Spatially and temporally resolved evaluation of local thermodynamic equilibrium for laser-induced plasma in a high vacuum

Abstract

An analytical method to validate local thermodynamic equilibrium (LTE) in laser-induced plasmas is reported in this article. A more universal and general than Maxwellian electron energy distribution function (EEDF) is...

Influence of ambient pressure on spatial–temporal evolution of local thermodynamic equilibrium for laser-induced plasma

“…It is worth noting that the coefficient v at 1 Pa decreases with the increase of radius r when r > 0.8 mm, which means that the ion energy level population of LIP gets closer to the Boltzmann distribution as radius r increases. The same phenomenon was also found in the works of Zelin Liu; 6 the reason of this phenomenon is that if the collisional frequency is adequate, the ion energy level population of LIP can be in the Boltzmann distribution again. Due to the stronger confinement of ambient gas on LIP and the smaller expansion volume of LIP in the environment of 10 Pa than that in the environment of 10 −3 Pa and 10 −1 Pa, the edge of the LIP can be detected, resulting in the phenomenon that the ion energy level population of LIP gets closer to the Boltzmann distribution with the increase of radius r at 10 Pa, T d = 50 ns.…”

“…According to the EEDF, Zelin Liu et al proposed a new approach to estimate the LTE of LIP, which is different from the McWhirter criterion. 6 This criterion does not need to calculate the plasma electron density and temperature under the assumption of LIP in LTE. The intensity I of the spectral line can be expressed as 6 where I nm z is the spectral intensity measured experimentally, and the spectral intensity is the peak area of spectral line, which is generated by the transitions of energy levels n and m .…”

“…6 This criterion does not need to calculate the plasma electron density and temperature under the assumption of LIP in LTE. The intensity I of the spectral line can be expressed as 6 where I nm z is the spectral intensity measured experimentally, and the spectral intensity is the peak area of spectral line, which is generated by the transitions of energy levels n and m . z is the charge state ( z = 0 for neutral particles, z = 1 for singly ionized ions, and so on).…”

“…Researchers have made numerous studies on the LTE of LIP. Zelin Liu et al studied the spatially and temporally resolved spectra of Si at 10 −5 mbar and proposed a new analytical method “coefficient v ” to validate LTE in laser-induced plasmas, 6 and it was found that the LIP departs from LTE with the increase of distance from the target. Giacomo et al ablated titanium dioxide and monoxide targets in a low-pressure environment and found that 5–10 ns after the pulsed laser, 12 the LIP at 0.6 mm away from the target surface was in LTE by the state-to-state collisional radiative model.…”

Optical emission spectrometric diagnosis of laser-induced plasma and shock front produced at moderate pressure

“…Laser interferometric studies and Thomson scattering, for instance, are capable of obtaining plasma information, but the accuracy can be reduced by probe laser-induced heating effects [16] . As a non-contact optical measurement technique, OES is an alternative route to such quantitative parameters of LIPs [17] .…”