Electron Cloud Observations: a Retrospective

The study of electron clouds in high-intensity storage rings remains an important area of R&D 30 years after the first observations were made. The list of cloud-induced effects continues to grow as new observations are reported. Data from dedicated electron cloud diagnostics, used in combination with standard beam diagnostics, have contributed to a better understanding of the physics of the electron cloud and its interaction with the beam. Examples of electron cloud observations have been selected from the published record to highlight various important aspects of the physics. Comparisons of data among various rings show interesting similarities and differences. Such data, used in combination with modeling and analytical calculations, more fully characterize the electron cloud distribution and help to identify cures. Rather than attempt to be comprehensive, this paper is meant to illustrate what we have learned and what needs to be better understood.