Dissertations / Theses on the topic 'High power lasers. Laser-plasma interactions'

Recent advances in the development of high power short pulse laser systems has opened a new regime of laser plasma interactions for study. The thesis is presented in two parts. In Part I, we consider the implications of these high power laser pulses for the interaction with a uniform underdense plasma, with particular regard to plasma-based accelerators. We present a scheme for the resonant excitation of large electrostatic Wakefields in these plasmas using a train of ultra-intense laser pulses. We also present an analysis of the resonant mechanism of this excitation based on consideration of phase space trajectories. In Part II, we consider the transition from linear Resonance Absorption to nonlinear absorption processes in a linear electron density profile as the intensity of the incident radiation increases and the scale length of the density profile decreases. We find that the electron motion excited by an electrostatic field exhibits some extremely complicated dynamics with bifurcations to period doubling and chaotic motion as the strength of the driving field is increased or the density scale length is decreased. We also present some results obtained from particle simulations of these interactions.

In recent years, high intensity laser-matter interactions (> 1018 W/cm2) have been shown to produce bright, compact sources of many different particles. These include x-rays, neutrons, protons and electrons, which can be used in applications such as x-ray and electron radiography. The potential use of these sources for industrial applications is promising. However, the scalability and tuning of the sources needs to be understood at a fundamental level. This thesis reports on three aspects of the development and application of these sources; the first two discuss applications of laser-plasma interactions. Firstly, the generation, characterisation and tunability of high-energy x-rays (= 200 keV) produced by the hot-electrons generated inside a solid target for the application of x-ray radiography. The characterisation of the x-ray source is conducted using a novel scintillator based absorption spectrometer. This source of x-rays was then used to radiograph a high density test object. Secondly, a novel technique of x-ray backscatter is investigated numerically and demonstrated experimentally for the first time on a laser facility. This uses the high energy electrons generated via wakefield acceleration to probe deeper into materials than traditional backscatter techniques. Finally, an investigation is reported examining the fundamental dynamics of electrons escaping from solid targets under different irradiation conditions. Experimentally, the number of escaping electrons was shown to maximise for certain laser illumination conditions; this was also explored using PIC simulations. The new results discussed in these three sections produce important new understanding of laser-driven x-ray generation and its application to penetrative probing and imaging for possible future industrial applications as well as the understanding of escaping electron dynamics.

It is possible to reach the radiation-reaction–dominated regime in today’s high-intensity laser facilities, using the collision of a wakefield-accelerated GeV electron beam with a 30 fs laser pulse of intensity 10²² Wcm^-2. This would demonstrate that the yield of high energy gamma rays is increased by the stochastic nature of photon emission: a beam of 10⁹ electrons will emit 6300 photons with energy > 700 MeV, 60 times the number predicted classically. Detecting those photons, or a prominent low energy peak in the electron beam's post-collision energy spectrum, will provide strong evidence of quantum radiation reaction; we place constraints on the accuracy of timing necessary to achieve this. This experiment would provide benchmarking for the simulations that will be used to study the plasmas produced in the next generation of laser facilities. With focused intensities > 10²³ Wcm^-2, these will be powerful enough to generate high fluxes of gamma rays and electron-positron pairs from laser–laser and laser–solid interactions. It will become possible to test the physics of exotic astrophysical phenomena, such as pair cascades in pulsar magnetospheres, and explore fundamental aspects of quantum electrodynamics (QED). To that end we will discuss: classical theories of radiation reaction; QED processes in intense fields; and a Monte Carlo algorithm by which the latter may be included in particle-in-cell codes. The feedback between QED processes and classical plasma dynamics characterises a new regime we call QED-plasma physics.

