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We numerically show that ultraintense laser pulses interacting with magnetized gases having higher electron cyclotron than plasma frequency efficiently emit THz field bursts ≥ 100 GV.m − 1 that may be useful to probe and manipulate molecular matter.

Terahertz (THz) emitters attract more and more interest due to their various applications, e.g., in molecular spectroscopy. Whereas THz sources driven by two-color laser-based photocurrents have been widely exploited [1] , the question of enhancing their performances still remains. Better efficiency can be expected from the action of external, uniform magnetic fields ( B ), with 10-100 T strengths currently achievable in laser-plasma experiments [2] . The associated electron cyclotron frequency ( ω ce ) ranges from 1 to 10 THz, rendering it appropriate to modulate THz spectral contents. Here we describe the effect of strong magnetic fields depending on their orientation, for single- or two-color intense laser pulses linearly polarized along the z -axis.

We study numerically the mechanisms of proton acceleration in gas-foil targets driven by an ultraintense femtosecond laser pulse. The target consists of a near-critical-density hydrogen gas layer of a few tens of microns attached to a solid carbon foil with a contaminant thin proton layer at its back side. Two-dimensional particle-in-cell simulations show that, at optimal gas density, the maximum energy of the contaminant protons is increased by a factor of $\sim 4$ compared to a single foil target. This improvement originates from the near-complete laser absorption into relativistic electrons in the gas. Several energetic electron populations are identified, and their respective effect on the proton acceleration is quantified by computing the electrostatic fields that they generate at the protons' positions. While each of those electron groups is found to contribute substantially to the overall accelerating field, the dominant one is the relativistic thermal bulk that results from the nonlinear wakefield excited in the gas, as analyzed recently by Debayle et al. [New J. Phys. 19, 123013 (2017)]. Our analysis also reveals the important role of the neighboring ions in the acceleration of the fastest protons, and the onset of multidimensional effects caused by the time-increasing curvature of the proton layer., Comment: 9 pages, 5 figures

The advection by a flow of ponderomotively driven density fluctuations may lead to the deflection of a laser pulse. This effect, known as beam bending, may modify the irradiation geometry and energy deposition in high energy laser plasma experiments. A kinetic modeling of beam-bending of a Gaussian laser pulse is proposed and validated by means of “particle-in-cell” simulations over a vast parametric domain, demonstrating the importance of accounting for kinetic damping of driven ion-acoustic waves. The transient regime is also addressed and compared to kinetic simulations.

The direct and indirect drive schemes for Inertial Confinement Fusion (ICF) make use of a large number of laser beams arranged in a symmetric angular distribution. The preferential decay geometry of the three waves resonant couplings, mainly responsible for backscattered light in single beam experiments, may then be deeply modified in the region of crossing beams where collective laser plasma instabilities could develop. Such instabilities can occur for laser beams having a common symmetry axis along which they drive a common daughter wave. The collective coupling results in an increase of the growth gain with the increase of the number of interacting beams and produce energy losses in new backward directions. We have taken advantage of the multiple beams of the Omega laser facility and of its large battery of diagnostics to study the physics related to this multiple beams interaction in the regimes of high temperature plasmas relevant of the direct and indirect drive schemes to ICF. Experiments performed in a planar open geometry have evidenced the large amplification of stimulated Raman scattering (SRS) electromagnetic waves almost transverse to the density gradient as theoretically predicted 40 years ago. This was achieved in long scale-length high-temperature plasmas in which two beams couple to the same scattered electromagnetic wave further demonstrating this multiple-beams collective SRS interaction. The collective nature of the coupling and the amplification at large angles from the density gradient increase the global SRS losses and produce light scattered in novel directions out of the planes of incidence of the beams. Indirect drive experiments were performed in rugby ball shaped Hohlraum irradiated by 40 beams. Large instantaneous (peak reflectivity >30%) Brillouin sidescattering was evidenced to originate from the collective Brillouin amplification of a shared ion acoustic wave driven along the Hohlraum axis by a cone of 10 beams. In this paper, the scattering geometry is detailed for the two types of collective instabilities showing that they produce light scattered in novel very precise directions located far from the original aperture of the beams where the diagnostics are usually set-up. This scattered light could be measured on Omega thanks to the flexibility of the facility. Key features of the light scattered by collective instabilities are identified that would allow to recognize their signatures in more complex, less diagnosed experiments.

