Investigation Of The Generation Mechanisms And Spectral Characteristics Of Atmospheric Uniform And Diffuse Plasma Jets

Atmospheric-pressure low-temperature plasmas have been extensively used in various practical applications,such as biological medicine,material fabrication,film deposition,and nanoscience.In recent years,in the field of low-temperature plasma technology,many efforts have been devoted to developing large-scale uniform diffuse plasma jets,so as to improve the working efficiency.When inexpensive argon is used as working gas,the constriction of the discharge channel into filament at atmospheric pressure prevents from the formation of uniform diffuse plasma jet in the dielectric-barrier discharges(DBDs).Direct-current(DC)glow discharge is an effective method to produce diffuse plasmas.However,considerable Joule heat is generated in the ballast resistor and DC discharge space,resulting in great loss of energy and inefficient utilization of power.In terms of the scientific and technical issues above,we will further explore the characteristics and limitations of filamentary plasma jet,the effective route for the filament to transform into diffuse plasma jet,and the novel method to produce low-cost,low-power,large-scale,uniform,and diffuse plasma jet,by using the modern advanced optical diagnostic technology.The specific research contents and related results are shown as follows.??Firstly,we developed a large-gap cylinder-plane filamentary DBD plasma jet,where a simple but efficient method about setting the gate width of the intensified charge coupled device(ICCD)was proposed to accurately characterize the detailed behavior of the plasma jet.Observation of the plasma jet profile shows that the formation of jet is resulted from the overlap of many filaments.Optical emission examination shows a decrease of optical emissions from OH and Ar,but an increase of optical emissions from N₂ along the plasma jet.Comparative analysis reveals that the vapor in the working gas argon rather than in the ambient air dominates the optical emissions from the de-excitation of OH along the plasma jet.It was also found that near the ground electrode,the nitrogen emission spectra are mainly resulted from the direct electron effect on the ground state or metastable nitrogen and the collisions between nitrogen metastables.The rotational temperature is lower than the vibrational temperature,but much higher than the room temperature.This feature makes the filamentary plasma jet only suitable for processing samples bearing high temperature.Secondly,we broke the thinking set of producing uniform and diffuse plasmas by slowing down the ionization rate or increasing the pre-ionization level,proposed the model of filament expansion into diffuse plasma,and developed an atmospheric filamentary discharge diffuse plasma jet.Examination of emission spectra from the plasma jet shows that the emission intensities of OH and Ar increase with the argon flow near the quartz tube nozzle,while the N₂ emission intensity first increases,then decreases,and finally approximately remains unchanged with the increase in the argon flow of interest.It is also found that with the argon flow set at 0.4 l/min,most of the reactive species are gathered close to the nozzle,the OH and Ar emission intensities decrease quickly after the plasma propagates out of the nozzle,but the N₂emission is able to propagate over a larger distance.These distinct spectral emission features of OH,N₂,and Ar are attributed to the different generation and quenching mechanisms of their corresponding excited states,i.e.,OH(A²?⁺),N₂(C³?_?)and Ar(4p)Ar(4s)in the argon plasma jet.Additionally,the formation of the diffuse plasma jet has been clarified by observing the discharge burning phase and solving the Laplace equation for the electric field distribution in an argon cylindrical dielectric-barrier discharge.The filamentary discharge deposits charged particles onto the dielectric.The positive surface charges in the positive half cycle induce a relatively high field in the local region close to the dielectric.The relatively high field and the high pre-ionization in this local region play a key role in initiating the diffuse positive corona.As a result,the transition of filament into diffuse plasma is realized.Finally,we proposed a theory and method of V-I characteristic modulation enhanced gas discharge and developed an atmospheric non-self-sustained DC glow discharge laminar plasma jet array(LPJA)enhanced by V-I characteristic modulation.Theoretical analysis and experimental examination of the V-I characteristics show that arranging non-self-sustained DC discharge cells in parallel can increase the discharge cross-section,decrease the sustaining voltage,and fulfill the transition of discharge from sub-normal to normal mode,so as to enhance the discharge.The electric field profile solved by the Laplace demonstrates that the cylindrical electrodes can optimize the field distribution,reduce the electric field difference between each discharge cell,and empower the sustainable and stable discharge of the LPJA.Examining the optical emission spectra of the LPJA indicates that the V-I characteristic modulation can increase the plasma jet length and plasma chemical activity.It is also found that the nested parallel circuit and alternating anode–cathode arrangements can increase the plasma jet width,plasma uniformity,and device compactness.Comparing different discharge modes shows that the LPJA scale is surprisingly increased nearly by 4 times with the discharge power reduced to 65%of the original.Compared with conventional arrays,this LPJA possesses the widest size90 mm and raises its uniformity from 30%to 97%.The methodology provides a highly cost-efficient roadmap to break through the bottleneck of restricting generation of a low-cost,low-power,large-scale,uniform,and diffuse plasma jet.