La production de faisceaux énergétiques d'ions/protons avec des impulsions laser à intensités relativistes (I>10^{18}W/cm^2) a reçu, au cours des dernières années, un intérêt croissant parmi les scientifiques travaillant dans les domaines de l'optique, de la physique des plasmas et des accélérateurs. Une fraction des électrons est chauffée à haute température lors de l'interaction entre une impulsion laser femtoseconde et un plasma surdense. Les ions et les protons sont extraits et accélérés par la séparation de charge qui est produite pendant l'expansion du plasma. Les résultats présentés dans ce manuscrit décrivent la réalisation d'expériences d'accélération d'ions avec un système laser à haute puissance et à haut contraste (XPW). Deux expériences préparatoires sont réalisées, afin d'étudier l'interaction entre le piédestal d'une impulsion laser et une cible. L'expansion d'un plasma créé par laser à intensité moyenne est mesurée par interférométrie; l'évolution de la longueur de son gradient de densité est déduite par les cartes de densité électronique, mesurées à différents instants. La variation de la réflectivité absolue d'une cible mince d'aluminium est mise en corrélation avec la température électronique afin de contrôler le débouché du choc produit par le laser. La corrélation entre les deux expériences est finalement utilisée pour définir le conditions optimales pour l'accélération des protons. Des expériences d'accélération de protons avec un laser à haut contraste, la construction et la validation d'un spectromètre (Galette a Micro-canaux et Parabole Thomson), ainsi que des autres détails sur le montage sont présentés. Les résultats ainsi obtenus montrent que l'amélioration du contraste permet d'utiliser des cibles plus minces et de produire des conditions d'interaction plus stables et contrôlables. Des faisceaux des protons ayant énergie cinétique supérieure à 4MeV sont produits, avec une stabilité tir à tir meilleure de 4% rms. L'accélération des protons avec deux impulsions laser confirme que l'absorption d'énergie laser est augmentée dans le cas des cibles pre-chauffées par une impulsion laser avec les bons paramètres
The production of energetic proton/ion beams with laser pulses at relativistic intensities (I>10^{18}W/cm^2) has received, in the past few years, increasing interest from the scientific community in plasma, optics and accelerator physics. A fraction of electrons is heated to high temperature during the ultrafast interaction between a femtosecond laser pulse and an overdense plasma. Ions and protons are extracted and accelerated by the charge separation set up during the expansion of the plasma. The results presented in this manuscript report on the realization of ion acceleration experiments using a high contrast (XPW) multi-terawatt laser system. Two preparatory experiments are set up, aiming to study the pedestal of a laser pulse interacting with the target. The expansion of a plasma created by a laser at moderate intensity is measured by interferometry; the evolution of the density gradient length is deduced from the electron density maps at different moments. The variation of the absolute reflectivity of a thin aluminium foil is correlated to the electron temperature and is used to monitor the arrival time of the laser produced shock. The crossing between the two experiments is finally used to define the optimum condition for proton acceleration. Proton acceleration experiments with high contrast laser are reported, including the construction and the validation of a real-time, single shot ion spectrometer (Micro-channel Plate and Thomson Parabola), and other details of the realised setup. The obtained results show that the increased contrast enables the use of thinner targets and the production of more stable and controllable interaction conditions. Proton beams with kinetic energy higher than 4 MeV are produced, with a shot-to-shot stability better than 4% rms. Proton acceleration experiment with two laser beams confirms that the laser energy absorption is enhanced when the target is pre-heated by a laser pulse with proper parameters

[Truncated abstract] Advanced laser interferometer gravitational wave detectors require an extremely high optical power in order to improve the coupling between the gravitational wave signal and the optical field. This high power requirement leads to new physical phenomena arising from nonlinear interactions associated with radiation pressure. In particular, detectors with multi-kilometer-long arm cavities containing high density optical fields suffer the possibility of 3-mode opto-acoustic interactions. This involves the process where ultrasonic vibrations of the test mass cause the steady state optical modes to scatter. These 3-mode interactions induce transverse optical modes in the arm cavities, which then can provide positive feedback to the acoustic vibrations in the test masses. This may result in the exponential growth of many acoustic