低发射度C波段光阴极微波电子枪设计与优化

Design and optimization of low-emittance C-band photocathode RF electron gun

自由电子激光(Free Electron Laser,FEL)对电子束的发射度有极高的要求。近年来,可产生短电子束团并且具有高亮度性质的电子枪,是研发高性能FEL的核心部分,也是重要挑战与研究热点。一个很有前景的方向是提高微波电子枪(RF电子枪)内的加速梯度。提高现有电子枪的梯度,需要对电子枪微波场的优化、螺线管的设计和束流初始分布设置有全新的方案。为了解决这个需要多参数协调优化的问题,本文基于上海软X射线自由电子激光装置,率先从空间电荷力、加速电场与发射度补偿的角度,系统地分析和探索了高梯度C波段光阴极电子枪升级的设计与优化的方法。用ASTRA程序,对电子枪内不同电场强度对束流动力学的影响进行了模拟。详细分析了发射度补偿原理,对螺线管的主副螺线圈电流比进行优化,并研究了初始束团分布对发射度的影响。模拟结果表明:采用优化的电场强度、螺线管的主副螺线圈电流比与初始束团分布,电子枪输出束团的发射度有显著改善,满足FEL光源对于电子枪低发射度的要求。

[Background]Free electron laser(FEL)has extremely high requirements on emittance of the electron beam.In recent years,electron guns that can produce short electron bunches and have high brightness properties have become an important challenge and research hotspot for the development of high-performance FEL.[Purpose]This study aims to increase the acceleration gradient in the RF electron gun by design and optimization of the C-band photocathode of the RF electron gun.[Methods]Based on Shanghai XFEL facility,a new scheme was proposed to optimize the microwave field of electron gun,design the solenoid and set the initial beam distribution in terms of space charge effect.The effect of RF gradient in the RF gun on the beam dynamic was simulated.The principle of emittance compensation was analyzed in detail.The ASTRA code was used to optimize the current ratio of the solenoid’s main and auxiliary coils from the engineering point of view,and the influence of the initial bunch distribution on the emittance was investigated.[Results and Conclusions]The results show that by optimizing the number of electron gun cavities,the current ratio between the main and auxiliary solenoid coils of the solenoid and the initial bunch distribution,the emittance of the output bunch of the electron gun is significantly improved,satisfying the low emittance requirements of the FEL light source for the electron gun.