Thèses sur le sujet « Instabilités de type Weibel »
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Sarrat, Mathieu. « Physique des instabilités de type Weibel ». Thesis, Université de Lorraine, 2017. http://www.theses.fr/2017LORR0162/document.
Texte intégralWeibel-type instabilities occurs when the velocity distribution function of the charged particles displays a pronounced anisotropy. A long-lasting magnetic field is generated due to the formation of current filaments, and it is accompanied by an important electrostatic activity. These ``basic’’ phenomena have been greatly investigated because of their involvement in many physical problems, natural (solar wind, relativistic jets) or experimental (laser-plasma interaction) : they occurs in plasmas which can be collisional or not, magnetised or not, relativistic or not. One needs to choose a suitable model for their description. The kinetic theory is the most complete and somewhat complex theoretical framework which we will consider. Due to its complexity, it may be interesting to develop reduced models. The first work realised during this thesis is the utilisation of a non-relativistic fluid description, including the dynamics of the pressure tensor, in order to model the linear Weibel-type instabilities. We put in evidence the effect of the non-diagonal components of the tensor on the magnetic field generation. We discuss the ability of the model to reproduce quantitatively or qualitatively the kinetic results by introducing the hydrodynamics limit. The second part of this thesis work is dedicated to the development of the relativistic semi-lagrangian code VLEM, using a domain decomposition scheme : we present the main mathematical tools used in the code, then we deal with the problem of the charge conservation and propose a solution for VLEM, based on an adaptation of the Esirkepov method. Finally, we validate the code through simulations of Weibel-type
Inglebert, Aurélie. « Modèle Vlasov-Maxwell pour l'étude des instabilités de type Weibel ». Thesis, Université de Lorraine, 2012. http://www.theses.fr/2012LORR0149/document.
Texte intégralThe origin of magnetic fields observed in laboratory and astrophysical plasmas is one ofthe most challenging problems in plasma physics. In this respect, the Weibel type instabilities are considered of key importance. These instabilities are caused by a temperature anisotropy (Weibel instability) and electron momentum (current filamentation instability). The main objective of this thesis is the theoretical and numerical study of these instabilities in a collisionless plasma in the relativistic regime. The first aspect of this work is to study the nonlinear regime of these instabilities and the role of kinetic and relativistic effects on the structure of self-consistent electromagnetic fields. In this context, a key problem for the theory and applications, is the identification and analysis of coherent structures developed spontaneously in the nonlinear regime of kinetic scales. A second aspect of the work is the development of analytical and numerical techniques for the study of collisionless plasmas. A mathematical model of reference is the Vlasov-Maxwell model, where the Vlasov equation (mean field theory) is coupled to the Maxwell equations in a self-consistent way. A one-dimensional model, the multi-stream model, is also introduced. Based on a dimensional reduction technique, it is both an analytical model "simple" having the advantage of being able to solve a 1D Vlasov equation for each particle beam, and a numerical model less expensive than a complete model
Betar, Homam. « Kinetic Effects in Magnetic Reconnection ». Electronic Thesis or Diss., Université de Lorraine, 2021. http://www.theses.fr/2021LORR0043.
