The work described in this thesis is concerned with the experimental investigation of the acceleration of high energy proton pulses generated by relativistic laser-plasma interaction and their application. Using the high intensity 150 TW Ti:sapphire based ultra-short pulse laser Draco, a laser-driven proton source was set up and characterized. Conducting experiments on the basis of the established target normal sheath acceleration (TNSA) process, proton energies of up to 20 MeV were obtained. The reliable performance of the proton source was demonstrated in the first direct and dose controlled comparison of the radiobiological effectiveness of intense proton pulses with that of conventionally generated continuous proton beams for the irradiation of in vitro tumour cells. As potential application radiation therapy calls for proton energies exceeding 200 MeV. Therefore the scaling of the maximum proton energy with laser power was investigated and observed to be near-linear for the case of ultra-short laser pulses. This result is attributed to the efficient predominantly quasi-static acceleration in the short acceleration period close to the target rear surface. This assumption is furthermore confirmed by the observation of prominent non-target-normal emission of energetic protons reflecting an asymmetry in the field distribution of promptly accelerated electrons generated by using oblique laser incidence or angularly chirped laser pulses. Supported by numerical simulations, this novel diagnostic reveals the relevance of the initial prethermal phase of the acceleration process preceding the thermal plasma sheath expansion of TNSA. During the plasma expansion phase, the efficiency of the proton acceleration can be improved using so called reduced mass targets (RMT). By confining the lateral target size which avoids the dilution of the expanding sheath and thus increases the strength of the accelerating sheath fields a significant increase of the proton energy and the proton yield was observed.

Laser-Plasma-Beschleunigung mittels ultraintensiver Laserstrahlung ist eine vielversprechende Technologie für die Entwicklung kompakter Strahlungsquellen. Diese werden in einem breiten Spektrum technischer Anwendungsfälle genutzt, zum Beispiel zur Krebstherapie, in der Laborastrophysik und für die Trägheitsfusion, weshalb viele interdisziplinären Forschungsfelder ein großes Interesse an ihrer Entwicklung haben. Die ersten Machbarkeitsstudien zur Nutzung gepulster Protonenstrahlung zur Tumorbehandlung haben bereits erfreuliche Ergebnisse geliefert. Dennoch lagen die erzielten Parameter des Protonenstrahls weit unter den erwarteten Werten. Die bekannten Faktoren, die diese Performance einschränken, wurden fast ausschließlich durch Simulationen identifiziert. Der experimentelle Zugang zur Laser-Plasma-Wechselwirkung ist bisher auf die Auswertung der resultierenden Strahlung und auf makroskopische Oberflächeneffekte beschränkt, die mit optischen Messtechniken untersucht werden können. Diese Diagnostiken liefern allerdings keinerlei Informationen über die Vorgänge im Inneren des Plasmas, die letztlich die Parameter der beschleunigten Protonen bestimmen. Diese Prozesse werden in ihrer Größe und Zeitskala durch die Plasmaoszillation bzw. deren Frequenz und Wellenlänge bestimmt. Das Ziel dieses Forschungsprojekts war es, diese Lücke in der Auflösung bestehender Messmethoden zu schließen und eine Diagnostik zu entwickeln, die in der Lage ist, nanoskopische Plasma-Phänomene im Inneren der lasergetriebenen Probe zu untersuchen. Dieses Ziel konnten wir durch die Einführung von Röntgenkleinwinkelstreuung (SAXS) in Laserexperimenten an Röntgen-Freie-Elektronen-Lasern (XFELs) erreichen. In dieser Arbeit erläutere ich das technische Design und die methodische Auswertung des ersten dedizierten SAXS Experiments, das an der Matter in Extreme Conditions Messstation (auch MEC, Materie unter extremen Bedingungen) der Linac Coherent Light Source (auch LCLS, Linearbeschleuniger als kohärente Lichtquelle) durchgeführt wurde. Dieses Experiment war vorrangig eine Machbarkeitsstudie, die als Basis für die weitere Verwendung von SAXS in Laserexperimenten dienen soll. Meine Arbeit wird ausführlich die dafür nötigen experimentellen Techniken, den Aufbau, die Reinigung des gemessenen Beugungsbilds, das Probendesign und den Auswerteprozess erläutern. Um die experimentelle Durchführbarkeit dieser Methode zu testen, nutzten wir SAXS, um die