Le caractère éminemment cinétique et hors équilibre de l'interaction laser-plasma et du transport électronique nécessite de résoudre le système complet des équations de Vlasov-Maxwell. Cette thèse se concentre sur les méthodes PIC (‘‘Particle-In-Cell''), et vise à en accroître le régime de fonctionnement. Tout d'abord, nous présentons l'analyse de stabilité linéaire d'un algorithme PIC explicite incluant l'effet de la discrétisation spatio-temporelle. Cette analyse met en exergue l'instabilité d'aliasing, que nous relions au problème, plus général, du chauffage numérique dans les codes PIC en régime surcritique. Nous montrons l'influence bénéfique de la montée en ordre du facteur de forme pour réduire ce chauffage, permettant ainsi d'atteindre des régimes de densité jusque là inaccessibles. Les codes PIC implicites ne sont pas soumis aux mêmes contraintes de stabilité que leurs équivalents explicites. En particulier nous ne sommes plus tenus de résoudre les modes haute fréquence électroniques. Une telle propriété est particulièrement précieuse lorsqu'on modélise l'interaction entre un laser à ultra-haute intensité et un plasma fortement sur-critique. Nous présentons ici l'extension relativiste de la méthode implicite dite directe, en y incluant un paramètre d'amortissement ajustable et des facteurs de forme d'ordre élevé. Ce formalisme a été implémenté dans le code ELIXIRS, 2D en espace et 3D en vitesse. Ce code est validé sur de nombreux problèmes de physique des plasmas, allant de l'expansion d'un plasma à une ou deux températures électroniques, à l'interaction laser-plasma à haut-flux, en passant par les instabilités ‘‘deux faisceaux'' et de filamentation en régime relativiste. Nous montrons notamment la capacité du code à capturer les principales caractéristiques de l'interaction laser-plasma, malgré une discrétisation spatio-temporelle dégradée, autorisant ainsi des gains substantiels en temps de calcul.

The work described in this thesis is concerned with the experimental investigation of the acceleration of high energy proton pulses generated by relativistic laser-plasma interaction and their application. Using the high intensity 150 TW Ti:sapphire based ultra-short pulse laser Draco, a laser-driven proton source was set up and characterized. Conducting experiments on the basis of the established target normal sheath acceleration (TNSA) process, proton energies of up to 20 MeV were obtained. The reliable performance of the proton source was demonstrated in the first direct and dose controlled comparison of the radiobiological effectiveness of intense proton pulses with that of conventionally generated continuous proton beams for the irradiation of in vitro tumour cells. As potential application radiation therapy calls for proton energies exceeding 200 MeV. Therefore the scaling of the maximum proton energy with laser power was investigated and observed to be near-linear for the case of ultra-short laser pulses. This result is attributed to the efficient predominantly quasi-static acceleration in the short acceleration period close to the target rear surface. This assumption is furthermore confirmed by the observation of prominent non-target-normal emission of energetic protons reflecting an asymmetry in the field distribution of promptly accelerated electrons generated by using oblique laser incidence or angularly chirped laser pulses. Supported by numerical simulations, this novel diagnostic reveals the relevance of the initial prethermal phase of the acceleration process preceding the thermal plasma sheath expansion of TNSA. During the plasma expansion phase, the efficiency of the proton acceleration can be improved using so called reduced mass targets (RMT). By confining the lateral target size which avoids the dilution of the expanding sheath and thus increases the strength of the accelerating sheath fields a significant increase of the proton energy and the proton yield was observed.

The work described in this thesis is concerned with the experimental investigation of the acceleration of high energy proton pulses generated by relativistic laser-plasma interaction and their application. Using the high intensity 150 TW Ti:sapphire based ultra-short pulse laser Draco, a laser-driven proton source was set up and characterized. Conducting experiments on the basis of the established target normal sheath acceleration (TNSA) process, proton energies of up to 20 MeV were obtained. The reliable performance of the proton source was demonstrated in the first direct and dose controlled comparison of the radiobiological effectiveness of intense proton pulses with that of conventionally generated continuous proton beams for the irradiation of in vitro tumour cells. As potential application radiation therapy calls for proton energies exceeding 200 MeV. Therefore the scaling of the maximum proton energy with laser power was investigated and observed to be near-linear for the case of ultra-short laser pulses. This result is attributed to the efficient predominantly quasi-static acceleration in the short acceleration period close to the target rear surface. This assumption is furthermore confirmed by the observation of prominent non-target-normal emission of energetic protons reflecting an asymmetry in the field distribution of promptly accelerated electrons generated by using oblique laser incidence or angularly chirped laser pulses. Supported by numerical simulations, this novel diagnostic reveals the relevance of the initial prethermal phase of the acceleration process preceding the thermal plasma sheath expansion of TNSA. During the plasma expansion phase, the efficiency of the proton acceleration can be improved using so called reduced mass targets (RMT). By confining the lateral target size which avoids the dilution of the expanding sheath and thus increases the strength of the accelerating sheath fields a significant increase of the proton energy and the proton yield was observed.

No description available.