V. A. Balakirev, I. N. Onishchenko
The process of transition excitation of an electromagnetic field by a relativistic electron bunch in a plasma half-space is investigated. The influence of the plasma boundary on the spatial structure of plasma wake oscillations is studied. The shape and intensity of the transition electromagnetic pulse propagating in the plasma are determined.
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A train of short charged particle bunches can efficiently drive a strong plasma wakefield over a long propagation distance only if all bunches reside in focusing and decelerating phases of the wakefield. This is shown possible with equidistant bunch trains, but requires the bunch charge to increase along the train and the plasma frequency to be higher than the bunch repetition frequency.
Acceleration rate enhancement by negative plasma density gradient in multi-bunch driven plasma wakefield accelerator
本文提出在多束驱动的等离子体尾场加速器中引入负等离子体密度梯度，通过仅调节驱动束与波的相对相位并保持波相位稳定，从而在有限长度内提升被加速束的能量增益。
If a charged particle bunch propagates near a plasma-vacuum boundary, it excites a surface wave and experiences a force caused by the boundary. For the linearly responding plasma and ultra-relativistic bunch, the spatial distribution of excited fields is calculated, and the force exerted on a short and narrow bunch is approximated by elementary functions. The force attracts the bunch to the boundary, if the bunch is in the vacuum, and repels otherwise. There are also additional focusing and defocusing components of the force.
The expressions for wake fields, generated in plasma (in the plasma waveguide or unlimited plasma) by relativistic electron bunch, was received and analyzed for the cases of the presence and absence of strong external longitudinal magnetic field. For the both cases the comparative analysis of the dependence of field amplitudes on the parameters of the electron bunch was done.
Dynamics of self-injected electron bunches has been numerically simulated in blowout regime at self-consistent change of electron bunch acceleration by plasma wakefield, excited by a laser pulse, to additional their acceleration by wakefield, excited by self-injected bunch. Advantages of acceleration by pulse train and bunch self-cleaning have been considered.
A linear theory of a wakefield excitation in a plasma-dielectric accelerating structure by a drive electron bunch in the case of an off-axis bunch injection has been constructed. The structure under investigation is a round dielectric-loaded metal waveguide with a channel for the charged particles, filled with homogeneous cold plasma. Derived theory was used to investigate numerically the spatial distribution of the bunch-excited wakefield components, which act on both the drive and witness bunches.
We show in experiments that a long, relativistic proton bunch is focused by the plasma adiabatic response. The free plasma electrons migrate so as to neutralize the space charge field of the bunch, and the bunch is therefore focused by the azimuthal magnetic field generated by its own current, that is not balanced by the radial electric field. Since the length of the bunch is much longer than the plasma electron wavelength, the bunch also undergoes the self-modulation instability. Thus, the amplitude of the wakefields grows along the bunch and along the plasma, and the defocusing effect of the…
The plasma wakefield amplitudes which could be achieved via the modulation of a long proton bunch are investigated. We find that in the limit of long bunches compared to the plasma wavelength, the strength of the accelerating fields is directly proportional to the number of particles in the drive bunch and inversely proportional to the square of the transverse bunch size. The scaling laws were tested and verified in detailed simulations using parameters of existing proton accelerators, and large electric fields were achieved, reaching 1 GV/m for LHC bunches. Energy gains for test electrons bey…
High brightness, symmetric electron bunch generation in a plasma wakefield accelerator via a radially-polarized plasma photocathode
本文扩展等离子体光阴极方案，采用径向极化激光产生高电荷、高亮度且发射度对称的电子束，并通过贝叶斯优化评估性能，发现其在最优加载下优于线极化方案。
A plasma flow behind a relativistic electron bunch propagating through a cold plasma is found assuming that the transverse and longitudinal dimensions of the bunch are small and the bunch can be treated as a point charge. In addition, the bunch charge is assumed small. A simplified system of equations for the plasma electrons is derived and it is shown that, through a simple rescaling of variables, the bunch charge can be eliminated from the equations. These equations have a unique solution, with an ion cavity formed behind the driver. The equations are solved numerically and the scaling of th…
