Jurnal Edukasi
Not a member yet
4879 research outputs found
Sort by
Physical mechanisms controlling the initiation of convective self-aggregation in a General Circulation Model
No full textInternational audienceCloud-resolving models have shown that under certain conditions, the Radiative-Convective Equilibrium (RCE) could become unstable and lead to the spontaneous organization of the atmosphere into dry and wet areas, and the aggregation of convection. In this study, we show that this “self-aggregation” behavior also occurs in nonrotating RCE simulations performed with the IPSL-CM5A-LR General Circulation Model (GCM), and that it exhibits a strong dependence on sea surface temperature (SST). We investigate the physical mechanisms that control the initiation of self-aggregation in this model, and their dependence on temperature. At low SSTs, the onset of self-aggregation is primarily controlled by the coupling between low-cloud radiative effects and shallow circulations and the formation of “radiatively driven cold pools” in areas devoid of deep convection, while at high SSTs it is primarily controlled by the coupling between surface fluxes and circulation within convective areas. At intermediate temperatures, the occurrence of self-aggregation is less spontaneous and depends on initial conditions, but it can arise through a combination of both mechanisms. Through their coupling to circulation and surface fluxes, the radiative effects of low-level clouds play a critical role in both initiation mechanisms, and the sensitivity of boundary layer clouds to surface temperature explains to a large extent the temperature dependence of convective self-aggregation. At any SST, the presence of cloud-radiative effects in the free troposphere is necessary to the initiation, growth, and maintenance of convective aggregation
Deconfinement transition in SU(N) theories from perturbation theory
No full textInternational audienceWe consider a simple massive extension of the Landau-DeWitt gauge for SU(N) Yang-Mills theory. We compute the corresponding one-loop effective potential for a temporal background gluon field at finite temperature. At this order the background field is simply related to the Polyakov loop, the order parameter of the deconfinement transition. Our perturbative calculation correctly describes a quark confining phase at low temperature and a phase transition of second order for N=2 and weakly first order for N=3. Our estimates for the transition temperatures are in qualitative agreement with values from lattice simulations or from other continuum approaches. Finally, we discuss the effective gluon mass parameter in relation to the Gribov ambiguities of the Landau-DeWitt gauge
Caractérisation et modélisation d'un propulseur plasma à résonance cyclotronique des électrons
No full textThe purpose of this work is the characterization and theoretical investigation of an electron cyclotron resonance plasma thruster. The objectives is to study the physics of the thruster (energy transfer by cyclotron resonance, ionization process, coupling microwave/plasma and acceleration process) to improve his performances, efficiency and development dimensioning tools.An experimental prototype of the thruster was characterized around the operating freedoms degrees as frequency, magnetic field, the power, the geometry and the gas flow. The results are used to set the conditions for a nominal operation of the thruster in terms of performances and efficiency. It was shown that the position of the resonance area and the operating pressure are the two keys parameters for the optimization of the thruster.This research helped to increase performance and total efficiency of the thruster. For a power of 30 watts and a flow rate of 0.1 mg/s, the thrust provided 1 mN with a specific impulse of 1000 s for 16 % total efficiency.In parallel, a discharge model is adapted to the configuration of the thruster. He estimates the thruster performance, identifies key points and provides sizing prospects for a new version of the thruster. To complete the model, preliminary simulations of electromagnetic wave propagation and microwave plasma coupling magnetized are carried out. The results obtained make it possible to better understand the microwave power deposition in the plasma source and reproduce the influence of the magnetic field observed experimentally.L'objet de ce travail consiste à la caractérisation et à la modélisation d'un propulseur électrique à résonance cyclotronique des électrons. L’objectif est d’étudier la physique du propulseur (transfert d’énergie par résonance, processus d’ionisation, couplage micro-onde/plasma, processus d’accélération) afin d’améliorer ses performances, son efficacité ainsi que le développement d’outils de dimensionnement. Un prototype expérimental du propulseur a été caractérisé autour des degrés de libertés de fonctionnement tels que la fréquence, le champ magnétique, la puissance, la géométrie et le débit de gaz. Les résultats obtenus permettent de définir les conditions pour un fonctionnement nominal du propulseur en termes de performances et d’efficacité. Il a été montré que la position de la zone résonance ainsi que la pression de fonctionnement sont les deux paramètres clés pour l’optimisation du propulseur. Ces travaux de recherche ont permis d’augmenter les performances et le rendement total du propulseur. Pour une puissance de 30 Watts et un débit de 0.1 mg/s, le propulseur fourni une poussée de 1 mN avec une impulsion spécifique de 1000 s pour 16 % d’efficacité totale.En parallèle, un modèle de décharge est adapté au propulseur. Il estime les performances du propulseur, permet d’identifier les points importants et apporte des perspectives de dimensionnement pour une nouvelle version du propulseur. Pour compléter ce modèle, des simulations préliminaires de propagation d’ondes électromagnétiques et de couplage micro-onde plasma magnétisé sont réalisées. Les résultats obtenues permettent de mieux comprendre la déposition de puissance micro-onde dans le propulseur
