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Gaudefroy L.,CEA DAM Ile-de-France
Physical Review C - Nuclear Physics | Year: 2010

A systematic study of the low-lying structure of N=27, 28, and 29 isotones is performed within the shell model framework using the SDPF-U interaction. For each isotonic chain, correlation energy is found to increase while moving away from the stability line. Spherical shapes as well as small values of correlation energy are associated with the isotopes of 20Ca and 19K discussed in this study. Neutron intruder states appear at low excitation energy in the 18Ar and 17Cl studied isotopes. Coexistence between spherical and prolate deformed states is a systematic feature in 16S isotopes. Below Z=16 most of the studied nuclei are characterized by intruder ground states. The major role played by protons in determining the structure of N 28 nuclei is shown. © 2010 The American Physical Society.

An implementation of full self-consistency over the electronic density in the DFT+DMFT framework on the basis of a plane waveprojector augmented wave (PAW) DFT code is presented. It allows for an accurate calculation of the total energy in DFT+DMFT within a plane wave approach. In contrast to frameworks based on the maximally localized Wannier function, the method is easily applied to f electron systems, such as cerium, cerium oxide (Ce 2O 3) and plutonium oxide (Pu 2O 3). In order to have a correct and physical calculation of the energy terms, we find that the calculation of the self-consistent density is mandatory. The formalism is general and does not depend on the method used to solve the impurity model. Calculations are carried out within the Hubbard I approximation, which is fast to solve, and gives a good description of strongly correlated insulators. We compare the DFT+DMFT and DFT+U solutions, and underline the qualitative differences of their converged densities. We emphasize that in contrast to DFT+U, DFT+DMFT does not break the spin and orbital symmetry. As a consequence, DFT+DMFT implies, on top of a better physical description of correlated metals and insulators, a reduced occurrence of unphysical metastable solutions in correlated insulators in comparison to DFT+U. © 2012 IOP Publishing Ltd.

Pellegrini Y.-P.,CEA DAM Ile-de-France
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

A theoretical framework is proposed to derive a dynamic equation motion for rectilinear dislocations within isotropic continuum elastodynamics. The theory relies on a recent dynamic extension of the Peierls-Nabarro equation, so as to account for core-width generalized stacking-fault energy effects. The degrees of freedom of the solution of the latter equation are reduced by means of the collective-variable method, well known in soliton theory, which we reformulate in a way suitable to the problem at hand. Through these means, two coupled governing equations for the dislocation position and core width are obtained, which are combined into one single complex-valued equation of motion, of compact form. The latter equation embodies the history dependence of dislocation inertia. It is employed to investigate the motion of an edge dislocation under uniform time-dependent loading, with focus on the subsonic/transonic transition. Except in the steady-state supersonic range of velocities - which the equation does not address - our results are in good agreement with atomistic simulations on tungsten. In particular, we provide an explanation for the transition, showing that it is governed by a loading-dependent dynamic critical stress. The transition has the character of a delayed bifurcation. Moreover, various quantitative predictions are made, that could be tested in atomistic simulations. Overall, this work demonstrates the crucial role played by core-width variations in dynamic dislocation motion. © 2014 American Physical Society.

A nonlinear model giving the short-time dynamics of turbulent mixing layers of two incompressible miscible fluids submitted to strong accelerations is proposed. This model encompasses both the linear rapid distortion theory applied to unstably stratified flows and an equation of evolution for the mixing zone width L. The nonlinear mechanism coming from the interaction between the turbulent quantities and the mean concentration field leads to a self-similar regime. The convergence to this state is analyzed in depth using dynamical system techniques. In this framework, the existence of a central manifold is established and allows a reduction of dimension of the problem. This is associated with a Lagrangian formulation depending only on L, L so that the dynamics of L degenerates into a buoyancy-drag equation. Here, the expression for the buoyancy coefficient is explicit. It depends only on the global mixing parameter and a quantity called the dimensionality parameter characterizing the form of density turbulent structures inside the mixing zone. An extension of the rapid acceleration model to the classical self-similar Rayleigh-Taylor problem is presented leading to an analytical expression for the growth parameter α, which is compared to existing numerical simulations and experiments. © 2013 American Institute of Physics.

An analytic model is presented that predicts viscosity and diffusion of plasma for pure elements and multicomponent mixtures, from the high-temperature low-density weakly coupled regime to the low-temperature high-density strongly coupled regime. It relies on a pseudo-ion in jellium modeling that incorporates the effect of electron screening on the ion-ion interaction in the pseudo-ionization. Mixtures are treated using approximate kinetic expressions and mixing laws applied to the excess viscosity and self-diffusion of pure elements. Comparisons are made with classical and quantum molecular dynamics results to assess its accuracy. The mean deviations are in the range 20-40% with almost no predictions further than a factor of 2 over many decades of variation. Applications of this model in the inertial confinement fusion context could help in predicting the appearance and the growth of hydrodynamic instabilities. © 2013 Elsevier B.V.

