A. Lifschitz

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.

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.

GeV Wakefield acceleration of low energy electron bunches using Petawatt lasers

The possibility of accelerating electrons to the GeV level using a Petawatt laser focused in a uniform plasma is investigated. The proposed scheme relies on the wakefield acceleration of an electron bunch from a state-of-the-art radio-frequency accelerator. Using an analytical model as well as numerical simulations performed with WAKE [P. Mora and T. M. Antonsen, Phys. Plasmas 4, 217 (1997)], a systematical study of the injector parameters is carried out. In particular, it is found that the quality of the accelerated electron bunch—in terms of bunch length and energy spread—depends crucially on the injection energy. Injection energies of a few MeV lead to a GeV electron beam with sub-100fs bunches and 10% energy spreads. Most of the features of the acceleration process can be explained within the linear response framework, including both the reduction of energy spread and bunch length at low injection energies. The role of nonlinear effects is discussed.

Direct observation of betatron oscillations in a laser-plasma electron accelerator

During experiments performed on a laser-plasma–based accelerator, correlation of the electron output angle with the electron energy has been observed. These spectral oscillations of the electron beam centroid are attributed to betatron oscillations of the electron beam during its propagation. An analytical model for betatron oscillations including constant longitudinal acceleration is described and used to validate the scenario and retrieve physical parameters. The oscillations can arise from an off-axis injection of the electrons, which can be reproduced using an asymmetric laser intensity profile in Particle-In-Cell (PIC) simulations. This study emphasizes the influence of non-ideal interaction conditions inherent to experiments.

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.

Laser-plasma accelerators : a new tool for science and for society

The recent and continuous development of powerful laser systems has permitted the emergence of new approaches for generating energetic electron beams. By focusing light pulses containing a few joules of energy in a few tens of femtoseconds onto gas jets, extremely large electric fields can be generated, reaching the terravolts per metre level. Such fields are 10?000 times greater than those produced in the radio-frequency cavities of conventional accelerators. As a result, the length over which electrons extracted from the target can be accelerated to hundreds of MeV is reduced to a few millimetres. The reduction of the size and the cost of laser-plasma accelerators is a promising consequence, but these electron beams also reveal original properties, which make them a wonderful tool for science. By adjusting the interaction parameters, the electron energy distribution can be tuned from a maxwellian-like distribution to a quasi-monoenergetic one. The new properties of these laser-based particle beams are well suited to many applications in different fields, including medicine (radiotherapy), chemistry (ultrafast radiolysis), material science (non-destructive material inspection using radiography) and, of course, for accelerator physics.

Electron Rephasing in a Laser-Wakefield Accelerator

An important limit for energy gain in laser-plasma wakefield accelerators is the dephasing length, after which the electron beam reaches the decelerating region of the wakefield and starts to decelerate. Here, we propose to manipulate the phase of the electron beam in the wakefield, in order to bring the beam back into the accelerating region, hence increasing the final beam energy. This rephasing is operated by placing an upward density step in the beam path. In a first experiment, we demonstrate the principle of this technique using a large energy spread electron beam. Then, we show that it can be used to increase the energy of monoenergetic electron beams by more than 50%.

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.

Shock assisted ionization injection in laser-plasma accelerators

Ionization injection is a simple and efficient method to trap an electron beam in a laser plasma accelerator. Yet, because of a long injection length, this injection technique leads generally to the production of large energy spread electron beams. Here, we propose to use a shock front transition to localize the injection. Experimental results show that the energy spread can be reduced down to 10 MeV and that the beam energy can be tuned by varying the position of the shock. This simple technique leads to very stable and reliable injection even for modest laser energy. It should therefore become a unique tool for the development of laser-plasma accelerators.