Generation of low-divergence megaelectronvolt ion beams from thin foil irradiated with an ultrahigh-contrast laser

Megaelectronvolt (MeV) ion beams with low divergence (10 degrees) are experimentally generated from a thin foil irradiated by an ultrahigh-contrast laser at a peak intensity of similar to 10(18) W/cm(2). The high-contrast (similar to 10(11)) laser is obtained with a pulse cleaner based on noncollinear optical-parametric amplification and second-harmonic generation processes. The effects of the foil density, foil thickness, as well as the density gradients at the front and back sides of the foil are investigated with two-dimensional particle-in-cell simulations. The beam parameters of maximum energy and divergence strongly depend on the density gradients at the back side of the foil. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4766439]