The recent development of LAser Shock Adhesion Test (LASAT) as quantitative Non-Destructive Testing (NDT) process for evaluation of structural bonded assemblies brings new challenges. Applicative assemblies composed o f complex materials with poor transverse mechanical properties and highly resistant bonded joints require laser parameters optimization and a more accurate control on the whole process. The development of a numerical tool is then necessary to ensure laser parameters specification to evaluate the bond mechanical strength for a given assembly. In this document, the ability of ESTHER code for the description of laser-matter interaction on aluminum and ablation pressure prediction is exposed. The influence of the target initial reflectivity on ablation pressure is investigated. In this paper, validation o f the code in both direct (1 - 500 GW/cm(2)) and water-confined (0, 2 - 7 GW/cm(2)) irradiation regimes is achieved with comparison to suitable sets o f experimental data. Experiments were led on two laser facilities: the transportable laser shock generator (GCLT) at the CEA/DAM/DIF and the Hephaistos facility at the Processes and Engineering in Mechanics and Materials laboratory (PIMM lab). Numerical models developed in this work are compared to previous experimental data and to reference models. Ablation pressures defined by our predictive models can then be coupled to other codes which are able to describe 2D/3D shock propagation, in order to model the entire LASAT process on complex assemblies. Characterization of a 6061 Aluminum/ Epoxy/ 6061 Aluminum assembly is achieved using ESTHER, showing its ability to master the phenomena involved in the LASAT process. For the first time, results open the full numerical design of laser adhesion test with the same code.

We present a model to calculate the electrical and thermal electronic conductivities in plasmas using the Chester-Thellung-Kubo-Greenwood approach coupled with the Kramers approximation. The divergence in photon energy at low values is eliminated using a regularization scheme with an effective energy-dependent electron-ion collision-frequency. Doing so, we interpolate smoothly between the Drude-like and the Spitzer-like regularizations. The model still satisfies the well-known sum rule over the electrical conductivity. Such kind of approximation is also naturally extended to the average-atom model. A particular attention is paid to the Lorenz number. Its nondegenerate and degenerate limits are given and the transition towards the Drude-like limit is proved in the Kramers approximation.

The influence of a plasma density gradient on ions accelerated along the specular (back reflection) direction in the transparent Target Normal Sheath Acceleration regime is investigated. Enhanced acceleration of ions is experimentally observed in this regime using high-intensity and ultra-high contrast laser pulses and extremely thin foils of few nanometer thicknesses. The experimental trend for the maximum proton energy appeared quite different from the already published numerical results in this regime where an infinitely steep plasma gradient was assumed. We showed that for a realistic modelling, a finite density gradient has to be taken into account. By means of particle-in-cell (PIC) simulations, we studied for the first time the influence of the plasma density scale length on ion acceleration from these nanofoil targets. Through a qualitative agreement between our numerical particle-in-cell simulations and our experiments, the main conclusion with regard to the experimental requirements is that, in the transparent regime evidenced with nanofoils as compared to the opaque regime, the plasma expansion has to be taken into account and both the pulse contrast and the damage threshold of the material are essential parameters.

CO2 laser heating of silica glass is used in many scientific and industrial applications. Particularly, localized CO2 laser heating of silica glass has demonstrated its ability to mitigate surface damage on optics used for high power laser applications. To develop such applications, the control of temperature, heat affected area, and resulting mechanical stresses are critical. Therefore, it is necessary to understand the silica transformation, the material ejection, and the thermo-mechanical stresses induced by the laser heating and subsequent cooling. In this paper, we detail the development of comprehensive thermo-mechanical numerical simulations of these physical processes, based on finite-element method. The approach is developed for 2D or 3D cases to tackle the case of a moving beam at the surface of the sample, and we particularly discuss the choice of the different parameters based on bibliographic inputs. The thermal and mechanical numerical results have been compared to different dedicated experimental studies: infrared thermography measurements at the surface of the irradiated area, optical profilometry measurements of the laser-processed sites, and photo-elastic measurements. Very consistent results are obtained between numerical and experimental results for the description of the temperature gradients, the material ejection, and the residual stresses.

The one-dimensional magnetohydrodynamics of a plasma cylindrical liner is addressed in the case of a two components magnetic field. The azimuthal component is responsible for the implosion of the liner and the axial field is compressed inside the liner. A complete set of analytical profiles for the magnetic field components, the density, and the local velocity are proposed at the scale of the liner thickness. Numerical simulations are also presented to test the validity of the analytical formulas.

