Cette thèse s’inscrit dans le cadre de recherches menées sur la fusion par confinement inertiel (FCI). En particulier, l’étude proposée ici s’est concentrée sur les équations d’état tabulées de deux matériaux ablateurs synthétisés sur les capsules du Laser Mégajoule. Le but est alors de tester la modélisation théorique implémentée dans ces tables. Nous avons concentré notre étude sur un domaine restreint du diagramme de phase caractérisé par des pressions de quelques mégabars et de températures de quelques électronvolts qui peut être atteint sur des installations laser de tailles moyennes.Pour ce faire, nous nous sommes basés sur le modèle QEOS, car il est simple d’utilisation, paramétrable et donc facilement modifiable.Nous avons ensuite appliqué les méthodes de la dynamique moléculaire quantique pour générer la courbe froide et les courbes d’Hugoniot des deux matériaux étudiés. Ces calculs ont notamment mis en avant l’influence de la dissociation chimique sur la forme de ces courbes. Une comparaison avec le modèle QEOS a montré un écart important sur l’Hugoniot. Une modification de ce modèle, à travers le coefficient de Grüneisen, nous a ensuite permis de restituer les effets observés et d’étudier leurs impacts sur la chronométrie des chocs dans une capsule de FCI.Parallèlement à cette étude numérique, nous avons mesuré des états thermodynamiques le long de l’Hugoniot lors de trois campagnes sur les installations laser LULI2000 et GEKKO XII. L’utilisation de diagnostics VISAR et d’un diagnostic d’émission propre, nous a alors permis de sonder la matière sous choc. En outre, les données expérimentales ont confirmé les précédents résultats.En outre, cette étude a été réalisée sur deux matériaux ablateurs différents parmi lesquels on distingue le polymère non dopé CHO et le polymère dopé au silicium CHOSi. Elle montre un comportement universel de ces matériaux le long de l’Hugoniot. / This PhD thesis enters the field of inertial confinement fusion studies. In particular, it focuses on the equation of state tables of ablator materials synthetized on LMJ capsules. This work is indeed aims at improving the theoretical models introduced into the equation of state tables. We focused in the Mbar-eV pressure-temperature range because it can be access on kJ-scale laser facilities.In order to achieve this, we used the QEOS model, which is simple to use, configurable, and easily modifiable.First, quantum molecular dynamics (QMD) simulations were performed to generate cold compression curve as well as shock compression curves along the principal Hugoniot. Simulations were compared to QEOS model and showed that atomic bond dissociation has an effect on the compressibility. Results from these simulations are then used to parametrize the Grüneisen parameter in order to generate a tabulated equation of state that includes dissociation. It allowed us to show its influence on shock timing in a hydrodynamic simulation.Second, thermodynamic states along the Hugoniot were measured during three experimental campaigns upon the LULI2000 and GEKKO XII laser facilities. Experimental data confirm QMD simulations.This study was performed on two ablator materials which are an undoped polymer CHO, and a silicon-doped polymer CHOSi. Results showed universal shock compression properties.

<p>  </p> <p>This work explored the dynamic behavior and failure mechanisms of an additively manufactured high-entropy alloy (HEA) when subjected to high-strain rate shock impacts. A laser-induced projectile impact testing (LIPIT) setup was used to study the dynamic behavior of the Cantor alloy CoCrFeMnNi samples manufactured using a directed-energy deposition technique. HEA flyers were accelerated by a pulse laser to velocities up to 1 km/s prior to impact with lithium fluoride glass windows. A photon Doppler velocimetry (PDV) system recorded the velocity of the flyer during the acceleration and subsequent impact. From this velocity profile, the Hugoniot coefficient and sound speed of the HEA samples were determined.</p> <p><br></p> <p>Upon determination of key shock parameters, spallation occurring due to shock was analyzed. Using the same LIPIT and PDV systems as the earlier testing, aluminum flyers of various thicknesses were accelerated into HEA samples. The back-surface velocity profiles of the HEA samples showed a characteristic “pullback” caused by the interaction of the tensile stress waves indicative of spall occurrence in the material. The magnitude of this pullback and the material properties determined in the first experiments allow for the measurement of spall strength at various strain-rates. This data is compared to previous data looking at similar HEAs manufactured using traditional methods. A comparison of this data showed that the spall strength of the HEA samples was equivalent to that of similar alloys but at significantly higher strain rates. As an increased strain-rate tends to result in increased spall strengths, further examination was needed to determine the reasons for this decreased spall strength in the AM samples.</p> <p><br></p> <p>Post-shock specimen recovery allowed for the failure mechanisms behind the spallation to be observed. Scanning electron microscope (SEM) images of the cross-section of the samples showed ductile fracture and void growth outside of the predicted spall region. Further imaging using energy dispersive spectroscopy (EDS) showed the presence of potentially chromium-oxide deposits in regions outside of the predicted spall plane. It is hypothesized that these regions created nucleation points about which spallation occurred. Thus, to achieve spall strength in AM HEAs equivalent to strengths in traditionally-casted alloys, the AM sample must be refined to reduce the occurrence of these deposits and voids.  </p>

