Electron Self-injection Research Articles

Energy boosting and scaling of self-guided hybrid laser-plasma wakefield acceleration in a single uniform plasma

Abstract In laser-driven plasma wakefield acceleration, laser pump depletion and electron dephasing are the major constraints of the electron energy gain. Hybrid laser-plasma wakefield acceleration, which uses the laser-accelerated electron beam to drive a beam-driven plasma wakefield in separated stages, has been proposed to increase the beam brightness. However, the overall electron energy gain in hybrid acceleration is even lower than single-stage laser acceleration. In this paper, we report the observation of the energy boosting effect of the hybrid acceleration in single uniform plasmas through a series of particle-in-cell simulations. The self-injected electron beam from the laser-driven wakefield automatically moves forward to drive the beam-driven wakefield after laser depletion. The electrons at the beam tail are then accelerated by the beam-driven plasma wakefield, and the energy gain is at least doubled compared to previous single-stage experiments with the same laser energy. We also propose the scaling of the electron energy gain and the acceleration distance with the laser energy. For example, with a 9.1 J energy laser pulse and a 3.5 cm long plasma of $1.6 \times 10^{18}\ \rm cm^{-3}$ density, one can produce a quasi-monoenergetic electron beam at 3.5 GeV energy with 23 pC charge.

Evolution of autoresonant plasma wave excitation in two-dimensional particle-in-cell simulations

The generation of an autoresonantly phase-locked high-amplitude plasma waves to the chirped beat frequency of two driving lasers is studied in two dimensions using particle-in-cell simulations. The two-dimensional plasma and laser parameters correspond to those that optimized the plasma wave amplitude in one-dimensional simulations. Near the start of autoresonant locking, the two-dimensional simulations appear similar to one-dimensional particle-in-cell results (Luo et al., Phys. Rev. Res., vol. 6, 2024, p. 013338) with plasma wave amplitudes above the Rosenbluth–Liu limit. Later, just below wave breaking, the two-dimensional simulation exhibits a Weibel-like instability and eventually laser beam filamentation. These limit the coherence of the plasma oscillation after the peak plasma wave field is obtained. In spite of the reduction of spatial coherence of the accelerating density structure, the acceleration of self-injected electrons in the case studied remains at $70\,\%$ to $80\,\%$ of that observed in one dimension. Other effects such as plasma wave bowing are discussed.

The effect of laser pulse evolution on down-ramp injection in laser wakefield accelerators

Electron self-injection in laser wakefield accelerators (LWFAs) is an important determinator of electron beam parameters. Controllable and adjustable LWFA beams are essential for applications. Controlled injection by capturing sheath electrons can be achieved using plasma density down-ramps or bumps, which perturb the LWFA bubble phase velocity by varying the plasma frequency and by affecting relativistic self-focussing of the laser. We report on a comprehensive study, using particle-in-cell simulations, of the effect of laser pulse evolution on injection on density perturbations. We show how the LWFA can be optimised to make it suitable for use in a wide range of applications, in particular those requiring short duration, low slice-emittance and low energy spread, and high-charge electron bunches.

Correction to: Controllable electron self-injection in laser wakefield acceleration with asymmetric gas-jet nozzle

Correction to: Controllable electron self-injection in laser wakefield acceleration with asymmetric gas-jet nozzle

Effect of sharp vacuum–plasma boundary on the electron injection and acceleration in a few-cycle laser driven wakefield

The electron injection and acceleration driven by a few-cycle laser with a sharp vacuum–plasma boundary have been investigated through three-dimensional (3D) particle-in-cell simulations. It is found that an isotropic boundary impact injection (BII) first occurs at the vacuum–plasma boundary, and then carrier-envelope-phase (CEP) shift causes the transverse oscillation of the plasma bubble, resulting in a periodic electron self-injection (SI) in the laser polarization direction. It shows that the electron charge of the BII only accounts for a small part of the total charge, and the CEP can effectively tune the quality of the injected electron beam. The dependences of laser intensity and electron density on the total charge and the ratio of BII charge to the total charge are studied. The results are beneficial to electron acceleration and its applications, such as betatron radiation source.

