Laser-driven coherent betatron oscillation in a laser-wakefield cavity

Generation of attosecond micro bunched beam using ionization injection in laser wakefield acceleration

Micro bunched electron beams with periodic longitudinal density modulation at optical wavelengths give rise to coherent light emission. In this paper, we show attosecond micro bunched beam generation and acceleration in laser-plasma wakefield via particle-in-cell simulations. Due to the near-threshold ionization with the drive laser, the electrons with phase-dependent distributions are non-linearly mapped to discrete final phase spaces. Electrons can preserve this initial bunching structure during the acceleration, leading to an attosecond electron bunch train after leaving the plasma with separations of the same time scale. The modulation of the comb-like current density profile is about 2k₀ ∼ 3k₀, where k₀ is the wavenumber of the laser pulse. Such pre-bunched electrons with low relative energy spread may have potential in applications related to future coherent light sources driven by laser-plasma accelerators and broad application prospects in attosecond science and ultrafast dynamical detection.

Attosecond betatron radiation pulse train

High-intensity X-ray sources are essential diagnostic tools for science, technology and medicine. Such X-ray sources can be produced in laser-plasma accelerators, where electrons emit short-wavelength radiation due to their betatron oscillations in the plasma wake of a laser pulse. Contemporary available betatron radiation X-ray sources can deliver a collimated X-ray pulse of duration on the order of several femtoseconds from a source size of the order of several micrometres. In this paper we demonstrate, through particle-in-cell simulations, that the temporal resolution of such a source can be enhanced by an order of magnitude by a spatial modulation of the emitting relativistic electron bunch. The modulation is achieved by the interaction of the that electron bunch with a co-propagating laser beam which results in the generation of a train of equidistant sub-femtosecond X-ray pulses. The distance between the single pulses of a train is tuned by the wavelength of the modulation laser pulse. The modelled experimental setup is achievable with current technologies. Potential applications include stroboscopic sampling of ultrafast fundamental processes.

Coherent Optical Signatures of Electron Microbunching in Laser-Driven Plasma Accelerators

We report observations of coherent optical transition radiation interferometry (COTRI) patterns generated by microbunched ∼200-MeV electrons as they emerge from a laser-driven plasma accelerator. The divergence of the microbunched portion of electrons, deduced by comparison to a COTRI model, is ∼9× smaller than the ∼3  mrad ensemble beam divergence, while the radius of the microbunched beam, obtained from COTR images on the same shot, is <3  μm. The combined results show that the microbunched distribution has estimated transverse normalized emittance ∼0.4  mm mrad.

Radiation emission in laser-wakefields driven by structured laser pulses with orbital angular momentum

High-intensity X-ray sources are invaluable tools, enabling experiments at the forefront of our understanding of materials science, chemistry, biology, and physics. Laser-plasma electron accelerators are sources of high-intensity X-rays, as electrons accelerated in wakefields emit short-wavelength radiation due to betatron oscillations. While applications such as phasecontrast imaging with these betatron sources have already been demonstrated, others would require higher photon number and would benefit from increased tunability. In this paper we demonstrate, through detailed 3D simulations, a novel configuration for a laser-wakefield betatron source that increases the energy of the X-ray emission and also provides increased flexibility in the tuning of the X-ray photon energy. This is made by combining two Laguerre-Gaussian pulses with non-zero net orbital angular momentum, leading to a rotation of the intensity pattern, and hence, of the driven wakefields. The helical motion driven by the laser rotation is found to dominate the radiation emission, rather than the betatron oscillations. Moreover, the radius of this helical motion can be controlled through the laser spot size and orbital angular momentum indexes, meaning that the radiation can be tuned fully independently of the plasma parameters.

