CERN Accelerating science

In this thesis I observe experimentally and study in simulations the seeded selfmodulation of a relativistic proton bunch in AWAKE, the Advanced Proton Driven Plasma Wakefield Acceleration Experiment. The 400 GeV/c proton bunch from the CERN SPS with a rms length of 12 cm propagates in a 10m long plasma with a density adjustable between 2-10x10^14 electrons/cm3. The seeded self-modulation process results in focusing and defocusing of the protons, thereby forming a bunch train that resonantly drives wakefields to large amplitudes. I use the two-screen measurement setup, to observe the result of the proton bunch self-modulation and to learn about its physics (i.e. growth of the process). The idea is to obtain images of protons that were defocused by the transverse wakefields, 2 and 10m downstream the end of the plasma. From these images I determine the maximum transverse momentum of the defocused protons as well as infer their point of origin along the plasma. I use simulations to guide the understanding of the experimental results. At a plasma density of 7.7x10^14 electrons/cm3, the maximum defocused protons exit the wakefields with a transverse momentum of (390+-25) MeV/c, 4m after the plasma entrance. This measured transverse momentum is larger than that from the bunch emittance (sigma_pr =20MeV/c) plus that from the initial seed wakefield (15MV/m) integrated over the plasma interaction length. This therefore proves for the first time that the wakefields grow along the plasma and that the proton bunch strongly evolves in its transverse dimension, as a result of the seeded self-modulation process.