A method for measuring energy gain with variable plasma length at AWAKE

Abstract The AWAKE experiment investigates the acceleration of externally injected electrons into the wakefields driven by a self-modulated proton bunch. In Run 1, AWAKE successfully demonstrated proton bunch self-modulation and accelerated electrons from 19 MeV to 2 GeV. For Run 2b, upgrades to the rubidium vapour source enabled the introduction of a plasma density step and adjustments to the plasma length. This facilitated studies on how the density step sustains the longitudinal wakefield amplitude by measuring the electron energy gain as a function of the plasma length. This paper presents the analysis techniques for such energy measurements and the technical considerations for interpreting results under the varying plasma conditions.