Ausbreitung einer nanostrukturierten Probe in der Zeit kurz vor und während des Beginns des Laserpulses zu messen. Der Ausbreitungsparameter, den wir so aus den experimentellen Daten gewinnen konnten, liegt im einstelligen Nanometer- und teilweise im Subnanometer-Bereich und stimmte gut mit den Ergebnissen einer Particle In Cell (PIC) Simulation zur frühen Ausbreitungsphase überein. Dies zeigt, dass SAXS in der Lage ist, Plasma Prozesse zu messen, die für andere Diagnostiken bisher nicht zugänglich waren. Außerdem beobachteten wir eine Abweichung der experimentellen Daten von dem von uns entwickelten Modell zur Beschreibung der ungehinderten Ausbreitung des Plasmas ins Vakuum. Dies veranlasste uns zu einer genaueren Untersuchung der Ausbreitung mittels PIC Simulation und tatsächlich sahen wir darin die Bildung von Plasma-Strömen, die auch in der SAXS-Auswertung qualitativ bestätigt werden konnten. Die Komplexität des Ausbreitungsprozesses, die wir in diesem Forschungsprojekt aufdecken konnten, zeigt, dass weitere Studien dazu durchgeführt werden sollten. Wenn wir die Ergebnisse der hier präsentierten SAXS Modelle nutzen, um unser Verständnis des Effekts von Vorpulsen und Intensitäts-Plateaus auf die Protonenbeschleunigung mit nanostrukturierten Proben zu verbessern, werden wir zukünftig in der Lage sein, die damit erzielten Strahlparameter zu verbessern. Der entwickelte SAXS Aufbau wurde auch an die Gegebenheiten von Experimenten zur Schockwellenverdichtung mittels Hochenergielasern angepasst und angewendet. Es gibt großes wissenschaftliches Interesse an der Entmischung von Kohlenwasserstoffen im Zustand warmer dichter Materie (WDM). Viele Laborastrophysikexperimente untersuchen das Innere von Eisriesen wie Uranus und Neptun, insbesondere den Verlauf der Phasentrennung von leichten Elementen wie Kohlenstoff und Wasserstoff, die zu Diamantregen führt. Bisher war es bei diesen Messungen nicht möglich, nanoskopische Dichteänderungen im Inneren einer dichten Probe unter extremen Bedingungen zu untersuchen. Im Rahmen dieser Forschungsarbeit wurde SAXS als ergänzende Diagnostik in Hochenergiedichte-Experimenten mit Lasern an Einrichtungen wie an der MEC Messstation und an anderen XFELs etabliert. Ich wendete bekannte SAXS Auswerteroutinen auf den besonderen Fall eines sich von Schuss zu Schuss ändernden Dichtekontrasts an. Die verschiedenen Komponenten der SAXS Daten wurden mit den Informationen korreliert, die aus anderen Diagnostiken wie Beugung und VISAR gewonnen wurden. So konnte ich durch die Auswertung der Nanodiamant-Komponente eine Schätzung der Diamantgröße und des Diamant-Volumenanteils ableiten, indem ich spezifische Modelle fittete, die auf hydrodynamischen Simulationen basieren. Zukünftig möchten wir diese experimentellen Grundlagen auch auf die Untersuchung von Flüssig-Flüssig-Entmischung leichter Elemente im WDM Zustand anwenden. In dieser Arbeit erläutere ich die von mir entwickelten Auswerteprozesse, die auf weitere Messungen angewendet werden können, sobald deren Messbereich und Sensitivität so verbessert wurde, dass die Parameter von Interesse bestimmbar sind. Dieses Projekt half dabei, SAXS als Standarddiagnostik in Forschungseinrichtungen zu etablieren, die XFELs mit Hochleistungslaserexperimenten verbinden. Es bereitet sowohl die technische als auch die methodische Grundlage für weitere Experimente. / Laser plasma acceleration with ultra-high intensity (UHI) lasers is a promising technology for building compact radiation sources. These hold immense potential for a wide array of applications including cancer therapy, laboratory astrophysics and inertial confinement fusion and there is great interest in their development in many interdisciplinary fields of research. But while proof of concept experiments using proton pulses for tumor irradiation have delivered encouraging results, the achieved proton beam parameters fell short of the originally expected values. The limiting factors to this performance have mostly been identified in simulation only. Experimental access to the interaction between the drive laser and the dense plasma is so far limited to the analysis of the emitted radiation and the macroscopic surface effects that can be probed by visible light. These diagnostics cannot provide information about the processes in the bulk of the plasma that eventually determine the properties of the accelerated particles. Their spatial and temporal domain is dominated by the plasma oscillation