Cette thèse présente une série de diagnostiques tir-par-tir non invasifs pour des paquets d'électrons produits par un accélérateur laser-plasma (LPA). Trois phénomènes d'injection du LPA sont caractérisés : auto-injection canalisée et autoguidée, injection dans une rampe plasma et injection par collision de pulses laser. De nouvelles techniques sont démontrées : simplification des mesures de densité en utilisant un détecteur de front d'onde multiplie la sensitivité par 8, le fort couplage spatiotemporel du pulse THz focalisé est démontré par convolution des champs électriques (TEX) de deux pulses sondes et confirme la double structure du paquet observée avec le spectromètre à électrons, et des émittances transverses normalisées de 0.1 mm mrad sont démontrées pour des électrons de 0.5 GeV produits dans un LPA à capillaire en caractérisant la radiation bétatron émise par les électrons à l'intérieur du plasma en utilisant une nouvelle technique de spectrométrie X tir-par-tir.

Le contrôle au moyen d'impulsions laser ultracourtes de la transformation collective et concertée de molécules à l'état solide pouvant induire une commutation ultrarapide de l'état macroscopique d'un matériau offre des perspectives nouvelles. L'objectif est de réaliser à l'échelle du matériau ce qui est réalisé à l'échelle moléculaire en femtochimie. Ces processus sont hautement non linéaires et coopératifs, pouvant conduire à une auto-amplification et une auto-organisation au sein du matériau, et donc à une transition de phase photo-induite vers un nouvel ordre à longue distance (structural, magnétique, ferroélectrique...). Deux familles de matériaux ont été ici étudiées: tout d'abord, des matériaux à transition de spin, passant d'un état diamagnétique à paramagnétique, sous l'effet de la température, ou sous irradiation laser continue. Il s'agit de matériaux photo-actifs prototypes de la bistabilité moléculaire à l'état solide, dont la commutation est étudiée lors d'expériences de diffraction X, de réflectivité optique et de magnétisme. Une seconde partie des études a porté sur des complexes moléculaires à transfert de charge qui sont des composés prototypes pour les transitions de phase photo-induites ultra-rapides: neutre-ionique, isolant-métal... En plus des expériences d'optique temporelle ultra-rapide, la cristallographie X résolue en temps constitue une technique clé permettant de suivre au niveau atomique les différentes étapes de la transformation photo-induite et par conséquent d'observer les mécanismes mis en jeu. Ainsi, nous avons pu mettre en évidence un processus de photo-formation de nanodomaines unidimensionnels d'excitations de transfert de charge relaxées structuralement, pilotant la transition de phase photo-induite du TTF-CA, à l'aide de premières études de diffusion diffuse résolue en temps. Une nouvelle source laser-plasma femtoseconde et un dispositif de spectroscopie pompe sonde optique à détection hautement sensible ont aussi été développés dans le cadre de ce travail. Les résultats présentés dans cette étude seront une illustration des enjeux scientifiques actuels relatifs d'une part aux développements de projets de grande ampleur (nouvelles sources ultra-brèves) et d'autre part à la photo-commutation ultra-rapide.

L'accélération de protons par laser a connu une expansion exponentielle ces dernières années principalement grâce à une amélioration des lasers de puissance associée à une diminution de la taille et du coût de telles installations. Les applications envisagées de ces faisceaux sont nombreuses, tant dans le domaine médical (proton thérapie, création d'isotopes pour la TEP...) que dans le domaine énergétique (fusion inertielle, allumeur rapide...). L'interaction entre un faisceau laser intense et une cible solide permet de générer différents types de rayonnement ionisant, notamment des électrons, ions, neutrons, rayons X et protons. L'intérêt de ma thèse est de caractériser les faisceaux de protons produits par laser (divergence, energie, spectre, stabilité...) en fonction des différents paramètres laser, afin d'optimiser la conversion de l'énergie laser en protons énergétiques, pour, à plus long terme, une utilisation éventuelle de ce faisceau lors de traitements en proton thérapie. Ceci nous a amené, dans un premier temps, à développer des diagnostics adaptés pour une détection en temps réel du faisceau de protons puis, dans un deuxième temps, à ouvrir une discussion sur les paramètres laser d'intérêt intervenant dans la génération du faisceau de protons. L'énergie maximale des protons atteinte avec des impulsions courtes est de 10 MeV (LOA), en utilisant des impulsions plus longues (et plus d'énergie laser), le record est de 58 MeV (LNL). Ces résultats sont prometteurs et encourageants pour l'avenir, mais encore bien loin de la gamme 70 - 200 MeV nécessaire pour des traitements en proton thérapie.