Terahertz to far-infrared emission by two-color, ultrashort optical pulses interacting with underdense helium gases at ultrahigh intensities ($g{10}^{19}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$) is investigated by means of 3D particle-in-cell simulations. The terahertz field is shown to be produced by two mechanisms occurring sequentially, namely, photoionization-induced radiation (PIR) by the two-color pulse, and coherent transition radiation (CTR) by the wakefield-accelerated electrons escaping the plasma. We exhibit laser-plasma parameters for which CTR proves to be the dominant process, providing terahertz bursts with field strength as high as $100\text{ }\text{ }\mathrm{GV}/\mathrm{m}$ and energy in excess of 10 mJ. Analytical models are developed for both the PIR and CTR processes, which correctly reproduce the simulation data.

International audience; We investigate the electron heating induced by a relativistic-intensity laser pulse propagating through a near-critical plasma. Using particle-in-cell simulations, we show that a specific interaction regime sets in when, due to the energy depletion caused by the plasma wakefield, the laser front profile has steepened to the point of having a length scale close to the laser wavelength. Wave breaking and phase mixing have then occurred, giving rise to a relativistically hot electron population following the laser pulse. This hot electron flow is dense enough to neutralize the cold bulk electrons during their backward acceleration by the wakefield. This neutralization mechanism delays, but does not prevent the breaking of the wakefield: the resulting phase mixing converts the large kinetic energy of the backward-flowing electrons into thermal energy greatly exceeding the conventional ponderomotive scaling at laser intensities >10(21) W cm(-2) and gas densities around 10% of the critical density. We develop a semi-numerical model, based on the Akhiezer-Polovin equations, which correctly reproduces the particle-in-cell-predicted electron thermal energies over a broad parameter range. Given this good agreement, we propose a criterion for full laser absorption that includes field-induced ionization. Finally, we show that our predictions still hold in a two-dimensional geometry using a realistic gas profile.

The stationary ray tracing method, commonly used in hydrodynamic codes to describe the laser propagation and energy deposition, is reformulated to include energy exchanges between laser beams, referred to as cross-beam energy transfer (CBET), as well as laser beam backscatterings from acoustic (Brillouin) and electron (Raman) plasma waves. These energy exchanges and scatterings are described by a Monte Carlo method simulating the creation/annihilation of rays. The algorithm has been validated against other numerical solvers and, in the case of CBET, by means of kinetic simulations. The method is efficient and can be easily implemented in already existing ray tracing packages used in many hydrodynamic codes. It can be further extended to describe other kinds of wave mixing processes such as side-scatterings and collective scatterings.

Experiments have been performed evidencing significant stimulated Raman sidescattering (SRS) at large angles from the density gradient. This was achieved in long scale-length high-temperature plasmas in which two beams couple to the same scattered electromagnetic wave further demonstrating for the first time this multiple-beam collective SRS interaction. The collective nature of the coupling and the amplification at large angles from the density gradient increase the global SRS losses and produce light scattered in novel directions out of the planes of incidence of the beams. These findings obtained in plasmas conditions relevant of inertial confinement fusion experiments similarly apply to the more complex geometry of these experiments where anomalously large levels of SRS were measured.

Amplification of a picosecond pulse beam by a lower intensity nanosecond pulse beam was experimentally observed in a flowing plasma. Modifications of intensity distributions in beam focal spots due to nonhomogeneous energy transfer and its transient regime were investigated. The mean transferred power reached 57% of the incident power of the nanosecond pulse beam. An imaging diagnostic allowed the intensity profile of the picosecond pulse beam to be determined, bringing to evidence the spatial nonuniformity of energy transfer in the amplified beam. This diagnostic also enabled us to observe the temporal evolution of the speckle intensity distribution because of the transfer. These results are reproduced by numerical simulations of two complementary codes. The method and the observed effects are important for the understanding of experiments with multiple crossing laser beams in plasmas.

We numerically investigate terahertz (THz) pulse generation by linearly-polarized, two-color femtosecond laser pulses in highly-ionized argon. Major processes consist of tunneling photoionization and ponderomotive forces associated with transverse and longitudinal field excitations. By means of two-dimensional particle-in-cell (PIC) simulations, we reveal the importance of photocurrent mechanisms besides transverse and longitudinal plasma waves for laser intensities >1015 W/cm2. We demonstrate the following. (i) With two-color pulses, photoionization prevails in the generation of GV/m THz fields up to 1017 W/cm2 laser intensities and suddenly loses efficiency near the relativistic threshold, as the outermost electron shell of ionized Ar atoms has been fully depleted. (ii) PIC results can be explained by a one-dimensional Maxwell-fluid model and its semi-analytical solutions, offering the first unified description of the main THz sources created in plasmas. (iii) The THz power emitted outside the plasma channel mostly originates from the transverse currents.

