Laser Wakefield Electron Acceleration Research Articles

High-sensitivity optical tomography of instabilities in supersonic gas flow.

Characterization of gas targets relies largely on conventional optical techniques, providing millisecond time resolution, which not only overlooks the fluctuations occurring at shorter time scales but also often challenges the sensitivity limits of optical probing as their refractive index is close to unity. Hence, the ability to resolve these fluctuations needs to be addressed as it is paramount for accurate gas jet characterization for their applications, including laser-matter interaction in laser wakefield electron acceleration or plasma x ray sources. In this Letter, we introduce an advanced gas jet characterization system capable of visualizing fast density fluctuations by Schlieren imaging, combined with density characterization by interferometric tomography, both with increased sensitivity due to the four-pass probing configuration. We demonstrate that combining the two modalities provides a substantial advancement in achieving a comprehensive, both quantitative and qualitative, characterization of gas jets.

Laser Wakefield Electron Acceleration and Novel Radiation Sources (Invited)

Laser Wakefield Electron Acceleration and Novel Radiation Sources (Invited)

Laser wakefield acceleration of electrons in a magnetically controlled plasma

In the present study, the laser wakefield acceleration of electrons in a magnetically-controlled plasma is investigated. The results indicate that by employing a linearly-chirped laser pulse propagating through magnetized plasma with a reversed external magnetic field, higher energy electrons are obtained than with unmagnetized plasma and/or a nonchirped laser pulse. By considering an appropriate constant chirping magnitude and an axial external magnetic field, one can obtain remarkable GeV electron energies. It is also found that the effect of the external magnetic field direction on the electron energy is not sensed much in the mildly relativistic regime, while the reversed magnetic field increases the peak of electron energy gains compared with the forward magnetic field in the highly relativistic regime. For the mildly relativistic case, a peak energy of 412 MeV and a relative energy spread of 7% is obtained. In addition, for the highly relativistic case, a peak energy of 1.55 GeV is obtained.

Modeling of the driver transverse profile for laser wakefield electron acceleration at APOLLON research facility

The quality of electron bunches accelerated by laser wakefields is highly dependant on the temporal and spatial features of the laser driver. Analysis of experiments performed at APOLLON PW-class laser facility shows that spatial instabilities of the focal spot, such as shot-to-shot pointing fluctuations or asymmetry of the transverse fluence, lead to charge and energy degradation of the accelerated electron bunch. It is shown that PIC simulations can reproduce experimental results with a significantly higher accuracy when the measured laser asymmetries are included in the simulated laser's transverse profile, compared to simulations with ideal, symmetric laser profile. A method based on a modified Gerchberg–Saxton iterative algorithm is used to retrieve the laser electric field from fluence measurements in vacuum in the focal volume, and accurately reproduce experimental results using PIC simulations, leading to simulated electron spectra in close agreement with experimental results, for the accelerated charge, energy distribution, and pointing of the electron beam at the exit of the plasma.

Laser wakefield acceleration of electrons using Bessel–Gauss doughnut beams for accelerating beam guiding

A high-intensity laser pulse propagating through a gas target disturbs the uniform plasma distribution. Plasma density structures, created by high-order Bessel–Gauss beams for guiding the accelerating Gaussian beam and laser wakefield acceleration of electrons, are analysed using Wake-T and Fourier–Bessel particle-in-cell (FBPIC) simulation tools. The use of Bessel–Gauss doughnut beams increases the acceleration distance and energy of accelerated electrons up to 2.3 times at a 2 mm distance relative to the Gaussian beam of the same intensity.

Impact of He+N2 concentration on self-modulated laser wakefield acceleration driven by pulses of a few TW

Laser wakefield electron acceleration with ionization injection has rarely been studied in the low-power, self-modulated case. We performed simulations of such regimes using a mixture of He and N2 gases and driven by laser pulses with peak powers around 1 TW. Analyses show the generation of electron bunches with an average energy of up to 70 MeV, an energy spread as low as 18%, and an emittance as good as a fraction of a mm mrad. The obtained electron beam parameters lead to several trade-offs as a function of N2 concentration, allowing for many different designs.

