Fireball Chemistry and Hydrodynamics of Laser Ablation Plasmas

Developing measurement technologies to monitor nuclear-relevant materials in the environment aims to prevent the proliferation of nuclear weapons by detecting undeclared nuclear fuel cycle activities. Standoff, field-deployable optical detection systems based on laser spectroscopy offer the capability to measure spectrochemical signatures without sample preparation by exciting a target material via laser ablation followed by diagnostics to detect photons emitted from the ensuing laser-produced plasma. Spectroscopic signatures are dependent on plasma conditions, where the spatial and physicochemical properties of the plasma change rapidly with time and are highly sensitive to both experimental and environmental conditions. Uranium and plutonium plasmas react readily with oxygen in the atmosphere, leading to the formation of molecular species and considerably influencing signature interpretation. This work aims to address the complex plasma plume hydrodynamics and mixing processes with the surrounding atmosphere to explain the high-temperature gas-phase oxidation reactions and kinetics of laser ablation plasmas. Experimental measurements are presented alongside high-fidelity multi-physics simulations, providing visualizations of the expansion dynamics and thermochemical plume dynamics. Gasphase molecular species are observed to form in the thin plume periphery within the first few nanoseconds of the simulation in the presence of large temperature gradients (thousands of Kelvin) and strong shockwaves. Intermixing between reacting plasma-gas species is shown to be driven by diffusion and hydrodynamic processes such as vortex formation, while shockwaves are observed to pre-heat the air ahead of the plasma plume yet otherwise remain uninvolved in plasma plume chemistry.