We present an extensive numerical study by means of particle-in-cell simulations of the energy transfer that occurs during the crossing of two laser beams. In the linear regime, when ions are not trapped in the potential well induced by the laser interference pattern, a very good agreement is obtained with a simple linear stationary model, provided the laser intensity is sufficiently smooth. These comparisons include different plasma compositions to cover the strong and weak Landau damping regimes as well as the multispecies case. The correct evaluation of the linear Landau damping at the phase velocity imposed by the laser interference pattern is essential to estimate the energy transfer rate between the laser beams, once the stationary regime is reached. The transient evolution obtained in kinetic simulations is also analysed by means of a full analytical formula that includes 3D beam energy exchange coupled with the ion acoustic wave response. Specific attention is paid to the energy transfer when the laser presents small-scale inhomogeneities. In particular, the energy transfer is reduced when the laser inhomogeneities are comparable with the Landau damping characteristic length of the ion acoustic wave.We present an extensive numerical study by means of particle-in-cell simulations of the energy transfer that occurs during the crossing of two laser beams. In the linear regime, when ions are not trapped in the potential well induced by the laser interference pattern, a very good agreement is obtained with a simple linear stationary model, provided the laser intensity is sufficiently smooth. These comparisons include different plasma compositions to cover the strong and weak Landau damping regimes as well as the multispecies case. The correct evaluation of the linear Landau damping at the phase velocity imposed by the laser interference pattern is essential to estimate the energy transfer rate between the laser beams, once the stationary regime is reached. The transient evolution obtained in kinetic simulations is also analysed by means of a full analytical formula that includes 3D beam energy exchange coupled with the ion acoustic wave response. Specific attention is paid to the energy transfer when the ...

Interaction of high intensity laser pulses, (I > 1018W/cm2) with solid targets is an efficient way of production of high current relativistic electron beams (jb ~ 10 kA/μm2). Such currents can be transported only under the condition of their charge and current neutralization by the target electrons. This effect is highly dependent on the target conductivity. In a dielectric target, the free electrons are generated due to the field and collisional ionization self-induced by the relativistic electrons. The ionization process is unstable and it can lead to a beam filamentation. We demonstrate here that the electric field ionization is responsible for this instability, and it develops on spatial scales significantly larger than the ionization front thickness.

We show that photocurrents remain an efficient source of THz emission for two-color laser pulses interacting with argon at high intensities > 1015 W/cm2. Near the relativistic limit, their efficiency decreases while ponderomotive emitters prevail.

We studied the interaction of a high-intensity laser with mass-limited Ti-wires. The laser was focused up to , with contrast of to produce relativistic electrons. High-spatial-resolution X-ray spectroscopy was used to measure isochoric heating induced by hot electrons propagating along the wire up to 1 mm depth. For the first time it was possible to distinguish surface target regions heated by mixed plasma mechanisms from those heated only by the hot electrons that generate warm dense matter with temperatures up to 50 eV. Our results are compared to simulations that highlight both the role of electron confinement inside the wire and the importance of resistive stopping powers in warm dense matter.

We report on the first self-consistent numerical study of the feasibility of laser-driven relativistic pair shocks of prime interest for high-energy astrophysics. Using a QED-particle-in-cell code, we simulate the collective interaction between two counterstreaming electron-positron jets driven from solid foils by short-pulse (~60 fs), high-energy (~100 kJ) lasers. We show that the dissipation caused by self-induced, ultrastrong (>10^{6} T) electromagnetic fluctuations is amplified by intense synchrotron emission, which enhances the magnetic confinement and compression of the colliding jets.

Crossed-beam energy transfer between three laser beams has been experimentally investigated in a flowing plasma. Time-evolution measurements of the amplification of a first beam by a second beam highlighted the inhibition of energy transfer by hydrodynamic modifications of the plasma in the crossing volume due to the propagation of a third beam. According to 3D simulations and an analytical model, it appears that the long-wavelength expansion-velocity fluctuations produced by the propagation of the third beam in the crossing volume are responsible for this mitigation of energy transfer. This effect could be a cause of the over-estimation of the amount of the transferred energy in indirect-drive inertial confinement fusion experiments. Besides, tuning such long-wavelength fluctuations could be a way to completely inhibit CBET at the laser entrance holes of hohlraums.

