Uncoupled achromatic condition of a dog-leg system with the presence of RF cavities

To merge the beam from either of the two injectors to the main linac, a dog-leg system will be employed in the second Medium Energy Beam Transport(MEBT2) line of the China ADS driving accelerator. The achromatic condition has to be guaranteed to avoid beam center excursion against energy jitter. RF cavities were found to be indispensable to control the bunch length growth in the dog-leg system of MEBT2. The full uncoupling between transverse and longitudinal plane is desired to minimize the growth of projected rms emittances. The uncoupled achromatic condition of this dogleg system with the presence of RF bunching cavities will be deduced using the transfer matrices method. It is found that, to fulfill the uncoupling condition, the distance between the bunching cavities is uniquely determined by the maximum energy gain of the RF cavities. The theoretical analysis is verified by the simulation code TraceWin. The space charge effect on the uncoupled achromatic condition and the beam emittance growth will also be discussed.     

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。     

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.     

C波段2 MeV驻波加速管物理设计

 为了发展结构紧凑、小束斑加速器,进行了C波段驻波电子直线加速器加速管的物理设计和研究。利用多级聚束和大孔径注入方法,抑制空间电荷效应。加速管由3个聚束腔和4个加速腔组成,长约163 mm,馈入微波功率源1 MW。在最终的数值模拟中,得到脉冲电流约150 mA,俘获效率优于30％,在加速管出口外约14 mm处,对电子的横向分布进行非线性Gauss拟合,得到束斑直径（FWHM）约为0.55 mm。     

C波段2 MeV驻波加速管物理设计

为了发展结构紧凑、小束斑加速器,进行了C波段驻波电子直线加速器加速管的物理设计和研究。利用多级聚束和大孔径注入方法,抑制空间电荷效应。加速管由3个聚束腔和4个加速腔组成,长约163 mm,馈入微波功率源1 MW。在最终的数值模拟中,得到脉冲电流约150 mA,俘获效率优于30％,在加速管出口外约14 mm处,对电子的横向分布进行非线性Gauss拟合,得到束斑直径（FWHM）约为0.55 mm。     

Bunch evolution study in optimization of MeV 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.     

电荷束团横向非均匀密度分布引起的发射度增长

直线加速器中电荷束团的非线性效应是导致束流发射度增长的一个重要原因。本文给出了直线加速器中几种常见的横向非均匀电荷密度分布的有限长空间电荷束团所具有的非线性自场能,得到了由束团非均匀电荷密度分布引起的发射度增长。     

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.     

Betatron resonance crossing during the intense beam bunching

In the process of bunching an intense beam, the betatron oscillation frequency of particles changes due to a growth in their momentum rejection (in the presence of chromaticity) and changes in the Coulomb shift. These changes lead to the intersection of nonlinear betatron resonances. A mathematical model of the dynamics of ions in the process of bunching an intense beam has been built in order to assess the influence of transverse betatron resonances on the motion of particles. This model takes into account the influence of space charge on the longitudinal and transverse oscillations of particles in the presence of one- and two-dimensional betatron resonances. An analysis of the motion of particles near the one- and two-dimensional third-order resonances is given. Results of the numerical simulation of particle losses and emittance growth in the process of bunching an intense beam in the storage ring at the Institute for Theoretical and Experimental Physics (ITEP) are presented.     

BPM测量束流纵向发射度与Twiss参数

在C-ADS直线加速器样机中,为了描述束流相空间分布,需要精确测量束流在RFQ出口处的参数。束流横向信息已通过发射度重构测量获得,束流光学也得到了验证。采用了一种用BPM的SUM信号测量束流纵向参数的方法。在实验中,调节两台聚束器的聚束腔压并记录其下游BPM上的SUM信号。通过粒子群算法和TraceWin模拟相结合的方法,得到了考虑空间电荷效应的测量结果。测得的发射度与Toutatis中的模拟值相近。     

HIAF-BRing束团合并过程中的束流负载效应模拟

强流重离子加速器HIAF-BRing在加速完成后进行束团合并,为研究BRing中的束流负载效应对束团合并的影响,对238U35+束流进行了粒子跟踪模拟。模拟结果显示,在束团合并过程中,束流负载效应引起束团长度和束团中心位置的振荡,导致束流动量分散和束团长度的增长。束团合并过程中尾场电压以及不同束团间尾场的耦合导致的势阱畸变,是引起束团长度和束团中心振荡及束流发射度增长的原因。为了降低束流负载效应的影响,采用多谐波前馈系统进行补偿,达到了补偿束团合并过程中的束流负载效应的目的,从而确保了BRing中引出束流的品质,同时根据模拟结果确定了前馈系统需要覆盖的频率范围和需要补偿的最大尾场电压。     