The effect of the refraction of a laser beam propagating through three different phase objects, i.e. a laser produced plasma and two different gas media, is investigated in this thesis. It is shown that these effects have useful applications. As an introduction to the work performed, a basic discussion of the theory of light is given. In the first experimental study, the accuracy of using the Refractive Fringe Diagnostic, as a tool to determine the electron density profiles of laser produced plasmas, is investigated [Buccellato et al. (1992)]. A comparative study is performed between an established method of determining the electron density profiles of laser produced plasmas, i.e. Nomarski interferometry, and the Refractive Fringe Diagnostic, by comparing experimental data obtained from the same laser shot. For the electron density profiles investigated, it is shown that the Refractive Fringe Diagnostic over-estimates the electron density by an order of magnitude. It is suggested that the electron density errors are due to the inherent assumptions of the Refractive Fringe Diagnostic. To verify this, a numerical simulation into the accuracy of the RFD is performed on a mathematically modelled plasma. The discrepancy in the numerical results are consistent with those of the experimental results and these can be attributed to the assumptions made by the Refractive Fringe Diagnostic. Laser light refracted by a gas medium, with a specific density profile, may produce a near diffraction limited focal spot. The remaining two experimental investigations deal with two novel gas lenses: the Pulsed Gas Lens and the Colliding Shock Lens. A radially expanding cylinder of gas produces a suitable density structure to focus laser light. A design of a gas lens, the Pulsed Gas Lens, using this principle is proposed as a final focusing lens for a laser fusion power station [Buccellato et al. (1993a)]. To establish the feasibility of such a lens a proof-of- principle design for the lens is given. A numerical simulation of this lens is performed by modelling the gas flow from the lens and raytracing through the determined density profiles inside the lens. It is found that this lens can be used as a focusing element. To establish certain practical aspects of the proof-of- principle design, a beam deflection device was constructed and tested. This beam deflection device models the lensing principle of the proposed lens. The laser beam deflection observed did not match the computed deflection. The opening mechanism for the proof-of-principle design did not produce an instantaneous opening of the chamber as was assumed in the simulation. The opening mechanism must be modified to decrease the opening time. Diverging spherical shock waves, produced by pairs of opposing electrodes evenly spaced on a circumference, produce a converging cylindrically symmetric shock wave. After convergence a suitable density structure exists for near diffraction li.mited focusing to occur. It is found that the Colliding Shock Lens is a varifocal lens: the focal length and lens diameter increase with time [Buccellato et al. (1993b)]. A numerical simulation is performed to model the operation of the Colliding Shock Lens. The numerical results compare favourably with the experimental results. From the simulation it is established that the lens diameter can be scaled up by increasing the physical size of the lens and the input energy to the lens. Potential applications of the colliding shock lens are discussed. To conclude this thesis, the results of the separate investigations are summarised. / Thesis (Ph.D.)-University of Natal, 1994.

<p>Proton beam therapy has shown great promise for cancer treatment due to its high precision in irradiating tumor volumes. However, due to the massive size and expense of the cyclotrons/synchrotrons needed to accelerate the protons, the widespread use of proton therapy is limited. Laser plasma accelerated (LPA) proton beams may be a potential alternative to conventional proton beams: by shooting an ultraintense, ultrashort pulsed laser at a thin target, a plasma sheath electric field may be formed with the capability of accelerating protons to potentially therapeutic energies in very short distances. In addition to accessibility, there is significant uncertainty in proton range in heterogeneous tissues. Thermoacoustic computed tomographic (TACT) imaging has the potential to provide <em>in vivo</em> dose imaging and range verification to address these uncertainties. TACT measures thermoacoustic waves generated from the absorbed dose and implements a 3D filtered backprojection to reconstruct volumetric images of the dose. The purpose of this thesis is to determine the feasibility of integrating LPA proton beams with thermoacoustic imaging into a novel image-guided small animal therapy platform as an early step towards clinical  translation to address the issues of accessibility and dosimetric spatial uncertainty. A Monte Carlo (MC) method is used to simulate an LPA proton beam with characteristics based on literature, thermoacoustic waves are simulated on a voxel-wise basis of the MC dose, and 3D filtered backprojection is used to reconstruct a volumetric image of the dose. In Specific Aim 1, the dependence of image accuracy on transducer array angular coverage is investigated; in Specific Aim 2, an iterative reconstruction algorithm is implemented to improve image accuracy through increased sampling of projection space when transducer array angular coverage is insufficient; and in Specific Aim 3, the detector sensitivity to dose is determined for several therapeutic endpoints. The work presented in this thesis not only demonstrates the feasibility of integrating LPA and thermoacoustic technologies but necessary design changes to realize a functional small animal platform.</p>

No description available.

