C. A. Lindstrøm
Accelerating particles to high energies with a high-gradient wakefield accelerator may require use of multiple stages. Coupling beams from one stage to another can be difficult due to high divergence and non-negligible energy spreads. We review the challenges, technical requirements and currently proposed methods for solving the staging problem.
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Laser plasma accelerators capable of generating >10 GeV electron beams may require plasma mirrors to remove undepleted laser energy at the end of each accelerator stage. Near the plasma mirror surface, the electron bunch can interact with the reflected light, resulting in inverse Compton scattering. For realistic conditions, we show that a significant fraction of electrons emit one or more photons, increasing the energy spread of the electron bunch. We provide an analytical expression for calculating this effect, and use it to estimate the minimum drift space required before the plasma mirror …
The one-dimensional wakefield generation equations are solved for increasing levels of non-linearity, to demonstrate how they contribute to the overall behaviour of a non-linear wakefield in a plasma. The effect of laser guiding is also studied as a way to increase the interaction length of a laser wakefield accelerator.
Plasma Wakefield Accelerators promise huge acceleration gradients that are three orders of magnitude greater than today's conventional radio frequency (RF) accelerators. These novel accelerators show also the potential of diminishing the size of the future accelerators nearly by the same factor. This review gives brief explanations and the working principles of the Plasma Wakefield Accelerators and shows the recent developments of the field. The current challenges are given and the potential future use of the Plasma Wakefield Accelerators are discussed.
A new electron injection scheme is proposed in sub-relativistic plasma wakefield accelerators. A transverse laser ionizes a dopant gas and ponderomotively accelerates the released electrons in the direction of wake propagation. This process enables electron trapping in the wakefield even for a wakefield potential below the trapping threshold. We study the scheme theoretically and by means of particle-in-cell simulations to demonstrate high-quality beam formation and acceleration with sub-micrometer normalized emittances and sub-percent uncorrelated energy spreads.
We present a new concept for a plasma wakefield accelerator driven by magnetowaves (MPWA). This concept was originally proposed as a viable mechanism for the "cosmic accelerator" that would accelerate cosmic particles to ultra high energies in the astrophysical setting. Unlike the more familiar Plasma Wakefield Accelerator (PWFA) and the Laser Wakefield Accelerator (LWFA) where the drivers, the charged-particle beam and the laser, are independently existing entities, MPWA invokes the high-frequency and high-speed whistler mode as the driver, which is a medium wave that cannot exist outside of …
The self-modulation instability is a key effect that makes possible the usage of nowadays proton beams as drivers for plasma wakefield acceleration. Development of the instability in uniform plasmas and in plasmas with a small density up-step is numerically studied with the focus at nonlinear stages of beam evolution. The step parameters providing the strongest established wakefield are found, and the mechanism of stable bunch train formation is identified.
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…
Transformer ratio growth due to ion motion in plasma wakefield accelerators
本文报告了等离子体尾场加速器中Langmuir波破裂导致加速电场增强的反直觉效应，其机制是横向波破裂时通过电子的集体流入与单独流出从邻近区域汲取能量。
Laser wakefield acceleration offers the promise of a compact electron accelerator for generating a multi-GeV electron beam using the huge field gradient induced by an intense laser pulse, compared to conventional rf accelerators. However, the energy and quality of the electron beam from the laser wakefield accelerator have been limited by the power of the driving laser pulses and interaction properties in the target medium. Recent progress in laser technology has resulted in the realization of a petawatt (PW) femtosecond laser, which offers new capabilities for research on laser wakefield acce…
This paper has been withdrawn by the author. An assumption made in the analysis is not valid under the experimental conditions.
We investigate the effects of a Stern-Gerlach-type addition to the Lorentz force on electrons in a laser wakefield accelerator. The Stern-Gerlach-type terms are found to generate a family of trajectories describing electrons that surf along the plasma density wave driven by a laser pulse. Such trajectories could lead to an increase in the size of an electron bunch, which may have implications for attempts to exploit such bunches in future free electron lasers.
