Proton beam defocusing in AWAKE: comparison of simulations and measurements

TL;DR

This paper compares simulations and measurements of proton beam defocusing in AWAKE, showing good agreement under specific conditions and highlighting the importance of simulation box size for accurate modeling.

Contribution

It demonstrates the accuracy of 2D particle-in-cell simulations in modeling proton beam defocusing in AWAKE across various parameters.

Findings

Simulations agree with measurements for specific plasma densities and gradients.

A sufficiently large simulation box (at least five plasma wavelengths) is necessary for accurate results.

The study validates the use of LCODE for modeling proton beam self-modulation in plasma.

Abstract

In 2017, AWAKE demonstrated the seeded self-modulation (SSM) of a 400 GeV proton beam from the Super Proton Synchrotron (SPS) at CERN. The angular distribution of the protons deflected due to SSM is a quantitative measure of the process, which agrees with simulations by the two-dimensional (axisymmetric) particle-in-cell code LCODE. Agreement is achieved for beam populations between $10^{11}$ and $3 \times 10^{11}$ particles, various plasma density gradients ($-20 \div 20\%$) and two plasma densities ($2\times 10^{14} \text{cm}^{-3}$ and $7 \times 10^{14} \text{cm}^{-3}$). The agreement is reached only in the case of a wide enough simulation box (at least five plasma wavelengths).