A Tight Runtime Analysis of the (1+(λ, λ)) Genetic Algorithm on OneMax
No full textInternational audienc
Unphysical and physical solutions in many-body theories: from weak to strong correlation
No full textInternational audienceMany-body theory is largely based on self-consistent equations that are constructed in terms of the physical quantity of interest itself, for example the density. Therefore, the calculation of important properties such as total energies or photoemission spectra requires the solution of nonlinear equations that have unphysical and physical solutions. In this work we show in which circumstances one runs into an unphysical solution, and we indicate how one can overcome this problem. Moreover, we solve the puzzle of when and why the interacting Green's function does not unambiguously determine the underlying system, given in terms of its potential, or non-interacting Green's function. Our results are general since they originate from the fundamental structure of the equations. The absorption spectrum of lithium fluoride is shown as one illustration, and observations in the literature for some widely used models are explained by our approach. Our findings apply to both the weak and strong-correlation regimes. For the strong-correlation regime we show that one cannot use the expressions that are obtained from standard perturbation theory, and we suggest a different approach that is exact in the limit of strong interaction
Numerical study of a cylinder model of the diffusion MRI signal for neuronal dendrite trees
No full textInternational audienc
Matériaux pour la gestion de la lumière fabriqués par voie liquide
No full textLight management is a current hot issue for the development of new materials for optics and optoelectronics. In the energy field, more efficient systems at lower costs can be obtained by integrating these structures either to trap the light (photovoltaic) or to extract it (LED technology).Thin film solar cells allow decreasing the amount of photoactive materials but this induces a loss in the absorption due to a shortened path of light. A approach consists in the integration of new structures like photonic crystals to extend the path of light at specific wavelengths in the absorbing medium and thus lead to more coupled guided modes. Researchers focus on methods to elaborate such systems while reducing costs and liquid deposition techniques seem to be an interesting way around vacuum processes.We have developed a simple sol-gel method to fabricated optically clear Distributed Bragg Reflectors (DBR) with silica 1D-photonic crystal upon it made by soft thermal nanoimprint technology. DBRs show a high selective and specular reflection with a low number of layers thanks to the use of materials with a high refractive index contrast: dense titania layers (n=2.10 at λ=600nm) and 50%-macroporous silica layers (n=1,24) which porosity is obtained with a porogen PMMA latex degraded during a single calcination step. These DBRs are easily wavelength tunable by changing the thickness of the layers and they show stable optical properties in harsh environment or after immersion in solvents.Using the RCWA simulation method, we have optimized optical and geometrical parameters of the structure in order to maximize the absorption in a simple photoactive system made of a thin film amorphous silicon layer on glass. We have demonstrated the fabrication of the optimal system with a DBR, a 1D-photonic crystal upon it and a conformal deposited silicon layer. We show that the light trapping structure increases the absorption in a 100nm-thick absorbing layer by almost 70% between 350nm and 800nm. The comparison of the optimized system with other geometries shows the importance of the simulation approach. We went further by integrating a DBR in a real amorphous silicon solar cell. We observed a 5%-increase of the external quantum efficiency thanks to the reflector. This work put forward the sol-gel technology approach applied to photoactive systems.La gestion de la lumière est un élément clé dans le développement de nouveaux matériaux pour l’optique et l’optoélectronique. Dans le domaine énergétique, intégrer des systèmes permettant d’augmenter l’absorption de lumière (photovoltaïque) ou son extraction (technologie LED) conduit à des produits plus efficaces et à moindre coût.Les cellules solaires en couches minces permettent de diminuer la quantité de matériaux actifs utilisés, mais l’absorption de la lumière se trouve limitée par un trajet optique plus court dans le matériau. Une stratégie possible consiste à intégrer des cristaux photoniques pour allonger le trajet des photons de longueurs d’onde spécifiques dans la couche absorbante en les couplant à des modes guidés de celle-ci. De nombreuses recherches sont menées pour fabriquer de tels systèmes tout en limitant les coûts de fabrication et dans ce contexte, la synthèse par voie liquide apparaît comme une alternative intéressante aux méthodes de dépôt sous vide. Au cours de ce travail, nous avons développé une méthode simple par voie sol-gel pour fabriquer des miroirs de Bragg