Ion acceleration by ultra-intense lasers: Analysis of contrast effects

The temporal profile of a intense laser pulse impacts in various ways the physics and the evolution of the ion acceleration process. In most experimental cases, the amount of light that precedes the main intensity peak gets intense enough to initiate the ionization of the target; among the consequences, the modification of the target structure, the laser-plasma interaction and the dynamics of ion acceleration have been extensively investigated. In this contribution, we discuss the most important effects related to ion acceleration with ultra-intense, sub-picosecond laser pulses. The production of accelerated proton beams from laser-matter interaction has been firstly obtained by Gitomer [1], after the growing interest in the late seventies on the presence of ion signals of various species during the expansion of laser produced plasmas (see for instance [2]). Further studies have shown the presence of protons among the set of accelerated species, independently of the type of target material. In forthcoming decades, interest increased on the use of laser matter interaction at relativistic intensities as a way to produce accelerated bunches of charged particles. The main interest in the feasibility of plasma based accelerating structures resides in the fact that very high transient electrostatic fields can be held by the plasma medium (exceeding several TV=m), possibly opening the way to a new approach to the production of particle beams. The techniques for accelerating particle beams by laser interaction with solid targets have received a huge improvement during the past decade, made possible by the introduction and technological development of Ti:Sa (Titanium doped Sapphire.