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 3010 to 4350 K and the rotational temperature (Trot) measured from the OH and O2 diatomic molecular spectra ranged from 2250 to 3550 K. The electron density (ne) from the HΒ Stark broadening width at the plasma core was in the range of 6.6 to 7.6× 1014 cm-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.