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 $\gtrsim {10}^9$, confined in a cone angle of $40°\pm 10°$ 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 back side in a direction normal to the target surface. Acceleration field gradients $\sim 10\mathrm{GeV}/\mathrm{cm}$ are inferred. The maximum proton energy can be explained by the charge-separation electrostatic-field acceleration due to “vacuum heating.”