Laser Acceleration Of Electrons Research Articles

Mid-infrared dielectric laser acceleration in a silicon dual pillar structure

Dielectric laser accelerators use near-infrared laser pulses to accelerate electrons at dielectric structures. Driving these devices with mid-infrared light should result in relaxed requirements on the electron beam, easier fabrication, higher damage threshold, and thus higher acceleration gradients. In this paper, we demonstrate dielectric laser acceleration of electrons driven with 10 μm light in a silicon dual pillar structure. We observe the acceleration of 27 keV electrons by 1.4 keV, corresponding to a 93 MeV/m acceleration gradient. The damage threshold of the structures of 3.3 ± 0.6 GV/m peak field is significantly higher than for near-infrared accelerators. The dual pillar acceleration structure itself even survived 5.2 ± 0.9 GV/m, the highest field strength we could achieve with the current system. This together with the larger structure acceptance bodes well for future dielectric laser accelerators driven with mid-infrared light.

All-optical source size and emittance measurements of laser-accelerated electron beams

Novel schemes for generating ultralow emittance electron beams have been developed in past years and promise compact particle sources with excellent beam quality suitable for future high-energy physics experiments and free-electron lasers. Recent theoretical work has proposed a laser-based method capable of resolving emittances in the sub 0.1 mm mrad regime by modulating the electron phase space ponderomotively. Here we present the first experimental demonstration of this scheme using a laser wakefield accelerator. The observed emittance and source size are consistent with published values. We also show calculations demonstrating that tight bounds on the upper limit for emittance and source size can be derived from the “laser-grating” method even in the presence of low signal to noise and uncertainty in laser-grating parameters. Published by the American Physical Society 2024

The influence of laser focusing conditions on the direct laser acceleration of electrons

Direct laser acceleration of electrons during a high-energy, picosecond laser interaction with an underdense plasma has been demonstrated to be substantially enhanced by controlling the laser focusing geometry. Experiments using the OMEGA EP facility measured electrons accelerated to maximum energies exceeding 120 times the ponderomotive energy under certain laser focusing, pulse energy, and plasma density conditions. Two-dimensional particle-in-cell simulations show that the laser focusing conditions alter the laser field evolution, channel fields generation, and electron oscillation, all of which contribute to the final electron energies. The optimal laser focusing condition occurs when the transverse oscillation amplitude of the accelerated electron in the channel fields matches the laser beam width, resulting in efficient energy gain. Through this observation, a simple model was developed to calculate the optimal laser focal spot size in more general conditions and is validated by experimental data.

Electron acceleration and x-ray generation from near-critical-density carbon nanotube foams driven by moderately relativistic lasers

Direct laser acceleration of electrons in near-critical-density (NCD) carbon nanotube foams (CNFs) has its advantages in the high-efficiency generation of relativistic electrons and broadband x-rays. Here, we report the first simultaneous measurement on the spectra of laser-driven electrons and x-rays from CNFs at moderately relativistic intensities of around 5×1019 W/cm2. The density and thickness of the CNFs were scanned in the experiments, indicating the optimized electron temperature of 5.5 MeV and x-ray critical energy of 5 keV. Two-dimensional particle-in-cell simulations confirm that the electrons, with a temperature significantly higher than the pondermotive scale, are directly accelerated by the laser along the NCD plasma channel, while the bright x-rays are emitted by these electrons through betatron radiation or Thomson backscattering inside the channel. The simultaneously generated electrons and x-rays, automatically synchronized with the femtosecond laser driver, are suitable for applications such as bi-modal radiography.

Direct Laser Acceleration in Underdense Plasmas with Multi-PW Lasers: A Path to High-Charge, GeV-Class Electron Bunches.

The direct laser acceleration (DLA) of electrons in underdense plasmas can provide hundreds of nC of electrons accelerated to near-GeV energies using currently available lasers. Here we demonstrate the key role of electron transverse displacement in the acceleration and use it to analytically predict the expected maximum electron energies. The energy scaling is shown to be in agreement with full-scale quasi-3D particle-in-cell simulations of a laser pulse propagating through a preformed guiding channel and can be directly used for optimizing DLA in near-future laser facilities. The strategy towards optimizing DLA through matched laser focusing is presented for a wide range of plasma densities paired with current and near-future laser technology. Electron energies in excess of 10GeV are accessible for lasers at I∼10^{21} W/cm^{2}.