Using a periodic electron beam bunch train to resonantly excite plasma wakefields in the quasi-nonlinear (QNL) regime has distinct advantages over employing a single, higher charge bunch. Resonant excitation in the QNL regime can produce plasma electron blowout using very low emittance beams with a small charge per pulse: the local density perturbation is extremely nonlinear, achieving total rarefaction, yet the resonant response of the plasma electrons at the plasma frequency is preserved. Such a pulse train, with inter-bunch spacing equal to the plasma period, can be produced via inverse fre…
The AWAKE experiment relies on the self-modulation instability of a long proton bunch to effectively drive wakefields and accelerate an electron bunch to GeV-level energies. During the first experimental run (2016-2018) the instability was made phase reproducible by means of a seeding process: a short laser pulse co-propagates within the proton bunch in a rubidium vapor. Thus, the fast creation of plasma and the onset of beam-plasma interaction within the bunch drives seed wakefields. However, this seeding method leaves the front of the bunch not modulated. The bunch front could self-modulate …
We discuss the focusing effect experienced by a long relativistic proton bunch, when propagating in underdense plasma. We show with 2D quasi-static simulations that the response of the plasma to the presence of the bunch provides a focusing force for the protons. We discuss the impact of the finite transverse size of the plasma on the dynamics of the process and we introduce the measurements performed at the AWAKE experiment at CERN.
By means of hollow plasma, multiple proton bunches work well in driving nonlinear plasma wakefields and accelerate electrons to energy frontier with preserved beam quality. However, the acceleration of positrons is different because the accelerating structure is strongly charge dependent. There is a discrepancy between keeping a small normalized emittance and a small energy spread. This results from the conflict that the plasma electrons used to provide focusing to the multiple proton bunches dilute the positron bunch. By loading an extra electron bunch to repel the plasma electrons and meanwh…
A long, narrow, relativistic charged particle bunch propagating in plasma is subject to the self-modulation (SM) instability. We show that SM of a proton bunch can be seeded by the wakefields driven by a preceding electron bunch. SM timing reproducibility and control are at the level of a small fraction of the modulation period. With this seeding method, we independently control the amplitude of the seed wakefields with the charge of the electron bunch and the growth rate of SM with the charge of the proton bunch. Seeding leads to larger growth of the wakefields than in the instability case.
We analyze the electromagnetic field of a small bunch that uniformly moves in a circular waveguide and transverses a boundary between an area filled up with cold magnetized electron plasma and a vacuum area. The magnetic field is supposed to be strong but finite so the perturbation technique can be applied. Two cases are studied in detail: the bunch is flying out of the plasma into a vacuum, and, inversely, the bunch is flying into the plasma out of the vacuum area of waveguide. The investigation of the waveguide mode components is performed analytically with methods of the complex variable fu…
Plasma wake lens in which all short relativistic electron bunches of sequence are focused identically and uniformly is studied analytically and by numerical simulation. For two types of lenses necessary parameters of focused sequence of relativistic electron bunches are formulated. Verification of these parameters is performed by numerical simulation.
We study experimentally the effect of linear plasma density gradients on the self-modulation of a 400\,GeV proton bunch. Results show that a positive/negative gradient in/decreases the number of micro-bunches and the relative charge per micro-bunch observed after 10\,m of plasma. The measured modulation frequency also in/decreases. With the largest positive gradient we observe two frequencies in the modulation power spectrum. Results are consistent with changes in wakefields' phase velocity due to plasma density gradient adding to the slow wakefields' phase velocity during self-modulation grow…
Plasma lens for the focusing of positron bunches
针对等离子体加速器中正电子束团聚焦的难题，提出一种工作在线性区的等离子体透镜聚焦方法，经数值模拟验证可对两种构型的正电子束团实现高质量横向聚焦，还具备降低束团能散的潜力。
The process of the excitation of an electromagnetic field by a relativistic electron bunch at the input of a semi-infinite plasma waveguide is investigated. The shape and intensity of the short transition electromagnetic pulse are determined and its evolution during propagating in the plasma waveguide is studied. Besides, the influence of the plasma boundary on the spatial structure of plasma wake oscillations in plasma waveguide is considered.