mode amplitudes, known as Parametric Instability (PI). This thesis describes research on 3-mode opto-acoustic interactions in advanced interferometric gravitational wave detectors through numerical investigations of these interactions for various interferometer configurations. Detailed analysis reveals the properties of opto-acoustic interactions, and their dependence on the interferometer configuration. This thesis is designed to provide a pathway towards a tool for the analysis of the parametric instabilities in the next generation interferometers. Possible techniques which could be helpful in the design of control schemes to mitigate this undesirable phenomenon are also discussed. The first predictions of parametric instability considered only single interactions involving one transverse mode and one acoustic mode in a simple optical cavity. ... In Chapter 6, I was able to make use of a new analytical model due to Strigin et al., which describes parametric instability in dual recycling interferometers. To make the solution tractable, it was necessary to consider two extreme cases. In the worst case, recycling cavities are assumed to be resonant for all transverse modes, whereas in the best cases, both recycling cavities are anti-resonant for the transverse modes. Results show that, for the worst case, parametric gain values as high as ~1000 can be expected, while in the best case the gain can be as low as ~ 3. The gain is shown to be very sensitive to the precise conditions of the interferometer, emphasising the importance of understanding the behaviour of the detectors when the cavity locking deviates from ideal conditions. Chapter 7 of this thesis contains work on the observation of 3-mode interactions in an optical cavity at Gingin, which confirms the analysis presented here, and also a paper which shows how the problem of 3-mode interactions can be harnessed to create new devices called opto-acoustic parametric amplifiers. In the conclusions in Chapter 8, I discuss the next important steps in understanding parametric interactions in real interferometers – including the need for more automated codes relevant to the design requirements for recycling cavities. In particular, it is pointed out how the modal structure of power and signal recycling cavities must be understood in detail, including the Gouy phase for each transverse mode, to be able to obtain precise predictions of parametric gain. This thesis is organised as a series of papers which are published or have been submitted for publication. Such writing style fills the condition for Ph.D. thesis at the University of Western Australia.

Lors de la focalisation d’un laser femtoseconde ultra-intense [I>10¹⁶W/cm²] sur une cible solide, dès le début de l’impulsion le champ laser est suffisant pour totalement ioniser la surface de la cible. Le reste de l’impulsion est ensuite réfléchi dans la direction spéculaire par le plasma dense ainsi créé : c’est un miroir plasma. Le champ laser ultra-intense peut accélérer les électrons au sein du plasma à des vitesses relativistes. Certains sont éjectés vers le vide et ces miroirs plasmas sont ainsi des sources de faisceaux d’électrons énergétiques. De plus, ils rayonnent dans l’extrême ultra-violet (XUV) à chaque période laser, ce qui se traduit par de la génération d’harmoniques d’ordre élevé de la pulsation laser. L’objectif de cette thèse est de mieux comprendre l’interaction laser-plasma sur miroirs plasmas à l’aide de la caractérisation de ces deux observables physiques qui en sont issues : les faisceaux d’électrons relativistes et les faisceaux d’harmoniques d’ordre élevé. Une première partie traite de la mesure des faisceaux harmoniques. Du fait des conditions physiques extrêmes d’interaction, la détection ne peut se faire qu’à une distance macroscopique de la cible. Ainsi la caractérisation des propriétés angulaires de ces faisceaux (réalisée en fonction des conditions d’interaction au cours de travaux précédents) ne fournit que des informations partielles sur l’interaction en elle-même. La ptychographie, une technique de mesure par diffraction cohérente où une sonde est diffractée par un objet, est ici transposée à la génération d’harmoniques sur miroirs plasmas grâce à la micro-structuration optique du plasma à la surface de la cible. Les champs sources harmoniques sont ainsi reconstruits en amplitude et en phase spatiales directement dans le plan cible. Grâce à ces mesures dans différentes conditions d’interaction, des modèles théoriques analytiques d’interaction en régime non relativiste [I<10¹⁸W/cm²] et relativiste [I>10¹⁸W/cm²] développés précédemment sont validés expérimentalement. Une seconde partie de cette thèse est consacrée à l’étude expérimentale des propriétés angulaires et en énergie des faisceaux d’électrons relativistes issus des miroirs plasmas. Une étude théorique et numérique, permet de prouver que ces mesures sont la première observation claire de l’accélération d’électrons relativistes par laser dans le vide (VLA). Enfin, l’étude simultanée des efficacités de génération des faisceaux d’électrons et d’harmoniques montre une corrélation nette entre les deux processus en régime relativiste
