  The Advanced Wakefield Experiment (AWAKE) develops the first plasma wakefield
accelerator with a high-energy proton bunch as driver. The 400GeV bunch from
CERN Super Proton Synchrotron (SPS) propagates through a 10m long rubidium
plasma, ionized by a 4TW laser pulse co-propagating with the proton bunch. The
relativistic ionization front seeds a self-modulation process. The seeded
self-modulation transforms the bunch into a train of micro-bunches resonantly
driving wakefields. We measure the density modulation of the bunch, in time,
with a streak camera with picosecond resolution. The observed effect
corresponds to alternating focusing and defocusing fields. We present a
procedure recovering the charge of the bunch from the experimental streak
camera images containing the charge density. These studies are important to
determine the charge per micro-bunch along the modulated proton bunch and to
understand the wakefields driven by the modulated bunch.
