Portail HAL Ensta
Not a member yet
11080 research outputs found
Sort by
Seeded free-electron laser driven by a compact laser plasma accelerator
No full textFree-electron lasers generate high-brilliance coherent radiation at wavelengths spanning from the infrared to the X-ray domains. The recent development of short-wavelength seeded free-electron lasers now allows for unprecedented levels of control on longitudinal coherence[1], opening new scientific avenues as ultra-fast dynamics on complex systems and X-ray nonlinear optics. While those devices rely on state-of-the-art large-scale accelerators, advancements on laser-plasma accelerators, which harness giga-volt-per-centimeter accelerating fields, showcase a promising technology as compact drivers for free-electron lasers. Using such miniaturized accelerators, exponential amplification of a shot-noise type of radiation in a self-amplified spontaneous emission configuration was recently achieved [2]. However, employing this compact approach for the delivery of temporally coherent pulses in a controlled manner remained a major challenge. Here, we present the experimental demonstration of a laser-plasma accelerator driven free-electron laser in a seeded configuration, where control over the radiation wavelength is accomplished. Furthermore, the appearance of interference fringes, resulting from the interaction between the phase-locked emitted radiation and the seed, confirms longitudinal coherence. Building on our scientific achievements, we anticipate a straightforward scaling to extreme-ultraviolet wavelengths, paving the way towards university-scale free-electron lasers, unique tools for a multitude of applications. [1] Meyer, M. FELs of europe: Whitebook on science with free electron lasers 8–19 (2016). [2] Wang, W. et al. Free-electron lasing at 27 nanometres based on a laser wakefield accelerator
Data-Clustering Analysis of Scanning Ultrafast Acoustic Experiments: Revealing Acoustic and Structural Properties of a Motoneuron
No full textInternational audienceUltrafast acoustic imaging experiments are a powerful tool to investigate, at the nanometer scale, cell mechanical properties such as stiffness, viscosity, and adhesion, properties that play some roles in the life and death of cells. However, due to cell complex structures, the ultrafast acoustic signal analysis is quite intricate and depends on multiple parameters. Complex data analysis with poorly known parameters can be handled by a data clustering method as already shown in particle physics and biology. In this work, ultrafast acoustic data analysis is tackled by a spectral clustering method followed by a hierarchical agglomerating method. Coupled to conventional microscopy performed on the very same cell, the clustered data can be assigned to inner-cell features such as the nucleus, the cytoplasm, and the cytoskeleton. The signal dependency on the cell thickness and stiffness is highlighted. Moreover, thanks to the improvement of the signal-to-noise ratio, the nature of the adhesion is also assessed through observation and characterization of a polymerlike layer as thin as a few nanometers
Assessment of a non-conservative Residual Distribution scheme for solving a four-equation two-phase system with phase transition
No full textInternational audienceThis work focuses on a four-equation model for simulating two-phase mixtures with phase transition. The main assumption consists in a homogeneous temperature, pressure and velocity fields between the two phases. In particular, we tackle the study of time dependent problems with strong discontinuities and phase transition. This work presents the extension of a non-conservative residual distribution scheme to solve a four-equation two-phase system with phase transition. This non-conservative formulation allows avoiding the classical oscillations obtained by many approaches, that might appear for the pressure profile across contact discontinuities. The proposed method relies on a Finite Volume based Residual Distribution scheme which is designed for an explicit second-order time stepping. We test the non-conservative Residual Distribution scheme on several benchmark problems and assess the results via a cross-validation with the approximated solution obtained via a conservative approach, based on an HLLC solver. Furthermore, we check both methods for mesh convergence and show the effective robustness on very severe test cases, that involve both problems with and without phase transition
Design and field testing of a non-linear single-beam echosounder for multi-frequency seabed characterization
No full textInternational audienceSeabed mapping and characterization are best performed using several frequencies and several angles of incidence. This is often an issue because of the need to employ different sonars, with distinct frequencies but co-located as much as possible to image the same patch of seafloor. This article presents the design, calibration and field testing of a multiple-frequency single-beam echosounder (SBES), mounted on a mechanical pan-and-tilt head. It uses very high transmitting levels to produce non-linear effects and generate harmonics of a 100 kHz fundamental frequency. PZT transducers are used to transmit high acoustic powers and PDVF transducers enable the reception of scattering levels over a very broad frequency band (for the different harmonics). Tank experiments are used to verify effective harmonic generation. The shock distance (at which harmonics are at their maximum level) is measured as 2 m from the transmitter and recommended as the minimum far-field range. Non-linear transmission losses (distinct from linear losses) are calibrated using a full metal sphere 38.1 mm in diameter