Expansion dynamics and equilibrium conditions in a laser ablation plume of lithium: Modeling and experiment

The gas dynamics and atomic kinetics of a laser ablation plume of lithium, expanding adiabatically in vacuum, are included in a numerical model, using isothermal and isentropic self-similar analytical solutions and steady-state collisional radiative equations, respectively. Measurements of plume expansion dynamics using ultrafast imaging for various laser wavelengths $(266–1064nm)$⁠, fluences $(2–6.5Jcm−2)$⁠, and spot sizes $(50–1000μm)$ are performed to provide input parameters for the model and, thereby, study the influence of laser spot size, wavelength, and fluence, respectively, on both the plume expansion dynamics and atomic kinetics. Target recoil pressure, which clearly affects plume dynamics, is included in the model. The effects of laser wavelength and spot size on plume dynamics are discussed in terms of plasma absorption of laser light. A transition from isothermal to isentropic behavior for spot sizes greater than $50μm$ is clearly evidenced. Equilibrium conditions are found to exist only up to $300ns$ after the plume creation, while complete local thermodynamic equilibrium is found to be confined to the very early parts of the expansion.