An overview of the beam diagnostics for the laser wakefield acceleration project at the Texas Petawatt Laser facility is presented. In this experiment, short and intense laser pulses of 165 fs and up to 190 J will be used to accelerate electrons up to the GeV energy range using laser wakefield acceleration. The density variation of the plasma generated in a helium gas cell will be measured with different optical detection systems such as frequency domain holography. Spectra of the transmitted laser beam and optical transition radiation will yield information about the energy transfer to the plasma and the energy of the electrons, respectively. In addition, a calorimeter will measure accelerated electron energies. Prior to the final experiment, preliminary frequency shift measurements and simulations on optical transition radiation were performed. / text

This thesis presents results from two strands of work towards realizing a free-electron laser (FEL) driven by electron bunches generated by a laser-wakefield accelerator (LWFA). The first strand focuses on selecting operating parameters for such a light source, on the basis of currently achievable bunch parameters as well as near-term projections. The viability of LWFA-driven incoherent undulator sources producing nanojoule-level pulses of femtosecond duration at wavelengths of 5 nm and 0.5 nm is demonstrated. A study on the prospective operation of an FEL at 32 nm is carried out, on the basis of scaling laws and full 3-D time-dependent simulations. A working point is selected, based on realistic bunch parameters. At that working point saturation is expected to occur within a length of 1.6 m with peak power at the 0.1 GW-level. This level, as well as the stability of the amplification process, can be improved significantly by seeding the FEL with an external radiation source. In the context of FEL seeding, we study the ability of conventional simulation codes to correctly handle seeds from high-harmonic generation (HHG) sources, which have a broad bandwidth and temporal structure on the attosecond scale. Namely, they violate the slowly-varying envelope approximation (SVEA) that underpins the governing equations in conventional codes. For this purpose we develop a 1-D simulation code that works outside the SVEA. We carry out a set of benchmarks that lead us to conclude that conventional codes are adequately capable of simulating seeding with broadband radiation, which is in line with an analytical treatment of the interaction. The second strand of work is experimental, and focuses on on the use of coherent transition radiation (CTR) as an electron bunch diagnostic. The thesis presents results from two experimental campaigns at the MPI für Quantenoptik in Garching, Germany. We present the first set of single-shot measurements of CTR over a continuous wavelength range from 420 nm to 7 μm. Data over such a broad spectral range allows for the first reconstruction of the longitudinal profiles of electron bunches from a laser-wakefield accelerator, indicating full-width at half-maximum bunch lengths around 1.4 μm (4.7 fs), corresponding to peak currents of several kiloampères. The bunch profiles are reconstructed through the application of phase reconstruction algorithms that were initially developed for studying x-ray diffraction data, and are adapted here for the first time to the analysis of CTR data. The measurements allow for an analysis of acceleration dynamics, and suggest that upon depletion of the driving laser the accelerated bunch can itself drive a wake in which electrons are injected. High levels of coherence at optical wavelengths indicate the presence of an interaction between the bunch and the driving laser pulse.

Laser-plasma accelerators are of great interest because of their ability to sustain extremely large acceleration gradients, enabling compact accelerating structures. Laser-plasma acceleration is realized by using a high-intensity short pulse laser to drive a large plasma wave or wakefield in an underdense plasma. This thesis considers the effect of axial plasma density upramps on laser wakefield acceleration. Theoretical groundwork shows that tapered plasma channels can be used to mitigate one of the main limitations of laser plasma acceleration, that is, dephasing of an electron beam with respect to the plasma wave. It is shown that it is possible to maintain an electron bunch at constant phase in the longitudinal electric fields of the laser wake field. This leads to an increased energy gain of an electron trapped in the wakefield. The required shape of the density slope is difficult to implement in experiments. Therefore, a linear density ramp is also considered which is predicted to also increase the energy gain beyond that possible in a uniform density plasma. Towards an experimental implementation it was studied how a suitable gas density profile can be established in a capillary. This was done employing simulations using the computational fluid dynamics tool kit OpenFoam and comparing these to measurements of the axial density profile based on Raman scattering. It was demonstrated that a linear density ramp could be established by applying different pressures on the capillary gas inlets. The dependence of the density profile on the capillary parameters, such as, capillary diameter and length and inlet diameter were also studied. The results of the simulations and the measurement showed excellent agreement and demonstrate that approximately linear density ramps can be generated by flowing gas along a capillary of constant cross-section Laser wakefield acceleration in plasmas with longitudinally varying density was investigated in an experiment at the Astra Laser at Rutherford Laboratories. The experiment utilised ionisation injection in order to operate in the mildly non-linear regime of laser-wakefield acceleration. The measured electron energies agree well with the theoretical predictions. It was demonstrated that an increase in the energy gain can be obtained by driving the accelerator in a ramped plasma, the electron spectrum is more narrow and the injected charge increases significantly. Measurements of the X-ray spectrum emitted by the betatron motion of the accelerated electron bunch allowed the transverse radius of the bunch to be deduced. These measurements showed that retrieved electron bunch radius is inversely proportional to the longitudinal density gradient, that is a plasma density upramp (downramp) has a decreased (increased) electron bunch radius.

