ED Sciences Physiques et de l'Ingénieur
Investigating in the laboratory particle/magnetized plasma energy exchange processes relevant for astrophysics
by Tessa WALTENSPIEL (Centre Lasers Intenses et Applications)
The defense will take place at 14h00 - Amphithéâtre Becquerel École Polytechnique, Route de Saclay 91120 Palaiseau
in front of the jury composed of
- Emmanuel D'HUMIERES - Professeur - Université de Bordeaux - Directeur de these
- Patrizio ANTICI - Professeur - Institut National de la Recherche Scientifique - CoDirecteur de these
- Alexandre MARCOWITH - Directeur de recherche - Université de Montpellier - Rapporteur
- Bruno ALBERTAZZI - Chargé de recherche - CNRS LULI - Examinateur
- François VIDAL - Professeur - Institut National de la Recherche Scientifique - Examinateur
- Claire MICHAUT - Directrice de recherche - Observatoire de la Côte d'Azur - Rapporteur
The direct in situ observation in space of magnetized plasma interactions and particle energy exchange processes remain difficult nowadays, even with satellites orbiting around Earth or spacecrafts traveling within the Solar System. Indeed, these mechanisms are succinct (lasting only a few hours at best), very spatially localized and it is almost impossible to predict them. As a consequence, the rise of laser technology since the middle of the XXth century that lead to the development of the high power lasers enables to recreate these phenomena in the laboratory but at reduced scales. This is the framework of Laboratory Astrophysics with high power lasers. One of the main characteristics of the astrophysical magnetic fields is the magnetization of initially unmagnetized plasmas. This spontaneous magnetization can result from various mechanisms set off by the inner dynamic of the plasma or the ambient field of a specific medium. One of these mechanisms is the Biermann battery effect, where non-collinear density and temperature gradients within the plasma induce self-generated seed magnetic field. This field can be advected by plasma flows and be involved in perturbations between magnetized plasmas, from which important variations of field amplitude can result. Consequently, the Biermann battery effect can entail strong magnetic fields observed in a wide range of astrophysical objects and environments, including in regions initially devoid of primordial magnetic field. Our study focused on the interactions between magnetic fields in plasmas that are only relying on the Biermann battery effect. Our aim for this thesis was to study in the laboratory, using high power lasers, two fundamental processes between magnetized plasmas : magnetic compression and magnetic reconnection. These mechanisms, whose apparitions depend on the relative orientation and polarity of the involved magnetic fields, are both subject of intense researches as they are suspected of contributing to both plasma heating and particle energization. These effects are observed in various astrophysical events, such as in solar arches or within the interaction between the magnetic field carried by the solar wind and the Earth magnetosphere. Our study focused on the characteristic aspects of these magnetized processes and their evolution, detailing mechanisms such as the saturation of the compressed magnetic field during magnetic compression or the fragmentation of the reconnection sheet due to the apparition of inner tearing instabilities. Therefore, we developed an experimental setup to investigate these two processes in the laboratory. Our work relies partially on previous studies in order to deepen the understanding of these works, but also introduces new specificities, more particularly to focus on interactions between Biermann battery self-generated magnetic fields or to introduce more complex 3D configurations allowing to go further than ideal 2D topologies that were already investigated in the laboratory. In this work, we propose new elements of comprehension about these plasma phenomena, whose detailed studies are achievable in the laboratory. We present new results which contribute to broaden our understanding for the overall question of the production of highly energetic particles in the Universe.