热等离子体中的尾波加速

通过计算在等离子体温度不太高时尾波中本底等离子体的分布函数,推导出热等离子体中的尾波方程.这个方程对进一步研究如何最大限度地提高热等离子体中的尾波加速效率提供了出发点.讨论了温度对等离子体尾波加速的影响. 关键词：     

面向激光等离子体尾波加速的毛细管放电实验研究

具有合适径向密度分布的等离子体通道可以用于超短超强激光导引,这使得等离子体通道在激光尾波加速中有着重要的应用.本文介绍了在上海交通大学激光等离子体实验室开展的毛细管放电和光导引实验.通过光谱展宽法测量了充氦气的放电毛细管中的等离子体密度分布,在长度为3 cm、内径为300μm的毛细管中实现了轴向均匀,径向呈抛物线型的等离子体密度分布.通过改变放电延时和喷气时长,确定和优化了产生等离子体通道的参数区间,得到的最大通道深度为28μm,与实验中使用的激光焦斑半径匹配.在此基础之上,开展了不同能量的激光脉冲在放电等离子体通道中的导引研究,结果发现当通道深度与焦斑半径匹配时,激光可以不散焦地在通道中传输,实现激光导引.这项研究为未来的激光尾波级联加速和锁相加速等研究奠定了基础.     

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.     

第二讲 激光尾波场加速中的准单能电子束的产生

自从激光尾波场加速电子方案提出以来,经过二十多年的理论和实验研究,人们在激光尾波场加速方面已经取得了重大进步,相继在电子束能量、电子单色性等束流性能上取得重大突破.特别是在2004年对电子束的单色性研究取得重大突破,国际上几个著名实验室相继报道了准单能电子束产生的实验观测,掀起了激光尾波场研究的新高潮.对于准单能电子束的产生机制,虽然尚未达成统一认识,但普遍认为空泡加速可能是其中非常重要的机制之一.文章介绍了激光尾波场的基本概念,着重介绍了单能电子束产生的空泡加速模式里的两个关键物理过程：波破和电子的自捕获,同时介绍国际上相关的一些重要实验结果和理论进展.     

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。     

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。     

高等离子体密度区域的光子加速现象

本文研究了激光尾场实验中的透射光谱。和标准尾场实验不同的是,我们采用了更高的等离子体密度。实验中仍然观察到了明显的频谱加宽与频谱分裂,这是光子加速的显著标志。为解释该现象,我们做了二维粒子模拟。模拟结果证明在该高密度等离子体中, 光子加速仍然可以发生。     

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     

超强飞秒激光尾波场加速产生58MeV准单能电子束实验

超短超强激光与等离子体相互作用产生的激光尾波场可以在毫米尺度上加速产生高能量的准单能电子束.在SILEX-Ⅰ激光装置上进行的激光尾波场加速实验中,利用超强飞秒激光与超声速锥形喷嘴产生的2.7mm直径氦气气体柱相互作用,获得了能散为15.5%、发散角为15mrad、能量为58MeV的准单能电子束.在70TW激光照射下获得的电子束总电量达到15.4nC.介绍了实验条件、方法和主要实验结果.     

激光等离子体尾波加速器的发展和展望

超强激光在气体等离子体中传输时可以激发出大振幅的电子等离子体尾波。激光等离子体尾波加速器是利用该尾波对带电粒子（特别是电子和正电子）进行加速的一种新型装置。由于其加速梯度相较于现有的常规加速器可以提升1000倍,为建造超紧凑型的加速器和辐射源奠定了基础,也为将来建造基于等离子体的超高能正负电子对撞机和自由电子激光装置提供了可能。对该新型加速器的原理、特点、发展历程,尤其是近十年来的主要进展和未来发展趋势及面临的主要挑战进行简要梳理和介绍。     

激光尾波场中光子加速研究

在SILEX-Ⅰ激光装置上,测量了超短超强激光脉冲与稀薄等离子体相互作用之后的透射谱. 实验中发现,激光尾波场产生的密度扰动导致等离子体折射率随时间空间不断变化,导致光子的加速/减速. 透射谱上主要表现为激光频率谱峰的劈裂和随密度变化的展宽,没有发现与前向受激拉曼散射或自调制不稳定性相联系的边频波. 同时,利用LPIC++无碰撞粒子模拟程序模拟了超短超强激光与稀薄等离子体相互作用后的透射谱,模拟结果也发现了明显的光子加速过程. 关键词： 超短超强激光脉冲 透射谱 光子加速/减速     

