Investigation of the local thermodynamic equilibrium of laser-induced aluminum plasma by Thomson scattering technique

• Laser Thomson scattering method was applied to investigate aluminum laser-induced plasma.
• Spatio-temporal evolution of electron temperature and density was determined.
• Three criteria for existence of local thermodynamic equilibrium were verified.
• Criteria are much easier to satisfy for metallic plasma species than for nitrogen.
• Criteria are easier to satisfy at earlier times and in the plasma core.

A laser Thomson scattering method was applied to investigate the local Saha–Boltzmann equilibrium in aluminum laser-induced plasma. Plasma was created in ambient air using 4.5 ns pulses from a Nd:YAG laser at 532 nm, focused on an Al target. Spatially resolved measurements, performed for the time interval between 600 ns and 3 μs, show electron density and temperature to decrease from 3.4 × 1023 m− 3 to 0.5 × 1023 m− 3 and from 61,000 K to 13,000 K in the plasma core. The existence of local thermodynamic equilibria in the plasma was verified by comparing the rates of the collisional to radiative processes (the McWhirter criterion), as well as relaxation times and diffusion lengths of different plasma species, with the appropriate rate of electron density evolution and its gradients at given, experimentally determined, electron temperatures. We found these criteria to be much easier to satisfy for metallic plasma species than for nitrogen. The criteria are also easier to satisfy in the plasma core of higher electron density.