Oscillation damper for misaligned witness in plasma wakefield accelerator
针对线性或中等非线性等离子体尾场加速器中外部见证束对中困难的问题，本文提出在其前方亚毫米处加入同源阻尼束团，通过尾场扰动实现见证束自动回轴且无质量损失。
We discovered a novel effect that can cause witness emittance growth in plasma wakefield accelerators. The effect appears in linear or moderately nonlinear plasma waves. The witness experiences a time-varying focusing force and loses quality during the time required for the drive beam to reach transverse equilibrium with the plasma wave. The higher the witness charge, the lower the emittance growth rate because of additional focusing of the witness by its own wakefield. However, the witness head always degrades, and the boundary between degraded and intact parts gradually propagates backward a…
Coherent and incoherent electromagnetic radiation emitted from a laser wakefield accelerator is calculated based on Lienard-Wiechert potentials. It is found that at wavelengths longer than the bunch length, the radiation is coherent. The coherent radiation, which typically lies in the infrared range, shows features that reveal details of the acceleration process and properties of the electron bunch, such as its duration, charge, energy, and offset with respect to the wakefield axis. The incoherent range of the spectrum, which extends to the X-ray frequency range, consists of rather broad peaks…
The energy spread in laser-wakefield accelerators is primarily limited by the energy-chirp introduced during the injection and acceleration processes. Here we propose and demonstrate the use of longitudinal density tailoring to adapt the accelerating fields and reduce the chirp at the end of the accelerator. Experimental data supported by 3D PIC simulations show that broadband electron beams can be converted to quasi-monoenergetic beams of less than 10% energy spread while maintaining a high charge of more than 120 pC. In the linear and quasi-linear regimes of wakefield acceleration, the metho…
A new injection scheme using the interference of two coaxial laser pulses is proposed for generating high quality beams in laser wakefield accelerators. In this scheme, a relatively loosely focused laser pulse drives the plasma wakefield, and a tightly focused laser pulse with similar intensity triggers interference ring pattern which creates onion-like multi sheaths in the plasma wakefield. Due to the wavefront curvature change after the focal position of the tightly focused laser, the innermost sheath of the wakefield expands, which slows down the effective phase velocity of the wakefield an…
A laser pulse guided in a curved plasma channel can excite wakefields that steer electrons along an arched trajectory. As the electrons are accelerated along the curved channel, they emit synchrotron radiation. We present simple analytical models and simulations examining laser pulse guiding, wakefield generation, electron steering, and synchrotron emission in curved plasma channels. For experimentally realizable parameters, a ~2 GeV electron emits 0.1 photons per cm with an average photon energy of multiple keV.
Experimental Observation of Motion of Ions in a Resonantly Driven Plasma Wakefield Accelerator
实验证实等离子体尾场加速器中离子运动效应与离子质量成反比，由有质动力驱动，并随尾场振幅变化，表现为束团尾部。
We study the energetics of wake excitation during the laser-plasma interaction in application to laser wakefield accelerators. We find that both the wake amplitude and the accelerating efficiency (transformer ratio) can be maximized by properly shaping the longitudinal profile of the driving laser pulse. The corresponding family of laser pulse shapes is derived in the nonlinear regime of laser-plasma interaction. Such shapes provide theoretical upper limit on the magnitude of the wakefield and efficiency by allowing for uniform photon deceleration inside the laser pulse. We also construct real…
Single cycle laser pulse propagating inside a plasma causes controllable asymmetric plasma electron expulsion from laser according to laser carrier envelope phase (CEP) and forms an oscillating plasma bubble. Bubble's transverse wakefield is modified, exhibiting periodic modulation. Injection scheme for a laser wakefield accelerator combining a single cycle low frequency laser pulse and a many cycle high frequency laser pulse is proposed. The co-propagating laser pulses form a transversely oscillating wakefield which efficiently traps and accelerates electrons from background plasma. By tuning…
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。
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.
