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Uniform semigroup spectral analysis of the discrete, fractional & classical Fokker-Planck equations
No full textIn this paper, we investigate the spectral analysis (from the point of view of semi-groups) of discrete, fractional and classical Fokker-Planck equations. Discrete and fractional Fokker-Planck equations converge in some sense to the classical one. As a consequence, we first deal with discrete and classical Fokker-Planck equations in a same framework, proving uniform spectral estimates using a perturbation argument and an enlargement argument. Then, we do a similar analysis for fractional and classical Fokker-Planck equations using an argument of enlargement of the space in which the semigroup decays. We also handle another class of discrete Fokker-Planck equations which converge to the fractional Fokker-Planck one, we are also able to treat these equations in a same framework from the spectral analysis viewpoint, still with a semigroup approach and thanks to a perturbative argument combined with an enlargement one. Let us emphasize here that we improve the perturbative argument introduced in [7] and developed in [11], relaxing the hypothesis of the theorem, enlarging thus the class of operators which fulfills the assumptions required to apply it
Non line of sight signal analysis: Investigation of interferometry modes over urban area
No full textInternational audienceDue to complex geometry (urban canyons generate multiple scattering) SAR images are difficult to interpretate on urban areas. In order to characterize Non Line Of Sight (NLOS) scattering phenomenon at Ka band, measurement campaigns have been performed using the ONERA BUSARD motor glider and a dedicated Ka band SAR sensor. From 2011 to 2014 a large number of measurements flights have been done, leading to the acquisition of a radar dataset that covers different geometries and different acquisition modes: Single channel, cross-track or along-track interferometry modes. NLOS radar echoes have been investigated on two types of materials: concrete and metallic walls and different geometries of acquisition. NLOS echoes observation has been validated in various cases and interferometry modes have been proved to be efficient in NLOS echoes detection and validation
Vertex barycenter of generalized associahedra
No full textInternational audienceWe show that the vertex barycenter of generalized associahedra and permutahedra coincide for any finite Coxeter system
A new constellation for space-time coding
No full textInternational audienceIn this paper, we propose a new encoding scheme for space-time coding. This encoding relies on the generalisation [2] of Gabidulin codes to innite elds. This enables to make use of the properties of Gabidulin codes, namely, we can use algebraic decoding algorithms
A regularised singularity approach to phoretic problems
No full textInternational audienceAn efficient, accurate, and flexible numerical method is proposed for the solution of the swimming problem of one or more autophoretic particles in the purely diffusive limit. The method relies on successive boundary element solutions of the Laplacian and the Stokes flow equations using regularised Green’s functions for swift, simple implementations, an extension of the well-known method of “regularised stokeslets” for Stokes flow problems. The boundary element method is particularly suitable for phoretic problems, since no quantities in the domain bulk are required to compute the swimming velocity. For time-dependent problems, the method requires no re-meshing and simple boundaries such as a plane wall may be added at no increase to the size of the linear system through the method of images. The method is validated against two classical examples for which an analytical or semi-analytical solution is known, a two-sphere system and a Janus particle, and provides a rigorous computational pipeline to address further problems with complex geometry and multiple bodies
Correction: Formal [3+2] cycloaddition of Ugi adducts towards pyrrolines
No full textInternational audienceCorrection for ‘Formal [3+2] cycloaddition of Ugi adducts towards pyrrolines’ by Abdelbari Ben Abdessalem et al., Chem. Commun., 2015, 51, 1116–1119
Revealing the second harmonic generation in a femtosecond laser-driven cluster-based plasma by analyzing shapes of Ar XVII spectral lines
No full textInternational audienceWe present experiments dealing with a femtosecond laser-driven cluster-based plasma, where by analyzing the nonlinear phenomenon of satellites of spectral lines of Ar XVII, we revealed the nonlinear phenomenon of the generation of the second harmonic of the laser frequency. For performing this analysis we developed new results in the theory of satellites of spectral lines. From such lineshape analysis we found, in particular, that the efficiency of converting the short (40 fs) intense (3x10 18 W/cm 2) incident laser light into the second harmonic was 2%. This result is in the excellent agreement with the 2-Dimensional Particle-In-Cell (2D PIC) simulation that we also performed. There is also an order of magnitude agreement between the thresholds for the SHG found from the line shape analysis and from the 2D PIC simulations
Trajectoires professionnelles et santé en Europe