We present a predictive model of the nonlinear phase of the Weibel instability induced by two symmetric, counter-streaming ion beams in the non-relativistic regime. This self-consistent model combines the quasilinear kinetic theory of Davidson et al. [Phys. Fluids 15, 317 (1972)] with a simple description of current filament coalescence. It allows us to follow the evolution of the ion parameters up to a stage close to complete isotropization, and is thus of prime interest to understand the dynamics of collisionless shock formation. Its predictions are supported by 2-D and 3-D particle-in-cell simulations of the ion Weibel instability. The derived approximate analytical solutions reveal the various dependencies of the ion relaxation to isotropy. In particular, it is found that the influence of the electron screening can affect the results of simulations using an unphysical electron mass., Accepted for publication in Phys. Plasmas

Modifications of the spatial distribution of polarization in the focal spot of a picosecond-pulse beam after its amplification by a nanosecond pump in a plasma have been observed experimentally. The spatially resolved measurement of beam polarization was achieved thanks to the simultaneous imaging of the focal spot with two cameras, one of which was equipped with a polarizer, permitting to determine the polarization characteristics. The mean modification of the picosecond-pulse beam polarization after crossing the nanosecond beam depends on its mean amplification by crossed-beam energy transfer. A fine analysis of the spatial polarization modification reveals a large dispersion of the speckle polarization modification after their amplification. This dispersive modification originates from the spatial smoothing of the two crossing beams.

We present results on laser-driven relativistic electron beam propagation through aluminum samples, which are either solid and cold or compressed and heated by laser-induced shock. A full numerical description of fast electron generation and transport is found to reproduce the experimental absolute K-alpha yield and spot size measurements for varying target thicknesses, and to sequentially quantify the collisional and resistive electron stopping powers. The results demonstrate that both stopping mechanisms are enhanced in compressed Al samples and are attributed to the increase in the medium density and resistivity, respectively. For the achieved time-and space-averaged electronic current density, < j(h)> similar to 8 x 10(10) A/cm(2) in the samples, the collisional and resistive stopping powers in warm and compressed Al are estimated to be 1.5 keV/mu m and 0.8 keV/mu m, respectively. By contrast, for cold and solid Al, the corresponding estimated values are 1.1 keV/mu m and 0.6 keV/mu m. Prospective numerical simulations involving higher j(h) show that the resistive stopping power can reach the same level as the collisional one. In addition to the effects of compression, the effect of the transient behavior of the resistivity of Al during relativistic electron beam transport becomes progressively more dominant, and for a significantly high current density, j(h) similar to 10(12) A/cm(2), cancels the difference in the electron resistive stopping power (or the total stopping power in units of areal density) between solid and compressed samples. Analytical calculations extend the analysis up to j(h) = 10(14) A/cm(2) (representative of the full-scale fast ignition scenario of inertial confinement fusion), where a very rapid transition to the Spitzer resistivity regime saturates the resistive stopping power, averaged over the electron beam duration, to values of similar to 1 keV/mu m. (C) 2014 AIP Publishing LLC.

We developed a numerical code that describes both the energy transfer occurring when two or more laser beams overlap in a weakly non-homogeneous plasma, and the beam energy losses associated with the electron-ion collisions. The numerical solutions are validated with both the exact analytical solutions in homogeneous plasmas, and with new approximate analytical solutions in non-homogeneous plasmas that include the aforementioned inverse bremsstrahlung effect. Comparisons with kinetic particle-in-cell simulations are satisfactory, provided the acoustic wave-breaking limit and the self-focusing regime are not reached. An application of the Cross-Beam Energy Transfer model is shown for a typical case of indirect-drive implosion in a gold hohlraum.

Fast electrons produced by a 10 ps, 160 J laser pulse through laser-compressed plastic cylinders are studied experimentally and numerically in the context of fast ignition. K(alpha)-emission images reveal a collimated or scattered electron beam depending on the initial density and the compression timing. A numerical transport model shows that implosion-driven electrical resistivity gradients induce strong magnetic fields able to guide the electrons. The good agreement with measured beam sizes provides the first experimental evidence for fast-electron magnetic collimation in laser-compressed matter.

International audience; The high current electron beam losses have been studied experimentally with 0.7 J, 40 fs, 6 1019 Wcm-2 laser pulses interacting with Al foils of thicknesses 10-200 ?m. The fast electron beam characteristics and the foil temperature were measured by recording the intensity of the electromagnetic emission from the foils rear side at two different wavelengths in the optical domain, ?407 nm (the second harmonic of the laser light) and ?500 nm. The experimentally observed fast electron distribution contains two components: one relativistic tail made of very energetic (Thtail ? 10 MeV) and highly collimated (7° ± 3°) electrons, carrying a small amount of energy (less than 1% of the laser energy), and another, the bulk of the accelerated electrons, containing lower-energy (Thbulk = 500±100 keV) more divergent electrons (35 ± 5°), which transports about 35% of the laser energy. The relativistic component manifests itself by the coherent 2?0 emission due to the modulation of the electron density in the interaction zone. The bulk component induces a strong target heating producing measurable yields of thermal emission from the foils rear side. Our data and modeling demonstrate two mechanisms of fast electron energy deposition: resistive heating due to the neutralizing return current and collisions of fast electrons with plasma electrons. The resistive mechanism is more important at shallow target depths, representing an heating rate of 100 eV per Joule of laser energy at 15 ?m. Beyond that depth, because of the beam divergence, the incident current goes under 1012 Acm-2 and the collisional heating becomes more important than the resistive heating. The heating rate is of only 1.5 eV per Joule at 50 ?m depth. © EDP Sciences and Springer 2009.

