photoelectric effect. Measurements made at 100 GeV to study the variation of the pressure increase, of the gas composition and of the cleaning rates as a function of the beam dose and energy are presented and discussed in the context of the absorption of high- energy photons in aluminium. (8 refs).