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。     

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。     

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...     

强流低发射度光阴极微波电子枪的优化设计

为了实现高增益 FEL,对其关键部件光阴极微波电子枪进行了优化设计 ,还就发射度补偿技术进行了较详细的考虑 ,并用 PARMELA程序进行了粒子动力学的模拟 ,初步给出了为降低初始电子束团横向发射度的优化参数。     

A method to measure the nonlinear force caused emittance growth in a RF photoinjector

Based on the multi-slit method, a new method is introduced to measure the non linear force caused emittance growth in a RF photoinjector. It is possible to reconstruct the phase space of a beam under some conditions by the multi-slit method. Based on the reconstructed phase space, besides the emittance, the emittance growth from the distortion of the phase space can also be measured. The emittance growth results from the effects of nonlinear force acting on electron, which is very important for the high quality beam in a RF photoinjector.     

Ultra-low emittance X-band photocathode RF gun

In this paper, we present the simulation results of a 1.6 cell X-band photocathode RF gun for ultra-low emittance electron beams. It will work at 9.3 GHz. The emittance, bunch length, electron energy and energy spread at the gun exit are optimized at bunch charge of 1pC using PARMELA. Electron bunches with emittance about 0.1 mm·mrad and bunch length less than 100 fs can be obtained from this gun. A PITZ type coupler is adopted in this gun and an initial simulation by MAFIA is also given in this paper.     

RFQ加速器高频测量软件和分析软件开发

强流RFQ加速器要求严格控制束流损失和束流发射度增长,因此必须准确测量和正确分析RFQ加速器的场分布和模式的场分量,并使它们符合设计要求.LebView是一个广泛应用于自动测量和数据处理的计算机软件平台,介绍在此平台上开发的RFQ自动测量软件和数据分析软件,同时给出这些软件的一些应用实例.     

激光斜入射光阴极对束流质量的影响及其优化

 针对美国布鲁克海文国家实验室（BNL）型的光阴极微波电子枪,模拟了不同分布状态的驱动激光脉冲斜入射光阴极对束流质量的影响,给出了改变注入相位和补偿线圈磁场强度对发射度的优化结果。结果表明：光斑椭圆化将会导致发射度的大幅增长,优化效果不理想;波前不同步导致的发射度增长对于纵向高斯分布的脉冲可以得到理想的优化。对于斜入射引起的光斑椭圆化和波前不同步问题给出了光学校正方法及部分测量结果。此外,模拟结果显示,对于横向均匀分布的激光脉冲,适当椭圆度的光斑比圆形光斑更有利于提高电子束质量。     

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.     

A New Measurement of Electron Beam Emittance with Cerenkov Radiation “Double Imaging” Method

A new way in electron beam emittance measurement with Cerenkov radiation "double imaging" method is proposed in this paper. In the standard emittance measurements with optical diagnostics such as Cerenkov radiation, OTR (Optical Transition Radiation), fluorescence screen or BMP (beam profile monitor) etc., the emittance is indirectly calculated through quadrupole-scanning technique, with the prior ssumption that the beam phase space density distribution is ellipse, which will certainly induce systematic error when the beam profile is quite irregular or the space-charge effects can not be omitted. In proposed method, the Cerenkov radiation pass through a 1-meter focal-length thin convex lens, and a CCD camera is used to capture two images of Cerenkov radiation at the focal plane and at the image plane of the lens respectively. Then, with image processing technique, we acquire the angular divergence information of the electron beam from the image of the focal plane and the radial distribution information from the image of the image plane, which we call Cerenkov radiation "double imaging" method. Therefore, the emittance can be directly attained according to the definition of the RMS emittance. By this method, we can measure the actual phase space distributions without making any prior assumptions about the density distributions. Compared with other general measurements, this "double imaging" method has advantages of simpler equipment, higher precision and wider application. This paper also presents the computer simulation results of emittance measurement on the DC-SC (DC-Superconducting) photocathode injector of PKU-SCAF (Peking University Superconducting Accelerator Facility) being built at Peking University.     

光阴极直流高压电子枪束流偏轴发射研究

离子反轰会造成直流高压电子枪内光阴极的量子效率衰减,降低阴极的工作寿命和稳定性,是限制光阴极直流高压电子枪性能的重要因素之一。基于中国工程物理研究院太赫兹自由电子激光(THzFEL)的直流高压电子枪,通过束流动力学模拟和阴极量子效率测量,研究了束流偏轴发射对离子反轰的抑制作用及其应用限制。模拟结果表明:束流偏轴发射时,离子主要轰击阴极的电中心附近,光电发射区域的离子通量较轴上发射时降低约95%;但束流发射度增大约4πmm·mrad,横向运动幅度增加约1倍。通过电子枪运行前后阴极量子效率测量,发现束流偏轴发射时离子反轰引起电中心附近量子效率大面积的衰减,而光电发射区域量子效率损失较小,验证了束流偏轴发射抑制离子反轰的可行性。     

Mismatch study of C-ADS main linac

The ADS accelerator in China is a Continuous-Wave(CW) proton linac with 1.5 Ge V beam energy,10 m A beam current, and 15 MW beam power. To meet the extremely low beam loss rate and high reliability requirements, it is very important to study the beam halo caused by beam mismatch, which is one major sources of beam loss. To avoid envelope instability, the phase advances per period are all smaller than 90 degrees in the main linac design. In this paper, simulation results of the emittance growth and the envelope oscillations caused by mismatch in the main linac section are presented. To meet the emittance growth requirement, the transverse and longitudinal mismatch factors should be smaller than 0.4 and 0.3, respectively.     

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.     

