Near-threshold electron injection in the laser-plasma wakefield accelerator leading to femtosecond bunches

Theoretical model describing self-injection of electron in laser-plasma wakefield accelerator near the threshold regime. This process is crucial in determining the electron bunch properties.

In this paper we show that injection occurs due to the localised charge density build-up in the sheath crossing region at the rear of the bubble, which has the effect of increasing the accelerating potential to above a critical value. Injected electron bunch duration is determined by the dwell time above this critical value, which explains why single or multiple ultra-short electron bunches are formed in the first bubble. We have performed a series of 3-D particle-in-cell simulations (OSIRIS) to compared our model as well as experimental measurements on the ALPHA-X beam line.