Controllable electron self-injection in laser wakefield acceleration with asymmetric gas-jet nozzle

AbstractBeam charge control in the staging of laser wakefield acceleration (LWFA) is a crucial technique for developing full-optical jitter-free high-energy electron accelerators. Precise control of total charge in pre-accelerated electron bunches is necessary to achieve practical electron beam characteristics in the final acceleration stage(s). In contrast to the well-known cathode techniques in conventional accelerators, in LWFA the electron injection results from non-linear processes originating from plasma wave breaking. Therefore, the development of charge control requires a deep understanding of the electron self-injection processes and applications of non-trivial tools. The use of asymmetric gas-jet nozzles seems to be a promising way in developing charge control via tuning the target parameters such as plasma density, density slope, and acceleration length. Here, we demonstrate and characterize controllable electron self-injection, owing to a parametric resonance in slantwise density gas jets irradiated by 50 TW femtosecond laser pulses. The measured characteristics of the electron bunches, in which charge and energy distribution depend on the gas density and gas density gradient, agree well with those obtained by multidimensional particle-in-cell simulation and confirm the possibility of charge control.

Observation of transverse injection and enhanced beam quality in laser wakefield acceleration of isolated electron bunches using an optimized plasma waveguide.

The laser wakefield acceleration of monoenergetic multi-GeV electron beams in the bubble regime is investigated via particle-in-cell simulations considering laser guiding of sub-petawatt pulses by an optimized plasma waveguide. The density profile of the plasma has a transverse transition from a low value for the laser guiding central channel to an optimal higher value for the surrounding plasma. Multidimensional particle-in-cell simulations in the nonlinear bubble regime show that when the spot size of the Gaussian laser pulse is matched to the diameter of the low-density laser-guiding plasma channel, electron self-injection can be transversely provided from the surrounding high-density plasma mitigating the need for a minimum electron density of the low-density channel to trigger the self-injection. Accordingly, the pump depletion and electron dephasing lengths can be increased by reducing the electron density of the axial channel, and the electron bunch can be accelerated to considerably longer distances. As a result, the energy gain of the trapped electrons, injected from the surrounding high-density region, can be efficiently enhanced. Under such conditions, a completely localized electron bunch with considerably decreased energy spread (<2%) and enhanced peak energy (∼2.5GeV) is accelerated over a length of ∼6mm by a sub-petawatt laser pulse (∼86TW).

Investigation of the Way of Phase Synchronization of a Self-Injected Bunch and an Accelerating Wakefield in Solid-State Plasma

The electron acceleration, in a laser wakefield accelerator, controlled through plasma density inhomogeneity is studied on a basis of 2.5-dimensional particle-in-cell simulation. The acceleration requires a concordance of the density scale length and shift of the accelerated electron bunch relative to wake bubble during electron acceleration. This paper considers the excitation of a wakefield in plasma with a density equal to the density of free electrons in metals, solid-state plasma (the original idea of Prof. T. Tajima), in the context of studying the wakefield process. As is known in the wake process, as the wake bubble moves through the plasma, the self-injected electron bunch shifts along the wake bubble. Then, the self-injected bunch falls into the phase of deceleration of the wake wave. In this paper, support of the acceleration process by maintaining the position of the self-injected electron bunch using an inhomogeneous plasma is proposed. It is confirmed that the method of maintaining phase synchronization proposed in the article by using a nonuniform plasma leads to an increase in the accelerating gradient and energy of the accelerated electron bunch in comparison with the case of self-injection and acceleration in a homogeneous plasma.

