Theoretical studies of collisionless shocks for laser-acceleration of ions

摘要

Recently, strong effort has been done in exploring shock acceleration for the generation of highly energetic ion beams, with applications e.g. for medical purposes. The heating of a near-critical density plasma target with a laser, increases the electron temperature and excites ion acoustic waves, which can lead to electrostatic shock formation due to non-linear wave breaking. The higher inertia background ions are reflected and accelerated at the shock potential, showing a quasi-monoenergetic profile. For the first time, its feasibility has been demonstrated experimentally, gaining 20 MeV protons with a very narrow energy spread and a predicted scaling up to 200 MeV for lasers with a_0 = 10. In the quest for high proton energies, optimal conditions for shock formation have to be found. We developed a relativistic model that connects the initial parameters with the steady state shock Mach number, which is based on the Sagdeev approach, showing an increase of the ion energy for high upstream electron temperatures and low downstream to upstream density ratios and high temperature ratios, which has been confirmed by particle-in-cell simulations. In the context of producing a quasi-monoenergetic beam profile, we studied the enhancement of the Weibel instability in an electrostatic shock setup. Governing parameter regimes for the transition to an electromagnetic shock, which is associated with a broadening of the ion spectrum, were determined analytically and confirmed with simulations.