External electron injection into plasma wakefields for the AWAKE Run 2b experiments
External electron injection into plasma wakefields for the AWAKE Run 2b experiments
Plasma wakefield acceleration offers a promising route towards compact particle accelerators by enabling accelerating gradients orders of magnitude higher than those achievable with conventional radio-frequency technologies. In proton-driven plasma wakefield acceleration, long, highly energetic proton bunches can drive high amplitude wakefields after undergoing the self-modulation (SM) instability. Reproducible, high-quality electron (witness) beam acceleration in plasma wakefields requires precise understanding and control of both the SM and injection process.
This thesis of Nikita Gils presents experimental studies of external electron injection into proton-driven plasma wakefields conducted at the Advanced WAkefield Experiment (AWAKE) at CERN during its Run 2b (2023–2025). The work focuses on understanding and mitigating the challenges associated with witness bunch injection and wakefield stabilisation. A central result is the experimental demonstration that a plasma density step increases the accelerating gradient after saturation of SM, confirming numerical predictions. These measurements represent an initial experimental attempt at determining accelerating gradients in AWAKE.
Significant advances are made in electron beam alignment, optics and beam line modelling; strongly supported by hardware upgrades to the experimental setup such as the implementation of beam screens inside the vapour source. Thus enabling determination of beam crossing location, beam overlap with the wakefields, and trajectory stability. The limitations of off-axis injection are identified, motivating the transition to on-axis injection for future operation. In addition, a novel technique providing phase and amplitude reproducible SM based on truncating an electron bunch with a relativistic ionisation front is experimentally validated, overcoming difficulties encountered at AWAKE’s baseline plasma density.
The results of this thesis provide essential input for the design of AWAKE Run 2c, where SM and acceleration of a witness bunch will be separated into two plasma stages and high-quality electron beam acceleration will be pursued. In regard to this, scattering in rubidium vapour and its effect on electron beam quality is experimentally and numerically investigated.
This work contributes to establishing a solid experimental foundation for controlled proton-driven plasma wakefield acceleration, as well as advancing the development of plasma-based accelerators for future high-energy physics applications.