质子束尾波场中利用密度跃变实现电子捕获及加速的粒子模拟

 用2D3V粒子模拟程序研究了高能质子束驱动的尾波场加速电子的方案,及其在此方案中应用背景等离子体密度的跃变致使等离子体电子自注入加速相区的可能性。粒子模拟结果显示:密度跃变实现了电子的自注入,并且捕获的电子束处于加速相位,等离子体尾波场纵向电场对捕获的电子束起箍缩作用;捕获的电子束随着传输,表现为窄能谱分布;同时随着密度跃变大小的增大,可以增加等离子体电子的捕获。     

超短超强激光导引及对电子加速的影响

使用粒子模拟程序对30 fs超短超强激光在均匀与抛物型两种密度分布等离子体中的传输, 以及在稳定传输状态下尾场的电子注入与加速形成的电子能谱进行了模拟与分析. 固定入射激光束斑尺寸, 在(0.4-2)×10¹⁹/cm³等离子体密度范围, 对比分析了归一化峰值强度从1-6范围的激光脉冲在上述两种密度分布等离子 体中传输时激光束斑尺寸的演化, 结果表明抛物型分布的等离子体密度通道能够对超短超强脉冲实现良好的导引, 有利于高能电子加速. 对于较高密度情况,即使在均匀等离子体中依靠相对论自聚 焦等机制也可以实现良好的自导引传输,有利于实验简化以及产生更大电量的加速电子.     

激光脉冲的横向波形对弓形波电子俘获的影响

用粒子模拟研究了在激光尾波场电子弓形波注入过程中激光脉冲的横向波形对尾波场俘获电子数目的影响, 发现与高斯激光相比, 超高斯形激光更有利于拉动空泡闭合前侧边的电子团向空泡尾部汇聚形成高能量局域化的弓形波, 从而导致更多的电子注入到空泡的加速相, 使得被俘获的电子数目提高近5倍, 且电子束品质得到改善.该研究对于进一步理解尾波场加速中电子注入等有参考价值. 关键词： 尾波场 电子俘获 横向波形 粒子模拟     

等离子体界面附近激光场中电子的运动

分析了在等离子体中两个不同密度区的交界面附近激光场中电子的运动.导出了电子横向动量与纵向动量的相互关系的一般方程.分析发现,介质的非均匀性导致电子运动特性的重要改变.在一定条件下,这种非均匀性有利于注入电子获得加速.     

激光脉冲形状对弓形波电子俘获的影响

使用二维粒子模拟程序研究了电子弓形波注入机制中激光脉冲形状对电子俘获效果的影响. 研究结果表明, 激光脉冲时间上升沿陡峭的正扭曲脉冲激发的尾波场强度高, 加速区域分布广, 并且有利于电子获得更高的初速度, 从而推动更多的电子进入尾波场加速相位. 在其他条件相同的情况下, 正扭曲脉冲的电子俘获数目远高于激光脉冲时间分别为高斯形和负扭曲分布的情形, 使得电子束的品质得到改善. 研究结果对于理解尾波场加速中电子注入过程以及获得大电荷量高能电子束具有积极意义. 关键词： 尾波场 电子俘获 时间波形 粒子模拟     

锐真空-等离子体边界倾角对激光尾波场加速中电子注入的影响

超短超强激光脉冲在气体等离子体中激发的尾波场加速在过去40年里有了长足的发展,人们已经在厘米加速距离内获得了数GeV的准单能电子加速,激光尾波加速的最高电子能量已经达到8 GeV.为了进一步提升加速电子束的稳定性和品质,多种电子注入方式先后被提出.本文研究了基于锐真空-等离子体边界面的密度跃变注入,着重讨论了不同角度的倾斜边界面对注入电子品质的影响.二维粒子模拟研究表明,与倾角为0°的垂直边界面相比,在合适的倾斜边界角下,第二个尾波空泡内产生的注入电量可以有近三倍的提升,同时偏振方向与入射面平行的驱动激光可以增加第一个空泡内注入电子的电量.根据不同激光入射角度时尾波场中电子自注入的起始位置差异,分析了电子电量与横向振荡增强的原因.这些研究有利于提升基于Betatron运动的尾波场辐射及其应用.     

