A discharge plasma source for proton-driven plasma wakefield acceleration at CERN

This thesis presents the development, characterisation, and first beam operation of a Discharge Plasma Source (DPS) designed for the AWAKE proton-driven plasma-wakefield acceleration experiment at CERN. The system, first proposed by Instituto Superior Técnico (Lisbon) and Imperial College London as a scalable technology, was assembled at CERN to be compatible with both laboratory studies and beamline operation, combining flexibility for development with the operational robustness required in the CERN accelerator complex.

The thesis primarily describes the plasma diagnostics implemented to characterise the discharge properties. Interferometry was employed to measure the temporal evolution of the axially averaged electron density. Discharges in xenon, argon, and helium were studied, yielding peak plasma densities up to (1.78±0.15), (0.97±0.06), and (0.36±0.02)x1015cm-3 respectively, for discharge currents up to 500 A in 10 m plasma tubes of 13 mm radius.

Complementary Thomson-scattering measurements provided local electron density and temperature, confirming axial uniformity within ±6 % at peak density, close to the diagnostic current precision. The DPS was subsequently installed and operated in the AWAKE experiment during a dedicated proton-beam run, where it operated reliably during a three-week campaign. The proton bunch underwent self-modulation in plasmas produced by the DPS, representing the first demonstration of a discharge-based plasma technology in AWAKE.

Experiments with different plasma lengths indicated the concept’s potential for scalability, while the use of multiple gases enabled the study of ion-motion effects in the proton self-modulation instability. Overall, the work demonstrates that the DPS can generate long and reproducible plasmas suitable for plasma-based acceleration experiments. The results establish a solid experimental foundation for the future development of modular, extended plasmas capable of increasing the acceleration length in AWAKE or in other plasma-based accelerator projects.