Characterization of bright betatron radiation generated by direct laser acceleration of electrons in plasma of near critical density

Directed x-rays produced in the interaction of sub-picosecond laser pulses of moderate relativistic intensity with plasma of near-critical density are investigated. Synchrotron-like (betatron) radiation occurs in the process of direct laser acceleration (DLA) of electrons in a relativistic laser channel when the electrons undergo transverse betatron oscillations in self-generated quasi-static electric and magnetic fields. In an experiment at the PHELIX laser system, high-current directed beams of DLA electrons with a mean energy ten times higher than the ponderomotive potential and maximum energy up to 100 MeV were measured at 1019 W/cm2 laser intensity. The spectrum of directed x-rays in the range of 5–60 keV was evaluated using two sets of Ross filters placed at 0° and 10° to the laser pulse propagation axis. The differential x-ray absorption method allowed for absolute measurements of the angular-dependent photon fluence. We report 1013 photons/sr with energies >5 keV measured at 0° to the laser axis and a brilliance of 1021 photons s−1 mm−2 mrad−2 (0.1%BW)−1. The angular distribution of the emission has an FWHM of 14°–16°. Thanks to the ultra-high photon fluence, point-like radiation source, and ultra-short emission time, DLA-based keV backlighters are promising for various applications in high-energy-density research with kilojoule petawatt-class laser facilities.

Electron energy gain due to a laser frequency modulation experienced by electron during betatron motion

Direct laser acceleration of electrons is an important energy deposition mechanism for laser-irradiated plasmas that is particularly effective at relativistic laser intensities in the presence of quasi-static laser-driven plasma electric and magnetic fields. These radial electric and azimuthal magnetic fields provide transverse electron confinement by inducing betatron oscillations of forward-moving electrons undergoing laser acceleration. Electrons are said to experience a betatron resonance when the frequency of betatron oscillations matches the average frequency of the laser field oscillations at the electron position. In this paper, we show that the modulation of the laser frequency as seen by an electron performing betatron oscillations can be another important mechanism for net energy gain that is qualitatively different from the betatron resonance. Specifically, we show that the frequency modulation experienced by the electron can lead to net energy gain in the regime where the laser field performs three oscillations per betatron oscillation. There is no net energy gain in this regime without the modulation because the energy gain is fully compensated by the energy loss. The modulation slows down the laser oscillation near transverse stopping points, increasing the time interval during which the electron gains energy and making it possible to achieve net energy gain.

THz transition radiation of electron bunches laser-accelerated in long-scale near-critical-density plasmas

Direct laser electron acceleration in near-critical density plasma produces collimated electron beams with high charge Q (up to µC). This regime could be of interest for high-energy THz radiation generation, as many of the mechanisms have a scaling ∝Q2 . In this work, we focus specifically on the challenges that arise during numerical investigations of transition radiation in such interactions. Detailed analytical calculations that include both the diffraction and decoherence effects of the characteristics of transition radiation in the THz range were conducted with the input parameters obtained from 3D particle-in-cell and hydrodynamic simulations. The calculated characteristics of THz radiation are in good agreement with the experimentally measured ones. Therefore, this approach can be used both to optimize the properties of THz radiation and to distinguish the transition radiation contribution if several mechanisms of THz radiation generation are considered.

Electron-positron generation by irradiating various metallic materials with laser-accelerated electrons

We examined electron–positron pair production in solid iron, zinc, tungsten, and lead targets irradiated by a laser-accelerated electron beam generated with a 100TW laser. These targets were assessed at the target thickness of 0.5, 1.25, and 2.0radiation lengths for each material. Using a 0.75-T-magnetic spectrometer, we measured the electron and positron yields and spectra, producing 3 × 108 positrons per shot with a peak leptonic density of 4 × 1012cm−3. These experimental results agree very well with Monte Carlo simulations conducted with the simulation code Geant4. Importantly, our findings show that normalizing the target thickness to each material’s radiation length results in consistent electron and positron yields across the materials, effectively reducing discrepancies due to material differences.

Mitigation of electromagnetic pulses interfering with Thomson parabola ion spectrometers at XG-III laser facility.

The Thomson parabola ion spectrometer is vulnerable to intense electromagnetic pulses (EMPs) generated by a high-power laser interacting with solid targets. A metal shielding cage with a circular aperture of 1mm diameter is designed to mitigate EMPs induced by a picosecond laser irradiating a copper target in an experiment where additionally an 8-ns delayed nanosecond laser is incident into an aluminum target at the XG-III laser facility. The implementation of the shielding cage reduces the maximum EMP amplitude inside the cage to 5.2 kV/m, and the simulation results indicate that the cage effectively shields electromagnetic waves. However, the laser-accelerated relativistic electrons which escaped the target potential accumulate charge on the surface of the cage, which is responsible for the detected EMPs within the cage. To further alleviate EMPs, a lead wall and an absorbing material (ECCOSORB AN-94) were added before the cage, significantly blocking the propagation of electrons. These findings provide valuable insights into EMP generation in large-scale laser infrastructures and serve as a foundation for electromagnetic shielding design.

