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Microtearing turbulence: magnetic braiding and disruption limit
No full textInternational audienceA realistic reduced model involving a large poloidal spectrum of microtearing modes is used to probe the existence of some stochasticity of magnetic field lines. Stochasticity is shown to occur even for the low values of the magnetic perturbation δB/B devoted to magnetic turbulence that have been experimentally measured. Because the diffusion coefficient may strongly depend on the radial (or magnetic-flux) coordinate, being very low near some resonant surfaces, and because its evaluation implicitly makes a normal diffusion hypothesis, one turns to another indicator appropriate to diagnose the confinement: the mean residence time of magnetic field lines. Their computation in the microturbulence frame points to the existence of a disruption limit, namely of a critical order of magnitude of δB/B above which stochasticity is no longer benign yet leads to a macroscopic loss of confinement in some tens to hundred of electron toroidal excursions. Since the level of magnetic turbulence δB/B has been measured to grow with the plasma electron density this would also be a density limit
Ammonia emissions in tropical biomass burning regions: Comparison between satellite-derived emissions and bottom-up fire inventories
No full textInternational audienceVegetation fires emit large amounts of nitrogen compounds in the atmosphere, including ammonia (NH3). Excess NH3 is known to be responsible for several environmental issues: eutrophication of terrestrial and aquatic ecosystem, soil acidification, and loss of plant diversity. NH3 emissions, which are mainly estimated from bottom-up approaches, are still subject to large uncertainties. NH3 satellite measurements are now available since a few years and offer the possibility to enhance our knowledge of NH3 sources and to reduce the remaining uncertainties on their magnitude. Global bi-daily NH3 total columns can in particular be derived from the IASI infrared sounder onboard MetOp satellites using a retrieval method developed at the Universi
Tidal effects on stratospheric temperature series derived from successive advanced microwave sounding units
No full textInternational audienceStratospheric temperature series derived from the Advanced Microvave Sounding Unit on board successive NOAA satellites reveal during periods of overlap some bias and drifts. Part of the reason for these discrepancies could be associated with atmospheric tides as the orbits of these satellites drifted, inducing large changes in the actual time of measurements. NOAA 15 and 16, which exhibit a long period of overlap, allow deriving diurnal tides that can correct such temperature drifts. The characteristics of the derived diurnal tides during summer periods is in good agreement with those calculated with the Global Scale Wave model, indicating that most of the observed drifts is likely due to the atmospheric tides. Cooling can be biased by a factor of 2, if time of measurements are not considered. When diurnal tides are considered trends derived with temperature lidar series are in good agreement with AMSU series. Future adjustments of temperature time series based on successive AMSU instruments will require to consider corrections associated with the local time of measurements
Multiscale statistics of Lagrangian and Eulerian acceleration in turbulent stratified shear flows
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On Acceleration Statistics in Turbulent Stratified Shear Flows
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Design and current progress of the Apollon 10 PW project
No full textInternational audienceThe objective of the Apollon project is the generation of 10 PW peak power pulses of 15 fs at 1 shot/minute. In this paper the Apollon facility design, the technological challenges and the current progress of the project will be presented
Elasticity Optimism
No full textInternational audienceOn average, estimates of trade elasticities are smaller in aggregate data than at sector level. This is an artifact of aggregation. Estimations performed on aggregate data constrain sector elasticities to homogeneity, which creates a heterogeneity bias. The paper shows such a bias exists in two prominent approaches used to estimate elasticities, which has meaningful consequences for the calibration of the trade elasticity in one-sector, aggregative models. With elasticities calibrated to aggregate data, macroeconomic models can have predictions at odds with the implications of their multi-sector counterparts. They do not when elasticities are calibrated using a weighted average of sector elasticities
Sensibilité des précipitations extrêmes au couplage sous-mensuel atmosphère-océan en Méditerranée nord-occidentale : approche par la modélisation climatique régionale