Gamma-ray emission from wakefield-accelerated electrons wiggling in a laser field

Ultra-fast synchrotron radiation emission can arise from the transverse betatron motion of an electron in a laser plasma wakefield, and the radiation spectral peak is limited to tens of keV. Here, we present a new method for achieving high-energy radiation via accelerated electrons wiggling in an additional laser field whose intensity is one order of magnitude higher than that for the self-generated transverse field of the bubble, resulting in an equivalent wiggler strength parameter K increase of approximately twenty times. By calculating synchrotron radiation, we acquired a peak brightness for the case of the laser wiggler field of 1.2 × 10²³ ph/s/mrad²/mm²/0.1%BW at 1 MeV. Such a high brilliance and ultra-fast gamma-ray source could be applied to time-resolved probing of dense materials and the production of medical radioisotopes.

Collimated ultrabright gamma rays from electron wiggling along a petawatt laser-irradiated wire in the QED regime

Even though bright X-rays below mega-electron volt photon energy can be obtained from X-ray free electron lasers and synchrotron radiation facilities, it remains a great challenge to generate collimated bright gamma-ray beams over 10 mega-electron volts. We propose a scheme to efficiently generate such beams from submicron wires irradiated by petawatt lasers, where electron accelerating and wiggling are achieved simultaneously. With significant quantum electrodynamics effects existing even with petawatt lasers, our full 3D simulations show that directional gamma rays can be generated with thousand-fold higher brilliance and thousand-fold higher photon energy than those from synchrotron radiation facilities. In addition, the photon yield efficiency approaches 10%, 100,000-fold higher than those typical from betatron radiation and Compton scattering based on laser-wakefield accelerators.

Even though high-quality X- and gamma rays with photon energy below mega-electron volt (MeV) are available from large-scale X-ray free electron lasers and synchrotron radiation facilities, it remains a great challenge to generate bright gamma rays over 10 MeV. Recently, gamma rays with energies up to the MeV level were observed in Compton scattering experiments based on laser wakefield accelerators, but the yield efficiency was as low as 10−6, owing to low charge of the electron beam. Here, we propose a scheme to efficiently generate gamma rays of hundreds of MeV from submicrometer wires irradiated by petawatt lasers, where electron accelerating and wiggling are achieved simultaneously. The wiggling is caused by the quasistatic electric and magnetic fields induced around the wire surface, and these are so high that even quantum electrodynamics (QED) effects become significant for gamma-ray generation, although the driving lasers are only at the petawatt level. Our full 3D simulations show that directional, ultrabright gamma rays are generated, containing 1012 photons between 5 and 500 MeV within a 10-fs duration. The brilliance, up to 1027 photons s−1 mrad−2 mm−2 per 0.1% bandwidth at an average photon energy of 20 MeV, is second only to X-ray free electron lasers, while the photon energy is 3 orders of magnitude higher than the latter. In addition, the gamma ray yield efficiency approaches 10%—that is, 5 orders of magnitude higher than the Compton scattering based on laser wakefield accelerators. Such high-energy, ultrabright, femtosecond-duration gamma rays may find applications in nuclear photonics, radiotherapy, and laboratory astrophysics.

γ-Ray Generation from Plasma Wakefield Resonant Wiggler

A flexible gamma-ray radiation source based on the resonant laser-plasma wakefield wiggler is proposed. The wiggler is achieved by inducing centroid oscillations of a short laser pulse in a plasma channel. Electrons (self-)injected in such a wakefield experience both oscillations due to the transverse electric fields and energy gain due to the longitudinal electric field. The oscillations are significantly enhanced when the laser pulse centroid oscillations are in resonance with the electron betatron oscillations, extending the radiation spectrum to the gamma-ray range. The polarization of the radiation can be easily controlled by adjusting the injection of the laser pulse into the plasma channel.