Hosing of a long relativistic particle bunch in plasma
实验首次表征了由短而错位的前置束流尾流场诱发的长粒子束在等离子体中的摇摆现象，揭示其发展取决于错位方向、程度及质子束电荷，结果与理论模型相符且适用于其他等离子体加速器。
The paper presents the results of numerical PIC-simulation of positron bunch focusing when acceleration in a plasma dielectric wakefield accelerator. The wakefield was excited by drive electron bunch in quartz dielectric tube, embedded in cylindrical metal waveguide. The internal area of dielectric tube has been filled with radially homogeneous plasma having in general case the vacuum channel along waveguide axis. Results of numerical PIC simulation have shown that it is possible a simultaneous acceleration and focusing of test positron bunch in the wakefield. The dependence of transport and a…
Nonlinear effects responsible for elongation of the plasma wave period are numerically studied with the emphasis on two-dimensionality of the wave. The limitation on the wakefield amplitude imposed by detuning of the wave and the driver is found.
The paper presents the results of numerical PIC-simulation of accelerated and drive bunches dynamics in a dielectric waveguide filled with radially inhomogeneous plasma. The wakefield was excited by the electron bunch in a quartz (permittivity 3.75) dielectric tube with outer and inner diameters of 1.2 mm and 1.0 mm, respectively, which was nested into a cylindrical metallic waveguide. The drive bunch characteristics were chosen to be: 5 GeV for electron energy, 3 nC for the charge, 0.2 mm - the bunch length, 0.9 mm - the bunch diameter. The accelerated bunch had the same parameters, except fo…
We study experimentally the longitudinal and transverse wakefields driven by a highly relativistic proton bunch during self-modulation in plasma. We show that the wakefields' growth and amplitude increase with increasing seed amplitude as well as with the proton bunch charge in the plasma. We study transverse wakefields using the maximum radius of the proton bunch distribution measured on a screen downstream from the plasma. We study longitudinal wakefields by externally injecting electrons and measuring their final energy. Measurements agree with trends predicted by theory and numerical simul…
Experimental Observation of Space-Charge Field Screening of a Relativistic Particle Bunch in Plasma
本研究通过测量无涂层介质毛细管中的介质尾场，实验观测相对论粒子束在等离子体中的空间电荷场屏蔽及其对尾场的抑制条件。
At the laser acceleration of self-injected electron bunch by plasma wakefield it is important to form bunch with small energy spread and small size. It has been shown that laser-pulse shaping on radius, intensity and shape controls characteristics of the self-injected electron bunch and provides at certain shaping small energy spread and small size of self-injected and accelerated electron bunch.
