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Influence of N2, O2, and H2 admixtures on the electron power balance and neutral gas heating in microwave Ar plasmas at atmospheric pressure

A combination of optical emission and absorption spectroscopy of argon 2p –1s transitions (Paschen notation) combined with collisional-radiative (CR) modelling of argon 2p  states was used to characterize microwave argon plasmas at atmospheric pressure in presence of N2, O2, and H2 admixtures. In particular, the neutral gas temperature (obtained from the broadening of argon 2p 2–1s2 and 2p 3–1s2 emission lines), the number density of argon 1s5 atoms (obtained from absorption spectroscopy of the argon 2p 9–1s5 transition using a tunable laser diode), the electron temperature (obtained from the comparison between measured and simulated argon 2p -to-1s relative line emission intensities), and the electron density (obtained from the Stark broadening of the Hβ line and argon relative line emission intensities) were recorded as a function of the axial distance along the microwave plasma column. The results show that, for a given position in the plasma and a higher amount of admixture in the nominally pure argon plasma, the neutral gas temperature increases and the electron number density decreases, while the electron temperature and the population of argon metastable atoms first decreases and then increases at higher concentrations. With such information, a detailed analysis of the electron power balance was performed. It is found that less than 1% of the admixture in the argon plasma already absorbed more than 80% of the microwave power. Part of this energy is used for neutral gas heating, mostly through electron-impact excitation of rotational levels.