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     

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     

Coherent wake emission of high-order harmonics from overdense plasmas

We present a new mechanism for high-order harmonic generation by reflection of a laser beam from an overdense plasma, efficient even at moderate laser intensities (down to Igamma2 approximately 4x10(15) W cm-2 microm2). In this mechanism, a transient phase matching between the electromagnetic field and plasma oscillations within a density gradient leads to the emission of harmonics up to the plasma frequency. These plasma oscillations are periodically excited in the wake of attosecond electron bunches which sweep across the density gradient. This process leads to a train of unevenly spaced chirped attosecond pulses and, hence, to broadened and chirped harmonics. This last effect is confirmed experimentally.     

Generation of short electron bunches by a laser pulse crossing a sharp boundary of inhomogeneous plasma

The formation of short electron bunches during the passage of a laser pulse of relativistic intensity through a sharp boundary of semi-bounded plasma has been analytically studied. It is shown in one-dimensional geometry that one physical mechanism that is responsible for the generation of electron bunches is their self-injection into the wake field of a laser pulse, which occurs due to the mixing of electrons during the action of the laser pulse on plasma. Simple analytic relationships are obtained that can be used for estimating the length and charge of an electron bunch and the spread of electron energies in the bunch. The results of the analytical investigation are confirmed by data from numerical simulations.     

Attosecond electron bunches

Electron bunches of attosecond duration may coherently interact with laser beams. We show how p-polarized ultraintense laser pulses interacting with sharp boundaries of overdense plasmas can produce such bunches. Particle-in-cell simulations demonstrate attosecond bunch generation during pulse propagation through a thin channel or in the course of grazing incidence on a plasma layer. In the plasma, due to the self-intersection of electron trajectories, electron concentration is abruptly peaked. A group of counterstream electrons is pushed away from the plasma through nulls in the electromagnetic field, having inherited a peaked electron density distribution and forming relativistic ultrashort bunches in vacuum.     

Self-Injection and Acceleration of Monoenergetic Electron Beams from Laser Wakefield Accelerators in a Highly Relativistic Regime

Self-injection and acceleration of monoenergetic electron beams from laser wakefield accelerators are first investigated in the highly relativistic regime, using 100 TW class, 27 fs laser pulses. Quasi-monoenergetic multi- bunched beams with energies as high as multi-hundredMeV are observed with simultaneous measurements of side-scattering emissions that indicate the formation of self-channelfing and self-injection of electrons into a plasma wake, referred to as a ＇bubble＇. The three-dimensional particle-in-cell simulations confirmed multiple self-injection of electron bunches into the bubble and their beam acceleration with gradient of 1.5 GeV/cm.     

Plasma-density-gradient injection of low absolute-momentum-spread electron bunches

Plasma density gradients in a gas jet were used to control the wake phase velocity and trapping threshold in a laser wakefield accelerator, producing stable electron bunches with longitudinal and transverse momentum spreads more than 10 times lower than in previous experiments (0.17 and 0.02 MeV/c FWHM, respectively) and with central momenta of 0.76+/-0.02 MeV/c. Transition radiation measurements combined with simulations indicated that the bunches can be used as a wakefield accelerator injector to produce stable beams with 0.2 MeV/c-class momentum spread at high energies.     

Micro-bunch production with radio frequency photoinjectors

In this paper we discuss the generation of ultra-short electron bunches using laser-driven RF guns. The designs are tailored for future plasma accelerators. Second generation plasma accelerators are expected to be very demanding in terms of bunch length, since the accelerated beam is expected to be short with respect to the wavelength of the excited Langmuir space-charge plasma wave. Since the anticipated wavelength ranges from 100 to 300 μm, 10-50 μm-long bunches are required with a bunch population of the order of 10⁸ particles. The laser-driven RF gun is a promising candidate to attain such beams. The rationale for this choice as well as the main limitations in terms of minimum bunch length will be analyzed and discussed in the following. Two possible configurations are evaluated: the direct production at the photocathode surface of ultra-short electron bunches by illumination of the cathode with 160-fs-long laser pulses and the acceleration of a 1-ps electron bunch with further magnetic compression in a wiggler     

