Le contexte général de cette étude est la fusion thermonucléaire contrôlée par Confinement Inertiel (FCI) et, plus particulièrement, l'Allumeur Rapide (Fast Ignitor). Dans ce schéma la maîtrise de la génération et du transport des électrons sont vitaux. Cette thèse est une étude expérimentale de la génération et du transport des électrons rapides créés lors de l'interaction d'un laser ultra-intense (>= 10 19 W/cm2) avec une cible solide. Le diagnostic principal utilisé au cours de cette thèse est le rayonnement de transition. Ce rayonnement dépend des propriétés des électrons qui le produisent et donne des informations importantes sur ces électrons: leur énergie, température, géométrie de propagation, etc. L'analyse spectrale, spatiale et temporelle de ce rayonnement a permis de mettre en évidence l'accélération de paquets périodiques d'électrons qui dans ce cas émettent un rayonnement de transition cohérent (CTR - Coherent Transition Radiation). Nous avons développé des modèles théoriques au cours de cette thèse pour expliquer les résultats expérimentaux. On trouve ainsi deux types de paquets d'électrons émis périodiquement à la fréquence du laser (w0) et au double de la fréquence laser (2w0), mettant en jeu différents mécanismes d'accélération: vacuum heating/absorption résonnante et vxB respectivement. Ces paquets sont également observés par des simulations PIC. La température de ces électrons est de ~ 2 MeV dans nos conditions expérimentales. Ils sont émis à partir d'un point source qui est la tache focale du laser et se propagent de façon balistique; ils peuvent, dans certains cas, être réinjectés dans la cible par un phénomène de re-circulation. Cependant ce diagnostic n'est sensible qu'aux électrons cohérents et relativistes, ce qui explique la faible partie d'énergie totale qu'ils emportent (~ quelques mJ). Le CTR de ces électrons supra-thermiques domine largement le rayonnement émis par les électrons moins énergétiques qui emportent la majorité de l'énergie.

Ce manuscrit présente une nouvelle formulation de l’Interaction Laser-Plasma (ILP) à l’échelle hydrodynamique, qui couple la dynamique du plasma avec les processus d’ILP linéaires et non-linéaires. Le modèle standard du tracé de rayon (Ray-Tracing), basé sur l’Optique Géométrique, est peu adapté pour modéliser l’ILP non-linéaire car la distribution de l’intensité laser dans le plasma n’est pas directement disponible. Nous proposons un modèle alternatif spécifiquement formulé pour un code hydrodynamique Lagrangien, basé sur l’Optique Géométrique Complexe Paraxiale qui décrit la propagation de faisceaux Gaussiens. Cette méthode est ensuite adaptée à la description de faisceaux laser non Gaussiens, et permet de reproduire la statistique d’intensité, l’enveloppe et le contraste de faisceaux lissés par une Lame de Phase. Nous proposons des modèles en ligne pour décrire l’échange d’énergie entre faisceaux croisés (CBET) et la génération d’électrons rapides par l’ILP non-linéaire, en utilisant PCGO. Le modèle en ligne de CBET est validé par comparaison avec un code de propagation d’une onde électromagnétique paraxial conventionnel dans le cas d’un plasma inhomogène en vitesse. Un bon accord est trouvé après une période transitoire de l’ordre de la picoseconde, notamment en ce qui concerne la distribution spatiale de l’intensité laser et des perturbations de densité du plasma. Ce modèle appliqué à une configuration d’attaque directe de Fusion par Confinement Inertiel (FCI) montre que le CBET réduit le couplage laser-cible, réduit le facteur de convergence, et amplifie les modes basse fréquence de déformation de la capsule. Le modèle de génération d’électrons rapides par l’ILP non-linéaire modélise les propriétés des faisceaux d’électrons rapides, i.e. leur flux, énergie moyenne, dispersions angulaire et direction, à partir de l’intensité laser prédite par PCGO et à partir d’expressions simplifiées, basées sur des modèles théoriques et des lois d’échelles obtenues à l’aide de simulations cinétiques. La propagation et le dépôt d’énergie par les électrons rapides est décrite à partir d’une approximation de diffusion angulaire adaptée en deux dimensions, pour des faisceaux de profil transverse d’intensité Gaussien, de distribution d’énergie exponentielle et d’ouverture angulaire arbitraire. Ce modèle couplé rend compte de (i) la compétition pour l’énergie laser entre les différentes instabilités et avec l’absorption collisionnelle, (ii) le couplage entre l’ILP non-linéaire