Experimental Investigation On The Argon Discharge Characteristics Of Cross-field Plasma Brushes

Non-equilibrium plasmas with a low temperature have attracted much attention because chemically active species are abundant in these plasmas.They have enormous application potentials in various fields of science and technology.Large-scale plasmas are more desirable for some applications because they can improve working efficiency.However,it is difficult to generate the large-scale plasmas outside the electrodes at atmospheric pressure.Therefore,three cross-field discharge devices are designed to generate the large-scale plasmas,the discharge characteristics and mechanisms of the plasma plumes are investigated in this thesis.By a cross-field discharge device composed of two naked electrodes,a large-scale plasma plume at atmospheric pressure is generated downstream of two opposite electrodes.The plasma plume is made up of micro-discharge filaments periodically propagating along the argon flow.With the periodical propagations,amplitude of the gas voltage periodically varies.Under a lower dissipated power,a period of the gas voltage envelope can be divided into two regions: an increasing amplitude region,and a fluctuating amplitude one.With increasing the power,duration time increases for the increasing amplitude region,and decreases for the fluctuating amplitude region until it disappears completely.Details of the two regions are investigated as to waveforms of the gas voltage,the current and the integrated emission.Results indicate that there are one current pulse and one emission pulse per half voltage cycle in the increasing amplitude region.However,in the fluctuating amplitude region,there are one emission pulse and less current pulse on average per half voltage cycle.Moreover,two current pulses appear per period of the voltage-amplitude fluctuation.Comprehensive investigations are performed using fast photography over the planar plume,synchronized with simultaneously recorded waveforms.It is found that filament length and its minimal impedance increase with time during one voltage-envelope period,which almost equals to the filament lifetime.All of the results verify the periodical propagation of the filament results in the periodical voltage envelope.In order to avoid ballast resistors and fluctuating of the voltage in the naked electrodes discharge device,a wedged barrier discharge device is developed to generate a large-scale brush-shaped plasma plume.The parameter range for plume generation and its discharge characteristics are studied through electrical and optical methods.The spatial and temporal evolution is implemented by fast photography to investigate the formation mechanism of the plume.At a lower voltage,it is found that the large-scale plume is a superposition of micro-discharge filaments gliding along the argon flow direction,which operate in a glow discharge regime.However,streamer-discharge branches appear stochastically on the gliding micro-discharge filaments under an overvoltage.Results also indicate that the plasma is in a non-equilibrium condition.Then,using a cross-field transverse dielectric barrier discharge(TDBD)device,a laminar plasma plume with a fairly large volume is generated at atmospheric pressure.Its discharge characteristics are investigated through electrical and optical methods.Results indicate that the discharge power,based on Lissajous plot,increases with increasing the peak voltage.High-speed imaging indicates that the formation mechanism of the plasma plume corresponds to a series of micro-discharge filaments leaving the outlet of the gas duct,whose probable lifetime is about 2.7 ms.In order to improving its uniformity,an auxiliary dielectric barrier discharge(ADBD)is added upstream of TDBD device.Enhanced by the ADBD,a uniform TDBD plume with a fairly large scale is generated downstream of a narrow slit.Electrical and optical characteristics are compared for the two discharges with and without the ADBD.Results indicate that the plume with the ADBD is longer,more uniform,and dissipates a higher power.Moreover,its inception voltage is much lower.High-speed imaging presents that the uniform plasma plume with the ADBD comprises a series of moving micro-discharge filaments in a glow regime,which are much smoother than those without the ADBD.