Buffered spectrally-peaked proton beams in the relativistic-transparency   regime
  Spectrally-peaked proton beams ($E_{p}\approx 8$ MeV, $\Delta E\approx 4$ MeV) have been observed from the interaction of an intense laser ($> 10^{19 }$ Wcm$^{-2}$) with ultrathin CH foils, as measured by spectrally-resolved full beam profiles. These beams are reproducibly generated for foil thicknesses (5-100 nm), and exhibit narrowing divergence with decreasing target thickness down to $\approx 8^\circ$ for 5 nm. Simulations demonstrate that the narrow energy spread feature is a result of buffered acceleration of protons. Due to their higher charge-to-mass ratio, the protons outrun a carbon plasma driven in the relativistic transparency regime. 
N. P. Dover, M. J. V. Streeter, C. A. J. Palmer, H. Ahmed, B.   Albertazzi, M. Borghesi, D. C. Carroll, J. Fuchs, R. Heathco...
physics.plasm-ph