Simulation of Head-Tail Instability Caused by Electron Cloud in the Positron Ring at PEP-II ∗

Abstract

The head-tail instability caused by an electron cloud in positron storage rings is studied numerically using a simple model. In the model, the positron beam is longitudinally divided into many slices that have a fixed transverse size. The centroid of each slice evolves dynamically according to the interaction with a two-dimensional electron cloud at a given azimuthal location in the ring and a six-dimensional lattice map. A sudden and huge increase of the projected beam size and the mode coupling in the dipole spectrum are observed in the simulation at the threshold of the instability. Even below the threshold, the vertical beam size increases along a bunch train that has 8.5 ns bunch spacing. Above the threshold, a positive chromaticity can damp down the centroid motion but has very little effect on the blowup of the beam size. The results of the simulation are consistent with many observations at PEP-II.