Plasma wakefield dynamics over timescales up to 800 ps, approximately 100 plasma periods, are studied
experimentally at the Advanced Wakefield Experiment (AWAKE). The development of the longitudinal
wakefield amplitude driven by a self-modulated proton bunch is measured using the external injection of
witness electrons that sample the fields. In simulation, resonant excitation of the wakefield causes plasma
electron trajectory crossing, resulting in the development of a potential outside the plasma boundary as
electrons are transversely ejected. Trends consistent with the presence of this potential are experimentally
measured and their dependence on wakefield amplitude are studied via seed laser timing scans and electron
injection delay scan