Soulard O.,CEA DAM Ile-de-France
Physical Review Letters | Year: 2012

The aim of this letter is to assess existing theories for Rayleigh-Taylor small turbulent scales. For this purpose, we propose to adapt the Monin-Yaglom relation to the Rayleigh-Taylor turbulence context. A special emphasis is put on the inhomogeneity of the flow and on the effect of buoyancy forces. This relation is then used to show that, among existing theories, the standard Kolmogorov-Obukhov theory should apply to Rayleigh-Taylor turbulence in the limit of a large Reynolds number, large times, and small scales. © 2012 American Physical Society.

Larroche O.,CEA DAM Ile-de-France
Physics of Plasmas | Year: 2012

Recently performed inertial confinement fusion implosion experiments involving D-3He gas-filled microballoons have shown discrepancies between expected and measured nuclear fusion yields as the relative abundances of D and 3He are varied. The latter have been tentatively attributed to a sedimentation, or stratification phenomenon occurring in the target core. This work investigates the possibility of ion species sedimentation in a detailed way through multi-species ion-kinetic Vlasov-Fokker-Planck simulations of the implosion process. A noticeable amount of sedimentation is found to build up during the main shock propagation to the target center, but then disappears as the implosion proceeds. As a result, only the yield of the first burst of neutrons, associated with shock convergence, is appreciably modified, leaving the main neutron production phase during fuel compression and stagnation unaffected. The sedimentation of fuel ion species found, thus, cannot explain the experimental discrepancies. © 2012 American Institute of Physics.

Geneste G.,CEA DAM Ile-de-France
Journal of Physics Condensed Matter | Year: 2011

General features of the order parameter distribution in barium titanate in its paraelectric phaseand in its ferroelectric phases (tetragonal and orthorhombic) are presented. The density of probability of the polarization ΠLx ,Ly ,Lz (→u), defined by an average of the local order parameters over regions of various sizes and shapes (L x×Ly×Lz), is examined by molecular dynamics simulations using a first-principles derived effective Hamiltonian. The free energies F Lx ,Ly ,Lz (→u) associated with these probabilities are computed by thermodynamic integration. The evolution of these quantities are explained through the computation of pair correlations, which are found, as stated in several previous works, very anisotropic, needle-like, with longitudinal correlations (→→) having much longer range than transverse ones (↑↑). The correlations explain why the density of probability of the order parameter evolves from a multiple-peaked distribution with maxima along [111] (in the single cell), along [100] for small needle-like regions, towards a single-peaked distribution for larger regions. A useful expression in which the shape-dependence of the free energy is manifest is provided. © 2011 IOP Publishing Ltd.

Llor A.,CEA DAM Ile-de-France
European Journal of Mechanics, B/Fluids | Year: 2011

As empirically observed over some fifty years, the steepness of the "infrared" spectrum of velocity fluctuations in a turbulent flowE(κ)∝κμ at scales D∼1κ≫ℓ larger than energy-containing eddiesconstrains the decay of turbulent kinetic energy. However, the theoretical understanding of the physical process controlling decay is still patchy, even in the simplest case of homogeneous isotropic turbulence (HIT). Here, HIT decay laws are derived from angular momentum invariance at big scales Dan approach first mentioned by Landau in 1944, but unduly dismissed later. It is restated in terms of a stochastic equation similar to a Langevin equation, to which usual investigation techniques can be adapted. By deriving two forms of closed equations for the variance of angular momentum fluctuations, various new results on HIT are established: (i) the clear physical difference of the present approach with respect to previous investigations based on the KármánHowarth equationor its various integrated forms such as Loitsyanskii's integraldue to the separation of fast (noise) and slow (friction) components of the fluctuating fields; (ii) under the appropriate closure assumptions and necessary conditions, a proof of the so-far conjectured "permanence of big structures"; (iii) novel relationships relating μ and the velocity correlation function f(s), obtained from the higher order terms of the expansion in D; (iv) the relationship between the exponent of the infrared spectrum κμ and the decay invariant in self-similar regimes, both for μ≤3 and 3<μ≤4 which display different behavior; (v) the stringent conditions required to reach Kolmogorov's decay exponent n=107 in ideal HIT; (vi) a closed relationship between the velocity correlation function f(s) at big scales and the "turbulent viscosity" coefficient Cμ. Beyond an improved theoretical understanding and the present results, this framework will lead to new predictions on turbulence decay and dissipation in numerous other types of flows with practical impact. © 2011 Elsevier Ltd. All rights reserved.

Pineau N.,CEA DAM Ile-de-France
Journal of Physical Chemistry C | Year: 2013

We present molecular dynamic simulations of the shock compression of graphite with the LCBOPII potential. The range of shock intensities covers the full range of available experimental data, including near-terapascal pressures. The results are in excellent agreement with the available DFT data and point to a graphite-diamond transition for shock pressures above 65 GPa, a value larger than the experimental data (20 to 50 GPa). The transition mechanism leads preferentially to hexagonal diamond through a diffusionless process but is submitted to irreversible regraphitization upon release: this result is in good agreement with the lack of highly ordered diamond observed in post-mortem experimental samples. Melting is found for shock pressures ranging from 200 to 300 GPa, close to the approximate LCBOPII diamond melting line. A good overall agreement is found between the calculated and experimental Hugoniot data up to 46% compression rate. © 2013 American Chemical Society.

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