14 Décembre – Thesis defense - Adrien Pineau

14 h Amphi D - Building A29 (University of Bordeaux - Talence campus)

Modeling of laser-induced plasma formation within the framework of inertial confinement fusion.

Laser direct-drive inertial confinement fusion (ICF) aims at imploding a deuterium-tritium target with a large number of laser beams in order to create nuclear fusion reactions. To minimize the fuel ablation, the target is coated with a plastic layer (usually polystyrene which is a dielectric material), named ablator, which its thickness is about several tens of micrometers. The implosion is optimized with the spatial and temporal shaping of the laser pulse, in particular with a pre-pulse which the duration is about one hundred of picoseconds. However, the interaction between this pre-pulse and the ablator can create spatial inhomogeneities of density and pressure at the surface of the ablator (this is the so-called laser imprint process) leading to Rayleigh-Taylor instabilities lowering the efficiency of the target implosion. In particular, it has been shown experimentally that the initial solid state of the ablator can amplify these inhomogeneities. In addition, the hydrodynamic radiative codes dedicated to ICF simulation assume that the target, including the ablator, is in a plasma state initially.
The goal of this thesis is thus to model the transition from the solid state to the plasma state of a polystyrene ablator induced by a laser pre-pulse caracteristic of ICF. To do so, the variations of the free electron density and the variations of electron and lattice/ion temperatures are described, including the polystyrene fragmentation. The evolution of the electron density is mainly based on photo-ionisation, impact ionisation and electron recombination. The evolution of the temperatures is driven by the laser absorption and the energy transfer from the electrons to the lattice or the ions depending on the solid or plasma state of the ablator. These two processes are due to electron collisions which are desbribed in detail. This modeling of the solid-to-plasma transition is then coupled to the Helmholtz equation through the dielectric permittivity of the ablator in order to add one spatial dimension and described the laser propagation. The cases of homogeneous ablator and foam (enabling to mitigate the laser imprint) have been studied. The results show that the caracteristic timescale of the solid-to-plasma transition is about one hundred of picoseconds which is not negligible compared the timescales of ICF. We also show that the laser energy that is transmitted at the early time could modify the internal state of the target.

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