Effects of the pulse width and oxygen admixture on the production of reactive species in gas- and liquid-phases exposed by bipolar microsecond-pulsed atmospheric pressure helium plasma jets

Atmospheric pressure helium plasma jets driven by a low-frequency bipolar microsecond-pulsed voltage were characterized. Some characteristic features of plasma jets were discussed with an emphasis on the temporal evolution of discharge current and light emission. The electrical and optical characteristics of the jet exhibited quite a strong dependence on the pulse width and oxygen gas admixture. We investigated the effects of the pulse width and oxygen admixture on the production of reactive species in gas and liquid phases. The concentrations of reactive species were measured at the plasma-liquid surface and inside the plasma-treated liquids using ultraviolet absorption spectroscopy, the chemical probe method, and UV-VIS spectrophotometry. In the range of pulse widths of 1.5-5.5 mu s, the 3.5 mu s case was observed to exhibit higher values in discharge current and optical emission intensity. The OH densities estimated at the plasma-liquid interface and inside the plasma-treated liquid were observed to vary with the pulse width, and it exhibited a similar trend of change to those of discharge current, optical emission intensity, and the concentrations of the long-living reactive species, H2O2 and NO3-, in the plasma-treated liquids. Around the oxygen admixture of 10SCCM (=1% vol. %), the discharge current and wavelength-integrated optical intensity were maximal, but the estimated OH density inside the plasma-treated liquid exhibits a slight rise with the increasing oxygen admixture, reaching the highest at 20 SCCM, and then decreases slightly.