Cavity Beam Position Monitor (CBPM) adopting a resonant cavity structure and using the characteristic modes excited by the electron beam to measure the beam parameters, has the advantage of high position resolution and is widely used in Free-electron Laser (FEL) facilities. The wake field of different modes carries different bunch information, the amplitude and phase of the signals of different modes can be extracted through the signal processing method to obtain the characteristic parameters of the source bunch. In the application of bunch charge and position measurement, the accurate amplitude extraction method for cavity BPM signal is the primary issue to be considered when designing the data acquisition and processing system. In this paper, through theoretical analysis and numerical simulation, it is proved that the optimal algorithm of amplitude extraction for CBPM exists, and the dependence between the data processing window size and the decay time of the cavity BPM under the optimal design is given. In addition, the relationship between the optimized amplitude extraction uncertainty and the noise-to-signal ratio, sampling rate of data acquisition and processing system, and the decay time of the cavity BPM is also proposed, which can provide clear guidance for the design and optimization of the CBPM system. Accordingly, the CBPM system in Shanghai Soft X-ray FEL (SXFEL) facility is optimized, and the resolution improved from 273 nm to 177 nm at the bunch charge of 500 pC.

Cavity Beam Position Monitor (CBPM) 采用谐振腔结构并利用由谐振腔激发的特征模式 电子束测量光束参数，具有位置分辨率高的优点，广泛应用于自由电子激光（FEL）设施。不同模式的尾流场携带不同的束信息，通过信号处理方法可以提取不同模式信号的幅度和相位，得到源束的特征参数。在束电荷和位置测量应用中，腔体BPM信号的精确幅度提取方法是设计数据采集和处理系统时要考虑的首要问题。本文通过理论分析和数值模拟，证明了CBPM幅度提取的最优算法是存在的，给出了优化设计下数据处理窗口大小与腔BPM衰减时间的相关性。此外，还提出了优化后的幅度提取不确定度与信噪比、数据采集处理系统采样率、腔体BPM衰减时间之间的关系，可为设计和设计提供明确的指导。 CBPM 系统的优化。相应地，对上海软X射线FEL（SXFEL）设施的CBPM系统进行了优化，在500 pC的束电荷下，分辨率从273 nm提高到177 nm。并且还提出了腔体BPM的衰减时间，可为CBPM系统的设计和优化提供明确的指导。相应地，对上海软X射线FEL（SXFEL）设施的CBPM系统进行了优化，在500 pC的束电荷下，分辨率从273 nm提高到177 nm。并且还提出了腔体BPM的衰减时间，可以为CBPM系统的设计和优化提供明确的指导。相应地，对上海软X射线FEL（SXFEL）设施的CBPM系统进行了优化，在500 pC的束电荷下，分辨率从273 nm提高到177 nm。