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The laser wakefield acceleration (LWFA) scheme is now a superb and potent instrument for industry, medicine, fusion, and other uses. With the aid of this technique, high-quality electron beams can be generated, which might eventually be employed to create small, portable X-ray sources. Using a linearly polarized Gaussian laser pulse, the present work focuses on examining electron acceleration in an under-dense plasma having a ramp profile with different density-hill widths. Two-dimensional Fourier–Bessel Particle-In-Cell simulation (2D-FBPIC) code shows that optimizing the laser parameters and plasma density profile maximizes the energy gain and minimizes the normalized rms emittance in the x, y directions inside the plasma bubble. For 10 and 90 μ m density-hill widths, respectively, the optimal emittance value is reported to be ϵ x ∼ 3.10 mm.mrad in the x-direction and ϵ y ∼ 2.63 mm.mrad in the y-direction. At a density-hill width of 90 μ m , it is also observed that the charge trapped inside the bubble is 44.91 pC. With this density-hill width and a plasma channel length of 3.84 mm, our simulation findings show an energy gain of 0.38 GeV to the electron-bunch accelerated inside the bubble. [ABSTRACT FROM AUTHOR]

Ultrashort electron beams with femtosecond to picosecond bunch durations offer unique opportunities to explore active research areas ranging from ultrafast structural dynamics to ultra‐high dose‐rate radiobiological studies. It presents a straightforward method to generate relativistic electron beams in ambient air via the tight focusing of a few‐cycle, mJ‐class femtosecond infrared laser. It demonstrates experimentally that electrons can reach up to 1.4 MeV at a dose‐rate of 0.15 Gy/s, providing enough dose rate for radiation therapy applications. 3D Particle‐In‐Cell simulations confirm that the acceleration mechanism is based on the relativistic ponderomotive force and show theoretical agreement with the measured electron energies and divergence. Relativistic peak intensities up to 1019 Wcm−2 are reached in ambient air due to a very low B‐integral accumulation during focusing, which prevents intensity clamping. Furthermore, it discusses the scalability of this method with the continuing development of mJ‐class high average power lasers, and providing a promising approach for FLASH radiation therapy. [ABSTRACT FROM AUTHOR]

The nonlinear effects of an magnetized plasma of a system including two kinds of electrons with Kappa distribution, warm ions, in the presence of an electron beam have been investigated. The nonlinear differential equation governing this system is derived using the reduction perturbation approach. The propagation of waves in plasma settings is significantly impacted by the electron beam's existence as a unique component. The results demonstrate that soliton waves propagating in the proposed plasma environment experience changes in their amplitude, phase speed, and nonlinear coefficient owing to the presence of the electron beam. The current results demonstrate that the electron number density of the beam may be used to regulate the propagation of ion acoustic waves in plasmas. The outcomes obtained have applications in plasma laser interaction. [ABSTRACT FROM AUTHOR]

With the rapid development of high-power petawatt class lasers worldwide, exploring physics in the strong field QED regime will become one of the frontiers for laser–plasma interactions research. Particle-in-cell codes, including quantum emission processes, are powerful tools for predicting and analyzing future experiments where the physics of relativistic plasma is strongly affected by strong field QED processes. The spin/polarization dependence of these quantum processes has been of recent interest. In this article, we perform a parametric study of the interaction of two laser pulses with an ultrarelativistic electron beam. The first pulse is optimized to generate high-energy photons by nonlinear Compton scattering and efficiently decelerate electron beam through the quantum radiation reaction. The second pulse is optimized to generate electron–positron pairs by the nonlinear Breit–Wheeler decay of photons with the maximum polarization dependence. This may be experimentally realized as a verification of the strong field QED framework, including the spin/polarization rates. [ABSTRACT FROM AUTHOR]

