We study proton acceleration from a foil target with a transversely varying density using multi-dimensional Particle-in-Cell (PIC) simulations. In order to reduce electron heating and deformation of the target, circularly polarized Gaussian laser pulses at intensities on the order of 1022 Wcm−2 are used. It is shown that when the target density distribution fits that of the laser intensity profile, protons accelerated from the center part of the target have quasi-monoenergetic spectra and are well collimated. In our two-dimensional PIC simulations, the final peak energy can be up to 1.4 GeV with the full-width of half maximum divergence cone of less than 4°. We observe highly efficient energy conversion from the laser to the protons in the simulations.

Monoenegetic ion beam generation from circularly polarized laser-pulse interaction with a double-layer target is considered. The front layer consists of heavy-ion plasma, and the rear layer is a small thin coating of light-ion plasma. Particle-in-cell simulation shows that the multi-dimensional effects in the ion radiation pressure acceleration are avoided and a highly monoenergetic light-ion beam can be produced. Our simulations reveal that the charge-mass ratio of heavy ions in the front layer and the thicknesses of both layers can strongly affect the proton energy spectra.