Evolution of the LHC Beam Screen Surface Conditioning Upon Electron Irradiation

For the vacuum scientists and the accelerator community, finding solutions to mitigate pressure rises induced by electron, photon, and ion desorption, and also beam instabilities induced by ion and electron clouds is a major issue. Moreover, it is worth noting that the OFE copper beam screen of the LHC is initially cleaned with standard industrial processes, leading to residual chemical contamination. Along the time, changes in the surface chemistry of vacuum chambers are observed during beam operations, leading to modifications of outgassing rates, stimulated desorption processes, and secondary emission yields (SEY). The impact of ions on molecule desorption and electron production was investigated to identify their influence on the global pressure rises and to quantify the ion conditioning effect on copper surfaces: (i) SEY evolution was measured to understand the changes of surface conditioning upon particle irradiation; (ii) surface chemistry evolution after electron irradiation was investigated by both XPS and TOF-SIMS analyses using the ANDROMEDE facility at IJCLab. Finally, the relationship between surface chemistry and the conditioning phenomenon will be discussed.