Dissertations / Theses on the topic 'Particle-In-Cell code'

The particle-in-cell code EPOCH was extended to include field and collisional ionisation for use in simulating initially neutral or partially-ionised targets in laser-plasma inter- actions. The means by which particles ionise in the the field of an intense laser was described and physical models were included to determine the instantaneous ionisa- tion rate at particles within the simulation domain for multiphoton, tunnelling, barrier- suppression and electron-impact ionisation. The algorithms used to implement these models were presented and demonstrated to produce the correct ionisation statistics. A scheme allowing for modelling small amounts of ionisation for an arbitrarily low number of superparticles was also presented for comparison and it was shown that for sufficient simulation time the two schemes converge. The three major mechanisms of ionisation in laser-plasma interactions were described as being ionisation-induced defocussing, fast shuttering and ionisation injection. Simulations for these three effects were presented and shown to be in good agreement with theory and experiment. For fast-shuttering, plasma mirrors were simulated using the pulse profile for the Astra Gemini laser at the Central Laser Facility. Rapid switch-on and the theoretical maximum for contrast ratio was observed. For ionisation injection, simulations for laser wakefield acceleration in a helium gas were performed and the accelerated electron population was shown to be greatly increased through use of a 1% nitrogen dopant consistent with the experimental results of McGuffey et al. A study of the laser filamentation instability due to SRS backscatter at the relativistically corrected quarter critical surface (RCQCS) was per- formed in collaboration with C.S. Brady and T.D. Arber at the University of Warwick [1]. It was found that for hydrogen and plastic the instability was unaffected by the in- clusion of ionisation. Further study with argon revealed a attening of the RCQCS and it was demonstrated that for a material with multiple ionisation levels ionising strongly near the self-focussed intensities at the RCQCS, rapid ionisation caused an inversion of the RCQCS that suppressed the filamentation instability.

Thesis (S.M. and S.B.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering; and, (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2012.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 105-107).
In this thesis, I designed and implemented a particle-in-cell (PIC) code on a graphical processing unit (GPU) using NVIDA's Compute Unified Architecture (CUDA). The massively parallel nature of computing on a GPU nessecitated the development of new methods for various steps of the PIC method. I investigated different algorithms and data structures used in the past for GPU PIC codes, as well as developed some of new ones. The results of this research and development were used to implement an efficient multi-GPU version of the 3D3v PIC code SCEPTIC3D. The performance of the SCEPTIC3DGPU code was evaluated and compared to that of the CPU version on two different systems. For test cases with a moderate number of particles per cell, the GPU version of the code was 71x faster than the system with a newer processor, and 160x faster than the older system. These results indicate that SCEPTIC3DCPU can run problems on a modest workstation that previously would have required a large cluster.
by Joshua Estes Payne.
S.B.
S.M.and S.B.

This thesis presents the parallel version of Coliseum, the Air Force Research Laboratory plasma simulation framework. The parallel code was designed to run large simulations on the world fastest supercomputers as well as home mode clusters. Plasma simulations are extremely computationally intensive as they require tracking millions of particles and solving field equations over large domains. This new parallel version will allow Coliseum to run simulations of spacecraft-plasma interactions in domain large enough to reproduce space conditions. The parallel code ran on two of the world fastest supercomputers, the NASA JPL Cosmos supercomputer ranked 37th on the TOP500 list and Virginia Tech's System X, ranked 7th. DRACO, the Virginia Tech PIC module to Coliseum, was modified with parallel algorithms to create a full parallel PIC code. A parallel solver was added to DRACO. It uses a Gauss-Seidel method with SOR acceleration on a Red-Black checkerboard scheme. Timing results were obtained on JPL Cosmos supercomputer to determine the efficiency of the parallel code. Although the communication overhead limits the code's parallel efficiency, the speed up obtained greatly decreases the time required to run the simulations. A speed up of 51 was reached on 128 processors. The parallel code was also used to simulate the plume expansion of an ion thruster array composed of three NSTAR thrusters. Results showed that the multiple beams merge to form a single plume similar to the plume created by a single ion thruster.
Master of Science

There has been a great deal of interest in investigating numerous unique types of electrostatic and electromagnetic waves and instabilities in dusty plasmas. Dusty plasmas are characterized by the presence of micrometer or submicrometer size dust grains immersed in a partially or fully ionized plasma. In this study, a two-dimensional numerical model is presented to study waves and instabilities in dusty plasmas. Fundamental differences exist between dusty plasmas and electron-ion plasmas because of dust charging processes. Therefore, a primary goal of this study is to consider the unique effects of dust charging on collective effects in dusty plasmas. The background plasma electrons and ions here are treated as two interpenerating fluids whose densities vary by dust charging. The dust is treated with a Particle-In-Cell PIC model in which the dust charge varies with time according to the standard dust charging model. Fourier spectral methods with a predictor-corrector time advance are used to temporally evolve the background plasma electron and ion equations. The dust charge fluctuation mode and the damping of lower hybrid oscillations due to dust charging, as well as plasma instabilities associated with dust expansion into a magnetized background plasma are investigated using our numerical model. Also, an ion acoustic streaming instability in unmagnetized dusty plasmas due to dust charging is investigated. The numerical simulation results show good agreement with theoretical predictions and provide further insight into dust charging effects on wave modes and instabilities in dusty plasmas.
Ph. D.

