| Atmospheric-pressure(AP)microwave plasma has great potential in industrial fields due to its distinct advantages,e.g.electrodeless configuration,high power density and safety etc.,and thus has been attracted extensively by both domestic and foreign laboratories.Compared with resonant cavity microwave discharge,plasma sustained by surface wave could extend far away from the wave launcher without any special waveguide to confine and guide the surface wave propagation.Most of the previous work on the AP surface wave plasma(SWP)has been focused on the discharges in continuous working mode.With the increasing of operating frequency and gas pressure,the plasma gas temperature will be enhanced significantly.On the one hand,usually a special cooling system is employed to protect the discharge tube from the thermal deterioration.On the other,the high gas temperature also restricts its applications at low temperature.In this thesis,a pulse-modulated microwave power supply technique is proposed to manipulate the plasma parameters of SWP including the gas temperature.It is also provide a method to investigate the production and evolution of the fundamental processes of AP SWP.Moreover,during the formation of SWP,a novel phenomenon is observed experimentally.Optical emission spectroscopy(OES)as a passive spectroscopic diagnostics has been employed for plasma diagnosis of AP microwave discharge.Due to the advantages of being fast time response,nonintrusive,inexpensive and easy to implement,OES has been employed for on-line plasma diagnostics.In present work,the main contents are summarized as follows:1.An atmospheric-pressure,pulse-modulated surface wave argon plasma is investigated with respect to its propagation of the ionization front.The time-resolved photographs about the advance of the ionization front are taken using a high speed camera.The ionization front velocity and its rise time when propagating along the discharge tube are measured with respect to a series of values of input power,duty ratio,and pulse modulated frequency.And it is found that the reduced electric field and memory effect from previous discharge impose the influence on both the ionization front velocity and its rise time strongly.Based on OES technique,the temporal evolution of the gas temperature,the electron temperature and density,the radiative species of atomic Ar,and the molecular band of OH(A)and N2(C)are investigated experimentally by altering the instantaneous power,pulse repetitive frequency,and duty ratio.We focused on the physical phenomena occurring at the onset of the time-on period and after the power interruption at the start of the time-off period.It is found that the gas temperature can be tuned by changing the duty ratio effectively.In the steady state,the electron temperature equals to the excitation temperature of Ar I and the electron density is estimated to reach up to;he order of 1015 cm-3,which can be manipulated by the absorbed power.During the post-discharge,the decay time constant of electron density is estimated to be 3.5 ?s,and the population of the Ar I exhibits a Saha excitation balance.2.A propagating standing wave pattern is observed experimentally during the development of AP SWP by the high-speed camera.The temporal evolution of the electron density and temperature influenced by the standing wave is investigated by time-resolved optical emission spectroscopy.The results indicate that the enhanced emission intensity in plasma column corresponds to the node of standing wave where the electron density and temperature decreases.This behavior is attributed to the fact that the excited Ar I in the plasma are govern by the Saha balance.The physical mechanism with respect to the formation of propagating standing wave is studied based on the electromagnetic wave model.It is the velocity of the ionization front that is much smaller than that of surface wave in plasma column resulting in the wave reflection at the interface between the ionization front and neutral gas.In contrast,a stationary standing wave pattern is produced by restricting the geometry size of the discharge tube.The incident surface wave along the plasma column is reflected at the interface of tube end and then interferes with the reflection wave produced.The enhanced emission intensity in the plasma column is the place where the antinode of the standing wave is located and thus the electron density and temperature increases.3.In order to obtain large-scale surface wave plasma,the axial profiles of the pulse modulated atmospheric pressure surface-wave argon plasma confined in discharge tubes of different diameters are measured by means of optical emission spectroscopy with respect to the emission intensity,gas temperature,and electron density and temperature.Then,the role of the discharge tube is investigated with respect to its diameter.Furthermore,a theoretical model for high collision frequency is proposed for analysis of the influence of tube diameter on the plasma electron density.Theoretical results show that with the increase of diameter of discharge tube,the electron density decreases and its axial gradient becomes smaller,which achieves a good agreement with the experimental results of electron density obtained by the Stark broadening technique.It is suggested that the tube of small diameter is a better candidate for applications requiring higher electron density and emission intensity,e.g.,element analysis,while the tube of larger diameter is more suitable for high throughput processing,e.g.gas conversion.4.When the diameter of discharge tube reaches the maximum value that is equal to the aperture holes on the wave launcher,the discharge presents a shape of plasma torch.Atmospheric-pressure microwave induced N2 plasma is diagnosed by optical emission spectroscopy with respect to the plasma gas temperature.The spectroscopic measurement of plasma gas temperature is discussed with respect to the spectral line broadening of Ar I and the various emission rotational-vibrational band systems of N2(B-A),N2(C-B)and N2+(B-X).It is found that the Boltzmann plot of the selective spectral lines from N2+(B-X)at 391.4 nm is preferable to others with an accuracy better than 5%for an atmospheric-pressure plasma of high gas temperature.On the basis of the thermal balance equation,the dependences of the plasma gas temperature on the absorbed power,the gas flow rate,and the gas composition are investigated experimentally with photographs recording the plasma morphology.Moreover,the species N2(C),N2(B),and N2+(B),in the post-discharge are investigated by means of optical emission diagnosis of the spatial distribution of emission intensities of N2(B-A),N2(C-B)and N2+(B-X)transitions.Correspondingly,the post-discharge can be divided into two distinct regimes,the early and late afterglows.It is found that not only atomic N survives in the late afterglow regime of the post-discharge but also the N2+ ions are produced even far from the microwave launcher.This is attributed to the fact that the vibrationally excited N2(X,v)and atomic N with a long lifetime can be conveyed at far distance and act as the precursor of generating N2+ions locally. |
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