Electron trapping and reinjection in prepulse-shaped gas targets for laser-plasma accelerators

A novel mechanism for injection, emittance selection, and postacceleration for laser wakefield electron acceleration is identified and described. It is shown that a laser prepulse can create an ionized plasma filament through multiphoton ionization and this heats the electrons and ions, driving an ellipsoidal blast-wave aligned with the laser-axis. The subsequent high-intensity laser-pulse generates a plasma wakefield which, on entering the leading edge of the blast-wave structure, encounters a sharp reduction in electron density, causing density down-ramp electron injection. The injected electrons are accelerated to $\ensuremath{\sim}2\text{ }\text{ }\mathrm{MeV}$ within the blast-wave. After the main laser-pulse has propagated past the blast-wave, it drives a secondary wakefield within the homogenous background plasma. On exiting the blast-wave structure, the preaccelerated electrons encounter these secondary wakefields, are retrapped, and accelerated to higher energies. Due to the longitudinal extent of the blast-wave, only those electrons with small transverse velocity are retrapped, leading to the potential for the generation of electron bunches with reduced transverse size and emittance.