The electron cloud instability: summary of measurements and understanding

Electron-cloud effects presently limit the performance of several accelerators operating with high beam current, notably the SLAC and KEK B factories, the CERN SPS, the CERN PS, and the Los Alamos PSR. They are a major concern for many future projects, e.g. the CERN LHC and the SNS. An electron cloud is generated in the vacuum chamber by photoemission or beam-induced multipacting and subsequent electron accumulation during a bunch or bunch-train passage. Both coupled and single bunch instabilities, pressure rise, malfunctioning of beam diagnostics and failures of multi-bunch feedback systems have all been attributed to the cloud of electrons. We compare observations from various laboratories with computer simulations and analytical estimates, and we address mechanisms by which the electrons may dilute the beam emittance. Possible cures and future research directions are also discussed.