Brilliant X-rays using a Two-Stage Plasma Insertion Device

Particle accelerators have made an enormous impact in all fields of natural sciences, from elementary particle physics, to the imaging of proteins and the development of new pharmaceuticals. Modern light sources have advanced many fields by providing extraordinarily bright, short X-ray pulses. Here we present a novel numerical study, demonstrating that existing third generation light sources can significantly enhance the brightness and photon energy of their X-ray pulses by undulating their beams within plasma wakefields. This study shows that a three order of magnitude increase in X-ray brightness and over an order of magnitude increase in X-ray photon energy is achieved by passing a 3 GeV electron beam through a two-stage plasma insertion device. The production mechanism micro-bunches the electron beam and ensures the pulses are radially polarised on creation. We also demonstrate that the micro-bunched electron beam is itself an effective wakefield driver that can potentially accelerate a witness electron beam up to 6 GeV.

Large-charge quasimonoenergetic electron beams produced by off-axis colliding laser pulses in underdense plasma

Electrons can be efficiently injected into a plasma wave by colliding two counterpropagating laser pulses in a laser wakefield acceleration. However, the generation of a high-quality electron beam with a large charge is difficult in the traditional on-axis colliding scheme due to the growth of the electron beam duration coming from the increase of the beam charge. To solve this problem, we propose an off-axis colliding scheme, in which the collision point is away from the axis of the driver pulse. We show that the electrons injected from the off-axis region are highly concentered on the tail of the bubble even for a large trapped charge, thus feeling almost the same accelerating field. As a result, quasimonoenergetic electron beams with a large charge can be produced. The validity of this scheme is confirmed by both the particle-in-cell simulations and the Hamiltonian model. Furthermore, it is shown that a Laguerre-Gauss (LG) laser can be adopted as the injection pulse to realize the off-axis colliding injection in three dimensions symmetrically, which may be useful in simplifying the technical layout of the real experiment setup.

Generation of femtosecond γ-ray bursts stimulated by laser-driven hosing evolution

The promising ability of a plasma wiggler based on laser wakefield acceleration to produce betatron X-rays with photon energies of a few keV to hundreds of keV and a peak brilliance of 10²²–10²³ photons/s/mm²/mrad²/0.1%BW has been demonstrated, providing an alternative to large-scale synchrotron light sources. Most methods for generating betatron radiation are based on two typical approaches, one relying on an inherent transverse focusing electrostatic field, which induces transverse oscillation, and the other relying on the electron beam catching up with the rear part of the laser pulse, which results in strong electron resonance. Here, we present a new regime of betatron γ-ray radiation generated by stimulating a large-amplitude transverse oscillation of a continuously injected electron bunch through the hosing of the bubble induced by the carrier envelope phase (CEP) effect of the self-steepened laser pulse. Our method increases the critical photon energy to the MeV level, according to the results of particle-in-cell (PIC) simulations. The highly collimated, energetic and femtosecond γ-ray bursts that are produced in this way may provide an interesting potential means of exploring nuclear physics in table top photo nuclear reactions.

Characteristics of betatron radiation from direct-laser-accelerated electrons

Betatron radiation from direct-laser-accelerated electrons is characterized analytically and numerically. It is shown here that the electron dynamics is strongly dependent on a self-similar parameter S(≡n_{e}/n_{c}a_{0}). Both the electron transverse momentum and energy are proportional to the normalized amplitude of laser field (a_{0}) for a fixed value of S. As a result, the total number of radiated photons scales as a_{0}^{2}/sqrt[S] and the energy conversion efficiency of photons from the accelerated electrons scales as a_{0}^{3}/S. The particle-in-cell simulations agree well with the analytical scalings. It is suggested that a tunable high-energy and high-flux radiation source can be achieved by exploiting this regime.

Resonantly Enhanced Betatron Hard X-rays from Ionization Injected Electrons in a Laser Plasma Accelerator

Ultrafast betatron x-ray emission from electron oscillations in laser wakefield acceleration (LWFA) has been widely investigated as a promising source. Betatron x-rays are usually produced via self-injected electron beams, which are not controllable and are not optimized for x-ray yields. Here, we present a new method for bright hard x-ray emission via ionization injection from the K-shell electrons of nitrogen into the accelerating bucket. A total photon yield of 8 × 10(8)/shot and 10(8 )photons with energy greater than 110 keV is obtained. The yield is 10 times higher than that achieved with self-injection mode in helium under similar laser parameters. The simulation suggests that ionization-injected electrons are quickly accelerated to the driving laser region and are subsequently driven into betatron resonance. The present scheme enables the single-stage betatron radiation from LWFA to be extended to bright γ-ray radiation, which is beyond the capability of 3(rd) generation synchrotrons.