frequency and wavelength. The aim of this project was to bridge this resolution gap with a diagnostic that is capable of investigating nanoscopic plasma features in the bulk of a laser-driven sample on a femtosecond scale. This was achieved by establishing the use of Small Angle X-Ray Scattering (SAXS) at UHI laser experiments at X-Ray Free Electron Lasers. My thesis will outline the technical design and scientific analysis of the first dedicated SAXS experiment at the Matter in Extreme Conditions (MEC) instrument of the Linac Coherent Light Source. The primary goal of the experiment was proof of concept as a foundation for regular use of SAXS in UHI experiments in the future. I will discuss the experimental procedures, the setup, the cleaning of the diffraction pattern, the target design and the analysis process that were developed for this new diagnostic in detail. To test the feasibility of this method, we used SAXS to measure the expansion of a nanostructured target in the femtosecond time span before and around the onset of a low intensity drive laser pulse. The expansion parameter that was extracted from the experimental data is in the in the sub- to single nanometer range and was in good agreement with the results of a particle-in-cell (PIC) simulation describing the early expansion phase. This demonstrates that SAXS is capable of measuring plasma processes on scales that were previously unobtainable by other diagnostics. We also identified a deviation of the experimental data from the simple model that we developed to describe an unobstructed expansion of plasma into vacuum. This lead us to examine the expansion in more detail via PIC simulation and indeed we discovered the formation of plasma jets at a later phase of the plasma expansion in simulation for a grating target. This additional effect was confirmed qualitatively by the SAXS analysis. The complexity of the plasma expansion process for a structured target we found in this project demonstrates the need for further studies. If we use the SAXS models presented here to improve our understanding of the effect of prepulses and pedestals on proton acceleration using nanostructured targets, we can apply this knowledge to the improvement of the proton beam parameters in future developments. %Additionally the technical implementation of SAXS for UHI laser experiments was developed in the framework of this thesis and established as a useful tool for the investigation of other nanoscopic plasma features. The developed experimental setup for SAXS was also adapted and applied to laser shock compression experiments using high energy drive lasers. There is great research interest in the demixing of hydrocarbons in the Warm Dense Matter (WDM) state. Many laboratory astrophysics experiments investigate the internal structure of ice giants like Uranus and Neptune, specifically the dynamics of the phase separation of light elements like carbon and hydrogen which can result in diamond rain. So far these measurements lacked a diagnostic that is capable of probing nanoscopic density modulations in the bulk of a dense target in an extreme state of matter. SAXS allowed us to gain access to the parameters of the demixing process. In the framework of this project SAXS was established as a complementary diagnostic to the standard setup for high energy density laser experiments at the MEC instrument and at other XFELs. I applied existing SAXS analysis procedures to the special case of a density contrast that changes on every shot. The different components of the SAXS data were correlated to information from other standard diagnostics including diffraction and VISAR. I was able to quantitatively analyze the component caused by nanodiamonds and retrieved an estimate of the diamond size and volume fraction from fits to custom models that are based on hydrodynamic simulations. In the future, we would like to extend this experimental basis to the investigation of liquid-liquid demixing of light elements in the WDM state. In this thesis I will discuss the SAXS analysis procedures that I dweveloped so that they can be applied to future measurements, once the experimental range and sensitivity has been improved to retrieve the parameters of interest. This project helped to establish SAXS as a standard diagnostic at facilities combining XFELs with high power laser experiments. It is supposed to lay both the technical and methodical groundwork for further experiments.

