Seeded self-modulation of an entire long proton bunch in plasma using a short electron bunch

The AWAKE experiment at CERN [1] relies on the self-modulation of a long proton bunch in plasma [2] to effectively drive wakefields and accelerate an externally injected electron bunch to GeV-level energies [3]. The control of the acceleration requires that the self-modulation process is reproducible from event to event: this is achieved by seeding the instability. Seeding using a relativistic ionization front was demonstrated during AWAKE Run 1 [4], but this method leaves the head of the bunch un-modulated. Therefore we are going to use, for the first time, a 18MeV short electron bunch placed ahead of the proton bunch to seed the self-modulation of the entire bunch [5].  We will present preliminary results of the current experimental campaign where we study the phase reproducibility of the proton bunch self-modulation, varying the parameters of the seed electron bunch at the plasma entrance. 

[1] P. Muggli et al. (AWAKE Collaboration), Plasma Phys. and Contr. Fus., 60(1) 014046 (2017).

[2] M. Turner et al. (AWAKE Collaboration), Phys. Rev. Lett. 122, 054801 (2019).

[3] AWAKE Collaboration, Nature 561, 363 (2018),

[4] F. Batsch et al. (AWAKE Collaboration), Phys. Rev. Lett. 126, 164802 (2021).

[5] P. Muggli et al., J. Phys.: Conf. Ser.1596 012066 (2020).