Gas temperature measurement by atomic line broadening using the LIBS technique

Abstract

Accurate temperature measurement of the gas flow field is of great significance in theoretical research and engineering applications. Here, we found that when the laser-induced breakdown spectroscopy (LIBS) technique is performed in gas, there is a monotonic correlation between the line broadening of the atomic spectrum in the plasma and the initial gas temperature. Based on this monotonic correlation, we propose a method for gas temperature measurements under constant pressure. First, a series of experimental parameters that can sharply capture the linewidth variation were found through preliminary experiments, including the appropriate gate width, ICCD camera delay, and laser pulse energy. We found that the full width at half maximum (FWHM) of the H Balmer α line varies significantly with the increase of the flow field temperature, and the two show a power function dependence. The effect of the initial gas temperature on the plasma parameters and how this effect is passed on to the FWHM of the spectral lines were discussed from the perspective of electron density, which well explains the calibration curve obtained experimentally. The FWHM of the H Balmer α line at 656.3 nm, serving as an indicator, exhibits a decrease of about 30% in the temperature range of 300–1100 K, while the FWHM change tends to flatten out in the combustion field with higher temperatures. In exploring the effect of matrix effects (gas component changes) on this thermometry technique, we did not observe this nuisance, which is usually prevalent in solid LIBS, demonstrating the potential of this technique for wide applications. For flow fields without hydrogen atoms, we propose to deal by injecting the inert gas Kr. We found that the FWHM of the Kr I 810 nm spectral line follows the same trend with temperature as that of the H Balmer α line and also shows good applicability independent of the components. The results provide an experimental basis for developing LIBS-based gas temperature measurement techniques. They may provide support in potential application situations such as flow field organization and thermal system design.