Two-Stream Effects in Present and Future Accelerators

Electron beams are perturbed by ions in a similar way as proton and positron beams are by electrons, generated via gas ionization, photoemission, or multipacting. Fast beam-ion or electron-cloud "two-stream" instabilities become more severe for higher beam current or closer bunch spacing, and they may limit the ultimate performance of an accelerator. For the Large Hadron Collider (LHC), the heat load deposited by electrons on the beam screen inside the superconducting magnets is a concern. I compare theories and simulations of these two-stream effects with observations at various storage rings, e.g., ALS, PLS, SPS, and KEKB, and comment on future machines, such as the LHC, the damping rings of a future linear collider, and a possible LHC upgrade operating with long "superbunches".