Verification of electron beam alignment and optics for external off-axis injection in AWAKE Run 2b

The Advanced Wakefield Experiment (AWAKE) has the long term goal to accelerate electrons to particle physics relevant energies using self-modulated proton bunches as drivers in plasma. AWAKE is currently in its Run 2b (2023–2025), where the goal is to stabilise the wakefield amplitude after the saturation of the self-modulation process by introducing a plasma density step. To optimise witness electron injection, retractable YAG screens have been installed inside the vapour source. These screens enable, for the first time, direct measurements of electron bunch sizes at the injection location and estimates of the spatial overlap with the wakefields. This manuscript presents an overview of the upgraded experimental setup and measurements of the transverse distribution of the electron bunches, crucial for improved control over the injection process. Additionally, results show agreement between simulated and measured transverse bunch sizes and positions, and the influence of various factors (e.g., plasma density, electron bunch charge, and witness bunch timing) on the electron charge overlapping with the wakefields. Furthermore, alignment challenges as well as potential solutions are discussed.