This thesis focuses on the design, development and testing of a two dimensional particle in cell (PIC) code (PICSIE) written in Matlab. The code is applied to the specific problem of modelling the performance of dual stage ion thrusters. The code simulates one full aperture within dual stage ion thruster systems, focusing on the flow of ions through the aperture. Only the ions have been included in the simulation in order to minimize running time. The results produced by the simulation code are compared with results obtained from the vacuum chamber testing of the DS4G prototype, along with results from other simulation codes and research papers in order to verify the performance of the simulation code. The Dual-Stage 4-Grid (DS4G) and Dual-Stage 3-Grid (DS3G) thrusters are both sim- ulated in order to compare the performance of the two thrusters and assess the benefits and disadvantages of including the fourth grid in a dual stage thruster system. Different grid configurations are simulated in order to find the most efficient configuration of the ion optics and accelerating voltages for each thruster, with the aim being to find the con- figurations that produce the maximum particle momentum, thrust and specific impulse while minimizing the rate of erosion of the ion optics and maximising the efficiency of the thruster. These simulations are applied to the problem of deciding if the advantages provided in using a 4th grid outweigh the disadvantages compared to the 3 grid design. The results show that if erosion due to backstreaming ions is disregarded, including the fourth grid in the thruster design results in no apparent advantages in terms of the perfor- mance parameters studied in this work. The only noticeable difference between the three and four grid cases is a significant increase in the change in ion momentum observed when the fourth grid is not included in the design. The conclusion of the work is that the fourth grid should not be included in the dual stage design unless a very long lifetime is required and it is thought that erosion due to backstreaming will prevent the three grid thruster from fulfilling this criteria. The concept of propagating waves through the plasma within the ion thruster discharge chamber is investigated, with the aim of discovering any benefits and improvements in performance that may arise and forming a conclusion on whether further study on the topic of waves within the discharge chamber may be beneficial. No improvements in per- formance parameters were observed in this work, although further study in the area may show benefits to introducing waves into the plasma.

This thesis discusses the recent developments to the electric propulsion plume code DRACO as well as a validation and sensitivity analysis of the code using data from an AFRL experiment using a Busek 200 W Hall Thruster. DRACO is a PIC code that models particles kinematically while using finite differences schemes to solve the electric potential and field.

The DRACO code has been recently modified to improve simulation results, functionality and performance. A particle source has been added that uses the Hall Thruster device code HPHall as input for a source to model Hall Thrusters. The code is now also capable of using a non-uniform mesh that uses any combination of uniform, linear and exponential stretching schemes in any of the three directions. A stretched mesh can be used to refine simulation results in certain areas, such as the exit of a thruster, or improve performance by reducing the number of cells in a mesh. Finally, DRACO now has the capability of using a DSMC collision scheme as well as performing recombination collisions.

A sensitivity analysis of the newly upgraded DRACO code was performed to test the new functionalities of the code as well as validate the code using experimental data gathered at AFRL using a Busek 200 W Hall Thruster. A simulation was created that attempts to numerically recreate the AFRL experiment and the validation is performed by comparing the plasma potential, polytropic temperature, ion number density of the thruster plume as well as Faraday and ExB probe results. The study compares the newly developed HPHall source with older source models and also compares the variations of the HPHall source. The field solver and collision model used are also compared to determine how to achieve the best results using the DRACO code. Finally, both uniform and non-uniform meshes are tested to determine if a non-uniform mesh can be properly implemented to improve simulation results and performance.

The results from the validation and sensitivity study show that the DRACO code can be used to recreate a vacuum chamber simulation using a Hall Thruster. The best results occur when the newly developed HPHall source is used with a MCC collision scheme using a projected background neutral density and CEX collision tracking. A stretched mesh was tested and proved results that are as accurate as a uniform mesh, if not more accurate in locations of high mesh refinement.
Master of Science

L'avènement d'une nouvelle génération de lasers ultra-relativistes (d'éclairement supérieur à 10^22 W/cm2), tels le laser APOLLON sur le plateau de Saclay, donnera lieu à un régime d'interaction laser-matière sans précédent, couplant physique des plasmas relativistes et effets électrodynamiques quantiques. Sources de particules et de rayonnements aux propriétés énergétiques et spatio-temporelles inédites, ces lasers serviront, entre autres applications, à la mise au point de nouveaux concepts d'accélérateurs et de diagnostics radiographiques, au chauffage de plasmas denses, comme à la reproduction de configurations astrophysiques en laboratoire. En prévision des futures expériences, les codes particle-in-cell (PIC), qui constituent les outils de référence pour la simulation de l'interaction laser-plasma, doivent être enrichis des processus radiatifs et quantiques propres à ce nouveau régime d'interaction. C'est le cas du code CALDER développé au CEA/DAM, qui modélise désormais l'émission de photons énergétiques et la conversion de ceux-ci en paires électron-positron ; autant d'effets susceptibles d'affecter le bilan d'énergie de l'interaction laser-cible et, plus précisément, le rendement du laser en particules et rayonnements énergétiques. L'objet de ce stage théorique est d'étudier, à l'aide du code CALDER, l'influence de ces processus dans un certain nombre de scénarios physiques en champ extrême (accélération électronique et ionique dans un plasma surcritique, production de rayonnement, génération de choc non-collisionnel…)
Forthcoming multi-petawatt laser systems, such as the French Apollon and European Extreme Light Infrastructure facilities, are expected to deliver on-target laser intensities exceeding 10^22 W/cm^2. A novel regime of laser-matter interaction will ensue, where ultra-relativistic plasma effects are coupled with copious generation of high-energy photons and electron-positron pairs. This will pave the way for many transdisciplinary applications in fundamental and applied research, including the development of unprecedentedly intense, compact particle and radiation sources, the experimental investigation of relativistic astrophysical scenarios and tests of quantum electrodynamics theory.In recent years, most theoretical studies performed in this research field have focused on the impact of synchrotron photon emission and Breit-Wheeler pair generation, both directly induced by the laser field and believed to be dominant at intensities >10^22 W/cm^2. At the lower intensities (≲10^21 Wcm^(-2)) currently attainable, by contrast, photon and pair production mainly originate from, respectively, Bremsstrahlung and Bethe-Heitler/Trident processes, all triggered by atomic Coulomb fields. The conditions for a transition between these two regimes have, as yet, hardly been investigated, particularly by means of integrated kinetic numerical simulations. The purpose of this PhD is precisely to study the aforementioned processes under various physical scenarios involving extreme laser-plasma interactions. This work is carried out using the particle-in-cell CALDER code developed at CEA/DAM which, over the past few years, had been enriched with modules describing the synchrotron and Breit-Wheeler processes.Our first study aimed at extending the simulation capabilities of CALDER to the whole range of photon and positron generation mechanisms arising during relativistic laser-plasma interactions. To this purpose, we have implemented modules for the Coulomb-field-mediated Bremsstrahlung, Bethe-Heitler and Trident processes. Refined Bremsstrahlung and Bethe-Heitler cross sections have been obtained which account for electronic shielding effects in arbitrarily ionized plasmas. Following validation tests of the Monte Carlo numerical method, we have examined the competition between Bremsstrahlung/Bethe-Heitler and Trident pair generations by relativistic electrons propagating through micrometer copper foils. Our self-consistent simulations qualitatively agree with a 0-D theoretical model, yet they show that the deceleration of the fast electrons due to target expansion significantly impacts pair production.We then address the competition between Bremsstrahlung and synchrotron emission from copper foils irradiated at 10^22 Wcm^(-2). We show that the maximum radiation yield (into >10 keV photons) is achieved through synchrotron emission in relativistically transparent targets of a few 10 nm thick. The efficiency of Bremsstrahlung increases with the target thickness, and takes over synchrotron for >2μm thicknesses. The spectral properties of the two radiation processes are analyzed in detail and correlated with the ultrafast target dynamics.Finally, we investigate the potential of nanowire-array targets to enhance the synchrotron yield of a 10^22 Wcm^(-2) femtosecond laser pulse. Several radiation mechanisms are identified depending on the target parameters and as a function of time. A simulation scan allows us to identify the optimal target geometry in terms of nanowire width and interspacing, yielding a ∼10% radiation efficiency. In this configuration, the laser-driven nanowire array rapidly expands to form a quasi-uniform, relativistically transparent plasma. Furthermore, we demonstrate that uniform sub-solid targets can achieve synchrotron yields as high as in nanowire arrays, but that the latter enable a strong emission level to be sustained over a broader range of average plasma density

