Beam test results of high Q CBPM prototype for SXFEL

In pursuit of high-precision beam position measurements at micrometers or submicrometers for the Shanghai soft X-ray free-electron laser(SXFEL) facility which is under construction in the vicinity of the Shanghai Synchrotron Radiation Facility,a high Q cavity beam position monitor(CBPM) with a resonant frequency of4.7 GHz is developed by the Shanghai Institute of Applied Physics,and the relevant BPM electronics with a dedicated RF front end,and a digital BPM,are completed.The cavity design,cold test,system architecture,and first beam test are performed at the Shanghai deep ultraviolet free-electron laser(Zhao et al.in Nucl Instrum Meth A 528(1-2):591-594,2004.doi:10.1016/j.nima.2004.04.108) facility.Results of the beam experiment show that the performance of the CBPM is consistent with basic expectations,and the beam position resolution can fulfill the requirements for the SXFEL project if the beam conditions are optimized.

Optimization of the cavity beam-position monitor system for the Shanghai soft X-ray free-electron laser user facility

To achieve high-efficiency operation of the highgain free-electron laser(FEL),the electron beams and radiated photon beams need to be overlapped precisely and pass through the entire undulator section.Therefore,a high-resolution beam-position monitor(BPM)is required.A cavity BPM(CBPM)with a resonant cavity structure was developed and used in the Shanghai Soft X-ray FEL(SXFEL)test facility and can achieve a position resolution of<1μm.The construction and operation of the SXFEL user facility also bring about higher requirements for beamposition measurement.In this case,the factors that affect the performance of the CBPM system were further analyzed.These included the amplitude and phase stability of the local oscillator,stability of the trigger signal,performance of the radio frequency front-end,signal processing electronics,and signal processing algorithms.Based on the upgrade and optimization of the system,a beam test platform was built at the end of the linear acceleration section of the SXFEL,and the experimental results show that the position resolution of the system can reach 177 nm at a bunch charge of 500 pC,and the dynamic range is controlled within±300μm,and the relative measurement uncertainty of the bunch charge can reach 0.021%,which are significant improvements compared to the attributes of the previous system.

Study of the crosstalk evaluation for cavity BPM

In order to pursue high-precision beam position measurements for free-electron laser facilities, a cavity beam position monitor(CBPM) is employed to measure the transverse position that can meet the requirement of position resolution at a sub-micrometer or even nanometer scale. However, for the pill-box cavity BPM, crosstalk between the cavities will have an effect on the accurate measurement of beam position. To reduce the effect of crosstalk on CBPM performance and ease the measurement of the isolation between the cavities, the cavities with a slight difference in resonant frequency were designed and applied in the Dalian coherent light source and Shanghai soft X-ray free-electron laser facilities. Furthermore, two methods, the principal component analysis method and the method of harmonic analysis, are proposed in this paper to evaluate the crosstalk. The results demonstrate that the two methods are feasible in evaluating the crosstalk between the cavities.