Formation Mechanisms On Morphologies Of Plasma Plume In Argon Plasma Jet Excited By A Sinusoidal Wave Ranges From Several Hundred To Several Kilo Hertz

Atmospheric pressure plasma jet can produce plasma plumes abundant with active species and has great potential application in many fields,such as pollution control,surface modification,and flow control.The morphology of plasma plume determines the spatial distribution of the active species,hence it is one of important parameters of plasma jet.However,research on plume morphology is still absent.Aim to this status,several morphologies of plasma plumes are generated and studied in this thesis.Main contents are given as follows.Through using an argon plasma jet excited by a biased sinusoidal voltage with several kHz frequency,a conical plasma plume is produced.With increasing bias voltage(V_b),the plasma plume changes from solid to hollow.Length of the solid plume decreases as V_b increases,while length of the hollow plume decreases as V_b increases.The solid plume has a discharge pulse only during the negative half voltage cycle.The hollow plume has a discharge pulse only in the positive half voltage cycle.Through fast photography,it is found that there is a"plump"bullet propagation in the solid plume,which is in negative streamer mechanism.The hollow plume is in positive streamer mechanism.The positive streamer first appears as a guided streamer,which is then transits into a branching streamer.Because of the mutual effect of Penning ionization and residual negative ions,the branching streamer tends to propagate at the flow periphery,thus forming the hollow structure.This mechanism is verified by mixing different concentrations of nitrogen,oxygen and air into the working gas.For several hundred hertz excitation frequency,regularly distributed swells are found on the plume.The regularly swelling plume can be solid or hollow depending on the polarity of the biased voltage.When the driving frequency is higher than a certain value,the swells becomes almost indistinguishable.It is found by fast photography that the solid swelling plume is in negative streamer mechanism,while the hollow one is in positive streamer mechanism.For both of the two plumes,the swells are attributed to the regular distribution of active species downstream of the nozzle,which are provided by strong periodic discharges near the nozzle.For a higher frequency,a diffuse-and-filamentary plume is produced.By comparison of the photoelectric signals,it is found that the diffuse plume and the filament correspond to the negative discharge pulse and the positive discharge pulse,respectively.Moreover,length of the plume increases with increasing frequency.Through fast photography,formation mechanism the diffuse-and-filamentary plume is a combination of two streamers with different mechanisms.The positive discharge corresponds to thin positive streamer,while the negative discharge corresponds to"plump"negative streamer.Finally,residual active species provided by the negative streamer can reduce the breakdown critical value of field along the central axis of the gas flow,thereby,the forthcoming positive streamer tends to propagate along the axis of the gas flow.