Excitation of accelerating wakefields in inhomogeneous plasmas

The excitation of the wakefields in an inhomogeneous plasma by a short laser pulse is investigated theoretically. A general equation for the wake excitation in transversely nonuniform plasma is derived. This equation is applied to the step-function density profile model of hollow channel laser wakefield accelerator. A more realistic model, in which the transition between the evacuated channel and the homogeneous surrounding plasma occurs over a finite radial extent, is then analyzed. It is shown that the excited channel made can interact resonantly with the plasma electrons inside the channel wall, leading to secular growth of the electric field. This eventually results in wavebreaking and the dissipation of the accelerating mode. We introduce an effective quality factor Q for the hollow channel laser wakefield geometry. This resonance limits the number of electron bunches that can be accelerated in the wake of single laser pulse     

Suppression of multiple ion bunches and generation of monoenergetic ion beams in laser foil-plasma

In one-dimensional particle-in-cell simulations, this paper shows that the formation of multiple ion bunches is disadvantageous to the generation of monoenergetic ion beams and can be suppressed by choosing an optimum target thickness in the radiation pressure acceleration mechanism by a circularly polarised laser pulse. As the laser pulse becomes intense, the optimum target thickness obtained by a non-relativistic treatment is no longer adequate. Considering the relativistic Doppler-shifted pressure, it proposes a relativistic formulation to determine the optimum target thickness. The theoretical predictions agree with the simulation results well. The model is also valid for two-dimensional cases. The accelerated ion beams can be compelled to be more stable by choosing the optimum target thickness when they exhibit some unstable behaviours.     

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.     

互相关法实现超短脉冲精确延时的探测及控制

在超短超强激光装置中, 强激光与靶物质相互作用产生等离子体, 为了获知等离子体的演化规律, 需要准确控制探针光与打靶主激光的相对延时, 精度须达到几十飞秒至几皮秒。为此设计了一台延时测量仪, 该测量仪基于超短脉冲强度二阶单次互相关测量原理, 利用非线性晶体的非共线和频效应, 采用线阵CCD采集探针光与打靶光产生的和频光空间光强分布曲线, 通过测量和频光强峰值的相对移动量, 反映探针光与打靶光的相对延时, 控制延时光路达到两光束的精确延时。详细阐述了该延时测量仪的构型, 讨论了超短脉冲强度互相关测量的原理, 理论分析了两光束的相对时间差与和频光峰值相对移动量之间的关系, 通过数值模拟给出了两光束夹角对延时测量精度的影响, 可根据不同要求改变两光束夹角更换晶体, 以满足延时精度及延时范围的要求。在理论分析的基础上进行了初步的实验验证。     

Plasma-based advanced accelerators at the Brookhaven Accelerator Test Facility

The Accelerator Test Facility at Brookhaven National Laboratory (BNL ATF) offers to its users a unique combination of research tools that include a high-brightness 70-MeV electron beam, a mid-infrared (λ = 10 μm) CO₂ laser of terawatt power, and a capillary discharge as a plasma source. These cutting-edge technologies have enabled us to launch a new R&;D program at the forefronts of advanced accelerators and radiation sources. The main subjects that we are researching are innovative methods of producing wakes in a linear regime using plasma resonance with the electron microbunch train periodic to the laser’s wavelength and so-called “seeded” laser wakefield acceleration (LWFA) that is driven and probed by a combination of electron and laser beams. We describe the present status of the ATF experimental program, including simulations and preliminary experiments; in addition, we review previous ATF experiments that were the precursors to the present program. They encompass our demonstration of longitudinal-and transverse-field phasing inside the plasma wave, plasma channeling of intense CO₂ laser beams, and the generation of e-beam microbunch trains by the inverse FEL technique.     

Control of femtosecond filamentation by field-free revivals of molecular alignment

We show that the filamentation dynamics of a femtosecond laser probe pulse can be readily controlled by properly matching it to the quantum revivals of pre-aligned molecules prepared through impulsive rotational Raman excitation with an advancing ultrashort pump pulse. Several features of the filamentation process including supercontinuum generation, the length of the plasma channel generated in the wake of the filament, the associated secondary radiations and the multiple filamentation pattern are all easily modified by tuning the cross phase modulation induced by the field-free revivals of molecular alignment, through the delay between the pump and the probe pulses. We show that molecular alignment can also be used to generate conical waves with extremely short intensity spike called shocked X-waves and to further tune the frequency of a few-cycle laser pulse in the wake of a self-guided intense filament.     

