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Flow dynamics and magnetic induction in the von-Karman plasma experiment
No full textThe von-Karman plasma experiment is a novel versatile experimental device designed to explore the dynamics of basic magnetic induction processes and the dynamics of flows driven in weakly magnetized plasmas. A high-density plasma column (10^16 - 10^19 particles.m^-3) is created by two radio-frequency plasma sources located at each end of a 1 m long linear device. Flows are driven through JxB azimuthal torques created from independently controlled emissive cathodes. The device has been designed such that magnetic induction processes and turbulent plasma dynamics can be studied from a variety of time-averaged axisymmetric flows in a cylinder. MHD simulations implementing volume-penalization support the experimental development to design the most efficient flow-driving schemes and understand the flow dynamics. Preliminary experimental results show that a rotating motion of up to nearly 1 km/s is controlled by the JxB azimuthal torque
What time-resolved measurements tell us about femtosecond laser damage?
No full textInternational audienc
Curvature of an elasto-plastic strip at finite strains : application to fast simulation of coils
No full textInternational audienceThis work is part of the framework of a fast modeling of winding aiming at improving knowledge of residual stress evolution in steel strips and therefore their flatness during the coiling process. An exact analytical solution of an elasto-plastic strip with isotropic hardening at finite strains under an imposedtransformation of curvature is developed. Issues related to flow rules for non-differentiable yield functions (Tresca) have been broached and a unique solution is obtained. The equivalence for this transformation, between von Mises and Tresca yield functions is demonstrated. This solution contributes to an efficient model by terms of computation times that aims at simulating coiling by taking into account inelastic deformations and enabling parametric studies in order to improve the process
Polarization analysis of CuXX-lines emitted from X-pinch
No full textInternational audienceSoft x-ray emission from CuXX L-shell lines emitted by a dense X-pinch plasma have been investigated with high-resolution curved Bragg crystals at different angles of orientation. Single shot time integrated spectra show clear evidences of polarization for the Ne-like spectral lines 2s22p6 1S0 → 2s22p53s 1P1 (λ = 12.570 Å), 2s22p6 1S0 → 2s22p53s 3P1 (λ = 12.8277 Å). The variation of the intensity ratio of these two well-separated L-shell lines is discussed in view of its application for suprathermal electron characterization under real experimental conditions of pinch plasmas. We demonstrated that the simultaneous use of two different polarization spectrometers (means 4 Bragg crystals) permitted a high level of confidence for the analysis of the variation of the line ratios due to polarization
Money for Nothing: Speeding Up Evolutionary Algorithms Through Better Initialization
No full textInternational audienceThat the initialization can have a significant impact on the performance of evolutionary algorithms (EAs) is a well known fact in the empirical evolutionary computation literature. Surprisingly, it has nevertheless received only little attention from the theoretical community.We bridge this gap by providing a thorough runtime analysis for a simple iterated random sampling initialization. In the latter, instead of starting an EA with a random sample, it is started in the best of k search points that are taken from the search space uniformly at random. Implementing this strategy comes at almost no cost, neither in the actual coding work nor in terms of wall-clock time.Taking the best of two random samples already decreases the Θ(n log n) expected runtime of the (1+1)~EA and Randomized Local Search on OneMax by an additive term of order √n. The optimal gain that one can achieve with iterated random sampling is an additive term of order √n log n}. This also determines the best possible mutation-based EA for OneMax, a question left open in [Sudholt, IEEE TEC 2013].At the heart of our analysis is a very precise bound for the maximum of k independent Binomially distributed variables with success probability 1/2
Validation of reactive gases and aerosols in the MACC global analysis and forecast system
