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Optical smoothing techniques are widely used in large high-power laser facilities designed for inertial confinement fusion research, such as the Laser Mégajoule (LMJ) in France. Those techniques are of primordial interest because they reduce laser–plasma instabilities as, for example, the stimulated backward Brillouin scattering (SBS). The latter takes place in the hot plasmas produced inside a hohlraum by the ablation of high-Z or gold material where ion acoustic waves are weakly damped, easing SBS. However, despite the current optical smoothing implementation, like smoothing by spectral dispersion (SSD), SBS is not sufficiently mitigated and may also damage the laser optics by following back the laser path. Moreover, SSD limits the available energy and may produce unwanted amplitude modulations in the laser chain, resulting in an additional damage. Using spectral distribution with two colors (Duluc et al 2019 Phys. Rev. Appl. 12 054055) has proven to limit the latter damage, keeping the same efficiency for limiting SBS. In this study, we go a step forward and generalize this approach by exploring multi-color spectral distribution schemes and the role of dispersion by means of three-dimensional simulations. This study paves the road to new laser concepts that could tackle more efficiently the laser–plasma interaction issues leading to a better laser energy coupling.

International audience; A new fluid model describing backward stimulated Raman scattering (SRS) is presented based on parametric three-wave coupling in multidimensional geometry. It takes into account kinetic effects in the description of the plasma wave via a non linear frequency shift due to trapped electrons. The model is valid in the regime of hot and weakly inhomogeneous plasmas under conditions relevant for inertial confinement fusion with the plasma parameter k L λ De 0.25 (k L standing for the plasma wave number and λ De for the Debye length). Benchmarks of the model have been performed against the Maxwell-particle-in-cell (PIC) code Emi2D in order to calibrate the adjustable parameters controlling the non linear frequency shift. Two major configurations have been tested, one in a homogeneous plasma, with the onset of laser pump depletion and the other in an inhomogeneous plasma, producing auto-resonant growth. Good agreement between fluid and PIC simulations has been found for both configurations, in particular for the growth of SRS, and further on in time for the average backscatter level. The model is a promising tool to be implemented in multi-dimensional laser-plasma interaction packages coupled to hydrodynamics codes in order to compute SRS in mm-size volumes, usually inaccessible with PIC codes.

Optical smoothing techniques are crucial for achieving well-controlled experiments at large-scale laser facilities dedicated to high-energy-density physics. Due to detrimental effects in the laser chain, however, those techniques can convert phase modulation into potentially large amplitude modulation, reducing laser performance at the target and causing laser damage. This study uses innovative smoothing paradigms to address this key issue. In particular, one simple solution is to distribute the laser bandwidth over the beams, a result that might have considerable impact on engineering solutions to such problems.

In the context of inertial confinement fusion, optical smoothing is a technique used to provide laser irradiation as homogeneous as possible. It is required for limiting the development of laser-plasma instabilities, in particular, stimulated Brillouin backscattering (SBS). Here, we carefully compare longitudinal and transverse smoothing by spectral dispersion in an ideal smoothing configuration for each case. With 3D codes, we have simulated SBS in a gold plasma. We show that, as opposed to common belief, the time evolution of the SBS reflectivity exhibits some differences between both smoothing schemes. First, the asymptotic values of the saturation levels are not quite the same. With a simple ray description and computing the SBS gain for each ray, we were able to explain this difference. Moreover, the dynamics of SBS are also somewhat different. We have shown that SBS dynamics are driven by the time evolution of speckle properties and in particular by the effective interaction length between the Brillouin backscattered light and the hot-spots. This effective interaction length depends both on the longitudinal velocity and on the length of the hot-spots. As a matter of fact, synchronizing the effective interaction lengths of both smoothing schemes also synchronizes the backscattering curves before reaching the saturation level.