Development of the Self-Modulation Instability of a Relativistic Proton Bunch in Plasma
本文实验研究相对论质子束在等离子体中的自调制不稳定性，发现增大束团横向尺寸与等离子体趋肤深度之比会延迟或抑制不稳定性，并讨论了对AWAKE实验升级等加速器设计的影响。
Filamentation of a Relativistic Proton Bunch in Plasma
实验证明长、欠稠密相对论质子束在等离子体中发生斜向不稳定性（丝化），确定阈值条件，并发现磁能随等离子体密度增加。
Seeded self-modulation in a plasma can transform a long proton beam into a train of micro-bunches that can excite a strong wakefield over long distances, but this needs the plasma to have a certain density profile with a short-scale ramp up. For the parameters of the AWAKE experiment at CERN, we numerically study which density profiles are optimal if the self-modulation is seeded by a short electron bunch. With the optimal profiles, it is possible to "freeze" the wakefield at approximately half the wavebreaking level. High-energy electron bunches (160 MeV) are less efficient seeds than low-ene…
Plasma Light As Diagnostic For Wakefields Driven By Developing Self-Modulation Of A Long Particle Bunch
本研究提出利用等离子体原子线辐射光诊断长质子束自调制驱动的尾场，设计了可设置不同位置和高度密度阶跃的新型蒸汽/等离子体源，模拟显示百分之几的密度阶跃可使束流在数百米等离子体中驱动GV/m级尾场。
Profiling and variation of laser pulse parameters as a way to preserve the stability of self-injected bunches during excitation of a wakefield in plasma
该研究针对等离子体尾场激发中自注入束团易因横向电子感应加速振荡损毁的问题，通过X射线激光脉冲链的参数整形与调控，找到了获得稳定高质量束团的参数，为解决该问题提供了可行方案。
We expand on the material that was published in the previous Proceedings of the CERN Accelerator School on Plasma Wakefield Acceleration. The material focused on Plasma Wakefield Acceleration in the short, narrow bunch regime. After a brief introduction, we describe Plasma Wakefield Acceleration driven by a bunch train. We then attempt to give simple and intuitive descriptions of bunch self-modulation, occurring when the bunch is long, and of current filamentation, occurring when the bunch is wide. Self-modulation is a means to use existing bunches carrying large amounts of energy to drive pla…
Techniques to seed the self-modulation instability of a long proton bunch in plasma
本文介绍AWAKE实验中两种质子束自调制种子注入方法的实验结果，并探讨其对含等离子体密度阶跃的后续实验及未来两段式等离子体实验设计的影响。
Earlier, the authors found a mechanism for the sequence of short relativistic electron bunches, which leads to resonant excitation of the wakefield, even if the repetition frequency of bunches differs from the plasma frequency. In this case, the synchronization of frequencies is restored due to defocusing of the bunches which get into the bad phases with respect to the plasma wave. However, in this case, the bunches are lost, which as a result of this do not participate in the excitation of the wakefield. In this paper, numerical simulation was used to study the dynamics of electron bunches an…
Bunch-excited wakefield in dielectric waveguide with hollow plasma channel
本文研究介质加载圆柱波导中空心等离子体通道内相对论电子束团激发的尾场，推导线性机制下纵横向尾场解析解并进行模式与频谱分析，PIC模拟验证了准线性机制下解析与数值结果的定性一致。
Wakefield acceleration of self-injected bunch in a conical plasma channels
针对激光尾场加速中自注入粒子束因超驱动速度移出最大加速相位的问题，提出采用锥形渐变通道通过缩小气泡尺寸同步束与尾波相位，所得相关参数变化关系可为后续研究提供参考。
Plasma wakefield dynamics of self-generated electron bunch trains
本研究通过激光驱动尾场加速器中的下坡注入产生具有周期性能量间隔的准单能电子束团，利用尾场的相对论性延长和相空间旋转实现能量压缩，并通过粒子模拟揭示其形成与演化机制。
A numerical investigation on the spatiotemporal evolution of an electron beam, externally injected in a plasma in the presence of a plasma wake field, is carried out. The latter is driven by an ultra-short relativistic axially-symmetric femtosecond electron bunch. We first derive a novel Poisson-like equation for the wake potential where the driving term is the ultra-short bunch density, taking suitably into account the interplay between the sharpness and high energy of the bunch. Then, we show that a channel is formed longitudinally, through the externally injected beam while experiencing the…
In the Phase 2 of the AWAKE first experimental run (from May to November 2018), an electron beam was used to probe and test proton-driven wakefield acceleration in a rubidium plasma column. In this work, we analyze the overall charge capture and shot-to-shot reproducibility of the proton-driven plasma wakefield accelerator with various electron bunch injection parameters. The witness electron bunches were produced using an RF-gun equipped with a Cs2Te photocathode illuminated by a tailorable ultrafast deep ultraviolet (UV) laser pulse. The construction of the UV beam optical system enabled app…
The spatio-temporal structure of the bulk wakefields excited by a relativistic electron bunch in plasma of semiconductors and semimetals is studied. It is shown that these wakefield consists of the field of longitudinal plasmons and Cherenkov electromagnetic radiation, which is a set of eigen electromagnetic waves of the semiconductor or semimetal waveguide. A branch of the surface plasmons appears in a waveguide with an axial vacuum channel. The process of wake excitation of the surface plasmons by the relativistic electron bunch is also investigated. The intensity of the excited wake surface…
It is shown that the requirements for high quality electron bunch generation and trapping from an underdense photocathode in plasma wakefield accelerators can be substantially relaxed through localizing it on a plasma density downramp. This depresses the phase velocity of the accelerating electric field until the generated electrons are in phase, allowing for trapping in shallow trapping potentials. As a consequence the underdense photocathode technique is applicable by a much larger number of accelerator facilities. Furthermore, dark current generation is effectively suppressed.
