Forest hydrology and watershed management = hydrologie forestière et aménagement des bassins hydrologiques

On compare les écoulements d'une parcelle (400 m²), de bassins-versants élémentaires (environ (1.5 ha) et d'un petit bassin-versant (4.5 km²) situés sur le même substratum géologique, en forêt tropicale humide. Le ruissellement annuel de la parcelle est de 8.8 % et celui du petit bassin de 23 % (1981), différence s'expliquant par l'analyse des systèmes pédologiques. Les écoulements des bassins élémentaires peuvent être semblables à ceux de la parcelle ou du petit bassin, selon le stade d'évolution de leur couverture pédologique. (Résumé d'auteur

Measurements of electron emission under electron impact on BN sample for incident electron energy between 10 eV and 1000 eV

International audienceElectron emission measurements have been performed on a BN sample by using a new specific protocol and experimental setup , which allows characterizing electron emission under electron impact on resistive material in a short time and with a wide variety of extracted data: total electron emission yield, emitted electron energy distribution, elastically backscattered electron emission yield and energy efficiency of electron-surface interaction. Methodology, calibration, biases corrections and results are presented in this letter. Results are compared to that measured on another material SiO2. As there are few published data on electron emission at low incident electron energy on BN sample, it is expected that these measurements could be useful for numerous studies implying electron emission on BN surface

Spacecraft worst-case surface charging: On the importance of measuring the electron emission yield under representative environmental conditions

International audienceTo represent the time spent by space materials on ground before launch, the measurement of secondary electron emission properties is performed after long duration exposure to ambient atmosphere and humidity. The observed change with respect to pristine samples have an impact on the estimation of worst-case surface charging levels in geostationary orbit, especially for a spacecraft in eclipse. It is therefore recommended to adequately outgas the samples with respect to the expected flight conditions and ageing effects

Drifting plasma collection by a positive biased tether wire in LEO-like plasma conditions: current measurement and plasma diagnostic

BETs is a three-year project financed by the Space Program of the European Commission, aimed at developing an efficient deorbit system that could be carried on board any future satellite launched into Low Earth Orbit (LEO). The operational system involves a conductive tape-tether left bare to establish anodic contact with the ambient plasma as a giant Langmuir probe. As a part of this project, we are carrying out both numerical and experimental approaches to estimate the collected current by the positive part of the tether. This paper deals with experimental measurements performed in the IONospheric Atmosphere Simulator (JONAS) plasma chamber of the Onera-Space Environment Department. The JONAS facility is a 9- m3 vacuum chamber equipped with a plasma source providing drifting plasma simulating LEO conditions in terms of density and temperature. A thin metallic cylinder, simulating the tether, is set inside the chamber and polarized up to 1000 V. The Earth's magnetic field is neutralized inside the chamber. In a first time, tether collected current versus tether polarization is measured for different plasma source energies and densities. In complement, several types of Langmuir probes are used at the same location to allow the extraction of both ion densities and electron parameters by computer modeling (classical Langmuir probe characteristics are not accurate enough in the present situation). These two measurements permit estimation of the discrepancies between the theoretical collection laws, orbital motion limited law in particular, and the experimental data in LEO-like conditions without magnetic fields. In a second time, the spatial variations and the time evolutions of the plasma properties around the tether are investigated. Spherical and emissive Langmuir probes are also used for a more extensive characterization of the plasma in space and time dependent analysis. Results show the ion depletion because of the wake effect and the accumulation of- ions upstream of the tether. In some regimes (at large positive potential), oscillations are observed on the tether collected current and on Langmuir probe collected current in specific sites

Multiscale Modeling of Dust Charging in Simulated Lunar Environment Conditions

International audienceA key aspect of dust adhesion to space equipment is the accumulation of charge under the space plasma environment. Recent models and experiments show possible negative charging of dust grains under VUV illumination. Macroscopic potential measurements conducted during a test campaign show that both positive and negative average charging can be reached under VUV irradiation pending on vacuum chamber configuration, suggesting that both situations can exist at lunar surface. Simulations of dust charging at microscopic scale are conducted with the SPIS software to evaluate electrical charge and electric field amplifications induced by the granular structure of the lunar regolith. A multi-layer pile of dust is modelled under lunar conditions. Grains from the first two layers tend to microscopically acquire both negative and positive charge patches when illuminated with a 45° VUV incidence angle, this differential charging being less pronounced under normal incidence angle. It is also found that dust deeply buried in the lunar soil may charge more negative due to the collection of environmental electron only. This effect is thought to reinforce the grain supercharging model presented by other authors. We show however that such charge development may be limited by electrical conduction pending on dust electrical properties

Propellantless de orbiting of space debris by bare electrodynamic tethers

A 3-year Project started on November 1 2010, financed by the European Commision within the FP-7 Space Program, and aimed at developing an efficient de-orbit system that could be carried on board by future spacecraft launched into LEO, will be presented. The operational system will deploy a thin uninsulated tape-tether to collect electrons as a giant Langmuir probe, using no propellant/no power supply, and generating power on board. This project will involve free-fall tests, and laboratory hypervelocity-impact and tether-current tests, and design/Manufacturing of subsystems: interface elements, electric control and driving module, electron-ejecting plasma contactor, tether-deployment mechanism/end-mass, and tape samples. Preliminary results to be presented involve: i) devising criteria for sizing the three disparate tape dimensions, affecting mass, resistance, current-collection, magnetic self-field, and survivability against debris itself; ii) assessing the dynamical relevance of tether parameters in implementing control laws to limit oscillations in /off the orbital plane, where passive stability may be marginal; iii) deriving a law for bare-tape current from numerical simulations and chamber tests, taking into account ambient magnetic field, ion ram motion, and adiabatic electron trapping; iv) determining requirements on a year-dormant hollow cathode under long times/broad emission-range operation, and trading-off against use of electron thermal emission; v) determining requirements on magnetic components and power semiconductors for a control module that faces high voltage/power operation under mass/volume limitations; vi) assessing strategies to passively deploy a wide conductive tape that needs no retrieval, while avoiding jamming and ending at minimum libration; vii) evaluating the tape structure as regards conductive and dielectric materials, both lengthwise and in its cross-section, in particular to prevent arcing in triple-point junctions

An universal system to de-orbit satellites at end of life

A 3-year Project financed by the European Commission is aimed at developing a universal system to de-orbit satellites at their end of life, as a fundamental contribution to limit the increase of debris in the Space environment. The operational system involves a conductive tapetether left bare to establish anodic contact with the ambient plasma as a giant Langmuir probe. The Project will size the three disparate dimensions of a tape for a selected de-orbit mission and determine scaling laws to allow system design for a general mission. Starting at the second year, mission selection is carried out while developing numerical codes to implement control laws on tether dynamics in/off the orbital plane; performing numerical simulations and plasma chamber measurements on tether-plasma interaction; and completing design of subsystems: electronejecting plasma contactor, power module, interface elements, deployment mechanism, and tether-tape/end-mass. This will be followed by subsystems manufacturing and by currentcollection, free-fall, and hypervelocity impact tests

Modélisation et simulation de la phase post-arc d'un disjoncteur sous vide

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF