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We report on the optimization of a BremsStrahlung Cannon (BSC) design for the investigation of laser-driven fast electron populations in a shock ignition relevant experimental campaign at the Laser Megajoule-PETawatt Aquitaine Laser facility. In this regime with laser intensities of 10(15) W/cm(2)-10(16) W/cm(2), fast electrons with energies

The “microdot spectroscopy” experiment [Barrios et al., “Electron temperature measurements inside the ablating plasma of gas-filled Hohlraums at the National Ignition Facility,” Phys. Plasmas 23, 056307 (2016); Barrios et al., “Developing an experimental basis for understanding transport in NIF Hohlraum plasmas,” Phys. Rev. Lett. 121, 095002 (2018).] allows for a simultaneous measurement of the electron temperature (Te) and position of a patch of Mn and Co inside a Hohlraum, as described by Barrios et al. [“Electron temperature measurements inside the ablating plasma of gas-filled Hohlraums at the National Ignition Facility,” Phys. Plasmas 23, 056307 (2016).] HYDRA simulations systematically predicted a dot location further away from its starting location than observed in the experiment. In the article, integrated radiation hydrodynamics simulations with TROLL have led to the same trend as HYDRA. A new ad hoc treatment of laser absorption, through what we have called absorption multipliers, has been implemented in TROLL in order to mimic the effect of absorption mechanisms other than inverse-bremsstrahlung. It led to the instrumental conclusion that whatever physical phenomenon was responsible for the position anomaly must have occurred in the early stage. More precise simulations of the dot region, from early to late time, show that the position discrepancy can be explained by a Rayleigh–Taylor mixing of the dot into the ablator as it expands in the Hohlraum. This mixing tends to shift the simulated dot closer to the location measured in the experiment. However, the mixing also changes the interpretation of the electron temperature from the spectral line ratios.

International audience; New data on some materials at 80ps pulse duration and 1057 nm wavelength give us the option of proportionally combining them to obtain arbitrary values between 35 (aluminum) and 800 N/MW (POM, polyoxymethylene) for momentum coupling coefficient Cm. Laser ablation physics lets us transfer to LEO from Earth, or to interplanetary space using repetitively pulsed lasers and Cm values appropriate for each mission. We discuss practical results for lifting small payloads from Earth to LEO, and space missions such as a cis-Mars orbit with associated laser system parameters.

International audience; At the École Polytechnique « LULI » facility, we have measured the impulse coupling coefficient Cm (target momentum per joule of incident laser light) with several target materials in vacuum, at 1057 nm and 400 fs and 80 ps pulse duration. A total of 64 laser shots were completed in a two-week experimental campaign, divided between the two pulse durations and among the materials. Our main purpose was to resolve wide discrepancies among reported values for Cm in the 100 ps region, where many applications exist. A secondary purpose was to compare Cm at 400 fs and 80 ps pulse duration. The 80 ps pulse was obtained by partial compression. Materials were Al, Ta, W, Au, and POM (polyoxymethylene, trade name Delrin). One application of these results is to pulsed laser ablation propulsion in space, including space debris re-entry, where narrow ranges in Cm and specific impulse Isp spell the difference between dramatic and uneconomical performance. We had difficulty measuring mass loss from single shots. Imparted momentum in single laser shots was determined using pendulum deflection and photonic Doppler velocimetry. Cm was smaller at the 400 fs pulse duration than at 80 ps. To our surprise, Cm for Al at 80 ps was at most 30 N/MW with 30 kJ/m2 incident fluence. On the other extreme, polyoxymethylene (POM, trade name Delrin) demonstrated 770 N/MW under these conditions. Together, these results offer the possibility of designing a Cm value suited to an application, by mixing the materials appropriately.

We present some experimental results which demonstrate the presence of electric inhibition in the propagation of relativistic electrons generated by intense laser pulses, depending on target conductivity. The use of transparent targets and shadowgraphic techniques has made it possible to evidence electron jets moving at the speed of light, an indication of the presence of self-generated strong magnetic fields.

We report on shadowgraphic measurements showing the first space- and time-resolved snapshots of ultraintense laser pulse-generated fast electrons propagating through a solid target. A remarkable result is the formation of highly collimated jets ( $l20\ensuremath{-}\ensuremath{\mu}\mathrm{m}$) traveling at the velocity of light and extending up to 1 mm. This feature clearly indicates a magnetically assisted regime of electron transport, of critical interest for the fast ignitor scheme. Along with these jets, we detect a slower ( $\ensuremath{\approx}c/2$) and broader (up to 1 mm) ionization front consistent with collisional hot electron energy transport.

Obtaining keV ion temperatures at solid density, i.e. “warm dense matter”, in the laboratory would be of great interest to measure opacity and equations of state of matter under extremes conditions. Here we report a new means to effectively confine the energetic electrons and localize the energy deposition to a small, more uniformly heated, volume at the tip of nanofabricated micro-cone targets. This is achieved with very high contrast laser irradiation, which interacts with the cone wall to generate strong (~10 MG) localized resistive magnetic fields within the target bulk. Temperatures of up to ~200 eV are observed, with an input laser energy of 10 J. This new means has been investigated both experimentally and with Particle-In-Cell simulations.

Short pulse laser plasma interaction experiments using diffraction limited beams provide an excellent platform to investigate the fundamental physics of stimulated Raman scattering. Detailed understanding of these laser plasma instabilities impacts the current inertial confinement fusion ignition designs and could potentially impact fast ignition when higher energy lasers are used with longer pulse durations (>1 kJ and >1 ps). Using short laser pulses, experiments can be modeled over the entire interaction time of the laser using particle-in-cell codes to validate our understanding quantitatively. Experiments have been conducted at the Trident laser facility and the Laboratoire pour l'Utilisation des Lasers Intenses (LULI) to investigate stimulated Raman scattering near the threshold of the instability using 527 nm and 1059 nm laser light, respectively, with 1.5–3.0 ps pulses. In both experiments, the interaction beam was focused into pre-ionized helium gas-jet plasma. Measurements of the reflectivity as a function of intensity and kλD were completed at the Trident laser facility, where k is the electron plasma wave number and λD is the plasma Debye length. At LULI, a 300 fs Thomson scattering probe is used to directly measure the density fluctuations of the driven electron plasma and ion acoustic waves. Work is currently underway comparing the results of the experiments with simulations using the VPIC particle-in-cell code. Details of the experimental results are presented in this manuscript.

We present experimental results on the interaction of short-pulse ultra-high-intensity laser beams with small size (“mass-limited”) targets. Several diagnostics (X-ray spectroscopy, Kα and optical imaging of target rear side) have been simultaneously used in order to characterize the laser-generated fast electron transport and energy deposition into the target material. Our results show that fast electrons are effectively confined inside the target by the induced space charge. This electrostatic confinement opens new opportunities to create “Warm Dense Matter” states characterized by solid-state density and temperatures of the order of a few tens of eV.

One of the main issues of the fast ignitor scheme is the role of fast electron transport in the solid fuel heating. Recent experiments used a new target scheme based on the use of cone to guide the PW laser and enhance the electron production. In this context it is fundamental to understand the physics underlying this new target scheme. We report here recent and preliminary results of ultra-intense laser pulse interaction with three layer targets in presence of the cone or without. Experiments have been performed at LULI with the 100TW laser facility, at intensities up to 3 10 19 W/cm 2 . Several diagnostics have been implemented (2D Kx imaging, Kα spectroscopy and rear side imaging, protons emission) to quantify the cone effect.

We present the results of some recent experiments performed at the LULI laboratory using the 100 TW laser facility concerning the study of the propagation of fast electrons in gas and solid targets. Novel diagnostics have been implemented including chirped shadowgraphy and proton radiography. Proton radiography images did show the presence of very strong fields in the gas probably produced by charge separation. In turn these imply a slowing down of the fast electron cloud as it penetrates in the gas and a strong inhibition of propagation. Indeed chirped shadowgraphy images show a strong reduction in time of the velocity of the electron cloud from the initial value, which is of the order of a fraction of c. We also performed some preliminary experiments with cone targets in order to verify the guiding effect and fast electron propagation in presence of the cone. Finally we compared results obtained by changing the target size.Here we only give a first presentation and preliminary analysis of data, which will be addressed in detail in a following paper.

