基于束流位置探测器的束团长度测量

分析了驱动电子束团的频域特性，研究了基于该特性进行长度测量的理论基础；使用三维模拟软件对束流位置探测器(BPM)进行建模，用模拟的方法对传输阻抗进行了数值计算；对不同长度的束团进行了测量和计算，并且分析了束团位置在真空管道中偏移对束团长度测量的影响。由测量结果可见，电子束团长度在10~100 ps(3~30 mm)时，测量误差均小于2%，满足中国工程物理研究院高平均功率自由电子激光太赫兹实验测量的使用要求。

Bunch length measurement based on beam position monitor

The electron beam of high average power Free Electron Laser Terahertz scientific research device in Chinese Academy of Engineering Physics (CAEP) has the characteristic of high average current. Non destructive measurement technique is required for the measurement of the beam parameters. The beam position monitor (BPM) is one of the basic measurement instruments. The method of extracting the signal spectrum of the BPM to obtain the bunch length is simple and can be used as a supplement or replacement of other bunch length measurement methods. This paper analyzes the frequency domain characteristics of the driving electron beam bunches, and analyzes the theoretical basement of the bunch length measurement based on the BPM. 3D simulation program was used for the numerical simulation of BPM transfer impedance modeling, the bunches of different length were measured and calculated, and the impact of the bunch’s off center position in the vacuum pipe on the bunch length measurements was also analyzed. The study results show that when the bunch length varies from 10 ps to 100 ps, all the measurement error is less than 2%, which meets the requirements of CAEP THz FEL operation.