Laser Wakefield Electron Acceleration Research Articles

Optimization of positrons generation based on laser wakefield electron acceleration

Laser based positron represents a new particle source with short pulse duration and high charge density. Positron production based on laser wakefield electron acceleration (LWFA) has been investigated theoretically in this paper. Analytical expressions for positron spectra and yield have been obtained through a combination of LWFA and cascade shower theories. The maximum positron yield and corresponding converter thickness have been optimized as a function of driven laser power. Under the optimal condition, high energy ($g100\text{ }\text{ }\mathrm{MeV}$) positron yield up to $5\ifmmode\times\else\texttimes\fi{}{10}^{11}$ can be produced by high power femtosecond lasers at ELI-NP. The percentage of positrons shows that a quasineutral electron-positron jet can be generated by setting the converter thickness greater than 5 radiation lengths.

Control and optimization of a staged laser-wakefield accelerator

We report results of an experimental study of laser-wakefield acceleration of electrons, using a staged device based on a double-jet gas target that enables independent injection and acceleration stages. This novel scheme is shown to produce stable, quasi-monoenergetic, and tunable electron beams. We show that optimal accelerator performance is achieved by systematic variation of five critical parameters. For the injection stage, we show that the amount of trapped charge is controlled by the gas density, composition, and laser power. For the acceleration stage, the gas density and the length of the jet are found to determine the final electron energy. This independent control over both the injection and acceleration processes enabled independent control over the charge and energy of the accelerated electron beam while preserving the quasi-monoenergetic character of the beam. We show that the charge and energy can be varied in the ranges of 2–45pC, and 50–450MeV, respectively. This robust and versatile electron accelerator will find application in the generation of high-brightness and controllable x-rays, and as the injector stage for more conventional devices.

Customizable electron beams from optically controlled laser plasma acceleration for γ-ray sources based on inverse Thomson scattering

Laser wakefield acceleration of electrons in the blowout regime can be controlled by tailoring the laser pulse phase and the plasma target. The 100nm-scale bandwidth and negative frequency chirp of the optical driver compensate for the nonlinear frequency red-shift imparted by wakefield excitation. This mitigates pulse self-steepening and suppresses continuous injection. The plasma channel suppresses diffraction of the pulse leading edge, further reducing self-steepening, making injection even quieter. Besides, the channel destabilizes the pulse tail confined within the accelerator cavity (the electron density “bubble”), causing oscillations in the bubble size. The resulting periodic injection generates background-free comb-like beams – sequences of synchronized, low phase-space volume bunches. Controlling the number of bunches, their energy, and energy spacing by varying the channel radius and the pulse length (as permitted by the large bandwidth) enables the design of a tunable, all-optical source of polychromatic, pulsed γ-rays using the mechanism of inverse Thomson scattering. Such source may radiate ~107 quasi-monochromatic 10MeV-scale photons per shot into a microsteradian-scale observation angle. The photon energy is distributed among several distinct bands, each having sub-25% energy spread dictated by the mrad-scale divergence of electron beam.

Laser Technology for Advanced Acceleration: Accelerating Beyond TeV

The implementation of the suggestion of thin film compression (TFC) allows the newest class of high power, ultrafast laser pulses (typically 20[Formula: see text]fs at near-infrared wavelengths) to be compressed to the limit of a single-cycle laser pulse (2[Formula: see text]fs). Its simplicity and high efficiency, as well as its accessibility to a single-cycle laser pulse, introduce a new regime of laser–plasma interaction that enhances laser acceleration. Single-cycle laser acceleration of ions is a far more efficient and coherent process than the known laser-ion acceleration mechanisms. The TFC-derived single-cycle optical pulse is capable of inducing a single-cycle X-ray laser pulse (with a far shorter pulse length and thus an extremely high intensity) through relativistic compression. The application of such an X-ray pulse leads to the novel regime of laser wakefield acceleration of electrons in the X-ray regime, yielding a prospect of “TeV on a chip.” This possibility of single-cycle X-ray pulses heralds zeptosecond and EW lasers (and zeptoscience). The additional invention of the coherent amplification network (CAN) fiber laser pushes the frontier of high repetition, high efficiency lasers, which are the hallmark of needed applications such as laser-driven LWFA colliders and other, societal applications. CAN addresses the crucial aspect of intense lasers that have traditionally lacked the above properties.