A general approach for optically controlled spatial structuring of overdense plasmas generated at the surface of initially plain solid targets is presented. We demonstrate it experimentally by creating sinusoidal plasma gratings of adjustable spatial periodicity and depth, and study the interaction of these transient structures with an ultraintense laser pulse to establish their usability at relativistically high intensities. We then show how these gratings can be used as a "spatial ruler" to determine the source size of the high-order harmonic beams produced at the surface of an overdense plasma. These results open new directions both for the metrology of laser-plasma interactions and the emerging field of ultrahigh intensity plasmonics.

This paper presents a laser-driven water-confined shock experiment into a commercial grade of porous graphite. An intensity of about 3 GW/cm2 led to a pressure above 2 GPa on the front surface of the 0.46 mm sample. The rear surface velocity, recorded by a Velocity Interferometer System (VISAR), reached 325 m/s. Two classical models for porous materials are discussed. The first one uses plates of dense graphite spaced out in order to obtain the correct average density. The second one models a continuous material and includes an experimental compaction curve of our porous graphite. They were implemented into hydrocodes and both gave quite correct maximum free surface velocities and shock break-out instants. Nevertheless, the continuous representation appeared to be more efficient to reproduce the experimental free surface velocity ramp. Discussions on the laser-matter interaction modeling are also provided. Finally, a protocol for the simulation of future laser experiments is proposed.

Optical diagnostics, such as VISAR (Velocity Interferometer System for Any Reflector) or SOP (Streaked Optical Pyrometry), have become essential in shock timing experiments. Their high precision allows for accurate measurements of shock velocities, chronometry, and brightness temperature. However, in some instances, these measurements can be compromised. In planar shock coalescence experiments recently performed at the LULI facility [Baton et al., Phys. Rev. Lett. 108, 195002 (2012)], VISAR signal loss was observed. In these experiments, a strong shock launched by a high-intensity spike catches up with a previously shock launched by an earlier, low-intensity beam. The disappearance of the VISAR signal is attributed to a preheating of the coronal plasma by x-rays generated by the high intensity spike. The signal does not disappear if the high-intensity spike starts after VISAR probe beam begins to reflect off of the first shock. The VISAR diagnostic, modeled using an assessment of the optical index in quartz, compares favorably to experimental results. This provides evidence that x-ray preheating can cause blanking of the VISAR signal in quartz.

Expressions for the electrical and thermal conductivities in dense plasmas are derived combining the Chester-Thellung-Kubo-Greenwood approach and the Kramers approximation. The infrared divergence is removed assuming a Drude-like behaviour. An analytical expression is obtained for the Lorenz number that interpolates between the cold solid-state and the hot plasma phases. An expression for the electrical resistivity is proposed using the Ziman-Evans formula, from which the thermal conductivity can be deduced using the analytical expression for the Lorenz number. The present method can be used to estimate electrical and thermal conductivities of mixtures. Comparisons with experiment and quantum molecular dynamics simulations are done.

The cratering process in brittle materials under hypervelocity impact (HVI) is of major relevance for debris shielding in spacecraft or high-power laser applications. Amongst other materials, carbon is of particular interest since it is widely used as elementary component in composite materials. In this paper we study a porous polycrystalline graphite under HVI and laser impact, both leading to strong debris ejection and cratering. First, we report new experimental data for normal impacts at 4100 and 4200 m s(-1) of a 500-mu m-diameter steel sphere on a thick sample of graphite. In a second step, dynamic loadings have been performed with a high-power nanosecond laser facility. High-resolution X-ray tomographies and observations with a scanning electron microscope have been performed in order to visualize the crater shape and the subsurface cracks. These two post-mortem diagnostics also provide evidence that, in the case of HVI tests, the fragmented steel sphere was buried into the graphite target below the crater surface. The current study aims to propose an interpretation of the results, including projectile trapping. In spite of their efficiency to capture overall trends in crater size and shape, semi-empirical scaling laws do not usually predict these phenomena. Hence, to offer better insight into the processes leading to this observation, the need for a computational damage model is argued. After discussing energy partitioning in order to identify the dominant physical mechanisms occurring in our experiments, we propose a simple damage model for porous and brittle materials. Compaction and fracture phenomena are included in the model. A failure criterion relying on Weibull theory is used to relate material tensile strength to deformation rate and damage. These constitutive relations have been implemented in an Eulerian hydrocode in order to compute numerical simulations and confront them with experiments. In this paper, we propose a simple fitting procedure of the unknown Weibull parameters based on HVI results. Good agreement is found with experimental observations of crater shapes and dimensions, as well as debris velocity. The projectile inclusion below the crater is also reproduced by the model and a mechanism is proposed for the trapping process. At least two sets of Weibull parameters can be used to match the results. Finally, we show that laser experiment simulations may discriminate in favor of one set of parameters. (C) 2013 Elsevier Ltd. All rights reserved.

