Forward ion acceleration in thin films driven by a high-intensity laser

A collimated beam of fast protons, with energies as high as 1.5 MeV and total number of greater than or equal to 10(9), confined in a cone angle of 40 degrees +/- 10 degrees is observed when a high-intensity high-contrast subpicosecond laser pulse is focused onto a thin foil target. The protons, which appear to originate from impurities on the front side of the target, are accelerated over a region extending into the target and exit out the hack side in a direction normal to the target surface. Acceleration field gradients similar to 10 GeV/cm are inferred. The maximum proton energy can be explained by the charge-separation electrostatic-field acceleration due to vacuum heating..