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Laser-plasma lens for laser-wakefield accelerators
No full textInternational audienceThanks to their compactness and unique properties, laser-wakefield accelerators are currently considered for several innovative applications. However, many of these applications—and especially those that require beam transport—are hindered by the large divergence of laser-accelerated beams. Here we propose a collimating concept that relies on the strong radial electric field of the laser-wakefield to reduce this divergence. This concept utilizes an additional gas jet, placed after the laser-wakefield accelerator. When the laser pulse propagates through this additional gas jet, it drives a wakefield which can refocus the trailing electron bunch. Particle-in-cell simulations demonstrate that this approach can reduce the divergence by at least a factor of 3 for realistic electron bunches
Fluid-solid-electric lock-in of energy-harvesting piezoelectric flags
No full textInternational audiencePlaced in a fluid flow, a flexible plate flaps spontaneously above a critical flow velocity. The resulting self-sustained vibrations of such a flag may be used to produce electrical energy and power an output electrical circuit using piezoelectric patches covering the flag that are deformed by its flapping motion (Fig. 1a). Previous work1,2 showed only moderate harvesting efficiency with a resistive output circuit, but proposed numerous directions for the improvement.In this work, we propose a numerical investigation of such a fluid-solid-electric system with inductive-resistive output circuits. The circuit connection in Fig. 1b is considered and both linear and nonlinear studies are conducted. We identified effects of such inductive output circuit on the coupled dynamics. In particular, we show that such resonant circuits lead to a destabilization of the system and a spontaneous flapping at lower fluid velocities. Also they significantly enhance the energy harvesting efficiency of the piezoelectric flag as a result of a frequency lock-in between the flag and the electrical circuit. These results suggest promising efficiency enhancements of such flow energy harvesters through the optimization of the output circuit
Femtosecond laser-induced damage threshold of electron-beam deposited materials for broadband high-reflective coatings on large optics
No full textInternational audienc
An algorithm to enumerate all possible protein conformations verifying a set of distance constraints
No full textInternational audienceBackground: The determination of protein structures satisfying distance constraints is an important problem in structural biology. Whereas the most common method currently employed is simulated annealing, there have been other methods previously proposed in the literature. Most of them, however, are designed to find one solution only. Results: In order to explore exhaustively the feasible conformational space, we propose here an interval Branch-and-Prune algorithm (iBP) to solve the Distance Geometry Problem (DGP) associated to protein structure determination. This algorithm is based on a discretization of the problem obtained by recursively constructing a search space having the structure of a tree, and by verifying whether the generated atomic positions are feasible or not by making use of pruning devices. The pruning devices used here are directly related to features of protein conformations. Conclusions: We described the new algorithm iBP to generate protein conformations satisfying distance constraints, that would potentially allows a systematic exploration of the conformational space. The algorithm iBP has been applied on three α-helical peptides
Capillary instability on an elastic helix
No full textInternational audienceWe present the results of a combined experimental and theoretical investigation of the capillary instability of an elastic helical thread bound within a fluid. The influence of the thread's elastic energy on the classic Rayleigh–Plateau instability is elucidated. The most unstable wavelength can be substantially increased by the influence of the helical coil. The relation between our system and the capture thread of the orb-spider is discussed
Cumulant expansion of the retarded one-electron Green function
No full textInternational audienceThe cumulant expansion is a powerful approach for including correlation effects in electronic structure calculations beyond the GW approximation. However, the expansion is not generally valid, as current implementations ignore terms that mix particle and hole states and lead to partial occupation numbers of one-electron states. These limitations are corrected here using a cumulant expansion of the retarded one-electron Green’s function that includes both particle and hole contributions. The approach provides a consistent framework that improves on the GW approximation to the spectral function without additional computational effort. The method is illustrated with results for the homogeneous electron gas and comparisons to experiment and other methods
Interaction des nuages magnétiques éjectés par le Soleil avec l'environnement terrestre
