AWAKE, a Self-modulated, Proton-driven Plasma Wakefield Acceleration Experiment at CERN

Proton ($p^+$) bunches available today carry large amounts of energy ($kJ$). They are therefore potential drivers for plasma wakefield acceleration experiments aiming at large energy gain along a single long plasma. However, these $p^+$ bunches are also long ($\sim10~cm$). In dense plasmas, such that the plasma period is shorter than the bunch length, they are subject to the self-modulation instability (SMI) [Kumar, Phys. Rev. Lett. 104, 255003 (2010)]. The SMI forms a train of short bunches that can resonantly drive accelerating wakefields to the $GV/m$ level. Based on this scheme, the AWAKE experiment at CERN will use the $400~GeV$ bunch with $3\times10^{11}~p^+$ of the SPS. Numerical simulations show that over $10~m$ of plasma with a density in the $1-10\times10^{14}~cm^{-3}$ range the SMI can grow, and saturate when seeded. Seeding also allows for deterministic injection of the witness bunch. Externally injected $MeV$ electrons can reach $GeV$ energies in $\sim GeV/m$ accelerating gradient. Operating at low plasma density, i.e., larger accelerating structure, but with large average gradient eases the injection process and the bunches production and alignment. The physics program and the experimental set-up of the AWAKE experiment will be presented.