Laser-driven quasimonoenergetic proton burst from water spray target

Abstract

A narrow band proton bursts at energies of 1.6±0.08 MeV were observed when a water spray consisting of (150 nm) -diameter droplets was irradiated by an ultrashort laser pulse of about 45 fs duration and at an intensity of 5× 1019 W/ cm2. The results are explained by a Coulomb explosion of sub-laser-wavelength droplets composed of two ion species. The laser prepulse plays an important role. By pre-evaporation of the droplets, its diameter is reduced so that the main pulse can interact with a smaller droplet, and this remaining bulk can be ionized to high states. In the case of water, the mixture of quite differently charged ions establishes an "iso-Coulomb-potential" during the droplet explosion such that protons are accelerated to a peak energy with a narrow energy spread. The model explains this crucial point, which differs critically from usual Coulomb explosion or ion sheath acceleration mechanisms. © 2010 American Institute of Physics.