Les accélérateurs par onde de sillage plasma produites par faisceaux de particules (PWFA) ou par faisceaux laser (LWFA) appartiennent à un nouveau type d’accélérateurs de particules particulièrement prometteur. Ils permettent d’exploiter des champs accélérateurs jusqu’à cent Gigaélectronvolt par mètre alors que les dispositifs conventionnels se limitent à cent Megaélectronvolt par mètre. Dans le schéma d’accélération par onde de sillage plasma, ou par onde de sillage laser, un faisceau de particules ou une impulsion laser se propage dans un plasma et créé une structure accélératrice dans son sillage : c’est une onde de densité électronique à laquelle sont associés des champs électromagnétiques dans le plasma. L’un des principaux résultats de cette thèse a été la démonstration de l’accélération par onde de sillage plasma d’un paquet distinct de positrons. Dans le schéma utilisé, un plasma de Lithium était créé dans un four, et une onde plasma était excitée par un premier paquet de positrons (le drive ou faisceau excitateur) et l’énergie était extraite par un second faisceau (le trailing ou faisceau témoin). Un champ accélérateur de 1,36 GeV/m a ainsi été obtenu durant l’expérience, pour une charge accélérée typique de 40 pC. Nous montrons également ici la possibilité d’utiliser différents régimes d’accélération qui semblent très prometteurs. Par ailleurs, l’accélération de particule par sillage laser permet quant à elle, en partant d’une impulsion laser femtoseconde de produire un faisceau d’électron quasi-monoénergétique d’énergie typique de l’ordre de 200 MeV. Nous présentons les résultats d’une campagne expérimentale d’association de ce schéma d’accélération par sillage laser avec un schéma d’accélération par sillage plasma. Au cours de cette expérience un faisceau d’électrons créé par laser est refocalisé lors d’une interaction dans un second plasma. Une étude des phénomènes associés à cette plateforme hybride LWFA-PWFA est également présentée. Enfin, le schéma hybride LWFA-PWFA est prometteur pour optimiser l’émission de rayonnement X par les électrons du faisceau de particule crée dans l’étage LWFA de la plateforme. Nous présentons dans un dernier temps la première réalisation expérimentale d’un tel schéma et ses résultats prometteurs
Plasma wakefield accelerators (PWFA) or laser wakefield accelerators (LWFA) are new technologies of particle accelerators that are particularly promising, as they can provide accelerating fields of hundreds of Gigaelectronvolts per meter while conventional facilities are limited to hundreds of Megaelectronvolts per meter. In the Plasma Wakefield Acceleration scheme (PWFA) and the Laser Wakefield Acceleration scheme (LWFA), a bunch of particles or a laser pulse propagates in a gas, creating an accelerating structure in its wake: an electron density wake associated to electromagnetic fields in the plasma. The main achievement of this thesis is the very first demonstration and experimental study in 2016 of the Plasma Wakefield Acceleration of a distinct positron bunch. In the scheme considered in the experiment, a lithium plasma was created in an oven, and a plasma density wave was excited inside it by a first bunch of positrons (the drive bunch) while the energy deposited in the plasma was extracted by a second bunch (the trailing bunch). An accelerating field of 1.36 GeV/m was reached during the experiment, for a typical accelerated charge of 40 pC. In the present manuscript is also reported the feasibility of several regimes of acceleration, which opens promising prospects for plasma wakefield accelerator staging and future colliders. Furthermore, this thesis also reports the progresses made regarding a new scheme: the use of a LWFA-produced electron beam to drive plasma waves in a gas jet. In this second experimental study, an electron beam created by laser-plasma interaction is refocused by particle bunch-plasma interaction in a second gas jet. A study of the physical phenomena associated to this hybrid LWFA-PWFA platform is reported. Last, the hybrid LWFA-PWFA scheme is also promising in order to enhance the X-ray emission by the LWFA electron beam produced in the first stage of the platform. In the last chapter of this thesis is reported the first experimental realization of this last scheme, and its promising results are discussed

