Parametric study of atmospheric pressure microwave-induced $Ar∕O2$ plasmas and the ambient air effect on the plasma

A torch type microwave-induced afterglow plasma was produced at atmospheric pressure using an open-ended fused silica concentric double tube assisted by Ar and $O2$ supply gases. The plasma emerged from the end of the discharge tube and was exposed to ambient air. A parametric study of the plasma characteristics was performed by measuring the temperature, density, and plasma volume as the operational parameters such as microwave power, gas flow rate, and its composition were varied. The excitation temperature $(Texc)$ obtained from the Ar I emission spectrum ranged from $3010to4350K$ and the rotational temperature $(Trot)$ measured from the OH and $O2$ diatomic molecular spectra ranged from $2250to3550K$⁠. The electron density $(ne)$ from the $Hβ$ Stark broadening width at the plasma core was in the range of $6.6to7.6×1014cm−3$⁠. The two-dimensional distribution of $Texc$ and $Trot$ was also obtained. Experiments while varying the Ar and $O2$ gas flow rate and the $O2∕Ar$ ratio showed that $ne$ was reduced but $Texc$ was increased as the $O2$ flow rate was increased. Using an additional dielectric tube for shielding the plasma from the ambient air demonstrated a significantly enlarged plasma length and lower $Trot$ due to the nitrogen entrainment, as compared to the unshielded case.