Colliding Pulse Injection of Polarized Electron Bunches in a Laser-Plasma Accelerator
针对现有激光等离子体加速器极化电子束产生方案的固有局限，提出对撞脉冲注入技术，结合贝叶斯优化参数，可利用100-TW级激光获得高电流、高极化度且品质优异的电子束团。
In plasma wakefield accelerators (e.g. AWAKE) the proton bunch self-modulation is seeded by the ionization front of a high-power laser pulse ionizing a vapour and by the resulting steep edge of the driving bunch profile inside the created plasma. In this paper, we present calculations in 2D linear theory for a concept of a different self-modulation seeding mechanism based on electron injection. The whole proton bunch propagates through a preformed plasma and the effective beam current is modulated by the external injection of a short electron bunch at the centre of the proton beam. The resulti…
Laser-driven Plasma Accelerators (LPA) have successfully generated high energy, high charge electron bunches which can reach many kA peak current, over short distances. Space charge issues, even in transport lines as simple as a drift section, have to be carefully taken into account since they can degrade the beam quality, preventing any further application of such electron beams. We analyse the space charge effects within an electron bunch with numerical simulations in order to assess their effect on the beam. We use LPA beam parameters published in previous experimental studies. These studie…
There has been much interest in the blowout regime of plasma wakefield acceleration (PWFA), which features ultra-high fields and nonlinear plasma motion. Using an exact analysis, we examine here a fundamental limit of nonlinear PWFA excitation, by an infinitesimally short, relativistic electron beam. The beam energy loss in this case is shown to be linear in charge even for nonlinear plasma response, where a normalized, unitless charge exceeds unity. The physical basis for this effect is discussed, as are deviations from linear behavior observed in simulations with finite length beams.
Here we discuss the possibility of employment of ultrarelativistic electron and proton bunches for generation of high plasma wakefields in dense plasmas due to the Cherenkov resonance plasma-bunch interaction. We estimate the maximum amplitude of such a wake and minimum system length at which the maximum amplitude can be generated at the given bunch parameters.