No description available.

Establishing precise control over the unique beam parameters of laser-accelerated ions from relativistic ultra-short pulse laser-solid interactions has been a major goal for the past 20 years. While the spatio-temporal coupling of laser-pulse and target parameters create transient phenomena at femtosecond-nanometer scales that are decisive for the acceleration performance, these scales have also largely been inaccessible to experimental observation. Computer simulations of laser-driven plasmas provide valuable insight into the physics at play. Nevertheless, predictive capabilities are still lacking due to the massive computational cost to perform these in 3D at high resolution for extended simulation times. This thesis investigates the optimal acceleration of protons from ultra-thin foils following the interaction with an ultra-short ultra-high intensity laser pulse, including realistic contrast conditions up to a picosecond before the main pulse. Advanced ionization methods implemented into the highly scalable, open-source particle-in-cell code PIConGPU enabled this study. Supporting two experimental campaigns, the new methods led to a deeper understanding of the physics of Laser-Wake eld acceleration and Colloidal Crystal melting, respectively, for they now allowed to explain experimental observations with simulated ionization- and plasma dynamics. Subsequently, explorative 3D3V simulations of enhanced laser-ion acceleration were performed on the Swiss supercomputer Piz Daint. There, the inclusion of realistic laser contrast conditions altered the intra-pulse dynamics of the acceleration process significantly. Contrary to a perfect Gaussian pulse, a better spatio-temporal overlap of the protons with the electron sheath origin allowed for full exploitation of the accelerating potential, leading to higher maximum energies. Adapting well-known analytic models allowed to match the results qualitatively and, in chosen cases, quantitatively. However, despite complex 3D plasma dynamics not being reflected within the 1D models, the upper limit of ion acceleration performance within the TNSA scenario can be predicted remarkably well. Radiation signatures obtained from synthetic diagnostics of electrons, protons, and bremsstrahlung photons show that the target state at maximum laser intensity is encoded, previewing how experiments may gain insight into this previously unobservable time frame. Furthermore, as X-ray Free Electron Laser facilities have only recently begun to allow observations at femtosecond-nanometer scales, benchmarking the physics models for solid-density plasma simulations is now in reach. Finally, this thesis presents the first start-to-end simulations of optical-pump, X-ray-probe laser-solid interactions with the photon scattering code ParaTAXIS. The associated PIC simulations guided the planning and execution of an LCLS experiment, demonstrating the first observation of solid-density plasma distribution driven by near-relativistic short-pulse laser pulses at femtosecond-nanometer resolution. / Die Erlangung präziser Kontrolle über die einzigartigen Strahlparameter von laserbeschleunigten Ionen aus relativistischen Ultrakurzpuls-Laser-Festkörper-Wechselwirkungen ist ein wesentliches Ziel der letzten 20 Jahre. Während die räumlich-zeitliche