Bunch length measurement using a traveling wave RF deflector

RF deflectors can be used for bunch length measurement with high resolution. This paper describes a completed S-band traveling wave RF deflector and the bunch length measurement of the electron beam produced by the photocathode RF gun of the Shanghai DUV-FEL facility. This is the first time that such a transverse RF deflector has been developed and used to measure the bunch length of picosecond order in China. The deflector＇s VSWR is 1.06, the whole attenuation 0.5 dB, and the bandwidth 4.77 MHz for VSWR less than 1.1. With a laser pulse width of 8.5 ps, beam energy of 4.2 MeV, and bunch charge of 0.64 nC, the bunch lengths for different RF input power into the deflector were measured, and an averaged rms bunch length of 5.25 ps was obtained. A YAG crystal is used as a screen downstream of the deflector, with the calibrated value of 1 pix = 136 μm.     

基于束流位置探测器的束团长度测量

分析了驱动电子束团的频域特性,研究了基于该特性进行长度测量的理论基础;使用三维模拟软件对束流位置探测器(BPM)进行建模,用模拟的方法对传输阻抗进行了数值计算;对不同长度的束团进行了测量和计算,并且分析了束团位置在真空管道中偏移对束团长度测量的影响。由测量结果可见,电子束团长度在10~100 ps(3~30 mm)时,测量误差均小于2%,满足中国工程物理研究院高平均功率自由电子激光太赫兹实验测量的使用要求。     

RF deflecting cavity for bunch length measurement in Tsinghua Thomson scattering X-ray source

An RF deflecting cavity used for bunch length measurement has been designed and fabricated at Tsinghua University for the Thomson Scattering X-Ray Source.The cavity is a 2856 MHz,π-mode,3-cell standing-wave cavity,to diagnose the 3.5 MeV beam produced by photocathode electron gun.With a larger power source,the same cavity will again be used to measure the accelerated beam with energy of 50 MeV before colliding with the laser pulse.The RF design using MAFIA for both the cavity shape and the power coupler is reviewed,followed by presenting the fabrication procedure and bench measurement results of two cavities.     

RF deflecting cavity design for bunch length measurement of photoinjector at Tsinghua University

RF deflecting cavity can be used for bunch length measurement and is designed to diagnose the beam produced by the photocathode electron gun which was built at Tsinghua University for the Thomson scattering experiment.Detailed discussion and calculation for measuring the 3.5 MeV bunch and another with further acceleration to 50 MeV,which is under development,are presented.A standing-wave deflecting cavity working at 2856 MHz is designed and the power feeding system has been planned.     

RF deflecting cavity design for bunch length measurement of photoinjector at Tsinghua University

RF deflecting cavity can be used for bunch length measurement and is designed to diagnose the beam produced by the photocathode electron gun which was built at Tsinghua University for the Thomson scattering experiment. Detailed discussion and calculation for measuring the 3.5 MeV bunch and another with further acceleration to 50 MeV, which is under development, are presented. A standing-wave deflecting cavity working at 2856 MHz is designed and the power feeding system has been planned.     

合肥光源单束团下束团长度和能散的测量

 根据同步光与储存环中的束流信号具有相同的时间结构的原理,测量同步光脉冲的半高全宽值可以计算出束团的长度。根据合肥光源的特点和实际需要,选择快速光电接收器搭配高速高带宽示波器作为在线测量束团长度和纵向分布等的主要手段。对单束团模式下束团长度随流强和高频腔腔压的变化趋势进行了测量。测量结果表明：束团长度与腔压的0.3次方成反比,比理论值0.5小;而束团长度随流强的增长率为2.0 ps/mA。通过测量纵向量子寿命进行了能散随流强变化的间接测量,结果表明,束团的拉伸是能散变化和势阱效应共同作用的结果。     

Design and studies on the traveling wave transverse RF deflecting structure

With the development of free electron laser (FEL) and the international linear collider (ILC), the electron bunch length is getting smaller and smaller. The traveling-wave transverse RF deflecting structure is an important part of the RF deflecting method for bunch length measurement and phase space diagnostics.The operation mode in RF deflector is the "TM11-like" mode. Since the TM11-like mode in this structure has a pair of degenerate dipole modes, two additional holes are provided on either side of each iris to stabilize the mode. The simulation and optimization have been done. A prototype has been fabricated and tested. The cold test results have been compared with the simulations of the first three modes.     