and of known frequency response, up to ranges commensurate with the depths expected in the field (⩽30 m). The −3 dB beamwidth varies from 5.8° at 100 kHz to 2.8° at 300 kHz. Harmonics are used to resolve phase ambiguities in detecting seabed depths. Backscattering strengths BS are matched to the Generic Seafloor Acoustic Backscatter (GSAB) model to derive the best-fitting parameters. Field validation took place in the Bay of Brest (France) in May 2016, over three different types of seafloor (namely: sandy mud; gravel; gravelly coarse sand with maerl). Additional in situ calibration was used. The echosounder was pointed at angles from 0° (nadir) to 60° by 5° steps. One of the areas surveyed (“Carré Renard”), commonly used for instrument calibration and comparison with other measurements, showed differences <1 dB at 200 kHz. Videos and photographs of the seafloor were used to ground truth interpretations of the BS curves. The results show that these BS curves measured with the echosounder are relevant for seabed classification and characterization. The different shapes and levels of BS when compared to ground truth are coherent with the Jackson model. The main limit of this prototype of echosounder is the signal to noise ratio, in particular for high frequency harmonics (⩾400 kHz). The in situ calibration is unavoidable because of the non-linear parameter variations with water characteristics (temperature, salinity…). Calibrated BS curves from 100 kHz to 300 kHz can be directly compared to other measurements, for example to calibrate other instruments
Coping with poachers in European stalked barnacle fisheries: Insights from a stakeholder workshop
No full textInternational audienceIn January 2020, a stakeholder workshop was organized as a knowledge sharing strategy among European stalked barnacle fisheries. Management of this fishery differs greatly among regions and ranges from less organized and governed at large scales (>100 km, coasts of SW Portugal and Brittany in France) to highly participatory systems which are co-managed at small spatial scales (10′s km and less, Galicia and Asturias). Discussions revealed that poaching is ubiquitous, hard to eradicate, and adapts to all types of management. The stakeholders identified some key management initiatives in the fight against poaching: granting professional harvesters with exclusive access to the resource, increasing social capital among harvesters through tenure systems (e.g. Territorial Use Rights in Fisheries) that empower them as stewards of their resource and intensification of surveillance with the active participation of the harvesters. Furthermore, increased cooperation between fishers associations and regional fisheries authorities, improved legal frameworks, adoption of new technologies and the implementation of market-based solutions can also help coping with this systemic problem
Stabilisation des équations des ondes
No full textThis thesis focuses on three problems in the context of the stabilization of wave equations. We consider different frameworks and we use techniques based on the multipliers method. First, we study the stability of the wave equation with non-linear localized damping in a standard Hilbertian framework in two dimensions. The proof is based on the work already existing in the case of a non-localized damping. We add a localization as well as disturbances. We prove the exponential stability of strong solutions in the absence of disturbances and also a weak Input-To-State stability property with respect to the considered disturbances. We next consider a more general functional framework, namely an L^p framework with p in (1,infty). We study the L^p stability of the wave equation with a linear and localized damping in one dimension since it is not always possible to define the wave operator in higher dimensions when p = 2. We prove the exponential stability of the problem by generalizing the multipliers of the Hilbertian framework in this new general framework, with a different proof for 1 2. We also prove in the same problem but with particular cases of a global constant damping, an exponential stability in the case p=1 and p=infty. We consider next the nonlinear case of the previous problem: relying on a linearizing technique, we reduce that study to that of the linear problem case in order to prove the exponential stability of the non-linear problem.Cette thèse traite trois problèmes liés à la stabilisation des équations des ondes. Nous considérons différents cadres fonctionnels et nous utilisons des techniques de démonstration basées sur la méthode des multiplicateurs. Tout d'abord, nous étudions la stabilité de l'équation des ondes avec un amortissement non-linéaire et localisé dans un cadre hilbertien standard en dimension deux. La preuve est basée sur des travaux existants dans le cas d'un amortissement non-localisé. Rajoutons une localisation ainsi que des perturbations. Nous démontrons la stabilité exponentielle le long des solutions fortes en l'absence de perturbation ainsi qu'une sorte stabilité au sens Input-To-State par rapport aux perturbations considérées. Dans un deuxième travail, nous considérons un cadre fonctionnel plus général non forcément hilbertien, c-a-d un cadre L^p avec p appartient (1,infini). Nous étudions la stabilité L^p de l'équation des ondes avec un amortissement linéaire et localisé. Cette étude n'est effectuée qu'en dimension un car il n'est pas toujours possible de définir l'opérateur des ondes en dimensions supérieures lorsque p différent 2. Nous démontrons la stabilité exponentielle du problème en généralisant les multiplicateurs du cadre hilbertien dans notre cadre plus général, avec des preuves différentes suivant que 1 2. Nous démontrons également dans le même problème mais avec des cas particuliers d'un amortissement global et constant, une stabilité exponentielle dans le cas p=1 et p=infini. Dans un troisième travail, nous considérons la version non-linéaire du problème précédent: en nous basant sur une technique de linéarisation, nous nous ramenons à la preuve du problème linéaire pour démontrer la stabilité exponentielle du non-linéaire.</p