Coherent control of plasma dynamics.

Coherent control of a system involves steering an interaction to a final coherent state by controlling the phase of an applied field. Plasmas support coherent wave structures that can be generated by intense laser fields. Here, we demonstrate the coherent control of plasma dynamics in a laser wakefield electron acceleration experiment. A genetic algorithm is implemented using a deformable mirror with the electron beam signal as feedback, which allows a heuristic search for the optimal wavefront under laser-plasma conditions that is not known a priori. We are able to improve both the electron beam charge and angular distribution by an order of magnitude. These improvements do not simply correlate with having the 'best' focal spot, as the highest quality vacuum focal spot produces a greatly inferior electron beam, but instead correspond to the particular laser phase front that steers the plasma wave to a final state with optimal accelerating fields.

Laser wakefield acceleration of electrons from a density-modulated plasma

AbstractThis research reports the increased electron energy gain from laser wakefield acceleration in density-modulated plasma with an external magnetic field. Periodic plasma density- modulation can excite higher harmonics of different phase velocities of fundamental wakefield that can assist in improving the self-trapping of pre-accelerated electrons to accelerate them for higher energy. Furthermore, the applied magnetic field assisted self-injection can also contribute in electron energy enhancement during the acceleration. The physical mechanism is described with a theoretical formulation for this scheme. Results of two-dimensional particle-in-cell simulations are reported to understand the proposed idea.

On electron betatron motion and electron injection in laser wakefield accelerators

We performed laser wakefield electron acceleration experiments using laser powers up to 100 TW in the ‘bubble’ regime. The measured angularly resolved energy spectra of the electron beam showed evidence of betatron oscillations during the acceleration process. Through diagnosis of these oscillations, electron injection into the wakefield could be controlled through adjustment of the shape of the laser focal spot or through changes in the plasma density. Several different acceleration regimes could be accessed including (i) injection of a single electron bunch into the wakefield ‘bubble’ (ii) multiple injection of several electron bunches and/or (iii) production of a transverse break up of the electron beam within the ‘bubble’ due to an asymmetry of the wakefield. We apply analytical formulae for electron motion in a wakefield to understand the experimental data.

Laser wakefield acceleration of electrons with ionization injection in a pure N5+ plasma waveguide

Ionization injection-assisted laser wakefield acceleration of electrons up to 120 MeV is demonstrated in a 1.5 mm long pure helium-like nitrogen plasma waveguide. The guiding structure stabilizes the high energy electron beam pointing and reduces the beam divergence. Our results are confirmed by 3D particle-in-cell simulations.

Measurements of high-energy radiation generation from laser-wakefield accelerated electron beams

Using high-energy (∼0.5 GeV) electron beams generated by laser wakefield acceleration (LWFA), bremsstrahlung radiation was created by interacting these beams with various solid targets. Secondary processes generate high-energy electrons, positrons, and neutrons, which can be measured shot-to-shot using magnetic spectrometers, short half-life activation, and Compton scattering. Presented here are proof-of-principle results from a high-resolution, high-energy gamma-ray spectrometer capable of single-shot operation, and high repetition rate activation diagnostics. We describe the techniques used in these measurements and their potential applications in diagnosing LWFA electron beams and measuring high-energy radiation from laser-plasma interactions.

Effect of chirp on self-modulation and laser wakefield electron acceleration in the regime of quasimonoenergetic electron beam generation

Strongly self-modulated regime of laser wakefield electron acceleration has been experimentally studied using positively/negatively chirped Ti:sapphire laser pulses with duration $\ensuremath{\ge}45\text{ }\text{ }\mathrm{fs}$, and intensity $\ensuremath{\le}1.2\ifmmode\times\else\texttimes\fi{}{10}^{18}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$ interacting with a plasma having density $>5\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$. The total charge of the high energy electrons produced from self-injection and acceleration in the wakefield was found to depend strongly and asymmetrically on the magnitude and sign of the chirp, which was also corroborated by the forward Raman scattering signals. Generation of low divergence ($\ensuremath{\le}10\text{ }\text{ }\mathrm{mrad}$), quasimonoenergetic electron beam was observed with a maximum energy of $\ensuremath{\sim}28\text{ }\text{ }\mathrm{MeV}$ for a positively chirped laser pulse of duration $\ensuremath{\sim}70\text{ }\text{ }\mathrm{fs}$. The observations are explained considering faster rise time associated with a positive chirped laser pulse which generates intense seed for stronger modulation of the laser pulse and consequently stronger wakefield excitation leading to higher electron beam charge and energy.