Laser plasma wakefield accelerators have seen tremendous progress in the last years, now capable of producing electron beams in the GeV energy range. The inherent few-femtoseconds short bunch duration of these accelerators leads to ultra-high peak-currents. Reducing the energy spread found in these accelerators, while scaling their output to hundreds of kiloampere peak current would stimulate the next generation of radiation sources covering high-field THz, high-brightness X-ray and -ray sources, compact free-electron lasers and laboratory-size beam-driven plasma accelerators. At such high currents, an accelerator operates in the beam loaded regime where the accelerating field is strongly modified by the self-fields of the injected bunch, potentially deteriorating key beam parameters. However, if appropriately controlled, the beam loading effect can be employed to improve the accelerator’s performance, specifically to reduce the energy spread. In this thesis the beam-loading effect is systematically studied at a quasi-monoenergetic nanocoulomb-class laser wakefield accelerator. For this purpose, a tailored scheme of the self-truncated ionisation injection process is introduced for the non-linear bubble regime. This scheme facilitates stable and tunable injection of high-charge electron bunches within a short and limited time-frame, ensuring low energy spread right after injection. Employing a three millimetres gas-jet acceleration medium and a moderate 150 TW short pulse laser system as driver, unprecedented charges of up to 0.5 nC within a quasi-monoenergetic peak and energies of ~0.5 GeV are achieved. Studying the beam loading mechanism, it is demonstrated that at the optimal loading condition, i.e. at a specific amount of injected charge, performance of the accelerator is optimised with a minimisation of the energy spread. At a relative energy spread of only 15%, the associated peak current is around 10 kA, while scaling this scheme to operate with a petawatt driver laser promises peak-currents up to 100 kA.

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.

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 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.

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.

This thesis investigates the use of high-power lasers for synchrotron radiation sources with high brilliance, from the EUV to the hard X-ray spectral range. Hereby lasers accelerate electrons by laser-wakefield acceleration (LWFA), act as optical undulators, or both. Experimental evidence shows for the first time that LWFA electron bunches are shorter than the driving laser and have a length scale comparable to the plasma wavelength. Furthermore, a first proof of principle experiment demonstrates that LWFA electrons can be exploited to generate undulator radiation. Building upon these experimental findings, as well as extensive numerical simulations of Thomson scattering, the theoretical foundations of a novel interaction geometry for laser-matter interaction are developed. This new method is very general and when tailored towards relativistically moving targets not being limited by the focusability (Rayleigh length) of the laser, while it does not require a waveguide. In a theoretical investigation of Thomson scattering, the optical analogue of undulator radiation, the limits of Thomson sources in scaling towards higher peak brilliances are highlighted. This leads to a novel method for generating brilliant, highly tunable X-ray sources, which is highly energy efficient by circumventing the laser Rayleigh limit through a novel traveling-wave Thomson scattering (TWTS) geometry. This new method suggests increases in X-ray photon yields of 2-3 orders of magnitudes using existing lasers and a way towards efficient, optical undulators to drive a free-electron laser. The results presented here extend far beyond the scope of this work. The possibility to use lasers as particle accelerators, as well as optical undulators, leads to very compact and energy efficient synchrotron sources. The resulting monoenergetic radiation of high brilliance in a range from extreme ultraviolet (EUV) to hard X-ray radiation is of fundamental importance for basic research, medical applications, material and life sciences and is going to significantly contribute to a new generation of radiation sources and free-electron lasers (FELs).