No full textMagnetic clouds are huge structures released from the Sun through violent eruptions, which then propagate into the solar system at supersonic speeds. They are characterised in the solar wind by an enhanced and smoothly-rotating magnetic field. They cause large disturbances in the Earth's environment which sometimes have an impact on human activity in space (telecommunications, GPS, …) and on the ground (electrical networks,...). When magnetic clouds arrive in the vicinity of Earth, they first encounter the bow shock. In this thesis, we focus on the alteration of the magnetic structure of the clouds at the bow shock's crossing and during their propagation in the downstream region. Any significant modification would indeed have important implications on the prediction of geomagnetic disturbances.Three different approaches complementing one another are employed to address this issue: we first analyse data from different spacecraft orbiting Earth, in particular from ESA's Cluster mission, then we develop a model describing as a whole the bow shock's crossing and the propagation of the magnetic cloud in the downstream region, and finally we use numerical simulations to study in more detail some aspects of the physics of this interaction.The results obtained with these three methods consistently show that the variation of the cloud's structure across the bow shock is strongly related to the magnetic configuration of the magnetic cloud relative to the shock. This can be quantified by the value of , the angle between the normal to the shock's surface and the magnetic cloud's magnetic field in the solar wind. We show that a quasi-perpendicular configuration () keeps the cloud's magnetic structure roughly unchanged. When the configuration becomes more oblique, a rotation of the magnetic cloud's structure is observed in some parts of the downstream region. In a quasi-parallel geometry, the magnetic cloud's structure is strongly altered. Its magnetic field direction can then reverse and a high level of turbulence is observed downstream of the shock. Using the model we developed, we estimate the location of the regions favourable to reconnection processes, which give rise to disturbances in the Earth's environment. The numerical simulations allow us to investigate the turbulent regions downstream and also upstream of the bow shock. Finally, we find that, owing to the modification of their magnetic structure across the bow shock, the impact of certain magnetic clouds on the Earth's environment can differ from that estimated from their characteristics in the solar wind.Les nuages magnétiques sont des structures émises par le Soleil lors d'éruptions violentes et qui se propagent ensuite dans le système solaire à des vitesses supersoniques. Ils se démarquent du vent solaire ambiant par un renforcement de l'intensité du champ magnétique et une rotation lente de sa direction. Ils sont à l'origine de fortes perturbations dans l'environnement magnétique terrestre qui peuvent avoir des conséquences importantes sur les activités humaines spatiales (satellites, liaisons GPS,...) voire au sol (réseaux électriques,...).Lorsque les nuages magnétiques arrivent au voisinage de la Terre, ils rencontrent tout d'abord le choc d'étrave. Dans ce travail de thèse, nous nous sommes intéressés à l'altération de la structure magnétique des nuages magnétiques à la traversée de l'onde de choc terrestre et lors de leur propagation en aval de celui-ci. En effet, toute modification significative aurait alors d'importantes implications pour la prédiction des perturbations induites dans l'environnement terrestre.Ce problème a été abordé sous trois angles différents et complémentaires : l'analyse comparative de données de satellites en orbite autour de la Terre, notamment à partir des observations de la mission Cluster de l’ESA, le développement d'un modèle décrivant de façon globale la traversée du choc et la propagation du nuage en aval de celui-ci, et enfin l'utilisation de simulations numériques qui reproduisent de façon plus réaliste certains éléments de la physique de cette interaction.Les résultats obtenus à l'aide de ces trois approches s'accordent pour montrer que la variation de la structure magnétique des nuages magnétiques au passage de l'onde de choc est étroitement liée à la configuration magnétique du nuage par rapport au choc ; on peut la quantifier par la valeur de , angle entre la normale au choc et le champ magnétique du nuage en amont de l'onde de choc. Nous montrons qu'une configuration quasi-perpendiculaire () au niveau du choc modifie peu la structure magnétique du nuage. Lorsque la configuration devient oblique, une rotation de la structure du nuage se produit dans la région située en aval du choc. La structure du nuage est complètement perturbée quand la configuration devient quasi-parallèle. Dans ce dernier cas, on peut observer une inversion du champ magnétique du nuage et le développement de turbulence en aval de l'onde de choc. La modélisation de la région aval permet de localiser des régions favorables à la reconnexion magnétique et donc au développement d'activité magnétique dans l'environnement terrestre. Les simulations numériques permettent en particulier de caractériser les régions turbulentes en aval et aussi en amont de l'onde de choc. Au final, du fait de l'altération de leur structure au passage du choc, certains nuages magnétiques peuvent avoir un impact sur l'environnement terrestre très différent de celui attendu à partir de leurs caractéristiques observées dans le vent solaire, en amont du choc