Two experiments have been performed to investigate heating by high-intensity laser-generated electrons, in the context of studies of the fast ignitor approach to inertial confinement fusion (ICF). A new spectrometer and layered targets have been used to detect Kα emission from aluminum heated by a fast electron beam. Results show that a temperature of about 40 eV is reached in solid density aluminum up to a depth of about 100 μm.

Relativistic electrons are produced, with energies up to 20MeV, by the interaction of a high-intensity subpicosecond laser pulse (1 {mu}m , 300 fs , 10{sup 19} W/cm{sup 2} ) with an underdense plasma. Two suprathermal electron populations appear with temperatures of 1 and 3MeV. In the same conditions, the laser beam transmission is increased up to 20{percent}{endash}30{percent}. We observe both features along with the evidence of laser pulse channeling. A fluid model predicts a strong self-focusing of the pulse. Acceleration in the enhanced laser field seems the most likely mechanism leading to the second electron population. {copyright} {ital 1997} {ital The American Physical Society}

A recent experiment was performed at the Rutherford Appleton Laboratory (UK) to study fast electron propagation in cylindrically compressed targets, a subject of interest for fast ignition. In this experiment, protons accelerated by a picosecond laser pulse have been used to radiograph a 220 µm diameter cylinder (10 µm wall filled with 0.1 g/cc foam), imploded with _ 200 J of green laser light in 4 symmetrically incident beams of wavelength and pulse length 1 ns. Point projection proton backlighting was used to measure the compression degree as well as the stagnation time. Results were also compared to those from a hard X-ray radiography diagnostics. Finally, Monte Carlo simulations of proton propagation in the cold and in the compressed targets allowed a detailed comparison with 2D numerical hydro simulations.

We present the results of experiments performed at the LULI laboratory, using the 100 TW laser facility, on the study of the propagation of fast electrons in gas targets. The implemented diagnostics included chirped shadowgraphy and proton imaging. Proton images showed the presence of very large fields in the gas (produced by charge separation). In turn, these imply a strong inhibition of propagation, and a slowing down of the fast electron cloud as it penetrates in the gas. Indeed chirped shadowgraphy images show a reduction in time of the velocity of the electron cloud from the initial value, of the order of a fraction of c, over a time scale of a few picoseconds.

We report on highly time- and space-resolved measurements of the evolution of electron plasma waves driven by stimulated Raman scattering of a picosecond, single laser speckle propagating through a preformed underdense plasma. Two-dimensional Thomson scatter spectra indicate that the dominant waves have significant transverse components. These results are supported by particle-in-cell simulations which pinpoint the dominant role of the wave front bowing and of secondary nonlinear electrostatic instabilities in the evolution of the plasma waves.

This second part of our review is devoted, in particular, to the description of the heating of a background material induced by fast electron propagation, the propagation geometry of fast electron beams (including the issues of possible collimation and filamentation instability), the formation of electron bunches, the refluxing of electrons in the target and finally, the direct detection of electric fields connected to fast electron propagation. General conclusions on the topic of fast electron propagation are presented.

This chapter gives a review of experimental results, which have been obtained in the last 10 years concerning the generation of very large relativistic currents of “fast” electrons in laser-matter interactions at ultra high intensities, and their propagation in matter. This subject is a key issue in the research concerning the new “fast ignition” approach to inertial confinement fusion. In particular, this chapter deals with the problems of fast electron generation (conversion efficiency of laser energy into energy of the fast electron beam, scaling of fast electron “temperature” vs. laser intensity; shape of the electron distribution function) and with the discussion of collisional effects vs. collective effects (due to electric and magnetic fields) in propagation, including the dependence of collective effects on material characteristics.

