Abstract

Here we compute the ion distribution produced by an electron beam when ion-clearing electrodes are installed. This ion density is established as an equilibrium between gas ionization and ion clearing. The transverse ion distributions are shown to strongly peak in the beam's center, producing very nonlinear forces on the electron beam. We will analyze perturbations to the beam properties by these nonlinear fields. To obtain reasonable simulation speeds, we develop fast algorithms that take advantage of adiabatic invariants and scaling properties of Maxwell's equations and the Lorentz force. Our results are very relevant for high current energy recovery linacs, where ions are produced relatively quickly, and where clearing gaps in the electron beam cannot easily be used for ion elimination. The examples in this paper therefore use parameters of the Cornell Energy Recovery Linac project. For simplicity we only consider the case of a circular electron beam of changing diameter. However, we parametrize this model to approximate nonround beams well. We find suitable places for clearing electrodes and compute the equilibrium ion density and its effect on electron-emittance growth and halo development. We find that it is not sufficient to place clearing electrodes only at the minimum of the electron beam potential where ions are accumulated.

Highlights

  • Several processes can contribute to the production of ions in the vicinity of accelerated electron beams [1].These positively charged ions are attracted to the negative beam

  • The ion distribution oscillates within the electron beam, while the electrons are attracted to the ion distribution

  • New ions are created, leading to a remnant equilibrium ion density that establishes itself in the presence of clearing electrodes

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Summary

INTRODUCTION

Several processes can contribute to the production of ions in the vicinity of accelerated electron beams [1]. In energy recovery linacs (ERLs) [1], where the beam’s energy is dumped in rf cavities and is immediately used to accelerate new electrons, one cannot turn off the beam (because this would interrupt the ERL process) and one can not introduce short gaps in the beam (because this would disrupt the gun or the linac that injects large currents into the ERL) In both of these cases, ion-clearing electrodes may have to be used [10,11]. Because these longitudinal forces are relatively weak, it typically takes a few milliseconds for an ion to move from the place where it is produced to an ion-clearing electrode a few meters down the beam line During this time, new ions are created, leading to a remnant equilibrium ion density that establishes itself in the presence of clearing electrodes.

The 3D beam-beam force
Adiabatic invariants of ion motion
Electron propagation
Ion-force driven emittance growth for the Cornell ERL
CONCLUSION
Outlook for nonround systems
Findings
Methods
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