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Enhanced proton flux in the MeV range by defocused laser irradiation

Green, J. S. and Carroll, D. C. and Brenner, C. and Dromey, B. and Foster, P. S. and Kar, S. and Li, Y. T. and Markey, K. and McKenna, P. and Neely, D. and Robinson, A. P. L. and Streeter, M. J. V. and Tolley, M. and Wahlstrom, C-G and Xu, M. H. and Zepf, M. (2010) Enhanced proton flux in the MeV range by defocused laser irradiation. New Journal of Physics, 12. ISSN 1367-2630

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Abstract

Thin Al foils (50 nm and 6 mu m) were irradiated at intensities of up to 2x10(19) W cm(-2) using high contrast (10(8)) laser pulses. Ion emission from the rear of the targets was measured using a scintillator-based Thomson parabola and beam sampling 'footprint' monitor. The variation of the ion spectra and beam profile with focal spot size was systematically studied. The results show that while the maximum proton energy is achieved around tight focus for both target thicknesses, as the spot size increases the ion flux at lower energies is seen to peak at significantly increased spot sizes. Measurements of the proton footprint, however, show that the off-axis proton flux is highest at tight focus, indicating that a previously identified proton deflection mechanism may alter the on-axis spectrum. One-dimensional particle-in-cell modelling of the experiment supports our hypothesis that the observed change in spectra with focal spot size is due to the competition of two effects: decrease in laser intensity and an increase in proton emission area.