Study Of The Cavity-based Bunch Length Diagnostic Technology For Linac

Cavity bunch length monitor uses the eigenmodes excited by the beam to measure the longitudinal root mean square(rms)length of the electron bunch in linac.As a noninvasive monitor,the cavity bunch length monitor has the advantages of large dynamic range,high signal-to-noise ratio,and simple structure.It is very suitable for the real-time non-destructive measurement for the short bunch in free-electron laser(FEL).In this paper,a study on the method of bunch length measurement based on the resonant cavity is carried out,and the basic principle,implementation approach,applications,and resolution of the method are analyzed.According to the beam characteristics of the FELiChEM,an infrared free-electron laser facility in the National Synchrotron Radiation Laboratory(NSRL)and the Shanghai Soft X-ray Free-Electron Laser facility(SXFEL),the cavity bunch length monitors were designed.This paper reports the design,machining,signal processing,and off-line test results of the monitor.Instead of the traditional cavity bunch length monitors that used TM010 mode,we adopt a higher-order mode,TM020 mode,to increase the working frequency and improve the resolution.According to the characteristics of the cavity output signal,a signal processing scheme based on FPGA is proposed.The signal sampling uses a high-speed data acquisition system based on dual-channel parallel time alternating sampling.The highest sampling rate can reach 1 Gsps.In order to study the performance of the cavity monitor,an improved offline test scheme using a thin wire to simulate the beam is proposed and exploited.Two tapered impedance matching with tension devices were exploited to minimize reflections and pull the wire tight.Based on the offline platform,the error analysis of bunch length measurement is conducted.By changing the transverse position of the wire,the effect of the beam transverse offset on the bunch length measurement is studied,and an error correction scheme is proposed.Using a two-dimensional network weighting method to simulate the beam with transverse distribution,the effect of the transverse distribution of the bunch on the bunch length measurement is studied.To improve the compactness of the pick-up,the single cavity bunch length monitors are proposed and designed.When the bunch passes through the cavity,two eigenmodes for measurement can be excited within a cavity at the same time.By using the selective coupling approach,the two modes are extracted without interference with each other.Thus,a single cavity is used to realize the function of measuring the bunch length.A series of simulated and experimental results show that the cavity monitor we designed can achieve the bunch length measurement with pretty high accuracy and resolution.