Electron acceleration by colliding laser beams in plasmas

All-optical controlled injection and acceleration of electrons in a laser wakefield has recently been achieved (Faure et al, Nature v. 444 p. 737, 2006). Injection was made using a second counterpropagating laser pulse with the same polarization as the main pulse . In this scheme, the interference pattern formed during the collision between the pulses pre-accelerates electrons that are then trapped in the wakefield. Numerical simulations of this process performed with a new Particle-in-Cell code are presented here. The results show the nature of the injection mechanism and explain some striking features found experimentally, such as the dependence of beam energy with the collision position and the reduction of the charge at high energies. Simulations reproduce well the main features of the experimental observations.