Development of a reliable platform for Laser-Plasma Accelerator driven Free-Electron Laser Studies

Abstract

Free-Electron Lasers (FELs) based on start-of-the-art radio-frequency (RF) accelerator facilities have been established as a reliable source of coherent, high-brightness x-ray beams, advancing and pushing the boundaries of a broad range of scientific applications. Their capabilities would unlock significant improvements to applications in medical imaging and semiconductor manufacturing, among others, but to make this technology available to a larger community, significant steps to reduce the cost and footprint of the accelerator facilities are required.

With the emergence of laser-plasma accelerators (LPAs) as powerful, compact alternatives to RF machines, leveraging these sources to drive FELs immediately arose as a promising application. First results of LPA-driven FEL radiation have been achieved by the community over the last few years, but reliable high-gain FEL operation, required to prove the potential of serving as a capable platform for future light source facilities, has yet to be demonstrated.

This dissertation was carried out at the Hundred TeraWatt Undulator (HTU) system, part of the Berkeley Lab Laser Accelerator (BELLA) group at Lawrence Berkeley National Laboratory (LBNL), and driven by the goal to demonstrate reliable operation of an LPA-driven FEL. The work presented here aims to achieve a more repeatable and stable LPA interaction, to enable a first demonstration of LPA-driven FEL lasing on the HTU system, and to develop an experimental platform for subsequent investigations of the LPA-driven FEL performance.

To achieve this goal, a large part of the work conducted as a part of this thesis project went towards improvements of the diagnostic capabilities of the experiment, to obtain new insights into the key parameters of the LPA interaction. These results were used to inform the need for additional stabilization systems to improve system performance, and their subsequent development, installation, and characterization are discussed as well.

The setup and commissioning of new, as well as the upgrade of existing diagnostics enabled the discovery of systematic instabilities of the laser system. Specifically variations of the laser pulse duration and energy, uncovered as part of this work, were shown to significantly impact the LPA performance. Implementing new stabilization capabilities targeted towards these variations, in combination with extensive efforts to further increase long-term reliability of the LPA interaction, yielded successful demonstration and characterization of LPA-driven FEL lasing at 420 nm. Through subsequent improvements to the laser system and operational procedures, first results exceeding 1000-fold gain from an LPA-driven FEL, with higher than 90% reliability, were achieved. During a dedicated follow-up campaign, continuous operation of the LPA-driven FEL over more than eight hours was demonstrated, while maintaining the performance of more than 90% of all shots exhibiting FEL gain. \newline

These results represent an unprecedented level of both shot-to-shot and long-term stability, as well as overall performance for LPA-driven FELs, and serve as a crucial step towards the demonstration of the technology as a viable platform for future compact FEL facilities.