Efficiency and energy spread in laser-wakefield acceleration

The theoretical limits on efficiency and energy spread of the laser-wakefield accelerator are investigated using a one-dimensional model. Modifications, both of the wakefield due to the electron bunch, and of the laser pulse shape due to the varying permittivity of the plasma, are described self-consistently. It is found that a short laser pulse gives a higher efficiency than a long laser pulse with the same initial energy. Energy spread can be minimized by optimizing bunch length and bunch charge such that the variation of the accelerating force along the length of the bunch is minimized. An inherent trade-off between energy spread and efficiency exists.     

Plasma wakefield acceleration for ultrahigh-energy cosmic rays

A cosmic acceleration mechanism is introduced which is based on the wakefields excited by the Alfvén shocks in a relativistically flowing plasma. We show that there exists a threshold condition for transparency below which the accelerating particle is collision-free and suffers little energy loss in the plasma medium. The stochastic encounters of the random accelerating-decelerating phases results in a power-law energy spectrum: f(epsilon) proportional, variant 1/epsilon(2). As an example, we discuss the possible production in the atmosphere of gamma ray bursts of ultrahigh-energy cosmic rays (UHECR) exceeding the Greisen-Zatsepin-Kuzmin cutoff. The estimated event rate in our model agrees with that from UHECR observations.     

Laser wakefield acceleration of short electron bunches

The theory of electron acceleration in a plasma wake wave is developed, and the dependence of the main characteristics of accelerated electron bunches on the wakefield parameters is investigated, It is shown that using a prebunching stage, under proper conditions, the final electron density of a compressed and accelerated bunch can exceed the initial electron beam density by orders of magnitude and that longitudinal bunch compression provides quasi-monoenergetic acceleration to high energies, It is demonstrated that, for an initial electron beam radius smaller than the optimal one for efficient beam trapping, the energy spread of the compressed and accelerated electron bunch and its length can be evaluated by using the simple analytical predictions of a one-dimensional (1-D) theory. The obtained analytical results are confirmed by three-dimensional (3-D) numerical modeling     

Observation of enhanced transformer ratio in collinear wakefield acceleration

One approach to future high energy particle accelerators is based on the wakefield principle: a leading high-charge drive bunch is used to excite fields in an accelerating structure or plasma that in turn accelerates a trailing low-charge witness bunch. The transformer ratio R is defined as the ratio of the maximum energy gain of the witness bunch to the maximum energy loss of the drive bunch. In general, R<2 for this configuration. A number of techniques have been proposed to overcome the transformer ratio limitation. We report here the first experimental study of the ramped bunch train (RBT) technique in a dielectric based accelerating structure. A single drive bunch was replaced by two bunches with charge ratio of 1:2.5 and a separation of 10.5 wavelengths of the fundamental mode. An average measured transformer ratio enhancement by a factor of 1.31 over the single drive bunch case was obtained.     

Energy spread in plasma-based acceleration

Electron acceleration in a one-dimensional plasma wave has been simulated, with emphasis on minimizing the energy spread of an accelerated electron bunch, while keeping the mean energy gain at a reasonable level. Bunch length, beam loading, and the injection phase are tuned to reach this goal. The simulation results show that, in a wide range of initial bunch lengths and beam loading parameters, an optimum acceleration distance exists, which combines low energy spread and high energy gain. The energy spread at the optimum is found to be weakly dependent on bunch length and beam loading, while it is highly sensitive to deviations in the injection phase     

Role of stochastic heating in wakefield acceleration monitored by optical injection

It is shown that stochastic heating can play an important role in Laser Wake Field Acceleration. When considering low density plasma interacting with a high intensity wave perturbed by a low intensity counterpropagating wave, stochastic heating can provide electrons with the right momentum for trapping in the wake field. The influence of stochastic acceleration on the trapping of electrons is compared to the one of cold injection by considering several polarizations of the colliding pulses. For some value of the plasma density and pulse duration, a transition from an injection due to stochastic acceleration to a cold injection dominated regime – regarding the trapped charge – has been observed from PIC code simulations. When the plasma density exceeds some value, stochastic heating becomes important and is necessary in some circumstances to get electrons trapped into the wakefield.