Characterisation and Manipulation of Proton Beams Accelerated by Ultra-Short and High-Contrast Laser Pulses

Abstract

Laser-driven acceleration of particle beams is a burgeoning field of research, based on the possibility of creating ultra-large electric fields in plasma, largely exceeding current limits of conventional acceleration technology. Research in this area has led to very significant progression regarding the acceleration of electrons and ions. Laser-driven ion sources have unique properties: high brightness (about 1013 protons/ions per shot), high current (in kiloAmpere range), ultra-low emittance, and short pulse duration (less than 1 ps), opening prospects for a broad range of applications. Recent advances in the laser technology have led to even further enhancements in the provision of extremely short and high-intensity pulses, which can further improve the accelerated ion beam specifications particularly regarding maximum ion energy and ion flux which are demanding most of the potential applications. These developments have stimulated an emergence of advanced diagnostics for measuring complex plasma effects. The absorption mechanisms of laser radiation at the target are the basic processes, which indeed defines the whole ion acceleration scenario. For laser intensities, where the classical normalized momentum of electrons quivering in the laser electric field: a = 8.53×10−10 (μm) I1/2 (W/cm2) > 1, the electrons become relativistic and the effect of the laser magnetic field is no longer negligible. The perpendicular component of the Lorentz force ev×B couples with the electric force to drive the electrons in the laser propagation direction (Wilks et al., 1992, Lefebvre & Bonnaud, 1997, Kruer & Estabrook, 1985) in contrast to inverse bremsstrahlung and resonance absorption, which causes the quiver motion of the electrons in the laser field. The ponderomotive force drives the electrons with a stepor plateau-like density profile and has a strong directionality along the laser propagation direction. Electron temperatures about 1 MeV have been measured (Malka & Miquel, 1996). The laser energy transfer to the hot electrons can also be out carried by fast plasma waves through the nonlinear ponderomotive force (Tajima & Dawson, 1979) and by laser field itself (Pukhov et al., 1999).