双束平行入射电子束引导的自注入电子加速效果的研究

在粒子束引导的等离子尾波场加速机制中,为了加速电子获得最大能量,大量研究集中于改变单束牵引粒子束的线度、形状、电荷性质等参数. 综合考虑已有的实验结果,本文提出了一种相比于单束电子牵引更为有效的加速方式,利用双束平行电子束来加速自注入的电子. 通过2.5维粒子程序模拟,发现在牵引电子束具有相同能量、电量、尺寸的条件下,通过双束平行电子束加速得到的电子具有长程加速、高能和准单能性的特性. 同时在空泡内形成了一束独特的回流电子,进一步使得自注入电子具有更好的准直性. 关键词： 电子束尾波场加速 双束平行电子束 粒子模拟     

增强辉光放电等离子体离子注入的三维PIC／MC模拟

采用三维粒子模拟／蒙特卡洛模型自洽地模拟了增强辉光放电等离子体离子注入过程中离子产生和注入,获得了放电空间的离子总数、电势分布、等离子体密度分布和离子入射剂量等信息．模拟结果表明,5μs时鞘层达到稳定扩展,15μs时离子的产生与注入达到平衡,证实了增强辉光放电等离子体离子注入能在一定条件下实现白持的辉光放电．注入过程中,在点状阳极正下方存在一个高密度的等离子体区域,证实了电子聚焦效应．除靶台边缘外,离子的注入速率稳定且入射剂量均匀．脉冲负偏压提高时注入速率增加但入射剂量的均匀性变差．     

光电子发射引起的柱腔内系统电磁脉冲的模拟

 用时域有限差分法结合PIC粒子模拟方法，对光电子发射引起的圆柱腔内电磁脉冲现象进行了模拟，并对单能电子发射时电场的空间分布和系统电磁脉冲波形特征进行了分析。利用粒子抽样和间隔时间粒子注入的方法，得到了特定电子发射谱下的计算结果，并与非抽样方法所得的结果进行了比较。计算结果显示，采用该方法后，噪声略有增加，但计算要求的条件大大降低，计算的粒子数有效地减少，适用于3维粒子模拟计算；计算结果还显示，发射电子能谱越高，注量越大，表面电场区与饱和电场区的长度越短。     

充气型放电毛细管的密度测量及磁流体模拟

在激光尾波场电子加速机理中,为了有效地加速电子,需要抑制衍射散焦等造成的激光传输不稳定性问题. 激光脉冲的稳定传输不仅有利于能量耦合给等离子体波,而且对电子束的注入及稳定加速有着重要影响,具有一定横向密度分布的充气型放电毛细管可以有效引导激光脉冲的传输. 利用等离子体的Stark展宽效应对毛细管产生的等离子体进行密度测量,给出了等离子体密度与充气压强之间的关系. 利用磁流体程序CRMHA对毛细管的放电特性进行了模拟,研究了毛细管引导效应的形成机理. 关键词： 充气型放电毛细管 Stark展宽 磁流体模拟 引导     

Stochastic heating and acceleration of electrons in colliding laser fields in plasma

We propose a mechanism that leads to efficient acceleration of electrons in plasma by two counterpropagating laser pulses. It is triggered by stochastic motion of electrons when the laser fields exceed some threshold amplitudes, as found in single-electron dynamics. It is further confirmed in particle-in-cell simulations. In vacuum or tenuous plasma, electron acceleration in the case with two colliding laser pulses can be much more efficient than with one laser pulse only. In plasma at moderate densities, such as a few percent of the critical density, the amplitude of the Raman-backscattered wave is high enough to serve as the second counterpropagating pulse to trigger the electron stochastic motion. As a result, even with one intense laser pulse only, electrons can be heated up to a temperature much higher than the corresponding laser ponderomotive potential.     

Acceleration of nonmonoenergetic electron bunches injected into a wake wave

The trapping and acceleration of nonmonoenergetic electron bunches in a wake field wave excited by a laser pulse in a plasma channel is studied. Electrons are injected into the region of the wake wave potential maximum at a velocity lower than the phase velocity of the wave. The paper analyzes the grouping of bunch electrons in the energy space emerging in the course of acceleration under certain conditions of their injection into the wake wave and minimizing the energy spread for such electrons. The factors determining the minimal energy spread between bunch electrons are analyzed. The possibility of monoenergetic acceleration of electron bunches generated by modern injectors in a wake wave is analyzed.     

Plasma electron trapping and acceleration in a plasma wake field using a density transition

A new scheme for plasma electron injection into an acceleration phase of a plasma wake field is presented. In this scheme, a single, short electron pulse travels through an underdense plasma with a sharp, localized, downward density transition. Near this transition, a number of background plasma electrons are trapped in the plasma wake field, due to the rapid wavelength increase of the induced wake wave in this region. The viability of this scheme is verified using two-dimensional particle-in-cell simulations. To investigate the trapping and acceleration mechanisms further, a 1D Hamiltonian analysis, as well as 1D simulations, has been performed, with the results presented and compared.     

