Bunch evolution study in optimization of Me V ultrafast electron diffraction

Megaelectronvolt ultrafast electron diffraction（UED） is a promising detection tool for ultrafast processes.The quality of diffraction image is determined by the transverse evolution of the probe bunch. In this paper, we study the contributing terms of the emittance and space charge effects to the bunch evolution in the MeV UED scheme, employing a mean-field model with an ellipsoidal distribution as well as particle tracking simulation. The small transverse dimension of the drive laser is found to be critical to improve the reciprocal resolution, exploiting both smaller emittance and larger transverse bunch size before the solenoid. The degradation of the reciprocal spatial resolution caused by the space charge effects should be carefully controlled.     

Transverse dynamics of space charge dominated beam during bunching

In this paper we present a technique to optimize the transport parameters for transverse matching of the space charge dominated beams in the presence of bunching. The increase in the current within the specified bunch width during the transport is obtained from the disc model and is included in the optimization method. A 2D particle in cell (PIC) code is then developed to study the details transverse dynamics of the bunch taking into account the effect of longitudinal compression by reweighting the charge and mass of macroparticles. The evolution of rms beam size obtained using PIC simulation agrees well with the results obtained by solving beam envelope equation. The PIC simulation shows emittance growth during bunching induced by the space charge effect for nonuniform distribution. The growth in the emittance increases with the nonlinearity of distribution and peaks near the time focus.     

高压直流连续波光阴极注入器中发射度补偿

空间电荷效应是引起发射度增长的主要因素,特别是在高压直流连续波光阴极注入器中。分析了高压直流连续波光阴极注入器中线性空间电荷力的特点及其对电子束横向发射度的影响,并从理论上解析地研究了螺线管发射度补偿的原理及特点。最后利用Parmela程序对中国工程物理研究院高压直流连续波光阴极注入器的发射度补偿作了模拟计算。结果表明,束团电荷量为80 pC的电子束在350 keV高压直流电子枪出口处的横向归一化发射度为5.14 mmmrad,经过螺线管补偿后,其最小横向发射度变为1.27 mmmrad。电子束的发射度得到了很好的补偿。     

Halo formation and emittance growth of positron beams in plasmas

An ultrarelativistic 28.5 GeV, 700-microm-long positron bunch is focused near the entrance of a 1.4-m-long plasma with a density n(e) between approximately equal to 10(13) and approximately equal to 5 x 10(14) cm(-3). Partial neutralization of the bunch space charge by the mobile plasma electrons results in a reduction in transverse size by a factor of approximately equal to 3 in the high emittance plane of the beam approximately equal to 1 m downstream from the plasma exit. As n(e) increases, the formation of a beam halo containing approximately 40% of the total charge is observed, indicating that the plasma focusing force is nonlinear. Numerical simulations confirm these observations. The bunch with an incoming transverse size ratio of approximately 3 and emittance ratio of approximately 5 suffers emittance growth and exits the plasma with approximately equal sizes and emittances.     

超紧凑型飞秒电子衍射仪的设计

由于空间电荷效应的限制,产生百飞秒的极短电子脉冲是超快电子衍射技术的一大难点.同时,电子的穿透深度随着电子能量的增加而增加,而电子的散射几率却具有相反的规律.因而,除了时间分辨的提升,还需要可宽范围调节的电子能量以优化不同厚度样品对其的需求.基于此,提出并设计了一种新型超紧凑电子枪,结合均匀场阴极和可移动阳极的配置,可在10-125 kV加速电压范围内实现100 fs量级时间分辨率.通过优化设计高压电极轮廓,使得其轴上和整个阴极面的场增强因子在不同阴阳极间距下均小于约4%,从而保证了不同加速电压下最大轴上场强均可达10 MV/m量级,有效地抑制了电子脉冲的展宽效应;进一步将阳极小孔设计成可放置致密电镜载网的阶梯孔,一方面可将载网支撑的样品紧贴小孔后方放置,最大程度上缩短了电子从阴极到样品的时间弥散,同时也可以有效地减弱阳极孔对电子束的散焦效应,提升电子束的横向聚焦性能.     