Dense Helical Electron Bunch Generation in Near-Critical Density Plasmas with Ultrarelativistic Laser Intensities

The mechanism for emergence of helical electron bunch(HEB) from an ultrarelativistic circularly polarized laser pulse propagating in near-critical density(NCD) plasma is investigated. Self-consistent three-dimensional(3D) Particle-in-Cell(PIC) simulations are performed to model all aspects of the laser plasma interaction including laser pulse evolution, electron and ion motions. At a laser intensity of 10²² W/cm², the accelerated electrons have a broadband spectrum ranging from 300 MeV to 1.3 GeV, with the charge of 22 nano-Coulombs(nC) within a solid-angle of 0.14 Sr. Based on the simulation results, a phase-space dynamics model is developed to explain the helical density structure and the broadband energy spectrum.

Synergistic laser-wakefield and direct-laser acceleration in the plasma-bubble regime

The concept of a hybrid laser plasma accelerator is proposed. Relativistic electrons undergoing resonant betatron oscillations inside the plasma bubble created by a laser pulse are accelerated by gaining energy directly from the laser pulse and from its plasma wake. The resulting phase space of self-injected plasma electrons is split into two, containing a subpopulation that experiences wakefield acceleration beyond the standard dephasing limit because of the multidimensional nature of its motion that reduces the phase slippage between the electrons and the wake.

Concurrence of monoenergetic electron beams and bright X-rays from an evolving laser-plasma bubble

Desktop laser plasma acceleration has proven to be able to generate gigaelectronvolt-level quasi-monoenergetic electron beams. Moreover, such electron beams can oscillate transversely (wiggling motion) in the laser-produced plasma bubble/channel and emit collimated ultrashort X-ray flashes known as betatron radiation with photon energy ranging from kiloelectronvolts to megaelectronvolts. This implies that usually one cannot obtain bright betatron X-rays and high-quality electron beams with low emittance and small energy spread simultaneously in the same accelerating wave bucket. Here, we report the first (to our knowledge) experimental observation of two distinct electron bunches in a single laser shot, one featured with quasi-monoenergetic spectrum and another with continuous spectrum along with large emittance. The latter is able to generate high-flux betatron X-rays. Such is observed only when the laser self-guiding is extended over 4 mm at a fixed plasma density (4 × 10(18) cm(-3)). Numerical simulation reveals that two bunches of electrons are injected at different stages due to the bubble evolution. The first bunch is injected at the beginning to form a stable quasi-monoenergetic electron beam, whereas the second one is injected later due to the oscillation of the bubble size as a result of the change of the laser spot size during the propagation. Due to the inherent temporal synchronization, this unique electron-photon source can be ideal for pump-probe applications with femtosecond time resolution.

Generating overcritical dense relativistic electron beams via self-matching resonance acceleration

We show a novel self-matching resonance acceleration regime for generating dense relativistic electron beams by using ultraintense circularly polarized laser pulses in near-critical density plasmas. When the self-generated quasistatic axial magnetic field is strong enough to pinch and trap thermal relativistic electrons, an overdense electron bunch is formed in the center of the laser channel. In the trapping process, the electron betatron frequencies and phases can be adjusted automatically to match the resonance condition. The matched electrons are accelerated continuously and a collimated electron beam with overcritical density, helical structure, and plateau profile energy spectrum is hence generated.