Texte intégralPlasmas are gaseous systems of ions and electrons which interact via electromagnetic fields and display collective properties. Among these, is the notion of the magnetic line "connection". This expresses the fact that, in regimes in which charged particles spiral sufficiently fast along lines of magnetic induction, the latter is linked to the bulk plasma motion and acquire a topological identity which forbids them to break, intersect and reconnect. This topological identity, however, can be locally violated thanks to a number of kinetic effects, such as particle collisions, when the currents in the plasma are sufficiently intense: one speaks of "magnetic reconnection". Magnetic reconnection is an important ingredient of the plasma self-organization and has significance for both space and laboratory plasmas since it is at the basis of natural phenomena like solar flares and polar lights, or of disruptive processes in thermonuclear fusion experiments. A long-standing problem in the study of laboratory and astrophysical plasmas is to understand the mechanisms of acceleration of electrons and ions, as a magnetic field reconnect and release energy. In this work, we studied kinetic effects on reconnection instabilities developing spontaneously in static current sheets (tearing modes) and in combination with a class of kinetic instabilities (Weibel instabilities) that are relevant both to astrophysical plasma jets and to laser-plasma interaction experiments. We performed this study using reduced-fluid and kinetic models and we investigated the competition between tearing-type modes and Weibel-type instabilities by means of both semi-lagrangian full kinetic Vlasov-Maxwell simulations and particles in cell simulations
Boutrouft, Keltoum. « Instabilités thermoconvectives de type Rayleigh-Taylor dans les fluides supercritiques ». Phd thesis, Paris, ENSAM, 2006. http://pastel.archives-ouvertes.fr/pastel-00002349.
Texte intégralRuyer, Charles. « Kinetic instabilities in plasmas : from electromagnetic fluctuations to collisionless shocks ». Thesis, Paris 11, 2014. http://www.theses.fr/2014PA112370/document.
Texte intégralCollisionless shocks play a major role in powerful astrophysical objects (e.g., gamma-ray bursts, supernova remnants, pulsar winds, etc.), where they are thought to be responsible for non-thermal particle acceleration and radiation. Numerical simulations have shown that, in the absence of an external magnetic field, these self-organizing structures originate from electromagnetic instabilities triggered by high-velocity colliding flows. These Weibel-like instabilities are indeed capable of producing the magnetic turbulence required for both efficient scattering and Fermi-type acceleration. Along with rapid advances in their theoretical understanding, intense effort is now underway to generate collisionless shocks in the laboratory using energetic lasers. In a first part we study the (w,k)-resolved electromagnetic thermal spectrum sustained by a drifting relativistic plasma. In particular, we obtain analytical formulae for the fluctuation spectra, the latter serving as seeds for growing magnetic modes in counterstreaming plasmas. Distinguishing between subluminal and supraluminal thermal fluctuations, we derived analytical formulae of their respective spectral contributions. Comparisons with particle-in-cell (PIC) simulations are made, showing close agreement in the subluminal regime along with some discrepancy in the supraluminal regime. Our formulae are then used to estimate the saturation time of the Weibel instability of relativistic pair plasmas. Our predictions are shown to match 2-D particle-in-cell (PIC) simulations over a three-decade range in flow energyWe then develop a predictive kinetic model of the nonlinear phase of the Weibel instability induced by two counter-streaming, symmetric and non-relativistic ion beams. This self consistent, fully analytical model allows us to follow the evolution of the beams' properties up to a stage close to complete isotropization and thus to shock formation. Its predictions are supported by 2D and 3D particle-in-cell (PIC) simulations of the ion Weibel instability in uniform geometries, as well as shock-relevant non-uniform configurations. Moreover, they are found in correct agreement with a recent laser-driven plasma collision experiment. Along with this comparison, we pinpoint the important role of electron screening on the ion-Weibel dynamics, which may affect the results of simulations with artificially high electron mass. We subsequently address the shock propagation resulting from the magnetic Weibel turbulence generated in the upstream region. Generalizing the previous symmetric-beam model to the upstream region of the shock, the role of the magnetic turbulence in the shock-front has been analytically and self-consistently characterized. Comparison with simulations validates the model. The interaction of high-energy, ultra-high intensity lasers with dense plasmas is known to produce copious amounts of suprathermal particles. Their acceleration and subsequent transport trigger a variety of Weibel-like electromagnetic instabilities, acting as additional sources of slowing down and scattering. Their understanding is important for the many applications based upon the energy deposition and/or field generation of laser-driven particles. We investigate the ability of relativistic-intensity laser pulses to induce Weibel instability-mediated shocks in overdense plasma targets, as first proposed by Fiuza in 2012. By means of both linear theory and 2D PIC simulations, we demonstrated that in contrast to the standard astrophysical scenario previously addressed, the early-time magnetic fluctuations (Weibel instability) generated by the suprathermal electrons (and not ions) are strong enough to isotropize the target ions and, therefore, induce a collisionless electromagnetic shock
Qiu, Ruihao. « Étude théorique des instabilités de type ferroïques dans des géométries confinées et des réseaux distordus ». Thesis, Bordeaux, 2017. http://www.theses.fr/2017BORD0660/document.
