Beam-driven plasma-based acceleration of electrons with density down-ramp injection at FLASHForward

We present simulations of injection of plasma electrons triggered by a negative gradient in the plasma-density profile for beam-driven plasma-wakefield acceleration. Studies show the potential of this trapping technique for the production of narrow-energy bandwidth, femtosecond, high-current electron beams with a small transverse normalised emittance in the hundred nm range. Moreover, the accelerated beams feature strong longitudinal phase-space correlations with sub-percent uncorrelated energy spreads. Tuning of density down-ramp injection is explored by scanning the slope and length of the downward transition. It is found that the mechanism causes particle trapping over a wide experimentally accessible parameter range. All simulations were performed in three spatial dimensions with the particle-in-cell code OSIRIS [1]. The investigated driver-beam properties and the plasma profile resemble expected experimental conditions at the FLASHForward facility at DESY.