Absolute energy calibration for relativistic electron beams with pointing instability from a laser-plasma accelerator

The pointing instability of energetic electron beams generated from a laser-driven accelerator can cause a serious error in measuring the electron spectrum with a magnetic spectrometer. In order to determine a correct electron spectrum, the pointing angle of an electron beam incident on the spectrometer should be exactly defined. Here, we present a method for absolutely calibrating the electron spectrum by monitoring the pointing angle using a scintillating screen installed in front of a permanent dipole magnet. The ambiguous electron energy due to the pointing instability is corrected by the numerical and analytical calculations based on the relativistic equation of electron motion. It is also possible to estimate the energy spread of the electron beam and determine the energy resolution of the spectrometer using the beam divergence angle that is simultaneously measured on the screen. The calibration method with direct measurement of the spatial profile of an incident electron beam has a simple experimental layout and presents the full range of spatial and spectral information of the electron beams with energies of multi-hundred MeV level, despite the limited energy resolution of the simple electron spectrometer.

Electron bow-wave injection of electrons in laser-driven bubble acceleration

An electron injection regime in laser wake-field acceleration, namely electron bow-wave injection, is investigated by two- and three-dimensional particle-in-cell simulation as well as analytical model. In this regime electrons in the intense electron bow wave behind the first bubble catch up with the bubble tail and are trapped by the bubble finally, resulting in considerable enhancement of the total trapped electron number. For example, with the increase of the laser intensity from 2 × 10(19) to 1 × 10(20) W/cm(2), the electron trapping changes from normal self-injection to bow-wave injection and the trapped electron number is enhanced by two orders of magnitude. An analytical model is proposed to explain the numerical observation.

Long-range persistence of femtosecond modulations on laser-plasma-accelerated electron beams

Laser plasma accelerators have produced femtosecond electron bunches with a relative energy spread ranging from 100% to a few percent. Simulations indicate that the measured energy spread can be dominated by a correlated spread, with the slice spread significantly lower. Measurements of coherent optical transition radiation are presented for broad-energy-spread beams with laser-induced density and momentum modulations. The long-range (meter-scale) observation of coherent optical transition radiation indicates that the slice energy spread is below the percent level to preserve the modulations.

Low emittance, high brilliance relativistic electron beams from a laser-plasma accelerator

Progress in laser wakefield accelerators indicates their suitability as a driver of compact free-electron lasers (FELs). High brightness is defined by the normalized transverse emittance, which should be less than 1π mm mrad for an x-ray FEL. We report high-resolution measurements of the emittance of 125 MeV, monoenergetic beams from a wakefield accelerator. An emittance as low as 1.1±0.1π mm mrad is measured using a pepper-pot mask. This sets an upper limit on the emittance, which is comparable with conventional linear accelerators. A peak transverse brightness of 5×10¹⁵ A m⁻¹ rad⁻¹ makes it suitable for compact XUV FELs.

Generating ultrarelativistic attosecond electron bunches with laser wakefield accelerators

Femtosecond electron bunches with ultrarelativistic energies were recently generated by laser wakefield accelerators. Here we predict that laser wakefield acceleration can generate even attosecond bunches, due to a strong chirp of the betatron frequency. We show how the bunch duration scales with the acceleration parameters and that, after acceleration, the bunches can propagate over many tens of centimeters without a significant increase in duration.

Monitoring the He+ ion channel formation by high-order harmonic generation

The macroscopic build-up of the high-order harmonic signal depends on the free electron density in the generation medium. The free electrons affect the harmonic yield and spectral shape through modifying the refractive index and the phase matching conditions. These dependences allow studying the He(+) ion channel formation in a He gas jet. The evolution of an ion channel created by an ultrashort laser pulse via optical field ionization was monitored using the harmonic signal generated by a collinear propagating second laser pulse. From the measured high harmonic signal as function of the delay we are able to gain information about the free electron density. Under our experimental condition, the ion channel has been fully formed 300 fs after the first laser pulse, resulting in an enhancement of harmonic yield of the second laser pulse by two orders of magnitude.