This thesis presents results from three strands of experimental work aimed towards establishing more reproducible, higher energy, and more accurately measured electron beams generated by a laser-driven plasma accelerator. The first experiment calibrated two types of detector frequently used to measure the bunch charge in laser wakefield accelerator experiments, namely scintillating screens and image plates. The experiments undertaken at the DAFNE beam test facility in Frascati, Italy, confirmed that the fluorescence signal from Kodak Lanex Regular screens varies linearly with the charge density for a nanosecond elec- tron bunch for charge densities in the range between ρ = 2 × 10⁻⁷ ^C/_m² to ρ = 10−5 ^C/_m². A sensitivity measurement of FUJIFILM BAS-IP MS image plates resulted in a sensitivity of SMS = (0.0487 ± 0.0028 ) PSL, which is 2.4 times higher than had been assumed prior to this work. The second strand aimed at improving the operation of the capillary discharge waveguide by re-designing the discharge circuit and the waveguide housing. The experiment showed that combining a glow discharge circuit with the pulsed discharge circuit of the capillary discharge waveguide reduced electrical noise, the timing jitter between the trigger pulse and the discharge, and the voltage required to initially break down the capillary gas for pressures below 10 mbar and above 150 mbar. The size of the housing of the capillary discharge waveguide was reduced in all three dimensions by an average of 60 %, enabling the device to be used in future staging experiments, and an open design of the housing eliminated the possibility of unwanted discharges. The new capillary design performed without flaw in the Astra-Gemini experiment and no disadvantages compared with the old housing were found. The third strand of work describes an experiment undertaken with the Astra-Gemini laser at the Central Laser Facility of the Rutherford Appleton Laboratory, United Kingdom. The improved capillary discharge waveguide was used to generate GeV-scale electron beams with good reproducibility. Beams of electrons with energies above 900 MeV, and with root- mean-square divergence of 3.5 mrad, were observed for a plasma density of 2.2 × 10¹⁸ cm⁻³ and a peak input laser power of 55 TW. The variation of the maximum electron energy with the plasma density was measured and found to agree well with simple models. The energy spectra of the generated electron beams exhibited good shot-to-shot reproducibility, with the observed variations attributable to the measured shot-to-shot jitter of the laser parameters. Two methods for correcting the effect of beam pointing variations on the measured energy spectrum were tested and it was found that using a thin Lanex screen in front of the electron spectrometer was easy to implement and did not degrade the recorded energy spectrum. The first observation of temporal compression of a laser pulse within a plasma channel with simultaneous electron acceleration to energies higher than 500 MeV is also presented. This measurement suggests that the pulse compresses linearly from the back as predicted by theory.

Cette thèse porte sur le rayonnement X bêtatron généré par des électrons accélérés par sillage laser plasma dans des tubes capillaires diélectriques. En l’état actuel de la technologie des impulsions laser multi-térawatts, on peut produire des faisceaux ayant une intensité crête élevée, de l’ordre de 1018 W/cm2 dans le plan focal. Une telle impulsion laser se propageant au sein d’un gaz sous-dense conduit à des phénomènes d’interaction laser-plasma non-linéaires, tels que la création d’une bulle de plasma, i.e. une bulle ne contenant aucun électron, suivant le laser. La séparation spatiale des charges en résultant crée des champs électriques très élevés au sein de la bulle, de l'ordre de 100 GV/m, ce qui offre la possibilité d'accélérer des électrons jusqu'au GeV après seulement quelques centimètres d’interaction. En outre, un rayonnement synchrotron ultra-bref, appelé rayonnement bêtatron, est produit lors de l’accélération des électrons puisque ces derniers, soumis au champ électrique radial de la bulle plasma, ont une trajectoire oscillante. Cette thèse présente des résultats expérimentaux sur la génération et l'optimisation de faisceaux d'électrons et de leur rayonnement X, en particulier lorsque le tube capillaire est utilisé pour recueillir l'énergie du halo laser dans le plan focal facilitant l’autofocalisation du laser sur de longues distances. Des faisceaux d’électrons de quelques dizaines de picocoulomb, avec une