Recently, it has been observed that intense, collimated beams of multi-MeV protons were generated in the forward direction by intense short-pulse lasers irradiating thin solid foils at the Laboratoire pour l’Utilisation des Lasers Intenses(LULI) of CNRS. This discovery of an intense (~10^11 protons/shot with a maximum energy of ~25 MeV), pulsed (duration less than 1 ps), laminar source of protons has opened new and fascinating perspectives.
he production of such high current protons and ions beams have been made possible by short-pulse, ultra-high intensity lasers (laser pulse duration less than 1 ps, power density more than 10^18 W/cm^2 um^2) such as the one at LULI(see photo on the right showing the compressor and target chamber). There are several mechanisms that can lead to laser-acceleration of protons in the forward direction. In experiments performed at LULI, it has been unambiguously demonstrated that rear-surface acceleration is the predominant mechanism to get protons of high energy. In this mechanism, relativistic electrons generated at the laser-irradiated interface by the intense laser and propagated through the target form on the non-irradiated rear-surface of the target a dense electron plasma sheath. Its strong (~TV/m) electric field can ionize atoms and rapidly accelerates ions normal to the initially unperturbed surface. The accelerated protons stem either from hydrogen-containing contaminants (e.g. water vapour) that are present on all target surfaces or from prepared layers.
J. Fuchs, et al,Laser-driven proton scaling laws and new paths towards energy increase, Nature Physics 2, 48-54 (2006).
Laser Interaction group of LULI and Ecole polytechnique