Extreme ultraviolet lithography(EUVL) is being developed worldwide as the next generation technology to be inserted in ~ 2009 for the mass production of IC chips with feature sizes <35 nm. One major challenge to its implementation is the development of a 13.5 nm EUV source of radiation that meets the requirements of current roadmap designs of the source of illumination in commercial EUVL scanners. The light source must be debris-free, in a free-space environment with the imaging EUV optics that must provide sufficient, narrow spectral band EUV power to print 100 wafers/hr. To meet this need, extensive studies on emission from a laser plasma source utilizing tin-doped droplet target was conducted. Presented in this work, are the many optical techniques such as spectroscopy, radiometry, and imaging, that were employed to characterize and optimize emission from the laser plasma source State of the art EUV spectrographs were employed to observe the source's spectrum under various laser irradiation conditions. Comparing the experimental spectra to those from theory, has allowed the determination of the Sn ion stages responsible for emitting into the useful EUV bandwidth. Experimental results were compared to spectral simulations obtained using Collisional-Radiative Equilibrium (CRE) model, as well. Moreover, extensive measurements surveying source emission from 2 nm to 30 nm, which is the region of the electromagnetic spectrum defined as EUV, was accomplished. Absolutely calibrated metrology was employed with the Flying Circus instrument from which the source's conversion efficiency (CE)--from laser to the useful EUV energy--was characterized under various laser irradiation conditions. Hydrodynamic simulations of the plasma expansion together with the CRE model predicted the condition at which optimum conversion could be attained. The condition was demonstrated experimentally, with the highest CE to be slightly above 2%, which is the highest value among all EUV source contenders. In addition to laser intensity, the CE was found to depend on the laser wavelength. For better understanding, this observation is compared to results from simulations. Through a novel approach in imaging, the size of the plasma was characterized by recording images of the plasma within a narrow band, around 13.5 nm. The size, approximately 100 ìm, is safely within the etendue limit set by the optical elements in the EUV scanner. Finally, the notion of irradiating the target with multiple laser beams was explored for the possibility of improving the source's conversion efficiency.

Laser-Induced Breakdown Spectroscopy (LIBS) and Raman spectroscopy are still growing analytical and sensing spectroscopic techniques. They significantly reduce the time and labor cost in analysis with simplified instrumentation, and lead to minimal or no sample damage. In this dissertation, fundamental studies to improve LIBS analytical performance were performed and its fusion with Raman into one single sensor was explored. On the fundamental side, Thomson scattering was reported for the first time to simultaneously measure the electron density and temperature of laser plasmas from a solid aluminum target at atmospheric pressure. Comparison between electron and excitation temperatures brought insights into the verification of local thermodynamic equilibrium condition in laser plasmas. To enhance LIBS emission, Microwave-Assisted LIBS (MA-LIBS) was developed and characterized. In MA-LIBS, a microwave field extends the emission lifetime of the plasma and stronger time integrated signal is obtained. Experimental results showed sensitivity improvement (more than 20-fold) and extension of the analytical range (down to a few tens of ppm) for the detection of copper traces in soil samples. Finally, laser spectroscopy systems that can perform both LIBS and Raman analysis were developed. Such systems provide two types of complimentary information – elemental composition from LIBS and structural information from Raman. Two novel approaches were reported for the first time for LIBS-Raman sensor fusion: (i) an Ultra-Violet system which combines Resonant Raman signal enhancement and high ablation efficiency from UV radiation, and (ii) a Ti:Sapphire laser based NIR system which reduces the fluorescence interference in Raman and takes advantage of femtosecond ablation for LIBS.

