Abstract
Electron clouds in the beam pipe of high-energy proton or positron storage rings can give rise to significant incoherent emittance growth, at densities far below the coherent-instability threshold. We identify two responsible mechanisms: namely, (1) a beam particle periodically crosses a resonance and (2) a beam particle periodically crosses a region of the bunch where its motion is linearly unstable. Formation of halo or beam-core blow up, respectively, are the result. Key ingredients for both processes are synchrotron motion and electron-induced tune shift. The mechanisms considered provide a possible explanation for reduced beam lifetime and emittance growth observed at several operating accelerators. Similar phenomena are likely to occur in other two-stream systems.
Highlights
Electron clouds in the beam pipe of high-energy proton or positron storage rings can give rise to significant incoherent emittance growth, at densities far below the coherent-instability threshold
Electron-cloud effects below the threshold of coherent instability are a concern, in particular, for proton storage rings like the Large Hadron Collider (LHC), under construction, where synchrotron-radiation damping is effective only after many hours, and small perturbations can lead to significant long-term emittance growth
Experience at operating storage rings hints at possible incoherent effects at ‘‘moderate’’ electron density: The lifetime of the LHC proton beam in the CERN Super Proton Synchrotron (SPS) is limited to 5–20 min at the injection momentum of 26 GeV=c [5]
Summary
Electron clouds in the beam pipe of high-energy proton or positron storage rings can give rise to significant incoherent emittance growth, at densities far below the coherent-instability threshold. For the standard LHC simulation parameters (see Tables I and II in [11]), the emittance growth below the coherent-instability threshold is found to be roughly independent of the number of macroparticles representing the electrons and the proton bunch, but to strongly depend on the number of electron-beam interaction points.
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