Study of electron acceleration and x-ray radiation as a function of plasma density in capillary-guided laser wakefield accelerators

Laser wakefield electron acceleration in the blow-out regime and the associated betatron X-ray radiation were investigated experimentally as a function of the plasma density in a configuration where the laser is guided. Dielectric capillary tubes were employed to assist the laser keeping self-focused over a long distance by collecting the laser energy around its central focal spot. With a 40 fs, 16 TW pulsed laser, electron bunches with tens of pC charge were measured to be accelerated to an energy up to 300 MeV, accompanied by X-ray emission with a peak brightness of the order of 1021 ph/s/mm2/mrad2/0.1%BW. Electron trapping and acceleration were studied using the emitted X-ray beam distribution to map the acceleration process; the number of betatron oscillations performed by the electrons was inferred from the correlation between measured X-ray fluence and beam charge. A study of the stability of electron and X-ray generation suggests that the fluctuation of X-ray emission can be reduced by stabilizing the beam charge. The experimental results are in good agreement with 3D particle-in-cell (PIC) simulation.

Numerical modeling of multi-GeV laser wakefield electron acceleration inside a dielectric capillary tube

Numerical modeling of laser wakefield electron acceleration inside a gas filled dielectric capillary tube is presented. Guiding of a short pulse laser inside a dielectric capillary tube over a long distance (∼1 m) and acceleration of an externally injected electron bunch to ultra-relativistic energies (∼5-10 GeV) are demonstrated in the quasi-linear regime of laser wakefield acceleration. Two dimensional axisymmetric simulations were performed with the code WAKE-EP (Extended Performances), which allows computationally efficient simulations of such long scale plasma. The code is an upgrade of the quasi-static particle code, WAKE [P. Mora and T. M. Antonsen, Jr., Phys. Plasmas 4, 217 (1997)], to simulate the acceleration of an externally injected electron bunch (including beam loading effect) and propagation of the laser beam inside a dielectric capillary. The influence of the transverse electric field of the plasma wake on the radial loss of the accelerated electrons to the dielectric wall is investigated. The stable acceleration of electrons to multi-GeV energy with a non-resonant laser pulse with a large spot-size is demonstrated.

Development of a Superconducting Transverse-Gradient Undulator for Laser-Wakefield Accelerators

Relativistic electrons with small energy spread propagating through undulators produce monochromatic radiation with high spectral intensity. The working principle of undulators requires a small energy spread of the electron beam in the order of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\Delta E/E \sim 0.1\%$</tex></formula> . Laser-wakefield accelerators can produce electron bunches with an energy of several 100 MeV within a few millimeters acceleration length, but with a relatively large energy spread <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(\Delta E/E \sim 1{-}10\%)$ </tex></formula> . In order to produce monochromatic undulator radiation with these electrons, a novel scheme involving transverse-gradient superconducting undulators was proposed in an earlier work. This paper reports on the design-optimization and construction of an iron-free cylindrical superconducting undulator tailored to the particular beam properties of the laser-wakefield electron accelerator at the University of Jena, Germany.

High repetition-rate wakefield electron source generated by few-millijoule, 30 fs laser pulses on a density downramp

We report on an experimental demonstration of laser wakefield electron acceleration using a sub-TW power laser by tightly focusing 30 fs laser pulses with an 8 mJ pulse energy on a 100 μm scale gas target. The experiments are carried out at an unprecedented 0.5 kHz repetition rate, allowing ‘real-time’ optimization of accelerator parameters. Well-collimated and stable electron beams with quasi-monoenergetic peaks in the 100 keV range are measured. Particle-in-cell simulations show excellent agreement with the experimental results and suggest an acceleration mechanism based on electron trapping on the density downramp, due to the time-varying phase velocity of the plasma waves.

