Scaling laws for electron cold injection in the narrow collision pulse approximation

Abstract

The latest advances in laser wakefield electron acceleration show a better beam quality, but much progress is still needed concerning the control and tunability of the electron beam. The recently proposed cold injection scheme offers a solution to this problem. It involves the use of two counter-propagating laser pulses to dephase a certain number of electrons into the wakefield of the main pulse, so that they are accelerated to high energies. As circular polarization is used, there is no stochastic heating and the injection process becomes much easier to model. We show that cold injection can be reduced to a one-dimensional problem in the case of a narrow collision pulse. The dephasing process can then be seen as competition between the longitudinal ponderomotive force of the main pulse and the stationary beatwave force arising from collision of the two circularly polarized lasers. This analysis leads to scaling laws for cold injection in the narrow collision pulse approximation and to a condition for its realization. Three-dimensional particle-in-cell simulations support both these scaling laws and the condition for cold injection to occur.