基于腔式探头的高分辨率束团电荷量测量方法研究

Methods study on high-resolution bunch charge measurement based on cavity monitor

束团电荷量是粒子束流最基本的特征参数,精确的电荷量测量和控制对于装置的运行以及束损的监测和安全联锁具有非常重要的作用。上海软X射线自由电子激光装置(Shanghai Soft X-ray Free Electron Laser Device,SXFEL)中加入了电荷量反馈环路,以期实现束团电荷量的精确控制并满足单束团实时电荷测量分辨率好于0.5%的要求,为此开展了高分辨率束团电荷量测量研究。由于腔式探头具有灵敏度高、原始信号信噪比高、近轴情况下位置无关等特点,因此被用作本研究中的束团电荷量信息拾取探头。传统的腔式探头通常采用单腔外本振的信号处理方案,要求额外提供特定频率的本振信号。为进一步简化系统,在此基础上提出一种新的双腔混频方案,该方案具有独立于本振频综及同步定时信号工作的特点。为了对比两种方案的性能优劣,对两种方案的实施效果进行了定性和定量分析。基于SXFEL装置的束流实验结果表明:单腔外本振混频方案电荷测量分辨率好于0.07%,双腔混频方案分辨率好于0.2%,前者的分辨率好于后者但二者均满足SXFEL用户装置要求。

[Background]Bunch charge is the most fundamental characteristic parameter of the particle beam.The Shanghai Soft X-ray Free Electron Laser Device Facility(SXFEL)needs to add a charge feedback loop to accurately control the bunch charge and the resolution of a real-time single bunch charge measurement is required to be better than 0.5%.At present,commonly used monitors that can realize online non-intercepting bunch charge measurement include beam current transformers,button-type or strip-type electrodes,and cavity-type probes,etc.Among them,the beam current transformer is susceptible to various electromagnetic interference,and its resolution is difficult to be improved;the button probe and the stripline probe have obvious bunch position dependence.The cavity probe has high sensitivity and high signal-to-noise ratio(SNR),position independence under paraxial conditions,thus it is very suitable for high-resolution bunch charge measurement.[Purpose]This study aims to explore the methods of highresolution bunch charge measurement based on cavity monitors.[Methods]Based on the traditional cavity probe signal measurement system using an external local oscillator signal mixing scheme,a new dual-cavity mixing scheme with a simple system structure without local oscillator frequency synthesis was proposed.It could work independently without synchronization timing signals.Both schemes were evaluated by beam experiments in the SXFEL facility.[Results]The experimental results show that the resolution of the single-cavity external IF(intermediate frequency)mixing scheme is better than 0.07%while the resolution of the dual-cavity mixing scheme is better than 0.2%.[Conclusions]Both schemes can achieve a high-resolution bunch charge measurement and satisfy the requirements of SXFEL user facility.