Buildup of electron cloud in the PEP-II particle accelerator in the presence of a solenoid field and with different bunch pattern

M. A. Furman,Y. Cai,M. Pivi

doi:10.1103/physrevstab.7.024402

Abstract

We have augmented the code POSINST to include solenoid fields, and used it to simulate the build up of electron cloud due in the PEP-II positron ring. We find that the distribution of electrons is strongly affected by the resonances associated with the cyclotron period and bunch spacing. In addition,we discover a threshold beyond which the electron density grows exponentially until it reaches the space charge limit. The threshold does not depend on the bunch spacing but does depend on the positron bunch population.

Highlights

It has been well established by many experimental evidences [1,2] at KEKB and PEP-II that the instabilities caused by electron impose a severe limitation upon the luminosity in eeÿ storage rings
Based on the experiments [1] at KEKB, there exists a current threshold beyond which the vertical beam size at the interaction point starts to grow like y / Np2=Sb, where Np is the bunch population and Sb is the spacing between two sequential bunches
We know from the simulation performed by Zimmermann [4] that longitudinal solenoid field Bs confines the electrons near the wall of the vacuum chamber and reduces the cloud density near the positron beam

Summary

INTRODUCTION

It has been well established by many experimental evidences [1,2] at KEKB and PEP-II that the instabilities caused by electron impose a severe limitation upon the luminosity in eeÿ storage rings. Since Np is normally set at the limit allowed by the beam-beam interaction, this observation implies that Sb cannot be too small otherwise the vertical blowup degrades the luminosity As a result, both B-factories are currently operated at Sb 2 m, which is larger than its design value. We know from the simulation performed by Zimmermann [4] that longitudinal solenoid field Bs confines the electrons near the wall of the vacuum chamber and reduces the cloud density near the positron beam. All this indicates that both Sb and Bs play vital roles in the physics of electroncloud instability. We study the dynamics between the positron beam and electron cloud with different Sb and Bs to reveal the physics indicated from the simulations and experiments in the PEP-II low-energy ring (LER)

Sources of electrons

Secondary emission process

Simulation model

Electron motion

Bunch train

Cyclotron resonance

ELECTRON-CLOUD INTENSITY THRESHOLD

Calculation of the vertical dipole wake field induced by the electron cloud

Effect of the solenoid field on the wake field

DISCUSSION