Approaching the radiation pressure regime of proton acceleration with high intensity lasers

• Main Author: Palmer, Charlotte Alexandra Jocelyn
• Published: Imperial College London 2012
• Subjects: 539.73
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.556643

This thesis presents experimental measurements, supported by particle-in-cell simulations, of ion beams accelerated by high intensity laser plasma interactions, spanning both the sheath acceleration and radiation pressure dominated regimes. For interactions of laser intensities rv 1020 W I cm? with micron thick gold targets rear surface sheath acceleration w~ seen to dominate. Proton beam spatial profiles revealed multiple concentric rings for target thicknesses ~ 20 tun. The number of rings increased with decreasing target thickness with no rings for thicknesses > 50 uu: It is postulated that these concentric rings stem from a larger number of recirculations of laser accelerated electrons through thinner targets. In following investigations structured proton beam profiles were observed from the interaction of nanometer scale targets at intensities of rv 1021 W Icm2. The most striking bubble-like structure, observed for the thinnest (5 nm) targets, closely resembles a Rayleigh- Taylor-like instability, a clear indication of radiation pressure driven acceleration. Both the experimental results and simulations also exhibit narrow energy spread features in the carbon ion spectra. Finally, a rv 5 ps, CO2 laser (A = 10.6 p,m) with intensity rv 1015 W Icm2 was focused onto an overdense hydrogen gas target. Transverse probing revealed a hole boring shock front associated with the first phase of radiation pressure acceleration. The accelerated proton beams contained up to 3 X 1012 protons/MeV [et, with energy spreads as low as r-;» 4 %. The ion energies scaled linearly with increasing integrated laser energy as expected from acceleration by hole boring.