Experimental Investigations About Effects Of Parallel Magnetic Field On Plasmas Produced By Nanosecond Pulsed Discharge At Atmospheric Pressure

Atmospheric pressure discharge,especially using open air as working gas,has always been a hot issue in the field of plasma sciences and technologies,because it does not need a vacuum system and it has a low operation cost.Dielectric barrier discharge is a commonly used method for generating a cold plasma in atmospheric air,and the dielectric barrier discharge is usually driven by traditional AC high voltage.Recently,it has been found that nanosecond pulsed discharge has many excellent characteristics such as better discharge uniformity,higher electron temperature,lower gas temperature,and more kinds of chemical active particles.This is because a nanosecond pulse has the sharp rising edge and the short pulse duration.Nanosecond pulsed discharge has very broad application prospects in fields,such as biomedicine,aerospace,material modification,energy chemical industry and environmental governance.However,the dielectric barrier discharge is a kind of "self-extinguishing"discharge due to the surface charge accumulated on the dielectric surface.Moreover,the dielectric barrier discharge mode in atmospheric pressure air is usually the filamentary mode,and uniform discharge has more advantage over the filamentary discharge in certain applications.Therefore,it is desirable to obtain a dielectric barrier discharge plasma with higher intensity and better uniformity in atmospheric air,based on the wide application of atmospheric cold plasma.Researchers found that an external magnetic field is capable of improving both the intensity and uniformity of a discharge at atmospheric pressure.However,the study concerning effect of magnetic field on the dielectric barrier discharge at atmospheric pressure is relatively few so far.In addition,relevant researches are mainly focused on the influence of the perpendicular magnetic field(when the magnetic field is with the direction perpendicular to the electric field)on a discharge.Little attention has been paid to the influence of the parallel magnetic field(when the magnetic field is with the direction parallel to the electric field)on a discharge.In this paper,nanosecond pulsed high voltage is applied to a parallel-plate dielectric barrier discharge structure,and the discharge plasma in atmospheric air is generated.Influence of parallel magnetic field generated by permanent magnets on the discharge characteristics was studied through electrical and optical diagnostics.The following work is conducted in this thesis:1.Influence of a parallel magnetic field on the bipolar nanosecond pulsed discharge in atmospheric air is investigated.It is found that electrons in plasma are magnetized by the parallel magnetic field,which leads to the change in discharge characteristics.In the discharge driven by the bipolar nanosecond pulse,a primary discharge occurs in the rising front,while the secondary and third discharge occur in the falling front.It is indicated that the intensity of the primary and third discharge is increased by the parallel magnetic field,and the enhancement effect on the third discharge is more prominent.The parallel magnetic field almost has no effects on the intensity of the secondary discharge.Discharge images show that the discharge uniformity is improved by the parallel magnetic field.Discharge channels at the edge of the electrodes develop along the magnetic field lines,and the number of discharge channels is increased by the parallel magnetic field.The discharge mode under different pulse repetitive frequencies is the filamentary mode.However,with the decrease of the pulse repetition frequency,the discharge state changes from the filamentary discharge state into the superimposed discharge state of the filamentary channels and the diffuse background emission.Moreover,diffuse background emission is stronger in the discharge with the parallel magnetic field.It is indicated in ICCD images that the stronger diffuse background emission in the discharge with the parallel magnetic field corresponds to the enhanced third discharge.And it is further deduced that the increase in the number of discharge channels corresponds to the increase in the intensity of the primary discharge.Electrons in discharge channels are confined by the parallel magnetic field,and dissipation of electrons in the avalanche head is decreased.Therefore,the number of discharge channels is increased and discharge uniformity is improved.The radial expansion of surface electrons on the dielectric surface is confined by the parallel magnetic field,and dissipation of surface electrons is decreased.As a result,the third discharge is enhanced.The confinement effect of the parallel magnetic field on the surface electrons is not obvious in the limited radial expansion,because the time interval between the primary discharge and the secondary discharge is short.2.Comparison between characteristics of the discharge driven by bipolar and unipolar nanosecond is conducted,and the output waveform polarity is changed by a water resistance which is connected in parallel with the discharge circuit.For the dielectric barrier discharge driven by the unipolar nanosecond pulse,only the primary discharge occurs at the rising front,and thus the residual charge remaining from the primary discharge is not consumed by the reverse discharges.After analyzing lifetime of the residual charge,it is found that the density of the residual charge in the unipolar nanosecond pulsed discharge is higher when the following pulsed discharge is ignited.Experimental results indicated that compared with the bipolar nanosecond pulsed discharge,intensity of the unipolar nanosecond pulsed discharge is slightly increased,and discharge uniformity is significantly improved.It is considered that the improvement of discharge uniformity in the unipolar nanosecond pulsed discharge is caused by two reasons.One reason is that a "virtual anode" is located in the region closer to the cathode when the following pulsed discharge is ignited.The discharge is equivalent to begin with the uniform breakdown within smaller gas gap.Another reason is that the enhanced electric field near the cathode during the pre-breakdown phase in the unipolar nanosecond pulsed discharge is higher.As a result,the ionization of the secondary discharge is strengthened in the higher enhanced electric field,and density of avalanche-initiating electrons is increased in the breakdown phase.The discharge state transits from filamentary discharge state to quasi-uniform discharge state,as pulse repetition frequency decreases in the unipolar nanosecond pulsed discharge.The role of gas temperature and nitrogen metastable states is excluded,after analyzing memory effect of the discharge under different pulse repetition frequencies.It is reckoned that the inhomogeneously distributed space charge is responsible for the change of the discharge state.The pulsed interval is longer under lower pulse repetition frequencies,and the space charges are fully diffused achieving a more homogeneous spatial distribution.Consequently,uniform of the following pulsed discharge is improved.3.Influence of the parallel magnetic field on the unipolar nanosecond pulsed discharge is investigated,and it is found that the parallel magnetic field has a more pronounced effect on the unipolar nanosecond pulsed discharge than that on the bipolar nanosecond pulsed discharge.Experimental results show that intensity of the unipolar nanosecond pulsed discharge is significantly increased,and meanwhile,the discharge uniformity is dramatically improved.On the one hand,the improved uniformity of the discharge with the magnetic field is caused by the increase in the number of discharge channels.On the other hand,it is caused by the increased density of avalanche-initiating electrons both in the pre-breakdown and breakdown phase.By using line-ratio technique of temporally and spatially averaged optical emission spectra,it is found that the average electron density and the electron temperature are both increased by the parallel magnetic field.Because the parallel magnetic field has a stronger confinement effect on the high-energy electrons in the avalanche head,and dissipation of high-energy electrons is significantly reduced.However,dissipation of low-energy electrons is probably unchanged by the parallel magnetic field.Therefore,proportion of high-energy electrons is increased by the parallel magnetic field,and consequently,the plasma electron temperature is higher.The phenomenon that the average electron density is increased by the parallel magnetic field is consistent with the increase in the electron temperature.The incremental degree of average electron density under different pulse repetition frequencies is basically the same,while the incremental degree of electron temperature increasing with pulse repetition frequency.The intensity of the single microdischarge is stronger when the pulse repetition frequency is higher,and thus the radial expansion extent of the avalanches is larger.Therefore,the confinement effect by the parallel magnetic field is more pronounced,and the incremental degree of electron temperature is greater when the pulse repetition frequency is higher.