Dynamic cratering of graphite : experimental results and simulations

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-μm-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

Optically Controlled Solid-Density Transient Plasma Gratings

International audienceA 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

Experimental simulation of bonding processes by laser spallation

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Ion acceleration in the transparent regime and the critical influence of the plasma density scale length

International audienceThe 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

Unraveling the Solid-Liquid-Vapor Phase Transition Dynamics at the Atomic Level with Ultrafast X-Ray Absorption Near-Edge Spectroscopy

X-ray absorption near-edge spectroscopy (XANES) is a powerful probe of electronic and atomic structures in various media, ranging from molecules to condensed matter. We show how ultrafast time resolution opens new possibilities to investigate highly nonequilibrium states of matter including phase transitions. Based on a tabletop laser-plasma ultrafast x-ray source, we have performed a time-resolved (∼3  ps) XANES experiment that reveals the evolution of an aluminum foil at the atomic level, when undergoing ultrafast laser heating and ablation. X-ray absorption spectra highlight an ultrafast transition from the crystalline solid to the disordered liquid followed by a progressive transition of the delocalized valence electronic structure (metal) down to localized atomic orbitals (nonmetal—vapor), as the average distance between atoms increases

REVIEW OF X-RAY LASER RESEARCH AT CEL-V

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Dynamique ultra-rapide de la transition de phase solide-liquide-vapeur par spectroscopie XANES résolue en temps

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

Picosecond Short-Range Disordering in Isochorically Heated Aluminum at Solid Density

International audienceUsing ultrafast x-ray probing, we experimentally observed a progressive loss of ordering within solid-density aluminum as the temperature raises from 300 K to >104  K. The Al sample was isochorically heated by a short (∼ps), laser-accelerated proton beam and probed by a short broadband x-ray source around the Al K edge. The loss of short-range ordering is detected through the progressive smoothing of the time-resolved x-ray absorption near-edge spectroscopy (XANES) structure. The results are compared with two different theoretical models of warm dense matter and allow us to put an upper bound on the onset of ion lattice disorder within the heated solid-density medium of ∼10  ps