et la dynamique du plasma à travers les faisceaux d’électrons rapides, et(iii) la perte de couplage laser-plasma due à la diffusion Raman arrière. Les performances de ce modèle sont évaluées par comparaisons avec des expériences d’allumage par choc conduites sur les installations laser Omega et Pals. Ce modèle multi-échelle est ensuite utilisé pour interpréter plusieurs expériences. On trouve notamment que les électrons générés par l’ILP non-linéaire augmentent la vitesse du choc et la pression en aval de ce dernier, tout en réduisant sa force et la pression d’ablation. Une application à la phase fortement non-linéaire de l’allumage par choc en FCI suggère que ces électrons sont néfastes pour l’implosion de la capsule en ce qui concerne les cibles conventionnelles : ceux-ci causent une augmentation de la masse du point chaud et des pertes radiatives. Ce modèle peut être appliqué à la modélisation hydrodynamique des expériences laser-cible de physique des hautes densités d’énergie pour les régimes d’interaction pertinents pour les instabilités évoquées ci dessus. / This manuscript presents a novel formulation of the Laser-Plasma Interaction (LPI) at hydrodynamical scales, that couples the plasma dynamics with linear and nonlinear LPI processes. The standard Ray Tracing model, based on Geometrical Optics, is not well suited for that purpose because it does not readily describe the laser intensity distribution in plasma. We propose an alternative model formulated for a Lagrangian hydrodynamic code. It is based on the ray-based Paraxial Complex Geometrical Optics (PCGO) that describes Gaussian optical beamlets. A method for modeling non-Gaussian laser beams smoothed by Phase Plates is presented, that allows to create intensity variations that reproduce the beam envelope, contrast and high-intensity statistics predicted by paraxial laser propagation codes. We propose inline reduced models for the non-linear laser-plasma interaction, in the case of the Cross-Beam Energy Transfer (CBET) and the generation of Hot Electrons (HE). The inline CBET model is validated against a time-dependent conventional paraxial electromagnetic wave propagation code, in a well-defined plasma configuration with density and velocity profiles corresponding to an inhomogeneous plasma. Good agreement is found past a transient period on the picosecond time scale, notably for the spatial distribution of density perturbations and laser intensities in the interaction region. Application of the model to a direct-drive Inertial Confinement Fusion (ICF) configuration shows that CBET significantly degrades the irradiation symmetry by amplifying low frequency modes and reducing the laser-capsule coupling efficiency, ultimately leading to large modulations of the shell areal density and lower convergence ratios. The LPI/HE model predicts the HE fluxes, temperatures, angular dispersion and direction from the laser intensity of PCGO beamlets from simplified expressions based on theoretical models and scaling laws obtained in kinetic simulations. The HE beams propagation and energy deposition in plasma is described in the angular scattering approximation, adapted to two-dimensional, transversally Gaussian, multigroup HE beams of arbitrary angular distribution. This model accounts for (i) competition for the laser energy between the various instabilities and with the linear collisional absorption, (ii) coupling between nonlinear LPIs and plasma dynamics via the high energy electron beams and(iii) loss of coupling due to backscattered Raman light. Its performance is confirmed by comparison with measurements of shock timing, laser absorption, HE fluxes and temperatures in experiments conducted on Omega and Pals laser facilities. This multiscale inline LPI-HE model is used to interpret several Shock Ignition experiments. It is found that HEs from parametric instabilities significantly increase the shock pressure and velocity in the target, while decreasing its strength and the overall ablation pressure. Applications to the high-intensity regime of shock ignition ICF suggest that HEs generated by the nonlinear LPI are nefarious to the capsule implosion in conventional target designs, as they lead to a dramatic increase in the hotspot mass and losses by Bremsstrahlung radiation. This model is readily applicable to hydrodynamic description of laser-target experiments of High Energy Density Physics, in the interaction regimes involving the above-mentioned non-linear LPI processes.

