Conditions for efficient and stable ion acceleration by moderate circularly polarized laser pulses at intensities of 1020 W/cm2

Conditions for efficient and stable ion radiation pressure acceleration (RPA) from thin foils by circularly polarized laser pulses at moderate intensities are theoretically and numerically investigated. It is found that the unavoidable decompression of the co-moving electron layer in Light-Sail RPA leads to a change of the local electrostatic field from a ''bunching'' to a ''debunching'' profile, ultimately resulting in premature termination of ion acceleration. One way to overcome this instability is the use of a multispecies foil where the high-Z ions act as a sacrificial species to supply excess co-moving electrons for preserving stable acceleration of the lower-Z ion species. It is shown by 2D particle-in-cell simulations that 100 MeV/u monoenergetic C6+ ion beams are produced by irradiation of a Cu-C-mixed foil with laser pulses at intensities 5 x 1020 W/cm2, which can be easily achieved by current day lasers.