Extreme Ultraviolet (EUV) sources rely on droplet laser plasmas for EUV generation. These sources consist of a small (30 µm diameter) droplet which is excited into plasma emitting EUV around 13.5 nm, the industry’s chosen wavelength for EUV lithography (EUVL). These sources are the best candidates for the commercialization of EUVL allowing mass production of computer chips with 32 nm or even smaller feature size. However, the biggest challenges which EUV source developers encounter today are the issues of conversion efficiency (CE) and debris. In order to satisfy the technology requirements, the source will need to meet high levels of stability, performance, and lifetime. Our tin-doped droplet plasma has demonstrated high CE and low debris resulting in long lifetime. Long term stability is obtained through the use of novel tracking techniques and active feedback. The laser plasma targeting system combines optical illumination and imaging, droplet technology innovation, advanced electronics, and custom software which act in harmony to provide complete stabilization of the droplets. Thus, a stable, debris-free light source combined with suitable collection optics can provide useful EUV radiation power. Detailed description of the targeting system and the evaluation of the system will be presented.

No description available.

Cette thèse porte sur l’étude théorique du transport d’un faisceau intense d’électrons relativistes dans une cible solide. Dans la première partie nous présentons les interprétations théoriques d’une partie des résultats d’une campagne d’expérience portant sur la production et le transport d’électrons relativistes dans une cible d’aluminium. Nous y démontrons la prédominance des e?ets collectifs sur les e?ets collisionels dans la première dizaine de microns de propagation grâce à des modèles de transports déjà existant au début de cette thèse. Ces modèles deviennent insu?sants dans le cas du transport de faisceau dans un isolant. Aussi, dans la deuxième partie, nous présentons un modèle de propagation du faisceau d’électrons relativistes dans un diélectrique incluant l’e?et de l’ionisation de la cible par le faisceau. Nous y quanti?ons les pertes d’énergies des électrons en fonction des paramètres du faisceau et du milieu environnant, et nous démontrons l’existence d’un régime de propagation pour lequel les électrons du plasma ne sont pas à l’équilibre thermodynamique local avec les ions. Ces résultats ont été comparés et con?rmés avec un code cinétique qui prend en compte l’ionisation par champ électrique et par collisions entre les électrons du plasma et les ions. Nous avons examiné la stabilité du faisceau et montré que ce dernier pouvait exciter deux types d’instabilités transverses sur des longueurs de propagation de l’ordre de 30 à 300 µm en fonction de la taille de la perturbation. / This PhD thesis is a theoretical study of high-current relativistic electron beam transport through solid targets. In the ?rst part, we present an interpretation of a part of experimental results of laser– produced electron beam transport in aluminium foil targets. We have estimated the fast electron beam characteristics and we demonstrated that the collective e?ects dominate the transport in the ?rst tens of µm of propagation. These quantitative estimates were done with the transport models already existing at the beginning of this thesis. These models are no longer su?cient in the case a fast electron beam propagation in insulator targets. Thus, in the second part, we have developed a propagation model of the beam that includes the e?ects of electric ?eld ionization and the collisional ionization by the plasma electrons. We present estimates of the electron energy loss induced by the target ionization, and we discuss its dependence on the beam and target parameters. In the case of a relatively low fast electron density, we demonstrated that the beam creates a plasma where the electons are not in a local thermodynamic equilibrium with ions. We have examined the beam stability and we demonstrated that transverse instabilities can be excited by the relativistic electron beam over the propagation distances of 30 - 300 µm depending on the perturbation wavelength.

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.

