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Mesure de l'équation d'état de matériaux poreux à l'aide d'ondes de choc générées par laser.

Abstract : A few words about the general context of the study The physics of high pressures, and particularly the extent of equations of state, has grown meet the needs of other disciplines where quantitative knowledge of the behavior of matter highly compressed is fundamental. Thus, geophysics, understanding the internal structure of the earth.1 depends on the equation of state of iron and its physical properties 2 . A similar situation occurs in Planetology for other elements (structure of Jupiter and the equation of state of the hydrogène3 Structure of Uranus and equation of state of the eau4 ...). More generally, the equation of state of materials forming an object Astrophysics necessarily involved in the theoretical study of its internal structure to close the system equations hydrostatique5 . The first study methods were static compression methods, such as cell- diamond anvil. Although these methods are currently undergoing improvements that extend the scope, limitations (low pressure maximum character isothermal compression) have led to the development of new ways. The shock waves generated by laser have emerged as a tool for this research. Since the cessation of nuclear tests (whose contribution to the extent state equation is summarized in articles Ragan III 6 American side and Trunin7 the Russian side), this method is one that achieves the highest pressure (750 Mbar in 1993 in Livermore by colliding gold leaf 8 ). If this possibility existed for the development of large power lasers for research on inertial confinement fusion, its use for accurate measurements is recent 9 . This is indeed become possible thanks to the previous work on the prerequisites of quality of impact: the introduction of optical smoothing to ensure uniformity of the shock, the study of its stationarity and preheating. This is an area not yet fully explored, and the materials to be studied many. Among these, porous materials are of a particular character and this thesis attempts to extent of their equation of state. The study of porous materials (foams) high pressure is applied to their many applications in the field of inertial confinement fusion, astrophysics experiments laboratory, as well as the purpose of better understanding of these exotic materials in extreme conditions. In the context of inertial confinement fusion, plastic foams are used in some schemes targets (in direct attack) or cavity filling (in collateral attack). They have indeed interesting properties in terms of hydrodynamics, and for smoothing thermal inhomogeneities lighting 10.11 . The goal of inertial confinement fusion is to trigger the ignition of reactions thermonuclear fusion by compressing a target consisting of a mixture of deuterium and tritium. In said direct attack scenario, we use the removal of the outer layers of the spherical target by lighting Laser jet to generate shock waves converging at the center in order to create the conditions temperature and density specific to the ignition of the first fusion reactions. They must then file enough energy, and the mixture around the hot spot must be sufficiently dense so that the combustion spreads to the entire target. The hydrodynamic instability amplifies the smallest irregularities the shock wave, it is essential to the success of this method to have the energy deposit as homogeneous possible. In addition to optical methods, use of foam proved an attractive option. Indeed, wear a target of low density foam can quickly create a plasma supercritical (That is to say, in which the laser light can not propagate) large size (comparable to the thickness foam). Through this area, the energy is transmitted by thermal conduction to the front of ablation where the shock is generated. This is essentially lateral thermal conduction which will broadcast the energy and allow smoothing actual profile at the front of ablation, that is to say its "footprint" on the hydrodynamics. For a reliable comparison with the patterns of target competitors, such as those using a ablateur beryllium, currently favored Twelve It is necessary to know their properties, particularly their equation of state. Indeed, the final choice and optimization of the target are made primarily on the basis of simulations hydrodynamic, whose validity depends on the particular equations of state used. In the context of laboratory astrophysics experiments, foams offer a means of simulate low-gravity environments. It can be used for example as analogous stellar atmospheres and interstellar medium. These include the experiences of Drake et al. 13.14 and Benuzzi et al. 15 , Simulating the explosion of a supernova. The major advantage of currently feasible experiments with intense laser seems to lie in the validation of hydrodynamic codes used by the astrophysicists 16 . A detailed comparison of numerical results and experimental inquiry again a good knowledge of the equation of state of similar materials used as media astrophysical. Because of their low density foams also have an interest in the context of experiments measurement equation of state of other materials by laser shock 17 : Their use as ablateur achieves high shock pressures due to the phenomenon of impedance mismatch, while minimizing the radiative preheating due to their low atomic number. Finally, a good knowledge of the state equation involves the ability to explore a wide range of space phases. However, all accessible states of a material with a shock wave are on the same curve (Hugoniot curve). To overcome this limitation, several approaches are used. One can for example resorting to multiple shocks 18 , To be closer to the isentropic compression curve of the material, or measure its relaxation from a state under choc19 . Another way is from a different state of material, either by compressing it in advance by a static method, either, from an initial density reduced by using a material poreux20 . The study of porous materials is therefore in the general the exploration of exotic states of matter. State of knowledge before the thesis When we undertook this study the equations of state available for foam plastics based Experimental results on very few, relatively low pressure (kbar) obtained using gas guns. It is essentially the work of Holmes 21 , Providing a single measuring point on the polar Impact of a styrofoam mg/cm3 139 At 40 kbar. The recent advent of accurate methods of measurement equation of state using shock waves generated by laser 9 thus enabled to consider their application to the case of plastic foam, in an area unexplored pressures and temperatures.
Mots-clés : Optique Laser Compression
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Submitted on : Friday, July 16, 2010 - 2:00:57 PM
Last modification on : Sunday, June 26, 2022 - 5:30:33 AM
Long-term archiving on: : Friday, October 22, 2010 - 11:56:15 AM


  • HAL Id : pastel-00503036, version 1


Franck Philippe. Mesure de l'équation d'état de matériaux poreux à l'aide d'ondes de choc générées par laser.. Optique [physics.optics]. Ecole Polytechnique X, 2001. Français. ⟨pastel-00503036⟩



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