腔式束流位置探头初步束流实验与参数仿真

Preliminary beam experiment and parameter simulation for cavity beam position monitor

针对下一代FEL装置束流位置精确测量的需求,中国科学院上海应用物理研究所研制了5712MHz高Q腔式BPM测量系统样机。其中ADC分辨率及数据长度是影响系统位置分辨率的关键参数。为寻求参数的最优化,采用束流实验结合蒙特卡洛的方法对此问题进行了研究。利用上海深紫外自由电子激光装置标定了探头位置因予,在此基础上采用蒙特卡洛方法,分别对射频采样、中频采样数据采集处理方法中的数据长度、ADC的分辨率要求进行了仿真。结果表明,射频采样方法由于现有超高速ADC分辨率的限制,难以实现亚微米量级的精确测量,更适合于样机的快速参数验证与评估;中频采样方法可充分发挥百MHz ADC高分辨率的优点,当ADC分辨率高于14位时,系统位置分辨率即可好于亚微米,是腔式BPM系统在线数据采集处理的最优选择。采用FFT方法对本样机信号进行处理,射频采样和中频采样的最优数据长度分别为8192点、256点。

Background： Aiming at precise beam position measurement of next generation FEL facility, Shanghai institute of applied physics developed 5712 MHz high Q CBPM prototype. Purpose： ADC bits and data length of data acquisition system are key parameters influencing spatial resolution of CBPM. In order to find the optimized configuration parameters, dedicated beam experiments and corresponding Monte Carlo simulations were used to solve the problems. Methods： The position calibration factors of CBPM prototype have been obtained through beam based calibration experiment at the Shanghai deep ultraviolet FEL facility. The dependence between system spatial resolution and ADC bits and the data length have been simulated with the measured position calibration factors by using Monte Carlo method. Results： Equipped with 14bits or better ADCs IF sampling technique is capable to achieve submicron position resolution, which is the optimal choice for CBPM on-line data acquisition system. Applying FFT signal processing algorithm, optimal data lengths in RF and IF sampling techniques were respectively 8192 and 256 points. Conclusions： RF sampling technique is hard to accomplish precise measurement of submicron level due to limited bits of present ultra- high-speed ADC, but suitable for plug ＆ play prototype parameter test and evaluation.