PIC mode simulation of solenoid collimation channel

A new SSC(Separated Sector Cyclotron)-Linac is being designed to serve as an injector for the SSC at the HIRFL(Heavy Ion Research Facility Lanzhou).The beam intensity at the LEBT(Low Energy Beam Transport)for the heavy ions after the selection is typically low and the space charge effects are inconspicuous.The space charge effects become obvious when the beam current increases to a few hundred microamperes.The emittance growth deriving from the space charge effects may be particularly troublesome for the following linac and cyclotron.An optical system containing three solenoids has been designed for the LEBT to limit the beam emittance and to avoid the unnecessary beam loss in the cyclotron,as well as for the purpose of immunizing the LEBT emittance growth due to the space charge effects.The results of the PIC(Particle-In-Cell)mode simulation illustrate that this channel could limit the beam emittance growth and increase the beam brightness.     

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...     

作为BEPCII预注入器的光阴极注入器

 为了提高并且采用双束加速技术以进一步提高BEPCII正电子从直线加速器注入到储存环的速率,BEPCII(Beijing Electron Positron Collider Upgrade Project) 正在建造带有两个次谐波聚束腔的新预注入器。提出了使用光阴极注入器作为BEPCII预注入器的新方案,其性能优于次谐波聚束系统：发射度和能散至多为次谐波聚束系统的1/4,100%的传输效率,没有卫星束团的干扰,等。此外,还分别对光阴极注入器在高电荷量和低电荷量两种情况下的束流动力学进行了模拟计算和优化研究,并将其性能与正在建造的次谐波聚束系统和旧聚束系统的性能进行了比较和讨论。由此光阴极注入器产生的电子束还可以在将来用作激光-等离子体或激光-非传导性加速结构的尾场加速研究。     

直线加速器中双向高斯密度分布电荷束团发射度增长的比率关系

采用直线加速器中的空间电荷束团的有限圆柱模型，并假设该有限圆柱体电荷密度分布在横向ｒ和纵向ｚ都满足高期分布，推导得到了该电荷束团的自场能与发射度增长公式；通过数值模拟计算，给出了束流发射度增长随束团参数和加速器系统参数变化的图表曲线，讨论了该双向高期密度分布电荷束团发射度增长的比率关系．     

射频腔光阴极注入器发射度研究

在激光驱动的光阴极注入器产生的高亮度电子束中,空间电荷引起发射度的增长。分析了射频腔中引起发射度增长的因素以及解决这个问题的办法——在射频腔的阴极附近加一个螺旋聚焦磁场进行补偿,也给出了补偿后电子束的发射度并和数值模拟结果进行比较,实验测试表明,所得结果比较符合。     

射频腔光阴极注入器发射度研究

 在激光驱动的光阴极注入器产生的高亮度电子束中，空间电荷引起发射度的增长。分析了射频腔中引起发射度增长的因素以及解决这个问题的办法——在射频腔的阴极附近加一个螺旋聚焦磁场进行补偿，也给出了补偿后电子束的发射度并和数值模拟结果进行比较，实验测试表明，所得结果比较符合。     

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.     

Space charge effect,coherence of charge oscillations,and emittance

The space charge effect plays an important role in accelerator injectors, in which the particle energy is relatively low. The emittance compensation technique makes it possible to effectively analyze and optimize such beamlines. This method is used for obtaining simple and reliable analytical and numerical estimates for the increase in emittance in beamlines with prevailing effect of space charge and guns. The longitudinal and transverse nonuniformity in charge density, acceleration, and bunching are taken into account. The parameters of optimal beamlines and guns are estimated.     

Study on space charge effects of the CSNS/RCS

The Rapid Cycling Synchrotron (RCS) is a key component of the China Spallation Neutron Source (CSNS). The space charge effect is one of the most important issues in the CSNS/RCS, which limits the maximum beam intensity, as well as the maximum beam power. Space charge effects are the main source of emittance growth and beam loss in the RCS. Space charge effects have been studied by simulation for the CSNS/RCS. By optimizing the painting orbit, the optimized painting distribution was obtained. The space charge effects during the acceleration are studied and dangerous resonances, which may induce emittance growth and beam loss, are investigated. The results are an important reference for the design and commissioning of the CSNS/RCS.