We present experimental results on a plasma wakefield accelerator (PWFA) driven by high-current electron beams from a laser wakefield accelerator (LWFA). In this staged setup stable and high quality (low divergence and low energy spread) electron beams are generated at an optically-generated hydrodynamic shock in the PWFA. The energy stability of the beams produced by that arrangement in the PWFA stage is comparable to both single-stage laser accelerators and plasma wakefield accelerators driven by conventional accelerators. Simulations support that the intrinsic insensitivity of PWFAs to driv…
Laser wakefield acceleration modeling using the Lorentz-boosted frame technique in the particle-in-cell code has demonstrated orders of magnitude speedups. A convergence study was previously conducted in cases with external injection in the linear regime and without injection in the nonlinear regime, and the obtained results have shown a convergence within the percentage level. In this article, a convergence study is carried out to model electron self-injection in the 2-1/2D configuration. It is observed that the Lorentz-boosted frame technique is capable of modeling complex particle dynamics …
Effects of the transversely nonuniform plasma density in a blowout regime of a plasma wakefield accelerator
本文基于等离子体电子弹道模型解析研究PWA吹离区横向密度梯度的影响，推导赝势与尾场解，发现其改变气泡形状、产生偶极尾场并使电磁中心偏移，但不会导致不稳定性。
Plasma-based accelerators have achieved tremendous progress in the past few decades, thanks to the advances of high power lasers and the availability of high-energy and relativistic particle beams. However, the electrons (or positrons) accelerated in the plasma wakefields are subject to radiation losses, which generally suppress the final energy gains of the beams. In this paper, radiation reaction in plasma-based high-energy accelerators is investigated using test particle approach. Energy-frontier TeV colliders based on a multiple stage laser-driven plasma wakefield accelerator and a single-…
Self-Adaptive Stabilization and Quality Boost for Electron Beams from All-Optical Plasma Wakefield Accelerators
本文提出利用等离子体光阴极技术提升激光等离子体谐振加速器电子束质量，通过补偿波动实现更稳定的输出。
Radiative depolarization of high-energy electron beams in wakefield accelerators
研究高能电子束在wakefield加速器中的辐射去偏极化效应，分析辐射效应如何影响束流偏极化，并探讨高能条件下束流与wakefield的对齐关系。
Wakefield regeneration in a resonantly driven plasma accelerator
针对等离子体加速器亮度受重复率和单次电荷量限制的问题，提出利用驱动束团列周期性加载尾场消除能量转移限制，通过AWAKE方案模拟验证，仅用部分驱动列实现加速电荷量翻倍以提升亮度。
A laser pulse traveling through a plasma can excite large amplitude plasma waves that can be used to accelerate relativistic electron beams in a very short distance---a technique called laser wakefield acceleration. Many wakefield acceleration experiments rely on the process of wavebreaking, or self-injection, to inject electrons into the wave, while other injection techniques rely on operation without self-injection. We present an experimental study into the parameters, including the pulse energy, focal spot quality and pulse power, that determine whether or not a wakefield accelerator will s…
We propose to use a frequency doubled pulse colliding with the driving pulse at an acute angle to trigger ionization injection in a laser wakefield accelerator. This scheme effectively reduces the duration that injection occurs, thus high injection quality is obtained. Three-dimensional particle-in-cell simulations show that electron beams with energy of ~500 MeV, charge of ~40 pC, energy spread of ~1% and normalized emittance of a few millimeter milliradian can be produced by ~100 TW laser pulses. By adjusting the angle between the two pulses, the intensity of the trigger pulse and the gas do…
It is demonstrated that the performance of the self-modulated proton driver plasma wakefield accelerator (SM-PDPWA) is strongly affected by the reduced phase velocity of the plasma wave. Using analytical theory and particle-in-cell simulations, we show that the reduction is largest during the linear stage of self-modulation. As the instability nonlinearly saturates, the phase velocity approaches that of the driver. The deleterious effects of the wake's dynamics on the maximum energy gain of accelerated electrons can be avoided using side-injections of electrons, or by controlling the wake's ph…
The influence of initial position on the acceleration of externally-injected electrons in a plasma wakefield is investigated. Test-particle simulations show previously unobserved complex structure in the parameter space, with quasi-stable injection channels forming for particles injected in narrow regions away from the wake centre. Particles injected into these channels remain in the wake for a considerable time after dephasing, and as a result achieve significantly higher energy than their neighbours. The result is relevant to both the planning and optimisation of experiments making use of ex…
The initial beam emittance determines the maximum wakefield amplitude that can be reached as a result of beam self-modulation in the plasma. The wakefield excited by the fully self-modulated beam decreases linearly with the increase of the beam emittance. There is a value of initial emittance beyond which the self-modulation does not develop even if the instability is initiated by a strong seed perturbation. The emittance scale at which the wakefield is twice suppressed with respect to the zero-emittance case (the so called critical emittance) is determined by inability of the excited wave to …
Plasma-based accelerators (PBAs) driven by either intense lasers (laser wakefield accelerators, LWFAs) or particle beams (plasma wakefield accelerators, PWFAs), can accelerate charged particles at extremely high gradients compared to conventional radio-frequency (RF) accelerators. In the past two decades, great strides have been made in this field, making PBA a candidate for next-generation light sources and colliders. However, these challenging applications necessarily require beams with good stability, high quality, controllable polarization and excellent reproducibility. To date, such beams…
High-charge 10 GeV electron acceleration in a 10 cm nanoparticle-assisted hybrid wakefield accelerator
该研究通过纳米颗粒辅助的激光与粒子驱动混合尾场机制，在10厘米装置中实现340 pC、约10.4 GeV电子加速，为紧凑型高电荷量高能加速器提供新路径。
The effect of nonlinearity on plasma wake wave excited by a relativistic cylindrical charged bunch is investigated. It is shown that owing to the nonlinearity the amplitude of wake wave gets modulated in the longitudinal direction. The nonlinear wavelength in the excited field changes in the transverse direction with the result that the phase front is distored and a turbulence developed. The nonlinear phase front distortion may be compensated by radial change of unperturbed plasma density.
