Measurement of Stimulated Raman Side-Scattering Predominance and Energetic Importance in the Compression Stage of the Double-Cone Ignition Approach to Inertial Confinement Fusion

Due to its particular geometry, stimulated Raman side-scattering (SRSS) drives scattered light emission at non-conventional directions, leading to scarce and complex experimental observations. Experimental campaigns at the SG-II UP facility have measured the scattered light driven by SRSS over a wide range of angles, showing an emission at large polar angles, sensitive to the plasma profile and laser polarization. Furthermore, direct comparison with back-scattering measurement has evidenced SRSS as the dominant Raman scattering process in the compression stage, leading to the scattering loss of about 5\% of the total laser energy. The predominance of SRSS was confirmed by 2D particle-in-cell simulations, and its angular spread has been corroborated by ray-tracing simulations. The main implication is that a complete characterization of the SRS instability and an accurate measurement of the energy losses require the collection of the scattered light in a broad range of directions. Otherwise, spatially limited measurement could lead to an underestimation of the energetic importance of stimulated Raman scattering

Advantages to a diverging Raman amplifier

The plasma Raman instability can efficiently compress a nanosecond long high-power laser pulse to sub-picosecond duration. Although, many authors envisaged a converging beam geometry for Raman amplification, here we propose the exact opposite geometry; the amplification should start at the intense focus of the seed. We generalise the coupled laser envelope equations to include this non-collimated case. The new geometry completely eradicates the usual trailing secondary peaks of the output pulse, which typically lower the efficiency by half. It also reduces, by orders of magnitude, the initial seed pulse energy required for efficient operation. As in the collimated case, the evolution is self similar, although the temporal pulse envelope is different. A two-dimensional particle-in-cell simulation demonstrates efficient amplification of a diverging seed with only 0.3 mJ energy. The pulse has no secondary peaks and almost constant intensity as it amplifies and diverges

Measurement of magnetic cavitation driven by heat flow in a plasma

We describe the direct measurement of the expulsion of a magnetic field from a plasma driven by heat flow. Using a laser to heat a column of gas within an applied magnetic field, we isolate Nernst advection and show how it changes the field over a nanosecond timescale. Reconstruction of the magnetic field map from proton radiographs demonstrates that the field is advected by heat flow in advance of the plasma expansion. This changes the dynamics of high energy density plasmas, in which heat flows and fields are strongly coupled, and may disrupt magnetised inertial confinement fusion schemes

Étude du comportement collectif des speckles dans le développement de la diffusion Raman stimulée lors de l’interaction laser-plasma

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

Measurement of magnetic cavitation driven by heat flow in a plasma

We describe the direct measurement of the expulsion of a magnetic field from a plasma driven by heat flow. Using a laser to heat a column of gas within an applied magnetic field, we isolate Nernst advection and show how it changes the field over a nanosecond timescale. Reconstruction of the magnetic field map from proton radiographs demonstrates that the field is advected by heat flow in advance of the plasma expansion. This changes the dynamics of high energy density plasmas, in which heat flows and fields are strongly coupled, and may disrupt magnetised inertial confinement fusion schemes.Comment: 5 pages, 4 figure