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

Abstract

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.

Influence of spot size on propagation dynamics of laser-produced tin plasma

“…27 The threedimensional plume propagation is governed by the initial pressure gradients and shape of the plume at the end of the laser pulse. We studied the expansion behavior of the plume with different spot sizes in vacuum using fast photography and noticed no difference in plume behavior.…”

Optical Thomson scatter from a laser-ablated magnesium plume

“…We have fitted power laws to the subsequent decay of the electron density with time and find that the density falls off more rapidly than the n e ϰ t −3 expected from a simple three-dimensional ͑3D͒ adiabatic expansion of an initially thin, uniform 1 mm 2 slab of plasma. 31 The intensity of the Rayleigh scattering signal, which is proportional to the density of atoms, does, however, follow this density scaling power law. We can see this in Fig.…”

“…We can compare the density data to simple plume expansion models 31 assuming either isentropic or isothermal expansions. In the former, it is usual to expect a Gaussian spatial profile of density, and in the latter a parabolic spatial and temperature profile.…”

“…31 include 32 indicated a vapor mass of ϳ2 g with an axial expansion speed of 1.2ϫ 10 6 cm s −1 . This latter number is in broad agreement with experimental 13 expansion velocities of 1.7ϫ 10 6 cm s −1 .…”

Ambient gas effects on the dynamics of laser-produced tin plume expansion

“…However, a systematic study on the effect of ambient gas on tin plasma dynamics is still lacking. The plume expansion into vacuum is adiabatic 17 and treated well by Anisimov et al 18,19 and Narayan and co-workers. 20,21 Compared to the expansion into vacuum, the interaction of the plume with an ambient gas is a far more complex gas dynamic process that involves deceleration, attenuation, and thermalization of the ablated species, as well as the formation of shock waves.…”