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.

Modular supersonic nozzle for the stable laser-driven electron acceleration.

The sharp density down-ramp injection (shock injection) mechanism produces the quasi-monoenergetic electron beam with a bunch duration of tens of femtoseconds via laser wakefield acceleration. The stability of the accelerated electron beam strongly depends on the stability of the laser beam and the shock structure produced by the supersonic gas nozzle. In this paper, we report the study of a newly designed modular supersonic nozzle with a flexible stilling chamber and a converging-diverging structure. The performance of the nozzle is studied both numerically and experimentally with the computational fluid dynamics simulation and the Mach-Zehnder interferometry method. The simulation results and the experimental measurements are well consistent, and both prove the effectiveness of the stilling chamber in stabilizing the gas flow.

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.

Coupling Effects in Multistage Laser Wake-field Acceleration of Electrons.

Staging laser wake-field acceleration is considered to be a necessary technique for developing full-optical jitter-free high energy electron accelerators. Splitting of the acceleration length into several technical parts and with independent laser drivers allows not only the generation of stable, reproducible acceleration fields but also overcoming the dephasing length while maintaining an overall high acceleration gradient and a compact footprint. Temporal and spatial coupling of pre-accelerated electron bunches for their injection in the acceleration phase of a successive laser pulse wake field is the key part of the staging laser-driven acceleration. Here, characterization of the coupling is performed with a dense, stable, narrow energy band of <3% and energy-selectable electron beams with a charge of ~1.6 pC and energy of ~10 MeV generated from a laser plasma cathode. Cumulative focusing of electron bunches in a low-density preplasma, exhibiting the Budker-Bennett effect, is shown to result in the efficient injection of electrons, even with a long distance between the injector and the booster in the laser pulse wake. The measured characteristics of electron beams modified by the booster wake field agree well with those obtained by multidimensional particle-in-cell simulations.

Decomposition of powerful axisymmetrically polarized laser pulses in underdense plasma.

Interaction of relativistically intense axisymmetrically polarized (radially or azimuthally polarized) laser pulses (RIAPLP) with underdense plasma is shown experimentally and theoretically to be essentially different from the interaction of conventional Gaussian pulses. The difference is clearly observed in distinct spectra of the side-scattered laser light for the RIAPLP and Gaussian pulses, as well as in the appearance of a spatially localized strong side emission of second harmonic of the laser pulse in the case of RIAPLP. According to our analysis based on three-dimensional particle-in-cell simulations, this is a result of instability in the propagation of RIAPLP in uniform underdense plasma.

Revealing the second harmonic generation in a femtosecond laser-driven cluster-based plasma by analyzing shapes of Ar XVII spectral lines.

We present experiments dealing with a femtosecond laser-driven cluster-based plasma, where by analyzing the nonlinear phenomenon of satellites of spectral lines of Ar XVII, we revealed the nonlinear phenomenon of the generation of the second harmonic of the laser frequency. For performing this analysis we developed new results in the theory of satellites of spectral lines. From such lineshape analysis we found, in particular, that the efficiency of converting the short (40 fs) intense (3x10¹8 W/cm²) incident laser light into the second harmonic was 2%. This result is in the excellent agreement with the 2-Dimensional Particle-In-Cell (2D PIC) simulation that we also performed. There is also an order of magnitude agreement between the thresholds for the SHG found from the line shape analysis and from the 2D PIC simulations.

Electron self-injection during interaction of tightly focused few-cycle laser pulses with underdense plasma.

