低发射度L波段光阴极微波电子枪物理设计

Physical design of low-emittance L-band photocathode microwave electron gun

太赫兹(THz)光源对电子束的能量、能散及发射度有极高要求,研发高性能电子源是基于自由电子激光(Free Electron Laser,FEL)的THz光源的重要挑战。对电子腔中束团发射度增长机制的研究,有助于设计针对有效的发射度补偿方案。本文首先描述了L波段光阴极微波电子枪腔的设计,利用POISSON Superfish软件给出了腔内电磁场分布,详细分析了束流发射度增长的因素,讨论发射度补偿原理。由此提出基于主副螺线管线圈抑制发射度增长的补偿方案,并用ASTRA程序对补偿效果进行模拟计算。结果表明,采用该补偿方案后,电子腔输出束团的能散和发射度有显著改善,达到THz光源对于电子源的要求。

Background： The development of high performance electron beam source has been a challenge for Free-electron Laser （FEL） based THz source which is critical for the energy, energy spread and emittance of the electron beam. Purpose： This work aims to find out the mechanism of emittance growth and developing a compensation technique for this emittance growth in the Radio Frequency （RF） gun cavity. Methods： First of all, a RF gun cavity is designed using POISSON Superfish to calculate its electro-magnetic field distribution for analysis of the emittance growth mechanism. Then, a compensation scheme is developed by employing a pair of solenoids to reduce the space charge emittance. Finally, the effectiveness of scheme is evaluated using the commonly used reliable simulation program ASTRA. Results： The simulation results show that the normalized transverse emittance of a 5.67-MeV electron bunch with 200 pC is reduced to 0.373 mm＇mrad while the energy spread is reduced to 2.3‰. For a 5.67-MeV electron bunch with 1 000 pC, the emittance and energy spread are reduced to 0.934 mm-mrad and 7.6%0, respectively. Conclusion： Simulation of the final design shows that the performance of the electron beam produced by the RF gun has achieved the requirements for THz source.