Tuning the electron energy by controlling the density perturbation   position in laser plasma accelerators
  A density perturbation produced in an underdense plasma was used to improve the quality of electron bunches produced in the laser-plasma wakefield acceleration scheme. Quasi-monoenergetic electrons were generated by controlled injection in the longitudinal density gradients of the density perturbation. By tuning the position of the density perturbation along the laser propagation axis, a fine control of the electron energy from a mean value of 60 MeV to 120 MeV has been demonstrated with a relative energy-spread of 15 +/- 3.6%, divergence of 4 +/- 0.8 mrad and charge of 6 +/- 1.8 pC. 
P. Brijesh, C. Thaury, K. Ta Phuoc, S. Corde, G. Lambert, V. Malka, S.   P. D. Mangles, M. Bloom, S. Kneip
physics.plasm-ph physics.acc-ph