Stable GeV Ion-Beam Acceleration from Thin Foils by Circularly Polarized Laser Pulses

A stable relativistic ion acceleration regime for thin foils irradiated by circularly polarized laser pulses is suggested. In this regime, the “light-sail” stage of radiation pressure acceleration for ions is smoothly connected with the initial relativistic “hole-boring” stage, and a defined relationship between laser intensity I₀, foil density n₀, and thickness l₀ should be satisfied. For foils with a wide range of n₀, the required I₀ and l₀ for the regime are theoretically estimated and verified with the particle-in-cell code ILLUMINATION. It is shown for the first time by 2D simulations that high-density monoenergetic ion beams with energy above GeV/u and divergence of 10° are produced by circularly polarized lasers at intensities of 10²²  W/cm², which are within reach of current laser systems.