Multi-millijoule terahertz emission from laser-wakefield-accelerated electrons

High-power terahertz radiation was observed to be emitted from a gas jet irradiated by 100-terawatt-class laser pulses in the laser-wakefield acceleration of electrons. The emitted terahertz radiation was characterized in terms of its spectrum, polarization, and energy dependence on the accompanying electron bunch energy and charge under various gas target conditions. With a nitrogen target, more than 4 mJ of energy was produced at <10 THz with a laser-to-terahertz conversion efficiency of ~0.15%. Such strong terahertz radiation is hypothesized to be produced from plasma electrons accelerated by the ponderomotive force of the laser and the plasma wakefields on the time scale of the laser pulse duration and plasma period. This model is examined with analytic calculations and particle-in-cell simulations to better understand the generation mechanism of high-energy terahertz radiation in laser-wakefield acceleration.

High-flux and bright betatron X-ray source generated from femtosecond laser pulse interaction with sub-critical density plasma.

Recent progress on betatron X-ray source enables the exploration of new physics in fundamental science; however, the application range is still limited by the source flux and brightness. In this Letter, we show the generation of more than 1 × 1012 photons (energy > 1 keV) with a peak brightness of 7.8 × 1022 photons/(s mm2 mrad2) at 0.1% bandwidth (BW) at 10 keV, driven by a femtosecond laser pulse of ≈5.5 J and a sub-critical density plasma (SCDP). The source flux is more than two orders of magnitude higher than that from typical laser wakefield electron acceleration. This method to produce high-flux and bright X-ray source would open a wide range of applications.

Supersonic gas jet stabilization in laser–plasma acceleration

Abstract Supersonic gas jets generated via a conical nozzle are widely applied in the laser wakefield acceleration of electrons. The stability of the gas jet is critical to the electron injection and the reproducibility of the wakefield acceleration. Here we discussed the role of the stilling chamber in a modified converging–diverging nozzle to dissipate the turbulence and to stabilize the gas jets. By the fluid dynamics simulations and the Mach–Zehnder interferometer measurements, the instability originating from the nonlinear turbulence is studied and the mechanism to suppress the instability is proposed. Both the numerical and experimental results prove that the carefully designed nozzle with a stilling chamber is able to reduce the perturbation by more than 10% compared with a simple-conical nozzle.

Wavefront Correction in Vacuum of SULF-1PW Laser Beamline

The focusing quality of high peak power lasers plays a crucial role in laser wakefield electron acceleration investigations. We report here an improvement in the focusing quality of the SULF-1PW laser beamline, planning to drive and generate 5~10 GeV electron beams. After the wavefront correction in vacuum with an adaptive optical system and the focusing with an f/56 off-axis parabolic mirror, near-diffraction-limited focal spots with a size of 52 × 54 μm2 at full width at half maximum are achieved, and the enclosed energy inside this size is ~36.6%. Consequently, the focused intensity of ~1.66 × 1019 W/cm2 can be achieved at 1 PW peak power. Moreover, we also examine the wavefront stability in air and vacuum, respectively. From the statistical analysis of 1900 shots of successive laser pulses at 1 Hz, we identify the wavefront fluctuation resulting from air turbulence and the better correction capacity in vacuum. This work demonstrates the importance and necessity of wavefront correction in vacuum for high peak power lasers.

Intense gamma-ray source based on focused electron beams from a laser wakefield accelerator

Laser wakefield accelerators generate ultrashort electron bunches with the capability to produce γ-rays. Here, we produce focused laser wakefield acceleration electron beams using three quadrupole magnets. Electron beams are then focused into a 3 mm lead converter to generate intense, focused bremsstrahlung γ beams. Experimental results demonstrate the generation and propagation of focused γ beams to a best focus spot size of 2.3 ± 0.1 × 2.7 ± 0.2 mm2 using a copper stack calorimeter. Monte Carlo simulations conducted using GEANT4 are in good agreement with experimental results and enable detailed examination of γ-ray generation. Simulations indicate that the focused γ beams contained 2.6 × 109 photons in the range of 100 keV to 33 MeV with an average energy of 6.4 MeV. A γ-ray intensity of 7 × 1010 W/cm2 was estimated from simulations. The generation of focused bremsstrahlung γ-ray sources can have important applications in medical imaging applications and laboratory astrophysics experiments.

