Formation Mechanism Of Patterns In Argon Single-needle Jet At Atmospheric Pressure Investigated By Optical Method

Without the need for vacuum devices,and having the ability to separate the generation and application of plasma,atmospheric pressure plasma jet（APPJ）has become one of the most popular research focuses in the field of low-temperature plasma,by which a plasma plume abundant with active particles can be generated.As is well known,the streamer head of APPJ is rich in charged particles,active particles,and ultraviolet photons,which can induce multiple biological effects and chemical reactions.Therefore,the rate of plasma polymerization and material growth along the discharge paths is much higher than at other locations.Results indicate that the streamer could propagate along inerratic tracks under appropriate conditions,leading to regular patterns of plumes.Hence,investigation on streamer dynamics and patterns in APPJ can assist to improve the accuracy of fixed-point treatment of plasma,which is of great significance for localized growth and etching of materials.In this thesis,several discharge patterns are produced by an argon APPJ with a single-needle structure at atmospheric pressure.Optical and electrical characteristics,temporal evolution,and plasma parameters of these patterns are investigated by means of electrical,optical,and spectroscopic methods,from which the mechanism of pattern formation and streamer behavior are analyzed.In addition,dynamics of positive streamer encountered with a cloud of positive ions with different densities is numerically investigated by employing a two-dimensional fluid model.The main research contents are as follows.（1）Patterns in open space produced by APPJ.Plasma plumes with various patterns are produced in open space with varying experimental parameters,including solid-cone,hollow-cone,hollow-swell,diffuse-swell,and planar plumes.Results show that the solid-cone plume originates from the propagation of negative and positive streamers,for which the negative streamer presents a diffuse cone shape and the positive streamer is filamentary and propagates straightly along the flow axis.The hollow-cone plume results from positive branching streamers which tend to appear at the interface between the argon stream and the surrounding air.The hollow-swell plume corresponds to two positive streamers in the same half cycle,for which the subsequent streamer will detour the tail of the previous streamer track（position where the streamer head stops）,leaving a curved track.However,for the diffuse-swell plume,the subsequent streamer can directly pass through the position where the previous streamer stops and presents stochastic branches.In addition,there are two modes of the planar plume,namely streamer mode and filamentary mode.The streamer-mode plume is composed of stochastically branching streamers,whereas the filamentary-mode plume results from a series of filaments moving along the flow.Plasma parameters of each pattern are diagnosed using optical emission spectroscopy.By considering the residual particles（including positive ions,negative ions,metastable states,etc.）,the spatial distribution of electric field and the effect of gas flow,formation mechanism of these patterns are analyzed qualitatively and streamer dynamics are revealed.（2）Patterns on dielectric target produced by APPJ.Single-ring,triple-ring,and petal-like patterns are observed on the dielectric target downstream of a single-needle APPJ.Results indicate the ring structures of patterns on the target are formed by the time superposition of positive streamers propagating along an arc.In the same half voltage cycle,the subsequent streamer tends to detour the track of the previous streamer.With an applied electric field high enough,the subsequent streamer can also cross the track of the previous streamer.The streamer behavior of petal-like patterns is more complex,which undergoes a stripe phase,a homogeneous phase,and a branch phase.By means of spectroscopy,the electric field distribution during streamer propagation is analyzed.Combining the electric field distribution,composition of working gas,and residual particles on the target,propagation behaviors of surface streamers are qualitatively analyzed and formation mechanism of patterns on the target is explained.（3）Numerical simulation of positive ion cloud effect on positive streamer dynamics.The dynamic behavior of a streamer encountered with a cloud of positive ions is investigated by employing a two-dimensional fluid model,through solving continuity,conservation,and Poisson’s equations.Results indicate that density of the ion cloud plays a decisive role in the propagation of positive streamer.With a rarefied ion cloud(1.0×10¹⁶ m^-3),the streamer always propagates along the axis,in other words,the streamer passes through the ion cloud directly which has an ignorable influence on the propagation of positive streamer.The streamer deflects upward when it approaches an ion cloud with a medium density(5.0×10¹⁶m^-3).Then it deflects downward after it departs from the ion cloud.Therefore,the streamer presents a detour behavior.When the positive streamer is encountered with a dense ion cloud(1.0×10¹⁷ m^-3),the primary streamer deflects upward in the vicinity of the cloud,and a secondary streamer initiates inside the ion cloud.A detouring track is fulfilled by the relay of these two streamers.Moreover,the velocity of the streamer and the deflection amplitude of the detouring track are simulated with varying ion density,cloud position,cloud scale,voltage amplitude,and gap width.