This article sums up the theoretical and experimental studies about ignition. Three experiments are salient this year on the Omega laser in collaboration with DOE laboratories (1) 3 cones of beams allow to mimic the LMJ configuration and to get symmetry measurements. (2) We measured perturbations due to hydro-instability in CHGe planar samples with face-on and side-on radiographs. (3) We improved our nuclear diagnostics, particularly the neutron image system tested on direct drive implosions. As far as LMJ target design is concerned, we defined a preliminary domain corresponding to the possible operation at 2ω. At 3ω we studied the low mode instability effects on the DT deformation (due to the laser or to the target) and on the yield. The stability is clearly improved with graded doped CH for our nominal capsule L1215.

The propagation and stimulated scattering of intense laser beams interacting with underdense plasmas are two important issues for inertial confinement fusion (ICF). The purpose of this work was to perform experiments under well-controlled interaction conditions and confront them with numerical simulations to test the physics included in the codes. Experimental diagnostics include time and space resolved images of incident and SBS light and of SBS-ion acoustic activity. New numerical diagnostics, including similar constraints as the experimental ones and the treatment of the propagation of the light between the emitting area and the detectors, have been developed. Particular care was put to include realistic plasma density and velocity profiles, as well as laser pulse shape in the simulations. In the experiments presented in this paper, the interaction beam was used with a random phase plate (RPP) to produce a statistical distribution of speckles in the focal volume. Stimulated Brillouin Scattering (SBS) was described using a decomposition of the spatial scales which provides a predictive modeling of SBS in an expanding mm-scale plasma. Spatial and temporal behavior of the SBS-ion acoustic waves was found to be in good agreement with the experimental ones for two laser intensities.

The multiple scattering of random laser beams on self-induced density fluctuations is responsible for the modification of the spatial and temporal coherence properties of the transmitted light. We investigate within a statistical approach the excitation of density fluctuations by randomized laser beams. Their spectrum is derived and their level is estimated as a few percents. The thermal effects are discussed. We characterize the evolution of the density perturbation level in time and along the propagation axis.

This paper investigates statistical properties of hot spots when speckle patterns are generated by the superimposition of multiple laser beams in a 2 cone and a 3 cone Laser MegaJoule configuration in the zone where all the beams overlap. Three different cases of polarizations are investigated: P polarization, S polarization and the case of Double Polarization Smoothing (DPS). It is found that the sizes of the speckles depend on the choice of the polarization and that DPS seems to be the best option in both configurations. It is shown that the longitudinal radius of the hot spots in a 2 cone configuration is bigger than in a 3 cone configuration.

International audience; The mechanisms of laser imprint reduction on a surface of a planar foil performed using an underdense foam are presented. The consequences on the Rayleigh–Taylor instability growth at the ablation front when the foil is accelerated are studied. The analysis is based on numerical simulations using a chain of codes: the electromagnetic paraxial code Parax provides the modifications of the intensity perturbation spectrum while the laser beam is crossing the foam. Two-dimensional axially symmetric simulations with the radiation hydrodynamic code CHIC describe the foam expansion and the foil dynamics. Finally, the perturbed flow calculations and the instability growth are investigated with the two-dimensional CHIC version in the planar geometry by using the initial and smoothed perturbation spectra. The dominant role of temporal laser smoothing during the time of foam crossing by the laser beam is demonstrated. Applications to the direct drive targets for inertial confinement fusion are discussed.

Reducing the detrimental effect of the Rayleigh-Taylor (RT) instability on the target performance is a critical challenge. In this purpose, the use of targets coated with low density foams is a promising approach to reduce the laser imprint. This article presents results of ablative RT instability growth measurements, performed on the OMEGA laser facility in direct-drive for plastic foils coated with underdense foams. The laser beam smoothing is explained by the parametric instabilities developing in the foam and reducing the laser imprint on the plastic (CH) foil. The initial perturbation pre-imposed by the means of a specific phase plate was shown to be smoothed using different foam characteristics. Numerical simulations of the laser beam smoothing in the foam and of the RT growth are performed with a suite of paraxial electromagnetic and radiation hydrodynamic codes. They confirmed the foam smoothing effect in the experimental conditions.