International audience; Beams of fast electrons have been generated from the ultra-intense laser interaction with Aluminium foil targets. The dynamics of the fast electrons propagation and the level of induced in-depth heating have been investigated using the optical emission from the foils rear side. Important yields of thermal emission, consequence of high target temperatures, were detected for targets thinner than 50 μm. We precisely characterized the targets in-depth temperature profile in order to reproduce the emission yields. At shallow depth, we show the important heating (estimated to > 100 eV till 15 μm depth) has a resistive origin upon the neutralizing return current. For deeper regions, because of the bulk component divergence, the fast electron energy losses and induced heating are due to collisions. Coupling the model to the experimental measurements, we were able to quantify the bulk of the fast electron population, corresponding to 35% of the laser energy and a 500 keV temperature.

International audience; The transport of an intense electron-beam produced by Ultra High Intensity laser pulses through metals and insulators has been studied by imaging with high resolution the optical emission (Optical Transition Radiation, Thermal and Cherenkov emission) due to electron transit through the targets. It is observed that if the target is an insulator the fast electron beam undergoes strong filamentation and the filaments increase in number with plastic thickness, with a characteristic growth rate and transversal scale in good agreement with analytical predictions based on electric field instabilities in the beam ionization front.

International audience; The transport of an intense electron beam produced by ultrahigh intensity laser pulses through metals and insulators has been studied by high resolution imaging of the optical emission from the targets. In metals, the emission is mainly due to coherent transition radiation, while in plastic, it is due to the Čerenkov effect and it is orders of magnitude larger. It is also observed that in the case of insulators the fast-electron beam undergoes strong filamentation and the number of filaments increases with the target thickness. This filamented behavior in insulators is due to the instability of the ionization front related to the electric field ionization process. The filamentary structures characteristic growth rate and characteristic transversal scale are in agreement with analytical predictions.

We present novel structured targets capable of collimating laser-generated fast electrons through dense plasmas. The proposed targets are made of narrow high- and low-Z filaments leading to a transversely modulated electrical resistivity profile. When featuring a spatially decreasing density, these targets permit both to guide the fast electrons and reduce their angular dispersion. The principle of our target design is explained by a theoretical model. Two-dimensional particle-in-cell simulations are performed to demonstrate its efficiency.

The interaction of an ultraintense laser pulse with a conical target is studied by means of numerical particle-in-cell simulations in the context of fast ignition. The divergence of the fast electron beam generated at the tip of the cone has been shown to be a crucial parameter for the efficient coupling of the ignition laser pulse to the precompressed fusion pellet. In this paper, we demonstrate that a focused hot electron beam is produced at the cone tip, provided that electron currents flowing along the surfaces of the cone sidewalls are efficiently generated. The influence of various interaction parameters over the formation of these wall currents is investigated. It is found that the strength of the electron flows is enhanced for high laser intensities, low density targets, and steep density gradients inside the cone. The hot electron energy distribution obeys a power law for energies of up to a few MeV, with the addition of a high-energy Maxwellian tail.

Beams of fast electrons have been generated from the ultra-intense laser interaction (6×1019W cm−2, 40fs) with aluminum foil targets. The dynamics of fast-electron propagation as well as the level of induced in-depth heating have been investigated using the optical emission from the foil’s rear side. The dependence of the emitted signals spectrum and size on the target thickness allowed the identification of the coherent (coherent transition radiation) and incoherent (thermal radiation) mechanisms of the optical emission. We demonstrate a two-temperature energy distribution for the laser-generated fast-electron population: a divergent bulk component (θbulk=35°±5°) with ≈35% of the laser focal spot energy and a 400–600keV temperature, plus a relativistic tail highly collimated (θtail=7°±3°), with a 10MeV temperature and a periodic modulation in microbunches, representing less than 1% of the laser energy. Important yields of thermal emission, observed for targets thinner than 50μm, are consequence of a hot ...