When focusing an ultra-intense femtosecond laser pulse [I>10¹⁶W/cm²] onto a solid target, this target is ionized at the very beginning of the laser pulse. The resulting dense plasma then reflects the laser in the specular direction: it is a plasma mirror. The ultra-intense laser field can accelerate electrons within the plasma at relativistic speeds. Some are ejected towards the vacuum and these plasma mirrors are therefore sources of relativistic electron beams. Moreover, at each optical cycle they radiate in the form of extreme ultraviolet light, resulting in the generation of high-order harmonics of the laser frequency (HHG). The objective of this PhD is to understand laser-plasma interaction though the characterization of high-order harmonics and relativistic electron beams generated from plasma mirrors. The first part deals with harmonic beam measurement. Due to the extreme physical conditions during the interaction, detection can only be performed at macroscopic distance from target. Thus, the characterization of the harmonic beams’ angular properties (carried out as a function of interaction conditions in previous works) only provides partial information on the interaction itself. A technique of coherent diffraction imaging, named ptychography, which consists of diffracting a probe onto an object, is transposed to HHG on plasma mirrors by optically micro-structuring the plasma on a target surface. Harmonic fields are then reconstructed spatially in amplitude and phase directly in the target plane. Thanks to this measurement in different interaction conditions, previously developed theoretical analytical models in non-relativistic regime [I<10¹⁸W/cm²] and relativistic regime [I>10¹⁸W/cm²] are experimentally validated. The second part of the PhD is dedicated to the experimental characterization of angular and spectral properties of relativistic electron beams. A theoretical and numerical study shows that this constitutes the first clear observation of vacuum laser acceleration (VLA). Finally, a simultaneous study of harmonic and electron signals highlights a strong correlation between both processes in the relativistic regime

Ce manuscrit présente le travail de thèse CIFRE/Amplitude réalisé pour mettre en place sur la ligne XUV de LASERIX une expérience de génération d'harmoniques par un faisceau infrarouge de pompe portant un moment angulaire non nul produit par ajout d'une lame de phase. L'originalité de notre démarche a été de générer les harmoniques dans une cellule de gaz relativement longue (de l'ordre de 10mm) et de caractériser les vortex optiques de l’harmonique 25 par un Hartmann Extreme Ultraviolet. Nous avons démontré que la sensibilité du détecteur et la fiabilité du traitement logiciel des données permettait de vérifier que l'harmonique 25 porte typiquement un moment angulaire de 25, conforme aux prédictions. L'analyse plus poussée des données expérimentales a permis de mettre en évidence le caractère intrinsèquement multimode des faisceaux produits. Elle a aussi permis de montrer le rôle de l'astigmatisme résiduel, même très faible du faisceau infrarouge de pompe dans la forme bi-lobale des vortex harmoniques produits. Forme que l'on a pu étudier et corriger grâce à une boucle de correction du front d'onde du laser de pompe. Le travail s'appuie sur des codes de propagation et de diffraction de faisceaux portant des moments angulaires non nuls, que ce soit dans l'XUV ou l'infrarouge. Ils permettent de quantifier le caractère multimode, en termes de modes LG, des faisceaux infrarouges traversant une lame de phase supposée d'abord parfaite, puis réelle. Puis en utilisant le modèle de l'atome unique de la GHOE que le processus non linéaire non perturbatif induisait intrinsèquement de nouveaux modes LG radiaux. L'objectif de tout ce travail pour l'équipe LASERIX était de démontrer la capacité d'un plasma amplificateur de type laser X pompé en cible solide à amplifier un vortex harmonique à la bonne longueur d'onde, tout en conservant sa structure modale et en tous cas la charge portée par le faisceau. Une collaboration avec Eduardo Oliva de Madrid qui réalise des simulations de type Maxwell Bloch montre que cela est possible du point de vue de la physique fondamentale de l'amplification dans ce domaine de longueurs d'onde