Plasma acceleration is considered a prospective technology for building a compact multi-TeV electron–positron collider in the future. The challenge of this endeavor is greater for positrons than for the electrons because usually the self-generated fields from laser–plasma interaction are not well-suited for positron focusing and on-axis guiding. In addition, an external positron source is required, while electrons are naturally available in the plasma. Here, we study electron–positron pair generation by an orthogonal collision of a multi-PW laser pulse and a GeV electron beam by the nonlinear Breit–Wheeler process. We studied conditions favorable for positron deflection in the direction of the laser pulse propagation, which favors injection into the plasma for further acceleration. We demonstrate using the OSIRIS particle-in-cell framework that the radiation reaction triggered by ultra-high laser intensity plays a crucial role in the positron injection. It provides a suppression of the initial transverse momentum gained by the positrons from the Breit-Wheeler process. For the parameters used in this work, the intensity of at least 2.2 × 10 23 W / cm 2 is needed in order to inject more than 1% of positrons created. Above this threshold, the percentage of injected positrons rapidly increases with intensity. Moreover, subsequent direct laser acceleration of positrons in a plasma channel, using the same laser pulse that created them, can ensure a boost of the final positron energy by a factor of two. The positron focusing and guiding on the axis is provided by significant electron beam loading that changes the internal structure of the channel fields. [ABSTRACT FROM AUTHOR]

High-field Terahertz (THz) with a broad frequency spectrum has important application in THz optics and imaging. In this paper we report a way for generating THz fields, which uses an electron beam propagating opposite to the laser pulse. The laser excites wakefields via charge separation associate with the ponderomotive force. The laser wakefield couples with the negative energy electron beam to drive the THz electromagnetic mode. The applied magnetic field provides an additional resonance to this parametric interaction between the laser and electron beam. The THz fields of 20 GV/cm with spectrum band covering the range from 1 to 15 THz can be realized. Several scaling laws are provided to optimize the THz fields. This work provides a reasonable approach to generate THz radiation fields during intense laser-plasma interactions. [ABSTRACT FROM AUTHOR]

A direct comparison of the properties of electron beam generated by antiparallel circularly polarized Laguerre–Gaussian (CPLG) laser pulse and parallel CPLG laser pulse has been performed with three-dimensional particle-in-cell simulations. It is known that the longitudinal field of an antiparallel CPLG laser pulse with opposite signs of spin and orbital quantum number preferentially accelerates electrons to high energy. However, a direct comparison of electron beam between the other combination of spin and orbital angular momentum, the parallel CPLG laser pulse with the same sign of spin and orbital angular quantum number, has not been conducted. While the two pulses have an identical transverse field envelope, the generated electron beam properties are different. Although the magnitude of the longitudinal field is about one order of magnitude less than that of the transverse field, it has a significant effect on beam divergence. For antiparallel CPLG laser pulse, collimated electron bunches are formed with small divergence (< 50 mrad); while for parallel CPLG laser pulse, a diverging (> 100 mrad) electron beam is formed. This difference in beam quality can indicate a field-induced acceleration in actual experiments. A few-cycle laser pulse and low-density plasma are used to rule out the effect of laser–plasma interaction. It is also shown that for antiparallel CPLG laser pulse, the maximum kinetic energy increases with the square root of incident laser power, consistent with the scaling law for field-induced acceleration. [ABSTRACT FROM AUTHOR]

Laser wakefield acceleration (LWFA) using PW-class laser pulses generally requires cm-scale laser–plasma interaction Rayleigh length, which can be realized by focusing such pulses inside a long underdense plasma with a large f-number focusing optic. Here, we present a new PW-based LWFA instrument at the SG-II 5 PW laser facility, which employs f/23 focusing. The setup also adapted an online probing of the plasma density via Nomarski interferometry using a probe laser beam having 30 fs pulse duration. By focusing 1-PW, 30-fs laser pulses down to a focal spot of 230 µm, the peak laser intensity reached a mild-relativistic level of 2.6 × 1018 W/cm2, a level modest for standard LWFA experiments. Despite the large aspect ratio of >25:1 (transverse to longitudinal dimensions) of the laser pulse, electron beams were observed in our experiment only when the laser pulse experienced relativistic self-focusing at high gas-pressure thresholds, corresponding to plasma densities higher than 3 × 1018 cm−3. [ABSTRACT FROM AUTHOR]

