Electron self-injection in the proton-driven-plasma-wakefield acceleration

Plasma wakefield acceleration and electron self-injection in the proton-driven-plasma-wakefield acceleration are investigated using a two-dimensional, electromagnetic particle-in-cell simulation method. Plasma electrons are self-injected into the back of the first acceleration bucket during the initial bubble formation period, where the wake phase velocity is low enough to trap sufficient electrons. As the wake phase velocity increases, the self-injection process terminates and these self-injected electrons are further accelerated by the wakefield to high energies. Most of the self-injected electrons are initially located within a distance of the skin depth c/Wpe to the beam axis. A decrease (or increase) in the beam radius (or length) leads to a significant reduction in the total charges of self-injected electron bunch. Compared to the uniform plasma, the energy spread, emittance and total charges of the self-injected bunch are reduced in the plasma channel case, due to a reduced injection of plasma electrons that initially located further away from the beam axis.