This document presents the CIFRE/Amplitude thesis work carried out to set up on the LASERIX XUV line an experiment of high harmonic generation by a pump infrared beam carrying a non-zero orbital angular momentum produced by adding a phase plate on the beam path. The originality of our approach was to generate the harmonics in a relatively long gas cell (around 10mm) and to characterize the optical vortices of harmonic 25 by a Extreme Ultraviolet Hartmann. We demonstrated that the sensitivity of the detector and the reliability of the software processing of the data made it possible to verify that harmonic 25 typically carries an orbital angular momentum of 25, as theoretically predicted. Further analysis of the experimental data allowed us to quantitatively demonstrate the intrinsically multimode nature of the produced beams. I have therefore detailed the different methods of analysis proposed in the literature, I could compare their reliability and their relevance to describe the physical phenomenon at work. I also studied in detail the robustness and convergence of the analytical methods applied to beams with typical orbital angular momenta as high as l=25. The analysis of the data allowed me to show the role of even very weak residual astigmatism contained in the pump infrared beam to produce the bi-lobal shape of the vortices. This characteristic shape was also obtained by other teams. One way to regain a truly annular shape is to set up an active optimization loop of the pump laser wavefront. I have also developed propagation and diffraction calculations of beams carrying non-zero orbital angular momenta, whether in EUV or infrared. This allowed me to quantify the multimode character, in terms of LG modes, of the infrared beams passing through a phase plate that was first assumed to be perfect, then real. Then I showed using the single atom model for harmonic generation that this non-linear, non-perturbative process intrinsically induced new radial LG modes. Finally, the objective of all this work for the LASERIX team was to demonstrate the ability of an X-ray laser amplifier plasma pumped from a solid target to amplify a harmonic vortex at the right wavelength, while maintaining its modal structure and in any case the charge carried by the beam. A collaboration with Eduardo Oliva from Madrid, who carries out Maxwell Bloch simulations, shows that this is possible from the point of view of the fundamental physics of the amplification in this wavelength range

Cette étude s’intéresse à l’amélioration des performances du laser solide Er3+:YAG, dont la longueur d’onde de 1,64 µm est dite « eye-safe ». L’une des solutions est le remplacement des monocristaux actuellement utilisés comme milieu amplificateur par des céramiques polycristallines Er:YAG transparentes, dont les propriétés thermomécaniques remarquables permettent une meilleure cohérence du faisceau de sortie et de ce fait, une augmentation des performances du laser. Cependant, la réunion des différents critères requis pour obtenir la transparence reste un réel challenge dans l’élaboration de ces céramiques. L’utilisation de poudres commerciales issues de deux voies de synthèse différentes a permis de souligner le rôle primordial des caractéristiques physiques de la poudre sur le comportement à la compaction et au frittage, effectué par Spark Plasma Sintering, tandis que la composition phasique et la pureté chimique conditionnent la qualité optique finale. Il ressort également que la coloration de la céramique observée lors du frittage résulte, non pas d’une contamination au carbone, mais de la formation de lacunes d’oxygène. Enfin, l’analyse et la compréhension du mode d’action du LiF utilisé comme aide au frittage ont permis d’établir des mécanismes réactionnels permettant d’optimiser le cycle de frittage. Cette démarche a conduit à l’obtention de céramiques polycristallines transparentes (Ø = 30 mm, e = 3 mm) à qualité optique élevée avec des valeurs de transmission de 80 % à 400 nm et 84 % à 1100 nm. Sur la base de ces résultats et de la simulation numérique, un changement d’échelle des céramiques (Ø = 50 mm, e = 5 mm) a été effectué dans le but de les évaluer en cavité laser
This work focus on the improvement of the solid state Er3+:YAG laser performances presenting an "eye-safe" wavelength at 1.64 µm. One way is the replacement of single crystals currently used as gain media by polycrystalline ceramics as they present improved thermo-mechanical properties allowing a longer use of the laser. However, the meeting of different criteria requested to get transparency remains a challenge in the development of these ceramics. The use of commercial powders produced by two different synthesis ways allowed to highlight the essential role of the physico-chemical characteristics of the powder on compaction and sintering behaviors, performed by Spark Plasma Sintering, Phase composition and chemical purity have an influence of the final optical quality. It was also figured out that the gray coloration of the ceramic observed after sintering is caused by the formation of oxygen vacancies, rather than a carbon contamination. Finally, the mode of action of LiF, used as sintering aid to increase optical transmittance, was studied in order to establish reaction mechanisms allowing an optimization of the SPS cycle. This approach helps to reach Er3+:YAG transparent polycrystalline ceramics (Ø = 30 mm, thk = 3 mm) with an optical transmittance of 80 at 400 nm and 84 % at 1100 nm. On the basis of these results and with the help of numerical simulation, an up-scaling of ceramics (Ø = 50 mm, thk = 5 mm) was undertaken in order to evaluate their laser performances through laser cavity tests

