Atmospheric pressure plasma jets (APPJs) have attracted considerable attention over the last decade, specifically for use in surface engineering. A comparative study of an APPJ, driven by pulsed DC voltage, is conducted in order to examine the plasma impingement onto different surfaces. In this paper, the effect of gas flow rate and composition is investigated using three kinds of substrates: dielectric glass, distilled water and metal plate using fast imaging. Alongside discharges associated with rising and falling voltage, a so-called third discharge is observed during the pulse for water and metal surfaces which corresponds to a restrike breakdown from surfaces to nozzle. The differences in plasma dynamics observed are mainly attributed to the differences in substrate conductivity. In addition, spatial and temporal distributions of OH and O density are investigated by means of laser induced fluorescence (LIF). The OH/O LIF intensity is found to be much higher for metal and water substrates compared to the glass plate. We attribute this effect to the differences in power dissipation associated with the presence and intensity of the third discharge. Effects of gas flow rate and seed gas (H 2 O and O 2 ) mixing on the LIF enhancement are also studied. The related results provide additional insights for optimizing the generation of reactive species.

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 µs. Starting from about 120 µs 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.

In this work, the dependence of the length of plasma plume, propagation velocity, electric field in the streamer head, and propagation mode transition on the tube diameter varied in the range of 0.07–4 mm is investigated for the first time. The atmospheric-pressure helium plasma plume, ignited by a positive pulsed direct current voltage with a pulse rising time of 60 ns, is confined inside a long glass tube. First, the decreased tube diameter results in the reduction of the length of plasma plume but the growth of aspect ratio of plasma plume. Second, as the tube diameter decreases, the average velocity of the propagation of plasma plume increases first, then reaches a maximum value at tube diameter of 1 mm, and finally decreases for the tube diameter decreasing further. Third, the electric field in the streamer head, determined by the method based on Stark polarization spectroscopy of He 447 nm line, increases monotonically from 9 kV/cm to 20 kV/cm with the tube diameter decreasing from 4 mm to 0.6 mm. Finally, when the tube diameter is further reduced to 0.07 mm, high-speed photography reveals that the propagation mode of the plasma plume transits from the plasma bullet to the continuous plasma column.

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