Dans le contexte de la fusion par confinement inertiel (FCI), le lissage optique est une technique utilisée pour obtenir une irradiation laser aussi homogène que possible, en modifiant les propriétés de cohérence temporelle et spatiale des faisceaux laser. L'utilisation du lissage optique est une nécessité sur les lasers de puissance comme le Laser MégaJoule (LMJ) pour limiter le développement des instabilités paramétriques issues de l'intéraction laser-plasma, et parmi elles, la rétrodiffusion Brillouin stimulée (RBS). Ces instabilités entraînent des défauts d'irradiation sur cible et peuvent aussi être une source d'endommagement dans la chaîne optique. Cependant ces techniques peuvent entraîner d'autres problèmes au niveau de la chaîne laser, tels que la conversion de modulation de phase en modulation d'amplitude (FM-AM), néfastes au bon déroulement des expériences et pouvant également endommager les chaînes laser.On comprend donc qu'il est nécessaire de trouver un compromis autour du lissage optique. L’évolution du compromis du lissage est cependant compliquée car la quantification des gains et des pertes est très difficile à établir. Ainsi, tant que la quantification n’est pas faite, le compromis n’évolue pas : le lasériste souhaite toujours moins de lissage et « l’expérimentateur » toujours plus de lissage mais aucun des deux ne peut apporter suffisamment d’éléments quantitatifs pour faire pencher la balance. Cette thèse propose donc de poser les premières briques permettant d'arriver à ce compromis pour le LMJ, à l'aide d'études théoriques et numériques.Nous comparons soigneusement le lissage longitudinal (LSSD) et transversal (TSSD) par dispersion spectrale dans une configuration de lissage idéale pour chaque cas. Avec des codes 3D, nous avons simulé la RBS dans un plasma d'or, typique des expériences de FCI et favorable au développement de la RBS. Nous montrons que, contrairement aux idées reçues, l'évolution temporelle de la RBS présente certaines différences entre les deux systèmes de lissage. Premièrement, les valeurs asymptotiques des niveaux de saturation ne sont pas tout à fait les mêmes. Avec une simple description des rayons et le calcul du gain RBS pour chaque rayon, nous avons pu expliquer cette différence. En outre, la dynamique de la RBS est également quelque peu différente. Nous avons montré que la dynamique RBS est déterminée par l'évolution temporelle des propriétés des surintensités et en particulier par la longueur d'interaction effective entre la lumière rétrodiffusée Brillouin et les points chauds. Cette longueur d'interaction effective dépend à la fois de la vitesse longitudinale et de la longueur des points chauds. En effet, la synchronisation des longueurs d'interaction effectives des deux schémas de lissage synchronise également la croissance des courbes de rétrodiffusion avant saturation.Nous montrons, également qu'il est possible de faire évoluer les paramètres de lissage du LMJ en illustrant une nouvelle façon de réduire la conversion FM-AM inévitablement présente dans les lasers de forte puissance. En répartissant le spectre total habituellement utilisé par un quadruplet (regroupement de 4 faisceaux), en deux parties de spectres identiques plus petits sur les faisceaux de gauche et de droite, la conversion FM en AM est considérablement réduite de 30% à 5% tout en maintenant la performance de lissage pour la RBS. Nous avons également montré que le temps de cohérence qui en résulte n'a aucun effet sur le niveau maximal de RBS atteint. De la même façon, il faudra étudier l'impact de ces évolutions sur d'autres instabilités telles que le diffusion Raman stimulée ou le transfert d'énergie par croisement de faisceaux. / In the context of inertial confinement fusion (ICF), optical smoothing is a technique used to obtain the most homogeneous laser irradiation possible, by modifying the temporal and spatial coherence properties of the laser beams. The use of optical smoothing is a necessity on high-power lasers such as the Laser Mégajoule (LMJ) to limit the development of parametric instabilities resulting from laser-plasma interaction, and among them, stimulated Brillouin backscattering (SBS). These instabilities lead to target irradiation defects and can also be a source of damage in the optical lines. However, these techniques can lead to other problems in the laser lines, such as the conversion of phase modulation to amplitude modulation (FM-to-AM), which is harmful to the proper conduct of the experiments and can also damage the laser optics.It is therefore a necessity to find a compromise around optical smoothing. The evolution of the smoothing compromise is however complicated because the quantification of gains and losses is very difficult to establish. Thus, as long as quantification is not done, the compromise does not evolve: