用于自由电子激光装置的丝扫描束流横向截面测量系统原型机

Prototype of Wire Scanner System for Beam Profile Measurement for Free Electron Laser

基于丝扫描系统的束流横向截面测量为半阻拦式的测量方法,相比于截面靶测量对束流影响较小。上海高重复频率硬X射线自由电子激光装置(SHINE)束团截面在线测量系统计划采用丝扫描技术实现,为此设计研制了由丝靶探头、机械运动机构、束损探测器、电子学系统构成的丝扫描束流横向截面测量系统原型机。本文介绍了该原型机的工作原理、具体设计及在丝扫描探头上游安装腔式束流位置测量(CBPM)系统进行束流中心位置和束团电荷量精确补偿的优化方案。为精确评估该样机系统性能,在上海软X射线自由电子激光(SXFEL)中相邻位置布局了丝扫描探头和YAG靶截面探头,通过同时测量并对比同样束流截面的方法,对丝扫描系统原型机性能进行了束流实验评估。束流实验结果表明,在束团电荷量500 pC、电子能量0.84 GeV的条件下,丝扫描技术可用于电子束团的在线半阻拦式截面测量,测量不确定度好于30μm,测量结果与SXFEL装置标配的YAG靶截面靶测量系统结果一致。

Beam profile measurement via wire scanner is a semi-invasive method,which has advantage of less beam influence compared with a profile target,and is widely used in particle accelerator all over the world.Wire scanner is adopted to be applied to Shanghai High Repetition Rate X-ray Free Electron Laser aNd Extreme Light Facility(SHINE).For this purpose,a prototype of wire scanner composed of a wire target,a set of mechanical mechanism,a beam loss monitor and a set of electronics was designed and developed.A tungsten wire with an outer diameter of 20μm was used as the wire target,and the safety of this tungsten wire was proved by thermal simulation.The beam loss monitor in the wire scanner prototype consists of a quartz fiber that generates Cherenkov light,and a photomultiplier tube.The data acquisition module was developed based on the ADQ14AC-4C data acquisition platform from Teledyne SP Devices,which is equipped with ADC chip with nominal bit of 14 bit,effective bit of about 9 bit,and analog 3 dB bandwidth of 1.2 GHz.The principle and specific design of the prototype were introduced in this paper.The optimization scheme of installing a cavity beam position monitor(CBPM)upstream of the wire target to accurately compensate the beam center position and the beam charge was also mentioned.In order to accurately evaluate performance of the prototype,we measured the beam profile simultaneously by arranging wire scanner system and a YAG target adjacent to each other in Shanghai Soft X-ray Free Electron Laser(SXFEL).The comparison of the experimental results show that the wire scanner can be used for the online semi-invasive beam profile measurement of the 0.84 GeV electron beam with a bunch charge of 500 pC.The resolustion of the prototype wire scanner is better than 30μm,and especially less than 10μm in the direction of low beam jitter.When the beam position changes greatly,the measurement accuracy of the prototype wire scanner is sharply improved by the CBPM system.Furthermore,the relative charge quantity calculated by the CBPM reference cavity signal is used to compensate the beam loss signal,and the measurement result of the prototype wire scanner is slightly improved.The results of the profile target agree with those of the prototype wire scanner well which verify the function of the device applied to SXFEL.The prototype of wire scanner system has laid a good technical foundation for SHINE construction,and at the same time,wire scanner system with CBPM can be used as a supplement for beam profile measurement in SXFEL.