This work focuses on the analysis of plasma irregularities generated during two active space experiments: the injection of an artificial dust layer, and high-power radio waves. The objective of the "first experiment is to examine the effects of artificially created dust layers on the scatter of radars from plasma irregularities embedded in dusty plasma in space. This is an alternate approach for understanding the mechanisms of enhanced radar scatter from plasma irregularities embedded in Noctilucent Clouds and Polar Mesospheric Summer Echoes. The second experiment involves a transmission of high power electromagnetic waves into the ionospheric plasma from the ground, which can excite stimulated electromagnetic emissions offset from the transmitter frequency. These stimulated electromagnetic emissions provide diagnostic information of the ionosphere and thus can be used to investigate fundamental physical principles which govern the earth\'s ionosphere, so that present and future transmission technologies may take into account the complexities of the ionosphere. The interaction altitude of the artificial dust layer and high power radio waves is approximately 250 km and 160 km respectively, thus dealing with uniquely different regions of the ionosphere. Each experiment is discussed separately using theoretical, observational and advanced computational methodologies. The study first investigates plasma turbulence associated with the creation of an artificial dust layer in the earth's ionosphere. Two scenarios are considered for plasma irregularity generation as dust is injected at an oblique angle across the geomagnetic field. The first is a shear-driven plasma instability due to inhomogeneities in the boundary layer between the injected charged dust layer and the background plasma. This begins to appear at very early times once the dust is released into the space plasma, which is of the order or less than the dust charging time period. The second mechanism is free streaming of the charged dust relative to the background plasma. This produces irregularities at times much longer than the dust charging period and also longer than the dust plasma period. Although both mechanisms are shown to produce turbulence in the lower hybrid frequency range, the resulting irregularities have important differences in their physical characteristics. A comparison between the processes is made to determine the consequences for upcoming observations. Both processes are shown to have the possibility of generating turbulence after the release of dust for the regimes of upcoming space experiments over a range of timescales. This work also presents the first observations of unique narrowband emissions ordered near the Hydrogen ion (H+) gyro-frequency (fcH) in the Stimulated Electromagnetic Emission (SEE) spectrum when the transmitter is tuned near the second electron gyro-harmonic frequency (2fce), during ionospheric modification experiments. The frequency structuring of these newly discovered emission lines is quite unexpected since H+ is known to be a minor constituent in the interaction region which is near 160 km altitude. The spectral lines are typically shifted from the pump wave frequency by harmonics of a frequency about 10% less than fcH (" 800 Hz) and have a bandwidth of less than 50 Hz which is near the O+ gyro-frequency fcO. A theory is proposed to explain these emissions in terms of a Parametric Decay Instability (PDI) in a multi-ion species plasma due to possible proton precipitation associated with the disturbed conditions during the heating experiment. The observations can be explained by including several percent H+ ions into the background plasma. The implications are new possibilities for characterizing proton precipitation events during ionospheric heating experiments.
Ph. D.

Une impulsion laser ultra-courte et ultra-intense se propageant dans un gaz de faible densité est capable d'accélérer une partie des électrons de ce gaz à des énergies relativistes, de l'ordre de quelques centaines de MeV, sur des distances de seulement quelques millimètres. Pendant leur accélération et dû à leur mouvement transverse, ces électrons émettent de plus un rayonnement X fortement collimaté et dirigé vers l'avant appelé rayonnement bétatron. Les caractéristiques de cette source la rendent intéressante pour son utilisation en imagerie à ultra-haute résolution.Dans ce manuscrit, nous explorons trois axes de travail autour de cette source à l'aide de simulations réalisées avec les codes Particle-In-Cell CALDER et CALDER-Circ. Nous commençons ainsi par étudier la création d'une source bétatron avec des impulsions laser de durée picoseconde et d'énergie kilojoule, donc plus longues et plus puissantes que celles habituellement utilisées par la communauté. Nous montrons que malgré les paramètres inhabituels de ces impulsions lasers il est toujours possibles de générer des sources X, et ce dans deux régimes différents.Ensuite, afin de comprendre une partie des différences généralement observées entre expériences et simulations, nous montrons dans une autre étude que l'utilisation dans les simulations de profils lasers réalistes au lieu de profils parfaitement Gaussiens dégrade fortement les performances de l'accélérateur laser-plasma et de la source bétatron. De plus, ceci conduit à un meilleur accord qualitatif et quantitatif avec l'expérience.Enfin nous explorons plusieurs techniques pour augmenter l'émission X basées sur une manipulation des profils de plasmas utilisés pour l'accélération. Nous trouvons que l'utilisation d'un gradient transverse ou d'une marche de densité conduisent tous deux à une augmentation de l'amplitude du mouvement transverse des électrons, et donc de l'énergie émise par la source bétatron. Alternativement, nous montrons que cet objectif peut-être atteint par la transition d'un régime de sillage laser vers un régime d'accélération par sillage plasma induit par une augmentation de la densité. L'accélération des électrons est optimisée dans le premier régime, tandis que l'émission X est fortement favorisée dans le second
An ultra-short and ultra-intense laser pulse propagating in a low-density gas can accelerate in its wake a part of the electrons ionized from the gas to relativistic energies of a few hundreds of MeV over distances of a few millimeters only. During their acceleration, as a consequence of their transverse motion, these electrons emit strongly collimated X-rays in the forward direction, which are called betatron radiations. The characteristics of this source turn it into an interesting tool for high-resolution imagery.In this thesis, we explore three different axis to work on this source using simulations on the Particles-In-Cells codes CALDER and CALDER-Circ. We first study the creation of a betatron X-ray source with kilojoule and picosecond laser pulses, for which duration and energy are then much higher than usual in this domain. In spite of the unusual laser parameters, we show that X-ray sources can still be generated, furthermore in two different regimes.In a second study, the generally observed discrepancies between experiments and simulations are investigated. We show that the use of realistic laser profiles instead of Gaussian ones in the simulations strongly degrades the performances of the laser-plasma accelerator and of the betatron source. Additionally, this leads to a better qualitative and quantitative agreement with the experiment.Finally, with the aim of improving the X-ray emission, we explore several techniques based on the manipulation of the plasma density profile used for acceleration. We find that both the use of a transverse gradient and of a density step increases the amplitude of the electrons transverse motions, and then increases the radiated energy. Alternatively, we show that this goal can also be achieved through the transition from a laser wakefield regime to a plasma wakefield regime induced by an increase of the density. The laser wakefield optimizes the electron acceleration whereas the plasma wakefield favours the X-ray emission