efficaces et robustes (DBR) revêtus des cristaux photoniques obtenus par nanoimpression thermique de couches de silice. Les DBRs présentent une réflexion sélective élevée pour un faible nombre de couches grâce à l’utilisation de matériaux ayant un contraste d’indice particulièrement important : le TiO2 dense (n= 2,10 à λ=600nm) et la silice macroporeuse (n= 1,24) dont la porosité (50%) est obtenue avec la dégradation d’un latex porogène lors d’une unique étape de recuit. Grâce à la flexibilité du procédé, des miroirs réfléchissant dans les Ultra-violets, le visible ou le proche Infra-Rouge peuvent être obtenus.A l’aide de la méthode de simulation RCWA, nous avons optimisé les paramètres optiques et géométriques de la structure pour maximiser l’absorption dans un système photoactif modèle à base de silicium amorphe. Après intégration dans ce système de la structure de piégeage optique optimale, nous avons montré qu’elle permet d’augmenter l’absorption de près de 70% dans 100nm de silicium sur la gamme de longueur d’onde 350 à 800nm. En comparant cette structure à d’autres avec des paramètres géométriques ayant été modulés, nous confirmons l’importance du travail d’optimisation préliminaire par la simulation. Nous avons ensuite appliqué nos résultats dans le cas d’une cellule solaire en couche mince. Nous avons ainsi montré que l’utilisation de nos structures de piégeage optique conduit à une augmentation de rendement quantique externe de l’ordre de 5% pour la structure étudiée. Ces résultats mettent en évidence les grandes possibilités de la technologie sol-gel au service des dispositifs optiquement actifs
Measuring Fast and Slow Enzyme Kinetics in Stationary Droplets
No full textInternational audienceWe present a new microfluidic platform for the study of enzymtatic reactions using static droplets on demand. This allows us to monitor both fast and slow reactions with the same device and minute amounts of reagents. The droplets are produced and displaced using confinement gradients, which allows the experiments to be performed without having any mean flow of the external phase. Our device is used to produce six different pairs of drops, which are placed side by side in the same microfluidic chamber. A laser pulse is then used to trigger the fusion of each pair, thus initiating a chemcial reaction. Imaging is used to monitor the time evolution of enzymatic reactions. In the case of slow reactions, the reagents are completely mixed before any reaction is detected. This allows us to use standard Michaelis–Menten theory to analyze the time evolution. In the case of fast reactions, the time evolution takes place through a reaction-diffusion process, for which we develop a model that incorporates enzymatic reactions in the reaction terms. The theoretical predictions from this model are then compared to experiments in order to provide measurements of the chemical kinetics. The approach of producing droplets through confinement gradients and analyzing reactions within stationary drops provides an ultralow consumption platform. The physical principles are simple and robust, which suggests that the platform can be automated to reach large throughput analyses of enzymes
Conservation and Role of Electrostatics in Thymidylate Synthase
No full textInternational audienceConservation of function across families of orthologous enzymes is generally accompanied by conservation of their active site electrostatic potentials. To study the electrostatic conservation in the highly conserved essential enzyme, thymidylate synthase (TS), we conducted a systematic species-based comparison of the electrostatic potential in the vicinity of its active site. Whereas the electrostatics of the active site of TS are generally well conserved, the TSs from minimal organisms do not conform to the overall trend. Since the genomes of minimal organisms have a high thymidine content compared to other organisms, the observation of non-conserved electrostatics was surprising. Analysis of the symbiotic relationship between minimal organisms and their hosts, and the genetic completeness of the thymidine synthesis pathway suggested that TS from the minimal organism Wigglesworthia glossinidia (W.g.b.) must be active. Four residues in the vicinity of the active site of Escherichia coli TS were mutated individually and simultaneously to mimic the electrostatics of W.g.b TS. The measured activities of the E. coli TS mutants imply that conservation of electrostatics in the region of the active site is important for the activity of TS, and suggest that the W.g.b. TS has the minimal activity necessary to support replication of its reduced genome. The electrostatic potential of a protein plays a crucial role in steering ligands to their binding sites, and orienting them correctly for binding 1. In enzymes, the active site electrostatic potential is important for stabilizing the transition state and thereby catalyzing the reaction 2. Therefore, conservation of protein function across a protein family is often accompanied by conservation of the electrostatic potential in the active site region, even though the rest of the protein may lack a conserved electrostatic potential 3,4. Consequently, comparison of protein electrostatic potentials has been employed as a tool to predict protein function and to derive similarities in protein function across protein families 5–7. Optimizing the electrostatic complementarity between a ligand and the binding site of a protein is also an important aspect in drug design 8,9 and may provide a route to gain target selectivity 10