Positron injection and acceleration on the wake driven by an electron beam in a foil-and-gas plasma

A novel approach for generating and accelerating positron bunches in a plasma wake is proposed and modeled. The system consists of a plasma with an embedded thin foil into which two electron beams are shot. The first beam creates a region for accelerating and focusing positrons and the second beam provides positrons to be accelerated. Monte Carlo and 3D PIC simulations show a large number of positrons (10(7) approximately 10(8)) are trapped and accelerated to approximately 5 GeV over 1 m with relatively narrow energy spread and low emittance.     

Compression of laser radiation in plasmas using electromagnetic cascading

Compressing high-power laser beams in plasmas via generation of a coherent cascade of electromagnetic sidebands is described. The technique requires two copropagating beams detuned by a near-resonant frequency Omega approximately < omega(p). The ponderomotive force of the laser beat wave drives an electron plasma wave which modifies the refractive index of plasma so as to produce a periodic phase modulation of the laser field with the beat period tau(b) = 2pi/Omega. A train of chirped laser beat notes (each of duration tau(b)) is thus created. The group velocity dispersion of radiation in plasma can then compress each beat note to a few-laser-cycle duration. As a result, a train of sharp electromagnetic spikes separated in time by tau(b) is formed. Depending on the plasma and laser parameters, chirping and compression can be implemented either concurrently in the same plasma or sequentially in different plasmas.     

Generation of Broadband High Harmonics through Linear Mode Conversion in Inhomogeneous Plasmas

The generation of high order harmonics from an inhomogeneous ovderdense plasma target irradiated by an ultrashort intense laser pulse is studied by numerical simulation. During such interaction, ultrafast electron bunches are generated and excite electron plasma oscillations as they pass through the overdense target. These plasma oscillations will emit high-frequency electromagnetic emission by linear mode conversion. Instead of the integer harmonies generation, the emission appears with a broadband and even continuous spectrum corresponding to the electron plasma frequency range of the inhomogeneous plasma density.     

再入体缩比模型湍流等离子体电磁散射特性测量

介绍了在弹道靶上利用二级轻气炮发射再入体缩比模型开展湍流等离子体电磁散射特性模拟试验方法、湍流等离子体的雷达散射截面积(RCS)测量方法。给出了模型尾迹湍流等离子体的电磁散射特性测量典型试验结果,对获得的试验结果进行了分析,并与一阶畸变波Born方法计算结果进行了比较。结果表明:合理选择缩比模型发射速度和飞行环境压力,在弹道靶上能够模拟产生湍流等离子体;利用激光阴影成像技术获得的锥模型尾迹流场图像证实了尾迹转捩的出现及其湍流形成;在给定的试验条件下,锥模型及其绕流RCS测量值比其尾迹RCS测量值高3个数量级,比背景散射电平高0.5~2.5个数量级,且信号没有周期性,幅度脉动范围为1~15dB,频率脉动范围0.4~40kHz;锥模型湍流尾迹RCS的脉动可能是尾迹电子密度的脉动引起的;单站X波段雷达系统测量的锥模型尾迹亚密湍流等离子体的散射信号测量值与计算结果变化规律一致;弹道靶RCS测量技术可用于再入体缩比模型湍流等离子体电磁散射特性研究,为开展相关研究提供了一种有效的地面模拟实验研究途径。     

Generation of Uniform Plasmas for Beat Wave Experiments

The laser plasma "beat wave" mechanism for the generation of ultrahigh electric fields requires plasmas of several meters length with density uniformity of about 1 percent. Multiphoton ionization of molecular hydrogen gas at a pressure of a few torr provides a scalable mechanism for generating these plasmas using the same laser beams that drive the beat wave. We describe measurements of electron density, temperature, and uniformity of plasmas generated by a frequency doubled Neodymium glass laser, at an irradiance of about 1014 W · cm-2. The plasma density corresponds to 100-percent ionization and is measured to be uniform to within the measurement errors over a length of 8 mm.     

Impact of kinetic processes on the macroscopic nonlinear evolution of the electromagnetic-beam-plasma instability

We present a new, fully kinetic mechanism of generation of spatial magnetic vortices that results from the resonant wave-particle interaction in a plasma. This phenomenon is of basic theoretical interest. It can be responsible for the magnetic vortices observed in numerical simulations in the wake of an ultrastrong, ultraintense laser pulse in an underdense plasma.