Electron acceleration in sub-relativistic wakefields driven by few-cycle laser pulses

Using particle-in-cell simulations, we study the interaction of few-mJ–fewcycle laser pulses with an underdense plasma at resonant density. In this previously unexplored regime, it is found that group velocity dispersion is a key ingredient of the interaction. The concomitant effects of dispersion and plasma nonlinearities cause a deceleration of the wakefield phase velocity, which becomes sub-relativistic. Electron injection in this sub-relativistic wakefield is enhanced and leads to the production of a femtosecond electron bunch with a picocoulomb of charge in the 5–10MeV energy range. Such an electron bunch is of great interest for application to ultrafast electron diffraction. In addition, in this dispersion dominated regime, it is shown that positively chirped laser pulses can be used as a tuning knob for compensating for plasma dispersion, increasing the laser amplitude during self-focusing and optimizing the trapped charge. S Online supplementary data available from stacks.iop.org/NJP/16/023023/ mmedia 1 Author to whom any correspondence should be addressed. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. New Journal of Physics 16 (2014) 023023 1367-2630/14/023023+15$33.00 © 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft New J. Phys. 16 (2014) 023023 B Beaurepaire et al