Bow-wake acceleration of ions toward a relativistic regime

Wakefield acceleration has been considered a possible mechanism for producing cosmic rays, including extremely high-energy ions, especially in astrophysical environments. In the laboratory, however, laser-driven wakefield acceleration of ions remains challenging because ions are much heavier than electrons. A central issue is how ions enter the accelerating field. One possible route with present laser facilities is to use dense plasma, in which the wake moves more slowly and ions can be trapped by the bow wake formed at the laser front. Here we experimentally demonstrate bow-wake acceleration of protons using resorcinol-formaldehyde foam targets. We obtain protons with energies up to 60 MeV. Three-dimensional particle-in-cell simulations support the interpretation that the protons are accelerated by the bow wake. These results show that dense plasma can enable ion injection into laser-driven wakes and provide a laboratory route toward bow-wake acceleration of high-energy ions.