The propagation through an insulator of a high-current monoenergetic fast electron beam is investigated in a one-dimensional model. The target ionization provides the charge and current neutralization and enables the beam propagation. The ionization process consists of two stages: (i) the self-consistent electric field ionization of atoms in the beam front and (ii) the collisional ionization of atoms by the return current in the beam body. The ionization in the beam front defines the propagation velocity. The charge neutralization quickly suppresses the electric field behind the beam front and the plasma heating by the return current supports the collisional ionization in the beam body. This constitutes the main mechanism of the energy loss for high beam densities.

Inertial confinement fusion experiments on large laser facilities such as the LMJ in Bordeaux or the NIF in the United States, involve the propagation of lasers through large plasmas (several millimeters). A large number of instabilities called wave coupling are likely to appear, and scatter the light in a different direction from the incident electromagnetic wave. In particular, Raman and Brillouin backscattering, as well as energy exchange between laser beams result from these wave couplings. Techniques called optical smoothing are used in large facilities to try to reduce these phenomena. The beams, once smoothed, are composed of many micrometric hot spots called speckles. During these experiments, the beams, focused around a target, are going to cross each other. The coupling of two coherent electromagnetic waves in a non-linear medium (the plasma), can induce an energy exchange between the two beams. The crossing of the lasers creates an interference grating, where the ponderomotive force expels the electrons. The ions follow the electrons due to the electrostatic spring force, generating a density modulation, or acoustic wave, which diffracts the electromagnetic waves from one beam to the other. This exchange, called Cross-beam Energy Transfer (CBET) takes place if the lasers have slightly different frequencies, or if the latter are equal but the plasma is moving in the direction of the acoustic wave. We have shown that, although these two situations are often considered equivalent in hydrodynamic models, they are in fact different. This is due to the fact that the exchange is commonly calculated by considering the laser beams as plane waves, i.e. neglecting the laser smoothing. In order to demonstrate this non-equivalence, we first studied an academic situation, considering the crossing of two laser beams each constituted of 4 Gaussian speckles. Different simulations have been performed with a particle-in-cell kinetic code, solving the Vlasov and Maxwell equations. The simulations showed that when the interaction is induced by a moving plasma, plane wave models are able to predict the exchange between the Gaussian beams. By contrast, when the exchange is induced by different laser frequencies, plane wave models overestimate the energy transfer. Moreover, we were able to distinguish two different configurations for the case where the laser frequencies are different. In the first case, the acoustic waves from different speckle crossings are in phase, and a constructive interference results from their interaction. In the other case, the waves are out of phase resulting in a destructive interference. It has been shown that although the exchange is greater in the in-phase case, the exchange remains lower than in the plasma flow case. The phase shift is therefore not the only source of difference between the two situations in which CBET appears. These results were obtained by considering a weakly Landau-damped plasma, i.e. a situation where the acoustic wave propagates and may encounter several speckle crossings before being damped. In a second step, a more realistic modeling of the smoothed beams has been adopted. For this purpose, the fields of a smoothed laser beam have been computed in an exact way, allowing to perform more accurate simulations and to build a model taking into account the real structure of the speckles. The previous results concerning the non-equivalence between the plasma flow and wavelength shift cases have