Ignition and dynamics of nanosecond pulsed helium streamers over a water electrode

Abstract

Applications of atmospheric pressure non-equilibrium plasma jets for a variety of biomedical applications have motivated fundamental studies of the gas-phase plasma in the presence of water. The initiation of guided streamers, excited with 164 ns, 5.2 kV pulses, as well as the dynamics of the streamer were investigated using a micro-hollow needle-to-plate configuration with and without water. Addition of water to the ground plate results in the enhanced ionization at the needle anode, earlier streamer formation, greater total energy deposition, and stronger emissions by excited states including N2, N2+, OH, and O. Importantly, spatiotemporally-resolved emission spectroscopy reveals that excitation transfer via collisions with heavy particles played an important role in the production of OH(A2Σ) in addition to direct electron impact.

Effects of pulse width on He plasma jets in contact with water evaluated by OH(A–X) emission and OHaq production

“…The positive and negative peaks in the jet current are associated with the discharges during the rising and falling phases of the voltage pulse, which are considered as the primary and secondary discharges, respectively. 14) The positive peaks of the jet currents for various pulse widths are the same at 52 mA. They are at least 10 times more than their correspondent negative peaks that occurred during the falling phase of the voltage pulses.…”

“…For this gap distance, a dent was observed at the water surface because of the helium gas flow. 14) The applied voltage and total current were measured at the load end of the transmission line using a HV probe (Tektronix 6015 A) and a current monitor (Pearson 6585), respectively. Another current monitor (Pearson 6585) was placed at the discharge gap for measurement of the jet current as shown in Fig.…”

“…The jet current, obtained by subtracting the assumed displacement current measured without He flow at the same pulsed voltage from that with He flow and hence the plasma was on, which is a good representation of the discharge dynamics of the plasma plume. 14) Figure 5 shown the jet currents for APNPJs driven by various pulse widths ranging from t pw = 200 to 1000 ns. The positive and negative peaks in the jet current are associated with the discharges during the rising and falling phases of the voltage pulse, which are considered as the primary and secondary discharges, respectively.…”

“…7,8) Previous studies showed that the presence of water could highly affect the spatial and temporal distribution of OH in plasma plumes by comparing plasma jets in air with in contact with water. 13,14) Along the discharge gap between the jet nozzle and water surface, a bimodal distribution of OH emissions was observed with strong peaks near the jet nozzle and the water surface via spatially-resolved optical emission spectroscopy. 14) By using laser-induced fluorescence, it was found that a large amount of ground state OH formed a conical shape within 2 mm above the water surface with a maximal concentration of 1.7 × 10 14 cm −3 .…”

“…13,14) Along the discharge gap between the jet nozzle and water surface, a bimodal distribution of OH emissions was observed with strong peaks near the jet nozzle and the water surface via spatially-resolved optical emission spectroscopy. 14) By using laser-induced fluorescence, it was found that a large amount of ground state OH formed a conical shape within 2 mm above the water surface with a maximal concentration of 1.7 × 10 14 cm −3 . 13) Computational studies showed that OH could be largely produced in the plasma column as well as near the water surface when a pulsed plasma jet was directed onto a water layer with 200 μm in thickness.…”