Electron cloud and ion effects

The significant progress in the understanding and control of machine impedances has allowed obtaining beams with increasing brilliance. Dense positively charged beams generate electron clouds via gas ionization, photoemission and multipacting. The electron cloud in turn interacts with the beam and the surrounding environment originating fast coupled and single bunch instabilities, emittance blow-up, additional loads to vacuum and cryogenic systems, perturbation to beam diagnostics and feedbacks and it constitutes a serious limitation to machine performance. In a similar way high brilliance electron beams are mainly affected by positively charged ions produced by residual gas ionization. Recent observations of electron cloud build-up and its effects in present accelerators are reviewed and compared with theory and with the results of state-of-the-art computer simulations. Two-stream instabilities induced by the interaction between electron beams and ions are discussed. The implications for future accelerators and possible cures are addressed [1].