been confirmed, even in the case of a strongly Landau-damped plasma. We were then able to show that the resonance conditions allowing the energy transfer to take place are also affected by the laser smoothing. In particular, the resonance width is broadened by the spatial smoothing.; Les expériences de fusion par confinement inertiel sur les grandes installations laser, telles que le LMJ à Bordeaux ou le NIF aux États-Unis, nécessitent la propagation des lasers à travers des plasmas de grandes tailles (plusieurs millimètres). Un grand nombre d'instabilités dites de couplages d'ondes peuvent apparaître, et diffusent la lumière dans une direction différente de l'onde électromagnétique incidente. En particulier, les diffusions arrières Raman et Brillouin, ainsi que l'échange d'énergie entre faisceaux laser en résultent. Des techniques dites de lissage optique sont utilisées pour tenter de réduire ces phénomènes. Les faisceaux, une fois lissés, sont constitués de nombreux points chauds micrométriques nommés speckles. Lors de ces expériences, les faisceaux, focalisés autour d'une cible, vont être amenés à se croiser.Le couplage de deux ondes électromagnétiques cohérentes dans un milieu non-linéaire (le plasma), peut induire un échange d'énergie entre les faisceaux. Le croisement des lasers crée un réseau d'interférences, où la force pondéromotrice expulse les électrons. Les ions suivent les électrons à cause de la force de rappel électrostatique. Cela crée une modulation de densité, ou onde acoustique, qui diffracte les ondes électromagnétiques d'un faisceau vers l'autre. Cet échange, nommé Cross-beam Energy Transfer (CBET) a lieu si les lasers ont des fréquences différentes, ou si ces dernières sont égales mais que le plasma est en mouvement dans la direction de l'onde acoustique. Nous avons montré que, bien que ces deux situations soient souvent considérées comme équivalentes dans les modèles hydrodynamiques, elles sont en réalité différentes. Ceci est dû au fait que l'échange est communément calculé en considérant les faisceaux laser comme des ondes planes, c'est-à-dire en négligeant le lissage laser. Afin de démontrer cette non-équivalence, nous avons, dans un premier temps, étudié une situation académique, en considérant le croisement de deux faisceaux lasers constitués chacun de 4 speckles Gaussiens. Différentes simulations ont été effectuées grâce à un code cinétique "particle-in-cell", résolvant les équations de Vlasov et de Maxwell. Les simulations ont montré que lorsque l'interaction est induite par un plasma en mouvement, les modèles du type onde plane sont en mesure de prédire l'échange entre les faisceaux Gaussiens. Au contraire, lorsque l'échange est induit par des fréquences laser différentes, ces modèles surestiment le transfert d'énergie. Nous avons aussi mis en évidence deux configurations distinctes dans le cas où les fréquences laser sont différentes. Dans la première situation, les ondes acoustiques issues de différents croisements de speckles sont en phase, et une interférence constructive résulte de leur interaction. Dans l'autre situation, les ondes sont déphasées ce qui donne une interférence destructive. Nous avons montré que, bien que l'échange soit plus important dans la situation en phase, il demeure inférieur au cas flot de plasma. Le déphasage n'est donc pas la seule cause de la différence entre une interaction avec ou sans flot. Nos résultats ont été obtenus en considérant un plasma faiblement amorti, où l'onde acoustique se propage et peut rencontrer plusieurs croisements de speckles avant que son amplitude n'ait significativement décru. Dans un second temps, nous avons considéré une situation plus proche de celle d'un croisement de faisceaux lissés. Nous avons calculé exactement le champ d'un faisceau laser lissé, ce qui a permis d'effectuer des simulations plus précises et de construire un modèle tenant compte de la structure réelle des speckles. Les résultats précédents ont été confirmés, même dans le cas d'un plasma fortement amorti. Nous avons également pu montrer que les conditions de résonance permettant au transfert d'énergie d'avoir lieu sont elles aussi affectées par le lissage laser. En particulier, le lissage spatial augmente la largeur de résonance.