No full textThe main objective of this thesis is to analyse the health consequences of careershocks in Europe. It considers two actual career shocks over the lifecourse : leaving full-timeeducation in a bad economy, and, at the other end of the age spectrum, retiring. In-betweenthese two critical periods, it investigates how an anticipated career shock – i.e. anticipatedjob loss – damages health. Empirical analyses are conducted using large European and Britishsurveys. We use institutional features and natural experiments to find neat instruments forcausal identification : the existence of compulsory schooling laws, the cross-country variation inemployment protection legislations, the cross-country variation in retirement systems and the1973 oil crisis. The results emphasise the causal and health-damaging impact of career shocks,both in the short and in the long-term.Cette thèse se propose d’analyser les effets des ruptures dans les trajectoires professionnellessur l’état de santé des individus en Europe. Nous considérons ici deux ruptures :l’une en début de carrière – l’entrée sur le marché du travail dans une économie dégradée –et l’autre en fin de carrière – le passage à la retraite. Entre ces deux périodes critiques, nousportons un intérêt spécifique à l’impact sur la santé d’une rupture cette fois anticipée : la peurde perdre son emploi. Nos analyses empiriques combinent des données d’enquêtes Européenneset Britanniques. Afin de pallier les problèmes d’endogénéité propres à toute analyse empiriquedu lien entre santé et trajectoire professionnelle, nous exerçons des chocs exogènes sur la carrièredes individus. Nous utilisons ainsi une expérience naturelle (la crise pétrolière de 1973)et les caractéristiques institutionnelles telles qu’elles sont définies dans la législation de chaquepays Européen (âges légaux de passage à la retraite, degrés de protection de l’emploi, règlesde scolarité obligatoire). Les résultats soulignent l’effet néfaste des ruptures au cours de la vieprofessionnelle sur la santé des individus, à la fois à court et à long terme
Étude et modélisation de la diffusion Raman stimulée en configuration multi-dimensionnelle pour la FCI
No full textInertial Confinement Fusion is one of the two major ways to achieve thermonuclear fusion of light nuclei in laboratory. The thermonuclear reactions are initiated with megajoule-class lasers, using the direct or the indirect drive scheme. Our main concern is the indirect drive scheme, where laser beams propagate through a hot and under-dense plasma in which they undergo substantial losses, because of laser-plasma instabilities. These losses are highly detrimental in the sense that they reduce the laser energy deposition needed to eventually compress the fuel via inertial confinement. More precisely, the incident laser light can be reflected in scattering processes, one of which is the Stimulated Raman Scattering (SRS) off electron plasma waves, leading to substantial energy losses (up to 50%). Although this instability has been investigated for many years, its modeling remains difficult particularly for millimeter-size and inhomogenous plasmas in long time scales (nanosecond). One major difficulty is the quick evolution toward non linear regimes related to the important modification of the electron distribution function. The challenging goal of this work is to develop a realistic model for SRS in ICF relevant plasmas, on long time scale, taking into consideration the different non linear effects associated with electron plasma waves. A new wave coupling approach, implemented in a 2D/3D code, was elaborated and successfully confronted with reference kinetic simulations from a PIC code. Ultimately, the developped model is applied to recent experimental contexts and allows a novel and relevant interpretation of some measurements.La Fusion par Confinement Inertiel (FCI) est l'une des voies qui permettrait de réaliser la fusion thermonucléaire de noyaux légers en laboratoire. Les réactions nucléaires sont amorcées en utilisant des lasers intenses de type mégajoule, par des schémas d'attaque directe ou indirecte. Dans le cas de l'attaque indirecte, approche retenue sur l'installation Laser Méga Joule qui est un projet majeur de la Direction des Applications Militaires du CEA, les lasers traversent un plasma chaud et sous-dense propice au développement d'instabilités générées par l'interaction laser-plasma. Ces instabilités peuvent conduire à des pertes importantes de l'énergie laser incidente. Plus particulièrement, la diffusion Raman stimulée (DRS), où le rayonnement laser diffuse sur les ondes plasma électroniques, peut renvoyer une partie importante de l'énergie laser (jusqu'à 50%), ce qui réduit inévitablement le dépôt d'énergie nécessaire pour réaliser la compression du milieu fusible par réaction d'inertie. Cette instabilité, bien qu'étudiée depuis de nombreuses années, reste difficile à modéliser sur des temps longs (nanoseconde) pour des plasmas millimétriques et, qui plus est, inhomogènes. En effet, l'instabilité évolue rapidement vers des régimes non linéaires, associés à une déformation importante de la fonction de distribution électronique. L'objectif ambitieux de ce travail est de modéliser de manière réaliste la diffusion Raman dans les plasmas relatifs à la FCI, sur des temps longs et en tenant compte des effets non linéaires associés aux ondes plasma électroniques. Dans ce but, un modèle original de couplage d'ondes prenant en compte la déformation de la fonction de distribution électronique a été développé. Ce modèle, mis en œuvre dans un code 2D/3D est comparé avec succès à des simulations cinétiques de références, issues d'un code PIC. Finalement, le modèle obtenu est appliqué à des situations expérimentales récentes et permet d'apporter, aux observations faites dans ce cadre, une interprétation originale et pertinente