Spectroscopic Diagnosis On Spatiotemporal Evolution Of Atmospheric Pressure Surface Discharge Plasma And OH Radical

As a low-temperature plasma generation technology,Atmospheric pressure surface discharge is an effective way to generate large-scale plasma.As some desirable plasma features,namely,safety,uniformity and flexible shape,surface discharge plasma has great application potentials in numerous fields,such as biomedicine,agriculture,food protection and environmental protection,etc.The reactive species generated by surface discharge plasma play a key role in these applications.Of all the reactive species,OH radical is one of the most strongly oxidative species produced by water containing plasmas and plays an important role in plasma biomedicine application.In this dissertation,the spatiotemporal evolution of AC surface discharge plasma confined by an array of hexagon electrode mesh in helium at atmospheric pressure and the transport behavior of OH radical are investigated adopting optical emission spectroscopy,laser induced fluorescence and high-speed imaging technologies.Details are presented as follows:1.Investigations of the spatiotemporal evolution of surface discharge plasma in helium at atmospheric pressure.The spatiotemporal evolutions of surface discharge plasma within one hexagon mesh unit and on the array electrode surface are studied via time-resolved optical emission spectroscopy and high-speed imaging technologies in helium with a constant repetition frequency of 30 k Hz.The temporal behaviors of the filtered image of four spectral lines（OH at 309 nm,N₂⁺at 391.4 nm,He at 706.5 nm and O at 777.4 nm）are also investigated.According to the investigation,we find:（i）The polarity of applied sinusoidal voltage could affect the characteristics of surface discharge plasma.The plasma has a discontinuous structure similar to a plasma bullet in the negative half cycle in one hexagon element while the discharge channel is continuous in the positive half cycle.（ii）With an increase in amplitude of applied voltage,multiple breakdowns will occur in the negative half cycle,resulting in multiple current pulses,while there is only one current pulse in the positive half cycle.In the negative half cycle,the position of breakdown within one mesh unit alternates between the middle part of the rim electrode and the corner of hexagon mesh.The number of strong current pulses in the negative half cycle could influence the breakdown position in the positive half cycle.When the number of the strong current pulses in negative half cycle is odd,the discharge event originates at the middle part of the rim electrode.When the number of the strong current pulses in negative half cycle is even,the discharge event starts at the middle part of the corner of the hexagon mesh.Namely,the discharge event in the positive half cycle originates at the same position with the last strong discharge event in the negative half cycle.（iii）The interactions between plasmas originated at different positions within one mesh unit in the positive half cycle are investigated.It is found that the applied electric field and the space-charge electric field formed by surface charges produced in the discharge would led to plasma branching.Considerable energy would lost in the process of establishing branches,causing a reduction in emission intensity.The distribution of surface charges could determine the plasma path to a certain extent.（iv）The spatiotemporal evolution process of plasma within different mesh units is not synchronized.An ignited plasma could ignite the adjacent elements that have not generated discharge.During the last discharge of the negative half cycle,the propagation direction of the plasma on the electrode array is opposite to that of the positive half cycle.The velocity of ionization wave on the array electrode surface is about 20 km/s.（v）Based on the analysis of the dynamic evolutions of spectral lines obtained by the filtered images,it is found that the direct electron impact excitation dominates the generation of the emission spectrum in the middle part of the rim electrode where the discharge event originates during the initial period.In contrast,the fan-shaped diffusion of the plasma patterns head is principally due to Penning ionization and charge transfer reaction in later phases.2.Investigations of OH radical generated by surface discharge plasma in helium at atmospheric pressure.The absolute density of ground state OH radical and spatiotemporal evolution of OH radical in the directions parallel and perpendicular to the surface discharge electrode array are studied in a pulse modulated AC surface discharge using laser induced fluorescence and high-speed imaging technologies.According to the investigation,we find:（i）When the laser sheet is parallel to the electrode,the effects of dissipated power,modulation on-time duration（duty cycle）and the distance from the electrode surface on OH radical are studied.It is found that dissipated power and modulation on-time duration have a greater impact on OH lifetime than OH density.The OH lifetime dependence on power is more pronounced as compared with the modulation time.The increase of the distance from the electrode surface results in the increase of OH lifetime and the decrease of OH density.（ii）When the laser sheet is perpendicular to the electrode,the spatiotemporal evolution of OH radical in the direction perpendicular to the electrode is investigated during the discharge and the post-discharge.It is demonstrated that the ionic wind caused by electro-hydro-dynamic force dominates the transport of OH radical from the surface plasma layer to the afterglow region.The maximum value of the propagation velocity is estimated as 1.86 m/s and the transport distance in perpendicular direction is up to 8 mm.After the plasma switches off,the impact of ionic wind is still present during the post-discharge.Besides,the dissipated power has no significant effect on the propagation velocity and delivery distance.（iii）When the laser sheet is perpendicular to the electrode,the effect of the geometric parameters of the electrode configuration on the spatiotemporal evolution of OH radical in the direction perpendicular to the electrode is investigated during the discharge.The findings show that the increased lattice distance of mesh electrode could cause the increase in the density,delivery distance and propagation velocity of OH radical.The increase of lattice distance of mesh electrode would increase the transmission distance of reactive chemical species while the reduction of the lattice distance facilitates a high degree of uniformity for reactive species on a downstream target.For the same hexagon mesh unit with electrode widths of 0.25 and 1 mm,it is found that a wide electrode width could improve the even distribution of reactive species and will increase the density of OH radical in the downstream region.