We present experimental and numerical results on the propagation and energy deposition of laser-generated fast electrons into conical targets. The first part reports on experimental measurements performed in various configurations in order to assess the predicted benefit of conical targets over standard planar ones. For the conditions investigated here, the fast electron-induced heating is found to be much weaker in cone-guided targets irradiated at a laser wavelength of 1.057μm, whereas frequency doubling of the laser pulse permits us to bridge the disparity between conical and planar targets. This result underscores the prejudicial role of the prepulse-generated plasma, whose confinement is enhanced in conical geometry. The second part is mostly devoted to the particle-in-cell modeling of the laser-cone interaction. In qualitative agreement with the experimental data, the calculations show that the presence of a large preplasma leads to a significant decrease in the fast electron density and energy flux...

Significantly collimated fast electron beam with a divergence angle 10° (FWHM) is generated through the interaction of ultra-intense laser light with a uniform critical density plasma in experiments and 2D PIC simulations. In the experiment, the uniform critical density plasma is created by ionizing an ultra-low density foam target. The spacial distribution of the fast electron is observed by Imaging Plate. 2D PIC simulation and post process analysis reveal magnetic collimation of energetic electrons along the plasma channel. Publisher PDF

With the addition of recent PW shots, the propagation of short-pulse laser generated electron beams have been studied using laser pulse energies from 30 J to 300 J, generating currents up to ∼15 MA in solid Al:Cu targets. This is ∼5% of the current that will be required in an ignition pulse. To this level, the current appears to simply scale with laser power, the propagation spread not change at all. The resistance of the aluminum does not seem to play a role in the propagation characteristics, though it might in setting the current starting parameters. We do find that at the highest currents parts of these targets reach temperatures high enough to modify the Cu-K2 emission spectrum rendering our Bragg imaging mirrors ineffective; spectrometers will be needed to collect data at these higher temperatures. © EDP Sciences.

Electron transport within solid targets, irradiated by a high-intensity short-pulse laser, has been measured by imaging ${K}_{\ensuremath{\alpha}}$ radiation from high-$Z$ layers (Cu, Ti) buried in low-$Z$ (CH, Al) foils. Although the laser spot is $\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ [full width at half maximum (FWHM)], the electron beam spreads to $\ensuremath{\geqslant}70\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ FWHM within $l20\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ of penetration into an Al target then, at depths $g100\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$, diverges with a $40\ifmmode^\circ\else\textdegree\fi{}$ spreading angle. Monte Carlo and analytic models are compared to our data. We find that a Monte Carlo model with a heuristic model for the electron injection gives a reasonable fit with our data.

International audience; We study the propagation of fast electrons in a gas at different densities. A large relativistic electron current is produced by focusing a short-pulse ultrahigh-intensity laser on a metallic target. It then propagates in a gas jet placed behind the foil. Shadowgraphy in the gas shows an electron cloud moving at sub-relativistic average velocities. The experiment shows (i) the essential role of the density of background material for allowing propagation of fast electrons, (ii) the importance of the ionization phase which produces free electrons available for the return current, and (iii) the effect of electrostatic fields on fast-electron propagation.

We report on rear-side optical self-emission results from ultraintense laser pulse interactions with solid targets. A prompt emission associated with a narrow electron jet has been observed up to aluminum target thicknesses of 400 microm with a typical spreading half-angle of 17 degrees. The quantitative results on the emitted energy are consistent with models where the optical emission is due to transition radiation of electrons reaching the back surface of the target or due to a synchrotron-type radiation of electrons pulled back to the target. These models associated with transport simulation results give an indication of a temperature of a few hundred keV for the fast-electron population.

The propagation of relativistic electrons in foam and solid density targets has been studied by means of K-\ensuremath{\alpha} spectroscopy. Experimental results point out the role of self-generated electric fields in propagation and the role of heating of matter induced by the passage of fast electrons. A simple analytical formulation has been given and Spitzer conductivity has been shown to be fairly compatible with experimental results.

Fast electron generation and propagation were studied in the interaction of a green laser with solids. The experiment, carried out with the LULI TW laser (350 fs, 15 J), used ${K}_{\ensuremath{\alpha}}$ emission from buried fluorescent layers to measure electron transport. Results for conductors (Al) and insulators (plastic) are compared with simulations: in plastic, inhibition in the propagation of fast electrons is observed, due to electric fields which become the dominant factor in electron transport.