Study on the effects of ion motion on laser-induced plasma wakes

A 2D analytical model is presented for the generation of plasma wakes (or bubbles) with an ultra-intense laser pulse by taking into account the response of plasma ions. It is shown that the effect of ion motion becomes significant at the laser intensity exceeding 1021 W/cm2 and plasma background density below 1019 cm−3. In this regime, ion motion tends to suppress the electrostatic field induced by charge separation and makes the electron acceleration less effective. As a result, the assumption of immobile ions overestimates the efficiency of laser wake-field acceleration of electrons. Based on the analytical model, the dynamics of plasma ions in laser-induced wake field is investigated. It is found that only one bubble appears as the plasmas background density exceeds the resonant density and the deposited laser energy is concentrated into the bubble, resulting in the generation of an ion bunch with extremely high energy density.

Laser wakefield electron acceleration for \gamma-ray radiography application

An electron beam is obtained using laser wakefield electron accelerator, and converted into a γ-ray source after undergoing bremsstrahlung radiation in a dense material. A quasi-monoenergetic structure is observed when the length of the plasma channel was modified. The structure has a 58-MeV peak energy, 15-mrad (full-width at half-maximum) divergence angle, and 340-pC charge. The γ-ray source generated by this high-quality electron beam is brighter and has higher spatial and temporal resolutions than other conventional sources. A γ-ray radiography demonstrational experiment is performed. Pictures of a ball with different layers made of different materials are taken. The results show a clear structure and density resolution.

Scaling laws for electron cold injection in the narrow collision pulse approximation

The latest advances in laser wakefield electron acceleration show a better beam quality, but much progress is still needed concerning the control and tunability of the electron beam. The recently proposed cold injection scheme offers a solution to this problem. It involves the use of two counter-propagating laser pulses to dephase a certain number of electrons into the wakefield of the main pulse, so that they are accelerated to high energies. As circular polarization is used, there is no stochastic heating and the injection process becomes much easier to model. We show that cold injection can be reduced to a one-dimensional problem in the case of a narrow collision pulse. The dephasing process can then be seen as competition between the longitudinal ponderomotive force of the main pulse and the stationary beatwave force arising from collision of the two circularly polarized lasers. This analysis leads to scaling laws for cold injection in the narrow collision pulse approximation and to a condition for its realization. Three-dimensional particle-in-cell simulations support both these scaling laws and the condition for cold injection to occur.

Laser wakefield acceleration of electron beams beyond 1 GeV from an ablative capillary discharge waveguide

Laser wakefield acceleration of electrons well beyond 1 GeV and optical guiding of ultraintense laser pulses of peak powers up to 160 TW over a 4-cm long ablative capillary discharge plasma channel were experimentally demonstrated. Electron beams, with energies up to 1.8 GeV, were generated by using the 130 TW, 55 fs driving laser pulses. A comparison of oxygen-containing acrylic resin (C:O:H = 4:2:7) capillary and no oxygen-containing polyethylene (C:O:H = 1:0:2) capillary measurements suggests that the injection of electron into the laser wakefield is assisted by the ionization of oxygen K-shell electrons.

Demonstration of a Narrow Energy Spread,∼0.5 GeVElectron Beam from a Two-Stage Laser Wakefield Accelerator

Laser wakefield acceleration of electrons holds great promise for producing ultracompact stages of GeV scale, high-quality electron beams for applications such as x-ray free electron lasers and high-energy colliders. Ultrahigh intensity laser pulses can be self-guided by relativistic plasma waves (the wake) over tens of vacuum diffraction lengths, to give >1 GeV energy in centimeter-scale low density plasmas using ionization-induced injection to inject charge into the wake even at low densities. By restricting electron injection to a distinct short region, the injector stage, energetic electron beams (of the order of 100 MeV) with a relatively large energy spread are generated. Some of these electrons are then further accelerated by a second, longer accelerator stage, which increases their energy to ∼0.5 GeV while reducing the relative energy spread to <5% FWHM.

Enhancement of injection and acceleration of electrons in a laser wakefield accelerator by using an argon-doped hydrogen gas jet and optically preformed plasma waveguide

A systematic experimental study on injection of electrons in a gas-jet-based laser wakefield accelerator via ionization of dopant was conducted. The pump-pulse threshold energy for producing a quasi-monoenergetic electron beam was significantly reduced by doping the hydrogen gas jet with argon atoms, resulting in a much better spatial contrast of the electron beam. Furthermore, laser wakefield electron acceleration in an optically preformed plasma waveguide based on the axicon-ignitor-heater scheme was achieved. It was found that doping with argon atoms can also lower the pump-pulse threshold energy in this experimental configuration.