Tuning the electron energy by controlling the density perturbation position in laser plasma accelerators

A density perturbation 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.