Abstract
Low-energy, background electrons are ubiquitous in high-energy particle accelerators. Under certain conditions, interactions between this electron cloud and the high-energy beam can give rise to numerous effects that can seriously degrade the accelerator performance. These effects range from vacuum degradation to collective beam instabilities and emittance blowup. Although electron-cloud effects were first observed two decades ago in a few proton storage rings, they have in recent years been widely observed and intensely studied in positron and proton rings. Electron-cloud diagnostics developed at the Advanced Photon Source enabled for the first time detailed, direct characterization of the electron-cloud properties in a positron and electron storage ring. From in situ measurements of the electron flux and energy distribution at the vacuum chamber wall, electron-cloud production mechanisms and details of the beam-cloud interaction can be inferred. A significant longitudinal variation of the electron cloud is also observed, due primarily to geometrical details of the vacuum chamber. Such experimental data can be used to provide realistic limits on key input parameters in modeling efforts, leading ultimately to greater confidence in predicting electron-cloud effects in future accelerators.
Highlights
In recent years, a growing number of observations of electron-cloud effects have been reported in various positron and proton rings [1,2,3,4,5,6], in some cases after operating them in a new configuration [7]
Experiments have been carried out at the Advanced Photon Source storage ring using dedicated diagnostics to measure the properties of the electron cloud
The diagnostic is based on the planar retarding field analyzer, and both the time-averaged electron-cloud flux and energy spectrum were measured for electrons striking the vacuum chamber wall for varying machine conditions
Summary
A growing number of observations of electron-cloud effects have been reported in various positron and proton rings [1,2,3,4,5,6], in some cases after operating them in a new configuration [7]. Prior to the development of electron-cloud diagnostics, the presence of electrons had to be inferred indirectly either by an anomalous vacuum pressure rise, by noise induced on beam electronics, or by qualitative similarities between theoretical instability predictions and beam experiments. Such indirect evidence was not always entirely convincing [4]. A dramatic amplification was observed in the electron cloud for positron beams with a 20-ns bunch spacing (7 rf wavelengths, rf) This gain is attributed to beam-induced multipacting (BIM), and was accompanied by an anomalous vacuum pressure rise, consistent with observations in the CERN Intersecting Storage Ring [16]. The effects of surface conditioning and an estimate of the electron-cloud density in the chamber is discussed
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