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.     

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.     

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.     

Collective emittance exchange with linear space charge forces and linear coupling

We show that pronounced collective nonlinear behavior is present in a beam with linear coupling and space charge as described by the complete second order moment equations. The collective effects result in a shifted and broadened resonance condition as well as saturation effects in the emittance transfer. For slow stop-band crossing either the beam evolves along a skewed matched solution with full emittance exchange or the exchange is hindered by collective space charge effects, depending on the emittance ratio and direction of crossing.     

Femtosecond and subfemtosecond x-ray pulses from a self-amplified spontaneous-emission-based free-electron laser

We propose a novel method to generate femtosecond and subfemtosecond photon pulses in a free-electron laser by selectively spoiling the transverse emittance of the electron beam. Its merits are simplicity and ease of implementation. When the system is applied to the Linac Coherent Light Source, it can provide x-ray pulses the order of 1 fs in duration containing about 10(10) transversely coherent photons.     

电子束亮度及多孔板法测量发射度的原理

本文探讨了利用多孔板法测量四维横向相空间中均匀分布以及高斯分布的电子束发射度的测量原理。结果表明,对于四维均匀分布的电子束,该法测量的是电子束的边发射度,并且光斑尺寸随着小孔位置不同而变化;对于高斯分布的电子束,该法测量的是电子束的均方根发射度,其光斑尺寸与小孔位置无关。     

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.     

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。     

Beam emittance investigation in high brightness injector using different driver laser profiles

Beam emittance plays an important role in any accelerator, and it is a main parameter to judge the performance of an accelerator. Emittance optimization is an indispensable part in conditioning and operation of the facility. For a laser-driven high brightness injector, different time structure of the laser pulse has different effects on transverse emittance. In order to compare Gaussian and flat-top laser pulse, systematic simulations of 500 pC have been done. From the simulation results, one can see that flat-top pulse laser will yield smaller minimal transverse beam size and transverse beam emittance than Gaussian pulse laser.     

低能离子束横向发射度的测量及分析

The transverse emittance of an ion beam describes its transverse size as the particles are transported from a source to a target.It allows for predicting beam losses in limiting apertures and the beam focus size at the target.Various definitions and issues are discussed.The most common and emerging measuring techniques are presented,including their advantages.Several methods of emittance data analysis,their accuracy and trustworthiness,are discussed.     

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。     

Emittance growth during bunch compression in the CTF-II

Measurements of the beam emittance during bunch compression in the CLIC Test Facility (CTF-II) are described. The measurements were made with different beam charges and different energy correlations versus the bunch compressor settings which were varied from no compression through the point of full compression and to overcompression. Significant increases in the beam emittance were observed with the maximum emittance occurring near the point of full (maximal) compression. Finally, evaluation of possible emittance dilution mechanisms indicates that coherent synchrotron radiation was the most likely cause.     

Transient beam response in synchrotrons with a digital transverse feedback system

The transient beam response to an external impulse force in synchrotrons with a digital transverse feedback system is studied. The experimental data on damping coherent transverse oscillations of bunch excited by a discrete-time unit impulse in the Large Hadron Collider (LHC) are analyzed. Good agreement between the measured and theoretically predicted decrements has been obtained. A method of feedback fine tuning based on measurements of the bunch response to a harmonic excitation impulse is discussed.     

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...     

Extremely low vertical-emittance beam in the accelerator test facility at KEK

Electron beams with the lowest, normalized transverse emittance recorded so far were produced and confirmed in single-bunch-mode operation of the Accelerator Test Facility at KEK. We established a tuning method of the damping ring which achieves a small vertical dispersion and small x-y orbit coupling. The vertical emittance was less than 1% of the horizontal emittance. At the zero-intensity limit, the vertical normalized emittance was less than 2.8 x 10(-8) rad m at beam energy 1.3 GeV. At high intensity, strong effects of intrabeam scattering were observed, which had been expected in view of the extremely high particle density due to the small transverse emittance.     

The Use of a Damping System in a Synchrotron for the Short-Time Excitation of Coherent Oscillations of Particles

Physics of Particles and Nuclei Letters - The possibility of applying a damping system of coherent transverse beam oscillations in a synchrotron using a positive feedback mode for the short-time...     

轴对称磁场的束流光学及其在ECR离子源中的应用

 带电粒子在轴对称磁场中一边沿着对称轴向前运动，一边绕对称轴旋转。所以横向运动可以分解为两部分：聚焦运动和子午面的旋转。因此，4维横向传输矩阵可以表示为聚焦矩阵和旋转矩阵的乘积。用旋转矩阵和聚焦矩阵重新推导了总的传输矩阵。然后给出了相应的相椭圆系数矩阵来进一步估算从ECR离子源引出束流的发射度.详细地讨论了束流分布函数的二次矩。     

直线加速器中的非线性空间电荷效应和发射度增长

直线加速器中束团的非线性空间电荷效应是引起发射度增长的重要原因之一．根据计算在屏蔽筒中非均匀分布京团的空间电荷效应的普遍方法，推导了直线加速器圆波导中几种非均匀分布束团的非线性场能公式，并给出数值计算结果．讨论了由非线性所引起发射度增长．