  In the Phase 2 of the AWAKE first experimental run (from May to November
2018), an electron beam was used to probe and test proton-driven wakefield
acceleration in a rubidium plasma column. In this work, we analyze the overall
charge capture and shot-to-shot reproducibility of the proton-driven plasma
wakefield accelerator with various electron bunch injection parameters. The
witness electron bunches were produced using an RF-gun equipped with a Cs2Te
photocathode illuminated by a tailorable ultrafast deep ultraviolet (UV) laser
pulse. The construction of the UV beam optical system enabled appropriate
transverse beam shaping and control of its pulse duration, size, and position
on the photocathode, as well as time delay with respect to the ionizing laser
pulse that seeds the plasma wakefields in the proton bunches. Variable
photocathode illumination provided the required flexibility to produce electron
bunches with variable charge, emittance, and injection trajectory into the
plasma column. We demonstrate charge capture rates exceeding 15% (40 pC of GeV
accelerated charge for a 385 pC injected electron bunch) under optimized
electron injection conditions.