This paper investigates hot spot characteristics generated by the superimposition of multiple laser beams in the Laser MegaJoule configuration. First, properties of speckle statistics are studied in the context of the superimposition of several laser beams. The case of a single quadruplet is studied. Values of the speckle width and of the speckle length as well as of the abundance of the speckles are compared to the results given by numerical simulations. Application to the speckle patterns generated in the Laser MegaJoule configuration in the zone where all the beams overlap is then presented. The case of three different polarizations is investigated: P polarization, S polarization and the case of Double Polarization smoothing (DPS). It is found that the sizes of the speckles and their abundance depend on the choice of the polarization and that DPS seems to be the best option.

This Letter presents first experimental results of the laser imprint reduction in fusion scale plasmas using a low-density foam layer. The experiments were conducted on the LIL facility at the energy level of 12 kJ with millimeter-size plasmas, reproducing the conditions of the initial interaction phase in the direct-drive scheme. The results include the generation of a supersonic ionization wave in the foam and the reduction of the initial laser fluctuations after propagation through 500 {mu}m of foam with limited levels of stimulated Brillouin and Raman scattering. The smoothing mechanisms are analyzed and explained.

A statistical model for forward stimulated Brillouin scattering (FSBS) is developed for a spatially incoherent, monochromatic, laser beam propagating in a plasma. A threshold for the average power in a speckle is found, well below the self-focusing one, above which the laser beam spatial incoherence can not prevent the coherent growth of FSBS. Three-dimensional simulations confirm its existence and reveal the onset of beam spray above it. From these results, we propose a new figure of merit for the control of the propagation through a plasma of a spatially incoherent laser beam., Comment: submitted to PRL

The forthcoming laser installations related to inertial confinement fusion, Laser Mégajoule (LMJ) (France) and National Ignition Facility (NIF) (USA), require multidimensional numerical simulation tools for interpreting current experimental data and to perform predictive modeling for future experiments. Simulations of macroscopic plasma volumes of the order of 1 mm3 and laser exposure times of the order of hundreds of picoseconds are necessary. We present recent developments in the PARAX code towards this goal. The laser field is treated in a standard paraxial approximation in three dimensions. The plasma response is described by single-fluid, two-temperature, fully nonlinear hydrodynamical equations in the plane transverse to the laser propagation axis. The code also accounts for the dominant nonlocal transport terms in spectral form originating from a linearized solution to the Fokker–Planck equation. The simulations of interest are hohlraum plasmas in the case of indirect drive or the plasma corona for direct drive. Recent experimental results on plasma-induced smoothing of RPP laser beams are used to validate the code.

A strong reduction of the spatial coherence of a laser beam after its propagation through a plasma has been measured using a Fresnel biprism interferometer. The laser beam was diffraction limited; the coherence width was reduced from 40 mm in vacuum down to a few mm with the plasma. Numerical results based on a paraxial model exhibit a coherence degree close to the experimental one; they also prove the importance of taking into account the nonlocal transport effects in numerical simulations for such plasma conditions.

Understanding and mitigating hydrodynamic instabilities and the fuel mix are the key elements for achieving ignition in Inertial Confinement Fusion. Cryogenic indirect-drive implosions on the National Ignition Facility have evidenced that the ablative Rayleigh-Taylor Instability (RTI) is a driver of the hot spot mix. This motivates the switch to a more flexible higher adiabat implosion design [O. A. Hurricane et al., Phys. Plasmas 21, 056313 (2014)]. The shell instability is also the main candidate for performance degradation in low-adiabat direct drive cryogenic implosions [Goncharov et al., Phys. Plasmas 21, 056315 (2014)]. This paper reviews recent results acquired in planar experiments performed on the OMEGA laser facility and devoted to the modeling and mitigation of hydrodynamic instabilities at the ablation front. In application to the indirect-drive scheme, we describe results obtained with a specific ablator composition such as the laminated ablator or a graded-dopant emulator. In application to the direct drive scheme, we discuss experiments devoted to the study of laser imprinted perturbations with special phase plates. The simulations of the Richtmyer-Meshkov phase reversal during the shock transit phase are challenging, and of crucial interest because this phase sets the seed of the RTI growth. Recent works were dedicated to increasing the accuracy of measurements of the phase inversion. We conclude by presenting a novel imprint mitigation mechanism based on the use of underdense foams. The foams induce laser smoothing by parametric instabilities thus reducing the laser imprint on the CH foil.