Wakefield-Dressed Relativistic Vortex Electrons in Plasma Accelerators
该研究发现等离子体尾场可作为移动量子结构支持相对论涡旋电子态，推导了相关电子旋量本征态，分析了波包与扰动，为高能涡旋电子制备及相对论粒子量子态控制提供了新途径。
High quality beam produced by tightly focused laser driven wakefield accelerators
提出利用紧聚焦激光在焦点后扩张引发尾波场空泡膨胀和电子注入，并通过最佳加速距离抑制啁啾，模拟获得纳库仑级、约1%能散的高品质电子束。
Energy Depletion and Re-Acceleration of Driver Electrons in a Plasma-Wakefield Accelerator
本文首次明确观测到等离子体尾场加速器中驱动电子的再加速现象，并测得驱动器将57%能量沉积于195毫米等离子体，结合其他效率表明其总能量转移效率可达到或超越传统加速器。
The acceleration gradients generated in a laser- or beam-driven plasma wakefield accelerator are typically three orders of magnitude greater than those produced by a conventional accelerator, and hence plasma accelerators can open a route to a new generation of very compact machines. In addition, plasma-based accelerators can generate beams with unique properties, such as tens of kiloamp peak currents, attosecond bunch duration, ultrahigh brightness and intrinsic particle beam-laser pulse synchronization. In this roadmap we review the status of plasma accelerator research in the UK. We outline…
Phase-dependent bubble hosing and resonant amplification of betatron oscillation in few-cycle laser wakefield accelerator
本研究通过PIC模拟与理论分析，揭示了少周期激光尾场加速器中气泡呼吸效应引发电子束感应加速振荡参数放大、导致束质量下降的机制，为长尺度激光尾场加速研究提供了关键参考。
The effects of plasma ion motion in self-modulated plasma based accelerators is examined. An analytical model describing ion motion in the narrow beam limit is developed, and confirmed through multi-dimensional particle-in-cell simulations. It is shown that the ion motion can lead to the early saturation of the self-modulation instability, and to the suppression of the accelerating gradients. This can reduce the total energy that can be transformed into kinetic energy of accelerated particles. For the parameters of future proton-driven plasma accelerator experiments, the ion dynamics can have …
In this report we explore resonant multi-bunch, transverse wakefield effects in a linear accelerator in which the dipole modes of the accelerator structures have been detuned.
The effect of laser focusing conditions on the evolution of relativistic plasma waves in laser wakefield accelerators is studied both experimentally and with particle-in-cell simulations. For short focal length ($w_0 < λ_p$) interactions, beam break-up prevents stable propagation of the pulse. High field gradients lead to non-localized phase injection of electrons, and thus broad energy spread beams. However for long focal length geometries ($w_0 > λ_p$), a single optical filament can capture the majority of the laser energy, and self-guide over distances comparable to the dephasing length, ev…
We generate inverse Compton scattered X-rays in both linear and nonlinear regimes with a 250 MeV laser wakefield electron accelerator and plasma mirror by retro-reflecting the unused drive laser light to scatter from the accelerated electrons. We characterize the X-rays using a CsI(Tl) voxelated scintillator that measures their total energy and divergence as a function of plasma mirror distance from the accelerator exit. At each plasma mirror position, these X-ray properties are correlated with the measured fluence and inferred intensity of the laser pulse after driving the accelerator to dete…
The energy gain in laser wakefield accelerators is limited by dephasing between the driving laser pulse and the highly relativistic electrons in its wake. Since this phase depends on both the driver and the cavity length, the effects of dephasing can be mitigated with appropriate tailoring of the plasma density along propagation. Preceding studies have discussed the prospects of continuous phase-locking in the linear wakefield regime. However, most experiments are performed in the highly non-linear regime and rely on self-guiding of the laser pulse. Due to the complexity of the driver evolutio…
Matched beam loading in laser wakefield acceleration (LWFA), characterizing the state of flattening of the acceleration electric field along the bunch, leads to the minimization of energy spread at high bunch charges. Here, we demonstrate by independently controlling injected charge and acceleration gradients, using the self-truncated ionization injection scheme, that minimal energy spread coincides with a reduction of the normalized beam divergence. With the simultaneous confirmation of a constant beam radius at the plasma exit, deduced from betatron radiation spectroscopy, we attribute this …
Phase space matching between two plasma-accelerator (PA) stages and between a PA and a traditional accelerator component is a critical issue for emittance preservation of beams accelerated by PAs. The drastic differences of the transverse focusing strengths as the beam propagates between different stages and components may lead to a catastrophic emittance growth in the presence of both finite energy spread and lack of proper matching. We propose using the linear focusing forces from nonlinear wakes in longitudinally tailored plasma density profiles to provide exact phase space matching to prop…
