Atomic emission spectroscopy method for mixing studies in high power thermal plasmas

Atomic emission spectrometry was used to measure the distribution of concentration ratios and temperature in thermal plasmas produced by high power torches for process engineering. The spectroscopic method is based on absolute line intensity measurement and Abel inversion. Assuming local thermal equilibrium, the temperature is deduced from the absolute emission of an argon line and the concentration ratio is deduced from the emission ratio of two lines. Using the two dimensions of the camera for spatial and spectral resolution, fast measurements with high resolution can be done. The method has been tested on a 60 kW inductively coupled plasma torch at atmospheric pressure. The results show that the concentration ratios n(O)/n(Ar) and n(H)/n(Ar) can be measured with an accuracy of 25% and that errors due to deviations from LTE are small. Demixing occurs in the induction zone. The application of the method showed that hydrogen diffuses much more than oxygen. The main disadvantage of this method is that, using emission, it does not permit to measure the concentration in the "cold" zones in the center and at the edge of the plasma. - Highlights: • Optical emission spectroscopy applied to the detailed analysis of a thermal plasma torch • Measurement of the distribution of concentration ratios between two elements • Analysis of error sources: fluctuations and deviations from local thermal equilibrium • Demixing occurs in the induction zone. • The concentration ratio can be measured with an accuracy of 25%