DTL cavity design and beam dynamics for a TAC linear proton accelerator

A 30 mA drift tube linac (DTL) accelerator has been designed using SUPERFISH code in the energy range of 3-55 MeV in the framework of the Turkish Accelerator Center (TAC) project. Optimization criteria in cavity design are effective shunt impedance (ZTT), transit-time factor and electrical breakdown limit. In geometrical optimization we have aimed to increase the energy gain in each RF gap of the DTL cells by maximizing the effective shunt impedance (ZTT) and the transit-time factor. Beam dynamics studies of the DTL accelerator have been performed using beam dynamics simulation codes of PATH and PARMILA. The results of both codes have been compared. In the beam dynamical studies, the rms values of beam emittance have been taken into account and a low emittance growth in both x and y directions has been attempted.     

用修正三梯度法测量束流发射度

 介绍了修正三梯度法的原理和利用该方法测量发射度的实验装置。编写了基于束包络方程的数值拟合程序，并进行了模拟计算。用修正三梯度法对3.5MeV注入器出口处脉冲电子束的发射度做了时间分辨的测量。结果表明，当空间电荷力不可忽略时，该注入器输出的电子束脉冲期间中间部分束的发射度为1 040π·mm·mrad。     

DC-SC光阴极注入器的束载实验的调试进展

 北京大学射频超导实验室设计了新型超导光电子枪——DC-SC光阴极注入器，目标是为自由电子激光平台提供能量在2~3MeV，脉宽小于10ps，脉冲重复频率为81.25MHz，平均流强约为1mA的低发射度电子束。现在已经建成了DC-SC光阴极注入器实验平台，包括激光驱动光阴极系统，Pierce直流高压加速结构，1.3GHz 1＋1/2纯铌超导腔，恒温器低温系统，4.5kW连续波微波系统，1/16分频与同步控制系统，束流诊断系统和能量分析系统等。并且完成了超导腔的静态实验，直流加速结构也经过了100μA低电流测试。实验结果符合设计要求，整体调试后即可以进行束载实验。     

RF腔光阴极注入器中热发射度的测量研究

论文介绍RF腔光阴极注入器中热发射度的测量方法，在注入器中有三个因素影响热发射度的测量，它们是：射频场效应、空间电荷效应和发射度测量误差. 在注入器出口处，电子束发射度由：热发射度、射频场效应引起的发射度增长和空间电荷效应的发射度增长三部分组成. 论文从注入器中发射度增长理论和模拟出发，给出了一个能够消除射频场效应和空间电荷效应的热发射度测量方法.     

Enhancing the accelerated beam current in the booster synchrotron by optimizing the transport line beam propagation

In this paper, we present the results of transverse beam emittance and twiss parameter measurement of an electron beam, delivered by a 20 MeV microtron which is used as a pre-injector system for a booster synchrotron in the Indus Accelerator Facility at RRCAT Indore. Based on these measured beam parameters, beam optics of a transport line was optimized and its results are also discussed in this paper. This beam transport line is used to transport the electron beam from the 20 MeV microtron to the booster synchrotron. The booster synchrotron works as a main injector for Indus-1 and Indus-2 synchrotron radiation facilities. To optimize the beam optics of a transport line for proper beam transmission through the line as well as to match the beam twiss parameters at the beam injection point of another accelerator, it is necessary to know the transverse beam emittance and twiss parameters of the beam coming from the first one. A MATLAB-based GUI program has been developed to calculate the beam emittance and twiss parameters, using quadrupole scan method. The measured parameters have been used for beam transport line optimization and twiss parameters matching at booster injection point. After this optimization, an enhancement of ～50% beam current has been observed in the booster synchrotron.     

重离子治癌装置4 MeV/u IH型漂移管直线注入器的动力学设计

基于KONUS束流动力学理论,完成了一台工作频率为162.5 MHz、占空比为1%的交叉指型漂移管直线注入器（IH-DTL）的动力学设计。该IH-DTL内置两套三组合四极透镜,共有41个加速单元,可为同步加速器提供流强400 eμA、能量4 MeV/u的C4+束流。在动力学设计过程中着重对每个加速间隙的同步能量偏差、注入相位和间隙电压等参数进行优化,使得该IH-DTL的横向归一化RMS接收度达到0.24 πmm·mrad,且横纵向归一化发射度增长小于10%,有利于提高同步环的注入效率。然后根据动力学设计的结构参数进行IH-DTL的高频仿真计算,将得到的三维电磁场分布导入PIC粒子跟踪程序中进行束流动力学模拟。动力学模拟结果显示,束流在IH-DTL出口的横向自然发射度小于13 πmm·mrad,达到了同步环的注入要求,而且在7%的垂直二极场分量下,束流中心的横向偏移在±0.5 mm以内,整体的束流传输效率高于99%。An interdigital H-mode drift tube linac (IH-DTL) with KONUS beam dynamic has been designed, which operation frequency was chosen 162.5 MHz. This IH-DTL consists of 41 accelerating cells and two quadrupole magnets triplets, can provide the C4+ with the current of 400 eμA and energy of 4 MeV/u for the synchrotron. In the beam dynamic design, the synchronous particle energy, inject RF phase and the acceleration voltage of each gap are optimized carefully to make the transverse normalized RMS acceptance of the IH-DTL to be 0.24 πmm·mrad and the beam emittance growth small than 10%. Then the RF structure was designed and the 3D electromagnetic field was imported into the PIC particle tracking code for the beam dynamic simulation. The transverse beam emittance at the exit of the IH-DTL is small than 13πmm·mrad which meets the injection requirement of the synchrotron. What is more, under the 7% vertical dipole fields component, the offset between the beam center and the drift tube's axis is ±0.5 mm at most. The transmission efficiency of the IH-DTL is higher than 99% for the whole beam in the acceptance.