Texte intégralIn this thesis, we present a theoretical study of two types of ferroic instabilities: the ferroelectric instability in novel confined geometries and magnetic instabilities controlled by the distortion of the underlying crystal lattice. On the one hand, we consider in detail the ferroelectric instability, specifically, in the nanotubes and the spherical nanoshells and develop a phenomenological theory for describing such an instability. We determine how the emergence of polarization is affected bythe thickness of the nanoparticle, the dielectric properties of the surrounding media and the interfacial boundary conditions. We finnd an intriguing topological finite-size effect that can promote an unexpected competition between two different types of distribution of polarization - irrotational and vortex-like - in the ultra-thin limit. One the other hand, we employ a different formalism to investigate the structural, electronic and magnetic properties of the rare-earth manganites. Specifically,we conduct a theoretical investigation from first-principles calculations. First, we predict a pressure-induced A-AFM insulator to FM metal transition on EuMnO3 under hydrostatic pressure, that is unprecedented in the multiferroic rare-earth manganites RMnO3. This investigation is extended to the study to the epitaxial strain effects on both EuMnO3 and TbMnO3 thin films. We show that epitaxial strain generates a much richer phase diagram compared to hydrostatic pressure. We predict novel magnetically-induced insulator { metal and polar { non-polar transitions. More specifically, we find that both the multiferroic E-AFM order and the polar metallic E*-AFM state are stabilized in TbMnO3 by means of epitaxial strain. In the contrast, we find a novel epitaxial-strain-induced multiferroic E-AFM state in EuMnO3 that cannot be obtained by means of just hydrostatic pressure
Simon, Patrice. « Ferroélectricité, mouvement des protons et instabilités dans les cristaux du type KH₂PO₄ : étude par réflexion infrarouge ». Orléans, 1986. http://www.theses.fr/1986ORLE3024.
Texte intégralPrioul, Mathieu. « Etude expérimentale des propulseurs de type Hall : processus collisionnels, comportement dynamique, micro-instabilités et phénomènes de transport ». Orléans, 2002. http://www.theses.fr/2002ORLE2058.
Texte intégralLe, Bourdiec Solène. « Méthodes déterministes de résolution des équations de Vlasov-Maxwell relativistes en vue du calcul de la dynamique des ceintures de Van Allen ». Phd thesis, Ecole Centrale Paris, 2007. http://tel.archives-ouvertes.fr/tel-00146258.
Texte intégralLe travail de cette thèse a consisté à concevoir un schéma numérique original pour la résolution du système d'équations modélisant ces interactions : les équations de Vlasov-Maxwell relativistes. Notre choix s'est orienté vers des méthodes d'intégration directe. Nous proposons trois nouvelles méthodes spectrales pour discrétiser en impulsion les équations : une méthode de Galerkin et deux méthodes de type collocation. Ces approches sont basées sur des fonctions de Hermite qui ont la particularité de dépendre d'un facteur d'échelle permettant d'obtenir une bonne résolution en vitesse.
Nous présentons dans ce manuscript les calculs conduisant à la discrétisation et à la résolution du problème de Vlasov-Poisson monodimensionnel ainsi que les résultats numériques obtenus. Puis nous étudions les extensions possibles des méthodes au problème complet relativiste. Afin de réduire les temps de calcul, une parallélisation et une optimisation des algorithmes ont été mises en \oe uvre. Enfin, les calculs de validation du code 1Dx-3Dv, à partir d'instabilités de types Weibel et whistlers, à une ou deux espèces d'électrons, sont détaillés.
