Paris, France
Paris, France

The ENSTA ParisTech, École nationale supérieure de techniques avancées , is one of the most prestigious and selective French grande école in engineering. It is a member of ParisTech .It is located in Palaiseau , next to École polytechnique.Every year about 150 engineers graduate from it.ENSTA offers its students general engineering training with the aim of enabling them to design, produce and oversee complex industrial systems, while meeting strict economic constraints and dealing with an international environment. To do this, the School provides high-level scientific and technological training, which is frequently updated to keep pace with changes in the leading edge technologies and supplemented by language, general culture, law and economics teaching.The teaching is given by research professors at ENSTA with the participation of numerous auxiliary teachers from the economic and industrial world familiar with the latest technical developments in a wide variety of fields.Research, which is one of the School's primary missions, makes a dynamic contribution in both the fundamental and applied fields, to the School's pedagogical project and to meeting the needs of business. Half is the responsibility of the School's research professors, and the other half is carried out by researchers from the CNRS, the INSERM and the École polytechnique working in ENSTA's premises.The general nature of the training given enables ENSTA graduates to find a career in a large number of sectors such as the automotive or naval industry, networks and telecommunications, space propulsion, robotics, oceanology and the environment. Most of the ENSTA graduate engineers are much sought-after by companies and generally find their first job in R&D departments and design offices, rapidly moving towards supervisory and project management posts.ENSTA is a public teaching and research establishment operating under the supervision of the Ministry of Defence. It is headed by a general officer of the Corps of Ordnance Ingineers . Some former graduates of École polytechnique attend ENSTA before joining the military Corps of Ordnance Ingineers, which staffs the DGA. Wikipedia.

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Cosso A.,Polytechnic of Milan | Russo F.,ENSTA ParisTech
Infinite Dimensional Analysis, Quantum Probability and Related Topics | Year: 2016

Functional Itô calculus was introduced in order to expand a functional F(t,X.+t,Xt) depending on time t, past and present values of the process X. Another possibility to expand F(t,X.+t,Xt) consists in considering the path X.+t = {Xx+t, x ∈ [-T, 0]} as an element of the Banach space of continuous functions on C([-T, 0]) and to use Banach space stochastic calculus. The aim of this paper is threefold. (1) To reformulate functional Itô calculus, separating time and past, making use of the regularization procedures which match more naturally the notion of horizontal derivative which is one of the tools of that calculus. (2) To exploit this reformulation in order to discuss the (not obvious) relation between the functional and the Banach space approaches. (3) To study existence and uniqueness of smooth solutions to path-dependent partial differential equations which naturally arise in the study of functional Itô calculus. More precisely, we study a path-dependent equation of Kolmogorov type which is related to the window process of the solution to an Itô stochastic differential equation with path-dependent coefficients. We also study a semilinear version of that equation. © 2016 World Scientific Publishing Company.

Gottlieb J.,Columbia University | Oudeyer P.-Y.,French Institute for Research in Computer Science and Automation | Oudeyer P.-Y.,ENSTA ParisTech | Lopes M.,French Institute for Research in Computer Science and Automation | And 2 more authors.
Trends in Cognitive Sciences | Year: 2013

Intelligent animals devote much time and energy to exploring and obtaining information, but the underlying mechanisms are poorly understood. We review recent developments on this topic that have emerged from the traditionally separate fields of machine learning, eye movements in natural behavior, and studies of curiosity in psychology and neuroscience. These studies show that exploration may be guided by a family of mechanisms that range from automatic biases toward novelty or surprise to systematic searches for learning progress and information gain in curiosity-driven behavior. In addition, eye movements reflect visual information searching in multiple conditions and are amenable for cellular-level investigations. This suggests that the oculomotor system is an excellent model system for understanding information-sampling mechanisms. © 2013 Elsevier Ltd.