Emittance measurement & optimization for the photocathode RF gun with laser profile shaping

The Laser Undulator Compact X-ray source(LUCX) is a test bench for a compact high brightness X-ray generator,based on inverse Compton Scattering at KEK,which requires high intensity multi-bunch trains with low transverse emittance.A photocathode RF gun with emittance compensation solenoid is used as an electron source.Much endeavor has been made to increase the beam intensity in the multi-bunch trains.The cavity of the RF gun is tuned into an unbalanced field in order to reduce space charge effects,so that the field gradient on the cathode surface is relatively higher when the forward RF power into gun cavity is not high enough.A laser profile shaper is employed to convert the driving laser profile from Gaussian into uniform.In this research we seek to find the optimized operational conditions for the decrease of the transverse emittance.With the uniform driving laser and the unbalanced RF gun,the RMS transverse emittance of a 1 nC bunch has been improved effectively from 5.46 πmm.mrad to 3.66 πmm.mrad.     

Preserving high multibunch luminosity in linear colliders

An analytic theory of cumulative multibunch beam breakup in linear colliders is developed. Included is a linear variation of transverse focusing across the bunch train as might be applied, e.g., by chirping the radio frequency power sources or by using radio frequency quadrupole magnets. The focusing variation saturates the exponential growth of the beam breakup and establishes an algebraic decay of the transverse bunch displacement versus bunch number. A closed-form expression for the transverse bunch displacement is developed. It is used to quantify the total normalized emittance and thereby isolate the region of parameter space corresponding to high multibunch luminosity.     

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。     

Compressing ultrafast electron pulse by radio frequency cavity

An ultrafast electron diffraction technique with both high temporal and spatial resolution has been shown to be a powerful tool to observe the material transient structural change on an atomic scale.The space charge forces in a multi-electron bunch will greatly broaden the electron pulse width,and therefore limit the temporal resolution of the high brightness electron pulse.Here in this work,we design an ultrafast electron diffraction system,and utilize a radio frequency cavity to realize the ultrafast electron pulse compression.We experimentally demonstrate that the stretched electron pulse width of14.98 ps with an electron energy of 40 keV and the electron number of 1.0 ×10~5 can be maximally compressed to about0.61 ps for single-pulse measurement and 2.48 ps for multi-pulse measurement by using a 3.2-GHz radiofrequency cavity.We also theoretically and experimentally analyze the parameters influencing the electron pulse compression efficiency for single-and multi-pulse measurements by considering radiofrequency field time jitter,electron pulse time jitter and their relative time jitter.We suggest that increasing the electron energy or shortening the distance between the compression cavity and the streak cavity can further improve the electron pulse compression efficiency.These experimental and theoretical results are very helpful for designing the ultrafast electron diffraction experiment equipment and compressing the ultrafast electron pulse width in a future study.     

用超快电子衍射技术研究Al薄膜的超快动力学行为

超快电子衍射（UED）技术因其同时具有亚皮秒的时间分辨和亚毫埃的空间分辨能力,成为研究物质瞬态结构变化,特别是研究晶格材料超快动力学的有力工具.应用国内首台自行研制的UED系统,我们实时测量了超快激光脉冲激发下,20 nm金属Al多晶薄膜产生的相干声子和晶格热运动.实验结果显示,在晶格热运动加剧的同时,热应力的作用使晶格产生了相干振荡,并最终膨胀达到新的平衡位置.实验中测得的振荡周期以及晶格上升的温度与理论计算的结果符合较好,展示了UED技术在超快晶格动力学研究方面的广阔应用前景 关键词： 超快电子衍射 相干声子 晶格热运动     

Velocity bunching in travelling wave accelerator with low acceleration gradient

We present the analytical and simulated results of our study of the influence of the acceleration gradient in the velocity bunching process, which is a bunch compression scheme that uses a travelling wave accelerating structure as a compressor. Our study shows that the bunch compression application with low acceleration gradient is more tolerant to phase jitter and more successful in obtaining a compressed electron beam with symmetrical longitudinal distribution and low energy spread. We also present a transverse emittance compensation scheme to compensate the emittance growth caused by the increase of the space charge force in the compressing process, which is easy to adjust for different compression factors.     

How to realize an ultrafast electron diffraction experiment with a terahertz pump: A theoretical study

To integrate a terahertz pump into an ultrafast electron diffraction (UED) experiment has attracted much attention due to its potential to initiate and detect the structural dynamics both directly. However, the deflection of the electron probe by the electromagnetic field of the terahertz pump alters the incident angle of the electron probe on the sample, impeding it from recording structural information afterwards. In this article, we studied this issue by a theoretical simulation of the terahertz-induced deflection effect on the electron probe, and came up with several possible schemes to reduce such effect. As a result, a terahertz-pump-electron-probe UED experiment with a temporal resolution comparable to the terahertz period is realized. We also found that MeV UED was more suitable for such terahertz pump experiment.     