School science education prepares learners to study science at a higher level, prepares them to follow a career in science and to become scientific literate citizens. It is the responsibility of the educator to ensure the learners’ conceptual framework is developed to the extent that secures success at higher level studies. The purpose of this study was to test the effectiveness of conceptual change teaching strategies on the conceptual development of grade 11 learners on Newton’s second law of motion. The two strategies employed were the cognitive conflict strategy and the development of ideas strategy. A sequential explanatory mixed-method research design was used during this study. The qualitative data were used to elucidate the quantitative findings. The quantitative research consisted of a quasi-experimental design consisting of a single-group pre-test–post-test method. During the qualitative part of the research a phenomenological research approach was utilised to gain a better understanding of participants’ learning experiences during the intervention. The quantitative research made use of an adapted version of the Force Concept Inventory (FCI). The data collected from the pre-test were used to inform the intervention. The intervention was videotaped and the video analysis or qualitative data analysis was done. After the intervention the post-test was written by the learners. Hake’s average normalised learning gain <g> from pre- to post-scores was analysed to establish the effectiveness of the intervention. The two sets of results (quantitative and qualitative) were integrated. Information from the qualitative data analysis was used to support and explain the quantitative data. The quantitative results indicate that there was an improvement in the students’ force conception from their initial alternative conceptions, such as that of an internal force. Especially the learners’ understanding of contact forces and Newton’s first law of motion yielded significant improvement. The qualitative data revealed that the understanding of Newton’s second law of motion by the learners who partook in this study did improve, since the learners immediately recognised the mistakes made when confronted with the anchor concept. The cognitive conflict teaching strategy was effective in establishing the anchor concept of force which proved to be useful as bridging concept in the development of ideas teaching strategy. The data from both datasets revealed that the cognitive conflict teaching strategy for the initial part of the intervention was effective. It was evident that for development of the idea teaching strategy the two data sets revealed mixed results. Recommendations were made for future research and implementation of conceptual development teaching strategies. / MEd (Natural Sciences Education), North-West University, Potchefstroom Campus, 2014

School science education prepares learners to study science at a higher level, prepares them to follow a career in science and to become scientific literate citizens. It is the responsibility of the educator to ensure the learners’ conceptual framework is developed to the extent that secures success at higher level studies. The purpose of this study was to test the effectiveness of conceptual change teaching strategies on the conceptual development of grade 11 learners on Newton’s second law of motion. The two strategies employed were the cognitive conflict strategy and the development of ideas strategy. A sequential explanatory mixed-method research design was used during this study. The qualitative data were used to elucidate the quantitative findings. The quantitative research consisted of a quasi-experimental design consisting of a single-group pre-test–post-test method. During the qualitative part of the research a phenomenological research approach was utilised to gain a better understanding of participants’ learning experiences during the intervention. The quantitative research made use of an adapted version of the Force Concept Inventory (FCI). The data collected from the pre-test were used to inform the intervention. The intervention was videotaped and the video analysis or qualitative data analysis was done. After the intervention the post-test was written by the learners. Hake’s average normalised learning gain <g> from pre- to post-scores was analysed to establish the effectiveness of the intervention. The two sets of results (quantitative and qualitative) were integrated. Information from the qualitative data analysis was used to support and explain the quantitative data. The quantitative results indicate that there was an improvement in the students’ force conception from their initial alternative conceptions, such as that of an internal force. Especially the learners’ understanding of contact forces and Newton’s first law of motion yielded significant improvement. The qualitative data revealed that the understanding of Newton’s second law of motion by the learners who partook in this study did improve, since the learners immediately recognised the mistakes made when confronted with the anchor concept. The cognitive conflict teaching strategy was effective in establishing the anchor concept of force which proved to be useful as bridging concept in the development of ideas teaching strategy. The data from both datasets revealed that the cognitive conflict teaching strategy for the initial part of the intervention was effective. It was evident that for development of the idea teaching strategy the two data sets revealed mixed results. Recommendations were made for future research and implementation of conceptual development teaching strategies. / MEd (Natural Sciences Education), North-West University, Potchefstroom Campus, 2014