Abstract
The present paper elaborates on the cold injection scheme, which was recently proposed in the context of laser wakefield acceleration (Davoine et al 2009 Phys. Rev. Lett. 102 065001). This scheme allows one to inject a bunch of electrons into a laser wakefield, which is possible thanks to the collision between the main and a counter-propagating laser pulse. Unlike in the conventional colliding pulse schemes, in this process, a beatwave is created during the collision, which allows the injection of electrons with negligible heating. In this paper, we show that the injection of on-axis electrons observed in simulations is well described by a one-dimensional (1D) model, as long as conditions given here are satisfied. Injection of off-axis electrons is also influenced by transverse effects, but the basic mechanisms remain the same. Then, a comparison with the conventional colliding pulse schemes shows that each scheme can occur in different regimes. In particular, cold injection proves to be more interesting regarding the energy spread issue. Indeed, the simulations demonstrate that electron bunches with sub-MeV absolute energy spreads can be injected, leading, after acceleration, to electrons at several GeV and relative energy spread below 1%.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. Laser wakefield acceleration is an efficient way to accelerate electrons to high energy over a few millimetres or centimetres. The properties of the accelerated beam depend on the characteristics of the electron bunch injected into the wakefield, and controlling the injection can improve the quality of the final beam. Several methods have been proposed for this injection. Among them, the cold injection scheme has recently been proposed and can lead to the production of electrons with low energy spread.
Main results. This article provides a better understanding of the advantages and limits of the cold injection scheme. A one-dimensional model describing the injection mechanism is first presented and validated by comparing its predictions to detailed laser–plasma interaction simulations. Transverse effects are then discussed. Conditions for the occurrence of cold injection are also given. Eventually, thanks to a comparison with a previous injection scheme and to simulations modeling both the injection and acceleration, we show that this new scheme can advantageously lead to the injection of an electron bunch with a low initial energy spread. As long as this energy spread is not dramatically increased during the acceleration process, beams with low final energy spreads can be obtained.
Wider implications. Reducing the electron beam energy spread is highly relevant to some applications. For example, x-rays produced by an electron beam wiggling in an undulator or free electron laser both depend dramatically on the spectral quality of the electron beam.