The properties of laser wakefield accelerated electrons in supersonic gas flows of hydrogen and helium are investigated. At identical backing pressure, we find that electron beams emerging from helium show large variations in their spectral and spatial distributions, whereas electron beams accelerated in hydrogen plasmas show a higher degree of reproducibility. In an experimental investigation of the relation between neutral gas density and backing pressure, it is found that the resulting number density for helium is ∼30% higher than for hydrogen at the same backing pressure. The observed differences in electron beam properties between the two gases can thus be explained by differences in plasma electron density. This interpretation is verified by repeating the laser wakefield acceleration experiment using similar plasma electron densities for the two gases, which then yielded electron beams with similar properties.

The dynamics of electron acceleration driven by laser wakefield inside a 30.5 mm long dielectric capillary tube is analyzed using radiation emitted in the x-ray range. 3D particle-in-cell simulations, performed with parameters close to the experimental ones, show that in long plasmas, the accelerated electrons catch up and finally overrun the driving laser owing to a higher velocity of the electrons in the plasma. The electrons are then transversely scattered by the laser pulse, and penetrate the capillary wall where they generate bremsstrahlung radiation, modeled using GEANT4 simulations. The signature of bremsstrahlung radiation is detected using an x-ray camera, together with the betatron radiation emitted during electron acceleration in the plasma bubble. The reflection of betatron radiation from the inner capillary surface also accounts for a fraction of the observed signal on the x-ray camera. The simulation results are in agreement with the experimental ones and provide a detailed description of the electron and radiation properties, useful for the design of laser wakefield accelerators or radiation sources using long plasma media.

Laser wakefield electron acceleration in the blow-out regime and the associated betatron X-ray radiation were investigated experimentally as a function of the plasma density in a configuration where the laser is guided. Dielectric capillary tubes were employed to assist the laser keeping self-focused over a long distance by collecting the laser energy around its central focal spot. With a 40 fs, 16 TW pulsed laser, electron bunches with tens of pC charge were measured to be accelerated to an energy up to 300 MeV, accompanied by X-ray emission with a peak brightness of the order of 1021 ph/s/mm2/mrad2/0.1%BW. Electron trapping and acceleration were studied using the emitted X-ray beam distribution to map the acceleration process; the number of betatron oscillations performed by the electrons was inferred from the correlation between measured X-ray fluence and beam charge. A study of the stability of electron and X-ray generation suggests that the fluctuation of X-ray emission can be reduced by stabilizing the beam charge. The experimental results are in good agreement with 3D particle-in-cell (PIC) simulation.

A novel approach to implement and control electron injection into the accelerating phase of a laser wakefield accelerator is presented. It utilizes a wire, which is introduced into the flow of a supersonic gas jet creating shock waves and three regions of differing plasma electron density. If tailored appropriately, the laser plasma interaction takes place in three stages: Laser self-compression, electron injection, and acceleration in the second plasma wave period. Compared to self-injection by wave breaking of a nonlinear plasma wave in a constant density plasma, this scheme increases beam charge by up to 1 order of magnitude in the quasimonoenergetic regime. Electron acceleration in the second plasma wave period reduces electron beam divergence by � 25%, and the localized injection at the density downramps results in spectra with less than a few percent relative spread.

Laser wakefield is generated inside capillary tubes in order to study the conditions for self-injection of plasma electrons and their acceleration inside a large domain of parameters. Dielectric capillary tubes are employed to guide the laser pulse and collect laser energy around the central focal spot to favor laser propagation. Electrons are observed to be self-injected and accelerated to the 200 MeV range when a peak laser intensity as low as 5x10(17) W/cm(2) is used. X-rays emitted by betatron radiation constitute a precise diagnostic of the electron acceleration process. Furthermore, the peak brightness of X-rays is increased to 10(21) ph/s/mm(2)/mrad(2)/0.1%BW when the laser pulse is focused to 5x10(18) W/cm(2), which is about 30 times higher than the value obtained by using a 2 mm gas jet.

Electrons accelerated in the nonlinear regime in a laser wakefield accelerator experience transverse oscillations inside the plasma cavity, giving rise to ultra-short pulsed x-rays, also called the betatron radiation. We show that the fluence of x-ray can be enhanced by more than one order of magnitude when the laser is guided by a 10 mm long capillary tube instead of interacting with a 2 mm gas jet. X-rays with a synchrotron-like spectrum and associated critical energy ∼5 keV, with a peak brightness of ∼1×1021 ph/s/mm2/mrad2/0.1%BW, were achieved by employing 16 TW laser pulses.

