Ultra-short electron bunches by Velocity Bunching as required for plasma wave accelerations

The generation of ultra-short bunches is nowadays a critical requirement for plasma wave accelerators, on which many laboratories world-wide are investigating or are close to starting with experimental activities. This requirement is true for both: external injection into “laser wake field accelerators”, where injected beams need lengths around or shorter than 10 fs; and the “plasma wake field accelerators”, where the wake field intensity scales like the driver bunch charge over the square of the rms bunch length (Qb/σz2). This work presents beam dynamic simulations, which show how to use the Velocity Bunching (VB) technique to obtain ultra-short bunches. The VB technique is applied to small bunch charges (0.5-15 pc) and it is driven with a proper control of the bunch density versus the bunch energy gain, which permits one to control the transverse beam emittance, the bunch length and the correlated longitudinal energy spread, in a peculiar manner. The compression optimizations by VB, shown in this work, are obtained using a layout very similar to SPARC's Linac one, which is a Linac designed to maximize VB performances.