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International audience

International audience

International audience

International audience

International audience

International audience

International audience

International audience

International audience; Water is a trace gas of strong interest for plasma-based medical applications. We use a hybrid global model to simulate a chemically complex Ar/ atmospheric pressure, radio frequency capacitive discharge, including 47 species with positive ion clusters up to . For a discharge gap of 1.5 mm driven at 27.12 MHz, we determine the discharge properties over a range of rf currents (150–500 A m−2) and initial concentrations (0.25–2%). An isothermal plug-flow model is used with a gas residence time of 0.2 s for most calculations, with the gas temperature calculated self-consistently from the input power. The cluster density distributions are determined, and we find that the higher mass cluster densities decrease rapidly with increasing gas temperature. A simplified cluster dynamics analytic model is developed and solved to determine the cluster density distributions, which is in good agreement with the hybrid simulation results.

International audience; Hall-effect thrusters, which are electrostatic devices based on an E B ´ plasma discharge, have successfully been used as satellite propulsion systems for the last few decades. However, the presence of anomalous electron cross-field transport is still poorly understood, and involves complex and strongly coupled mechanisms such as azimuthal electron drift instabilities and intense secondary electron emission (SEE) from the thruster walls. The present work focuses on the relative importance of these two phenomena. We use a 2D particle-in-cell/Monte Carlo collision model configured to simulate the radial-azimuthal directions near the thruster exit plane. A constant radial magnetic field and axial electric field are imposed, and electron drift instabilities are observed in the azimuthal (E B ´) direction. A simplified SEE model is implemented and an extensive parametric study is performed to directly determine the effect on electron transport. It is found that, for the operating conditions used in our simulations, SEE enhances the near-wall electron mobility by a factor 2, while reducing the bulk plasma mobility by about 20% (due to electron cooling). However, the dominant contribution to anomalous electron transport is still observed to be caused by electron drift instabilities driven by the E B ´ discharge configuration. SEE modifies the electron mobility profile, but the spatially-averaged value remains relatively constant. Three different operating regimes are identified depending on the SEE rate value: two that are stable, and a third which shows an oscillatory behaviour. In addition to electron transport, the kinetic simulations give new insight into the plasma sheath formation at the radial walls, and comparison with typical analytical sheath models demonstrate important differences.

International audience; Hall-effect thrusters (HETs) operated with xenon are one of the most commonly used electric propulsion technologies for a wide range of space missions, including drag compensation in low Earth orbit, station-keeping, and orbital insertion, as access to space becomes more affordable. Although anomalous electron transport, the electron drift instability (EDI), and secondary electron emission (SEE) have been studied experimentally and numerically in xenon-based HETs, the impact of alternative propellants is still poorly characterized. In this work, a two-dimensional particle-in-cell/Monte Carlo collision code is used to model the (r – θ) plane of a HET operated separately with four different noble gases: xenon, krypton, argon, and helium. Models for electron induced secondary electron emission (SEE) and dielectric walls are implemented in order to investigate the coupling between the propellant choice and the radial thruster walls. For all conditions and propellants studied, an EDI and enhanced electron cross-field transport are observed. The frequency of the instability, as well as the electron mobility, is compared with analytical expressions from a recently developed kinetic theory. Confirming this theory, it is shown that while the frequency of the EDI depends on the propellant mass, the electron mobility appears to be almost independent of the propellant choice.

From 71 forest soils classified as leached brown soils, brown calcic soils and humocalcic soils, the relationships were studied between the organic carbon and the variables: exchangeable calcium, cation exchange capacity, saturation level, pH, clay and silt content and elevation. The variables were selected from correlation coefficients and principal components analysis and multiple regressions. Exchangeable calcium explained 77% of the organic carbon variation. The saturation ratio and elevation explained 5,9 and 1,1% of the variation. The high dependence of organic carbon on calcium probably resulted from organo-metallic complexation and the protective effect of calcium against microbial degradations.

International audience

International audience

International audience

Les auteurs sont partis d'un complexe organo-mineral calcique provenant d'un sol humifere du Jura et ont provoque une diminution progressive au cours du temps du taux de calcium echangeable. L'etude de l'evolution de divers parametres, menee parallelement, a permis d'analyser les effets directs et indirects de la baisse du calcium sur le complexe organo-mineral. Les resultats obtenus concernent: 1. les effets directs de l'action mineralisatrice de la microflore sur la degradation de la matiere organique (Fig. 7) et la production d'azote mineral (Fig. 3). 2. les effets indirects de cette mineralisation sur la destabilisation du complexe argilo-humique (Fig. 8).

Les sols etudies se rattachent aux sols lessives a pseudogley, aux sols colluviaux brunifies, aux sols bruns a pellicules calcaires et aux sols humo-calciques. Les teneurs en azote nitrique et ammoniacal, determinees au printemps, en ete et a l'automne, opposent les sols collineens aux sols montagnards: dans les premiers, les teneurs estivales passent par un minimum, les valeurs de printemps et d'automne etant plus elevees; dans les seconds, en revanche, les teneurs estivales deviennent maximales. Le deficit hydrique climatique constitue le facteur limitant de la mineralisation d'azote a l'etage collineen (chenaie-charmaie); en revanche, l'absence de deficit hydrique en ete, associee a des temperatures favorables a l'ammonification et a la nitrification, stimule la mineralisation estivale d'azote a l'etage montagnard (sapiniere-hetraie et pessiere)

Plusieurs series d’experiences sont en cours a Besancon pour preciser le role des gibberellines dans le determinisme des mouvements des tiges volubiles. Nous allons ici en relater quelques unes.

Resume: Deux especes de Begonia ont ete cultivees: B. gracilis et B. semperflorens. Pour chacune des 2 especes nous avons etabli deux groupes de plantes. Celles du premier groupe ont ete cultivees en lumiere continue (conditions generales sensiblement uniformes) et celles du deuxieme groupe ont ete cultivees en serres. L’action de l’acide gibberellique a ete etudiee dans les deux groupes. En lumiere continue aucune floraison ne se manifeste apres 4 mois de culture chez les sujets traites; en serres, la floraison se produit avec un retard d’un mois par rapport aux temoins. Une etude anatomique montre une stimulation de l’activite cambiale sous l’action de l’acide gibberellique.