Review of Single Bunch Instabilities Driven by an Electron Cloud

Electrons generated and accumulated inside the beam pipe form an ‘electron cloud’ that interacts with a charged particle beam. If the number of electrons is sizable, this beam-cloud interaction can give rise to a two-stream instability, resulting in beam loss or emittance growth. The instability can occur within a single bunch, e.g., passing through the cloud on successive turns in a storage ring, or it can be a multi-bunch instability, where the motion of successive bunches is coupled via the electron cloud. In this paper, I review the experimental evidence for, simulation approaches to, and analytical treatments of singlebunch two-stream instabilities caused by an electron cloud. Depending on the parameter regime, this type of instability may resemble a coasting-beam instability, classical beam break-up, or transverse mode coupling. It can also cause long-term emittance growth. Despite of the apparent similarities, a few fundamental differences distinguish the two-stream instability from a conventional impedancedriven instability, and limit the applicability of established accelerator-physics concepts, like ‘wake field’. On the other hand, if, in addition to the electron cloud, spacecharge forces, conventional impedance, or beam-beam interaction are also present, these can conspire so as to enhance the growth rate.