Egly, Hugues. « Contribution à la modélisation et à la simulation des instabilités de type Rayleigh-Taylor ablatif pour la FCI ». Paris 6, 2007. http://www.theses.fr/2007PA066684.
Texte intégralThis thesis deals with the dynamics of accelerated ablative front spreading in Inertial Confinement Fusion experiments. ICF is designed for the implosion of a mdeuterium or tritium spherical target. The outer shell, the ablator, is irradiated providing a high level pressure inside the target. During this first stage, the ablation front propagating inward is perturbed by hydrodynamics instabilities, which can prevent the fusion reaction in the decelerated stage. We propose here a study on Rayleigh-Taylor instabilities during ablation process, in the two dimensional case. In order to obtain a numerical solution, we perform an asymptotic analysis in the limit of a high temperature ratio, between the remaining cold ablator and the hot ablated plasma. This study is divided in two steps. First, the thermo-diffusive part of the set of equations is approximated by a Hele-Shaw model, which is then perturbed by the hydrodynamics part. Using a vortex method, we have to solve the advection of a vortical sheet moving with the ablation front. We compute the numerical solution on an eulerian mesh coupled with a front tracking method. The thermal part is calculated by implementing the Fat Boundary Method, recently developped. The hydrodynamic part is obtained from a Finit Volume scheme
Ould-Hademine, Elhhousseine. « Etude expérimentale des structures d'écoulement dans l'espace rotor-stator d'un moteur électrique rapide de type ouvert ». Poitiers, 1996. http://www.theses.fr/1996POIT2282.
Texte intégralIvanov, Andrey. « Etude théorique et numérique des instabilités Rayleigh-Taylor en plasmas magnétisés ». Phd thesis, Ecole Polytechnique X, 2001. http://tel.archives-ouvertes.fr/tel-00005775.
Texte intégralOn a étudié l'influence de la diffusion Hall du champ magnétique sur le taux de croissance de l'instabilité. On a obtenu des solutions auto-similaires pour l'élargissement du profil initial et pour l'onde de pénétration du champ magnétique. Puis, on a étudié l'évolution postérieure du champ magnétique dans les commutateurs à ouverture de plasmas (COP) et on a montré la possibilité d'existence d'une onde de raréfaction forte, dans les cas avec et sans collisions. Cette onde peut expliquer le phénomène d'ouverture de COP.
Le phénomène de suppression de l'IRT par des oscillations forcées de la frontière entre deux fluides permet de proposer quelques idées pour les expériences de fusion inertielle. On considère le cas général de l'instabilité, c'est-à-dire deux fluides incompressibles visqueux dans un champ de gravitation. On obtient l'expression précise analytique du taux de croissance et on analyse l'influence des paramètres de " pompage " du système sur l'instabilité. Les résultats peuvent être appliqués à une grande diversité de situations, à partir de l'hydrodynamique classique, jusqu'aux plasmas astrophysiques.
Le schéma des coquilles à fils est récemment devenu une méthode très populaire d'obtention de rayonnement haute puissance ou d'implosion de haute qualité dans le domaine des Z-pinches (plasmas de striction magnétique). Il se trouve que les résultats expérimentaux sont bien meilleurs dans le cas d'une implosion de fils fins multiples situés sur un cylindre que dans le cas du schéma ordinaire du " liner ". On a examiné un problème modélisant la stabilisation de IRT dans ce cas due aux modulations régulières de la surface de séparation entre le plasma et le champ magnétique. Ce type de modulation peut apparaître grâce aux explosions initiales et à l'évolution postérieure des fils solides. On a montré que le couplage des modes de l'instabilité en présence du champ magnétique peut effectivement réduire le taux de croissance.