La spectroscopie d'absorption X près des seuils (XANES) est un outil puissant pour sonder la structure atomique (ordre local) et électronique (électrons de valence) dans de nombreux types de milieux, allant des molécules jusqu'à la matière condensée. À partir d'une source X laser-plasma ultra-brève compacte, nous avons réalisé des expériences de XANES avec une résolution temporelle (∼ 3 picosecondes) permettant de sonder, à l'échelle atomique, l'évolution d'une feuille d'aluminium illuminée par un laser ultra-bref. Les spectres d'absorption X révèlent, à haut flux, une transition ultra-rapide du solide cristallin au liquide désordonné, suivie par une relocalisation progressive des électrons de valence vers des orbitales atomiques (transition métal/non-métal et solide/vapeur). À plus bas flux, la matière reste dans une phase partiellement ordonnée aux temps longs, ce qui permet de suivre la perte progressive de l'ordre local et d'en estimer la durée à 5,7 ± 3 ps.

The purpose of our research is to study porous polycrystalline graphite under various loading conditions. New experimental data are provided. Some of them concern impacts of a 500μm diameter steel sphere at velocities above 4000 m/s on thick carbon targets, leading to strong debris ejection and cratering. A high speed frame camera showed the debris velocity distribution to lie in the 10-200 m/s range. Post-mortem tomographies have also been performed. They reveal some subsurface cracks, but also provide some evidence that the fragmented sphere lies below the target surface. Dynamic loadings involving similar energy densities (above 2000J/cm2) can also be reached through the interaction of a nanosecond intense laser focalized on a carbon target. An experimental result obtained on a laser facility is presented. Numerical simulations have been performed in order to explain the observed results. An Eulerian hydrocode has been chosen because of the large deformation occurring under considered experiments. We have used a classical model to describe the porous behavior, including equation of state, elasticity, shear strength and densification. In this paper, we focus on the effect of the addition to this porous model of a failure criterion relying on the Weibull theory. The selection of the parameters is based on one set of data and the fit is demonstrated against another test. The overall agreement with the experimental data is good.

We investigate the efficiency of local CO₂laser processing of scratches on silica optics in order to enhance the nanosecond UV-laser damage resistance. The surface deformations induced by the process have been measured for different CO₂laser parameters and then the pulse duration and the beam diameter have been chosen accordingly to limit those deformations below 1 µm. From the study of the laser damage resistance as a function of different material modifications we identify a range of optimal radiation parameters allowing a complete elimination of scratches associated with a high threshold of laser damage. Calculation of the temperature of silica using a two-dimensional axi-symmetric code was compared with experiment, supporting an optimization of the laser parameter as a function of the maximal dimensions of scratches that could be removed by this process.

We have probed, with time-resolved x-ray absorption near-edge spectroscopy (XANES), a femtosecond-laser-heated aluminum foil with fluences up to 1  J/cm2. The spectra reveal a loss of the short-range order in a few picoseconds. This time scale is compared with the electron-ion equilibration time, calculated with a two-temperature model. Hydrodynamic simulations shed light on complex features that affect the foil dynamics, including progressive density change from solid to liquid (∼10  ps). In this density range, quantum molecular dynamics simulations indicate that XANES is a relevant probe of the ionic temperature.

Using an optimal control hydrodynamic modeling approach and irradiation adaptive time-design, we indicate excitation channels maximizing heat load in laser ablated aluminum at low energy costs. The primary relaxation paths leading to an emerging plasma are particularly affected. With impulsive pulses on ps pedestals, thermodynamic trajectories are preferentially guided in ionized domains where variations in ionization degree occur. This impinges on the gas-transformation mechanisms and triggers a positive bremsstrahlung absorption feedback. The highest temperatures are thus obtained in the expanding ionized matter after a final impulsive excitation, as the electronic energy relaxes recombinatively. The drive relies on transitions to weakly coupled front plasmas at the critical optical density, favoring energy confinement with low mechanical work. Alternatively, robust collisional heating occurs in denser regions above the critical point. This impacts the nature, the excitation degree and the energy content of the ablated matter. Adaptive modeling can therefore provide optimal strategies with information on physical variables not readily accessible and, as experimentally confirmed, databases for pulse shapes with interest in remote spectroscopy, laser-induced matter transfer, laser material processing and development of secondary sources.

We describe laser-driven spall experiments on aluminum, tantalum, steel and gold targets. The free-surface velocity is measured by using a VISAR diagnostic and is compared with numerical simulations at early spallation stage based on the Johnson fracture model. For each material, we first launch one-dimensional simulations with different values of the model parameters and determine the ones which reproduce correctly the experimental data. Then we use two-dimensional simulations to take into account spatial inhomogeneity of the loading pressure over the laser spot. We show non planar ejected spalls which are in agreement with experimental results. We finally study the fragmentation of thin targets and show differences in material behavior for steel in comparison to gold, tantalum and aluminum.