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Une coupe très intéressante dans l'éponge de Menger -itération 2-
No full textAn interesting cross-section inside the Menger sponge -iteration 2- (Une coupe très intéressante dans l'éponge de Menger -itération 2-
Une coupe très intéressante dans l'éponge de Menger -itération 1-
No full textAn interesting cross-section inside the Menger sponge -iteration 1- (Une coupe très intéressante dans l'éponge de Menger -itération 1-
Constraining the thick disc formation scenario of the Milky Way
No full textInternational audienceWe study the shape of the thick disc using photometric data at high and intermediate latitudes from SDSS and 2MASS surveys. We use the population synthesis approach using an ABC-MCMC method to characterize the thick disc shape, scale height, scale length, local density and flare, and we investigate the extend of the thick disc formation period by simulating several formation episodes. We find that the vertical variation in density is not exponential, but much closer to an hyperbolic secant squared. Assuming a single formation epoch, the thick disc is better fitted with a sech2 scale height of 470 pc and a scale length of 2.3 kpc. However if one simulates two successive formation episodes, mimicking an extended formation period, the older episode has a higher scale height and a larger scale length than the younger episode, indicating a contraction during the collapse phase. The scale height decreases from 800 pc to 340 pc, and the scale length from 3.2 kpc to 2 kpc. The star formation increases from the old episode to the young one. During the fitting process, the halo parameters are also determined. The constraint on the halo shows that a transition between a inner and outer halo, if exists, cannot be at a distance of less than about 30 kpc, which is the limit of our investigation using turnoff halo stars. Finally, we show that extrapolating the thick disc towards the bulge region explains well the stellar populations observed there, that there is no longer need to invoke a classical bulge. To explain these results, the most probable scenario for the thick disc is that it formed while the Galaxy was gravitationally collapsing from well mixed gas-rich giant clumps sustained by high turbulence which awhile prevent a thin disc to form, as proposed by Bournaud et al. (2009). This scenario explains the observations in the thick disc region as well as in the bulge region. (abridged
Concentration rate and consistency of the posterior distribution for selected priors under monotonicity constraints
No full textInternational audienceIn this paper, we consider the well known problem of estimating a density function under qualitative assumptions. More precisely, we estimate monotone non-increasing densities in a Bayesian setting and derive concentration rate for the posterior distribution for a Dirichlet process and finite mixture prior. We prove that the posterior distribution based on both priors concentrates at the rate (n/log(n))−1/3, which is the minimax rate of estimation up to a log(n) factor. We also study the behaviour of the posterior for the point-wise loss at any fixed point of the support of the density and for the sup-norm. We prove that the posterior distribution is consistent for both loss functions
Etude de l'interaction d'un faisceau laser intense avec un plasma long et chaud dans le contexte du schéma d'allumage par choc
No full textShock ignition is an alternative direct-drive scheme for inertial fusion that consists in two steps. The first one is a several nanoseconds long compression with low intensity beams. The second one is a several hundred of picoseconds stage using high intensity beams to create a converging shock leading to ignition. During the second phase, the laser beam goes through a long and hot under-critical plasma. However, the coupling of this intense pulse with the coronal plasma has not been much studied experimentally or numerically. Then, the energy absorbed as well as the role of parametric instabilities regarding reflected or transmitted intensity cannot be predicted. In this PhD dissertation, we describe an experimental study of an intense laser pulse between 2.1015 W/cm2 and 2.1016 W/cm2 interacting with millimetric plasma heated close to one keV. We begin with a theoretical description of the interaction conditions in the coronal plasma. Brillouin scattering is in strongly coupled regime, Raman instability is kinetic regime and laser intensity is above ponderomotive filamentation threshold. We recreate these interaction conditions experimentally by means of pre-heated targets which are foams or thin plastic foils. Then, we present the first measurements of time resolved backscattered spectra from the smoothed picosecond beam as well as transmitted intensity distribution through the plasma. We find that Brillouin instability can be responsible for up to 60% reflectivity in plasmas with electronic density close to critical while Raman reflectivity stays at low levels. Transmitted intensity distribution is smoothed by the propagation and its diameter increases compared to the laser focal spot in vacuum. Finally, we discuss interaction measurements in nanosecond regime to highlight the fact that parametric instabilities reduction is essential for shock ignition to be a successful scheme.L'allumage par choc est un schéma de fusion réalisé en attaque directe qui se décompose en deux étapes. La première consiste en une compression de plusieurs nanosecondes à l'aide de faisceaux de quelques 1014 W/cm2 et la seconde en l'utilisation de faisceaux d'intensité supérieure à plusieurs 1015 W/cm2 pour créer un choc convergent déclenchant l'ignition. Le couplage du laser lors de la seconde étape soulève plusieurs questions. En effet, le laser doit traverser un plasma sous critique chaud, de taille millimétrique avant de déposer son énergie et créer le choc. Dans ce domaine mal connu expérimentalement et numériquement, on ne sait pas quelle sera la fraction d'énergie absorbée ou comment vont influer les instabilités paramétriques sur la propagation. Dans ce manuscrit, nous proposons une étude expérimentale de l'interaction d'un faisceau d'intensité comprise entre 2.1015 W/cm2 et 2.1016 W/cm2 dans un plasma de l'ordre du millimètre et à une température proche du keV. D'abord, nous présentons les conditions d'interaction dans le plasma coronal. L'instabilité Brillouin est en régime de couplage fort, l'instabilité Raman est en régime cinétique et l'intensité est au-dessus du seuil de filamentation pondéromotrice. Expérimentalement, nous approchons au mieux les conditions hydrodynamiques données par l'étude théorique en utilisant des cibles de mousses et des feuilles minces préformées. Nous présentons les premières mesures résolues en temps des spectres rétrodiffusés du faisceau picoseconde, lissé par une lame de phase aléatoire, ainsi que la distribution d'intensité transmise dans ce type de plasma. Nous mettons en évidence des mesures d'énergie rétrodiffusée par l'instabilité Brillouin pouvant atteindre 60% de l'énergie incidente pour les densités électronique de plasma les plus importantes. L'instabilité Raman n'induit quant à elle, pas de perte d'énergie conséquente. En ce qui concerne la distribution de l'intensité transmise, nous observons un lissage et une augmentation du diamètre de cette dernière par rapport à la tache focale à vide. Enfin nous surlignons l'importance de la prise en compte des instabilités paramétriques quant à la réussite du schéma d'allumage par choc en complétant nos observations avec résultats obtenus pour des impulsions nanoseconde moins intenses
Etude expérimentale de la modification des instabilités paramétriques en plasmas multiples.
