Gabriel Rigon

High-resolution x-ray radiography (HRXR) is a critical diagnostic tool for studying high-energy-density plasmas in large laser facilities such as the National Ignition Facility, the Laser Mega Joule, and Omega Extended Performance. However, geometric constraints in these facilities pose significant challenges for experimental design. This paper presents a comprehensive characterization of a picosecond laser-driven x-ray source tailored for HRXR applications. Through systematic experimental investigations, we analyzed the influence of various parameters on the performance of the x-ray source, focusing on spatial resolution and spectral distribution across a broad energy range. Our findings reveal that a single thin wire, comparable in diameter to the laser focal spot, positioned at 90° relative to the laser axis, serves as the optimal backlighter configuration. This configuration maximizes spatial resolution and spectral quality, addressing the geometric limitations inherent in large-scale laser facilities. The results provide a robust foundation for designing HRXR experiments, enabling more precise and reliable imaging of high-energy-density plasmas.

Le développement des lasers de puissance au cours du 20ème siècle a permis d’atteindre le régime que l'on appelle des hautes densités d’énergie (HDE), où la matière est portée dans des conditions extrêmes. Cela a permis l’essor d’une nouvelle discipline: l’astrophysique de laboratoire. Celle-ci vise à reproduire en laboratoire des conditions comparables à celles observées en astrophysique au sein de planètes, d’étoiles, ou lors de phénomènes cataclysmiques par exemple.Cette thèse porte sur l’étude expérimentale et numérique d’instabilités hydrodynamiques pouvant être trouvées dans de telles situations. Ces instabilités influencent l’évolution d’objets astrophysiques et en gênent l’observation. Ici nous nous intéresserons plus particulièrement aux instabilités de Rayleigh-Taylor (IRT) et de Richtmyer-Meshkov (IRM). La première apparaît dès lors qu’un fluide dense repose sur un fluide moins dense. La seconde peut être considérée comme un cas particulier de la première où la force responsable de l’instabilité est due à une onde de choc. Ces deux instabilités se retrouvent en astrophysique dans les restes de supernovæ, qui sont constitués de la matière éjectée lors de la mort (explosion) d’une étoile massive. Nous pouvons aussi les trouver en fusion par confinement inertiel, où elles sont en partie responsable des difficultés à atteindre l’ignition.Dans cette thèse nous présentons les résultats d’expériences réalisées sur les installations LULI2000 à Palaiseau (France), GEKKO XII à Osaka (Japon) et SACLA (Japon) visant l’étude de ces instabilités. Ces travaux expérimentaux nous ont permis d’observer directement et de reconstituer l’évolution de l’IRT de sa phase linéaire, aux temps courts, à sa phase turbulente, aux temps longs. Nous avons ainsi effectué une étude paramétrique de l’IRT où nous faisions varier les paramètres classiques l’influençant : longueur d’onde de perturbation, rapport des densités au travers de l’interface (nombre d’Atwood). Cela constitue donc une étude expérimentale complète de l’IRT et présente un caractère inédit quant à l’observation de la turbulence avec une résolution sans précédent dans le domaine de la HDE.Cette étude expérimentale est complétée d’une étude numérique réalisée à l’aide de FLASH, un code de magnéto-hydrodynamique développé par l’université de Chicago. Les simulations effectuées lors de cette thèse ont permis à la fois de préparer l’ensemble de l’étude expérimentale, et d’interpréter et de comprendre les résultats.En parallèle de ces études d’instabilités, un diagnostic de radiographie à rayons X à haute résolution spatiale a été développé. Il utilise un cristal de LiF comme détecteur. Une étude des caractéristiques de ce diagnostic a été effectuée (réponse spectrale, variation de résolution…) sur le synchrotron SOLEIL. Cela nous a alors permis d'utiliser le LiF comme détecteur principal lors de nos expériences sur SACLA où une résolution spatiale sub-micronique et temporelle de l’ordre de 10 fs ont été obtenues.

X-ray Free Electron Laser (XFEL) facilities offer unprecedented opportunities to advance instrumentation for studying matter under extreme conditions. In this study, we harnessed the enhanced x-ray capabilities of XFELs to demonstrate dark field imaging in laser-driven experiments at XFEL facilities. Utilizing a Talbot x-ray interferometer, we simultaneously captured transmission, dark-field, and differential phase contrast radiographs of laser-driven metallic foils. Our work showcases the feasibility of single-shot grating-based Talbot x-ray dark-field imaging in pump-probe experiments at XFEL facilities, opening doors to a wide range of hard x-ray imaging applications in material science and high-energy density physics.

Optically-transparent LiF film radiation imaging detectors of three different thicknesses, grown by thermal evaporation on Si(100) substrates and irradiated with monochromatic 7 keV X-rays at several doses in the range between 13 and 4.5×10³ Gy, were investigated by fluorescence microscopy to evaluate their visible photoluminescence response of X-ray induced colour centres and their spatial resolution. By edge-enhancement imaging experiments, performed by irradiating LiF film detectors with an Au mesh in front of them, a spatial resolution of ∼0.3 μm was estimated. This high spatial resolution, combined with the large field of view and the wide dynamic range offered by LiF detectors, encourages their use in X-ray imaging and for high-energy density experiments in synchrotron facilities.

