Survey of recent results on electron cloud effects in photon machines

Electron cloud interactions with high-energy beams are believed,. responsible, for various undesirable effects ranging from vacuum degradation to collective beam instabilities. An important source of uncertainty in predicting electron cloud effects for a given machine lies in understanding in detail the processes relating to the generation of the cloud. The Advanced Photon Source (APS) has taken a lead role in the development of techniques for and the interpretation of direct measurements of the electron cloud distribution. Through such data, good progress has been made in placing realistic limits on critical input parameters in computer models relating to cloud production for actual accelerator chamber surfaces and geometries. Specially constructed electron detectors, using designs based on those first implemented at the APS storage ring, have been installed or are planned at a number of electron, positron, and proton rings to directly measure the properties of the electron cloud. Highlights will be presented of recent results from the APS, and a comparison is made between measurements at the APS and recent results at the Beijing Electron Positron Collider. These results illuminate machine conditions in which secondary processes become important.