Abbadi, Mohammed. « Influence de la température et de la vitesse de chargement sur les instabilités plastiques du type PLC dans l'alliage 5182 O ». Metz, 2000. http://docnum.univ-lorraine.fr/public/UPV-M/Theses/2000/Abbadi.Mohammed.SMZ0030.pdf.
Texte intégralIn certain material containing solutes diffusing towards dislocations, one observes a heterogeneous macroscopic deformation during tensile tests for certain loading rates and temperatures, this macroscopic heterogeneity of the deformation is called “Portevin – Le Chatelier” effect ; it results in the presence of serrations or stairs on the traction diagrams. Each serration of stair corresponds to the propagation of deformation bands located along the tensile specimen. The microscopic interpretation of PLC effect is often associated with a dynamic interaction between the clouds of impurities or solute atoms and mobile dislocations. These instabilities which develop during forming process lead to a fall of ductility of material as well as in a bad surface quality of the finished product. This work consists in studying the influence of the temperature and loading rate on plastic instabilities of type Portevin-Le Chatelier in an aluminium alloy of 5000 series in the case of uniaxial tensile tests carried out using hard and soft tensile machines respectively at imposed strain and stress rates. The domain of occurrence of PLC effect in log [varepsilon]. - 1/T, then log [sigma]. - 1/T diagram is closed at high temperatures and loading rates. This result, envisaged by theoretical approaches, is confirmed by the experimental results of this stuudy. This result, envisaged by theoretical approaches, is confirmed by the experimental results of this study. Experimental measurements of the critical strain [varepsilon]c for the oneset of repeated yielding show that [varepsilon]c presents a normal behaviour for high loading rates. However, an inverse behavior is observed at low loading rates. The stress drop [delta][sigma] and reloading times tR increase with strain and generally tend to a maximum value of saturation. Further, the average value of saturation presents a decrease when strain rate increases with two values of slopes rather close to those envisaged by the theory of the diffusion. The results obtained starting from tests carried out on soft machine enabled us to investigate different characteristics of localised deformation bands and to discuss their evolution during the deformation as function of stres rate and variopus test temperatures. The variations of band strain [delta][varepsilon]b present a similarity with those of the critical strain [varepsilon]c : one observes for [delta][varepsilon]b the same transition between inverse and normal behaviours. Band propagation velocities Vb, which present the same variations with strain that the bandwidths Wb, decrease when the stress rate [sigma]. Increases. Certain experimental results were compared with those envisaged by the theoretical models of Hähner, Kubin-Estrin and Brechet-Estrin
Santanach, Carreras Enric. « Stabilité en extrusion des polymères fondus : effets de la pression et de la structure des copolymères triblocs de type ABA ». Phd thesis, Université Joseph Fourier (Grenoble), 2005. http://tel.archives-ouvertes.fr/tel-00011316.
Texte intégralLa présente thèse a pour objectif de mieux connaître l'influence des paramètres de pression et contrainte sur les propriétés des polymères et la stabilité de leurs écoulements, et ce en relation avec leur structure mésoscopique.
Les effets de la pression ont été étudiés sur quatre polyéthylènes (PE) de structures différentes. D'abord, les conditions expérimentales précises ont été définies pour isoler les effets de la pression des effets de la température et pour différencier les écoulements stables ou instables. Nos résultats montrent que les effets de la pression sont les mêmes en cisaillement et en élongation, sauf pour l'un des PE où ils sont 30% supérieurs en cisaillement. En outre, c'est un critère de contrainte critique qui caractérise l'apparitions des instabilités viscoélastiques quelle que soit la pression moyenne.
Dans la deuxième partie de cette thèse, trois copolymères de la famille des SEBS montrant une séparation de phase aux échelles nanoscopiques ont été considérés. L'étude des défauts macroscopiques d'extrusion, et de la propagation de fissures surfaciques, de ces copolymères à blocs a permis d'identifier l'origine du défaut dit de « refente d'extrudat » ainsi qu'un nouveau régime d'extrusion : le « pelage continu ». Ces défauts ont été mis en relation avec la structure mésoscopique par des essais de diffusion de rayons-X aux petits angles.
