Modeling of Resonant TE Waves for Electron Cloud Density Measurements in CesrTA

Maximizing the current and stability of the beam is an important goal in high-energy accelerators. Low-energy electron clouds can be produced from photoemission caused by synchrotron radiation, ionization of residual gas in beam-pipe, and secondary electrons. In lepton accelerators like CesrTA, low-energy electron clouds form primarily from synchrotron radiation of a positron beam and cause destabilization and lower the maximum possible total beam current [1]. The resonant TE wave technique is used to measure the electron cloud density in an accelerator. This method uses microwaves to resonantly excite standing waves the beam-pipe. The presence of an electron cloud within this standing wave will shift its resonant frequency slightly and the effect can be observed with a spectrum analyzer. In some beam-pipe geometries, the standing waves can be highly localized. For example, the geometries of some sections of CesrTA have lower cutoff frequencies than the surrounding beam-pipe. This can confine some of the resonances – and their sensitivity to the electron cloud to those short sections. This paper contains simulation and experimental results to show how resonant TE wave measurements can be localized.