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Strong solutions to the equations of electrically conductive magnetic fluids
No full textInternational audienceWe study the equations of flow of an electrically conductive magnetic fluid, when the fluid is subjected to the action of an external applied magnetic field. The system is formed by the incompressible Navier-Stokes equations, the magnetization relaxation equation of Bloch type and the magnetic induction equation. The system takes into account the Kelvin and Lorentz force densities. We prove the local-in-time existence of the unique strong solution to the system equipped with initial and boundary conditions. We also establish a blow-up criterion for the local strong solution
Backward Lasing of Air plasma pumped by Circularly polarized femtosecond pulses for the saKe of remote sensing (BLACK)
No full textInternational audienceRecently, S. Mitryukovskiy et al. presented experimental evidence showing that backward Amplified Spontaneous Emission (ASE) at 337 nm can be obtained from plasma filaments in nitrogen gas pumped by circularly polarized 800 nm femtosecond pulses (Opt. Express, 22, 12750 (2014)). Here, we report that a seed pulse injected in the backward direction can be amplified by ~200 times inside this plasma amplifier. The amplified 337 nm radiation can be either linearly or circularly polarized, dictated by the seeding pulse, which is distinct from the non-polarized nature of the ASE. We performed comprehensive measurements of the spatial profile, optical gain dynamics, and seed pulse energy dependence of this amplification process. These measurements allow us to deduce the pulse duration of the ASE and the amplified 337 nm radiation as well as the corresponding laser intensity inside the plasma amplifier. It indicates that the amplification is largely in the unsaturated regime and that further improvement of laser energy is possible. Moreover, we observed optical gain in plasma created in ambient air. This represents an important step towards future applications exploiting backward lasing for remote atmospheric sensing
Superfilamentation in Air
No full textInternational audienceThe interaction between a large number of laser filaments brought together using weak external focusing leads to the emergence of few filamentary structures reminiscent of standard filaments, but carrying a higher intensity. The resulting plasma is measured to be one order of magnitude denser than for short-scale filaments. This new propagation regime is dubbed superfilamentation. Numerical simulations of a nonlinear envelope equation provide good agreement with experiments
Spatio-temporal cleaning of a femtosecond laser pulse through interaction with counter-propagating filaments in air
No full textInternational audienceWe demonstrate spatiotemporal cleaning of a femtosecond laser pulse impinging on two counterpropagating filaments in air. The retroreflected signal essentially has a perfect beam profile. Prepulses present in the incident pulse are also efficiently removed. The performance of the “filament mirror” is explained by a plasma-mediated wave mixing process that can be viewed as a transient three-dimensional hologram
A data-assimilation method for Reynolds-averaged Navier–Stokes-driven mean flow reconstruction
No full textInternational audienceWe present a data-assimilation technique based on a variational formulation and a Lagrange multipliers approach to enforce the Navier–Stokes equations. A general operator (referred to as the measure operator) is defined in order to mathematically describe an experimental measure. The presented method is applied to the case of mean flow measurements. Such a flow can be described by the Reynolds-averaged Navier–Stokes (RANS) equations, which can be formulated as the classical Navier–Stokes equations driven by a forcing term involving the Reynolds stresses. The stress term is an unknown of the equations and is thus chosen as the control parameter in our study. The data-assimilation algorithm is derived to minimize the error between a mean flow measurement and the measure performed on a numerical solution of the steady, forced Navier–Stokes equations; the optimal forcing is found when this error is minimal. We demonstrate the developed data-assimilation framework on a test case: the two-dimensional flow around an infinite cylinder at a Reynolds number of Re = 150. The mean flow is computed by time-averaging instantaneous flow fields from a direct numerical simulation (DNS). We then perform several 'measures' on this mean flow and apply the data-assimilation method to reconstruct the full mean flow field. Spatial interpolation, extrapolation, state vector reconstruction and noise filtering are considered independently. The efficacy of the developed identification algorithm is quantified for each of these cases and compared with more traditional methods when possible. We also analyse the identified forcing in terms of unsteadiness characterization, present a way to recover the second-order statistical moments of the fluctuating velocities and finally explore the possibility of pressure reconstruction from velocity measurements
Faster polynomial multiplication over finite fields
No full textLet p be a prime, and let M_p(n) denote the bit complexity of multiplying two polynomials in F_p[X] of degree less than n. For n large compared to p, we establish the bound M_p(n)=O(n log n 8^(log^∗ n) log p), where log^∗ is the iterated logarithm. This is the first known Fürer-type complexity bound for F_p[X], and improves on the previously best known bound M_p(n)=O(n log n log log n log p)
Analyzing SBGN-AF Networks Using Normal Logic Programs: Chapter 9
No full textInternational audienceThis chapter focuses on the systems biology graphical notation activity flow language (SBGN-AF). A large part of the signaling networks present in the biological literature are represented in a graphical form that is close to SBGN-AF. Moreover, an increasing number of databases store such networks represented in SBGN-AF or in graphical forms that can be easily translated into SBGN-AF. The SBGN-AF language contains a set of glyphs that represent biological objects and relations between these objects. SBGN-AF contains five groups of glyphs: activity nodes, auxiliary units, container nodes, modulation arcs and logical operators. Activity nodes, container nodes and logical operators are the nodes of the network; modulation arcs are the edges. The chapter gives succinctly the signification of each type of glyph. It defines a particular type of logic programs; the normal logic programs (NLPs), and gives the semantics of these programs that will be used
Regularization of multiplicative iterative algorithms with nonnegative constraint
No full textInternational audienc
Matching in Closed-Form: Equilibrium, identification, and comparative statics
No full textThis paper provides closed-form formulas for a multidimensional two-sided matching problem with transferable utility and heterogeneity in tastes. When the matching surplus is quadratic, the marginal distributions of the characteristics are normal, and when the heterogeneity in tastes is of the continuous logit type, as in Choo and Siow (2006), we show that the optimal matching distribution is also jointly normal and can be computed in closed form from the model primitives. Conversely, the quadratic surplus function can be identified from the optimal matching distribution, also in closed-form. The analytical formulas make it computationally easy to solve problems with even a very large number of matches and allow for quantitative predictions about the evolution of the solution as the technology and the characteristics of the matching populations change
Measurement of jet fragmentation in PbPb and pp collisions at sqrt(s[NN]) = 2.76 TeV
No full textSubmitted to Phys. Rev. C ; see paper for full list of authorsThe jet fragmentation function of inclusive jets with transverse momentum pt above 100 GeV in PbPb collisions has been measured using reconstructed charged particles with pt above 1 GeV in a cone of radius 0.3 around the jet axis. A data sample of PbPb collisions collected in 2011 at a nucleon-nucleon center-of-mass energy of sqrt(s[NN]) = 2.76 TeV corresponding to an integrated luminosity of 150 inverse microbarns is used. The results for PbPb collisions as a function of collision centrality and jet transverse momentum are compared to reference distributions based on pp data collected at the same center-of-mass energy in 2013, with an integrated luminosity of 5.3 inverse picobarns. A centrality-dependent modification of the fragmentation function is found. For the most central collisions, a significant enhancement is observed in the PbPb/pp fragmentation function ratio for charged particles with pt less than 3 GeV. This enhancement is observed for all jet pt bins studied