Study of the isomeric yield ratio in the photoneutron reaction of natural holmium induced by laser-accelerated electron beams

Introduction: An accurate knowledge of the isomeric yield ratio (IR) induced by the photonuclear reaction is crucial to study the nuclear structure and reaction mechanisms. 165Ho is a good candidate for the investigation of the IR since the Ho target has a natural abundance of 100% and the residual nuclide has a good decay property.Methods: In this study, the photoneutron production of 164m, gHo induced by laser-accelerated electron beams is investigated experimentally. The γ-ray spectra of activated Ho foils are off-line detected. Since the direct transitions from the 164mHo are not successfully observed, we propose to extract the IRs of the 164m, gHo using only the photopeak counts from the ground-state decay.Results: The production yields of 164m, gHo are extracted to be (0.45 ± 0.10) × 106 and (1.48 ± 0.14) × 106 per laser shot, respectively. The resulting IR is obtained to be 0.30 ± 0.08 at the effective γ-ray energy of 12.65 MeV.Discussion: The present data, available experimental data, and TALYS calculations are then compared to examine the role of the excitation energy. It is found that besides the giant dipole resonance, the excitation energy effect also plays a key role in the determination of the IRs.

Excitation of an upper hybrid wave by Laguerre–Gaussian pulse in plasma with a density ramp and electron acceleration

In this paper, the excitation of an upper hybrid wave (UHW) by propagation of a Laguerre–Gaussian (LG) laser pulse in an exponential density ramp plasma has been investigated in relativistic regime. A uniform static magnetic field was externally applied perpendicular to the propagation of the laser. The spatiotemporal dynamics of laser pulse have been studied by two coupled differential equations obtained via the moment theory approach. The propagating laser pulse excites an electron plasma wave, which further interacts with the laser pulse in the presence of applied transverse magnetic field and generates an UHW. The excited UHW further traps the plasma electrons and accelerates them. It has been observed that the presence of density ramp affects the laser dynamics, excited UHW, and electron acceleration. The applied magnetic field and modes of LG laser have also shown significant effect on electron acceleration. Energy gain of the order of 400 MeV/mm has been obtained for electrons.

Laser-driven electrodynamic implosion of fast ions in a thin shell

Collision of laser-driven subrelativistic high-density ion flows provides a way to create extremely compressed ion conglomerates and study their properties. This paper presents a theoretical study of the electrodynamic implosion of ions inside a hollow spherical or cylindrical shell irradiated by femtosecond petawatt laser pulses. We propose to apply a very effective mechanism for ion acceleration in a self-consistent field with strong charge separation, based on the oscillation of laser-accelerated fast electrons in this field near the thin shell. Fast electrons are generated on the outer side of the shell under irradiation by the intense laser pulses. It is shown that ions, in particular protons, may be accelerated at the implosion stage to energies of tens and hundreds of MeV when a sub-micrometer shell is irradiated by femtosecond laser pulses with an intensity of 1021–1023 W cm−2.

Low energy electron injection for direct laser acceleration

The feasibility of ionization injection for Direct Laser Acceleration (DLA) of electrons up to hundreds of MeV has been studied analytically. Criteria for effective injection determining a range of background and in-channel plasma parameters, laser intensity, etc., were found using phase portraits of the system deduced from the simplified analytical model. The found optimal trajectory in the phase space corresponds to the electron with low (few eV) initial energy experiencing ∼109 times energy gain. For this to occur, electron density should be a few percent of the critical density, while the in-channel electron density should be ∼3 times lower. The analytically obtained dependence of the energy gain on the initial electron longitudinal and transverse momenta corresponds well to the results of exact numerical simulations of an electron motion in the plasma channel. To test the theory, a series of PIC simulations were carried out. PIC simulation confirms the model if the plasma channel has appropriate parameters. The developed approach can form the basis for further studies of electron injection in DLA varying plasma and laser parameters as well as initial electron energies.

Laser-accelerated electron beams at 1 GeV using optically-induced shock injection

In recent years, significant progress has been made in laser wakefield acceleration (LWFA), both regarding the increase in electron energy, charge and stability as well as the reduction of bandwidth of electron bunches. Simultaneous optimization of these parameters is, however, still the subject of an ongoing effort in the community to reach sufficient beam quality for next generation’s compact accelerators. In this report, we show the design of slit-shaped gas nozzles providing centimeter-long supersonic gas jets that can be used as targets for the acceleration of electrons to the GeV regime. In LWFA experiments at the Centre for Advanced Laser Applications, we show that electron bunches are accelerated to {1}text {GeV} using these nozzles. The electron bunches were injected into the laser wakefield via a laser-machined density down-ramp using hydrodynamic optical-field-ionization and subsequent plasma expansion on a ns-timescale. This injection method provides highly controllable quasi-monoenergetic electron beams with high charge around {100}text {pC}, low divergence of {0.5}text {mrad}, and a relatively small energy spread of around {10}% at {1}text {GeV}. In contrast to capillaries and gas cells, the scheme allows full plasma access for injection, probing or guiding in order to further improve the energy and quality of LWFA beams.

