Ion acceleration driven by intense laser pulses

Abstract

Laser pulses incident on plasma targets are capable of exciting very intense accelerating fields, that allow the acceleration of ions to high energies in very short distances. This is why a lot of interest has been developed on the topic of laser-driven ion acceleration over the past twenty years. Such a compact and affordable ion source would have many potential applications in physics and medicine, but several requirements are still far from being fulfilled. In this thesis two mechanisms of ion acceleration are investigated: shock wave acceleration and Coulomb explosion. Ultraintense lasers shot on plasma targets are capable of driving strong electrostatic shock waves that accelerate the plasma ions to high energies with a narrow energy spectrum. In the present work, the mechanism of shock formation and propagation in near-critical density plasmas is studied in detail. An idealized scenario where shock waves arise from the interpenetration of plasma slabs is studied. A theoretical kinetic model is derived and compared with simulation results. The conditions to accelerate ions to high energies with low energy spread are derived. The role of the laser in exciting shock waves is analyzed. The factors leading to high energy ion beams with narrow energy spectrum obtained in the simpler configuration are verified in this more complex and realistic scenario. A scaling for the ion energy with the pulse intensity is inferred for the ideal case of a plane wave and for a more realistic case of a finite size laser spot. The second mechanism of ion acceleration that has been considered is the Coulomb explosion of pure ion nanoplasmas, an important subject in the field of laser-cluster interaction. In this thesis, a detailed study of Coulomb explosion in hetero-nuclear clusters consisting of different atomic species is carried out. Numerical results indicate that, in the presence of different ion species, lighter ions are accelerated in a quasi-monoenergetic way, in contrast with the well known results on Coulomb explosion of clusters composed by a single ion species, where the energy spectrum is much broader. A study on the formation of shock shells, nonlinear structures that arises during Coulomb explosion of homo-clusters when the initial density exhibits radial non-uniformity, is also presented. The analysis is carried out comparing N-body simulation results, that represent the exact solution since no approximations have been made, to the collisionless kinetic theory. The study shows that there are consistent differences between the real dynamics and the model based on the Vlasov-Poisson equations