紧凑型自聚束微波电子枪设计研究

 提出一种新的紧凑型强流电子枪结构。紧凑型自聚束微波电子枪是一个多驻波腔结构,最大特点是腔与腔之间相互没有耦合,每腔可分别独立馈入功率和调节相位。通过选择合适的馈入功率和高频相位搭配,能够得到较优的束流品质。在此基础上采用外注入电子束的结构,能进一步增大流强和降低能散度,同时消除电子反轰对阴极工作的不利影响。利用Superfish和Parmela程序对腔体结构和束流动力学反复进行计算,确定了一组结构和工作参数。模拟结果表明出束微脉冲峰值流强可以到18 A,能散度小于2‰,横向发射度小于6 πmm·mrad。最后给出腔体冷侧结果,与模拟计算结果吻合得较好。     

新型金属丝靶电子源实验研究

电子束在基础科学研究、工农业生产和医疗领域发挥了重要作用。提出了一种新型的电子源技术方案:高功率激光脉冲轰击金属丝靶,可以产生大量能量在百keV量级的热电子,一部分热电子在丝靶表面自生电磁场的作用下沿着丝靶运动,丝靶后方可以获得指向性良好的电子束。实验上成功在金、钨和铜丝靶后方获得了电子束团,测量了束团束斑、电荷量和能谱。铜丝靶单发实验收集到的电子束团总电荷量可达3 nC,能量分布在0～240 keV区间内,能谱在100 keV附近呈现峰值。提出了微波压缩方案,设计了2腔微波聚束腔,利用ASTRA对微波腔压缩过程进行了模拟计算。结果显示,可以将电荷量1 nC、长度55 ps的束团压缩至27 ps,满足后续微波加速器对电子源的要求。     

Electro-optical sampling non-synchronous delay scanning measurement of electron beam bunch length at BFEL

The Electro-optical sampling delay scanning technique can be used for electron beam bunch length measurement.A novel non-synchronous delay scanning technique based on the electro-optical sampling measurements is presented.Based on Beijing Free Electron Laser(BFEL),the electron beam bunch length was measured with the electro-optical sampling technique for the first time in China.The result shows that the electron beam bunch length at BFEL is about 5.6±1.2 ps.     

Bunch length measurement at Tsinghua Thomson scattering X-ray source

The length of electron beam from a photocathode RF gun is determined by a spectrometer, according to the relative energy spread induced by the bunch length during the acceleration in a linac. For a photocathode RF gun, different laser injected phase and beam charge are studied. The compression is changed for the different laser phases, as from 10° to 30°, and the bunch length is lengthened due to the strong longitudinal space charge force, caused by the increased charge.     

Experimental Characterization of Sub-picosecond Electron Bunch Length with Coherent Diffraction Radiation

Diffraction radiation is one of the most promising candidates for electron beam diagnostics for the International Linear Collider, x-ray free electron lasers and energy recovery linac due to its non-intercepting characteristics. We report the non-intercepting measurement of sub-ps electron bunch length with coherent diffraction radiation. The bunch length is measured with a Martin--Puplett interferometer and the detailed longitudinal bunch shape is reconstructed with the Kramers--Kronig relation. The rms bunch length is found to be about 0.73ps, which confirms a successful commissioning of the bunch compressor and the interferometer.     

W–band光注入器的模拟研究

模拟研究了1.6个腔、高梯度的W-band光阴极微波电子枪系统,该系统能产生和加速300pC的电子束团.设计系统由频率91.392GHz光阴极微波电子枪以及频率91.392GHz行波加速结构组成.基于射频直线加速器标度律与数值模拟结果,设计系统能产生能量1.74MeV,电量300pC,束团长度0.72ps,归一化横向发射度0.55mm·mrad的电子束团.研究了高频、高梯度下的束流动力学.由于高梯度,有质动力效应在束流动力学中起重要作用,且由于横向与纵向之间的耦合,在基次空间谐波的情形下,仍然存在着有质动力聚焦效应.     

