CERN Accelerating science

The AWAKE experiment at CERN aims to drive strong plasma wakefields with a self-modulated proton drive bunch, and to use these wakefields for electron acceleration. During the self-modulation process, protons are defocused by the transverse plasma wakefields and form a halo around the focused bunch core. The two-screen setup integrated in AWAKE measures the transverse, time-integrated proton bunch distribution downstream the \unit[10]{m} long plasma to detect defocused protons. By measuring the maximum radii of the defocused protons we attempt calculate properties of the self-modulation. In this article, we develop a routine to identify the maximum radius of the defocused protons, based on a standard contour method. We compare the maximum radius to the logarithmic lineouts of the image to show that the radius identifies the edge of the signal.