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.

Impact of wiggler magnetic field on wakefield generation and electron acceleration by Gaussian, super-Gaussian and Bessel–Gaussian laser pulses propagating in collisionless plasma

In this research, the process of electron acceleration and wakefield generation by Gaussian-like (GL), super-Gaussian (SG) and Bessel–Gaussian (BG) laser pulses through cold collisionless plasma in the presence of a planar magnetostatic wiggler are studied. Three different types of laser spatial profiles, GL, SG and BG, are considered. Additionally, using the hydrodynamics fluid equations, Maxwell's equations as well as the perturbation technique for GL, SG and BG laser pulses in the weakly nonlinear regime and in the presence of a planar magnetostatic wiggler, governing equations for analysing the laser wakefield and electron acceleration have been derived and compared correspondingly. In addition, the effect of some important factors, including the wiggler field strengths, laser intensity, pulse length, plasma electron density and laser frequency on the wakefield and the electron energy gain, have been investigated. Numerical results show that enhancing the wiggler magnetic field results in an increase in the amplitude of the wakefield. Furthermore, it is observed that in comparison with the wakefield amplitude excited by SG and GL laser pulses, the amplitude of the wakefield excited by BG laser pulse is larger when the wiggler field is enhanced. Moreover, it is realized that the type of the laser profile, selected laser parameters and wiggler magnetic field are the most decisive and effective factors in the wakefield amplitude and shape of wakefield generation through cold collisionless plasma. Also, it is seen that as the pulse length declines, the amplitude of the wakefield increases, and correspondingly the resonance positions shift to higher ${({\varOmega _w}/{\varOmega _p})_{max}}$ values.

Characterization of relativistic electron–positron beams produced with laser-accelerated GeV electrons

The characterization of an electron–positron beam generated from the interaction of a multi-GeV electron beam with a lead plate is performed using GEANT4 simulations. The dependence of the positron beam size on driver electron beam energy and lead converter thickness is investigated in detail. A pancake-like positron beam structure is generated with a monoenergetic multi-GeV driver electron beam, with the results indicating that a 5 GeV driver electron beam with 1 nC charge can generate a positron beam with a density of 1015–1016 cm−3 at one radiation length of lead. In addition, we find that electron–positron beams generated using above-GeV electron beams have neutralities greater than 0.3 at one radiation length of lead, whereas neutralities of 0.2 are observed when using a 200 MeV electron beam. The possibility of observing plasma instabilities in experiments is also examined by comparing the plasma skin depth with the electron–positron beam size. A quasi-neutral electron–positron plasma can be produced in the interaction between a 1 nC, 5 GeV electron beam and lead with a thickness of five radiation lengths. Our findings will aid in analyzing and interpreting laser-produced electron–positron plasma for laboratory astrophysics research.

Collimated gamma beams with high peak flux driven by laser-accelerated electrons

Abstract Laser-accelerated electrons are promising in producing gamma-photon beams of high peak flux for the study of nuclear photonics, obtaining copious positrons and exploring photon–photon interaction in vacuum. We report on the experimental generation of brilliant gamma-ray beams with not only high photon yield but also low divergence, based on picosecond laser-accelerated electrons. The 120 J 1 ps laser pulse drives self-modulated wakefield acceleration in a high-density gas jet and generates tens-of-MeV electrons with 26 nC and divergence as small as $1.51{}^{\circ}$ . These collimated electrons produce gamma-ray photons through bremsstrahlung radiation when transversing a high-Z solid target. We design a high-energy-resolution Compton-scattering spectrometer and find that a total photon number of $2.2\times {10}^9$ is captured within an acceptance angle of $1.1{}^{\circ}$ for photon energies up to $16\;\mathrm{MeV}$ . Comparison between the experimental results and Monte Carlo simulations illustrates that the photon beam inherits the small divergence from electrons, corresponding to a total photon number of $2.2\times {10}^{11}$ and a divergence of $7.73{}^{\circ}$ .

Resistivity contribution to stopping power and plasma heating by laser-accelerated electrons

A model of plasma stopping power with respect to laser-accelerated electrons is generalized. The model includes both main mechanisms of fast electron energy loss caused by Coulomb collisions with plasma electrons and ions (“collisional” mechanism), and by the field of resistive plasma current (“Ohmic” mechanism). The results are used to develop an analytical model of plasma heating by fast electrons with energies corresponding to the wide range of energy flux density of laser pulse capable to provide an intense generation of these particles. It is shown that the contribution of the Ohmic stopping power component has a significant effect on the dynamics of plasma heating by high-current fast electron beam generated by short femtosecond and picosecond laser pulses of petawatt power.