No full textEvery year in autumn, heavy precipitation events (HPEs) occur in the northwestern Mediterrranean. This thesis uses coupled atmosphere-ocean regional climate modeling to tackle the sensitivity of these events to sea surface temperature (SST) changes coming either from model biases or from the oceanic mixed layer response to atmospheric forcing. Two case studies show the particular sensitivity of moisture convergence zones to SST changes. The use of synthetic indexes of precipitation changes and SST changes in the upstream zones shows a linear relationship between the two indexes in several regions (Cévennes, the region of Valencia, Calabria) in the modeling platforms MORCE and CNRM-RCSM4. Furthermore, we show that the HPEs in the region of Valencia are often preceded by a Mistral event which cools the upstream zone whithin 5 days before the HPEs. In turn, this cooling tends to reduce the intensity of the HPE.Chaque automne, des événements de précipitations intenses (HPEs) ont lieu en Méditerranée nord-occidentale. Cette thèse adopte une approche par la modélisation climatique régionale couplée atmosphère-océan pour traiter de la sensibilité de ces événements à des changements de température de surface de la mer (SST) résultant soit de biais dans le modèle couplé, soit de la réponse de la couche de mélange océanique à des forçages atmosphériques. Deux cas d’études mettent en évidence la sensibilité particulière des zones de convergence d’humidité aux changements de SST. L’élaboration d’indices synthétiques de changements dans les précipitations et de changements de SST en amont des zones précipitantes met en lumière dans plusieurs régions (Cévennes, région de Valence, Calabre) une relation linéaire entre ces deux quantités dans deux plateformes de modélisation différentes : MORCE et CNRM-RCSM4. Dans la région de Valence, en Espagne, nous montrons en outre que les événements de précipitations intenses sont souvent précédés d’un épisode de Mistral qui refroidit la zone amont des précipitations dans les jours précédant celles-ci, refroidissement qui tend ensuite à réduire l’intensité de l’événement précipitant
Étude du comportement collectif des speckles dans le développement de la diffusion Raman stimulée lors de l’interaction laser-plasma
No full textLaser-plasma instabilities which occur in millimetric hot plasma, typical in ICF experiments, are still a preoccupying topic because they lead to reflectivity levels higher than predicted. In this context, this work focuses on stimulated Raman scattering (SRS) through a multi-scale analysis: mono, bi and multi speckle scales. This picosecond experimental study highlights the difference between isolated and collective speckle behaviors in SRS development. A weak speckle, stable if isolated, may become unstable under the influence of SRS driven local plasma modifications generated by an intense nearby speckle: kinetic perturbations with suprathermal electrons and/or wave coupling with electromagnetic and electrostatic seeds. From set of experimental campaigns, featuring a highly spatially- and temporally-resolved Thomson-scattering diagnostic and backward Raman imaging, we evidenced and characterized this collective behavior. An experimental difference between these two kinds of perturbations was observed which lead to differentiate their contributions. In parallel, 2D PIC simulations using CALDER were performed to interpret bispeckle experimental results and in particular to understand the influence from both kinds of perturbations.Les instabilités paramétriques qui se développent dans l’interaction laser-plasma à haute intensité dans les plasmas chauds de plusieurs millimètres typiques des expériences de FCI, entrainent des taux de rétrodiffusion importants, ce qui en fait un sujet toujours préoccupant. Les travaux de cette thèse se concentrent sur l’étude d’une des instabilités paramétriques, la diffusion Raman stimulée, dans les cas mono-, bi- et multi-speckles ou points chauds que l’on retrouve dans les taches focales des gros lasers de puissance. Cette étude expérimentale, effectuée en régime picoseconde, a établi la différence des comportements en régime isolé ou collectif de la rétrodiffusion Raman : la modification des propriétés locales du plasma induite par un point chaud intense peut rendre instable un point chaud voisin, stable vis-à-vis de l’instabilité s'il était isolé, par l'intermédiaire d’un couplage cinétique et/ou d'un couplage d'ondes. Une série d’expériences utilisant des diagnostics de diffusion Thomson à hautes résolutions spatiale et temporelle couplées avec une imagerie de la rétrodiffusion Raman a permis de mettre en évidence l'existence de ce comportement collectif, de saisir sa dynamique puis de caractériser à l’échelle picoseconde son influence dans le développement global de l'instabilité Raman en discriminant l’aspect cinétique (électrons suprathermiques) de l’aspect ondulatoire (couplage par l’intermédiaire des ondes excitées par l’instabilité).Les mesures expérimentales bispeckle ont été interprétées à l’aide de simulations particulaires 2D du code CALDER qui reproduit cette discrimination entre ces deux types de perturbations
Large volume susy breaking with a solution to the decompactification problem
No full textInternational audienceWe study heterotic ground states in which supersymmetry is broken by coupling the momentum and winding charges of two large extra dimensions to the R-charges of the supersymmetry generators. The large dimensions give rise to towers of heavy string thresholds that contribute to the running of the gauge couplings. In the general case, these contributions are proportional to the volume of the two large dimensions and invalidate the perturbative string expansion. The problem is evaded if the susy breaking sectors arise as a spontaneously broken phase of N = 4 → N = 2 → N = 0 supersymmetry, provided that N = 4 super-symmetry is restored on the boundary of the moduli space. We discuss the mechanism in the case of Z 2 × Z 2 orbifolds, which requires that the twisted sector that contains the large extra dimensions has no fixed points. We analyze the full string partition function and show that the twisted sectors distribute themselves in non-aligned N = 2 orbits, hence preserving the solution to the string decompactification problem. Remarkably, we find that the contribution to the vacuum energy from the N = 2 → N = 0 sectors is suppressed, and the only substantial contribution arises from the breaking of the N = 4 sector to N = 0