Collimated Ion Beam by a Laser-Illuminated Tailored Target

Abstract

Suppression of transverse proton beam divergence is demonstrated by a tailored thin foil target with a hole at the opposite side of laser illumination. When an intense short pulse laser illuminates the thin foil target with the hole, transverse edge effects of an accelerated electron cloud and an ion cloud are eliminated by a protuberant part of the hole: the edge effects of the electron cloud and the ion cloud induce the proton beam divergence. Therefore the transverse proton beam divergence is suppressed well. Firstly this paper presents the robustness of the hole target against laser parameter changes in a laser spot size and in a laser pulse length, and against a contaminated proton source layer. It may be also difficult to make the laser axis coincide with the target hole-center line in realistic experiments and uses, when the target has only one hole. 2.5-dimensional PIC (particle-in-eell) simulations also present that a multiple-hole target is robust against the laser alignment error and the target positioning error. The multi-hole target may serve a robust target for practical uses to produce a collimated proton beam. Recent researches have demonstrated acceleration of ions to high energies (>MeV) in an interaction between an intense laser pulse and a thin foil target. These ion beams are expected to be useful for basic particle physics, medical therapies, controlled nuclear fusion, high-energy sources, and so on. One of the important issues of ion beam parameters is a quality of the ion beam. When an intense laser illuminates a thin foil, electrons obtain energies from the laser pulse and cause an electric charge separation. These electrons, accelerated by the laser, form an electron cloud, and ions are accelerated by the strong electric field, which is produced by the electric charge separation. Ion energies of a few MeV or more have been already observed in recent experiments and numerical calculations. The quality of an ion beam is one of the critical issues in the ion beam generation. Mono-energetic ion beams are also generated in recent studies. On the other hand, a suppression of the transverse ion beam is also important. We propose a thin foil target with multiple holes at the opposite side of the laser illumination, in order to eliminate the transverse edge fields of the ion cloud and the electron cloud.