Particle simulation on electron acceleration process by the laser ponderomotive force in inhomogeneous underdense plasma layers

The mechanism of electron ponderomotive acceleration due to increasing group velocity of laser pulse in inhomogeneous underdense plasma layers is studied by two-dimensional relativistic parallel particle-in-cell code. The electrons within the laser pulse move with it and can be strongly accelerated ponderomotively when the duration of laser pulse is much shorter than the duration of optimum condition for acceleration in the wake. The extra energy gain can be attributed to the change of laser group velocity. More high energy electrons are generated in the plasma layer with descending density profile than that with ascending density profile. The process and character of electron acceleration in three kinds of underdense plasma layers are presented and compared.     

Acceleration and radiation of externally injected electrons in laser plasma wakefield driven by a Laguerre-Gaussian pulse

By using three-dimensional particle-in-cell simulations, externally injected electron beam acceleration and radiation in donut-like wake fields driven by a Laguerre-Gaussian pulse are investigated. Studies show that in the acceleration process the total charge and azimuthal momenta of electrons can be stably maintained at a distance of a few hundreds of micrometers. Electrons experience low-frequency spiral rotation and high-frequency betatron oscillation, which leads to a synchrotron-like radiation. The radiation spectrum is mainly determined by the betatron motion of electrons. The far field distribution of radiation intensity shows axial symmetry due to the uniform transverse injection and spiral rotation of electrons. Our studies suggest a new way to simultaneously generate hollow electron beam and radiation source from a compact laser plasma accelerator.     

Laser wake field acceleration: the highly non-linear broken-wave regime

We use three-dimensional particle-in-cell simulations to study laser wake field acceleration (LWFA) at highly relativistic laser intensities. We observe ultra-short electron bunches emerging from laser wake fields driven above the wave-breaking threshold by few-cycle laser pulses shorter than the plasma wavelength. We find a new regime in which the laser wake takes the shape of a solitary plasma cavity. It traps background electrons continuously and accelerates them. We show that 12-J, 33-fs laser pulses may produce bunches of 3×10¹⁰ electrons with energy sharply peaked around 300 MeV. These electrons emerge as low-emittance beams from plasma layers just 700-μm thick. We also address a regime intermediate between direct laser acceleration and LWFA, when the laser-pulse duration is comparable with the plasma period. Received: 12 December 2001 / Published online: 14 March 2002     

On the production of flat electron bunches for laser wakefield acceleration

We suggest a novel method for the injection of electrons into the acceleration phase of particle accelerators, producing low-emittance beams appropriate even for the demanding high-energy linear collider specifications. We discuss the injection mechanism into the acceleration phase of the wakefield in a plasma behind a high-intensity laser pulse, which takes advantage of the laser polarization and focusing. The scheme uses the structurally stable regime of transverse wakewave breaking, when the electron trajectory self-intersection leads to the formation of a flat electron bunch. As shown in three-dimensional particle-in-cell simulations of the interaction of a laser pulse elongated in the transverse direction with an underdense plasma, the electrons injected via the transverse wakewave breaking and accelerated by the wakewave perform betatron oscillations with different amplitudes and frequencies along the two transverse coordinates. The polarization and focusing geometry lead to a way to produce relativistic electron bunches with an asymmetric emittance (flat beam). An approach for generating flat laser-accelerated ion beams is briefly discussed. The text was submitted by the authors in English.     

Laser-wakefield acceleration of monoenergetic electron beams in the first plasma-wave period

Beam profile measurements of laser-wakefield accelerated electron bunches reveal that in the monoenergetic regime the electrons are injected and accelerated at the back of the first period of the plasma wave. With pulse durations ctau >or= lambda(p), we observe an elliptical beam profile with the axis of the ellipse parallel to the axis of the laser polarization. This increase in divergence in the laser polarization direction indicates that the electrons are accelerated within the laser pulse. Reducing the plasma density (decreasing ctau/lambda(p)) leads to a beam profile with less ellipticity, implying that the self-injection occurs at the rear of the first period of the plasma wave. This also demonstrates that the electron bunches are less than a plasma wavelength long, i.e., have a duration <25 fs. This interpretation is supported by 3D particle-in-cell simulations.