Controlled injection of electrons in a plasma wave

In a laser-plasma accelerator, an intense laser pulse drives a longitudinal plasma wave, which is able to accelerate electrons in shorter distances than in conventional accelerators. Recently, several groups have shown that laser plasma accelerators can produce high quality electrons beams with quasi monoenergetic energy distribution at the 100 MeV level. However in these experiments the mechanism responsible for injecting electrons in the plasma wave is highly nonlinear (self-focusing, self-compression, wavebreaking) which leads to undesirably high shot to shot variations.This paper demonstrates how this problem can be addressed by externally injecting the electrons in the plasma wakefield by using a second counterpropagating laser pulse, thus providing stable and controlled optical injection of electrons.