Evaluation of the NEG Coating Saturation Level after 3 Years of LHC Beam Operation

The room temperature vacuum system of the Large Hadron Collider (LHC) at CERN has been designed to ensure vacuum stability and beam lifetime of 100 h with nominal current of 0.56 A per beam at 7 TeV of energy. The requirements for the interaction regions are moreover driven to minimize the background noise of the experiments, to keep the equivalent hydrogen gas density below 1013 molecules of H2 per m3. \nDuring the last two years, the LHC operated with proton beams at a maximum energy of 4 TeV, coasting for several hours each time, causing vacuum pressure increase owing to different effects: synchrotron radiation, electron cloud and localized temperature increase due to high order modes (HOMs). \nAll these phenomena liberated an important gas load from the vacuum chamber walls, which led in some cases to a partial or a total saturation of the NEG coating. To match the design vacuum performances and hence to schedule technical interventions for NEG vacuum reactivation, it is necessary to take into account all these aspects and to regularly evaluate the saturation level of the NEG coating. \nThis study analyses the saturation level of the NEG coated beam pipes in the LHC accelerator. Pressure reading variation of the Bayard-Alpert gauges without circulating proton beams are analysed and combined with laboratory studies of the NEG saturation behaviour. In addition, Vacuum Stability Code (VASCO) simulations are used to define the gas density profile change due to the saturation process in the NEG coated beam pipes.