Fiber-Optic Based Laser Wakefield Accelerated Electron Beams and Potential Applications in Radiotherapy Cancer Treatments

Ultra-compact electron beam technology based on laser wakefield acceleration (LWFA) could have a significant impact on radiotherapy treatments. Recent developments in LWFA high-density regime (HD-LWFA) and low-intensity fiber optically transmitted laser beams could allow for cancer treatments with electron beams from a miniature electronic source. Moreover, an electron beam emitted from a tip of a fiber optic channel could lead to new endoscopy-based radiotherapy, which is not currently available. Low-energy (10 keV–1 MeV) LWFA electron beams can be produced by irradiating high-density nano-materials with a low-intensity laser in the range of ~1014 W/cm2. This energy range could be useful in radiotherapy and, specifically, brachytherapy for treating superficial, interstitial, intravascular, and intracavitary tumors. Furthermore, it could unveil the next generation of high-dose-rate brachytherapy systems that are not dependent on radioactive sources, do not require specially designed radiation-shielded rooms for treatment, could be portable, could provide a selection of treatment energies, and would significantly reduce operating costs to a radiation oncology clinic.

Laser wakefield electron acceleration with PW lasers and future applications

Laser wakefield electron acceleration with PW lasers and future applications

Quasi-phase-matched laser wakefield acceleration of electrons in an axially density-modulated plasma channel

Quasi-phase matching in corrugated plasma channels has been proposed as a way to overcome the dephasing limitation in laser wakefield accelerators. In this study, the phase-lock dynamics of a relatively long electron bunch injected in an axially-modulated plasma waveguide is investigated by performing particle simulations. The main objective here is to obtain a better understanding of how the transverse and longitudinal components of the wakefield as well as the initial properties of the beam affect its evolution and qualities. The results indicate that the modulation of the electron beam generates trains of electron microbunches. It is shown that increasing the initial energy of the electron beam leads to a reduction in its final energy spread and produces a more collimated electron bunch. For larger bunch diameters, the final emittance of the electron beam increases due to the stronger experienced transverse forces and the larger diameter itself. Increasing the laser power improves the maximum energy gain of the electron beam. However, the stronger generated focusing and defocusing fields degrade the collimation of the bunch.

Compact spectroscopy of keV to MeV X-rays from a laser wakefield accelerator

We reconstruct spectra of secondary X-rays from a tunable 250–350 MeV laser wakefield electron accelerator from single-shot X-ray depth-energy measurements in a compact (7.5 × 7.5 × 15 cm), modular X-ray calorimeter made of alternating layers of absorbing materials and imaging plates. X-rays range from few-keV betatron to few-MeV inverse Compton to > 100 MeV bremsstrahlung emission, and are characterized both individually and in mixtures. Geant4 simulations of energy deposition of single-energy X-rays in the stack generate an energy-vs-depth response matrix for a given stack configuration. An iterative reconstruction algorithm based on analytic models of betatron, inverse Compton and bremsstrahlung photon energy distributions then unfolds X-ray spectra, typically within a minute. We discuss uncertainties, limitations and extensions of both measurement and reconstruction methods.

Gas density distribution in a clustered-gas jet produced from a supersonic slit nozzle under high backing pressure

A gas jet produced by adiabatic expansion of gas through a slit nozzle into a vacuum has been served as a target in the study of high harmonic generation or laser wakefield electron acceleration. In this work, Mach–Zehnder interferometry was utilized to obtain the gas density distribution in an argon cluster gas jet produced from a supersonic slit nozzle. The interference fringe distortion caused by the gas jet along the slit width under high backing pressure was recorded and inverted to a gas density profile. The gas backing pressure was up to 80 bars to obtain a gas jet with a higher density. It is found that the gas density in the jet is not uniform along the width direction of the slit nozzle and is the highest at the center of the jet. Along the gas jet, the highest gas density roughly decreases linearly. However, a steep density gradient is observed at P0 = 60–80 bars. Meanwhile, the highest gas density depends linearly on the gas backing pressure and the degree of dependence gradually decreases along the gas jet.

