Quasi-monoenergetic electron beams production in a sharp density   transition
  Using a laser plasma accelerator, experiments with a 80 TW and 30 fs laser pulse demonstrated quasi-monoenergetic electron spectra with maximum energy over 0.4 GeV. This is achieved using a supersonic He gas jet and a sharp density ramp generated by a high intensity laser crossing pre-pulse focused 3 ns before the main laser pulse. By adjusting this crossing pre-pulse position inside the gas jet, among the laser shots with electron injection more than 40% can produce quasi-monoenergetic spectra. This could become a relatively straight forward technique to control laser wakefield electron beams parameters. 
S. Fourmaux, K. Ta Phuoc, P. Lassonde, S. Corde, G. Lebrun, V. Malka,   A. Rousse, and J. C. Kieffer
physics.plasm-ph physics.acc-ph