Diagnostic of laser wakefield acceleration with ultra – Short laser pulse by using SMILEI PIC code

• Generation of high energy beam with LWFA is the most efficient and low in cost as compared with conventional accelerator. • In LWFA, we firstly performed this acceleration with the combination of up ramp and down ramp density region. • We firstly performed the work related with production of high energy beam with self-injected electrons. • We firstly performed the work with ultra – short laser pulse and polygonal plasma density profile for LWFA. High-energy particle beam is used to probe the local or long range structure and properties of materials. Particle accelerators are also used in material science to understand radiation damage, particularly in studies of structural material to be applied in fusion power generator and satellites. Laser Wakefield Acceleration (LWFA) is a promising technique to build compact and powerful particle accelerators for generating multi-GeV electron beams. The efficient coupling of laser energy with plasma is very important to generate high energy electrons, which is the subject to optimum laser and plasma profiles. In this article, we have used high intensity ultra-short laser pulse and a polygon plasma density profile to analyse the behaviour of LWFA in the bubble regime, which corresponds to the evacuation of plasma electrons in the wake of the laser pulse. Many electrons get self-injected in this wakefield and get accelerated behind the laser pulse up to very high energies, producing high energy beam useful for material processing along with their other applications. We have diagnosed some parameters such as electron density variation, wakefield magnitude and energy spectrum of the electrons accelerated in this mechanism.

Effect of pulse group velocity on charge loading in laser wakefield acceleration

Total charge and energy evaluations for the electron beams generated in the laser wakefield acceleration (LWFA) is the primary step in the determination of the required laser and target parameters. Particle-in-cell (PIC) simulation is an efficient numerical tool that can provide such evaluations unless the effect of numerical dispersion is not diminished. The numerical dispersion, which is specific for the PIC modeling, affects not only the dephasing lengths in LWFA but also the total amount of the self-injected electrons. A numerical error of the order of 10−4−10−3 in the calculation of the speed of the driving laser pulse results in a significant error in the total injected charge, energy gain and other critical parameters of the accelerated electron bunches. In the standard numerical approach, the numerical correction in the speed of the laser pulse either requires infinitely small spatial grid resolution (which needs large computation platform) or force to compromise with the numerical accuracy. Here, we present a simple and easy to implement numerical scheme to suppress the numerical dispersion of the electromagnetic pulse in PIC simulations even with a modest spatial resolution, and without any special treatments to the core structure of the numerical algorithm. Evaluated charges of the self-injected electron bunches become essentially lower owing to the better calculations of the wake phase velocity.

Controlled electron injection into beam driven plasma wakefield accelerators employing a co-propagating laser pulse

A novel technique for generating high current electron bunches in electron beam driven plasma wakefield accelerators (PWFAs) is suggested based on co-propagation of an electron beam and a laser pulse. It is observed that propagation of a laser pulse in front of an electron beam driver leads to bubble expansion and consequently electron injection into a PWFA. The acceleration structure is extensively studied in this scheme and the bubble evolution process is discussed. The difference in propagation velocity of the laser pulse and the beam driver in the plasma and variation of electron beam driver density in presence of the laser pulse cause the bubble radius grows. Using a laser pulse in a PWFA leads to the generation of an ultra short (10 fs) electron bunch with charge three times larger than the electron beam driver total charge. It is shown by altering the initial electron beam driver density and the laser pulse intensity, the external control of the amount of loaded charge is possible. The number of self-injected electrons is enhanced by increasing the laser pulse intensity and the density of the electron beam driver. The results represent that the accelerator operates in a highly loaded regime. Therefore, by raising the density of the electron beam driver and the laser pulse intensity, the final energy spread of the generated electron bunch increases. An interpretive approach to find the appropriate parameters for the laser pulse and the electron beam is proposed in this scheme.