This Letter reports the first complete observation of magnetized collisionless shock precursors formed through the compression of Biermann-battery magnetic fields in laser produced plasmas. At OMEGA, lasers produce a supersonic CH plasma flow which is magnetized with Biermann-battery magnetic fields. The plasma flow collides with an unmagnetized hydrogen gas jet plasma to create a magnetized shock precursor. The situation where the flowing plasma carries the magnetic field is similar to the Venusian bow shock. Imaging 2ω Thomson scattering confirms that the interaction is collisionless and shows density and temperature jumps. Proton radiographs have regions of strong deflections and FLASH magnetohydrodynamic (MHD) simulations show the presence of Biermann fields in the Thomson scattering region. Electrons are accelerated to energies of up to 100 keV in a power-law spectrum. OSIRIS particle-in-cell (PIC) simulations, initialized with measured parameters, show the formation of a magnetized shock precursor and corroborate the experimental observables.

The Particle Time of Flight (PTOF) detector is a chemical vapor deposition diamond-based detector used to measure bang times in low-yield (≲ 1015 neutrons) experiments at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL). Historically, the impulse response for PTOF diamond detectors has been obtained from x-ray timing shots on the NIF and shots on the MegaRay pulsed electron accelerator at LLNL. The impulse response may alternatively be obtained using single particle interactions with the detector, at substantially less cost and higher frequency compared to NIF timing shots, which typically occur months apart. Here, the response of a PTOF detector setup is characterized by statistically averaging a large number of single particle waveforms. A high fidelity instrument response function can be constructed in this way. This is confirmed by comparison of the single particle counting-constructed response to the impulse response function measured for the same detector at LLNL’s MegaRay facility.

The directional energy spectrum of neutrons generated from the in-flight fusion reaction of 1-MeV tritons contains information about the hot-spot symmetry. The National Ignition Facility (NIF) fields Symmetry Capsule (Symcap) implosions, which have historically measured the symmetry of the radiation, drive by measuring the hot-spot shape via x-ray self-emission. Symcaps are used to tune the hot-spot symmetry for ignition experiments at the NIF. This work shows the relationship between directional secondary DT-n spectra and x-ray imaging data for a large database of Symcap implosions. A correlation is observed between the relative widths of the DT-n spectra measured with nTOFs and the shape measured with x-ray imaging. A Monte Carlo model, which computes the directional secondary DT-n spectrum, is used to interpret the results. A comparison of the x-ray and secondary DT-n data with the Monte Carlo model indicates that 56% of the variance between the two datasets is explained by a P2 asymmetry. More advanced simulations using HYDRA suggest that the unaccounted variance is due to P1 and P4 asymmetries present in the hot spot. The comparison of secondary DT-n data and x-ray imaging data to the modeling shows the DT-n data contain important information that supplements current P2 measurements and contain new information about the P1 asymmetry.

Accurate modeling of astrophysical jets is critical for understanding accretion systems and their impact on the interstellar medium. While astronomical observations can validate models, they have limitations. Controlled laboratory experiments offer a complementary approach for qualitative and quantitative demonstration. Laser experiments offer a complementary approach. This article introduces a new platform on the OMEGA laser facility for high-velocity (1500 kms−1), high-aspect-ratio (∼36) jet creation with strong cylindrical symmetry. This platform's capabilities bridge observational gaps, enabling controlled initial conditions and direct measurements of plasma characteristics, crucial for refining astrophysical jet dynamics and improving the models accuracy.

The motion of line defects (dislocations) has been studied for more than 60 years, but the maximum speed at which they can move is unresolved. Recent models and atomistic simulations predict the existence of a limiting velocity of dislocation motion between the transonic and subsonic ranges at which the self-energy of dislocation diverges, though they do not deny the possibility of the transonic dislocations. We used femtosecond x-ray radiography to track ultrafast dislocation motion in shock-compressed single-crystal diamond. By visualizing stacking faults extending faster than the slowest sound wave speed of diamond, we show the evidence of partial dislocations at their leading edge moving transonically. Understanding the upper limit of dislocation mobility in crystals is essential to accurately model, predict, and control the mechanical properties of materials under extreme conditions.

Understanding the behavior of matter at extreme pressures of the order of a megabar (Mbar) is essential to gain insight into various physical phenomena at macroscales—the formation of planets, young stars, and the cores of super-Earths, and at microscales—damage to ceramic materials and high-pressure plastic transformation and phase transitions in solids. Under dynamic compression of solids up to Mbar pressures, even a solid with high strength exhibits plastic properties, causing the induced shock wave to split in two: an elastic precursor and a plastic shock wave. This phenomenon is described by theoretical models based on indirect measurements of material response. The advent of x-ray free-electron lasers (XFELs) has made it possible to use their ultrashort pulses for direct observations of the propagation of shock waves in solid materials by the method of phase-contrast radiography. However, there is still a lack of comprehensive data for verification of theoretical models of different solids. Here, we present the results of an experiment in which the evolution of the coupled elastic–plastic wave structure in diamond was directly observed and studied with submicrometer spatial resolution, using the unique capabilities of the x-ray free-electron laser (XFEL). The direct measurements allowed, for the first time, the fitting and validation of the 2D failure model for diamond in the range of several Mbar. Our experimental approach opens new possibilities for the direct verification and construction of equations of state of matter in the ultra-high-stress range, which are relevant to solving a variety of problems in high-energy-density physics.