énergie maximale allant jusqu’à 300 MeV, et dont le spectre est soit piqué à haute énergie soit exponentiellement décroissant, ont été produits dans des tubes capillaires de 10 mm de long avec l’installation laser du Lund Laser Center (LLC, en Suède) par une impulsion laser de 40 fs d’un 16 TW Ti: Saphir. Un rayonnement bêtatron a également été mesuré, il se compose de de photons X dont l’énergie est comprise entre 1 et 10 keV et atteint une luminosité maximale d’environ 1021 photons/s/mm²/mrad²/0.1%BW. Cela équivaut à environ 30 fois l’intensité des faisceaux générés dans le cas des jets de gaz de longueur 2 mm ne disposant pas de guidage optique externe. La compensation des fluctuations de pointé laser permet de minimiser les fluctuations des propriétés du faisceau d’électrons. On obtient des faisceaux d'électrons dont les fluctuations tir-a-tir sont de 1 mrad en pointé, de quelques pourcents en énergie et d’environ 20% RMS en charge. La fluctuation en charge du faisceau, qui peut être considérée comme relativement grande, s’avère être principalement corrélée à la fluctuation en puissance du laser. De plus, il a été montré que le rayonnement bêtatron pouvait être utilisé pour caractériser le processus d'accélération des électrons en caractérisant le nombre moyen d'oscillations bêtatron effectuées par les électrons à l'intérieur de la bulle plasma. La taille typique des sources de rayonnement X (dimension pour laquelle l’intensité gaussienne est égale à 1/e² de la valeur crête) est estimée à ~ 2.5 µm en utilisant un modèle de diffraction de Fresnel induite par une lame de rasoir. Cela correspond à une émittance RMS normalisée pour le faisceau d'électrons d’environ 0,83π mm.mrad. Des simulations tridimensionnelles particle-in-cell (PIC) ont été effectuées et confirment les résultats expérimentaux. Elles indiquent également que les paquets d'électrons générés ainsi que les flashs X directionnels sont ultra-brefs : ~ 10 fs. / This dissertation addresses electron acceleration and the associated betatron X-ray radiation generated by laser wakefield inside dielectric capillary tubes. Focusing the state-of-the-art multi-terawatt laser pulses, high peak intensity, of the order of 1018 W/cm2, can be achieved in the focal plane, where a plasma bubble free of electron is formed just behind the laser. Owing to space charge separation ultrahigh electric fields, of the order of 100 GV/m, occur inside the plasma bubble, providing the possibility to accelerate electrons up to GeV-class over merely a centimetre-scale distance. Furthermore, ultra-short synchrotron-like X-ray radiation, known as betatron radiation, is produced simultaneously when the accelerated electrons are transversely wiggled by the radial electric field inside the plasma bubble. This thesis reports experimental results on the generation and optimization of electron and X-ray beams, particularly when a capillary tube is used to collect the energy of laser halos in the focal plane to facilitate the laser keeping self-focused over a long distance. Employing the 40 fs, 16 TW Ti:sapphire laser at the Lund Laser Centre (LLC) in Sweden, either peaked or widely-spread accelerated electron spectra with a typical beam charge of tens of pC were measured with a maximum energy up to 300 MeV in 10 mm long capillary tubes. Meanwhile, betatron X-ray radiation consisting of 1-10 keV photons was measured with a peak brightness of the order of 1021 photons/s/mm2/mrad2/0.1%BW, which is around 30 times higher than that in the case of a 2 mm gas jet without external optical guiding. When the laser pointing fluctuation is compensated, exceptionally reproducible electron beams are obtained with fluctuations of only 1 mrad RMS in beam pointing, a few percent in electron energy, and around 20% RMS in beam charge. The relatively large instability of beam charge is found to be essentially correlated to laser power fluctuation. Moreover, betatron radiation is able to provide the diagnostics about electron acceleration process and average number of betatron oscillations fulfilled by electrons inside the plasma bubble. The typical X-ray source size (waist of Gaussian distribution at 1/e2 intensity) is quantified to be ~2.5 μm using Fresnel diffraction induced by a razor blade, which furthermore yields the corresponding normalized RMS emittance of electron beam 0.83π mm mrad. Three dimensional particle-in-cell (PIC) modelings are in good agreement with the experimental findings. The PIC simulations also reveal the generated electron bunches (or X-ray bursts) have pulse durations as short as 10 fs.