In the last decade, laser-plasma acceleration of high-energy electrons has attracted strong attention in different fields. Electrons with maximum energies in the GeV range can be laser-accelerated within a few cm using multi-hundreds terawatt (TW) lasers, yielding to very high beam currents at the source (electron bunches with up to tens-hundreds of pC in a few fs). While initially the challenge was to increase the maximum achievable electron energy, today strong effort is put in the control and usability of these laser-generated beams that still lack of some features in order to be used for applications where currently conventional, radio-frequency (RF) based, electron beam lines represent the most common and efficient solution. Several improvements have been suggested for this purpose, some of them acting directly on the plasma source, some using beam shaping tools located downstream. Concerning the latter, several studies have suggested the use of conventional accelerator magnetic devices (such as quadrupoles and solenoids) as an easy implementable solution when the laser-plasma accelerated beam requires optimization. In this paper, we report on a parametric study related to the transport of electron beams accelerated by laser-plasma interaction, using conventional accelerator elements and tools. We focus on both, high energy electron beams in the GeV range, as produced on petawatt (PW) class laser systems, and on lower energy electron beams in the hundreds of MeV range, as nowadays routinely obtained on commercially available multi-hundred TW laser systems. For both scenarios, our study allows understanding what are the crucial parameters that enable laser-plasma accelerators to compete with conventional ones and allow for a beam transport. We show that suitable working points require a tradeoff-combination between low beam divergence and narrow energy spread. [ABSTRACT FROM AUTHOR]

Excitation of electron plasma wave by an intense short laser pulse is relevant to electron acceleration process in laser plasma interactions. This work presents the self-focusing of an intense cosh-Gaussian laser beam in collissionless plasma with relativistic and ponderomotive nonlinearities in the extended-paraxial region. Further, the effect of self-focusing of the cosh-Gaussian laser beam on the excitation of electron plasma wave and on subsequent electron acceleration has been investigated. Analytical expressions for the beam width parameter/intensity of cosh-Gaussian laser beam and the electron plasma wave have been established and solved numerically. The energy of the accelerated electrons has also been obtained. The strong self-focusing of the cosh-Gaussian laser beam in plasmas stimulates a large amplitude electron plasma wave, which further accelerates the electrons. The well-established laser and plasma parameters have been used in numerical computation. The results have been compared with the paraxial ray approximation, the Gaussian profile of the laser beam, and only relativistic nonlinearity. The numerical results show that the focusing of the cosh-Gaussian laser beam, the amplitude of the electron plasma wave, and the energy gain by the electrons increases in the extended-paraxial region, when relativistic and ponderomotive nonlinearities are operative simultaneously. In addition, it has also been observed that the electron plasma wave is driven more efficiently by a cosh-Gaussian laser beam that accelerates plasma electrons to higher energies. [ABSTRACT FROM AUTHOR]

In experiments on the effect of magnetic field on electron transportation in laser–plasma interaction, the magnetic field is often produced by two coils and is mirror-like. In this paper, the transport and the reflection of fast electron beam generated in laser–plasma interaction in solid-density plasma immersed in a mirror magnetic field are studied using particle-in-cell simulation. The helicoidal motion of fast electrons in the field and the convergence of magnetic induction lines leads to the collimated transport and focusing of the fast electrons. The reflection of the fast electrons can lead to the decrease in the transmission ratio, and this reflection increases with the magnetic mirror ratio, but saturates at a certain level. [ABSTRACT FROM AUTHOR]

Results of numerical modeling of the interaction of an electron beam propagating across relativistic plasma waves indicate that electron beam filamentation and focusing may occur under certain conditions. The model is based on solving the relativistic equation of motion in three dimensions for the individual electrons in a tenuous Gaussian beam, as they pass through a relativistic plasma wave. Several electron beam and plasma wave parameters were varied, and the results are summarized. One of the results is that the spacing of the electron beam filaments correlates with the wavelength of the plasma wave. The electron beam filaments appear as vertical slabs after the beam exits the plasma waves. The electron beam also compresses to a focus after it exits the plasma, and the focal distance depends on several parameters including the electron beam energy and phase velocity of the relativistic plasma wave. It is suggested that these focusing and filamentation phenomena may be the basis for diagnostics schemes for laser plasma interactions. The parameters used in the model were electron beam energies in the 5–50 keV range and plasma wave properties typical for the beat-wave produced by CO2 lasers, which correspond to the facilities available in our laboratory. The limitations of these results to lower energy density beam and plasma regimes and to higher energy density regimes will be discussed. [ABSTRACT FROM AUTHOR]

