Clément Rechatin

Controlled injection and acceleration of electrons in plasma wakefields by colliding laser pulses.

In laser-plasma-based accelerators, an intense laser pulse drives a large electric field (the wakefield) which accelerates particles to high energies in distances much shorter than in conventional accelerators. These high acceleration gradients, of a few hundreds of gigavolts per metre, hold the promise of compact high-energy particle accelerators. Recently, several experiments have shown that laser-plasma accelerators can produce high-quality electron beams, with quasi-monoenergetic energy distributions at the 100 MeV level. However, these beams do not have the stability and reproducibility that are required for applications. This is because the mechanism responsible for injecting electrons into the wakefield is based on highly nonlinear phenomena, and is therefore hard to control. Here we demonstrate that the injection and subsequent acceleration of electrons can be controlled by using a second laser pulse. The collision of the two laser pulses provides a pre-acceleration stage which provokes the injection of electrons into the wakefield. The experimental results show that the electron beams obtained in this manner are collimated (5 mrad divergence), monoenergetic (with energy spread <10 per cent), tuneable (between 15 and 250 MeV) and, most importantly, stable. In addition, the experimental observations are compatible with electron bunch durations shorter than 10 fs. We anticipate that this stable and compact electron source will have a strong impact on applications requiring short bunches, such as the femtolysis of water, or high stability, such as radiotherapy with high-energy electrons or radiography for materials science.

Particle-in-Cell modelling of laser-plasma interaction using Fourier decomposition

A new Particle-in-Cell code developed for the modelling of laser-plasma interaction is presented. The code solves Maxwell equations using Fourier expansion along the poloidal direction with respect to the laser propagation axis. The goal of the code is to provide a three-dimensional description of the laser-plasma interaction in underdense plasmas with computational load similar to bidimensional calculations. Code results are successfully compared with three-dimensional calculations.

Injection and acceleration of quasimonoenergetic relativistic electron beams using density gradients at the edges of a plasma channel

The injection of quasimonoenergetic electron beams into a laser wakefield accelerator is demonstrated experimentally using density gradients at the edges of a plasma channel. In the experiment, two laser pulses are used; the main laser pulse drives a wakefield, while a second countercrossing laser beam produces a plasma whose expansion creates a channel with significant density gradients. Local injection of electrons in the wakefield is triggered by wave breaking in the density ramp. The injection is localized spatially and leads to the generation of collimated and narrow energy spread relativistic electron beams at the 100 MeV level, with charges in the range of 20–100 pC. The stability of this injection process is compared to the stability of the colliding pulse injection process and is found to be inferior for our experimental conditions. On the other hand, it is found that as the electron beam divergence is smaller in the case of gradient injection, the transverse emittance might be better.

Treatment planning for laser-accelerated very-high energy electrons

In recent experiments, quasi-monoenergetic and well-collimated very-high energy electron (VHEE) beams were obtained by laser-plasma accelerators. We investigate their potential use for radiation therapy. Monte Carlo simulations are used to study the influence of the experimental characteristics such as beam energy, energy spread and initial angular distribution on the dose distributions. It is found that magnetic focusing of the electron beam improves the lateral penumbra. The dosimetric properties of the laser-accelerated VHEE beams are implemented in our inverse treatment planning system for intensity-modulated treatments. The influence of the beam characteristics on the quality of a prostate treatment plan is evaluated. In comparison to a clinically approved 6 MV IMRT photon plan, a better target coverage is achieved. The quality of the sparing of organs at risk is found to be dependent on the depth. The bladder and rectum are better protected due to the sharp lateral penumbra at low depths, whereas the femoral heads receive a larger dose because of the large scattering amplitude at larger depths.

Laser-driven accelerators by colliding pulses injection: A review of simulation and experimental results

A review of recent simulation and experimental studies of the colliding pulse injection scheme is presented. One dimensional particle in cell simulations show that when the colliding pulses have parallel polarizations, the dominant effects that have to be considered for modeling electron injection in plasma waves are (i) stochastic heating and (ii) wakefield inhibition at the collision. With cross polarized pulses, injection of an electron beam is still possible because stochastic heating still occurs. However, it is found numerically that the injection threshold is higher in this case. The simulations also underline the possibility of tuning the electron beam parameters by modifying the injection laser pulse. Experiments (i) validate these scenarios and show that stable and high quality electron beams are produced when two counterpropagating laser pulses collide in an underdense plasma and (ii) confirm very clearly the existence of a threshold for injection, which is higher with cross polarized pulses than with parallel polarized pulses.

Observation of beam loading in a laser-plasma accelerator.

Beam loading is the phenomenon which limits the charge and the beam quality in plasma based accelerators. An experimental study conducted with a laser-plasma accelerator is presented. Beam loading manifests itself through the decrease of the beam energy, the reduction of dark current, and the increase of the energy spread for large beam charge. 3D PIC simulations are compared to the experimental results and confirm the effects of beam loading. It is found that, in our experimental conditions, the trapped electron beams generate decelerating fields on the order of 1 (GV/m)/pC and that beam loading effects are optimized for trapped charges of about 20 pC.

Simulation of quasimonoenergetic electron beams produced by colliding pulse wakefield acceleration

The collision of two laser pulses can inject electrons into a wakefield accelerator, and has been found to produce stable and tunable quasimonoenergetic electron beams [J. Faure et al., Nature 444, 737 (2006)]. This colliding pulse scheme is studied here with 3D particle-in-cell simulations. The results are successfully compared with experimental data, showing the accuracy of the simulations. The involved mechanisms (laser propagation, wake inhibition, electron heating and trapping, beam loading) are presented in detail. We explain their interplay effects on the beam parameters. The experimental variations of beam charge and energy with collision position are explained.

Compact and high-quality gamma-ray source applied to 10 μm-range resolution radiography

Gamma-ray beams with optimal and tuneable size, temperature, and dose are of great interest for a large variety of applications. These photons can be produced by the conversion of energetic electrons through the bremsstrahlung process in a dense material. This work presents the experimental demonstration of 30 μm resolution radiography of dense objects using an optimized gamma-ray source, produced with a high-quality electron beam delivered by a compact laser-plasma accelerator.

Characterization of the beam loading effects in a laser plasma accelerator

In this study, electrons were injected into a laser plasma accelerator using colliding laser pulses. By varying the parameters of the injection laser pulse, the amount of charge accelerated in the plasma wave could be controlled. This external control of the injected load was used to investigate beam loading of the accelerating structure and especially its influence on the electron beam energy and energy spread. Information on the accelerating structure and bunch duration was then derived from these experimental observations.

Plasma wake inhibition at the collision of two laser pulses in an underdense plasma

An electron injector concept for a laser-plasma accelerator was developed by E. Esarey et al. [Phys. Rev. Lett. 79, 2682 (1997)] and G. Fubiani et al. [Phys. Rev. E 70, 016402 (2004)]; it relies on the use of counterpropagating ultrashort laser pulses. In the latter work, the scheme is as follows: the pump laser pulse generates a large-amplitude laser wakefield (plasma wave). The counterpropagating injection pulse interferes with the pump laser pulse to generate a beatwave pattern. The ponderomotive force of the beatwave is able to inject plasma electrons into the wakefield. In this paper, this injection scheme is studied using one-dimensional Particle-in-Cell simulations. The simulations reveal phenomena and important physical processes that were not taken into account in previous models. In particular, at the collision of the laser pulses, most plasma electrons are trapped in the beatwave pattern and cannot contribute to the collective oscillation supporting the plasma wave. At this point, the fluid app...