No full textThe presence of several plasmas along the path of an energetic laser beam is likely to produce noticeable changes in the behaviour of parametric instabilities compared to the case of interaction with single plasma. In order to identify the different effects that can come into play in this kind of situation, a tunable double target system has been designed, allowing controlled variation in the distance betwwen the two targets. The first target was low density foam while the second one was a thin plastic foil. Part of this thesis is devoted to characterization of laser-plasma interaction into the foams since this subject has been not much explored yet for such low densities. In the cas of interaction with two plasmas, three effects have been identified and analysed: hydrodynamic coupling, the effect of laser-induced incoherence and electromagnetic coupling of the plasmas. For the first time it has been evidenced that laser-induced incoherence can lead to the suppression of the stimulated Brillouin backscattering generated into separate plasma. On the other hand, laser -induced incoherence benefits are lost when hydrodynamic effects occur due to the collision of the two plasmas if not enough distance is put between the targets. Stimulated Raman scattering is more sensitive to the electromagnetic coupling between the plasmas. The latter occurs when the backscattered light from the foil target propagates through the foam plasma. In this case, a reamplification of the scattered light can be observed leading to an increasing in relfectivity levels. This phenomenon is particularly desired for plasma amplification of short pulses. It at also been observed that in the low density region of the foam, bacskattered light from the foil leads to Raman amplification in the kinetic regime.La présence simultanée de plusieurs plasmas le long de la trajectoire d'un faisceau laser énergétique est susceptible de modifier de manière importante le comportement des instabilités paramétriques par rapport au cas où un seul plasma est présent. Afin d'identifier les différents effets pouvant intervenir dans ce type de situation, un système de cible double dont l'écartement est ajustable a été développé, la première cible étant une mousse de faible densité et la seconde une feuille de plastique. Une partie de cette thèse est consacrée à la caractérisation de l'interaction dans les mousses seules, cette dernière étant encore peu connue à l'heure actuelle pour de si faibles densités. Dans le cas d'interaction en présence de deux plasmas, nous analysons successivement trois effets : celui du mélange hydrodynamique des deux plasmas, celui de l'incohérence induite par plasma et enfin celui du couplage électromagnétique entre les deux plasmas. Pour la première fois, il a été mis en évidence que l'incohérence induite par plasma sur un faisceau conduisait à la réduction de la rétrodiffusion Brillouin dans un plasma séparé. En revanche, cette incohérence ne parvient plus à compenser les effets hydrodynamiques se produisant par collision des plasmas lorsque ceux-ci sont suffisamment proches. Le couplage électromagnétique entre les plasmas affecte plus particulièrement la rétrodiffusion Raman, lorsque la rétrodiffusion du plasma de feuille traverse le plasma de mousse. Outre une réamplification du signal rétrodiffusé qui est un phénomène recherché dans l'amplification d'impulsions laser en milieu plasma, nous avons observé le développement de l'instabilité Raman dans une zone de plasma dominée par l'amortissement Landau ce qui est la signature d'effets cinétiques importants
Characterization of laser-driven ultrafast shockless compression using gold targets
No full textInternational audienceIndirect laser-driven shockless compression experiments on gold targets were performed to characterize pressure loading processes and target states. Free surface velocities of the gold target under ramped pressure loading were measured using line-imaging velocity interferometers. From the velocity data and the equation of state, the maximum pressure and strain rate attained under compression were estimated to be 50 GPa and 4 107 s 1, respectively. Optical reflectivity was measured simultaneously with the velocity, the result suggesting no significant or unexpected temperature increases in the ultrafast shockless compression proces
Agricultural Trade, Biodiversity Effects and Food Price Volatility
No full textProduction risks in agriculture due to biotic elements such as pests create biodiversity effects that impede productivity. Pesticides reduce these effects but are damaging for the environment and human health. When regulating farming practices, governments weigh these side-effects against the competitiveness of their agriculture. In a Ricardian two-country setup, we show that free trade results in an incomplete production specialization, that restrictions on pesticides are generally more stringent than under autarky and that trade increases the price volatility of crops produced by both countries and some of the specialized crops. If biodiversity effects are large, the price volatility of all crops is larger than under autarky
