A numerical study of the effect of water content on OH production in a pulsed-dc atmospheric pressure helium-air plasma jet is presented. The generation and loss mechanisms of the OH radicals in a positive half-cycle of the applied voltage are studied and discussed. It is found that the peak OH density increases with water content in air (varying from 0% to 1%) and reaches 6.3×10 18 m −3 when the water content is 1%. Besides, as the water content increases from 0.01% to 1%, the space-averaged reaction rate of three-body recombination increases dramatically and is comparable to those of main OH generation reactions.

Recently, C/H/Ar plasma discharges found enormous potential and possibility in carbonaceous compounds conversion and production. In this work, a pulsed-dc CH 3 OH/Ar plasma jet generated at atmospheric pressure is investigated by means of optical and electrical diagnosis concerning the variation of its basic parameters, absolute concentration of OH radicals, and plasma temperature with different CH 3 OH/Ar volume ratios, in the core region of discharge with needle-to-ring electrode configuration. The voltage-current characteristics are also measured at different CH 3 OH/Ar ratios. Laser-induced fluorescence (LIF) results here show that only small amounts of added methanol vapor to argon plasma (about 0.05% CH 3 OH/ Ar volume ratio) is favorable for the production of OH radicals. The optical emission lines of CH, CN, and C 2 radicals have been detected in the CH 3 OH/Ar plasma. And, the plasma temperatures increase with successive amount of added methanol vapor to the growth plasma. Moreover, qualitative discussions are presented regarding the mechanisms for methanol dissociation and effect of the CH 3 OH component on the Ar plasma discharge at atmospheric pressure.

In this paper, a two-dimensional coupled model of neutral gas flow and plasma dynamics is presented to explain the gas flow dependence of discharge characteristics in helium plasma needle at atmospherics pressure. The diffusional mixing layer between the helium jet core and the ambient air has a moderate effect on the streamer propagation. The obtained simulation results present that the streamer shows the ring-shaped emission profile at a moderate gas flow rate. The key chemical reactions which drive the streamer propagation are electron-impact ionization of helium neutral, nitrogen and oxygen molecules. At a moderate gas flow rate of 0.5 slm, a significant increase in propagation velocity of the streamer is observed due to appropriate quantity of impurities air diffuse into the helium. Besides, when the gas flow rate is below 0.35 slm, the radial density of ground-state atomic oxygen peaks along the axis of symmetry. However, when the gas flow rate is above 0.5 slm, a ring-shaped density distribution appears. The peak density is on the order of 1020 m−3 at 10 ns in our work.

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A two-dimensional coupled model of neutral gas flow and plasma dynamics is presented to explain the two distinctive patterns of ground-state atomic oxygen density profiles that have been observed experimentally in the helium plasma needle discharge. When the gas flow rate is 0.25 standard liter per minute (SLM), the discharge is substantially sustained by the electron impact ionization of air near a dielectric surface, corresponding to the radial density peaks along the axis of the symmetry. However, as the flow rate is 1.1 SLM, Penning ionization between helium metastables and surrounding air dominates the ionization reactions and peaks at an off-center position (𝑟 = 0.9 mm), which indicates the ring-shaped density distribution. The critical feeding gas flow rate is found to be around 0.4 SLM. The peak density is on the order of 10 20 m −3 in our case. Previous reports of a flow-dependent bacterial killing pattern and ground-state atomic oxygen measurement support our simulation results.