Laser pulse-electron beam synergy effect on electron self-injection and higher energy gain in laser wakefield accelerators

A new scheme for injection and acceleration of electrons in wakefield accelerators is suggested based on the co-action of a laser pulse and an electron beam. This synergy leads to stronger wakefield generation and higher energy gain in the bubble regime. The strong deformation of the whole bubble leads to electron self-injection at lower laser powers and lower plasma densities. To predict the practical ranges of electron beam and laser pulse parameters an interpretive model is proposed. The effects of altering the initial electron beam position on self-trapping of plasma electrons are studied. It is observed that an ultra-short (25 fs), high charge (340 pC), 1 GeV electron bunch is produced by injection of a 280 pC electron beam in the decelerating phase of the 75 TW laser driven wakefield.

Optical shaping of plasma cavity for controlled laser wakefield acceleration

Laser wakefield accelerators rely on relativistically moving micron-sized plasma cavities that provide extremely high electric field >100GV/m. Here, we demonstrate transverse shaping of the plasma cavity to produce controlled sub-GeV electron beams, adopting laser pulses with an axially rotatable ellipse-shaped focal spot. We showed the control capability on electron self-injection, charge, and transverse profile of the electron beam by rotating the focal spot. We observed that the effect of the elliptical focal spot was imprinted in the profiles of the electron beams and the electron energy increased, as compared to the case of a circular focal spot. We performed 3D particle-in-cell (PIC) simulations which reproduced the experimental results and revealed dynamics of a new asymmetric self-injection process. This simple scheme offers a novel control method on laser wakefield acceleration to produce tailored electron beams and x-rays for various applications.

Periodic self-injection of electrons in a few-cycle laser driven oscillating plasma wake

Millijoule-energy few-cycle laser pulses excite the plasma wakefield and accelerate electrons at kilohertz repetition rate, generating mega-electron volt (MeV) electron bunches with femtosecond temporal duration for ultrafast electron diffraction applications. By simulating few-cycle laser pulses interacting with the underdense nitrogen plasma, we have studied the mechanism of periodic electron self-injection, which manifests the laser carrier envelop phase (CEP) effect in few-cycle laser wakefield acceleration. A few-cycle laser pulse experiencing a significant wavelength redshift induces the transverse oscillation of the plasma bubble at the same frequency as the laser CEP changes by 2π. The oscillation of the plasma bubble periodically injects free electrons into the bubble at a doubled frequency, broadening the electron energy spread.

Plasma bubble evolution in laser wakefield acceleration in a petawatt regime

The bubble regime of the laser wakefield acceleration is one of the most recent and promising mechanisms for generating quasi-monoenergetic electron beams. In this work, we propose to study the dynamics of bubble (a wake structure devoid of electrons created in underdense plasma by a highly-intense ultrashort laser pulse) in a petawatt regime. The dependence of the electron beam energy and the quality of the electron beam on the shape of the bubble is the main motivation behind this work. The bubble length as well as bubble shape have been investigated using two-dimensional particle-in-cell simulations. The evolution of the bubble with time, and the correlation of bubble length (longitudinal and transverse radius) with the intensity of laser pulse have been revealed in this study. The change of bubble dimensions can be estimated by various determining factors such as the laser pulse focusing, the beam loading, the residual electrons, and the bubble velocity. It has also been confirmed that the shape of the bubble cannot be predicted using fixed shape models as spherical or elliptical. Simulations unveil that as the bubble traverses in plasma, it evolves from spherical shape to the highly elliptical shape. And, as it approaches the dephasing length, the eccentricity decreases further. Consequently, the self-injection of plasma electrons in the bubble is seriously affected by the bubble evolution. Comparison of the electron energy gain at different intensities of laser pulse has also been provided. Various scaling laws for electron beam energy estimations are predicted in this investigation. High quality electron beam can be obtained by controlling the bubble evolution, which may have significant applications in future coherent light sources, biomedical, condensed matter physics, and x-ray generation by table-top FEL.

