基于腔式探头的束团到达时间测量算法优化

腔式探头具有高信噪比和高灵敏度的特性，TM010模式下在近轴附近的腔体输出信号与束流横向位置无关，信号幅度与电荷量呈正比，信号相位由束团到达时间决定，因此非常适用于高分辨率的束团到达时间测量。上海硬X射线自由电子激光装置（SHINE）对束团到达时间的测量精度要求非常高，期望运用基于谐振腔的束团到达时间监测系统，可实现在100 pC电荷量下分辨率好于25 fs的技术指标。对于高Q的谐振腔信号，单点采样的信噪比随信号的衰减而降低，因此肯定存在一最佳的信号处理窗口。为分析最佳处理窗口与相关参数的定量关系，采用数值仿真和束流实验相结合的办法来研究这个问题。分析发现，存在最佳数据处理时间窗口，该参数只与信号衰减时间常数有关，而与系统采样率、系统信噪比、系统模拟带宽无关。在最佳数据处理时间窗口条件下，束流到达时间测量不确定性可获得最佳值。

Optimization of Bunch Arrival Time Measurement Algorithm for Cavity Probe

High signal-to-noise ratio and high sensitivity are the biggest characteristics of cavity probes. In TM010 mode of the cavity, the output signal near the paraxial axis has nothing to do with the transverse position of the bunch. The signal amplitude is proportional to the bunch charge, and the signal phase is determined by the bunch arrival time. Therefore, it is very suitable for high resolution bunch arrival time measurement. The SHINE has very high requirements for the measurement accuracy of the bunch arrival time. It is expected that the resolution can be better than 25 fs under the charge of 100 pC by a cavity based bunch arrival time monitor. For a high Q resonant cavity signal, the signal to noise ratio of single point sampling will decrease as the signal attenuates. Therefore, there must be an optimal signal processing window to obtain the best system resolution. Numerical simulation and beam experiments were used to find the relationship between the system parameters and the measurement uncertainty. Finally, it is found that there is an optimal data processing window, and this parameter is only related to the signal decay time. Under this window, the measurement uncertainty of the bunch arrival time can obtain the best value.