Die Beschleunigung von Elektronen im Wechselwirkungsbereich hochintensiver Laserfelder mit einem Plasma wird als mögliche Alternative zu konventionellen Radiofrequenz basierten Beschleunigerkonzepten gehandelt. Die gezeigten Experimente sind die ersten Versuche zur Laser getriebenen Elektronenbeschleunigung am Max Born Institut. Im Rahmen dieser Dissertation konzentriere ich mich auf kontrollierte Injektion und es werden zwei verschiedene Methoden gezeigt. Die erste demonstrierte Variante einer stimulierten Injektion ist die Ionisationsinjektion, welche typischerweise zu einem kontinuierlichen Elektroneneinfang über einen ausgedehnten Bereich entlang der Propagation des Lasers führt. Die injizierten Elektronen werden dadurch über unterschiedliche Längen beschleunigt, was zu einem breiten Energiespektrum des beschleunigten Eletronenpaketes führt. Die zweite untersuchte Injektionsmethode basiert auf einem Überschallphänomen, welches eine quasi-instantane Injektion ermöglicht. Wird ein Überschall-Gasfluß durch eine scharfe Kante gestört, bildet sich ein scharfer Dichteübergang, bekannt als Schock Front, durch welchen eine Injektion stimuliert werden kann. Es wurde gezeigt, dass die Machzahl der Düse bzw. die Übergangshöhe der Schock Front dazu benutzt werden können, die injizierte Ladungsmenge zu kontrollieren. Eine Erhöhung der Ladungsmenge ist dabei mit einer Erhöhung der Energiebreite verknüpft. Es wurden Elektronenstrahlen demonstriert mit weniger als 2% Energiebreite bei einer Maximalenergie von 300MeV und 5 pC Ladung. Es zeigte sich, dass sowohl bei Shock-Front Injektion als auch bei Ionisationsinjektion die emittierte Ladung pro Energieintervall und Raumwinkel konstant blieb, bei einem Wert von (0.021+-0.001) pC/MeV/mrad^2. Dass sowohl eine kontinuierliche als auch eine instantane Injektion dieselbe Korrelation zwischen Ladung, Divergenz und Energiebreite aufweisen, lässt darauf schließen, dass es sich um eine Eigenschaft der Plasmawelle selbst handelt. / The acceleration of electrons in intense laser fields interacting with a plasma is widely considered as a possible alternative to conventional RF-based accelerator concepts. The presented measurements are the first demonstration of Laser Wakefield Acceleration at the Max Born Institut and a setup was build to perform the described experiments. This thesis focuses on controlled injection and two different methods will be compared. The first method of stimulated injection, presented in this thesis, is ionization injection, which typically causes electron trapping over an extended laser propagation distance. As electrons become injected at different positions, electrons will be accelerated over different distances, yielding a wide energy spread in the emitted electron beam. The second stimulated injection method utilizes a supersonic phenomenon called shock front to stimulate a quasi-instantaneous injection. When a supersonic gas flow is disturbed by a sharp edge, a shock front is created and injection is stimulated at the crossing of the propagating laser pulse and the shock-front region. It is found that the Mach number of the flow or the density transition in the shock front respectively, can be used to tune the total charge injected. This increase in total charge comes at the expense of an increased energy spread. Electron beams are demonstrated with an energy spread of less than 2% at peak energies of 300MeV with 5 pC of charge. For the ionization injection as well as for the shock-front injection it is found, that the charge per energy interval and solid angle is constant and amounts to (0.021+-0.001) pC/MeV/mrad^2 for all observed electron beams. The continuous injection and the quasi-instantaneous injection yield the same correlation between charge, divergence and energy spread. This implies that this correlation is a property of the wakefield structure itself.

