Effect of experimental laser imperfections on laser wakefield acceleration and betatron source

Stabilization and correction of aberrated laser beams via plasma channelling

High-power laser applications, and especially laser wakefield acceleration, continue to draw attention through various research topics, and may bring many industrial applications based on compact accelerators, from ultrafast imaging to cancer therapy. However, one main step towards this is the arch issue of stability. Indeed, the interaction of a complex, aberrated laser beam with plasma involves a lot of physical phenomena and non-linear effects, such as self-focusing and filamentation. Different outcomes can be induced by small laser instabilities (i.e. laser wavefront), therefore harming any practical solution. One promising path to be explored is the use of a plasma channel to possibly guide and correct aberrated beams. Complex and costly experimental facilities are required to investigate such topics. However, one way to quickly and efficiently explore new solutions is numerical simulations, especially Particle-In-Cell (PIC) simulations if, and only if, one is confidently implementing such aberrated beams which, contrary to a Gaussian beam, do not have analytical solutions. In this research, we propose two new advancements: the correct implementation of aberrated laser beams inside a 3D PIC code, showing a great consistency, under vacuum, compared to the calculations with Fresnel theory); and the correction of their quality via the propagation inside a plasma channel. We demonstrate improvements in the beam pattern, becoming closer to a single plasma mode with less distortions, and thus suggesting a better stability for the targeted application. Through this confident calculation technique for distorted laser beams, we are now expecting to proceed with more accurate PIC simulations, closer to experimental conditions, and obtained results with plasma channels indicate promising future research.

Effect of aberrations on the beam quality factor of Hermite-Gauss beams

The effect of aberrations on the beam quality factor (M2) of Hermite-Gauss (HG) beams is examined. Using the method of moments, we derive closed-form analytical expressions of M2 due to astigmatism and spherical aberration. Our analysis reveals that the radius of the HG beams plays a significant role in determining the effect of the aberrations on M2. For each aberration, we establish a critical width that separates the region where M2 changes infinitesimally from the region where it changes sharply. The analytical results are validated through numerical simulations.

Notable improvements on LWFA through precise laser wavefront tuning

Laser wakefield acceleration (LWFA) continues to grow and awaken interest worldwide, especially as in various applications it approaches performance comparable to classical accelerators. However, numerous challenges still exist until this can be a reality. The complex non-linear nature of the process of interaction between the laser and the induced plasma remains an obstacle to the widespread LWFA use as stable and reliable particle sources. It is commonly accepted that the best wavefront is a perfect Gaussian distribution. However, experimentally, this is not correct and more complicated ones can potentially give better results. in this work, the effects of tuning the laser wavefront via the controlled introduction of aberrations are explored for an LWFA accelerator using the shock injection configuration. Our experiments show the clear unique correlation between the generated beam transverse characteristics and the different input wavefronts. The electron beams stability, acceleration and injection are also significantly different. We found that in our case, the best beams were generated with a specific complex wavefront. A greater understanding of electron generation as function of the laser input is achieved thanks to this method and hopes towards a higher level of control on the electrons beams by LWFA is foreseen.

X-ray phase contrast imaging of spherical capsules

We demonstrate that a laser-based synchrotron X-ray source can be used to image and characterize in a single laser shot spherical capsules similar to ICF targets. Thus, we establish this source potential for real-time ultrafast imaging of the ICF laser driver interaction with the target. To produce the X-ray beam we used a 160 TW high power laser system with 3.2 J and 20 fs incident on a supersonic gas jet target at 2.5 Hz repetition rate. We produced 2.7 × 10⁹ photons/0.1% BW/sr/shot at 10 keV with a critical energy E_c = 15.1 keV. In our experimental conditions the spatial resolution was 4.3 μm in the object plane. We show that it is feasible to image the capsule structure and experimentally retrieve the phase information.

Laser-based synchrotron X-ray radiation experimental scaling

We review the results obtained in several experimental campaigns with the INRS high-power laser system and determine the X-ray emission scaling from synchrotron radiation produced during laser wakefield acceleration (LWFA) of electrons. The physical processes affecting the generation of intense and stable X-ray beams during the propagation phase of the high-intensity ultrashort pulse in the gas jet target are discussed. We successfully produced stable propagation in the gas jet target of a relativistic laser pulse through self-guiding on length larger than the dephasing and depletion lengths, generating very intense beams of hard X-rays with up to 200 TW on target. The experimental scaling law obtained for the photon yield in the 10-40 keV range is presented and the level of X-ray emission at the 1 PW laser peak power level, now available at several laser facilities, is estimated.

Laser and electron deflection from transverse asymmetries in laser-plasma accelerators

We report on the deflection of laser pulses and accelerated electrons in a laser-plasma accelerator (LPA) by the effects of laser pulse front tilt and transverse density gradients. Asymmetry in the plasma index of refraction leads to laser steering, which can be due to a density gradient or spatiotemporal coupling of the laser pulse. The transverse forces from the skewed plasma wave can also lead to electron deflection relative to the laser. Quantitative models are proposed for both the laser and electron steering, which are confirmed by particle-in-cell simulations. Experiments with the BELLA Petawatt Laser are presented which show controllable 0.1-1 mrad laser and electron beam deflection from laser pulse front tilt. This has potential applications for electron beam pointing control, which is of paramount importance for LPA applications.

Adaptive control of laser-wakefield accelerators driven by mid-IR laser pulses

There has been growing interest both in studying high intensity ultrafast laser plasma interactions with adaptive control systems as well as using long wavelength driver beams. We demonstrate the coherent control of the dynamics of laser-wakefield acceleration driven by ultrashort (∼ 100 fs) mid-infrared (∼ 3.9 μm) laser pulses. The critical density at this wavelength is 7.3 × 10¹⁹ cm^-3, which is achievable with an ordinary gas target system. Interactions between mid-infrared laser pulses and such near-critical-density plasma may be beneficial due to much higher absorption of laser energy. In addition, the normalized vector potential of the laser field a₀ increases with longer laser wavelength, lowering the required peak laser intensity to drive non-linear laser-wakefield acceleration. Here, MeV level, collimated electron beams with non-thermal, peaked energy spectra are generated. Optimization of electron beam qualities are realized through adaptive control of the laser wavefront. A genetic algorithm controlling a deformable mirror improves the electron total charge, energy spectra, beam pointing and stability at various plasma density profiles. Particle-in-cell simulations reveal that the optimal wavefront causes an earlier injection on the density up-ramp and thus higher energy gain as well as less filamentation during the interaction, which leads to the improvement in electron beam collimation and energy spectra.