This work focuses on understanding the impact-initiated combustion of aluminum powder compacts. Aluminum is typically one of the components of intermetallic-forming structural energetic materials (SEMs), which have the desirable combination of rapid release of thermal energy and high yield strength. Aluminum powders of various sizes and different levels of mechanical pre-activation are investigated to determine their reactivity under uniaxial stress rod-on-anvil impact conditions, using a 7.62 mm gas gun. The compacts reveal light emission due to combustion upon impact at velocities greater than 170 m/s. Particle size and mechanical pre-activation influence the initiation of aluminum combustion reaction through particle-level processes such as localized friction, strain, and heating, as well as continuum-scale effects controlling the amount of energy required for compaction and deformation of the powder compact during uniaxial stress loading. Compacts composed of larger diameter aluminum particles (~70µm) are more sensitive to impact initiated combustion than those composed of smaller diameter particles. Additionally, mechanical pre-activation by high energy ball milling (HEBM) increases the propensity for reaction initiation. Direct imaging using high-speed framing and IR cameras reveals light emission and temperature rise during the compaction and deformation processes. Correlations of these images to meso-scale CTH simulations reveal that initiation of combustion reactions in aluminum powder compacts is closely tied to mesoscale processes, such as particle-particle interactions, pore collapse, and particle-level deformation. These particle level processes cannot be measured directly because traditional pressure and velocity sensors provide spatially averaged responses. In order to address this issue, quantum dots (QDs) are investigated as possible meso-scale pressure sensors for probing the shock response of heterogeneous materials directly. Impact experiments were conducted on a QD-polymer film using a laser driven flyer setup at the University of Illinois Urbana-Champaign (UIUC). Time-resolved spectroscopy was used to monitor the energy shift and intensity loss as a function of pressure over nanosecond time scales. Shock compression of a QD-PVA film results in an upward shift in energy (or a blueshift in the emission spectra) and a decrease in emission intensity. The magnitude of the shift in energy and the drop in intensity are a function of the shock pressure and can be used to track the particle scale differences in the shock pressure. The encouraging results illustrate the possible use of quantum dots as mesoscale diagnostics to probe the mechanisms involved in the impact initiation of combustion or intermetallic reactions.

We have performed a series of experiments developing the techniques of volumetric, isochoric heating of matter to high energy density states, and the subsequent probing of the release isentrope. Using ultrafast, ultra intense laser systems with pulse lengths from 100fs - 1ps and pulse energies between 2 J and 100 J, we generated strong secondary radiation, in the form of K[subscript alpha] x-rays and directed proton beams, which we used to rapidly heat a foil sample to temperatures from ~ 1 eV to ~ 25 eV at solid density, thus entering the strongly coupled, partially ionized regime of warm dense matter, in which the equation of state is poorly understood. The first set of experiments examines the possibility of using laser generated K[subscript alpha] x-rays in isochoric heating experiments and concludes that this technique will require the use of higher energies and higher Z materials than were used in this thesis to achieve warm dense matter conditions. In the second set of experiments, we used an ultrafast, lasergenerated proton beam with a temperature of ~ 2 MeV and cutoff energy of ~ 40 MeV to volumetrically and isochorically heat a sample foil to > 20 eV. With singleshot diagnostics, we measured the evolution of the temperature with 3:3 ps resolution over the _rst 35 ps of expansion by streaked optical pyrometry, and measured the evolution of the target expansion over the same timescale with sub-ps resolution by chirped pulse interferometry. In this way we were able to verify the equation of state and ion-balance in the SESAME equation of state tables with a Saha ionization model and distinguish this as more accurate than other, simpler models. This thesis establishes an experimental framework for acquiring equation of state data in the regime of warm dense matter that is distinct and complimentary to that acquired by the techniques of shock heating.
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