Computational study of the afterglow in single and sequential pulsing of an atmospheric-pressure plasma jet

The spatial distribution of charged species in the afterglow of a helium plasma jet impinging atmospheric air has been computed using a 2D axisymmetric fluid model. The model is solved for two consecutive pulse periods of a rectangular voltage waveform (duration of 60 ns) and for two different frequencies (25 and 50 kHz). The most abundant ionic species in the afterglow are found to be ${\text{O}}_2^{+}$ and ${\text{O}}_2^{-}$ with their concentrations increasing by about an order of magnitude (up to about 10¹⁸ m⁻³ and 10¹⁷ m⁻³ respectively) in the initial 1 μs. In the first pulse, these species form a halo around the diffusing He⁺ and electron rich central channel, the shape of the former being strongly correlated with the shape of He–air mixing layer computed using a hydrodynamic model. In the next pulse, this general configuration is also observed; however ${\text{O}}_2^{+}$ is more concentrated on the axis of the jet, this being due to influence of residual electrons in the central channel. For ${\text{O}}_2^{-}$ there is little difference in their spatial distribution compared to the initial pulse. For higher frequency pulsing, the higher concentration of residual electrons lowers the necessary ignition electric field reducing the concentrations (by 25%) of charged species in a period of the applied waveform. This work provides new information on the concentration and distribution of ionic species generated by atmospheric-pressure capillary discharges of interest to those developing such sources for range of applications, particularly in the field of plasma medicine. (paper)