Numerical Studies of Electron Acceleration Behind Self-Modulating Proton Beam in Plasma with a Density Gradient

TL;DR

This paper uses simulations to explore how a plasma density gradient can control electron acceleration during the self-modulation of a proton beam in plasma, relevant to CERN's AWAKE project.

Contribution

It introduces the use of plasma density gradients to manipulate the self-modulation instability and electron acceleration in proton-driven plasma wakefield acceleration.

Findings

Density gradients influence the growth of the self-modulation instability.

Controlled gradients can optimize electron acceleration.

Results support experimental design for AWAKE.

Abstract

Presently available high-energy proton beams in circular accelerators carry enough momentum to accelerate high-intensity electron and positron beams to the TeV energy scale over several hundred meters of the plasma with a density of about 1e15 1/cm^3. However, the plasma wavelength at this density is 100-1000 times shorter than the typical longitudinal size of the high-energy proton beam. Therefore the self-modulation instability (SMI) of a long (~10 cm) proton beam in the plasma should be used to create the train of micro-bunches which would then drive the plasma wake resonantly. Changing the plasma density profile offers a simple way to control the development of the SMI and the acceleration of particles during this process. We present simulations of the possible use of a plasma density gradient as a way to control the acceleration of the electron beam during the development of the SMI of a 400 GeV proton beam in a 10 m long plasma. This work is done in the context of the AWAKE project --- the proof-of-principle experiment on proton driven plasma wakefield acceleration at CERN.