No full textInternational audienceThe European MACC (Monitoring Atmospheric Composition and Climate) project is preparing the operational Copernicus Atmosphere Monitoring Service (CAMS), one of the services of the European Copernicus Programme on Earth observation and environmental services. MACC uses data assimilation to combine in situ and remote sensing observations with global and regional models of atmospheric reactive gases, aerosols, and greenhouse gases, and is based on the Integrated Forecasting System of the European Centre for Medium-Range Weather Forecasts (ECMWF). The global component of the MACC service has a dedicated validation activity to document the quality of the atmospheric composition products. In this paper we discuss the approach to validation that has been developed over the past 3 years. Topics discussed are the validation requirements, the operational aspects, the measurement data sets used, the structure of the validation reports, the models and assimilation systems validated, the procedure to introduce new upgrades, and the scoring methods. One specific target of the MACC system concerns forecasting special events with high-pollution concentrations. Such events receive extra attention in the validation process. Finally, a summary is provided of the results from the validation of the latest set of daily global analysis and forecast products from the MACC system reported in November 2014
Publicité, sollicitation, intervention: Pistes pour une étude pragmatiste de l’expérience citoyenne
No full textInternational audienceCet article interroge les apports possibles du pragmatisme à une ethnographie de la citoyenneté, à côté de l’étude classique des mobilisations collectives, du vote ou des problèmes publics. Prenant au sérieux l’idée de la démocratie comme mode de vie, l’enquête consiste à saisir la formation de la culture et de la capacité d’agir politique depuis les expériences ordinaires de la coexistence. L’attention se porte sur des actions, apparemment mineures, témoignant d’un sens politique, visant le monde en tant qu’il est commun. Appliquée aux espaces publics urbains, cette démarche conduit à identifier ce qui trouble et sollicite les personnes au gré de leurs déplacements et à examiner la transformation éventuelle de ces sollicitations en interventions
A new chemistry option in WRF-Chem v. 3.4 for the simulation of direct and indirect aerosol effects using VBS: evaluation against IMPACT-EUCAARI data
No full textInternational audienceA parameterization for secondary organic aerosol (SOA) production based on the volatility basis set (VBS) approach has been coupled with microphysics and radiative scheme in WRF/Chem model. The new chemistry option called "RACM/MADE/VBS" was evaluated on a cloud resolving scale against ground-based and aircraft measurements collected during the IMPACT-EUCAARI campaign, and complemented with satellite data from MODIS. The day-to-day variability and the diurnal cycle of ozone (O3) and nitrogen oxides (NOx) at the surface is captured by the model. Surface aerosol mass of sulphate (SO4), nitrate (NO3), ammonium (NH4), and organic matter (OM) is simulated with a correlation larger than 0.55. WRF/Chem captures the vertical profile of the aerosol mass in both the planetary boundary layer (PBL) and free troposphere (FT) as a function of the synoptic condition, but the model does not capture the full range of the measured concentrations. Predicted OM concentration is at the lower end of the observed mass. The bias may be attributable to the missing aqueous chemistry processes of organic compounds, the uncertainties in meteorological fields, the assumption on the deposition velocity of condensable organic vapours, and the uncertainties in the anthropogenic emissions of primary organic carbon. Aerosol particle number concentration (condensation nuclei, CN) is overestimated by a factor 1.4 and 1.7 within PBL and FT, respectively. Model bias is most likely attributable to the uncertainties of primary particle emissions (mostly in the PBL) and to the nucleation rate. The overestimation of simulated cloud condensation nuclei (CCN) is more contained with respect to that of CN. The CCN efficiency, which is a measure of the ability of aerosol particles to nucleate cloud droplets, is underestimated by a factor of 1.5 and 3.8 in the PBL and FT, respectively. The comparison with MODIS data shows that the model overestimates the aerosol optical thickness (AOT). The domain averages (for one day) are 0.38 ± 0.12 and 0.42 ± 0.10 for MODIS and WRF/Chem data, respectively. Cloud water path (CWP) is overestimated on average by a factor of 1.7, whereas modelled cloud optical thickness (COT) agrees with observations within 10%. In a sensitivity test where the SOA was not included, simulated CWP is reduced by 40%, and its distribution function shifts toward lower values with respect to the reference run with SOA. The sensitivity test exhibits also 10% more optically thin clouds (COT < 40) and an average COT roughly halved. Moreover, the run with SOA shows convective clouds with an enhanced content of liquid and frozen hydrometers, and stronger updrafts and downdrafts. Considering that the previous version of WRF/Chem coupled with a modal aerosol module predicted very low SOA content (SORGAM mechanism) the new proposed option may lead to a better characterization of aerosol–cloud feedbacks