We study the interaction of short laser pulses tightly focused in a tiny volume proportional to the cube of the pulse wavelength (lambda3) with underdense plasma by means of real-geometry particle-in-cell simulations. Underdense plasma irradiated by relatively low-energy lambda3 (and lambda2) laser pulses is shown to be an efficient source of multi-MeV electrons, approximately 50 nC/J , and coherent hard x rays, despite a strong pulse diffraction. Transverse wave breaking in the vicinity of the laser focus is found to give rise to an immense electron charge loading to the acceleration phase of a laser wake field. A strong blowout regime provoked by the injected electrons resulting in the distribution of accelerated electrons is found for lambda3 pulses (further electron acceleration driving by lambda2 pulses runs in the usual way). With an increase of pulse energy, wiggling and electron-hose instabilities in the lambda3 pulse wake are recognized in the blowout regime. For higher-energy lambda3 pulses, the injected beams are well modulated and may serve as a good source of coherent x rays.

Giant electromagnetic vortex and MeV monoenergetic electrons generated by short laser pulses in underdense plasma near quarter critical density region.

Very efficient generation of monoenergetic, about 1MeV , electrons from underdense plasma with its electron density close to the critical, when irradiated by an intense femtosecond laser pulse, is found via two dimensional particle-in-cell simulation. The stimulated Raman scattering of a laser pulse with frequency omega< or =2omega(pl max) gives rise to a giant electromagnetic vortex. In contrast to electron acceleration by the well-known laser pulse wake, injected plasma electrons are accelerated up to vortex ponderomotive potential forming a quite monoenergetic distribution. A relatively high charge of such an electron source makes very efficient generation of soft gamma rays with homega>300 keV .

Effect of external static magnetic field on the emittance and total charge of electron beams generated by laser-Wakefield acceleration.

Significant enhancement of emittance and an increase of the total charge of femtosecond electron beams produced by a 12 TW, 40 fs laser pulse, tightly focused in a He gas jet, are observed after applying a static magnetic field, B> or =0.2 T, directed along the axis of laser pulse propagation. The effect appears when plasma produced by a laser prepulse becomes magnetized in the vicinity of the focus point: the electron Larmor frequency exceeds the collisional frequency, while periphery of the plasma remains unmagnetized. The entailed change in the shape of the plasma suppresses the diffraction of the main laser pulse that results in a much higher charge of electrons self-injected during the longitudinal wave breaking of the laser wake as well as the excellent stability of the beams.

Observation of strong correlation between quasimonoenergetic electron beam generation by laser wakefield and laser guiding inside a preplasma cavity.

We use a one-shot measurement technique to study effects of laser prepulses on the electron laser wakefield acceleration driven by relativistically intense laser pulses (lambda=790 nm, 11 TW, 37 fs) in dense helium gas jets. A quasimonoenergetic electron bunch with an energy peak approximately 11.5 MeV[DeltaE/E approximately 10% (FWHM)] and with a narrow-cone angle (0.04pi mm mrad) of ejection is detected at a plasma density of 8 x 10(19) cm(-3). A strong correlation between the generation of monoenergetic electrons and optical guiding of the pulse in a thin channel produced by picosecond laser prepulses is observed. This generation mechanism is well corroborated by two-dimensional particle-in-cell simulations.

Effect of a laser prepulse on a narrow-cone ejection of MeV electrons from a gas jet irradiated by an ultrashort laser pulse.

Spatial and energy distributions of energetic electrons produced by an ultrashort, intense laser pulse with a short focal length optical system (Ti:sapphire, 12 TW, 50 fs, lambda=790 nm, f/3.5) in a He gas jet are measured. They are shown to depend strongly on the contrast ratio and shape of the laser prepulse. The wave breaking of the plasma waves at the front of the shock wave formed by a proper laser prepulse is found to make a narrow-cone (0.1pi mm mrad) electron injection. These electrons are further accelerated by the plasma wake field generated by the laser pulse up to tens of MeV forming a Maxwell-like energy distribution. In the case of nonmonotonic prepulse, hydrodynamic instability at the shock front leads to a broader, spotted spatial distribution. The numerical analysis based on a two-dimensional (2D) hydrodynamic (for the laser prepulse) and 2D particle-in-cell (PIC) simulation justifies the mechanism of electron acceleration. The PIC calculation predicts that electrons with energy from 10 to 40 MeV form a bunch with a pulse duration of about 40 fs.