  Experiments on ion acceleration by irradiation of ultra-thin diamond-like
carbon (DLC) foils, with thicknesses well below the skin depth, irradiated with
laser pulses of ultra-high contrast and linear polarization, are presented. A
maximum energy of 13MeV for protons and 71MeV for carbon ions is observed with
a conversion efficiency of > 10%. Two-dimensional particle-in-cell (PIC)
simulations reveal that the increase in ion energies can be attributed to a
dominantly collective rather than thermal motion of the foil electrons, when
the target becomes transparent for the incident laser pulse.
