Study of external electron beam injection into proton driven plasma wakefields for AWAKE Run2

TL;DR

This paper proposes an external electron injection method into proton-driven plasma wakefields for AWAKE Run2, utilizing scattering in foils to optimize beam matching and ensuring effective entry into the plasma.

Contribution

It introduces a novel electron injection scheme using scattering in foils to improve beam matching in proton-driven plasma wakefield acceleration.

Findings

Foil scattering increases beam emittance by ~10 times.

Betatron function decreases by a factor of ~3 due to scattering.

Electron beam enters plasma through a sharp vapor-plasma boundary.

Abstract

We describe an external electron injection scheme for the AWAKE experiment. We use scattering in two foils, that are necessary as vacuum window and laser beam dump, to decrease the betatron function of the incoming electron beam for injection and matching into plasma wakefields driven by a self-modulated proton bunch. We show that, for a total aluminum foil thickness of $\sim 280\, \mu$m, multiple Coulomb scattering increases the beam emittance by a factor of $\sim 10$ and decreases the betatron function by a factor of $\sim 3$. The plasma in the accelerator is created by a ionizing laser pulse, counter-propagating with respect to the electron beam. This allows for the electron bunch to enter the plasma through an "infinitely" sharp vapor-plasma boundary, away from the foils.