Khadhraoui, Adel. « Modélisation et simulation interactive pour la navigation d'un robot sous-marin de type ROV Observer ». Thesis, Université Paris-Saclay (ComUE), 2015. http://www.theses.fr/2015SACLE014/document.
Texte intégralToday cross oceans can be done easily. However, it is not the same case for the seabed exploration. As this hostile and dangerous environment can be biologically rich and has exploitable resources, the man needs help in his discovery of the depths. Therefore, the intervention of underwater robots was a solution. In this context, the present manuscript deals with modeling and control of a ROV (Remotely Operated Vehicle). After identifying the different variables characterizing the fixed geometry, we study, at first, kinematics and dynamic modeling of the ROV. It is important to note that sets of inertia parameters of added mass and streaks coefficients is identified by means of the geometrical characteristics of the robot. A full nonlinear dynamic model of the submarine has been established. The second part of the thesis deals with the stabilization problem of the ROV’s model. We offer an explicit unsteady dependent control of both the state and time. A robust study of the control relative to external interference checking certain degree of homogeneity has been established. The autonomy of Rov also requires control movement along a reference path. We treated in the third part the Rov’s stability problem to ensure the tracking of a reference trajectory. These results are operated on a virtual platform, and implemented on the dedicated Virtools software for this application. To lighten the structure in terms of sensors and because of the high prices of various sensors, it is necessary to design a system called auxiliary observer who charge rebuild unmeasurable states using available information. A nonlinear observer has been proposed to the estimation of linear and non-measurable angular velocity, which will be considered as virtual sensors. These sensors will be implemented on the platform that will be used to animate the ROV in its virtual world
de, Moraes Franklin Erick. « Dynamique de dunes isolées dans un écoulement cisaillé ». Toulouse 3, 2008. http://thesesups.ups-tlse.fr/302/.
Texte intégralThe transport of solid particles entrained by a fluid flow as bed-load is frequently found in nature and in industrial environments. If bed-load takes place over a non-erodible ground, and if the particles flow rate is small enough and the fluid flow one-directional, an initial thin continuous layer of particles becomes discontinuous and composed of isolated dunes with a crescentic shape: the barchans. We present here an experimental study concerning barchans dynamics when submitted to a rectangular closed-conduit turbulent fluid flow. On the one hand, we investigate the migration of barchan dunes. On the other hand, we investigate the fluid flow perturbation caused by the dune, in particular the shear stress acting on its surface
Bernacki, Marc. « Méthodes de type Galerkin discontinu pour la propagation des ondes en aéroacoustique ». Phd thesis, Ecole des Ponts ParisTech, 2005. http://pastel.archives-ouvertes.fr/pastel-00001518.
Texte intégralGrassi, Anna. « Collisionless shocks in the context of Laboratory Astrophysics ». Thesis, Paris 6, 2017. http://www.theses.fr/2017PA066483/document.
Texte intégralThe 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
Théado, Sylvie. « Instabilités hydrodynamiques dans les étoiles en rotation en relation avec l'hélio- et l'astéro-sismologie et les contraintes imposées par l'observation des éléments légers ». Toulouse 3, 2002. http://www.theses.fr/2002TOU30169.
Texte intégralThe coupling between microscopic diffusion and meridional circulation is not yet well-known. In this PhD, we are interested in the effects of molecular weight gradients induced by microscopic diffusion on the meridional circulation. These effects had been neglected for a long time but we now know that they can be important and compared to the observations of chemical abundances and stellar oscillations. During the PhD we concentrated on several points : 1) Understanding and modeling the diffusion-circulation coupling using a 2D simulation. 2) Introduction of a deduced 1D modeling in the Toulouse-Geneva stellar evolution code and applications to different types of stars (pop. .