the laserist always wants less smoothing and the experimentalist always more smoothing, but neither of them can bring enough quantitative elements to tip the balance. This thesis therefore proposes to lay the first groundwork for reaching this compromise for the LMJ, using theoretical and numerical studies.We carefully compare longitudinal (LSSD) and transverse (TSSD) smoothing by spectral dispersion in an ideal smoothing configuration for each case. With 3D codes, we simulated SBS in a gold plasma, typical of ICF experiments and favourable to the development of SBS. We show that, contrary to popular belief, the temporal evolution of SBS shows some differences between the two smoothing schemes. First, the asymptotic values of saturation levels are not quite the same. With a simple description using light rays and the calculation of the SBS gain for each ray, we were able to explain this difference. In addition, the dynamics of SBS are also somewhat different. We have shown that the SBS dynamics is determined by the temporal evolution of the properties of the hot-spots and in particular by the effective interaction length between the Brillouin backscattered light and the hot-spots. This effective interaction length depends on both the longitudinal velocity and the length of the hot-spots. Indeed, the synchronization of the effective interaction lengths of the two smoothing schemes also synchronizes the growth of the backscatter curves before saturation.We also show that it is possible to change the smoothing parameters of the LMJ by illustrating a new way to reduce the FM-to-AM conversion inevitably present in high-power lasers. By splitting the total spectrum usually used by a quadruplet (grouping of 4 beams) into two parts of smaller identical spectra on the left and right beams, the FM-to-AM conversion is significantly reduced from 30% to 5% while maintaining the smoothing performance for SBS. We have also shown that the resulting coherence time of the laser has no effect on the maximum level of SBS achieved. Similarly, the impact of these developments on other instabilities such as stimulated Raman scattering or crossed beam energy transfer will also need to be investigated.

Un nouveau mod`ele r´eduit pour le transport de faisceaux d’´electrons relativistes dans des solide ou des plasma denses est propos´e. Il est bas´e sur la r´esolution des deux premiers moments angulaires de l’´equation cin´etique relativiste, compl´et´es par une relation de fermeture d´eduite du principe de maximisation de l’entropie angulaire de Minerbo. Le mod`ele prend en compte aussi bien les effets collectifs du transport avec les champs ´electromagn´etiques auto g´en´er´es que les effets collisionnels li´es au ralentissement des ´electrons par collision sur les plasmons, les ´electrons li´es et les ´electrons libres du milieu ainsi que leur diffusion angulaire par collisions sur les ´electrons et les ions. Le mod`ele permet une r´esolution num´erique rapide des ´equations du transport de faisceau d’´electrons rapides tout en d´ecrivant l’´evolution cin´etique de leur fonction de distribution. Malgr´e le fait de travailler avec les grandeurs angulaires moyennes, le mod`ele a ´et´e valid´e par comparaison avec des solutions analytiques d´eriv´ees dans un cas acad´emique de transport de faisceau mono ´energ´etique et collimat´e dans un plasma dense et chaud d’Hydrog`ene ainsi qu’avec une simulation PIC hybride dans un cas r´ealiste de transport d’´electrons acc´el´er´es par laser dans une cible solide. Le mod`ele est appliqu´e `a l’´etude de l’´emission de photons Kα lors d’exp´eriences laser-plasma ainsi qu’a` la g´en´eration d’ondes de choc. / A new hybrid reduced model for relativistic electron beam transport in solids and dense plasmas is presented. It is based on the two first angular moments of the relativistic kinetic equation completed with the Minerbo maximum angular entropy closure. It takes into account collective effects with the self-generated electromagnetic fields as well as collisional effects with the slowing down of the elec- trons in collisions with plasmons, bound and free electrons and their angular scattering on both ions and electrons. This model allows for fast computations of relativistic electron beam transport while describing the kinetic distribution function evolution. Despite the loss of information concerning the angular distribution of the electron beam, the model reproduces analytical estimates in the academic case of a collimated and monoenergetic electron beam propagating through a warm and dense Hydro- gen plasma and hybrid PIC simulation results in a realistic laser-generated electron beam transport in a solid target. The model is applied to the study of the emission of Kα photons in laser-solid experiments and to the generation of shock waves.