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Table-top extreme ultraviolet second harmonic generation
No full textInternational audienceThe lack of available table-top extreme ultraviolet (XUV) sources with high enough fluxes and coherence properties has limited the availability of nonlinear XUV and x-ray spectroscopies to free-electron lasers (FELs). Here, we demonstrate second harmonic generation (SHG) on a table-top XUV source by observing SHG near the Ti M 2,3 edge with a high-harmonic seeded soft x-ray laser. Furthermore, this experiment represents the first SHG experiment in the XUV. First-principles electronic structure calculations suggest the surface specificity and separate the observed signal into its resonant and nonresonant contributions. The realization of XUV-SHG on a table-top source opens up more accessible opportunities for the study of element-specific dynamics in multicomponent systems where surface, interfacial, and bulk-phase asymmetries play a driving role
StorNIR , a multi-objective replica placement strategy for cloud federations
No full textInternational audienc
Spectrum Sensing for Cognitive Radio: Recent Advances and Future Challenge
No full textInternational audienceSpectrum Sensing (SS) plays an essential role in Cognitive Radio (CR) networks to diagnose the availability of frequency resources. In this paper, we aim to provide an in-depth survey on the most recent advances in SS for CR. We start by explaining the Half-Duplex and Full-Duplex paradigms, while focusing on the operating modes in the Full-Duplex. A thorough discussion of Full-Duplex operation modes from collision and throughput points of view is presented. Then, we discuss the use of learning techniques in enhancing the SS performance considering both local and cooperative sensing scenarios. In addition, recent SS applications for CR-based Internet of Things and Wireless Sensors Networks are presented. Furthermore, we survey the latest achievements in Spectrum Sensing as a Service, where the Internet of Things or the Wireless Sensor Networks may play an essential role in providing the CR network with the SS data. We also discuss the utilisation of CR for the 5th Generation and Beyond and its possible role in frequency allocation. With the advancement of telecommunication technologies, additional features should be ensured by SS such as the ability to explore different available channels and free space for transmission. As such, we highlight important future research axes and challenging points in SS for CR based on the current and emerging techniques in wireless communications
Effect of Strain Rate on the Tensile Mechanical Properties of Electron Beam Welded OFE Copper and High-Purity Niobium for SRF Applications
No full textInternational audienceAn investigation of the tensile mechanical properties of electron beam welded OFE copper and high-purity niobium sheets is presented. Specimens were deformed in tension at strain rates ranging from 10−3 to ~ 1600 s−1. The 0.2% yield stress and ultimate tensile strength (UTS) of the welded niobium specimens are similar to those of unwelded specimens at strain rates lower or equal to 20 s−1. At higher strain rates, these mechanical properties are lower for welded niobium specimens. The 0.2% yield stress of welded OFE copper specimens is consistently lower than unwelded specimens over the range of strain rates studied, while the UTS is comparable at all strain rates. The elongation to failure of welded OFE copper specimens remains unchanged at all strain rates while the ductility of niobium specimens reduces at strain rates greater or equal to 20 s−1 and reaches a minimum at ~ 400 s−1. The effects of the weld on a non-standardized short specimen geometry, developed for this study to obtain strain rates in the order of 103 s−1, are more pronounced for niobium due to large grain sizes (up to 1200 μm) in the fusion region. However, comparable strength and ductility trends, with respect to a standard specimen, were measured at low strain rates. The conservation of strength and the relatively high ductility of the welded sheets, especially for OFE copper, suggest that bent and electron beam welded tubes could be used for the fabrication of seamless superconducting radiofrequency (SRF) cavities. These results are promising for the use of high-speed forming techniques, like electro-hydraulic forming, for the manufacturing of parts using welded tubes and sheets
Union and Intersection Operators for Thick Ellipsoid State Enclosures: Application to Bounded-Error Discrete-Time State Observer Design
No full textInternational audienceThis article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC B
Use of Ecoacoustics to Characterize the Marine Acoustic Environment off the North Atlantic French Saint-Pierre-et-Miquelon Archipelago
No full textInternational audienceVisual observations of the marine biodiversity can be difficult in specific areas for different reasons, including weather conditions or a lack of observers. In such conditions, passive acoustics represents a potential alternative approach. The objective of this work is to demonstrate how information about marine biodiversity can be obtained via detailed analysis of the underwater acoustic environment. This paper presents the first analysis of the Saint-Pierre-and-Miquelon (SPM) archipelago underwater acoustic environment. In order to have a better knowledge about the marine biodiversity of SPM, acoustic recordings were sampled at different time periods to highlight seasonal variations over several years. To extract information from these acoustic recordings, standard soundscape and ecoacoustic analysis workflow was used to compute acoustic metrics such as power spectral density, third-octave levels, acoustic complexity index, and sound pressure levels. The SPM marine acoustic environment can be divided into three main sound source classes: biophony, anthrophony, and geophony. Several cetacean species were encountered in the audio recordings including sperm whales (which were detected by visual observations and strandings of 3 males in 2014), humpback, and blue whales
