A novel regime is proposed where, by employing linearly polarized laser pulses at intensities
1021 Wcm2 (2 orders of magnitude lower than discussed in previous work [T. Esirkepov et al., Phys.
Rev. Lett. 92, 175003 (2004)]), ions are dominantly accelerated from ultrathin foils by the radiation pressure
and have monoenergetic spectra. In this regime, ions accelerated from the hole-boring process quickly catch
up with the ions accelerated by target normal sheath acceleration, and they then join in a single bunch,
undergoing a hybrid light-sail–target normal sheath acceleration. Under an appropriate coupling condition
between foil thickness, laser intensity, and pulse duration, laser radiation pressure can be dominant in this
hybrid acceleration. Two-dimensional particle-in-cell simulations show that 1.26 GeV quasimonoenergetic
C6þ beams are obtained by linearly polarized laser pulses at intensities of 1021 Wcm2.