We report on a detailed study of channel formation in the interaction of a nanosecond laser pulse with a He gas jet. A complete set of diagnostics is used in order to characterize the plasma precisely. The evolution of the plasma radius and of the electron density and temperature are measured by Thomson scattering, Schlieren imaging, and Mach-Zehnder interferometry. In gas jets, one observes the formation of a channel with a deep density depletion on axis. Because of ionization-induced defocusing which increases the size of the focal spot and decreases the maximum laser intensity, no channel is observed in the case of a gas-filled chamber. The results obtained in various gas-jet and laser conditions show that the channel radius, as well as the density along the propagation axis, can be adjusted by changing the laser energy and gas-jet pressure. This is a crucial issue when one wants to adapt the channel parameters in order to guide a subsequent high-intensity laser pulse. The experimental results and their comparison with one-dimensional (1D) and two-dimensional hydrodynamic simulations show that the main mechanism for channel formation is the hydrodynamic evolution behind a supersonic electron heat wave propagating radially in the plasma. It is also shown from 2D simulations that a fraction of the long pulse can be self-guided in the channel it creates. The preliminary results and analyses on this subject have been published before [V. Malka et al., Phys. Rev. Lett. 79, 2979 (1997)].

An experiment has been performed with the LULI Multi-TeraWatt Laser. The acceleration of electrons injected in a plasma wave generated by the laser wakefield mechanism has been observed with a maximum energy gain of 1.5 MeV. It has been shown that the electrons deflected during the interaction, could scatter on the walls of the experimental chamber, and fake a high-energy signal. A special effort has been given in the electron detection to separate the accelerated electrons signal from the background noise.The experimental results agree with theoretical predictions and numerical simulations when 3D effects on the electron beam are taken into account.

The second harmonic of the laser light ($2{\ensuremath{\omega}}_{0}$) is observed on the rear side of thick solid targets irradiated by a laser beam at relativistic intensities. This emission is explained by the acceleration by the laser pulse in front of the target of short bunches of electrons separated by the period (or half the period) of the laser light. When reaching the rear side of the target, these electron bunches emit coherent transition radiation at $2{\ensuremath{\omega}}_{0}$. The observations indicate that, in our conditions, the minimum fraction of the laser energy transferred to these electron bunches is of the order of 1%.

This paper reviews the physics of extremely high current propagation in dense materials. We consider explicitly the problem of the generation of high-current, high-particle energy propagation arisi...

