Ion acceleration from laser-plasma interaction in underdense to near-critical regime: wakefield effects and associated plasma structures

This work endeavoured to demonstrate theoretically and experimentally the interest of a spectral analysis of the radial ion emission, coupled to the quantitative observation of the excited plasma structures, to grasp more clearly the phenomena occurring when a laser interacts with a plasma. The fine tuning of the laser and plasma parameters made it possible to parametrically study in details the interaction. When the plasma peak density is varied from under- to quasi-overdense, the ion distributions change from a peak-like to a maxwellian-like shape, and exhibit strong modulations close to the laser wakefield resonance. This resonance is obtained when the pulse duration is close to half the plasma period. In this case, the accelerating mechanism is different from Coulomb explosion due to laser wakefield effects and influence of the plasma sheath at the edge of the laser-bored channel. Besides, for a given peak density, when the gradient scale length is increased, ion acceleration is suppressed and fundamental electromagnetic structures (soliton/vortex) appear and they were optically identified for the first time. Finally, close to the critical density, the efficient laser self-focusing leads to a very localized energy deposit, that entails an ultrafast electron expansion (within one picosecond) and the growth of an intense magnetic dipole heating further the electrons. Thanks to an innovative experimental setup, optical studies of magnetic fields within these structures are from now on possible. For these studies, technical means and diagnostics have been developed and simultaneously tested with success. They consist, on the one hand, in submillimetric very dense gas jets for a localized, stable and reproducible investigation within a wide range of plasma densities and profiles, and, on the other hand, in interferometer and polarimeter with high spatial and temporal resolutions, to inspect the laser propagation, along with densities and fields evolution inside the plasma.