Controlled electron injection into the wake wave using plasma density inhomogeneity

The electron injection, for the laser wake field accelerator, controlled through the plasma density inhomogeneity is studied on a basis of analytical estimates and two- and three-dimensional particle-in-cell simulations. The injection scheme requires a concordance of the density scale length and laser intensity. It is shown that at a sloping inhomogeneity of plasma the wave breaking produces stronger singularity of the electron density than at a density discontinuity, but develops slower. With the help of simulations for a moderate laser intensity, we demonstrate the optimal plasma density gradient, where the electron injection into the wake wave forms the electron beam with low divergence, small energy spread and high energy.