Electron Bunch Seeding of the Self-Modulation Instability in Plasma

Plasma wakefield acceleration uses relativistic charged particle bunches to accelerate electron bunches to high energies. For the last three decades, this method has been considered a solution for high-gradient acceleration and a valuable scheme for future accelerators. The Advanced WAKefield Experiment (AWAKE) makes use of the large amount of energy stored in relativistic proton bunches to accelerate electron bunches to high energy in a single stage. The length of the available proton bunches is ∼ 7 cm, that is too long to effectively drive wakefields with GV/m amplitude in plasmas with density > 1014 cm−3. Thus, the AWAKE scheme relies on the self-modulation instability of the proton bunch to drive wakefields with large amplitude. In this work, I demonstrate experimentally that the wakefields driven by a short electron bunch can seed the self-modulation instability of a long proton bunch in plasma. I show that the timing of the self-modulation is reproducible from event to event and that it is controlled by the timing of the seed bunch at the picosecond time scale. When seeding with the electron bunch, the growth of the self-modulation can be independently controlled by the amplitude of the seed wakefields and by the growth rate of the instability. I show that increasing the charge of the seed electron bunch increases the amplitude of the seed wakefields, leading to a larger growth of the self-modulation, while the growth rate remains constant. Analogously, when increasing the proton bunch charge, the growth rate of the self-modulation increases. Moreover, I show that the hosing instability of the proton bunch is seeded by purposely misaligning the trajectory of the seed electron bunch with respect to that of the proton bunch. I also discuss a method to obtain three-dimensional images of the self-modulated p+ bunch. These results are important milestones on the path towards high-energy physics applications of proton driven plasma wakefield acceleration.