Inertial Confinement Fusion (ICF) consists in compressing a millimeter-sized capsule of a mixture of Deuterium-Tritium using power lasers to reach the conditions of self-sustained thermonuclear fusion. Two methods of irradiation by power lasers are possible. For a direct drive, lasers are absorbed by the outer surface of the capsule. For an indirect drive, the capsule, located at the center of a gold cavity (hohlraum), is irradiated by an intense flux of X-rays resulting from laser absorption by the walls of the cavity. ICF experiments carried out for more than ten years in the USA at the National Ignition Facility have consistently shown that laser absorption by matter was still imperfectly modeled in the hydrodynamic-radiative simulations used for the design and post-shot modelling of these experiments.This thesis' goal is to improve the modelling of one of the main mechanisms of laser/matter coupling in ICF: radiation absorption by inverse bremsstrahlung (IB). We have proposed a parameterized model out of several models in the literature. The adjustable constants were evaluated for these various models from the literature (showing in passing the disparity) and we also evaluated these constants by classical molecular dynamics (CMD) simulations for two components (electrons-ions) weakly coupled plasmas (of interest to ICF) for ionization Z*=1 (weak) and Z*=10 (moderate). CMD simulation results seem to rule out certain hypotheses leading, in certain models, to a dependence of the Coulomb logarithm (which is a manifestation of microscopic collective effects in the absorption process) on the laser frequency regardless of intensity. We also implemented this parameterized model in the hydrodynamic-radiative code TROLL which was used to simulate a series of laser slab experiments (15 pure materials tested) carried out as part of this thesis on the GCLT facility at CEA DAM. The preliminary results of the comparison of these simulations with the experimental data (reflectivity over time, images of the evolution of the coronal plasma) allow an evaluation of the parameters which can be compared with the CMD values.; La Fusion par Confinement Inertiel (FCI) consiste à comprimer une capsule millimétrique d'un mélange Deutérium-Tritium à l'aide de lasers de puissance jusqu'à atteindre les conditions de fusion thermonucléaire auto-entretenue. Deux méthodes d'irradiation par les lasers de puissance sont possibles. En attaque directe, les lasers sont absorbés par la surface externe de la capsule. En attaque indirecte, la capsule, positionnée au centre d'une cavité en or (hohlraum), est irradiée par un rayonnement X intense issu de l'absorption des lasers par les parois de la cavité. Le retour des expériences de FCI réalisées depuis plus de dix ans aux USA au National Ignition Facility a montré que l’absorption laser par la matière était encore imparfaitement modélisée dans les simulations d'hydrodynamique-radiative utilisées pour les dimensionner et les restituer.Cette thèse a pour objectif d'améliorer la modélisation d'un des principaux mécanismes de couplage laser/matière en FCI~: l'absorption du rayonnement par bremsstrahlung inverse (BI). Nous avons proposé un modèle paramétré à partir d'un grand nombre de modèles de la littérature. Les constantes ajustables ont été évaluées pour ces différents modèles de la littérature (montrant au passage la disparité) et nous avons pu aussi les évaluer par des simulations de dynamique moléculaire classique (DMC) pour des plasmas à deux composantes (électrons-ions) faiblement couplés (d'intérêts pour la FCI) pour les degrés d'ionisation Z*=1 (faible) et Z*=10 (modéré). Les résultats de nos simulations de DMC semblent exclure certaines hypothèses qui mènent, dans certains modèles, à une dépendance du logarithme Coulombien (qui est une manifestation des effets collectifs microscopiques dans le processus d'absorption) à la fréquence du laser indépendamment de son intensité. Nous avons aussi implémenté ce modèle paramétré dans le code d'hydrodynamique-radiative TROLL qui a été utilisé pour simuler une série d'expériences de réflexion laser sur plaques (15 matériaux purs testés) réalisées dans le cadre de cette thèse sur l'installation GCLT au CEA DAM. Les résultats préliminaires de la comparaison de ces simulations avec les données expérimentales (réflectivité au cours du temps, imagerie de l'évolution du plasma coronal) permettent une évaluation des paramètres qui pourront être comparés aux valeurs issues de la DMC.

Laser-plasma instabilities which occur in millimetric hot plasma, typical in ICF experiments, are still a preoccupying topic because they lead to reflectivity levels higher than predicted. In this context, this work focuses on stimulated Raman scattering (SRS) through a multi-scale analysis: mono, bi and multi speckle scales. This picosecond experimental study highlights the difference between isolated and collective speckle behaviors in SRS development. A weak speckle, stable if isolated, may become unstable under the influence of SRS driven local plasma modifications generated by an intense nearby speckle: kinetic perturbations with suprathermal electrons and/or wave coupling with electromagnetic and electrostatic seeds. From set of experimental campaigns, featuring a highly spatially- and temporally-resolved Thomson-scattering diagnostic and backward Raman imaging, we evidenced and characterized this collective behavior. An experimental difference between these two kinds of perturbations was observed which lead to differentiate their contributions. In parallel, 2D PIC simulations using CALDER were performed to interpret bispeckle experimental results and in particular to understand the influence from both kinds of perturbations.; Les instabilités paramétriques qui se développent dans l’interaction laser-plasma à haute intensité dans les plasmas chauds de plusieurs millimètres typiques des expériences de FCI, entrainent des taux de rétrodiffusion importants, ce qui en fait un sujet toujours préoccupant. Les travaux de cette thèse se concentrent sur l’étude d’une des instabilités paramétriques, la diffusion Raman stimulée, dans les cas mono-, bi- et multi-speckles ou points chauds que l’on retrouve dans les taches focales des gros lasers de puissance. Cette étude expérimentale, effectuée en régime picoseconde, a établi la différence des comportements en régime isolé ou collectif de la rétrodiffusion Raman : la modification des propriétés locales du plasma induite par un point chaud intense peut rendre instable un point chaud voisin, stable vis-à-vis de l’instabilité s'il était isolé, par l'intermédiaire d’un couplage cinétique et/ou d'un couplage d'ondes. Une série d’expériences utilisant des diagnostics de diffusion Thomson à hautes résolutions spatiale et temporelle couplées avec une imagerie de la rétrodiffusion Raman a permis de mettre en évidence l'existence de ce comportement collectif, de saisir sa dynamique puis de caractériser à l’échelle picoseconde son influence dans le développement global de l'instabilité Raman en discriminant l’aspect cinétique (électrons suprathermiques) de l’aspect ondulatoire (couplage par l’intermédiaire des ondes excitées par l’instabilité).Les mesures expérimentales bispeckle ont été interprétées à l’aide de simulations particulaires 2D du code CALDER qui reproduit cette discrimination entre ces deux types de perturbations.