Injection of well-defined, high-quality electron populations into plasma waves is a key challenge of plasma wakefield accelerators. Here, we report on the first experimental demonstration of plasma density downramp injection in an electron-driven plasma wakefield accelerator, which can be controlled and tuned in all-optical fashion by mJ-level laser pulses. The laser pulse is directed across the path of the plasma wave before its arrival, where it generates a local plasma density spike in addition to the background plasma by tunnelling ionization of a high ionization threshold gas component. T…
We demonstrate that a long-propagating plasma bubble executing undulatory motion can be produced in the wake of two co-propagating laser pulses: a near-single-cycle injector and a multi-cycle driver. When the undulation amplitude exceeds the analytically derived threshold, highly-localized injections of plasma electrons into the bubble are followed by their long-distance acceleration. While the locations of the injection regions are controlled by the carrier-envelope phase (CEP) of the injector pulse, the mono-energetic spectrum of the accelerated sub-femtosecond high-charge electron bunches i…
Flat beam plasma wakefield accelerator
针对高能对撞机相互作用点的高不对称发射度比粒子束流，研究其驱动等离子体尾场的椭圆特性，推导匹配条件并给出AWA、FACET装置的应用示例。
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。
In a laser plasma accelerator (LPA), a short and intense laser pulse propagating in a plasma drives a wakefield (a plasma wave with a relativistic phase velocity) that can sustain extremely large electric fields, enabling compact accelerating structures. Potential LPA applications include compact radiation sources and high energy linear colliders. We propose and study plasma wave excitation by an incoherent combination of a large number of low energy laser pulses (i.e., without constraining the pulse phases). We show that, in spite of the incoherent nature of electromagnetic fields within the …
Simulations of proton-driven plasma wakefield accelerators have demonstrated substantially higher accelerating gradients compared to conventional accelerators and the viability of accelerating electrons to the energy frontier in a single plasma stage. However, due to the strong intrinsic transverse fields varying both radially and in time, the witness beam quality is still far from suitable for practical application in future colliders. Here we demonstrate efficient acceleration of electrons in proton-driven wakefields in a hollow plasma channel. In this regime, the witness bunch is positioned…
The interaction of the witness beam with the surrounding plasma particles and wakefields was studied. The impli- cations of the elastic scattering process on beam emittance and, emittance evolution under the focusing and acceleration provided by plasma wakefields were discussed. Simulations results from GEANT4 are presented in this paper.
Influence of ion motion in a resonantly driven wakefield accelerator
本研究通过模拟揭示了等离子体行波加速器中离子运动对束自调制发展的影响，发现共振损失和相混两种效应共同抑制行波场激发。
Laser-driven plasma accelerators can generate accelerating gradients three orders of magnitude larger than radio-frequency accelerators and have achieved beam energies above 1 GeV in centimetre long stages. However, the pulse repetition rate and wall-plug efficiency of plasma accelerators is limited by the driving laser to less than approximately 1 Hz and 0.1% respectively. Here we investigate the prospects for exciting the plasma wave with trains of low-energy laser pulses rather than a single high-energy pulse. Resonantly exciting the wakefield in this way would enable the use of different t…
We propose a new and simple strategy for controlled ionization-induced trapping of electrons in a beam-driven plasma accelerator. The presented method directly exploits electric wakefields to ionize electrons from a dopant gas and capture them into a well-defined volume of the accelerating and focusing wake phase, leading to high-quality witness-bunches. This injection principle is explained by example of three-dimensional particle-in-cell (PIC) calculations using the code OSIRIS. In these simulations a high-current-density electron-beam driver excites plasma waves in the blow-out regime insid…
Laser wakefield accelerators promise to revolutionise many areas of accelerator science. However, one of the greatest challenges to their widespread adoption is the difficulty in control and optimisation of the accelerator outputs due to coupling between input parameters and the dynamic evolution of the accelerating structure. Here, we use machine learning techniques to automate a 100 MeV-scale accelerator, which optimised its outputs by simultaneously varying up to 6 parameters including the spectral and spatial phase of the laser and the plasma density and length. Most notably, the model bui…
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