Lorentz E.,ENSTA ParisTech
International Journal of Fracture | Year: 2017

A rate-independent damage constitutive law is proposed to describe the fracture of plain concrete under tensile loading. Here, the target scale is the individual crack. In order to deal with localised damage, the model is inherently nonlocal: the gradient of the damage field is explicitly involved in the constitutive equations; it is parameterised by a nonlocal length scale which is interpreted as the width of the process zone. The model is defined so that its predictions are close to those of a cohesive law for vanishing nonlocal length scales. Therefore, the current model is plainly consistent with cohesive zone model analyses: the nonlocal length scale appears as a small parameter which does not need any specific identification. And four parameters—among which the tensile strength and the fracture energy—enable to adjust the softening cohesive response. Besides, a special attention has been paid to the shape of the initial damage surface and to the relation between damage and stiffness. The damage surface takes into account not only the contrast between tensile and compressive strengths but also experimental evidences regarding its shape in multiaxial tension. And the damage–stiffness relation is defined so as to describe important phenomena such as the stiffness recovery with crack closure and the sustainability of compressive loads by damaged structures. Finally, several comparisons with experimental data (global force/opening responses, size dependency, curved crack paths, crack opening profiles) enable to validate qualitatively and quantitatively the pertinence of the constitutive law in 2D and 3D. © 2017 Springer Science+Business Media B.V.

Corde S.,ENSTA ParisTech | Ta Phuoc K.,ENSTA ParisTech | Lambert G.,ENSTA ParisTech | Fitour R.,ENSTA ParisTech | And 4 more authors.
Reviews of Modern Physics | Year: 2013

Relativistic interaction of short-pulse lasers with underdense plasmas has recently led to the emergence of a novel generation of femtosecond x-ray sources. Based on radiation from electrons accelerated in plasma, these sources have the common properties to be compact and to deliver collimated, incoherent, and femtosecond radiation. In this article, within a unified formalism, the betatron radiation of trapped and accelerated electrons in the so-called bubble regime, the synchrotron radiation of laser-accelerated electrons in usual meter-scale undulators, the nonlinear Thomson scattering from relativistic electrons oscillating in an intense laser field, and the Thomson backscattered radiation of a laser beam by laser-accelerated electrons are reviewed. The underlying physics is presented using ideal models, the relevant parameters are defined, and analytical expressions providing the features of the sources are given. Numerical simulations and a summary of recent experimental results on the different mechanisms are also presented. Each section ends with the foreseen development of each scheme. Finally, one of the most promising applications of laser-plasma accelerators is discussed: the realization of a compact free-electron laser in the x-ray range of the spectrum. In the conclusion, the relevant parameters characterizing each sources are summarized. Considering typical laser-plasma interaction parameters obtained with currently available lasers, examples of the source features are given. The sources are then compared to each other in order to define their field of applications. © 2013 American Physical Society.

Depresseux A.,ENSTA ParisTech
Nature Photonics | Year: 2015

The advent of X-ray free-electron lasers has granted researchers an unprecedented access to the ultrafast dynamics of matter on the nanometre scale. Aside from being compact, seeded plasma-based soft X-ray lasers (SXRLs) turn out to be enticing as photon-rich sources (up to 1015 per pulse) that display high-quality optical properties. Hitherto, the duration of these sources was limited to the picosecond range, which consequently restricts the field of applications. This bottleneck was overcome by gating the gain through ultrafast collisional ionization in a high-density plasma generated by an ultraintense infrared pulse (a few 1018 W cm-2) guided in an optically pre-formed plasma waveguide. For electron densities that ranged from 3 × 1018 cm-3 to 1.2 × 1020 cm-3, the gain duration was measured to drop from 7 ps to an unprecedented value of about 450 fs, which paves the way to compact and ultrafast SXRL beams with performances previously only accessible in large-scale facilities. © 2015 Nature Publishing Group

A thermodynamic approach is proposed to determine the dissociation conditions of salt hydrates and semiclathrate hydrates. The thermodynamic properties of the liquid phase are described with the SAFT-VRE equation of state, and the solid-liquid equilibria are solved by applying the Gibbs energy minimization criterion under stoichiometric constraints. The methodology is applied to water + halide salt systems, and an excellent description of the solid-liquid coexistence curves is obtained. The approach is extended to the water + tetra-n-butylammonium bromide (TBAB) binary mixture, and an accurate representation of the solid-liquid coexistence curves and dissociation enthalpies is obtained. The van der Waals-Platteeuw (vdW-P) theory combined with the new model for salt hydrates is used to determine the dissociation temperatures of semiclathrate hydrates of TBAB + carbon dioxide. A good description of the dissociation pressures of CO2 semiclathrate hydrates is obtained over wide temperature, pressure, and TBAB composition ranges (AAD = 10.5%). For high TBAB weight fractions the new model predicts a change of hydrate structure from type A to type B as the partial pressure of CO2 is increased. The model can also capture a change of behavior with respect to TBAB concentration, which has been observed experimentally: an increase of the TBAB weight fraction leads to a stabilization of the gas semiclathrate hydrate at low initial TBAB concentrations below the stoichiometric composition but leads to a destabilization of the hydrate at TBAB concentrations above the stoichiometric composition. © 2010 American Chemical Society.

