Experimental characterization of argon/air mixture microwave plasmas using optical emission spectroscopy
Résumé
The plasma is generated by a microwave (MW) source inside a conditioned cell with an axial gas injection working at high pressure argon with a small admixture of air. Several characterization methods based on the analysis of the optical emission spectra are employed to determine rotational Trot, vibrational Tvib, excitation Tex and electron Te temperatures along the plasma flame axis. The densities of electrons and oxygen and nitrogen atoms are also determined from spectra analysis for the same plasma axis positions. The rotational and vibrational temperatures are simultaneously estimated using a new method based on the analysis of 3 consecutive band heads of the second positive system (SPS) of molecular nitrogen spectra. This new method, based on pre-calculated synthetic spectra databases, leads to Trot varying from about 2000 to 2500 K along the plasma axis while Tvib are higher (around 5000 K and more). Both temperatures are coherent when determined from N2SPS in the case of two distinct wavelength ranges (around 313 nm and 376 nm). These temperatures are in agreement with Trot determined from comparison between synthetic and experimental OH(A-X) spectra and Tvib determined from the first negative system N2+FNS spectra. The estimated electron temperatures Te are around 2.0 eV while the electron densities are in the range 1015 to 1016cm−3 using a literature method adapted in the present work to air diluted in argon buffer gas. The densities of atomic oxygen are estimated close to 1015 cm−3 and those of nitrogen close to 1016 cm−3 when using an actinometry method assuming a priori known the density of hot buffer argon gas at Trot temperature and also Tex determined using classical Boltzmann plots based on a chosen series of Ar lines. The significant deviations between the different temperatures Trot, Tvib, Tex and Te clearly underline a pronounced non thermal equilibrium of the present MW argon/air plasma. It is also shown an important chemical non-equilibrium since the measured electron, N and O densities are found much higher than the usual equilibrium densities generated by thermal dissociation and recombination processes. This therefore means that the energetic electron impacts play a significant role on the formation of the present non-equilibrium MW plasma.