Acceleration rate enhancement by negative plasma density gradient in multi-bunch driven plasma wakefield accelerator

In a plasma wakefield accelerator driven by a train of short particle bunches, it is possible to locally increase the acceleration rate by slightly decreasing the plasma density and introducing its small negative gradient. There is a regime in which changing the density affects only the relative phasing of the driver bunches and the wave, keeping the wave phase behind the driver stable. With this technique, it is possible to increase the energy gain of the accelerated witness bunch in a plasma section of limited length.