Malka V.,ENSTA ParisTech
Physics of Plasmas | Year: 2012

This review article highlights the tremendous evolution of the research on laser plasma accelerators which has, in record time, led to the production of high quality electron beams at the GeV level, using compact laser systems. I will describe the path we followed to explore different injection schemes and I will present the most significant breakthrough which allowed us to generate stable, high peak current and high quality electron beams, with control of the charge, of the relative energy spread and of the electron energy. © 2012 American Institute of Physics.

Nedjar B.,ENSTA ParisTech
International Journal of Solids and Structures | Year: 2016

The idea in this paper is to build a class of constitutive equations for highly compressible isotropic materials that, among others, are capable to describe a zero apparent Poisson's ratio in the whole finite strain range, not only for moderate straining. This remarkable property is, for instance, observed in many soft materials with micro-structures such as sponges and polymeric foams with high porosities. It would then be suitable to describe their behavior within a macroscopic modeling framework. More specifically, herein by means of elementary considerations, we deduce adequate forms of strain-energy functions that are a priori decomposed into purely volumetric and volume-preserving parts. A class of compressible hyperelastic materials of the general Odgen type is obtained. It can consequently be specialized, for instance, to neo-Hookean, Mooney-Rivlin, and Varga's model types as well. Furthermore, for the elastic parameters, a connection with the limiting case of linear elasticity is made whenever possible, in particular with the classical Poisson's ratio, and with the bulk to shear moduli ratio. © 2016.

Ciarlet Jr. P.,ENSTA ParisTech
Computers and Mathematics with Applications | Year: 2012

To solve variational indefinite problems, a celebrated tool is the Banach-Nečas-Babuška theory, which relies on the inf-sup condition. Here, we choose an alternate theory, T-coercivity. This theory relies on explicit inf-sup operators, both at the continuous and discrete levels. It is applied to solve Helmholtz-like problems in acoustics and electromagnetics. We provide simple proofs to solve the exact and discrete problems, and to show convergence under fairly general assumptions. We also establish sharp estimates on the convergence rates. © 2012 Elsevier Ltd. All rights reserved.

Baranes A.,ENSTA ParisTech | Oudeyer P.-Y.,ENSTA ParisTech
Robotics and Autonomous Systems | Year: 2013

We introduce the Self-Adaptive Goal Generation Robust Intelligent Adaptive Curiosity (SAGG-RIAC) architecture as an intrinsically motivated goal exploration mechanism which allows active learning of inverse models in high-dimensional redundant robots. This allows a robot to efficiently and actively learn distributions of parameterized motor skills/policies that solve a corresponding distribution of parameterized tasks/goals. The architecture makes the robot sample actively novel parameterized tasks in the task space, based on a measure of competence progress, each of which triggers low-level goal-directed learning of the motor policy parameters that allow to solve it. For both learning and generalization, the system leverages regression techniques which allow to infer the motor policy parameters corresponding to a given novel parameterized task, and based on the previously learnt correspondences between policy and task parameters. We present experiments with high-dimensional continuous sensorimotor spaces in three different robotic setups: (1) learning the inverse kinematics in a highly-redundant robotic arm, (2) learning omnidirectional locomotion with motor primitives in a quadruped robot, and (3) an arm learning to control a fishing rod with a flexible wire. We show that (1) exploration in the task space can be a lot faster than exploration in the actuator space for learning inverse models in redundant robots; (2) selecting goals maximizing competence progress creates developmental trajectories driving the robot to progressively focus on tasks of increasing complexity and is statistically significantly more efficient than selecting tasks randomly, as well as more efficient than different standard active motor babbling methods; (3) this architecture allows the robot to actively discover which parts of its task space it can learn to reach and which part it cannot. © 2012 Elsevier B.V. All rights reserved.

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