The propagation of an ionization wave through a subcritical foam is studied under inertial confinement fusion conditions. Independent measurements of the ionization wave velocity are compared with hydrodynamic simulations and analytical models. It is shown that simulations of a homogeneous material at equivalent density strongly overestimate the front velocity. The internal foam structure can be accounted for with a simple model of foam homogenization that allows improving agreement between experiment and calculations.

This paper investigates a statistical model to describe the spatial and temporal properties of hot spots generated by the superimposition of multiple laser beams. In the context of the Laser MegaJoule design, we introduce the formula for contrasts, trajectories and velocities of the speckle pattern. Single bundle of four beams, two-cones and three-cones configurations are considered. Statistical properties of the speckle in the zone where all the beams overlap are studied with different configurations of polarizations. These properties are shown to be very different from the case of one single bundle of four beams. The configuration of polarization has only a slight effect in the two-cones or three cones configuration. Indeed, the impact of the double polarization smoothing is reduced in the area in which all the beams overlap, while it is much more significant when they split. Moreover, the size of the hot-spots decreases as the number of laser beams increases, but we show that their velocity decreases. ...

This paper investigates hot spot characteristics generated by the superposition of multiple laser beams. First, properties of speckle statistics are studied in the context of only one laser beam by computing the autocorrelation function. The case of multiple laser beams is then considered. In certain conditions, it is shown that speckles have an ellipsoidal shape. Analytical expressions of hot spot radii generated by multiple laser beams are derived and compared to numerical estimates made from the autocorrelation function. They are also compared to numerical simulations performed within the paraxial approximation. Excellent agreement is found for the speckle width as well as for the speckle length. Application to the speckle patterns generated in the Laser MegaJoule configuration in the zone where all the beams overlap is presented. Influence of polarization on the size of the speckles as well as on their abundance is studied.

An experiment carried out on the LIL facility is presented. The experiment was designed to investigate the propagation of a multi-kJ laser beam through a plasma produced by the irradiation of a low-density plastic foam. Laser beam self-smoothing has been observed and its consequences on laser imprint and Stimulated Brillouin Scattering (SBS) are discussed.

This paper deals with a statistical approach for description of the laser field interaction with underdense plasmas and modification of the laser beam temporal coherence during its propagation through a plasma at power well below the filamentation threshold. The main properties of the plasma density perturbations driven by a randomized laser beam are derived from a stochastic wave equation. The laser spectral and angular broadening is shown to occur on a distance that depends essentially on the ratio of the average power in a speckle to the critical power for filamentation. The coherence time of the transmitted light is reduced to the plasma acoustic time of response to the laser. It is typically a few picoseconds. Dedicated diagnostics have been developed for the interaction code PARAX in order to analyze the laser and plasma statistical properties. The effect of the plasma length on the transmitted light coherence is found to be in good agreement with theoretical predictions. Forward stimulated Brilloui...

We have developed a diagnostic simulation tool for the code PARAX to interpret recent measurements of far-field images of the laser light transmitted through a preformed plasma. This includes the complete treatment of the propagation of the light coming from a well-defined region of plasma through the rest of the plasma and all the optics of the imaging system. We have modeled the whole light path, as well as the spatio-temporal integration of the instruments, and the limited collecting aperture for the light emerging out of the plasma. The convolution of computed magnitudes with the plasma and diagnostics transfer functions is indispensable to enable the comparison between experiments and simulations. This tool is essential in the study of the propagation of intense laser beams in plasma media.