Matching by solenoids in space charge dominated LEBTs

The betatron matching of a rotationally asymmetric beam in space charge dominated low-energy beam transports (LEBTs) where solenoids are used for the transverse matching has been studied.For better understanding, the coupling elements of a beam matrix are interpreted in special forms that are products of a term defined by the Larmor rotation angle and another by the difference between the beam matrix elements in the two transverse planes.The coupling form originally derived from the rotationally symmetric field in solenoids still holds when taking into account the rotationally asymmetric space charge forces that are due to the unequal emittance in the two transverse planes.It is shown in this paper that when an LEBT mainly comprising solenoids transports a beam having unequal emittance in the two transverse planes and the linear space charge force is taken into account, the initial Twiss parameters can be modified to obtain the minimum and equal emittance at the LEBT exit.The TRACE3D calculations also prove the principle.However, when quadrupoles that are also rotationally asymmetric are involved in between solenoids, the coupling between the two transverse planes becomes more complicated and the emittance increase is usually unavoidable.A matching example using the CSNS (China Spallation Neutron Source) LEBT conditions is also presented.     

光阴极微波电子枪中发射度补偿及模拟计算

介绍了上海深紫外自由电子激光用光阴极微波电子枪采用发射度补偿技术的结果 .详细分析了线性空间电荷力的特点及对束流发射度的影响 ,从束流动力学和相空间两方面讨论了发射度补偿原理 .给出了补偿线圈的设计结构及其轴向场分布 .利用PARMELA程序对补偿效果作了模拟计算 .结果表明 ,设计的腔体对单圈 1 .5nC束团 ,在枪出口后 1 .2m处 ,电子能量为 5 .7MeV ,横向归一化发射度εn ,RMS=1 .61 2πmm·mrad. The emittance compensation technology will be used on the photo cathode RF gun for Shanghai SDUV FEL. In this paper, the space charge force and its effect on electron beam transverse emittance in RF gunis is studied, the principle of emittance compensation in phase space is discussed. We have designed a compensation solenoid and calulated its magnetic field distribution. Its performance has been studied by the code PARMELA. A simulation result indicates that the normalized transverse RMS emi...     

Impact on the magnetic compressor due to CSR

When an electron bunch is compressed in a chicane compressor,the CSR (coherent synchrotron radiation) will induce energy redistribution along the bunch.Such energy redistribution will affect the longitudinal emittance as a direct consequence.It will also excite betatron oscillation due to the chromatic transfer functions,and hence a transverse emittance change.So,it is indispensable for us to find a way to alleviate the CSR-cansed emittance dilution and the bad result of chicane compressor in PKU-FEL.     

超快电子衍射技术及其应用

时间和空间上实时观测原子运动对于自然科学研究有着非常重大的意义, 而超快电子衍射(UED)技术同时具备飞秒激光脉冲的高时间分辨特性和电子衍射技术的高空间特性, 可以为实时观测原子级分辨尺度物质的结构变化提供一种有效工具. 本文综述了超快电子衍射技术的发展历史、实验方法以及相关应用, 并且展望了超快电子衍射技术未来的发展.     

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。     

Ultrafast Electron Diffraction with Spatiotemporal Resolution of Atomic Motion

Ultrafast electron diffraction （UED） is a rapidly advancing technique capable of recording the atomic-detail structural dynamics in real time. We report the establishment of the first UED system in China. Employing this UED apparatus, both the coherent and the concurrent thermal lattice motions in an aluminium thin-film, trigged by ultrafast laser heating, have been observed. These results demonstrate its ability to directly measure a sub-milli-angstrom lattice spacing change on a sub-picosecond time scale.     

Impact on the magnetic compressor due to CSR

When an electron bunch is compressed in a chicane compressor, the CSR (coherent synchrotron radiation) will induce energy redistribution along the bunch. Such energy redistribution will affect the longitudinal emittance as a direct consequence. It will also excite betatron oscillation due to the chromatic transfer functions, and hence a transverse emittance change. So, it is indispensable for us to find a way to alleviate the CSR-caused emittance dilution and the bad result of chicane compressor in PKU-FEL.     

Simulation Study of High Intensity S-Band Photoinjector

In this paper, we report the results of simulation study of KEK/U.Tokyo S-Band photoinjector under the present new experimental condition. The aim of the simulation study is to transport high bunch charge with low emittance evolution.