Donjat, David. « Étude et modélisation de l'hydrodynamique interne d'un injecteur mécanique de turboréacteur ». Toulouse, ENSAE, 2003. http://www.theses.fr/2003ESAE0005.
Texte intégralWatteaux, Romain. « Détection des grandes structures turbulentes dans les couches de mélange de type Rayleigh-Taylor en vue de la validation de modèles statistiques turbulents bi-structure ». Phd thesis, École normale supérieure de Cachan - ENS Cachan, 2011. http://tel.archives-ouvertes.fr/tel-00669707.
Texte intégralAbed-Meraim, Farid. « Contributions à la prédiction d'instabilités de type structure et matériau : modélisation de critères et formulation d'éléments finis adaptés à la simulation des structures minces ». Habilitation à diriger des recherches, Université de Metz, 2009. http://tel.archives-ouvertes.fr/tel-00655849.
Texte intégralChampmartin, Stéphane. « Matrice de résistance et description du mouvement d'une particule en interaction hydrodynamique et conséquences du confinement asymétrique sur les phénomènes de transfert ». Phd thesis, Université d'Angers, 2006. http://tel.archives-ouvertes.fr/tel-00102893.
Texte intégralTsai, Hsiang-Ming, et 蔡翔名. « A Preliminary Research on Alternative Lightwave Amplification Using Weibel-type Instabilities ». Thesis, 2011. http://ndltd.ncl.edu.tw/handle/6tszqg.
Texte intégral國立虎尾科技大學
光電與材料科技研究所
99
Even though the field of EM wave amplification for microwaves has been rather mature and enjoying many available technologies, amplifying a lightwave already left the laser resonant cavity can be a tough problem with very little choice. Long before erbium-doped optic fiber amplifier (EDFA) has become a reality and mass produced, other lightwave amplification schemes have been considered, even realized, and tested. Among them, the most noticed are the Stimulated Raman Scattering (SRS) and the Stimulated Brillouin Scattering (SBS). Comparing with the EDFA, a major difference in them both is that in their lightwave amplification mechanisms, no atomic (molecular) energy levels are involved, and only pure plasma physics processes are relevant. However, up to this day, it has been concluded that both the SBS and SRS amplification effects are so much weaker than the gain that stimulated emission provides in a doped-fiber amplifier that Raman and Brillouin amplifiers tend to involve very long distances and very high pump powers. On the other hand, since the EDFA amplification approach is entirely limited by quantum transition physics among energy levels, even though it may at best provide 980 nm and 1490 nm spectrum intervals for communication purposes, overall, it fails to give a real wide-band, flat-top working spectrum. Furthermore, apparently EDFA can only amplify lightwaves guided by optic fibers, not those flying in the open space and of arbitrary wavelengths and amplitudes. Nevertheless, mankind in fact has been longing for a lightwave-enhancing technology that is essentially unrestricted by available atomic energy levels, and at the same time can be applied on lights propagating in free space or within optic fibers. In man-made plasma sources, and in fusion experimental machines (such as the Tokamak), we often witness the working of a plasma instability called Weibel EM instability. However, in these cases, Weibel instability is an undesirable path for system energy loss. In natural environments, Weibel instability also plays a major role in causing gamma ray bursts observed by satellites. Here, we intend to direct the application of Weibel instability to a new direction, viz., amplifying essentially lightwaves of arbitrary wavelengths and amplitudes in either open or fiber-confined space, in a fashion more like the aforementioned SRS and SBS. The approach adopted is using controlled “plasma”, in the form of vertically (with respect to the incident light) oscillating electrons, to trigger the Weibel instability to further cause exponential growth of the incident lightwave amplitude. This current research mainly aims to test the feasibility of such lightwave Weibel amplification theory as a preliminary step toward the ultimate goal of air-borne lightwave amplification.