Kalman Filter-Based Real-Time Implementable Optimization of the Fuel Efficiency of Solid Oxide Fuel Cells
No full textInternational audienceThe electric power characteristic of solid oxide fuel cells (SOFCs) depends on numerous influencing factors. These are the mass flow of supplied hydrogen, the temperature distribution in the interior of the fuel cell stack, the temperatures of the supplied reaction media at the anode and cathode, and—most importantly—the electric current. Describing all of these dependencies by means of analytic system models is almost impossible. Therefore, it is reasonable to identify these dependencies by means of stochastic filter techniques. One possible option is the use of Kalman filters to find locally valid approximations of the power characteristics. These can then be employed for numerous online purposes of dynamically operated fuel cells such as maximum power point tracking or the maximization of the fuel efficiency. In the latter case, it has to be ensured that the fuel cell operation is restricted to the regime of Ohmic polarization. This aspect is crucial to avoid fuel starvation phenomena which may not only lead to an inefficient system operation but also to accelerated degradation. In this paper, a Kalman filter-based, real-time implementable optimization of the fuel efficiency is proposed for SOFCs which accounts for the aforementioned feasibility constraints. Essentially, the proposed strategy consists of two phases. First, the parameters of an approximation of the electric power characteristic are estimated. The measurable arguments of this function are the hydrogen mass flow and the electric stack current. In a second stage, these inputs are optimized so that a desired stack power is attained in an optimal way. Simulation results are presented which show the robustness of the proposed technique against inaccuracies in the a-priori knowledge about the power characteristics. For a numerical validation, three different models of the electric power characteristic are considered: (i) a static neural network input/output model, (ii) a first-order dynamic system representation and (iii) the combination of a static neural network model with a low-order fractional differential equation model representing transient phases during changes between different electric operating points
Niobium monocristallin et polycristallin et cuivre OFE pour les cavités SRF : caractérisation mécanique de basse à haute vitesse de déformation et investigation de la microstructure
No full textManufacturing of superconducting radiofrequency (SRF) cavities with high performances is paramount to increase the collision energy in new particle accelerators. The use of high-speed sheet forming techniques, such as electro-hydraulic forming, can be beneficial, but requires a detailed understanding of the mechanical properties of the materials being deformed and the consequence on their microstructure. This thesis focuses on the characterization of high-purity niobium single crystals, polycrystalline niobium sheets, and polycrystalline OFE copper sheets. The results from this study are separated in two parts. In Part I, the characterization of niobium single crystals focused on the mechanical properties in tension and compression at strain rates of 10-4 to 103 s-1 and on the microstructure (analyzed using SEM, EBSD, TEM, and nanoindentation) of the deformed specimens. The effect of crystal orientation, strain rate, and loading direction on the mechanical properties, the crystal rotation, and the dislocation substructures are presented. In Part II, the forming limit diagram (FLD) of polycrystalline niobium sheets and OFE copper sheets were measured at a quasi-static strain rate. The FLDs of those materials should provide important data for manufacturers using conventional techniques, such as deep-drawing and spinning. The second part also presents the mechanical properties of electron beam (EB) welded polycrystalline niobium sheets and OFE copper sheets deformed in tension and compression at strain rates of 10-3 to 103 s-1.La fabrication de cavités SRF à hautes performances est essentielle pour augmenter l’énergie de collision dans de nouveaux accélérateurs de particules. L’utilisation de procédés de fabrication à haute vitesse, comme l’électro-hydro formage, peut être bénéfique, mais requiert une compréhension détaillée des propriétés mécaniques des matériaux déformés à haute vitesse et de l’impact sur leurs microstructures. Les objectifs de cette thèse sont d’étudier les propriétés mécaniques de monocristaux de niobium et de tôles polycristallines de niobium à haute pureté et de cuivre OFE déformés à des taux de déformation d’environ 10−4 s−1 à 103 s−1. Les résultats de cette étude sont séparés en deux parties selon le matériau étudié. En partie I, la caractérisation de monocristaux de niobium se concentre sur les propriétés mécaniques en traction et en compression à des taux de déformation d’environ 10−4 s−1 à 103 s−1 et sur la microstructure (MEB, EBSD, MET et nanoindentation) d’éprouvettes déformées. Les effets de l’orientation des cristaux, la vitesse de déformation et de la direction de chargement sur les propriétés mécaniques, la rotation des cristaux et la structure de dislocations sont présentés. En partie II, la formabilité de tôles polycristallines de niobium et de cuivre OFE sont présentées à l’aide de courbes limites de formage (CLF) obtenues à un taux de déformation quasi-statique. Les CLFs de ces matériaux offrent des données importantes pour les techniques de formage conventionnel, tels que l’emboutissage et le repoussage. Cette seconde partie présente aussi les propriétés mécaniques d’éprouvettes de cuivre OFE recuites et de tôles de niobium polycristallines soudées par faisceau d’électron et déformées en traction et en compression à des taux de déformation de 10−3 à 103 s−1
Étude et modélisation d'impacts hautes et basses vitesses sur des matériaux et structures de satellites
