The AWAKE experiment harnesses the 400 GeV proton beam from the CERN SPS to drive plasma wakefields, which in turn accelerate a witness bunch of electrons to high energy.  Upgrades are currently being carried out to facilitate the experimental programme of Run 2c, which includes control of the witness bunch quality during acceleration.  We here present the first full simulations of the beam–plasma interaction for AWAKE Run 2c, including self-modulation of the proton drive beam and the acceleration of the witness bunch in the wakefields of the resulting train of driver microbunches.  We demonstrate that the length of the proton drive beam has a significant impact on the transverse wakefield dynamics which impact the quality of the accelerated electron witness.  These simulations inform the choice of parameters for the experiment.

Plasma wakefield acceleration (PWFA) is a promising route to compact, high-energy accelerators. Achieving TeV-scale electron energies for high-energy physics experiments requires multiple acceleration stages in electron-driven PWFA schemes, which remains a significant challenge. The AWAKE experiment at CERN explores a proton-driven approach, utilising the kJ-level energy of proton bunches from the Super Proton Synchrotron for single-stage acceleration. A key challenge in scaling this approach is the development of long, highly uniform plasma sources—capable of maintaining electron-density uniformity better than 0.25%—to extend acceleration lengths to 10–100 metres, enabling energy gains of several to tens of GeV. This work presents the development and characterisation of a length-scalable pulsed-DC discharge plasma, producing the required electron densities in direct-current discharges at 10–50 Pa in noble gases. A 10 m prototype was tested in AWAKE by propagating the 400 GeV proton bunch through the plasma and observing the induced self-modulation. The plasma density and uniformity were characterised, and length scalability was investigated by combining multiple discharges in series using shared electrodes and magnetic circuits for current balancing. The results presented here demonstrate the potential of this scalable plasma technology for future PWFA applications.