title: High Repetition-Rate Laser-Driven Particle Generation – Towards High Flux Fast Neutron Sources
Creators: name: Treffert, Franziska
Publisher: name: UNSPECIFIED
Year: 2024

description: High-flux, high repetition-rate pulsed neutron sources are of interest for probing studies such as neutron-induced damage processes in materials employed and considered for shielding purposes in fusion reactors.
Simulating the effect an intense neutron flux has on such materials will ultimately guide designs for future fusion reactors. Laser-driven neutron sources employing petawatt laser systems show great potential to fulfill the need for such a neutron source. One of the most common approaches for neutron generation utilizing lasers as drivers is the pitcher–catcher geometry in which a directional ion beam is generated from a pitcher target and impinges on a catcher target producing neutrons through nuclear reactions within the catcher material. Despite the fact that neutron generation using such setups have only recently gained attention, it has so far shown the highest neutron yields using short-pulse lasers. To date, experiments predominantly studied neutron generation on a single shot basis, especially since the development of a high repetition-rate laser-driven neutron source faces a variety of challenges.

In this thesis, the individual components of a successful high repetition-rate laser-driven neutron source were investigated and developed. The focus of this work was especially the development of a stable target
system compatible with high repetition-rate laser operations, the development of a design for the catcher target, allowing for optimization of ion beam-catcher overlap, and an efficient ion and neutron beam detection platform.

For this work, a robust version of the SLAC-developed converging liquid microjet target delivery systems was designed and fielded. This system was successfully implemented at two different laser facilities, surviving
more than 1000 shots on target with no apparent damage to the nozzle or degradation of the liquid target. The liquid microjet system was implemented to study high repetition-rate deuteron acceleration from heavy water microjet targets at the ALEPH laser facility reaching average fluxes of 1×10^12 deuterons/sr/min at a repetition rate of 0.5 Hz. Stable deuteron acceleration over 60 shots was observed at varying laser energies on target, suggesting a more favorable scaling of the ion beam cut-off energy than currently established in the literature.

A flexible, repetition-rate compatible neutron generation platform was designed around a stackable catcher target, which can be adjusted based on laser parameters and experimental conditions. This specific design aims at enhancing the generation of high-flux, directional neutron beams. A flexible detector setup simultaneously monitors the ion and neutron beam emission characteristics to study their individual shot-to-shot parameter changes and the correlations between them. Employing cryogenic or ambient-temperature liquid jet targets as a pitcher enables high-repetition-rate operation.

This novel platform was successfully tested using cryogenic liquid deuterium jet targets at the Texas Petawatt laser facility demonstrating efficient generation of forward directed neutron beams with fluxes reaching 7.2×10^9 neutrons/sr within a narrow divergence angle of ±20◦. As such, this work lays the foundation for future high-repetition-rate experiments towards pulsed, high-flux, fast neutron sources for radiation-induced effect studies relevant for fusion science and applications that require neutron beams with short pulse duration for the probing of fast evolving processes complementary to X-rays.