# The Two-Screen Measurement Setup to Indirectly Measure Proton Beam Self-Modulation in AWAKE

## TL;DR

This paper details the design, implementation, and commissioning of a two-screen measurement setup at CERN's AWAKE experiment, enabling indirect detection of proton beam self-modulation through imaging defocused protons after plasma passage.

## Contribution

It introduces a novel two-screen diagnostic system for indirect measurement of proton self-modulation in plasma wakefield experiments, validated during initial beam commissioning.

## Key findings

- System successfully commissioned and calibrated.
- Capable of single-shot imaging of self-modulated proton bunches.
- Effective in measuring proton defocusing due to plasma wakefields.

## Abstract

The goal of the first phase of the AWAKE \cite{AWAKE1,AWAKE2} experiment at CERN is to measure the self-modulation \cite{SMI} of the $\sigma_z = 12\,\rm{cm}$ long SPS proton bunch into microbunches after traversing $10\,\rm{m}$ of plasma with a plasma density of $n_{pe}=7\times10^{14}\,\rm{electrons/cm}^3$. The two screen measurement setup \cite{Turner2016} is a proton beam diagnostic that can indirectly prove the successful development of the self-modulation of the proton beam by imaging protons that got defocused by the transverse plasma wakefields after passing through the plasma, at two locations downstream the end of the plasma. This article describes the design and realization of the two screen measurement setup integrated in the AWAKE experiment. We discuss the performance and background response of the system based on measurements performed with an unmodulated Gaussian SPS proton bunch during the AWAKE beam commissioning in September and October 2016. We show that the system is fully commissioned and adapted to eventually image the full profile of a self-modulated SPS proton bunch in a single shot measurement during the first phase of the AWAKE experiment.