No full textThe protection of satellite against the threat of space debris hypervelocity impacts requires the development of protective structures, which have to be strong, lightweight and suited to the space environment. In order to meet this challenge, attention was turned to the integration of innovative material inside multi-shock shields, usually called Whipple Shield. These structures benefit from the progressive fragmentation and expansion of the projectile during its impact on successive shields. In this work, attention was focused on the comprehension of the phenomena involved during such impacts as well as their numerical modelling. First of all, all the materials under study were characterized through multiple experiments such as high velocity impacts, laser induced shocks and electrons beam induced shock. Based on literature data and dynamics tests, a numerical modelling of the materials was developed and confronted with experiments results. Impacts simulations with the codes HESIONE and LS-DYNA provided relevant results in terms of shapes and velocities of the debris clouds induced by the impacts, when compared with experimental results. They also brought satisfying results concerning sizes of the debris inside the clouds. These simulations strengthen our knowledges about the materials behavior under dynamic loading. Based on numerical and experimental results, an analysis was conducted in order to identify pros and cons of each material and furnish quantitative and qualitative information, useful for a protective structure concept phase. An optimization of a structure was finally proposed, which furnished a potential lightweight structure for protection against a wide range of projectiles and impacts velocities.La protection des satellites face à la menace d’impacts hypervéloces des débris spatiaux requiert le développement de structures de protection qui soient légères, résistantes et qui supportent l’environnement spatial. Pour relever ce défi, notre attention s’est tournée vers l’emploi de matériaux innovants au sein des structures multi-boucliers, communément appelées Whipple Shield. Ces structures exploitent la fragmentation et l’expansion progressive du projectile lors de l’impact sur des boucliers successifs. Dans ces travaux de thèse, l’attention est portée sur la compréhension des phénomènes mis en jeu lors de tels impacts et sur leur modélisation dans les simulations numériques associées. Dans un premier temps, chaque matériau d’étude a fait l’objet d’une caractérisation au travers des essais d’impacts, de choc laser et de choc induit par faisceau d’électrons. Une modélisation numérique est proposée en se basant sur des données de la littérature et divers essais dynamiques, avant d’être confrontée aux résultats d’essais. Les simulations sous les codes HESIONE et LS-DYNA des impacts par lanceur fournissent des résultats probants en termes de forme et de vitesse de nuage de débris induits, en comparaison des essais associés. Elles ont également permis de restituer les tailles des débris présents dans le nuage. Ces simulations viennent étoffer notre connaissance du comportement des matériaux sous choc. À partir des résultats d’essais et de simulations, une analyse est ensuite menée afin d’identifier les points forts et les points faibles de chaque matériau, afin d’obtenir des données qualitatives et quantitatives, nécessaires à l’élaboration d’un éventuel dossier de choix. Une optimisation de structure est enfin proposée pour assurer, avec une masse la plus faible possible, la résistance du bouclier face à une large gamme de débris et de vitesses d’impacts
An Interval Constraint Programming Approach for Quasi Capture Tube Validation
No full textProving that the state of a controlled nonlinear system always stays inside a time moving bubble (or capture tube) amounts to proving the inconsistency of a set of nonlinear inequalities in the time-state space. In practice however, even with a good intuition, it is difficult for a human to find such a capture tube except for simple examples. In 2014, Jaulin et al. established properties that support a new interval approach for validating a quasi capture tube, ie a candidate tube (with a simple form) from which the mobile system can escape, but into which it enters again before a given time. A quasi capture tube is easy to find in practice for a controlled system. Merging the trajectories originated from the candidate tube yields the smallest capture tube enclosing it.International audienceProving that the state of a controlled nonlinear system always stays inside a time moving bubble (or capture tube) amounts to proving the inconsistency of a set of nonlinear inequalities in the time-state space. In practice however, even with a good intuition, it is difficult for a human to find such a capture tube except for simple examples. In 2014, Jaulin et al. established properties that support a new interval approach for validating a quasi capture tube, i.e. a candidate tube (with a simple form) from which the mobile system can escape, but into which it enters again before a given time. A quasi capture tube is easy to find in practice for a controlled system. Merging the trajectories originated from the candidate tube yields the smallest capture tube enclosing it. This paper proposes an interval constraint programming solver dedicated to the quasi capture tube validation. The problem is viewed as a differential CSP where the functional variables correspond to the state variables of the system and the constraints define system trajectories that escape from the candidate tube "for ever". The solver performs a branch and contract procedure for computing the trajectories that escape from the candidate tube. If no solution is found, the quasi capture tube is validated and, as a side effect, a corrected smallest capture tube enclosing the quasi one is computed. The approach is experimentally validated on several examples having 2 to 5 degrees of freedom. 2012 ACM Subject Classification Applied computing → Operations research; Mathematics of computing → Ordinary differential equations; Mathematics of computing → Differential algebraic equations; Mathematics of computing → Interval arithmetic; Theory of computation → Constraint and logic programmin