We report stable laser wakefield acceleration using 17-50 TW laser pulses interacting with 4mmlong helium gas jet. The initial laser spot size was relatively large (28 lm) and the plasma densities were 0.48-2.0×1019cm-3. High-quality 100-MeV electron beams were generated at the plasma density of 7.5×1018cm-3, at which the beam parameters (pointing angle, energy spectrum, charge, and divergence angle) were measured and stabilized. At higher densities, filamentation instability of the laser-plasma interaction was observed and it has led to multiple wakefield accelerated electron beams. The experimental results are supported by 2D particle-in-cell simulations. The achievement presented here is an important step toward the use of laser-driven accelerators in real applications. [ABSTRACT FROM AUTHOR]

Very efficient generation of a high-charge electron beam by a laser pulse propagating in a self-trapping mode in near-critical density plasma makes it possible to produce a high yield of gamma rays for radiography of samples located deep in a dense medium. The three-dimensional particle-in-cell and Monte Carlo simulations performed with end-to-end modeling from laser–plasma interaction to the final gamma-imaging of deeply shielded objects located at distances up to several meters clearly demonstrate the promise of laser pulses of several hundred TW for single-shot radiography by using a high-performance scheme of electron acceleration in the laser pulse self-trapping regime. This is illustrated by two examples with the same laser–target design used for a bremsstrahlung gamma source and an all-optical nonlinear inverse Compton source. [ABSTRACT FROM AUTHOR]

Magnetic collimation via resistivity gradients is an innovative approach to electron beam control for the cone-guided fast ignition variant of inertial confinement fusion. This technique uses a resistivity gradient induced magnetic field to collimate the electron beam produced by the high-intensity laser–plasma interaction within a cone-guided fast ignition cone-tip. A variant of the resistive guiding approach, known as the "magnetic switchyard," has been proposed which uses shaped guiding elements to direct the electrons toward the compressed fuel. Here, the 1D radiation-hydrodynamics code HYADES is used to investigate and quantify the gross hydrodynamic motion of these magnetic switchyard guiding elements in conditions relevant to their use in fast ignition. Movement of the layers was assessed for a range of two-layer material combinations. Based upon the results of the simulations, a scaling law is found that enables the relative extent of hydrodynamic motion to be predicted based upon the material properties of the switchyard, thereby enabling optimization of material-combination choice on the basis of reducing hydrodynamic motion. A multi-layered configuration, more representative of an actual switchyard, was also simulated in which an outer Au layer is employed to tamp the motion of the outermost guiding element of the switchyard. [ABSTRACT FROM AUTHOR]

The article informs that developments in laser-wakefield accelerators have led to compact ultrashort X/gamma-ray sources that are comparable with conventional synchrotron sources. Topics include scheme to efficiently produce collimated ultrabright Gamma ray beams with photon energies tunable up to GeV with a multi-petawatt laser pulse into wakefield accelerator; and numerical simulations demonstrate that Gamma ray photons/shot are produced with energy conversion efficiency for photons.

We proposed a new method to yield brilliant keV-MeV x-ray emission, inspired by Cao et al (2010 Phys. Plasma 17 043103). When a short femtosecond pulse is coupling into a microwire array target, due to the weakly unbalanced quasi-static electric and magnetic fields, the energetic electrons accelerated by the laser will emit a large amount of photons per shot, comparable to that from the laser-wake betatron radiation. It was found that the critical energy of the photons and the total photon yields are strongly dependent on the laser intensity and structure parameters of the target. The peak spectral brightness of the radiation in our scheme was estimated to be 4 × 1019 photons s-1 mm² mrad² 0.1% bandwidth at the laser intensity 3 × 1020W cm-2. More important, the photon energies can be extended to hundreds of keV to MeV readily if electron beam accelerated by the laser is in the range of MeV to GeV. [ABSTRACT FROM AUTHOR]