The numerics of hyperbolic conservation laws, e.g., the Euler equations of gas dynamics, shallow water equations and MHD equations, is non-trivial due to the convective terms being highly non-linear and equations being coupled. Many numerical methods have been developed to solve these equations, out of which central schemes and upwind schemes (such as Flux Vector Splitting methods, Riemann solvers, Kinetic Theory based Schemes, Relaxation Schemes etc.) are well known. The majority of the above mentioned schemes give rise to very dissipative solutions. In this thesis, we propose novel low dissipative numerical algorithms for some hyperbolic conservation laws representing fluid flows. Four different and independent numerical methods which give low diffusive solutions are developed and demonstrated. The first idea is to regulate the numerical diffusion in the existing dissipative schemes so that the smearing of solution is reduced. A diffusion regulator model is developed and used along with the existing methods, resulting in crisper shock solutions at almost no added computational cost. The diffusion regulator is a function of jump in Mach number across the interface of the finite volume and the average Mach number across the surface. The introduction of the diffusion regulator makes the diffusive parent schemes to be very accurate and the steady contact discontinuities are captured exactly. The model is demonstrated in improving the diffusive Local Lax-Friedrichs (LLF) (or Rusanov) method and a Kinetic Scheme. Even when employed together with accurate methods of Roe and Osher, improvement in solutions is demonstrated for multidimensional problems. The second method, a Central Upwind-Biased Scheme (CUBS), attempts to reorganize a central scheme such that information from irrelevant directions is largely reduced and the upwind biased information is retained. The diffusion co-efficient follows a new format unlike the use of maximum characteristic speed in the Local Lax-Friedrichs method and the scheme results in improved solutions of the flow features. The grid-aligned steady contacts are captured exactly with the reorganized format of diffusion co-efficient. The stability and positivity of the scheme are discussed and the procedure is demonstrated for its ability to capture all the features of solution for different flow problems. Another method proposed in this thesis, a Central Rankine-Hugoniot Solver, attempts to integrate more physics into the discretization procedure by enforcing a simplified Rankine-Hugoniot condition which describes the jumps and hence resolves steady discontinuities very accurately. Three different variants of the scheme, termed as the Method of Optimal Viscosity for Enhanced Resolution of Shocks (MOVERS), based on a single wave (MOVERS-1), multiple waves (MOVERS-n) and limiter based diffusion (MOVERS-L) are presented. The scheme is demonstrated for scalar Burgers equation and systems of conservation laws like Euler equations, ideal Magneto-hydrodynamics equations and shallow water equations. The new scheme uniformly improves the solutions of the Local Lax-Friedrichs scheme on which it is based and captures steady discontinuities either exactly or very accurately. A Grid-Free Central Solver, which does not require a grid structure but operates on any random distribution of points, is presented. The grid-free scheme is generic in discretization of spatial derivatives with the location of the mid-point between a point and its neighbor being used to define a relevant coefficient of numerical dissipation. A new central scheme based on convective-pressure splitting to solve for mid-point flux is proposed and many test problems are solved effectively. The Rankine-Hugoniot Solver, which is developed in this thesis, is also implemented in the grid-free framework and its utility is demonstrated. The numerical methods presented are solved in a finite volume framework, except for the Grid-Free Central Solver which is a generalized finite difference method. The algorithms developed are tested on problems represented by different systems of equations and for a wide variety of flow features. The methods presented in this thesis do not need any eigen-structure and complicated flux splittings, but can still capture discontinuities very accurately (sometimes exactly, when aligned with the grid lines), yielding low dissipative solutions. The thesis ends with a highlight on the importance of developing genuinely multidimensional schemes to obtain accurate solutions for multidimensional flows. The requirement of simpler discretization framework for such schemes is emphasized in order to match the efficacy of the popular dimensional splitting schemes.