Radical Alliances. Solutions and Opportunities for Organic Synthesis
No full textInternational audienc
Palladium-catalyzed C-H alkoxycarbonylation of benzadehydes using a transient directing group strategy
No full textInternational audienc
Reconnaissance automatisée de points d’intérêts pour un robot d’inspection dans un environnement contraint et dégradé : inspection visuelle et chimique par un robot hexapode
No full textThe thesis subject concerns the automatic recognition of points of interest (PI) for an inspection robot in a constrained and degraded environment. The objective of this thesis work is to develop a robotic platform capable of carrying out autonomous missions based on detected visual and chemical PIs, a so-called bimodal problem. The combination of visual and chemical percepts optimizes localization accuracy and ensures information redundancy. The field of study concerns 3 application cases: case 1, the inspection is carried out in a confined space (industrial environment). Case 2, the inspection is carried out in an environment with a proven risk of loss of signal and predominantly rocky (mine, underground quarry). Case 3, the inspection is carried out in an environment that has undergone significant deformations and therefore a modified and chaotic geometry of the inspection sites (natural disasters such as earthquakes or landslides in an urban environment). In this study, a contextual case analysis method is proposed and presented in order to analyze the constraints of the different complex environments for the robotic solution. The thesis therefore brings together different issues: the study of environmental constraints, the choice of the robotic solution, autonomous navigation and visual and chemical servoing. Following this contextual analysis, a state of the art is oriented on the terrestrial robotic platform to determine the most suitable robotic solution to operate in the 3 application cases. The hexapod robots were chosen for their ability to overcome obstacles, their stability, and their carrying capacity for sensors, in particular. A method is proposed to reach the source of the percept in an unstructured environment by relying on visual and chemical PIs. For the evaluation of the proposed methodology, the visual PIs considered are of the QR code type and the detection of the concentration of a gas concerning chemical servoing. The effectiveness of the proposed scheme is first demonstrated by simulations. Finally, a hexapod prototype is designed, built and developed using the ROS software architecture. The developed hexapod carried out a mission within an industrial environment and inside a shipbuilding including a series of obstacles (case 1 of the study). The results of this robotic approach arefinally presented, commented and discussed.Le sujet de thèse porte sur la reconnaissance automatique de points d’intérêts (PI) pour un robot d’inspection dans un environnement contraint et dégradé. L’objectif de ces travaux de thèse est de développer une plateforme robotique capable d’effectuer des missions en autonomie en se basant sur des PI visuels et chimiques détectés, une problématique dite bimodale. La combinaison des percepts visuels et chimiques permet d’optimiser la précision de localisation etassure une redondance d’information. Le domaine d’étude concerne 3 cas d’application : le cas 1, l’inspection est réalisée dans un espace confiné (milieu industriel). Le cas 2, l’inspection est réalisée dans un environnement avec un risque avéré de perte de signal et à dominante rocheuse (mine, carrière souterraine). Le cas 3, l’inspection est réalisée dans un environnement ayant subi des déformations importantes et donc une géométrie des lieux d’inspection modifiée et chaotique (catastrophes naturelles de type séisme ou éboulement dans un environnement urbain). Dans cette étude, une méthode d’analyse contextuelle des cas est proposée et présentée afin d’analyser les contraintes des différents environnements complexes pour la solution robotique. La thèse regroupe donc différentes problématiques : l’étude des contraintes de l’environnement, le choix de la solution robotique, la navigation autonome et l’asservissement visuel et chimique. Suite à cette analyse contextuelle, un état de l’art est orienté sur la plateforme robotique terrestre pour déterminer la solution robotique la plus adaptée pour opérer dans les 3 cas d’application. Les robots hexapodes ont été choisis pour leurs capacités à franchir les obstacles, leurs stabilités, et leurs capacités d’emport pour les capteurs, notamment. Une méthode est proposée pour atteindre la source du percept dans un environnement non structuré en s’appuyant sur les PI visuels et chimiques. Pour l’évaluation de la méthodologie proposée, les PI visuels considérées sont de type QR code et la détection de la concentration d’un gaz concernant l’asservissement chimique. L’efficacité du schéma proposé est d’abord démontrée par des simulations. Enfin, un prototype d’hexapode est conçu, construit et développé en utilisant l’architecture logicielle ROS. L’hexapode développé a réalisé une mission au sein d’un environnement industriel et à l’intérieur d’une construction navale comprenant une série d’obstacles (cas 1 de l’étude). Les résultats de cette approche robotique sont enfin présentés, commentés et discutés