External injection of electron beams into plasma-wakefield accelerators

Abstract

Plasma wakefields enable GeV/m-level acceleration gradients, making them a promising avenue to reduce the size and the associated costs of future particle accelerators. High-energy physics facilities in particular place stringent demands on beam quality and energy efficiency, which necessitates precise control of the beam acceleration. Injecting an external electron bunch with predefined and controllable properties into beam-driven plasma wakefields is ideally suited to investigate the beam–plasma interaction in detail. This cumulative dissertation was carried out at the FLASHForward facility, and is driven by the overarching goal of advancing the understanding of this beam–plasma interaction from an experimental perspective and thereby promoting the precise control of the acceleration process.

Initial studies dealt with the preparation of the wake-driving as well as the injected electron bunch in both the transverse as well as the longitudinal plane for the interaction with the plasma. A new method for measuring transverse beam parameters has been implemented based on beam-jitter measurements in beam-position monitors, thereby enabling non-invasive fast-feedback control for the tedious process of matching the transverse phase space to the focusing forces prevalent in plasma wakes, which is essential for efficient and high-quality acceleration. Crucial acceleration parameters such as the transformer ratio, the energy transfer efficiency as well as the resulting energy spectrum of the accelerated bunch are strongly related to the detailed wakefield shape, which can be changed via beam loading and requires the ability to precisely shape the current profiles of both the wake-driving bunch and the accelerated bunch. To achieve this, a device of three finely adjustable collimators was implemented in the FLASHForward beam line, enabling current-profile modifications at the femtosecond-level through energy collimation in a dispersive section of an electron bunch with a strongly correlated longitudinal phase space. With the capabilities of these new tools, an operating point with an acceleration gradient of 1.3 GeV/m was accomplished at which the energy spread as well as the charge of the injected bunch was preserved while achieving an energy-transfer efficiency of 42%. The characteristic shape of the plasma wakefield can also be used to reduce a remaining correlated energy spread, which has been demonstrated with a dechirping strength of 1.8 GeV/mm/m. To achieve unprecedented control over plasma-based accelerators, new developments are required to diagnose the acceleration process, with the shape of the wakefields being of major interest. As the backbone of this work, a new method was invented to measure the longitudinal wakefield that effectively acts on the wake-driving and the externally injected electron bunch over the entire interaction length. This novel sampling method for beam-driven plasma wakefields enables femtosecond-resolved insights into the acceleration process and now permits it to be optimised routinely.