Investigation on the Ion Motion towards Clearing Electrodes in High-Brightness Electron Accelerators

Abstract

High-brightness electron beams provided by modern accelerators require several measures to preserve their high quality and to avoid instabilities. The mitigation of the impact of residual ions is one of these measures. It is particularly important if high bunch charges in combination with high repetition rates are aimed for. This is because ions can be trapped in the strong negative electrical potential of the electron beam causing emittance blow-up, increased beam halo and longitudinal and transverse instabilities.Over the last decades three ion-clearing strategies have been applied to counteract the degrading impact of the ions on the electron beam. These strategies are installation of clearing electrodes, introduction of gaps in filling pattern and beam shaking. Currently, their merit as clearing strategies for the next generation of high brightness accelerators such as energy recovery linacs (ERLs) are under intensive investigations by means of numerical and experimental studies. Of particular interest in this context is the performance of multi-purpose electrodes, which are designed such that they allow for a simultaneous ion-clearing and beam-position monitoring. Such electrodes will be installed in the bERLinPro facility.In this paper, we present numerical studies for the behavior of ions generated by electron bunches while passing through the field of clearing electrodes. The objective is to investigate the ion motion towards the electrodes and to study under which circumstances and up to which ratio an equilibrium between ion-generation and ion-clearing is established. Hereby, several ion species and configurations of electrodes are considered in combination with typical beam parameters of high-brightness electron accelerators. Furthermore, we present detailed numerical studies of the performance of multi-purpose clearing electrodes planned for bERLinPro.