Methods study on high-resolution bunch charge measurement based on cavity monitor

Abstract

BackgroundBunch charge is the most fundamental characteristic parameter of the particle beam. The Shanghai Soft X-ray Free Electron Laser Device Facility (SXFEL) needs to add a charge feedback loop to accurately control the bunch charge and the resolution of a real-time single bunch charge measurement is required to be better than 0.5%. At present, commonly used monitors that can realize online non-intercepting bunch charge measurement include beam current transformers, button-type or strip-type electrodes, and cavity-type probes, etc. Among them, the beam current transformer is susceptible to various electromagnetic interference, and its resolution is difficult to be improved; the button probe and the stripline probe have obvious bunch position dependence. The cavity probe has high sensitivity and high signal-to-noise ratio (SNR), position independence under paraxial conditions, thus it is very suitable for high-resolution bunch charge measurement.PurposeThis study aims to explore the methods of high-resolution bunch charge measurement based on cavity monitors.MethodsBased on the traditional cavity probe signal measurement system using an external local oscillator signal mixing scheme, a new dual-cavity mixing scheme with a simple system structure without local oscillator frequency synthesis was proposed. It could work independently without synchronization timing signals. Both schemes were evaluated by beam experiments in the SXFEL facility.ResultsThe experimental results show that the resolution of the single-cavity external IF (intermediate frequency) mixing scheme is better than 0.07% while the resolution of the dual-cavity mixing scheme is better than 0.2%.ConclusionsBoth schemes can achieve a high-resolution bunch charge measurement and satisfy the requirements of SXFEL user facility.

Keywords

• sxfel
• cavity monitor
• bunch charge
• dual-cavity mixing
• high resolution