Indirect Self-Modulation Instability Measurement Concept for the AWAKE Proton Beam

TL;DR

This paper proposes a method to measure the self-modulation instability in the AWAKE proton beam experiment by analyzing the transverse profile of the proton bunch downstream of the plasma, providing insights into the instability's growth and saturation.

Contribution

It introduces a novel measurement concept using transverse profile imaging to infer the occurrence and characteristics of self-modulation instability in proton beams.

Findings

Defocused protons at 1 mrad indicate GV/m electric fields and full self-modulation.

Edge measurements can determine the growth rate of the instability.

The method achieves a 0.6 mm resolution with a 1 mm scintillation screen.

Abstract

AWAKE, the Advanced Proton-Driven Plasma Wakefield Acceleration Experiment, is a proof-of-principle R&D experiment at CERN using a 400 GeV/c proton beam from the CERN SPS (longitudinal beam size sigma_z = 12 cm) which will be sent into a 10 m long plasma section with a nominal density of approx. 7x10^14 atoms/cm3 (plasma wavelength lambda_p = 1.2mm). In this paper we show that by measuring the time integrated transverse profile of the proton bunch at two locations downstream of the AWAKE plasma, information about the occurrence of the self-modulation instability (SMI) can be inferred. In particular we show that measuring defocused protons with an angle of 1 mrad corresponds to having electric fields in the order of GV/m and fully developed self-modulation of the proton bunch. Additionally, by measuring the defocused beam edge of the self-modulated bunch, information about the growth rate of the instability can be extracted. If hosing instability occurs, it could be detected by measuring a non-uniform defocused beam shape with changing radius. Using a 1 mm thick Chromox scintillation screen for imaging of the self-modulated proton bunch, an edge resolution of 0.6 mm and hence a SMI saturation point resolution of 1.2 m can be achieved.