Stimulated electromagnetic emission (SEE) produced by interactions of high-power radio waves with the Earth's ionosphere is currently a topic of significant interest in ionospheric modification physics. SEE is believed to be produced by nonlinear wave-wave interactions involving the electromagnetic and electrostatic plasma waves in the altitude region where the pump wave frequency is near the upper hybrid resonance frequency. The most prominent upshifted feature in the SEE spectrum is the broad upshifted maximum (BUM). In this study, the instability processes thought to be responsible to the BUM spectra in the SEE experiments are discussed and analyzed using theoretical and electrostatic particle-in-cell (PIC) models. From characteristics of this feature, a four-wave parametric decay process has been studied as a viable mechanism for its production. The object is to (1) investigate the early time nonlinear development of the four-wave decay instability by using theoretical and numerical simulation models, (2) study the variation of the four-wave decay instability spectral features for a wide range of plasma and pump wave parameters, and (3) access its possible role in the production of the BUM spectral feature. Results of this investigation show that there is good agreement between predictions of the proposed theoretical model and the numerical simulation experiments. The simulation electric field power spectrum exhibits many of the important features of the experimental observations. The numerical simulation results show that consideration of the full nonlinear development of the four-wave parametric instability is crucial in providing insight into the asymmetric nature of the wave frequency spectrum observed during the experiments. The velocity-space ring-plasma instability, another generation mechanism for the BUM spectra, is studied using a theoretical model. The theoretical calculations show that the growth rate is larger in the region of the upper hybrid wave than that of the electron Bernstein wave. In addition, the effects of various plasma parameters are analyzed and it is predicted that the BUM should be more prominent with a hotter ring, at the direction perpendicular to the magnetic field, or in a closer region of cyclotron harmonic. A detailed comparison of the velocity space ring-plasma instability and the four-wave parametric process is presented where both the differences and the possible relations are discussed.
Ph. D.

Une nouvelle approche, en utilisant un 3D code électromagnétique (PIC), est présentée pour étudier la sensibilité de la magnétosphère de la terre à la variabilité du vent solaire. Commençant par un vent solaire empiétant sur une terre magnétisée, le temps a été laissé au système ainsi une structure d'état d'équilibre de la magnétosphère a été atteinte. Une perturbation impulsive a été appliquée au système par changeant la vitesse du vent solaire pour simuler une dépression en sa pression dynamique, pour zéro, au sud et du nord du champ magnétique interplanétaire(IMF). La perturbation appliquée, un effet de trou d'air qui pourrait être décrit comme espace ~15Re est formé pour tous les cas d'état de IMF. Dès que le trou d'air a frappé le choc d'arc initial de la magnétosphère régulière, une reconnexion entre le champ magnétique de la terre et le IMF sud a été notée à la coté jour magnétopause(MP). Pendant la phase d'expansion du système, la frontière externe de la coté jour du MP a enfoncé quand IMF=0, et pourtant elle sa forme de balle quand un IMF au sud et nordique étaient inclus. La relaxation de temps du MP pour les trois cas de IMF a été étudiée. Le code est alors appliqué pour étudier l'événement d'Halloween de l'octobre 2003. Notre simulation a produit un nouveau genre de trou d'air, un espace raréfié qui a été produit après un gradient fort en IMF d'empiétement. Un tel dispositif est tout à fait semblable aux anomalies chaudes observées d'écoulement et peut avoir la même origine

Cette thèse s'inscrit dans le cadre de l'astrophysique de laboratoire. Nous abordons divers aspects de la physique des chocs non-collisionels en présence de flots de plasma relativistes dans des configurations d'intérêt pour les communautés astrophysique et de l’interaction laser-plasma (ILP). Notre approche repose sur la modélisation analytique et la simulation cinétique haute-performance, outil central pour décrire les processus d'ILP et la physique non linéaire à l'origine des chocs étudiés. Le code Particle-in-Cell SMILEI a été largement utilisé et développé au cours ce travail. Trois configurations physiques sont étudiées. L’instabilité Weibel en présence de faisceaux d'électrons contre-propagatifs alignés avec un champ magnétique externe est décrite. Les phases linéaires et non linéaires sont expliquées à l’aide de modèles théoriques confirmés par des simulations. La génération de chocs non-collisionels lors de l’interaction de deux plasmas relativistes de paires est étudiée en présence d’un champ magnétique perpendiculaire. L’accent est mis sur la comparaison des prédictions théoriques sur les grandeurs macroscopiques avec les simulations, ainsi que sur la définition du temps de formation du choc, l’ensemble de ces grandeurs étant d’une grande importance pour de futures expériences. Enfin, nous proposons un schéma permettant de recréer, en laboratoire, l’instabilité Weibel ionique par l'utilisation d'un laser intense. Les flots de plasmas produits ici sont plus rapides et denses que dans les expériences actuelles, conduisant à un taux de croissance et des champs magnétiques plus élevés. Ces résultats sont également important pour l’ILP à très haute intensité
The work presented in this thesis belongs to the general framework of Laboratory Astrophysics. We address various aspects of the physics of collisionless shocks developing in the presence of relativistic plasma flows, in configurations of interest for the astrophysical and the laser-plasma interaction (LPI) communities. The approach used throughout this thesis relied on both analytical modeling and high-performance kinetic simulations, a central tool to describe LPI processes as well as the non-linear physics behind shock formation. The PIC code SMILEI has been widely used and developed during this work. Three physical configurations are studied. First we consider the Weibel instability driven by two counter-streaming electron beams aligned with an external magnetic field. The linear and non-linear phases are explained using theoretical models confirmed by simulations.Then the generation of non-collisional shocks during the interaction of two relativistic plasma pairs is studied in the presence of a perpendicular magnetic field. We focus on the comparison of theoretical predictions for macroscopic variables with the simulation results, as well as on the definition and measurement of the shock formation time, all of which are of great importance for future experiments.Finally, we proposed a scheme to produce, in the laboratory, the ion-Weibel-instability with the use of an ultra-high-intensity laser. The produced flows are faster and denser than in current experiments, leading to a larger growth rate and stronger magnetic fields. These results are important for the LPI at very high intensity

