Speaker
Description
A particle bunch propagating through plasma will induce a non-linear response when $n_b \gg n_{e0}$ [1]. A negatively charged bunch will drive a blow-out, in which all plasma electrons are expelled from the propagation axis. A positively charged bunch will attract plasma electrons, which will flow-in to the propagation axis, creating a filament [2]. This will sustain focusing fields for the positively charged bunch and defocusing for negatively charged particles.
In the Advanced Wakefield Experiment (AWAKE) [3], in which a long proton bunch drives high-amplitude wakefields for electron acceleration through self-modulation [4], there is a plasma density ramp at the entrance and exit of the plasma [5]. The density in the plasma ramp can be up to five orders of magnitude lower than that of the long plasma. We present a numerical study performed with the particle-in-cell code LCODE [6] using parameters similar to those of the experiments. We show that the plasma ramp would have a detrimental effect on the propagation of both a seed electron bunch placed inside of the proton bunch and for an electron bunch injected in a second plasma for acceleration [7], if that plasma had a density ramp at the entrance.