Bunch beam cooling

Electron cooling is used for damping both transverse and longitudinal oscillations of heavy particle. The cooling of bunch ion beam (with RF voltage on) is important part of experiments with inner target, ion collision system, stacking and RF manipulation. The short length of an ion bunch increases the peak luminosity, gives a start-time point for using of the time-of-flight methods and obtains a short extraction beam pulse. This article describes the review of last experiments with electron cooling carried out on the CSRm, CSRe (China) and COSY (Germany) storage rings. The accumulated experience may be used for the project of electron cooler on 2.5 MeV (NICA) and 0.5 MeV HIAF for obtaining high luminosity, depressing beam-beam effects and RF manipulation.     

磁压缩对高频稳定性的要求

 曲柄式磁压缩系统是北京大学SASE自由电子激光装置中非常重要的部分，通过其对电子束团的压缩为扭摆器提供高流强、短脉冲的电子束，使电子束在扭摆器内较短的距离实现饱和出光。曲柄式磁压缩需要利用偏离高频峰位的加速相位使得电子束产生能量-位置关联，主要讨论高频相位抖动与能量-位置关联的相互关系，高频相位抖动使得束团的能量-位置关联不同，即束团内电子能量随位置分布不同。进而研究其对磁压缩性能的影响，即能量-位置关联不一样会导致磁压缩得到的束团长度出现涨落。     

光阴极RF腔注入器束流发射度研究

 分析了光阴极RF腔注入器中的RF场效应和空间电荷效应，给出了电子在加速腔中束流发射度的解析表达式，它说明在加速过程中束流发射度是振荡变化的。利用SUPERFISH和GPT程序模拟计算了光阴极1+1/2腔注入器输出束流发射度与加速场强、注入相位、束团大小和形状、束团电荷的关系。适当选择这些条件，可以获得横向发射度小于2πmm·mrad 的输出束流。     

合肥光源基于同步光的测量与研究

介绍了合肥光源同步光测量系统, 包括条纹相机系统、快速光电测量束团长度系统、束团横向截面测量系统和光位置测量系统. 利用条纹相机系统和快速光电测量束团长度系统进行了束团长度测量和束团伸长效应的研究. 利用束流截面测量系统进行了六极铁对横向不稳定性抑制效果和横向反馈系统反馈效果的测量研究. 光位置测量系统采用丝型光位置检测器和自行研制了对数处理器, 用于测量光源点的束流位置和角度.     

北京大学DC-SC光阴极注入器的升级设计

 基于直流电子枪-超导加速腔（DC-SC）光阴极注入器样机的初步实验结果，北京大学提出了新的注入器的改进设计。新注入器核心结构包括皮尔斯枪和3+1/2超导腔。文章给出了它们的详细结构参数，然后采用程序，对注入器的束流动力学进行了模拟。结果发现：新注入器可以提供具有高束流品质、高平均流强的电子束，束团的电荷量100 pC，横向发射度低于2 mm·mrad,脉宽5 ps,rms束斑可达0.5 mm，重复频率81.25 MHz；也可以提供电荷量为300 pC低重复频率的高峰值流强的电子束，其横向发射度小于3 mm·mrad,脉宽约为9 ps，以满足北京大学自由电子激光（PKU-FEL）实验平台的要求。     

皮秒量级电子脉冲宽度测量系统

提出了一种测量皮秒量级电子脉冲宽度的新方法; 并且在二次发射微波电子枪前期研究结果的基础上利用MAFIA等数值模拟程序建立起该测量方法的数值模型进行数值分析, 讨论了其可行性; 并根据本实验室现有条件给出了测量方法的最小分辨, 说明了测量方法的可行性.     

First experimental research of the bunch compressor at CAEPLI

Magnetic bunch compressor is one of the key technologies on the path to next generation accelerator driven facilities. In this paper we report the design principles and the first experimental research of the bunch compressor developed at Chinese Academy of Engineering Physics (CAEP). The length of the bunch after compressor is found to be about 0.7 ps (rms) and the peak current exceeds 500 A when operated in the optimized condition. The sensitivity of the bunch length on the phase of the acceleration field and magnetic field of the bunch compressor was also measured and analyzed.