A two-year experimental campaign is described during which diamond-like carbon and plastic targets with thicknesses from 20 nanometers to 15 micrometers were irradiated by the Texas Petawatt Laser. Target composition and thickness were varied to modify the specifics of the laser-matter interaction. Plasma mirrors were selectively implemented to affect the contrast of the laser system and provide additional control of the physical processes under investigation. A number of particle diagnostics were implemented to measure the distribution of laser accelerated ions and electrons. In addition, optical diagnostics were fielded to measure the intensity profile of the laser and measure the density of the target pre-plasma. The results of these experiments suggest that the Texas Petawatt laser pulse has pre-pulse and pedestal features with intensities at least 10⁻⁸ of the main pulse. Micronscale targets were able to survive these features and maintain a relatively sharp density gradient until the arrival of the main laser pulse, allowing for ion acceleration. Electron spectra measured in this configuration show an average temperature of 10 MeV, with no v angular dependence out to at least 60 degrees. By contrast, interferometric plasma density measurements and a lack of any observable ion acceleration suggest that nanoscale targets were destroyed well before the main pulse. In this case, the peak of the laser pulse interacted with a cloud of plasma between 10⁻³ and 10⁻² of critical density. The contrast improvement offered by the implementation of plasma mirrors was seen to increase the maximum energy of laser accelerated protons from targets thicker than 1 micrometer. In addition, the plasma mirrors allowed nanoscale targets to survive pre-pulse and pedestal features and support the production of ion beams. Proton spectra show that ions were accelerated to greater maximum energies from nanoscale targets than from more traditional micron-scale targets. This effect can be attributed to a reduction in the target pre-plasma scale length upon the introduction of plasma mirrors. These results indicate that the manipulation of target properties and laser contrast can significantly affect the interaction between an ultrahigh intensity laser and a target. / text

De part ses nombreuses applications scientifiques et sociétales comme la radiographie protonique ou encore la protonthérapie, l’accélération d’ions par laser suscite un grand intérêt. Cette thèse s’inscrit dans ce cadre et présente une étude de l’interaction d’une impulsion laser d’intensité relativiste avec un plasma de densité modérée. Dans ce régime, le plasma est transparent à l’onde laser et les électrons oscillent à des vitesses relativistes dans le champ de l’onde incidente. Ces conditions sont favorables à un transfert efficace de l’énergie laser vers le plasma, et donc sont intéressantes pour l’accélération d’ions par laser. Ce régime permet également la création de solitons électromagnétiques et acoustiques dont les mécanismes de formation et les propriétés nécessitent une meilleur compréhension. Nous réalisons une étude détaillée de simulations Particle-In-Cell (réalisées avec le code OCEAN) de l’interaction d’une impulsion laser intense avec un plasma sous dense. Nous montrons que la diffusion Raman stimulée (SRS) dans le régime relativiste est le principal processus responsable de l’absorption de l’énergie laser par le plasma et qu’il est, en outre, très efficace puisqu’il permet de transférer près de 70 % de l’énergie de l’impulsion laser aux électrons. Cette instabilité apparaît dans des plasmas dont la densité est nettement supérieure à la densité quart-critique du fait de la diminution de la fréquence plasma électronique et se développe sur des temps très courts. Il permet ainsi un chauffage homogène des électrons tout le long de la propagation de l’impulsion laser à travers le plasma. Ces électrons participent à la détente du plasma, et créent sur ses bords raids un champ électrostatique permettant l’accélération des ions. Ces derniers gagnent 30 % de l’énergie laser initiale. Nous avons aussi développé un modèle simple qui permet de prédire et donc d’optimiser le taux de rétro-diffusion du plasma du fait du développement de l’instabilité SRS. Nous nous intéressons également à la séquence des processus permettant la formation des cavités électromagnétiques. Cette analyse souligne le rôle joué par l’instabilité modulationnelle ou de Benjamin-Feir sur le front de l’impulsion laser qui est divisée en un train de plusieurs solitons électromagnétiques. À l’aide d’une étude détaillée, nous montrons que ces solitons excitent des ondes plasmas dans leur sillage en se propageant dans le plasma, perdent de l’énergie et finissent par être piégés. Ils forment également des dépressions (cavités) des densités électroniques et ioniques du plasma. Ces cavités sont des pièges pour les champs électromagnétiques rayonnés par le plasma (par exemple du fait de l’instabilité SRS) et survivent grâce à un équilibre entre la pression de radiation des champs piégés et les pressions cinétiques électroniques à leurs bords. Ces cavités absorbent une part importante de l’énergie laser mais elles n’en conservent qu’une partie sous forme d’énergie électromagnétique piégée. Le reste de l’énergie permet l’expansion de la cavité, la génération de solitons acoustiques supersoniques et l’accélération de particules. / The laser-accelerated ions draw an increasing interest due to their potential applications and to their unique properties. This manuscript presents a study of the interaction between a relativistic intense laser pulse and a low density plasma. In this regime, the plasma is transparent to the laser pulse and electrons oscillate with relativistic velocities in the field of the incident wave. These conditions make the transfer of the laser pulse energy to the plasma efficient, and therefore are interesting for the ion acceleration. This regime generates also electromagnetic and acoustic solitons whose formation mechanisms and properties need to be better understood. We carry out a detailed