Accurate single-shot visualization of laser wakefield structures can improve our fundamental understanding of plasma-based accelerators. Previously, frequency domain holography (FDH) was used to visualize weakly nonlinear sinusoidal wakes in plasmas of density n[subscript e] < 0.6 × 10¹⁹/cm³ that produced few or no relativistic electrons. Here, I address the more challenging task of visualizing highly nonlinear wakes in plasmas of density n[subscript e] ~ 1 to 3× 10¹⁹/cm³ that can produce high-quality relativistic electron beams. Nonlinear wakes were driven by 30 TW, 30 fs, 800 nm pump pulses. When bubbles formed, part of a 400 nm, co-propagating, overlapping probe pulse became trapped inside them, creating a light packet of plasma wavelength dimensions--that is, an optical "bullet"--that I reconstruct by FDH methods. As ne increased, the bullets first appeared at 0.8 × 10¹⁹/cm³, the first observation of bubble formation below the electron capture threshold. WAKE simulations confirmed bubble formation without electron capture and the trapping of optical bullets at this density. At n[subscript] >1× 10¹⁹/cm³, bullets appeared with high shot-to-shot stability together with quasi-monoenergetic relativistic electrons. I also directly observed the temporal walk-off of the optical bullet from the beam-loaded plasma bubble revealed by FDH phase shift data, providing unprecedented visualization of the electron injection and beam loading processes. There are five chapters in this thesis. Chapter 1 introduces general laser plasma- based accelerators (LPA). Chapter 2 discusses the FDH imaging technique, including the setup and reconstruction process. In 2006, Dr. N. H. Matlis used FDH to image a linear plasma wakefield. His work is also presented in Chapter 2 but with new analyses. Chapter 3, the main part of the thesis, discusses the visualization of LPAs in the bubble regime. Chapter 4 presents the concept of frequency domain tomography. Chapter 5 suggests future directions for research in FDH. / text

This thesis examines representations of trust in selected British sentimental novels of the eighteenth century. It focuses principally on the manner in which sentimental prose fiction reflects and participates in the shift from premodern to modern formations of trust. Commenting on the nature of modern trust, Anthony Giddens claims that, with the move to modernity, trust relations in the intimate sphere become increasingly dependent on emotional mutuality, while trust in institutions becomes increasingly impersonal and disengaged from assessments of moral character. / My work explores this dual shift in three sentimental novels. It first analyzes Samuel Richardson's Pamela (1740) and contends that Richardson denies the concept of honor its epistemological role in practical deliberations. The denial of the epistemology of honor uncouples the mechanism of personal trust from assessments of role and role performance and thus makes the trust in persons in the intimate sphere less dependent on institutional forms of trust. To replace honor's role in the formation of trust, Richardson proposes that the sentiments can provide reliable grounds for trust in the intimate sphere. However, he denies the sentiments a role in the formation of an encompassing social trust among strangers and mere acquaintances. The thesis proceeds to read Henry Fielding's Amelia (1751). In order to argue that Fielding envisioned divergent grounds for trust relations, it maintains that Fielding considers trust relations in the intimate sphere and trust relations in public life as based on the sentiments and fair distribution respectively. To conclude, the thesis investigates Oliver Goldsmith's The Vicar of Wakefield (1766) to uncover the manner in which Goldsmith distinguishes personal trust in the intimate sphere from general system trust, which Goldsmith ultimately envisions as an ontological trust in providence.