Lors de la propagation d'une impulsion laser ultra courte, ultra intense dans un gaz de faible densité, un plasma est créé et une partie des électrons vont pouvoir être accélérés grâce à la technique de sillage laser à des énergies de plusieurs GeV en quelques centimètres.Ces électrons, lors de leur accélération, émettent un rayonnement X appelé bêtatron, qui est fortement collimaté et possède de très bonnes propriétés spatiales et temporelles, lui donnant de nombreuses applications dont l'imagerie ultra-haute résolution.Dans cette thèse, on étudie comment améliorer les outils numériques utilisés pour simuler ces phénomènes physiques : les codes Particle-In-Cell (CALDER). On s'intéresse notamment à un artefact numérique appelé rayonnement Cherenkov numérique, qui survient lorsque les particules accélérés se déplacent à des vitesses proches de la vitesse de la lumière dans le vide.On démontre que cet artefact a un effet néfaste sur le comportement du faisceau d'électrons accélérés, en particulier sur son mouvement transverse, ce qui conduit à des erreurs importantes sur le calcul du rayonnement bêtatron à partir des simulations PIC. On propose alors une nouvelle approche pour limiter l'impact de ce rayonnement Cherenkov numérique sur les simulations d'accélération par sillage laser en modifiant la méthode d'interpolation des champs habituellement utilisée dans un code PIC. Les résultats obtenus avec cette nouvelle technique mettent en évidence une nette amélioration de la modélisation du mouvement des électrons, qui se rapproche du comportement attendu théoriquement. Fort de ces premiers résultats, d'autres applications de cette technique sont ensuite explorées, pour améliorer la modélisation des sources bêtatron, de l'accélération par laser dans le vide ou de l'accélération directe par laser.La plus grande précision sur le calcul du mouvement transverse des particules qu'apporte cette nouvelle méthode permet d'améliorer les résultats mais aussi d'étudier des phénomènes physiques aux effets subtils qui sont autrement cachés par le bruit numérique des simulations
When an ultra-short ultra-intense laser impulsion propagates through a low density gas jet, a plasma is created and a bunch of electrons can be accelerated through laser wakefield acceleration to Gev energies in only a few centimetres. Those accelerated electrons then emit what is called Betatron radiation: a highly focused X-ray source with extremely good spatial and temporal properties, which has a lot of possible applications including ultra-high resolution imaging.In this thesis, we investigate possible improvements to one of the main numerical tools used to simulate those phenomenons: the Particle-In-Cell codes (CALDER). We have especially studied a numerical artefact called the numerical Cherenkov radiation, that occurs when relativistic particles move at speeds aproaching the speed of light in a vaccuum.We show that this artefact has a negative impact on the behaviour of the accelerated electron beam, especially on its transverse motion, which leads to important errors on the betatron radiation calculated using PIC simulations.We then introduce a new approach to mitigate the impact of this numerical Cherenkov radiation on laser wakefield acceleration simulation with a simple modification of the electromagnetic field interpolation method used in PIC codes. The results obtained with this new technique show a meaningful improvement on the electron motion wich becomes close to the theoretically expected behaviour.We then explore other possible applications for this new technique, notably improving the modelization of betatron sources, vacuum laser acceleration or direct laser acceleration.The improvement of the computation of the particles transverse motion thanks to this new method leads to more accurate results but also enables us to study physical phenomenon with subtle effects that would otherwise be hidden among the numerical noise of the simulation

Today's supercomputers often consists of clusters of SMP nodes. Both OpenMP and MPI are programming paradigms that can be used for parallelization of codes for such architectures. OpenMP uses shared memory, and hence is viewed as a simpler programming paradigm than MPI that is primarily a distributed memory paradigm. However, the Open MP applications may not scale beyond one SMP node. On the other hand, if we only use MPI, we might introduce overhead in intra-node communication. In this thesis we explore the trade-offs between using OpenMP, MPI and a mix of both paradigms for the same application. In particular, we look at a physics simulation and parallalize it with both OpenMP and MPI for large-scale simulations on modern supercomputers. A parallel SOR solver with OpenMP and MPI is implemented and the effects of such hybrid code are measured. We also utilize the FFTW-library that includes both system-optimized serial implementations and a parallel OpenMP FFT implementation. These solvers are used to make our existing Particle-In-Cell codes be more scalable and compatible with current programming paradigms and supercomputer architectures. We demonstrate that the overhead from communications in OpenMP loops on an SMP node is significant and increases with the number of CPUs participating in execution of the loop compared to equivalent MPI implementations. To analyze this result, we also present a simple model on how to estimate the overhead from communication in OpenMP loops. Our results are both surprising and should be of great interest to a large class of parallel applications.

Cette thèse porte sur la caractérisation des champs magnétiques générés par l'interaction entre un laser d'intensité de 1017 W/cm2 à 1018 W/cm2 et de cibles solides, et leurs effets sur le faisceau d'électrons chauds. En effet, les différents champs magnétiques créés lors de cette interaction ont un rôle fondamental sur les caractéristiques du faisceau d'électrons chauds : sa source et son transport dans la matière. Des diagnostics de polarimétrie et d'interférométrie croisée ont été développés lors de cette thèse pour observer le champ magnétique en surface de la cible irradiée par laser et en particulier leurs évolutions spatiale et temporelle. Deux différents régimes ont été observés selon le contraste en intensité de l'impulsion laser : un possédant une montée rapide de champ magnétique suivie d'une décroissance plus lente créées par le déplacement des électrons chauds dans la matière, et un possédant une croissance plus lente de forme logarithmique créée par la pré-impulsion du laser par effet thermoélectrique. L'interprétation de nos résultats obtenues par ces diagnostics ont permis d'évaluer la résistivité du plasma. Cette résistivité nommée anormale dans la littérature se comprend en estimant l'influence du champ magnétique sur l'anisotropie du transport des électrons et donc sur la résistivité. Le dernier diagnostic permettant l'estimation du champ magnétique détaillé dans cette thèse est la déflectométrie protonique. Elle permet d'observer la déviation d'un faisceau de protons lors de sa propagation sous l'effet de champs électrique et magnétique. D'autres expériences se sont focalisées sur la divergence de ce faisceau d'électrons. Deux diagnostics principaux ont été utilisés : l'imagerie K α et l'imagerie du rayonnement de transition cohérente (C.T.R.) en face arrière de cibles
This thesis concerns magnetic fields, generated by the interaction between strong laser pulse (intensity up to1018 W/cm2) and solid target, and their effects on the fast electron beam. Indeed, the various magnetic fields created during this interaction can inuence the divergence of the fast electron beam. The magnetic field createdduring this interaction have a fundamental role on the fast electron beam characteristics : its source and its transportin the material. Diagnotics of polarimetry and crossed interferometry were developed during this thesis to observethe on-surface magnetic field of the target, and in particular, their spatial and temporal evolutions. Two types oftemporal evolution of the magnetic field were observed according to the contrast in intensity of the laser pulse : afast rise of magnetic field followed by a slower decrease created by the travel of the fast electrons in the material,and a slower growth of logarithmic form created by the pre-pulse of the laser by thermoelectric effect. The interpretation of our results obtained by these diagnotics allowed us to estimate the resistivity of the plasma.This resistivity named "anomalously high resistivity" in the literature can be explained by taking into account theinuence of the magnetic field on the electrons transport (creation of an anisotropy) and thus on the resitivity.The last diagnotic allowing the estimation of the magnetic field detailed in this thesis is the proton deectometry. itallows to observe the deviation of a proton beam during its propagation under the inuence of electric and magneticfields. Other experiments were focused on the fast electron beam divergence. Two main diagnotics were used : the K α imaging and the coherent transition radiation (C.T.R) imaging at the rear side of solid targets. These diagnoticsallowed to estimate the fast electron beam divergence for two distinct energetic electron populations. The differenceof divergence coming from characteristics of both diagnotics (electrons in charge of the emissions in different energies). The diagnotics of on-surface magnetic fields of target irradiated by intense laser, such as the technics of polarimetry and crossed interferometry developed in this thesis, are dedicated to be combined with diagnotics determining the evolution of the radial size of the fast electron beam generated by the laser-matter interaction. Their simultaneous use, and the correlation between their respective data, should allow to establish experimentally, in the short term, the inuence of the on-surface magnetic fields on the fast electron beam initial characteristics, in particular the angular and energy distributions. Our results of polarimetry on the spatio-temporal evolution of the magnetic fields of surface establish the state of the art for this type of measures. There are possible improvements, in particular as regards their use in conditions of irradiation by lasers of intensities > 1018 W/cm2. These perspectives are also the object of discussions in this manuscript