Electron self-injection threshold for the tandem-pulse laser wakefield accelerator

A controllable injection scheme is key to producing high quality laser-driven electron beams and x rays. Self-injection is the most straightforward scheme leading to high current and peak energies but is susceptible to variations in laser parameters and target characteristics. In this work, improved control of electron self-injection in the nonlinear cavity regime using two laser-pulses propagating in tandem is investigated. In particular, the advantages of the tandem-pulse scheme in terms of injection threshold, electron energy, and beam properties in a regime relevant to betatron radiation are demonstrated. Moreover, it is shown that the laser power threshold for electron self-injection can be reduced by up to a factor of two compared to the standard, single-pulse wakefield scheme.

Simulation study of ionization-induced injection in sub-terawatt laser wakefield acceleration

By using a thin, high-density gas cell, subterawatt laser wakefield acceleration (sub-TW LWFA) of electrons can be driven by few tens of megajoule pulses from diode-pumped lasers operated at high repetition rates. When a 0.5-TW, 1030-nm pulse interacts with a dense plasma, the self-focusing effect and the self-modulation instability are induced to enhance the pulse intensity to a level capable of exciting plasma bubbles. Through particle-in-cell simulations, this study investigates the sub-TW LWFA in which a H2-N2 mixture is applied for the gas target; in this fashion, the nitrogen doping ratio ρN can be varied to improve the output energy and the charge of accelerated electrons with the addition of ionization-induced injection. The results show that the acceleration efficiency is limited when using a pure hydrogen target, since the self-injection of electrons rarely occurs in the first plasma bubble having the highest accelerating field. By doping the hydrogen target with nitrogen, free electrons generated when the pulse peak ionizes the N5+ and N6+ ions can be injected into the first bubble. The optimal performance of sub-TW LWFA can be acquired with a nitrogen doping ratio between ρN = 1% and 3%, from which electrons can be produced with a maximum energy of &amp;gt; 40 MeV and a total charge ∼6 pC for the high-energy component (&amp;gt;20 MeV). Using a relatively high doping ratio, ρN≥ 5% will significantly degrade the properties of the output electrons, primarily because of the manifest ionization defocusing encountered by the driving pulse.

CONTROL OF CHARACTERISTICS OF SELF-INJECTED AND ACCELERATED ELECTRON BUNCH IN PLASMA BY LASER PULSE SHAPING ON RADIUS, INTENSITY AND SHAPE

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.

Diagnostics of the down-ramp gas and plasma density profiles in a gas jet and a capillary gas cell

Laser-aided plasma diagnostics is a very important tool for laser-driven plasma wakefield acceleration, where a gas jet or a gas cell is used to produce a plasma density of 1018∼1019/cm3. We developed special plasma sources based on a gas jet and a capillary gas cell with a sudden density down-ramp, which can induce easy self-injection of plasma electrons into the laser wakefield. In our research, the Mach-Zehnder laser interferometer with a Ti:sapphire laser of 40 fs pulse duration was used to diagnose the gas and laser-produced plasma densities in the gas jet and the capillary gas cell. The measurement results for the gas and plasma densities are reported in this paper.

Simulation study of CO2 laser-plasma interactions and self-modulated wakefield acceleration

3D numerical simulations of the interaction of a powerful CO2 laser with hydrogen jets demonstrating the role of ionization in the characteristics of induced wakes are presented. Simulations using SPACE, a parallel relativistic particle-in-cell code, are performed in support of the plasma wakefield accelerator experiments being conducted at the Brookhaven National Laboratory (BNL) Accelerator Test Facility (ATF). A novelty of the SPACE code is its set of efficient atomic physics algorithms that compute ionization and recombination rates on the grid and transfer them to particles. The influence of ionization on the spectrum of the pump laser has been studied for a range of gas densities. Simulations reproduce both Stokes and antiStokes shifts in the spectrum of the pump laser, similar to those observed in experiments in the spectrum of the probe. Good agreement has been achieved with the experiments on the effect of variation in gas density on Stokes/antiStokes intensity. In addition, self-injection and trapping of electrons into the self-modulated wakes have been observed and analyzed. The experimentally validated code SPACE will be used for predictive simulations to guide future experiments at BNL ATF.