Laser-plasma instabilities which occur in millimetric hot plasma, typical in ICF experiments, are still a preoccupying topic because they lead to reflectivity levels higher than predicted. In this context, this work focuses on stimulated Raman scattering (SRS) through a multi-scale analysis: mono, bi and multi speckle scales. This picosecond experimental study highlights the difference between isolated and collective speckle behaviors in SRS development. A weak speckle, stable if isolated, may become unstable under the influence of SRS driven local plasma modifications generated by an intense nearby speckle: kinetic perturbations with suprathermal electrons and/or wave coupling with electromagnetic and electrostatic seeds. From set of experimental campaigns, featuring a highly spatially- and temporally-resolved Thomson-scattering diagnostic and backward Raman imaging, we evidenced and characterized this collective behavior. An experimental difference between these two kinds of perturbations was observed which lead to differentiate their contributions. In parallel, 2D PIC simulations using CALDER were performed to interpret bispeckle experimental results and in particular to understand the influence from both kinds of perturbations.; Les instabilités paramétriques qui se développent dans l’interaction laser-plasma à haute intensité dans les plasmas chauds de plusieurs millimètres typiques des expériences de FCI, entrainent des taux de rétrodiffusion importants, ce qui en fait un sujet toujours préoccupant. Les travaux de cette thèse se concentrent sur l’étude d’une des instabilités paramétriques, la diffusion Raman stimulée, dans les cas mono-, bi- et multi-speckles ou points chauds que l’on retrouve dans les taches focales des gros lasers de puissance. Cette étude expérimentale, effectuée en régime picoseconde, a établi la différence des comportements en régime isolé ou collectif de la rétrodiffusion Raman : la modification des propriétés locales du plasma induite par un point chaud intense peut rendre instable un point chaud voisin, stable vis-à-vis de l’instabilité s'il était isolé, par l'intermédiaire d’un couplage cinétique et/ou d'un couplage d'ondes. Une série d’expériences utilisant des diagnostics de diffusion Thomson à hautes résolutions spatiale et temporelle couplées avec une imagerie de la rétrodiffusion Raman a permis de mettre en évidence l'existence de ce comportement collectif, de saisir sa dynamique puis de caractériser à l’échelle picoseconde son influence dans le développement global de l'instabilité Raman en discriminant l’aspect cinétique (électrons suprathermiques) de l’aspect ondulatoire (couplage par l’intermédiaire des ondes excitées par l’instabilité).Les mesures expérimentales bispeckle ont été interprétées à l’aide de simulations particulaires 2D du code CALDER qui reproduit cette discrimination entre ces deux types de perturbations.