analysis of Particle-In-Cell simulations (performed with the code OCEAN) of interaction of an intense laser pulse with a low density plasma.We show that the stimulated Raman scattering (SRS) is the main mechanism responsible for the absorption of laser energy in plasma. This process is very efficient : it leads to the transfer of 70 % of the laser pulse energy to electrons. This instability occurs in plasmas with a density larger than the quarter critical one due to the decrease of the electron plasma frequency and develops in a very short time scale. It leads to an homogeneous electron heating all along the distance of propagation of the laser pulse through the plasma. The ions are efficiently accelerated at the plasma edges and can get nearly 30%of the initial laser energy. This study is accompanied by a simple analytical model which is able to predict and so optimize the laser backscattering fraction due to the development of the SRS instability. We also present a sequence of stages which lead to the formation of electromagnetic cavities. This analysis highlights the role of the modulationnal or Benjamin-Feir instability in the front of the laser pulse, which is split in a train of electromagnetic solitons. Our detailed study shows that these solitons excite plasmas waves in their wake, lose energy and are finally trapped in the plasma. They lead to the formation of density depressions (cavities) which may trap the electromagnetic fields produced in the plasma (by the SRS instability, for example). These structures may survive for a long time thanks to an equilibrium of the trapped field radiation pressure and the electronic kinetic pressure at their borders. These cavities absorb an significant part of the laser energy but only a part of it is trapped inside. The remaining part is invested in the cavity expansion, generation of acoustic solitons and acceleration of charged particles.

This thesis presents results on the theoretical description of ion acceleration using ultra-short ultra-intense laser pulses. It consists of two parts. One deals with the very general and underlying description and theoretic modeling of the laser interaction with the plasma, the other part presents three approaches of optimizing the ion acceleration by target geometry improvements using the results of the first part. In the first part, a novel approach of modeling the electron average energy of an over-critical plasma that is irradiated by a few tens of femtoseconds laser pulse with relativistic intensity is introduced. The first step is the derivation of a general expression of the distribution of accelerated electrons in the laboratory time frame. As is shown, the distribution is homogeneous in the proper time of the accelerated electrons, provided they are at rest and distributed uniformly initially. The average hot electron energy can then be derived in a second step from a weighted average of the single electron energy evolution. This result is applied exemplary for the two important cases of infinite laser contrast and square laser temporal profile, and the case of an experimentally more realistic case of a laser pulse with a temporal profile sufficient to produce a preplasma profile with a scale length of a few hundred nanometers prior to the laser pulse peak. The thus derived electron temperatures are in excellent agreement with recent measurements and simulations, and in particular provide an analytic explanation for the reduced temperatures seen both in experiments and simulations compared to the widely used ponderomotive energy scaling. The implications of this new electron temperature scaling on the ion acceleration, i.e. the maximum proton energy, are then briefly studied in the frame of an isothermal 1D expansion model. Based on this model, two distinct regions of laser pulse duration are identified with respect to the maximum energy scaling. For short laser pulses, compared to a reference time, the maximum ion energy is found to scale linearly with the laser intensity for a simple flat foil, and the most important other parameter is the laser absorption efficiency. In particular the electron temperature is of minor importance. For long laser pulse durations the maximum ion energy scales only proportional to the square root of the laser peak intensity and the electron temperature has a large impact. Consequently, improvements of the ion acceleration beyond the simple flat foil target maximum energies should focus on the increase of the laser absorption in the first case and the increase of the hot electron temperature in the latter case. In the second part, exemplary geometric designs are studied by means of simulations and analytic discussions with respect to their capability for an improvement of the laser absorption efficiency and temperature increase. First, a stack of several foils spaced by a few hundred nanometers is proposed and it is shown that the laser energy absorption for short pulses and therefore the maximum proton energy can be significantly increased. Secondly, mass limited targets, i.e. thin foils with a finite lateral extension, are studied with respect to the increase of the hot electron temperature. An analytical model is provided predicting this temperature based on the lateral foil width. Finally, the important case of bent foils with attached flat top is analyzed. This target geometry resembles hollow cone targets with flat top attached to the tip, as were used in a recent experiment producing world record proton energies. The presented analysis explains the observed increase in proton energy with a new electron acceleration mechanism, the direct acceleration of surface confined electrons by the laser light. This mechanism occurs when the laser is aligned tangentially to the curved cone wall and the laser phase co-moves with the energetic electrons. The resulting electron average energy can exceed the energies from normal or oblique laser incidence by several times. Proton energies are therefore also greatly increased and show a theoretical scaling proportional to the laser intensity, even for long laser pulses.