Secondary Yield and Electron Clouds
“Electron cloud” is a special type of electron resonant multiplication effect, called multipactoring. A “cloud” of electrons forms around intense positively-charged circulating bunches. The cloud is spatially-coupled to the transverse motion of the bunches, leading to a potential disruption of trailing bunches. The source of the original electrons, that subsequently form the cloud, is synchrotron radiation striking the chamber walls. These photoelectrons generate secondary electrons that further multiply at the walls and can even be accelerated to the opposite wall by the passing positive bunches. The electron cloud density depends on characteristics of the positively-charged circulating beam (bunch length, charge and spacing) and the electron secondary yield (SEY) of the wall surface from which the starting electrons arise. PEL measures the SEY of chamber material and its SE-suppressing coatings, first for SPEAR and PEP, and now for the proposed ILC positron damping rings.
Electron-conditioning of TiN-coated Al alloy, originally exposed to ambient atmosphere. Inset shows SEY for first (A) and last (B) conditioning points.
Further reading: “Secondary electron emission yields from PEP-II accelerator materials”, Nucl. Instr. And Meth. In Phys. Research A469, 1-12 (2001). (SLAC Document Server)