Foot and pedestal pulses that precede the main pulse from a high-intensity laser greatly affect laser-plasma interactions. Especially in fast ignition schemes, preceding pulses generate a plasma prior to irradiation by the main pulse. This results in a too energetic and divergent electron beam being generated in the preformed plasma, which reduces the energy coupling efficiency from the heating laser to the dense fuel core. A preformed plasma with a density scale length of 40-60 μm was observed by a time- and space-resolved x-ray backlight technique using the LFEX laser system at the Institute of Laser Engineering, Osaka University. Preceding pulses (i.e., the foot and pedestal) of the LFEX were characterized by comparing observations with calculations results obtained using a two-dimension (2D) radiation-hydrodynamic simulation code. In a separate experiment, the 2D code was benchmarked with the experimentally observed hydrodynamic behavior of a gold plasma produced by a nanosecond laser pulse that mimicked foot and pedestal pulses (intensity: 1 ×1011-1×1012W/cm2). The preceding pulses were estimated to have an intensity of 1 ×1012-1013W/cm2, a duration of 2.0 ns, and a spot diameter at the target of 200-600 μm by comparing the measured hydrodynamics of the preformed plasma with that calculated by the 2D hydrodynamic simulation code. [ABSTRACT FROM AUTHOR]

Abstract Over the past decade the production of multi-GeV electron beams from laser-driven plasma accelerators has been successfully demonstrated. However, demanding applications such as compact electron-driven X-ray sources require further improvements on beam energy spread and transverse emittance. One promising candidate to satisfy these requirements is to externally inject an electron beam generated by an RF linac into a laser-driven plasma accelerator. We present studies on the optimization of the final quality of the externally injected electron beam accelerated to GeV energy, using simulations with the particle-in-cell code OSIRIS. We quantified the effect of the injection phase on the quality of the accelerated beam. Tailoring the longitudinal plasma density profile by a smooth vacuum-plasma transition has been used to minimize the induced emittance growth. [ABSTRACT FROM AUTHOR]

Abstract Plasma wakefield acceleration is the most promising acceleration technique for compact and cheap accelerators, thanks to the high accelerating gradients achievable. Nevertheless, this approach still suffers of shot-to-shot instabilities, mostly related to experimental parameters fluctuations. Therefore, the use of single shot diagnostics is needed to properly understand the acceleration mechanism. In this work, we present two diagnostics to probe electron beams from laser–plasma interactions, one relying on Electro Optical Sampling (EOS) for laser–solid matter interactions, the other one based on Optical Transition Radiation (OTR) for single shot measurements of the transverse emittance of plasma accelerated electron beams, both developed at the SPARC_LAB Test Facility. [ABSTRACT FROM AUTHOR]

The recent development of ultra-high intensity laser facilities is finally opening up the possibility of studying high-field quantum electrodynamics in the laboratory. Arguably, one of the central phenomena in this area is that of quantum radiation reaction experienced by an ultra-relativistic electron beam as it propagates through the tight focus of a laser beam. In this paper, we discuss the major experimental challenges that are to be faced in order to extract meaningful and quantitative information from this class of experiments using existing and near-term laser facilities. [ABSTRACT FROM AUTHOR]

The generation of stable electron beams produced by the laser wakefield acceleration mechanism with a few-terawatt laser system (600 mJ, 50 fs) in a supersonic synthetic air jet is reported and the requirements necessary to build such a stable electron source are experimentally investigated in conditions near the bubble regime threshold. The resulting electron beams have stable energies of (17.4 ± 1.1) MeV and an energy spread of (13.5 ± 1.5) MeV (FWHM), which has been achieved by optimizing the properties of the supersonic gas jet target for the given laser system. Due to the availability of few-terawatt laser systems in many laboratories around the world these stable electron beams open possibilities for applications of this type of particle source. [ABSTRACT FROM AUTHOR]

