Computational model of the interaction of a helium atmospheric-pressure jet with a dielectric surface

Using a time-dependent two-dimensional axisymmetric fluid model the interaction of a plasma jet with a dielectric surface has been studied. The model is solved for two consecutive periods of a positive unipolar pulsed waveform. The study concentrates on determining the fluxes of the main oxygen ion species, O-2(+), O-2(-) and the total accumulated charge on the surface. Approaching the dielectric surface, the streamer head is seen to divert its direction of propagation, spreading out radially approximately 0.2 mm above the dielectric surface. For O-2(+) generated near the streamer head, this leads to a maximum in their flux to the surface which moves radially outwards with the streamer propagation, driven by the applied electric field in pulse on-time. In the off-time, the flux of O-2(+) drops by at least two orders of magnitude. As a result, the total number of O-2(+) ions arriving at the surface over one entire pulse period (fluence) has an annular shape limited by the effective contact area of the streamer on the surface. In contrast O-2(-) ions generated in the pulse on-time do not reach the surface due to the direction of the applied electric field. In the off-time, O-2(-) ions generated at the edges of the deformed streamer are pushed by the accumulated surface charge outwards. As a result, the O-2(-) fluence has an annular structure with its maximum being outside the area of the dielectric surface covered by the plasma channel. Solving for the second pulse period shows small changes in the predicted fluences, with largest difference seen with O-2(-). We see that increasing the flow rate (by a factor of three) shifts the position of the maximum fluence of O-2(+) outwards, and decreasing the O-2(-) fluence in the second pulse period.