Collider-quality electron bunches from an all-optical plasma photoinjector
本文提出利用全光学等离子体光电注入器生成高质电子束的方法，通过激光脉冲时空控制产生离子化前沿，实现电子束的均匀加速场，模拟结果展示出高电荷和低发散角的电子束，满足碰撞机需求。
Laser-plasma acceleration in a conical plasma channel with longitudinally inhomogeneous plasma profile
本文研究在纵向非均匀等离子体密度的锥形通道中提升激光等离子体加速性能，证明该结构能维持自注入束团加速相位并压缩激光脉冲，从而显著提高加速梯度和束团纵向动量。
Dependence of self-injected bunch parameters on the plasma density gradient and laser pulse amplitude at LWFA in a conical plasma channel
本文通过WarpX数值模拟，探究锥形等离子体通道中自注入束参数对激光振幅和等离子体密度梯度的依赖关系，获得特定条件下的自注入束参数，为LWFA的应用提供依据。
We investigate numerically the detection of the self-modulation instability in a virtual detector located downstream from the plasma in the context of AWAKE. We show that the density structures, appearing in the temporally resolving virtual detector, map the transverse beam phase space distribution at the plasma exit. As a result, the proton bunch radius that appears to grow along the bunch in the detector results from the divergence increase along the bunch, related with the spatial growth of the self-modulated wakefields. In addition, asymmetric bunch structures in the detector are a result …
Nanometer-scale pre-bunched electron beams generated from all-optical plasma-based acceleration
本文提出一种基于全光学等离子体加速的方案，通过激光调制产生纳米尺度预压缩电子束，为超快X射线应用提供新可能。
Generation and Acceleration of Isolated-Attosecond Electron Bunch in a Hollow-Channel Plasma Wakefield
提出利用电子束驱动空心通道等离子体靶同时产生并加速十GeV级孤立阿秒电子束的新方案，通过辐射尾场注入并加速等离子体电子，获得高电荷量、高能量、高稳定性的电子束，适用于超快与高能物理。
Drive particle beams in linear or weakly nonlinear regimes of the plasma wakefield accelerator quickly reach a radial equilibrium with the wakefield, which is described in detail for the first time. The equilibrium beam state and self-consistent wakefields are obtained by combining analytical relationships, numerical integration, and first-principle simulations. In the equilibrium state, the beam density is strongly peaked near the axis, the beam radius is constant along the beam, and longitudinal variation of the focusing strength is balanced by varying beam emittance. The transverse momentum…
All-optical GeV electron bunch generation in a laser-plasma accelerator via truncated-channel injection
提出截断通道注入方案，利用全光等离子体通道引导驱动脉冲产生无暗电流的高质量GeV电子束，并通过实验和模拟验证了其有效性及进一步提升能量的潜力。
Electron source based on emergence of self-injected electron bunch at plasma wakefield excitation by a TW laser pulse
本文通过模拟数十毫焦、1-2太瓦激光脉冲激发等离子体尾场，研究均匀与高斯等离子体轮廓对自注入电子束能量的影响，证明可产生相对论性自注入束团，作为电子源用于激光加速实验。
The transverse self-modulation of ultra-relativistic, long lepton bunches in high-density plasmas is explored through full-scale particle-in-cell simulations. We demonstrate that long SLAC-type electron and positron bunches can become strongly self-modulated over centimeter distances, leading to wake excitation in the blowout regime with accelerating fields in excess of 20 GV/m. We show that particles energy variations exceeding 10 GeV can occur in meter-long plasmas. We find that the self-modulation of positively and negatively charged bunches differ when the blowout is reached. Seeding the s…
The present numerical investigation of a Plasma Wakefield Acceleration scenario in the weakly non linear regime with external injection is motivated by the upcoming campaigns at the SPARC\_LAB test facility where the final goal is to demonstrate modest gradient acceleration ($\sim$1 GV/m) with no quality loss. The accelerated bunch can be envisioned to seed a free electron laser. The numerical study has been conducted with the particle-in-cell code ${\tt ALaDyn}$, an exhaustive description of the plasma-acceleration version is provided. The configuration consider a two bunches setup with param…
Efficient Acceleration of High-Quality GeV-Electron Bunches in a Hybrid Laser- and Beam-Driven Plasma Wakefield Accelerator
该研究通过混合激光与束驱动等离子体wakefield加速器，实现了高效率的能量传输，显著提升了驱动束到见证束的能量转换效率。
We give direct experimental evidence for the observation of the full transverse self-modulation of a relativistic proton bunch propagating through a dense plasma. The bunch exits the plasma with a density modulation resulting from radial wakefield effects with a period reciprocal to the plasma frequency. We show that the modulation is seeded by using an intense laser pulse co-propagating with the proton bunch which creates a relativistic ionization front within the bunch. We show by varying the plasma density over one order of magnitude that the modulation period scales with the expected depen…
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