Kopplung von Laserpuls und Targetparametern transiente Phänomene auf Femtosekunden- und Nanometerskalen erzeugt, die für den Beschleunigungsprozess entscheidend sind, waren diese Skalen der experimentellen Beobachtung bisher weitgehend unzugänglich. Computersimulationen von lasergetriebenen Plasmen liefern dabei wertvolle Einblicke in die zugrunde liegende Physik. Dennoch mangelt es noch an Vorhersagemöglichkeiten aufgrund des massiven Rechenaufwands, um Parameterstudien in 3D mit hoher Auflösung für längere Simulationszeiten durchzuführen. In dieser Arbeit wird die optimale Beschleunigung von Protonen aus ultradünnen Folien nach der Wechselwirkung mit einem ultrakurzen Ultrahochintensitäts-Laserpuls unter Einbeziehung realistischer Kontrastbedingungen bis zu einer Pikosekunde vor dem Hauptpuls untersucht. Hierbei ermöglichen neu implementierte fortschrittliche Ionisierungsmethoden für den hoch skalierbaren, quelloffenen Partikel-in-Zelle-Code PIConGPU von nun an Studien dieser Art. Bei der Unterstützung zweier Experimentalkampagnen führten diese Methoden zu einem tieferen Verständnis der Laser-Wake eld-Beschleunigung bzw. des Schmelzens kolloidaler Kristalle, da nun experimentelle Beobachtungen mit simulierter Ionisations- und Plasmadynamik erklärt werden konnten. Im Anschluss werden explorative 3D3V Simulationen verbesserter Laser-Ionen-Beschleunigung vorgestellt, die auf dem Schweizer Supercomputer Piz Daint durchgeführt wurden. Dabei veränderte die Einbeziehung realistischer Laserkontrastbedingungen die Intrapulsdynamik des Beschleunigungsprozesses signifikant. Im Gegensatz zu einem perfekten Gauß-Puls erlaubte eine bessere räumlich-zeitliche Überlappung der Protonen mit dem Ursprung der Elektronenwolke die volle Ausnutzung des Beschleunigungspotentials, was zu höheren maximalen Energien führte. Die Adaptation bekannter analytischer Modelle erlaubte es, die Ergebnisse qualitativ und in ausgewählten Fällen auch quantitativ zu bestätigen. Trotz der in den 1D-Modellen nicht abgebildeten komplexen 3D-Plasmadynamik zeigt die Vorhersage erstaunlich gut das obere Limit der erreichbaren Ionen-Energien im TNSA Szenario. Strahlungssignaturen, die aus synthethischen Diagnostiken von Elektronen, Protonen und Bremsstrahlungsphotonen gewonnen wurden, zeigen, dass der Target-Zustand bei maximaler Laserintensität einkodiert ist, was einen Ausblick darauf gibt, wie Experimente Einblicke in dieses bisher unbeobachtbare Zeitfenster gewinnen können. Mit neuen Freie-Elektronen-Röntgenlasern sind Beobachtungen auf Femtosekunden-Nanometerskalen endlich zugänglich geworden. Damit liegt ein Benchmarking der physikalischen Modelle für Plasmasimulationen bei Festkörperdichte nun in Reichweite, aber Experimente sind immer noch selten, komplex, und schwer zu interpretieren. Zuletzt werden daher in dieser Arbeit die ersten Start-zu-End-Simulationen der Pump-Probe Wechselwirkungen von optischem sowie Röntgenlaser mit Festkörpern mittels des Photonenstreu-Codes ParaTAXIS vorgestellt. Darüber hinaus dienten die zugehörigen PIC-Simulationen als Grundlage für die Planung und Durchführung eines LCLS-Experiments zur erstmaligen Beobachtung einer durch nah-relativistische Kurzpuls-Laserpulse getriebenen Festkörper-Plasma-Dichte, dessen Auflösungsbereich gleichzeitig bis auf Femtosekunden und Nanometer vordrang.

