We discuss the physical processes, which take place in a multi-component plasma set in expansion by a minority of energetic electrons. The expansion is in the form of a collisionless rarefaction wave associated with three types of electrostatic shocks. Each shock manifests itself in a potential jump and in the spatial separation of plasma species. The shock front associated with the proton-electron separation sets the maximum proton velocity. Two other shocks are due to the hot-cold electron separation and the light-heavy ion separation. They result in the light ion acceleration and their accumulation in the phase space. These structures open possibilities for control of the number and the energy spectrum of accelerated ions. Simple analytical models are confirmed in numerical simulations where the ions are described kinetically, and the electrons assume the Boltzmann distribution.

We present experimental results on ion acceleration with circularly polarized, ultrahigh contrast laser pulses focused to peak intensities of 5 Â 10 19 W cm À2 onto polymer targets of a few 10 nanometer thickness. We observed spatially and energetically separated protons and carbon ions that accumulate to pronounced peaks around 2 MeV containing as much as 6.5% of the laser energy. Based on particle-incell simulation, we illustrate that an early separation of heavier carbon ions and lighter protons creates a stable interface that is maintained beyond the end of the radiation pressure dominated acceleration process.