Optical Emission Study of a Direct-Current, Atmospheric-Pressure Non-Thermal Plasma Microjet

This work aims to study a direct-current driven, atmospheric-pressure non-thermal plasma microjet (PMJ) operated in air with mixtures of He or He/O$_{2}$ as the working gases with a typical current of 5-40 mA and a gas flow rate of 2-5 slm. Optical emission spectra were taken from end-on and in some cases from side-on (at different distances from the exit nozzle). End-on spectra show major He emission lines as well as weak emissions the N$_{2}$ 2$^{nd}$ positive system (C$^{3}\Pi _{u}$-B$^{3}\Pi _{g})$. Strong atomic oxygen emission was also observed. Similar emissions were observed when the He/O$_{2}$ PMJ was submerged in water. The strong emission of oxygen can result from direct He* penning ionization of O$_{2}$ molecules followed by the electron-impact dissociation of O$_{2}^{+}$. The relative intensity of the oxygen was found to increase with the increase of the operating current and peaked at an O$_{2}$ volume concentration of 0.1{\%}-0.3{\%}.