No description available.

Laser-driven plasma accelerators can produce high-energy, high peak current ion beams by irradiating solid materials with ultra-intense laser pulses. This innovative concept attracts a lot of attention for various multidisciplinary applications as a compact and energy-efficient alternative to conventional accelerators. The maturation of plasma accelerators from complex physics experiments to turnkey particle sources for practical applications necessitates breakthroughs in the generated beam parameters, their robustness and scalability to higher repetition rates and efficiencies. This thesis investigates viable optimisation strategies for enhancing ion acceleration from thin foil targets in ultra-intense laser-plasma interactions. The influence of the detailed laser pulse parameters on plasma-based ion acceleration has been systematically investigated in a series of experiments carried out on two state-of-the-art high-power laser systems. A central aspect of this work is the establishment and integration of laser diagnostics and operational techniques to advance control of the interaction conditions for maximum acceleration performance. Meticulous efforts in continuously monitoring and enhancing the temporal intensity contrast of the laser system, enabled to optimise ion acceleration in two different regimes, each offering unique perspectives for applications. Using the widely established target-normal sheath acceleration (TNSA) scheme and adjusting the temporal shape of the laser pulse accordingly, proton energies up to 70 MeV were reliably obtained over many months of operation. Asymmetric laser pulses, deviating significantly from the standard conditions of an ideally compressed pulse, resulted in the highest particle numbers and an average energy gain ≥ 37 %. This beam quality enhancement is demonstrated across a broad range of parameters, including thickness and material of the target, laser energy and temporal intensity contrast. To overcome the energy scaling limitations of TNSA, the second part of the thesis focuses on an advanced acceleration scheme occurring in the relativistically induced transparency (RIT) regime. The combination of thin foil targets with precisely matched temporal contrast conditions of the laser enabled a transition of the initially opaque targets to transparency upon main pulse arrival. Laser-driven proton acceleration to a record energy of 150 MeV is experimentally demonstrated using only 22 J of laser energy on target. The low-divergent high-energy component of the accelerated beam is spatially and spectrally well separated from a lower energetic TNSA component. Start-to-end simulations validate these results and elucidate the role of preceding laser light in pre-expanding the target along with the detailed acceleration dynamics during the main pulse interaction. The ultrashort pulse duration of the laser facilitates a rapid succession of multiple known acceleration regimes to cascade efficiently at the onset of RIT, leading to the observed beam parameters and enabling ion acceleration to unprecedented energies. The discussed acceleration scheme was successfully replicated at two different laser facilities and for different temporal contrast levels. The results demonstrate the robustness of this scenario and that the optimum target thickness decreases with improved laser contrast due to reduced pre-expansion. Target transparency was found to identify the best-performance shots within the acquired data sets, making it a suitable feedback parameter for automated laser and target optimisation to enhance stability of plasma accelerators in the future. Overall, the obtained results and described optimisation strategies of this thesis may become the guiding step for the further development of laser-driven ion accelerators.:1 Introduction 1.1 Motivation 1.2 Thesis outline 2 Fundamentals of laser-matter interactions 2.1 Plasma 2.1.1 Plasma properties 2.1.2 Dispersion relation of a plasma 2.1.3 Laser propagation in a plasma 2.2 Laser-matter interactions 2.2.1 Ionisation processes 2.2.2 Electron dynamics in the laser field 2.2.3 Ponderomotive force 2.2.4 Plasma heating processes 2.3 Laser-driven ion acceleration mechanisms 2.3.1 Target normal sheath acceleration 2.3.2 Radiation pressure acceleration 2.3.3 Acceleration in the relativistically induced transparency regime 3 Methodology for high-power laser experiments 3.1 High-power lasers 3.1.1 High-power laser techniques 3.1.2 Temporal contrast of high-power laser systems 3.1.3 DRACO laser system 3.1.4 J-KAREN-P laser system 3.2 Experimental Area 3.2.1 Short-f chamber at HZDR 3.2.2 Short-f chamber at KPSI 3.3 Targets 3.4 Optical diagnostic 3.4.1 Transmitted and reflected laser light 3.4.2 Spectral phase measurements 3.5 Particle diagnostic 3.5.1 Thomson parabola spectrometer 3.5.2 Time of flight measurements 3.5.3 Spatial proton beam profiler 3.5.4 Radiochromic films 3.5.5 Nuclear activation measurements 4 Optimisation of sheath acceleration for high-quality proton beams 4.1 Introduction 4.2 Temporal contrast at experimental environment 4.3 Plasma mirror 4.3.1 Plasma mirror implementation at DRACO-PW 4.3.2 Plasma mirror characterisation at DRACO-PW 4.4 Temporal pulse shaping by spectral phase modification 4.4.1 Theory on temporal pulse shaping 4.4.2 Experimental realisation and results 4.5 Proton acceleration under optimised temporal contrast conditions 4.6 Experimental results 4.7 Discussion on numerical simulations 4.8 Conclusions 5 Enhanced ion acceleration in the relativistic transparency regime 5.1 Introduction 5.2 Experimental setup using the J-KAREN-P laser 5.3 Experimental results 5.4 Laser-induced breakdown and target pre-expansion 5.5 Elucidating ion acceleration in the relativistically induced transparency regime 5.5.1 Details on simulation methodology 5.5.2 Simulation results 5.6 Acceleration in the RIT regime for modified temporal contrast 5.6.1 Experimental setup using the DRACO-PW laser 5.6.2 Experimental results using the DRACO-PW laser 5.6.3 Simulation results for modified temporal contrast 5.7 Conclusions 6 Ion acceleration beyond the 100 MeV frontier from cascading acceleration schemes 6.1 Introduction 6.2 Experimental setup 6.3 Experimental results 6.3.1 Analysis of acceleration performance 6.3.2 Spatial proton beam profile 6.3.3 Nuclear activation measurement 6.3.4 Scaling of maximum proton energy 6.4 Numerical simulations 6.4.1 Simulation setup 6.4.2 Simulation results & discussion 6.5 Conclusions 7 Summary and outlook Appendix References