Cette thèse s'inscrit dans le cadre de la recherche sur la fusion nucléaire par confinement inertiel, et vise notamment à contribuer à la validation du schéma d'allumage rapide. Elle consiste en une étude expérimentale des différents processus impliqués dans la propagation d'un faisceau d'électrons relativistes, produit par une impulsion laser ultra-intense (10^{19} W.cm-2), au sein de la matière dense qu'elle soit solide ou comprimée. Dans ce travail de recherche nous présentons les résultats de trois expériences réalisées sur des installations laser distinctes afin de générer des faisceaux d'électrons dans diverses conditions et d'étudier leur propagation dans différents états de la matière, du solide froid au plasma dense et tiède.La première expérience a été réalisée à très haut contraste temporel sur l'installation laser UHI100 du CEA de Saclay. L'étude du dépôt d'énergie par le faisceau d'électrons dans l'aluminium solide a mis en évidence un important chauffage à faible profondeur, où les effets collectifs sont prédominants, générant ainsi un gradient important de température entre les faces avant (300eV) et arrière (20eV) sur 20µm d'épaisseur. Une modélisation numérique de l'expérience montre que ce gradient induit la formation d'une onde de choc débouchant en face arrière, donnant alors lieu à une augmentation de l'émission thermique. La chronométrie expérimentale du débouché du choc permet de valider le modèle de transport collectif des électrons.Deux autres expériences ont porté sur l'étude de la propagation de faisceaux d'électrons rapides au sein de cibles comprimées. Lors de la première expérience sur LULI2000 (LULI), la géométrie de compression plane a permis de dissocier de manière précise les pertes d'énergie liées aux effets résistifs de celles liées aux effets collisionnels. En comparant nos résultats expérimentaux à des simulations, nous avons mis en évidence l'augmentation significative des pertes d'énergie du faisceau d'électrons avec la compression et le chauffage de la cible à des température proches de la température de Fermi, et ce, pour les deux mécanismes. La seconde expérience, réalisée en géométrie cylindrique sur Vulcan (RAL), a permis de mettre en évidence un phénomène de guidage du faisceau d'électrons rapides sous l'effet d'un intense champ magnétique, auto-généré en présence d'importants gradients radiaux de résistivité. Par ailleurs, dans les conditions de température et de densité atteintes, l'augmentation des pertes d'énergie collisionnelles avec la densité s'avère être compensée par une diminution des pertes résistives du fait du passage de la conductivité du milieu dans le régime des hautes températures de Spitzer
The framework of this PhD thesis is the validation of the fast ignition scheme for the nuclear fusion by inertial confinement. It consists in the experimental study of the various processes involved in fast electron beams propagation, produced by intense laser pulses (10^{19} W.cm-2), through dense matter either solid or compressed. In this work we present the results of three experiments carried out on different laser facilities in order to generate fast electron beams in various conditions and study their propagation in different states of matter, from the cold solid to the warm and dense plasma.The first experiment was performed with a high intensity contrast on the UHI100 laser facility (CEA Saclay). The study of fast electron energy deposition inside thin aluminium targets highlights a strong target heating at shallow depths, where the collectivs effects are predominant, thus producing a steep temperature profile between front (300eV) and rear (20eV) sides over 20µm thickness. A numerical simulation of the experiment shows that this temperature gradient induces the formation of a shock wave, breaking through the rear side of the target and thus leading to increase the thermal emission. The experimental chronometry of the shock breakthrough allowed validating the model of the collective transport of electrons.Two other experiments were dedicated to the study of fast electron beam propagation inside compressed targets. In the first experiment on the LULI2000 laser facility, the plane compression geometry allowed to precisely dissociate the energy losses due to resistive effects from those due to the collisional ones. By comparing our experimental results with simulations, we observed a significative increase of the fast electron beam energy losses with the compression and the target heating to temperatures close to the Fermi temperature. The second experiment, performed in a cylindrical geometry, demonstrated a fast electron beam guiding phenomenon due to self-generated magnetic fields in presence of sharp radial resistivity gradients. Furthermore, in the temperature and density conditions achieved here, the increase of collisional energy losses with density is compensated by the decreasing resistive energy losses due to the transition of the conductivity into the high-temperatures Spitzer regime