In the fields of stellar interiors and inertial confinement fusion, determination of dense and hot plasmas opacity is a major goal. Laboratory plasmas, close to both solid density and hundreds of electron-volt temprature are needed to improve theoretical and numerical models. This work is focused on the characterization of such plasmas formed by ultra-high-intensity laser interaction with solid matter. Experimental measurements on aluminium plasmas are presented. They highlight the complexity arising in the interpretation of K-shell x-ray emission spectra, typically used to infer the plasma density and temperature. To reproduce these experimental results, simulations using 3 codes were carried out (PIC code - CALDER, radiationhydrodynamics code - MULTI and atomic population code - SPECT3D). This work concludes on the need to account for many physical phenomena to further characterize the plasma: non-LTE equilibrium, hot electrons, time evolution and spatial gradients. All these issues are addressed and discussed. This kind of experiment opens bright perspectives in the study of very-high-density plasmas. Good temporal and spatial resolution is nevertheless necessary to ensure an accurate characterization of the plasma density and temperature., Les propriétés radiatives de plasmas denses et chauds conditionnent de nombreux phénomènes rencontrés dans les intérieurs stellaires et dans le cadre de la fusion par confinement inertielle. Afin d’améliorer les modèles dans un régime à la fois proche du solide (de l’ordre du g.cm-3) et de plusieurs centaines d’électron-Volts, des mesures en laboratoire sont aujourd’hui nécessaires. L’étude menée dans cette thèse consiste en lacaractérisation de tels plasmas formés par interaction laser à ultra-haute intensité. Dans le cas d’un plasma d’aluminium, les mesures expérimentales obtenues ont permis de mettre en évidence le caractère complexe de l’interprétation d’un spectre d’émission de couche K habituellement utilisé pour estimer la densité et la température du plasma. Pour restituer les résultats expérimentaux, une suite de simulations numériques utilisant plusieurs codes (codes PIC CALDER, hydro-radiatif MULTI et de physique atomique SPECT3D) a été menée. Ces simulations ont permis de conclure sur la nécessité de prendre en compte de nombreux phénomènes physiques pour caractériser au mieux le plasma : les conditions d’équilibre, la présence d’électrons chauds, l’évolution temporelle rapide et les gradients spatiaux. Ces points sont abordés et discutés dans ce travail. Ces études montrent que ce type d’expériences ouvre de vastes perspectives dans l’étude des plasmas à très haute densité. Une bonne résolution temporelle et spatialeest néanmoins nécessaire pour assurer une caractérisation précise de la densité et de la température du plasma produit.

The inertial confinement fusion (ICF) concept is widely studied nowadays. It consists in quickly compressing and heating a small spherical capsule filled with fuel, using extremely energetic lasers. Since approximately 15 years, the fast-ignition (FI) technique has been proposed to facilitate the fuel heating by adding a particle beam - electrons generated by an ultra-intense laser - at the exact moment when the capsule compression is at its maximum. This thesis constitutes an experimental study of these electron beams generated by picosecond-scale lasers. We present new results on the characteristics of these electrons after they are accelerated by the laser (energy, divergence, etc.) as well as their interaction with the matter they pass through. The experimental results are explained and reveal different aspects of these laser-accelerated fast electrons. Their analysis allowed for significant progress in understanding several mechanisms: how they are injected into solid matter, how to measure their divergence, and how they can be automatically collimated inside compressed matter., Le concept de fusion par confinement inertiel (FCI) est aujourd'hui largement étudié. Il s'agit de comprimer et chauffer brièvement une petite capsule sphérique remplie de combustible, à l'aide de lasers extrêmement énergétiques. Depuis une quinzaine d'année, la technique d'allumage rapide (AR) propose de faciliter le chauffage de ce combustible en ajoutant un faisceau de particules - des électrons générés par un laser ultra-intense - au moment exact où la compression de la capsule est maximale. La présente thèse constitue une étude expérimentale de ces faisceaux d'électrons générés par lasers picosecondes. Nous présentons des nouveaux résultats sur les caractéristiques de ces électrons après leur accélération par laser (énergie, divergence, etc.) ainsi que sur leur interaction avec la matière qu'ils traversent, qu'elle soit solide ou comprimée. Les résultats expérimentaux exposés révèlent différentes facettes de ces électrons rapides créés par laser, et leur analyse nous a permis de progresser dans la compréhension de certains mécanismes : comment ils sont injectés à l'intérieur de la matière solide, comment mesurer leur divergence, et comment ils peuvent être automatiquement collimatés à l'intérieur de la matière comprimée.