Phase velocity and particle injection in a self-modulated proton-driven plasma wakefield accelerator

TL;DR

This paper investigates how the phase velocity reduction in self-modulated proton-driven plasma wakefield accelerators impacts performance, and proposes methods to mitigate adverse effects for improved electron acceleration.

Contribution

It provides analytical and simulation insights into phase velocity dynamics and introduces strategies to control wake phase velocity for better acceleration outcomes.

Findings

Phase velocity is significantly reduced during the linear stage of self-modulation.

Nonlinear saturation of the instability restores phase velocity to that of the driver.

Electron injection strategies can mitigate the effects of phase velocity reduction.

Abstract

It is demonstrated that the performance of the self-modulated proton driver plasma wakefield accelerator (SM-PDPWA) is strongly affected by the reduced phase velocity of the plasma wave. Using analytical theory and particle-in-cell simulations, we show that the reduction is largest during the linear stage of self-modulation. As the instability nonlinearly saturates, the phase velocity approaches that of the driver. The deleterious effects of the wake's dynamics on the maximum energy gain of accelerated electrons can be avoided using side-injections of electrons, or by controlling the wake's phase velocity by smooth plasma density gradients.