Silicium microcristallin déposé par plasmas de SiF4/H2/Ar et ses applications au photovoltaïque
No full textSilicon tetrafluoride is a good candidate to replace silane as silicon precursor in plasmas used to deposit thin films. It has been demonstrated by previous Ph.D. thesis at LPICM that microcrystalline silicon, grown from SiF4/H2/Ar gas mixtures, leads to excellent thin film transistors. This thesis is dedicated to another application: photovoltaic devices. A new challenge arises because several micron thick layers are required in microcrystalline silicon thin film PIN solar cells. Therefore a better and deeper understanding of SiF4/H2/Ar plasmas is necessary to optimize the deposition rate of microcrystalline silicon.SiF4/H2/Ar plasmas have been studied thanks to mass spectrometry and the specificity of such gas mixtures is demonstrated to be the formation HF molecules via the recombination of atomic F and molecular H2. We found that a H2-limited process is associated to amorphous growth and all H2 is used to form HF. On the contrary, an excess of H2 leads to microcrystalline growth. A simple yet accurate model, strongly based on experimental data, has been developed to explain the amorphous-to-microcrystalline transition and enables a controlled tuning of plasma parameters leading to an increase of deposition rate. Moreover, the contribution of plasma generated nanoparticles has been clarified, in particular their crystallization in the plasma phase. The microcrystalline silicon, fabricated from SiF4/H2/Ar, is a high quality material: a density of defects as low as 3x10-3cm-1 has been deduced from absorption below the bandgap (at 0.8eV). By using the fluorinated chemistry for the deposition of the intrinsic absorber layers in thin film PIN solar cells, open-circuit voltage as high as 536mV has been obtained for highly crystallized layers, leading to an efficiency of 9.2%.Le tétrafluorure de silicum est une excellente alternative pour remplacer le silane comme précurseur dans les plasmas de dépôt de couches minces. Il a été montré dans les précédentes thèses du LPICM que le silicium microcristallin, crû par des mélanges de SiF4/H2/Ar, conduit à d’excellents transistors en couches minces. Cette thèse est consacrée à une autre application : les dispositifs photovoltaïques. Un nouveau défi apparait du fait que l’épaisseur nécessaire des couches est de plusieurs microns. C’est pourquoi une compréhension des plasmas de SiF4/H2/Ar meilleure et plus approfondie est requise pour optimiser la vitesse de dépôt du silicium microcristallin.Les plasmas de SiF4/H2/Ar ont été étudiés par spectroscopie de masse et il a été démontré que la spécificité de tels mélanges est la formation de molécules de HF par la recombinaison de fluor atomique avec l’hydrogène moléculaire. Nous avons montré que dans des conditions où le H2 est limitant, la couche obtenue est amorphe et l’hydrogène est principalement utilisé pour former du HF. Inversement, un excès de H2 mène à des couches microcristallines. Un modèle simple mais fidèle, basé sur nos données expérimentales, a été développé pour expliquer la transition amorphe vers microcristallin. Ce modèle nous permet d’augmenter la vitesse de croissance de nos couches par un ajustement raisonné des paramètres du plasma. De plus, la contribution des nanoparticules générées en phase plasma a été clarifiée comme par exemple les conditions de leur cristallisation en phase plasma. Le silicium microcristallin fabriqué à partir de SiF4/H2/Ar est de haute qualité : une densité de défauts aussi basse que 3x10-3cm-1 a été déduite par absorption sous la bande interdite (précisément à 0.8eV). En utilisant une chimie fluorée pour le dépôt de la couche intrinsèque absorbante dans des cellules PIN en couches minces, une tension en circuit ouvert aussi élevée que 536mV a été obtenue avec des couches bien cristallisées, aboutissant à des rendements de 9.2%
A finite elements method to solve the Bloch–Torrey equation applied to diffusion magnetic resonance imaging
No full textInternational audienceThe complex transverse water proton magnetization subject to diffusion-encoding magneticfield gradient pulses in a heterogeneous medium can be modeled by themultiple compartment Bloch-Torrey partial differential equation (PDE).In addition, steady-state Laplace PDEs can be formulated to produce the homogenized diffusion tensor that describes the diffusion characteristicsof the medium in the long time limit.In spatial domains that model biological tissues at the cellular level, these two types of PDEs have to be completed with permeability conditions on the cellular interfaces.To solve these PDEs, we implemented a finite elements method thatallows jumps in the solution at the cell interfaces by using doublenodes. Using a transformation of the Bloch-Torrey PDE we reduced oscillations in the searched-for solution and simplified the implementationof the boundary conditions. The spatial discretizationwas then coupled to the adaptive explict Runge-Kutta-Chebychev time-stepping method. Our proposed method is second order accurate in space and second order accurate in time.We implemented this method on the FEniCSC++ platform and show time and spatial convergence results.Finally, this method is applied to study some relevant questions in diffusionMRI