A study on the temporally and spatially resolved OH radical distribution of a room-temperature atmospheric-pressure plasma jet by laser-induced fluorescence imaging

In this paper, the time and spatially resolved OH distribution of a room-temperature atmospheric-pressure plasma jet is investigated using a laser-induced fluorescence (LIF) method. The plasma jet is generated in room air by applying a nanosecond pulsed high voltage onto a ceramic tube with helium gas flow. It is found that, before the plasma 'bullet' propagates through the region detected by the laser, there are low OH LIF signals, which are from the OH left from previous discharge pulses. After the propagation of the primary plasma 'bullet' (generated by the primary discharge at the rising edge of the voltage pulse) through the detected region, the OH LIF signals are more than doubled. Furthermore, after the propagation of the secondary plasma 'bullet' (generated by the secondary discharge at the falling edge of the voltage pulse) through the detected region, the OH LIF signals are doubled again. Then, after the voltage pulse, the OH LIF signals decay slowly until about 120 mu s. Starting from about 120 mu s after the voltage pulse, the OH LIF signals have a third increase; its peak value is more than doubled. Detailed investigations find that this is due to the gas flow, which blows the OH generated inside the discharge tube to the detected region. In addition, spatially resolved OH LIF signals show that the signal intensity is stronger on both edges, which gives rise to the donut shape of the OH distribution. Further studies reveal that this might be due to the interaction of the plasma plume with the surrounding water vapor in the air.