Intense lasers interacting with dense targets accelerate relativistic electron beams, which transport part of the laser energy into the target depth. However, the overall laser-to-target energy coupling efficiency is impaired by the large divergence of the electron beam, intrinsic to the laser-plasma interaction. Here we demonstrate that an efficient guiding of MeV electrons with about 30MA current in solid matter is obtained by imposing a laserdriven longitudinal magnetostatic field of 600 T. In the magnetized conditions the transported energy density and the peak background electron temperature at the 60-μm-thick target's rear surface rise by about a factor of five, as unfolded from benchmarked simulations. Such an improvement of energy-density flux through dense matter paves the ground for advances in laser-driven intense sources of energetic particles and radiation, driving matter to extreme temperatures, reaching states relevant for planetary or stellar science as yet inaccessible at the laboratory scale and achieving high-gain laser-driven thermonuclear fusion. [ABSTRACT FROM AUTHOR]

We show the results of a systematic study of the properties recently evidenced for the asymmetric lateral coherence of broadband radiation emitted by particles in betatron motion. 2-dimensional Montecarlo simulations have been used in order to investigate changes of the coherence properties by varying the spatial distribution of an ensemble of particles, whose parameters match those of the electron beams at the SPARC_LAB test facility. A clear interpretation is given under several conditions, and applications to diagnostics of particle beams appear a natural exploitation of the coherence measurements. [ABSTRACT FROM AUTHOR]

We demonstrate laser-plasma acceleration of high charge electron beams to the ~10 MeV scale using ultrashort laser pulses with as little energy as 10 mJ. This result is made possible by an extremely dense and thin hydrogen gas jet. Total charge up to ~0.5 nC is measured for energies >1 MeV. Acceleration is correlated to the presence of a relativistically self-focused laser filament accompanied by an intense coherent broadband light flash, associated with wave breaking, which can radiate more than ~3% of the laser energy in a ~1 fs bandwidth consistent with half-cycle optical emission. Our results enable truly portable applications of laser-driven acceleration, such as low dose radiography, ultrafast probing of matter, and isotope production. [ABSTRACT FROM AUTHOR]

Hot electrons created in laser plasma interaction at laser intensities 1 - 10PWcm-2 in shock ignition scheme can deposit their energy in the shell of the target, augmenting the strength of the ignitor shock. Here, we present a model that describes the effect of the spatial profile of fast electron energy deposition on the dynamics of shock wave formation. A criterion of a strong shock formation is obtained for an arbitrary electron beam distribution function. It is shown that the time and the position of the shock formation are defined by the electron average stopping range, while the strength of the shock decreases as the width of electron energy distribution increases. The latter feature is explained by the fast electron target preheat. The conclusions of theoretical model are confirmed in numerical simulations. The pressure, the strength of the shock, and the efficiency of shock generation are calculated for different electron distributions with the same average stopping range. [ABSTRACT FROM AUTHOR]

We report on the development of a Monte Carlo application, based on the GEANT4 toolkit, for the characterization and optimization of electron beams for clinical applications produced by a laser-driven plasma source. The GEANT4 application is conceived so as to represent in the most general way the physical and geometrical features of a typical laser-driven accelerator. It is designed to provide standard dosimetric figures such as percentage dose depth curves, two-dimensional dose distributions and 3D dose profiles at different positions both inside and outside the interaction chamber. The application was validated by comparing its predictions to experimental measurements carried out on a real laser-driven accelerator. The work is aimed at optimizing the source, by using this novel application, for radiobiological studies and, in perspective, for medical applications. [ABSTRACT FROM AUTHOR]

Optimized-quality monoenergetic target surface electron beams at MeV level with low normalized emittance (0.03p mm mrad) and high charge (30 pC) per shot have been obtained from 3 TW lasersolid interactions at a grazing incidence. The 2-Dimension particle-in-cell simulations suggest that electrons are wake-field accelerated in a large-scale, near-critical-density preplasma. It reveals that a bubble-like structure as an accelerating cavity appears in the near-critical-density plasma region and travels along the target surface. A bunch of electrons are pinched transversely and accelerated longitudinally by the wake field in the bubble. The outstanding normalized emittance and monochromaticity of such highly collimated surface electron beams could make it an ideal beam for fast ignition or may serve as an injector in traditional accelerators. [ABSTRACT FROM AUTHOR]