Ponderomotive laser self-focusing poses a threat to the success of the inertial confinement fusion (ICF) program: it locally enhances the laser intensity, which causes two detrimental effects: i) undermining the uniformity of the shock wave launched into the target, and ii) increasing the onset of laser-plasma instabilities. Despite several optical techniques have been implemented to smooth ponderomotive effects, they still remain a concern in case of crossing beams or in spike pulse-plasma interaction as in shock ignition.In order to ameliorate the interpretation capability of radiation-hydrodynamics simulations, the Paraxial Complex Geometrical Optics (PCGO) module has been implemented in the hydrodynamics code CHIC in two-dimensional planar geometry, and is an improved version of the standard Ray-Tracing technique. PCGO accounts for nonlinear laser-plasma interaction such as ponderomotive force and hot electrons generation and propagation, usually neglected in hydrodynamic simulations. This approach is also used for creating spatially modulated laser beams by superposing Gaussian PCGO beamlets in the far-field. Their intensity envelope generates the intensity fluctuations. Although this PCGO-based method has improved the accuracy of CHIC simulations, the superposition of PCGO beamlets produces larger and longer speckles than real ones, and their self-focusing may be overestimated.In this thesis, we develop a method for describing and controlling the excessive ponderomotive self-focusing developing in PCGO speckles while performing CHIC simulations. This study has been conducted in stationary plasmas. First, we investigate self-focusing of a single Gaussian PCGO beamlet in a homogeneous nonabsorbing plasma by comparing its behavior to a Gaussian-shaped beam modeled with the paraxial electromagnetic code HARMONY. This comparison allows to define the domain of beam power where the PCGO approximation is valid. We found that within 4 times the critical power, PCGO correctly reproduces HARMONY results.Afterwards, we consider the self-focusing of a PCGO speckle created by superposition of several beamlets, referred to as a multi-beamlet speckle. This speckle stands for a reference for any PCGO speckle created in CHIC. The reduction of the speckle intensity enhancement is quantified as a function of the number of superposed beamlets and by considering two strategies for multi-beamlet speckle shaping: random and regular.The latter configuration demonstrates better performances in controlling and reducing ponderomotive effects for a number of beamlets equal to three: our results show that the critical power of a three-beamlet speckle is twice higher compared to the critical power of a Gaussian beam with same characteristics.This novel speckle configuration has been implemented in CHIC and employed to generate multi-speckle beams whose speckle intensity distribution obeys to an exponential law. We then studied the self-focusing of a spatially modulated beam (multi-speckle beam) in a homogeneous nonabsorbing plasma and show that our configuration allows to properly treat ponderomotive effects for different laser intensities: this method describes the speckle intensity statistics modification induced by speckle self-focusing and inter-speckle interaction as observed in electromagnetic simulations.The last part of the thesis is devoted to establish a baseline towards modelling of laser self-focusing in real ICF conditions. For this purpose, our results are extended to absorbing plasmas with a linear density profile. Speckle self-focusing is investigated here for different plasma lengths, and the effect of laser absorption is discussed. It is demonstrated that the proposed method of creation of a multi-beamlet speckle pattern operates in the conditions relevant to the direct-drive ICF. It allows to control efficiently the speckle self-focusing and its effect on the speckle intensity distribution in plasma.; L'auto-focalisation laser due à la force pondéromotrice constitue un obstacle au succès du programme de fusion par confinement inertiel (FCI) car elle augmente localement l'intensité laser, qui provoque deux effets néfastes: i) diminue l'uniformité de l'onde de choc lancée dans la cible, et ii) accroit la probabilité d'excitation des instabilités paramétriques. Bien que plusieurs techniques optiques aient été mises en œuvre pour atténuer les effets pondéromoteurs, ils restent une préoccupation en cas de croisement de faisceaux ou d'interaction avec une intensité laser élevée comme dans l'allumage par choc.Afin d'améliorer la capabilités interprétatives des codes hydrodynamiques, un module appelé Paraxial Complex Geometrical Optics (PCGO) a été implémenté dans le code hydrodynamique CHIC en géométrie plane bidimensionnelle : une telle méthode est une version améliorée de la technique standard du Ray-Tracing (RT). PCGO tient compte de l'interaction non linéaire laser-plasma telle que la force pondéromotrice et la génération et la propagation d'électrons chauds, généralement négligés dans les simulations hydrodynamiques. Cette approche est également utilisée pour créer des faisceaux laser spatialement modulés par superposition de faisceaux gaussiens PCGO : l'enveloppe d'intensité de ces faisceaux génère des fluctuations d'intensité (`` speckles '') en champ lointain. Bien que cette méthode basée sur PCGO ait amélioré la précision des simulations CHIC, la superposition de faisceaux PCGO produit des speckles laser plus grands et plus longs que dans les expériences, et leur auto-focalisation peut être surestimée.Dans cette thèse, nous développons une méthode pour décrire et contrôler l'auto-focalisation pondéromotrice des speckles dans un plasma stationnaire en utilisant CHIC avec PCGO. Dans un premier temps, nous étudions l'auto-focalisation d'un faisceau PCGO gaussien dans un plasma homogène non absorbant en comparant son comportement à un faisceau de forme gaussienne modélisé avec le code électromagnétique HARMONY. Nous montrons que jusqu’à 4 fois la puissance critique, PCGO reproduit correctement les résultats d'HARMONY.Ensuite, nous considérons l'auto-focalisation d'un speckle PCGO créé par superposition de plusieurs sous-faisceaux, appelé `` multi-beamlet speckle ''. Ce speckle représente une référence pour tout speckle PCGO créé dans CHIC. Nous considérons deux stratégies pour la mise en forme du speckle à faisceaux multiples: aléatoire et régulière. Cette dernière configuration permet d’obtenir de meilleures performances pour contrôler et réduire les effets pondéromoteurs pour un nombre de faisceaux égal à trois.Cette nouvelle configuration a été implémentée dans CHIC et utilisée pour générer des faisceaux multi-speckle dont la distribution des intensités des speckles obéit à une loi exponentielle. Nous avons ensuite étudié l'auto-focalisation d'un faisceau modulé spatialement (faisceau multi-speckle) dans un plasma homogène non absorbant et montré que notre configuration de speckles permet de traiter correctement les effets pondéromoteurs pour différentes intensités laser: cette méthode décrit la modification des statistiques d'intensité de speckle induite par auto-focalisation du speckle et l'interaction entre speckles comme c'est observé dans des simulations électromagnétiques de référence.La dernière partie de la thèse est consacrée à établir une base pour la modélisation de l'autofocalisation laser dans des conditions FCI réelles. A cet effet, nos résultats sont étendus aux plasmas absorbants dont la densité présente un profile linéaire. L'auto-focalisation est étudiée ici pour différentes longueurs de plasma, et l'effet de l'absorption laser est discuté. Il est démontré que la méthode proposée pour la création d'un distribution de speckles à faisceaux multiples fonctionne dans les conditions pertinentes pour l'ICF à entraînement direct.