Efficient carbon ion beam generation from laser-driven volume acceleration
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Experimental data on laser-driven carbon C6+ ion acceleration with a peak intensity of 5 × 1020 W cm−2 are presented and compared for opaque target normal sheath acceleration (TNSA) and relativistically transparent laser–plasma interactions. Particle numbers, peak ion energy and conversion efficiency have been investigated for target thicknesses from 50 nm to 25µm using unprecedented full spectral beam profile line-out measurements made using a novel high-resolution ion wide-angle spectrometer. For thicknesses of about 200 nm, particle numbers and peak energy increase to 5 × 1011 carbon C 6+ particles between 33 and 700 MeV (60 MeV u−1 ), which is a factor of five higher in particle number than that observed for targets with micron thickness. For 200 nm thick targets, we find that the peak conversion efficiency is 6% and that up to 55% of the target under the laser focal spot is accelerated to energies above 33 MeV. This contrasts with the results for targets with micron thickness, where surface acceleration with TNSA is dominant. The experimental findings are consistent with two-dimensional particle-in-cell simulations.