This dissertation addresses electron acceleration and the associated betatron X-ray radiation generated by laser wakefield inside dielectric capillary tubes. Focusing the state-of-the-art multi-terawatt laser pulses, high peak intensity, of the order of 1018 W/cm2, can be achieved in the focal plane, where a plasma bubble free of electron is formed just behind the laser. Owing to space charge separation ultrahigh electric fields, of the order of 100 GV/m, occur inside the plasma bubble, providing the possibility to accelerate electrons up to GeV-class over merely a centimetre-scale distance. Furthermore, ultra-short synchrotron-like X-ray radiation, known as betatron radiation, is produced simultaneously when the accelerated electrons are transversely wiggled by the radial electric field inside the plasma bubble. This thesis reports experimental results on the generation and optimization of electron and X-ray beams, particularly when a capillary tube is used to collect the energy of laser halos in the focal plane to facilitate the laser keeping self-focused over a long distance. Employing the 40 fs, 16 TW Ti:sapphire laser at the Lund Laser Centre (LLC) in Sweden, either peaked or widely-spread accelerated electron spectra with a typical beam charge of tens of pC were measured with a maximum energy up to 300 MeV in 10 mm long capillary tubes. Meanwhile, betatron X-ray radiation consisting of 1-10 keV photons was measured with a peak brightness of the order of 1021 photons/s/mm2/mrad2/0.1%BW, which is around 30 times higher than that in the case of a 2 mm gas jet without external optical guiding. When the laser pointing fluctuation is compensated, exceptionally reproducible electron beams are obtained with fluctuations of only 1 mrad RMS in beam pointing, a few percent in electron energy, and around 20% RMS in beam charge. The relatively large instability of beam charge is found to be essentially correlated to laser power fluctuation. Moreover, betatron radiation is able to provide the diagnostics about electron acceleration process and average number of betatron oscillations fulfilled by electrons inside the plasma bubble. The typical X-ray source size (waist of Gaussian distribution at 1/e2 intensity) is quantified to be ~2.5 μm using Fresnel diffraction induced by a razor blade, which furthermore yields the corresponding normalized RMS emittance of electron beam 0.83π mm mrad. Three dimensional particle-in-cell (PIC) modelings are in good agreement with the experimental findings. The PIC simulations also reveal the generated electron bunches (or X-ray bursts) have pulse durations as short as 10 fs.

La thèse est axée sur l'étude des effets collectifs dans un anneau de stockage d'électrons de 50 MeV, ThomX, en l'absence d'amortissement synchrotron et d’adaptation longitudinale. Cette thèse est divisée en deux parties distinctes. La première partie correspond à la conception du modèle d'impédance (champ de sillage géométrique, résistif et rayonnement synchrotron cohérent) de l'anneau de stockage afin de simuler la dynamique faisceau. Le modèle d'impédance géométrique de l'anneau de stockage a été obtenu par la simulation des éléments individuels et a été vérifié en utilisant des mesures RF sur des prototypes. Le rayonnement synchrotron cohérent a été simulé en tenant compte d'une chambre à vide rectangulaire. Des simulations de la dynamique faisceau, de la cathode de canon RF à l'anneau de stockage, comprenant les effets collectifs sont présentées. Les simulations sont utilisées pour optimiser la dynamique faisceau dans l'anneau de stockage dans le régime de micro-paquet. La deuxième partie concerne l'étude du nuage d'ions produit par l'ionisation des molécules du vide résiduel et l'optimisation des techniques de nettoyage des ions. Les points d'accumulation longitudinaux des ions et le piégeage dans les champs magnétiques sont tous les deux étudiés analytiquement et en utilisant un programme développé à cet effet. Les électrodes de nettoyage et les espaces de nettoyages sont simulés et optimisés en utilisant ce code et la stratégie choisie pour la limitation des effets induits par les ions est décrite. Enfin, les effets de la multi-ionisation et de la dissociation ionique sont pris en compte et l'effet des ions sur le faisceau d'électrons est estimé. / The thesis is focused on the study of collective effects in a 50 MeV electron storage ring, ThomX, in the absence of synchrotron radiation damping and of longitudinal matching. This thesis is divided in two distinct parts. The first part corresponds to the design of the impedance model (geometric and resistive wakefields, coherent synchrotron radiation) of the storage ring in order to simulate the beam dynamics. The geometric impedance model of the storage ring was obtained via simulation of the individual elements and was checked using wire measurements on prototypes. The coherent synchrotron radiation was simulated taking into account a rectangular vacuum chamber. Beam dynamics simulations, from the RF gun cathode to the storage ring, including collective effects are presented. The simulations are used to optimise the beam dynamics in the storage ring in the micro-bunching regime. The second part is the study of the ion cloud produced by the ionisation of the residual vacuum molecules and the optimisation of the ion clearing techniques. The longitudinal ion accumulation points and the trapping in magnetic fields are both studied analytically and by using a tracking code developed for this purpose. Clearing electrodes and clearing gaps are simulated and optimised using this code and the strategy chosen for the limitation of ion induced effects is described. Finally, the effect of multi-ionisation and ion dissociation is taken into account and the ion effect on the electron beam is estimated.