Multi-GeV Laser Wakefield Electron Acceleration with PW Lasers

Laser wakefield electron acceleration (LWFA) is an emerging technology for the next generation of electron accelerators. As intense laser technology has rapidly developed, LWFA has overcome its limitations and has proven its possibilities to facilitate compact high-energy electron beams. Since high-power lasers reach peak power beyond petawatts (PW), LWFA has a new chance to explore the multi-GeV energy regime. In this article, we review the recent development of multi-GeV electron acceleration with PW lasers and discuss the limitations and perspectives of the LWFA with high-power lasers.

Study of quasimonoenergetic electron bunch generation in self-modulated laser wakefield acceleration using TW or sub-TW ultrashort laser pulses

This work presents a study on laser wakefield electron acceleration in the self-modulated regime (SM-LWFA) using 50-fs laser pulses with energy on the mJ scale, at λ = 0.8 µm, impinging on a thin H2 gas jet. Particle-in-cell simulations were performed using laser peak powers ranging from sub-terawatt to a few terawatts and plasma densities varying from the relativistic self-focusing threshold up to values close to the critical density. The differences in the obtained acceleration processes are discussed. Results show that bunched electron beams with full charge on the nC scale and kinetic energy in the MeV range can be produced and configurations with peak density in the range 0.5–5 × 1020 atoms/cm3 generate electrons with maximum energies. In this range, some simulations generated quasimonoenergetic bunches with ∼0.5% of the total accelerated charge and we show that the beam characteristics, process dynamics, and operational parameters are close to those expected for the blowout regime. The configurations that led to quasimonoenergetic bunches from the sub-TW SM-LWFA regime allow the use of laser systems with repetition rates in the kHz range, which can be beneficial for practical applications.

Highly efficient few-cycle laser wakefield electron accelerator

A significant part of the laser wakefield acceleration (LWFA) research effort focuses on studying high-energy, quasi-monoenergetic electron beams. For other applications, such as the production and application of intense betatron x-ray radiation, Bremsstrahlung γ-rays and positron beams, the beam’s spectral quality is secondary to the number of electrons produced. This work discusses 3D particle-in-cell simulations of a highly efficient LWFA acceleration process, generating a broad spectrum of electrons, driven by a 12 TW few-cycle laser on high-density gas targets. In some cases, laser absorption in plasma exceeds 80%, and up to 27% of the driving laser energy is transferred to electrons over 20 MeV leaving the plasma. We also observe a deceleration of the accelerated beam at the plasma downramp and plasma exit, which arises from transitioning from laser-driven to beam-dominated wake, and also from the induced axial electric field. This effect is similar to magnetic vortex acceleration, where the induced axial electric field, instead of accelerating plasma ions, would slow down the opposite-charged electron beam and also a strong return current and backward electron beam.

Pulse-length Effect on Laser Wakefield Acceleration of Electrons by Skewed Laser Pulses

Laser pulse-length-related control and optimization of electron acceleration from laser wakefield acceleration (LWFA) by a shaped laser pulse is proposed to investigate in this study. The laser pulse shape changes and becomes skewed due to the variation in refractive index during propagation in plasmas. The optimum laser pulse-length for excitation of plasma wave in linear regime $(a_{0} is $\tau _{L}=\lambda _{p} / \sqrt {2\pi c} $ , but, in nonlinear regime $(a_{0}>1)$ , is not quite certain. Here, laser strength parameter $(a_{0})$ is defined as the ratio $eE_{L}/\text {m}\omega \text {c}$ . In nonlinear case, the variation in pulse skewedness at an optimized laser pulse-length produces different evolution of the wakefield that might affect the electron beam parameters. The present investigation predicts an optimized laser pulse-length (with an optimize ratio of the leading to the trailing pulse edge duration) 35 fs and laser strength parameter $a_{0}=2$ to control the electron bunch charge, emittance, and energy wakefield. Injected bunch charge is enhanced about 20%, and beam emittance is reported to reduce about 40% by employing the skewed Gaussian laser pulse in place of Gaussian pulse for LWFA. Energy-spread of accelerated electron bunch is also reduced from 18% to 6.6% for $s_{\alpha }=0.45$ for 35-fs laser pulse. The electron bunch parameters can also be controlled and optimized using skewed laser pulse and optimized pulse-length.