L'interaction d'impulsions lasers brèves et intenses avec un plasma est une source intéressante d'ions énergétiques. Les travaux effectués au cours de cette thèse s'articulent autour de deux grandes thématiques : la production de protons par laser, et leur utilisation pour le chauffage isochore, avec, pour principal outil d'étude, la simulation à l'aide de codes numériques (cinétique particulaire et hydrodynamique). Dans un premier temps, nous avons étudié le comportement de l'énergie cinétique maximale des protons qu'il est possible d'accélérer avec le mécanisme du Target Normal Sheath Acceleration (TNSA), en régime sub-ps, en fonction de différents paramètres, notamment de la durée d'impulsion laser. Nous avons montré que l'allongement de la durée d'impulsion, à énergie laser constante, est responsable du préchauffage et de la détente du plasma avant l'arrivé du pic d'intensité. Les gradients de densité ainsi produits (face avant et face arrière) peuvent favoriser, ou au contraire pénaliser, le gain en énergie cinétique des protons. Les résultats obtenus ont servi à l'interprétation d'une étude expérimentale réalisée au Laboratoire d'Optique Appliquée. Nos efforts se sont ensuite concentrés sur l'élaboration d'un modèle semi-analytique rendant compte de l'énergie cinétique maximale des protons accélérés par le biais du TNSA. Ce modèle permet de retrouver l'ordre de grandeur des intensités nécessaires, de l'ordre de 6x1021 W/cm², pour atteindre des énergies de proton supérieures à 150 MeV avec des impulsions laser de quelques joules et plusieurs dizaines de fs. Dans la dernière partie de cette thèse, nous nous sommes intéressés à l'utilisation de ces faisceaux de protons pour le chauffage isochore. Nous avons caractérisé, dans un premier temps, les fonctions de distribution produites par des cibles composées d'un substrat lourd (A >> 1) sur la face arrière duquel est déposé un plot d'hydrogène (schéma d'Esirkepov). Ensuite, à partir de simulations hydrodynamiques, nous avons étudié le temps caractéristique de détente de la cible chauffée en modifiant des paramètres tels que la distance à la source de protons, l'intensité et la tache focale du laser, et la densité surfacique du plot. Nous avons enfin étendu cette étude aux cibles cylindriques et nous avons montré qu'il est possible de réduire les effets liés à la divergence naturelle du faisceau de protons et ainsi d'obtenir des températures plus élevées.

Cette thèse a pour contexte la résolution numérique du système de Vlasov–Poisson (modèle utilisé en physique des plasmas, par exemple dans le cadre du projet ITER) par les méthodes classiques particulaires (PIC pour "Particle-in-Cell") et semi-Lagrangiennes. La contribution principale de notre thèse est une implémentation efficace de la méthode PIC pour architectures multi-coeurs, écrite dans le langage C, dont le nom est Pic-Vert. Notre implémentation (a) atteint un nombre quasi-minimal de transferts mémoires avec la mémoire principale, (b) exploite les instructions vectorielles (SIMD) pour les calculs numériques, et (c) expose une quantité suffisante de parallélisme, en mémoire partagée. Pour mettre notre travail en perspective avec l'état de l'art, nous proposons une métrique permettant de comparer différentes implémentations sur différentes architectures. Notre implémentation est 3 fois plus rapide que d'autres implémentations récentes sur la même architecture (Intel Haswell)
In this thesis, we are interested in solving the Vlasov–Poisson system of equations (useful in the domain of plasma physics, for example within the ITER project), thanks to classical Particle-in-Cell (PIC) and semi-Lagrangian methods. The main contribution of our thesis is an efficient implementation of the PIC method on multi-core architectures, written in C, called Pic-Vert. Our implementation (a) achieves close-to-minimal number of memory transfers with the main memory, (b) exploits SIMD instructions for numerical computations, and (c) exhibits a high degree of shared memory parallelism. To put our work in perspective with respect to the state-of-the-art, we propose a metric to compare the efficiency of different PIC implementations when using different multi-core architectures. Our implementation is 3 times faster than other recent implementations on the same architecture (Intel Haswell)

L’obtention d’un Laser à Électrons Libres X (LEL-X) compact est un objectif majeur pour le développement des lasers. Plusieurs schémas prometteurs de LEL-X ont été proposés en utilisant à la fois l’accélération d’électrons dans les plasmas et des onduleurs optiques en régime Compton ou Compton inverse. Nous avons proposé un nouveau concept de LEL-X compact baptisé Raman XFEL, en combinant la physique des LEL en régime Compton, des lasers XUV conventionnels basés sur l’interaction laser plasma, et de l’optique non-linéaire. Nous étudions dans cette thèse les étapes préalables pour déclencher un effet laser à rayons X lors de l’interaction entre un paquet d’électrons libres relativistes et un réseau optique créé par l’interférence transverse de deux impulsions laser intenses. Dans cet objectif j’ai développé un code particulaire baptisé RELIC. Les études menées avec le code RELIC nous ont permis d’étudier la dynamique d’électrons relativistes et les processus d’injection du paquet d’électrons dans le réseau optique. Grâce à RELIC, nous avons distingué de nouveaux régimes d’interaction en fonction des paramètres du paquet d’électrons, ainsi que de la géométrie du réseau optique. Ces études ont été appliquées à l’amplification du rayonnement X et appuyées par des simulations PIC. RELIC a également permis de modéliser et d’analyser la première expérience réalisée en octobre 2015 sur l’installation laser ’Salle Jaune’ au Laboratoire d’Optique Appliquée (LOA). Cette première expérience a été une étape très importante pour la validation des modèles théoriques, et pour la réalisation future d’un laser à électrons libre X Raman
The quest for a compact X-ray laser has long been a major objective of laser science. Several schemes using optical undulators are currently considered, in order to trigger the amplification of back scattered radiation, in Compton or inverse Compton regime. We have proposed a new concept of compact XFEL based on a combination between the physics of free electron lasers, of laser-plasma interactions, and of nonlinear optics. In this thesis, we study the necessary steps to trigger a X-ray laser during the interaction between a free relativistic electron bunch and an optical lattice created by the interference of two intense transverse laser pulses. For this purpose I developed a particular tracking code dubbed RELIC. RELIC allowed us to study the dynamics and injection process of a bunch of relativistic electrons into the optical lattice. Thanks to RELIC, we distinguished several interaction regimes depending on the relativistic electron bunch parameters, and on those of the optical lattice and its geometry. These studies are applied to the X ray amplification and supported by PIC simulations. RELIC also allowed us to model and analyze the first experiment conducted in october 2015 on the ”Salle Jaune” laser facility at LOA. This first experiment was very important to validate our theoretical models, and should prove to be an essential milestone for the development of a Raman X-ray free electron laser