Une impulsion laser intense se propageant dans un plasma sous-dense (ne<<nc) déplace les électrons sur son passage et crée une onde de sillage à même d'accélérer des électrons. Lorsque l'impulsion est très intense (I₀> 10¹⁸ W.cm⁻²) et de durée très courte (τ₀< 100 fs), , on atteint le régime de la bulle. Les champs électriques dans ces bulles, de l’ordre de 100 GV/m, peuvent accélérer un faisceau d’électrons jusqu’au GeV sur des distances de l’ordre du centimètre. Dans ce régime, les électrons expulsés par la force pondéromotrice du laser forment une fine et dense couche à la surface d'une cavité d'ions restés immobiles. Les propriétés de ce régime sont examinées par l’intermédiaire d’un modèle analytique, que nous avons développé en nous inspirant du travail de W. Lu et S. Yi. En nous plaçant dans ce régime prometteur, nous avons étudié les mécanismes d’injection et de piégeage dans l'onde de sillage. Dans l’injection optique, les polarisations parallèles ou circulaires positives conduisent respectivement à une injection mettant en jeu du chauffage stochastique, ou à l’injection froide. Un paramètre de similarité est introduit, celui-ci permet de déterminer la méthode d’injection la plus appropriée pour maximiser la charge injectée. Enfin, le modèle analytique présenté en première partie est étendu afin d’étudier l’onde de sillage dans le régime de la bulle lorsqu’un champ magnétique longitudinal initial est appliqué au plasma. Lorsque le plasma est magnétisé deux phénomènes remarquables se manifestent, d'une part une ouverture apparaît à l'arrière de la bulle et d'autre part un mécanisme d'amplification du champ magnétique longitudinale est induit par la variation du flux magnétique. Les prédictions de notre modèle analytique sont confrontées aux résultats de simulations PIC 3D issues du code CALDER-Circ. La conséquence immédiate de la déformation de l'onde de sillage est la réduction, voire la suppression de l'auto-injection. L’application d’un champ magnétique longitudinal, combinée à un choix judicieux des paramètres laser-plasma, permet de réduire la dispersion en énergie des faisceaux d’électrons produits après injection optique. / An intense laser pulse propagating in an under dense plasma (ne<<nc) expels electrons and a wakefield is created which can accelerate efficiently electrons. When the laser pulse is very intense (I₀> 10¹⁸ W.cm⁻²) and short(τ₀< 100 fs), the bubble regime is reached. Within the bubble the electric field can exceed 100 GV/m and a trapped electron beam is accelerated to GeV energy with few centimetres of plasma.In this regime, the electrons expelled by the laser ponderomotive force are brought back and form a dense sheath layer. First, an analytic model was derived using W. Lu and S. Yi formalisms in order to investigate the properties of the wakefield in the blowout regime. In a second part, the trapping and injection mechanisms into the wakefield were studied. When the optical injection scheme is used, electrons may undergo stochastic heating or cold injection depending on the lasers’ polarisations. A similarity parameter was introduced to find out the most appropriate method to maximise the trapped charge. In a third part, our analytic model is extended to investigate the influence of an initially applied longitudinal magnetic field on the laser wakefield in the bubble regime. When the plasma is magnetized two remarkable phenomena occur. Firstly the bubble is opened at its rear, and secondly the longitudinal magnetic field is amplified - at the rear of the bubble - due to the azimuthal current induced by the variation of the magnetic flux. The predictions of our analytic model were shown to be in agreement with 3D PIC simulation results obtained with Calder-Circ. In most situations the wake shape is altered and self-injection can be reduced or even cancelled by the applied magnetic field. However, the application of a longitudinal magnetic field, combined with a careful choice of laser-plasma parameters, reduces the energy spread of the electron beam produced after optical injection.

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