The beam pointing of a multi-terawatt laser [corrected] is stabilized on a millisecond time scale using an active control system. Two piezo mirrors, two position sensing detectors, and a computer based optimization program ensure that both near- and far-field are stable, even during single shot operation. A standard deviation for the distribution of laser shots of 2.6 μ rad is achieved.

Electron beams and betatron X-ray radiation generated by laser wakefield acceleration in long plasma targets are studied. The targets consist of hydrogen filled dielectric capillary tubes of diameter 150 to 200 microns and length 6 to 20 mm. Electron beams are observed for peak laser intensities as low as 5×10 17 W/cm 2 . It is found that the capillary collects energy outside the main peak of the focal spot and contributes to keep the beam self-focused over a distance longer than in a gas jet of similar density. This enables the pulse to evolve enough to reach the threshold for wavebreaking, and thus trap and accelerate electrons. No electrons were observed for capillaries of large diameter (250 μm), confirming that the capillary influences the interaction and does not have the same behaviour as a gas cell. Finally, X-rays are used as a diagnostic of the interaction and, in particular, to estimate the position of the electrons trapping point inside the capillary.

A general approach to the modifications of the spectrum of a laser pulse interacting with matter is elaborated and used for spectral diagnostics of laser wakefield generation in guiding structures. Analytical predictions of the laser frequency red shift due to the wakefield excited in a capillary waveguide are confirmed by self-consistent modeling results. The role of ionization blue shift, and nonlinear laser pulse and wakefield dynamics on the spectrum modification, is analyzed for recent experiments on plasma wave excitation by an intense laser pulse guided in hydrogen-filled glass capillary tubes up to 8 cm long. The dependence of the spectral frequency shift, measured as a function of filling pressure, capillary tube length and incident laser energy, is in excellent agreement with the simulation results, and the associated longitudinal accelerating field is in the range 1-10 GV m(-1).

L'interaction d'une impulsion laser intense et de courte duree avec un plasma permet de produire une onde de plasma de grande amplitude dans son sillage, auquel est associe un champ electrique longitudinal. Celui-ci peut etre utilise pour accelerer des electrons relativistes injectes dans l'onde jusqu'a de grandes energies - de l'ordre du GeV - sur de courtes distances - quelques centimetres - au regard des distances dans les accelerateurs conventionnels. Le controle des caracteristiques du faisceau d'electrons lors du processus d'acceleration est fondamental pour realiser un etage d'acceleration laser-plasma utilisable. Le travail de these a porte sur la creation et la caracterisation d'une onde de plasma en regime faiblement non lineaire sur une longueur de plusieurs centimetres. Des tubes capillaires sont utilises pour guider le faisceau laser sur ces distances tout en maintenant une intensite suffisante (~ 1E17 W/cm²). Le faisceau laser guide ionise le gaz contenu dans le tube et cree l'onde de plasma. Un diagnostic optique reposant sur la modification du spectre de l'impulsion laser a ete utilise pour determiner l'amplitude de l'onde de plasma le long du tube. Sa dependance en fonction de la pression de remplissage du gaz, de la longueur du capillaire et de l'energie laser, a ete etudiee. Les resultats experimentaux compares aux resultats analytiques et de modelisation sont en excellent accord, et montrent que le champ electrique associe a l'onde de plasma est compris entre 1 et 10 GV/m sur une longueur allant jusqu'a 8 cm. Ce travail a permis de montrer la possibilite de creer de facon controlee une onde de plasma en regime faiblement non lineaire.

The excitation of plasma waves over a length of up to 8 cm is demonstrated using laser guiding of intense laser pulses through hydrogen-filled glass capillary tubes. The plasma waves are diagnosed by spectral analysis of the transmitted laser radiation. The dependence of the spectral redshift-measured as a function of filling pressure, capillary tube length, and incident laser energy-is in excellent agreement with simulation results. The longitudinal accelerating field inferred from the simulations is in the range of 1-10 GV/m.

This paper has been withdrawn by the author. An assumption made in the analysis is not valid under the experimental conditions.

By tailoring the wavefront of the laser pulse used in a laser-wakefield accelerator, we show that the properties of the x-rays produced due to the electron beam’s betatron oscillations in the plasma can be controlled. By creating a wavefront with coma, we find that the critical energy of the synchrotronlike x-ray spectrum can be significantly increased. The coma does not substantially change the energy of the electron beam, but does increase its divergence and produces an energy-dependent exit angle, indicating that changes in the x-ray spectrum are due to an increase in the electron beam's oscillation amplitude within the wakefield.