Experimental and simulated investigation of microdischarge characteristics in a pin-to-pin dielectric barrier discharge (DBD) reactor

Both experimental and simulated studies of microdischarge (MD) are carried out in a dielectric barrier discharge with a pin-to-pin gap of 3.5 mm, ignited by a sinusoidal voltage with a peak voltage of 10 kV and a driving frequency of 5 kHz. Statistical results have shown that the probability of the single current pulse in the positive half-period (HP) reaches 73.6% under these conditions. Experimental results show that great luminous intensity is concentrated on the dielectric surface and the tip of the metal electrode. A 1D plasma fluid model is implemented by coupling the species continuity equations, electron energy density equations, Poisson equation, and Helmholtz equations to analyze the MD dynamics on the microscale. The simulated results are in good qualitative agreement with the experimental results. The simulated results show that the MD dynamics can be divided into three phases: the Townsend phase, the streamer propagation phase, and the discharge decay phase. During the streamer propagation phase, the electric field and electron density increase with the streamer propagation from the anode to the cathode, and their maximal values reach 625.48 Td and 2.31 × 1019 m−3, as well as 790.13 Td and 3.58 × 1019 m−3 in the positive and negative HP, respectively. Furthermore, a transient glow-like discharge is detected around the anode during the same period of streamer propagation. The formation of transient glow-like discharge is attributed to electrons drifting back to the anode, which is driven by the residual voltage in the air gap.     

Plasma propagation in single-particle packed dielectric barrier discharges: joint effects of particle shape and discharge gap

In this work, a single Al₂O₃ particle packed dielectric barrier discharge (DBD) reactor with adjustable discharge gap is built, and the influences of the particle shape (ball and column) and the residual gap between the top electrode and particle on the electrical and optical characteristics of plasma are studied. Our research confirms that streamer discharge and surface discharge are the two main discharge patterns in the single-particle packed DBD reactor. The strong electric field distortion at the top of the ball or column caused by the dielectric polarization effect is an important reason for the formation of streamer discharge. The length of streamer discharge is proportional to the size of the residual gap, but the number of discharge times of a single voltage cycle shows an opposite trend. Compared to the column, a smooth spherical surface is more conducive to the formation of large and uniform surface discharges. The surface discharge area and the discharge intensity reach a maximum when the gap is equal to the diameter of the ball. All in all, the results of this study will provide important theoretical support for the establishment of the synergistic characteristics of discharge and catalysis in plasma catalysis.     

Characteristics of long-gap AC streamer discharges under low pressure conditions

The generation and propagation of a streamer is a significant physical process of air gap discharge. Research on the mechanism of streamers under low-pressure conditions is helpful for understanding the process of long-gap discharge in a high-altitude area. This paper describes laboratory investigations of streamer discharge under alternating current(AC) voltage in a low pressure test platform for a 60 cm rod–plane gap at 30 kPa, and analyzes the characteristics of streamer generation and propagation. The results show that the partial streamer and breakdown streamer all occur in the positive half-cycle of AC voltage near the peak voltage at 30 kPa. The partial streamer could cause the distortion of current and voltage waveform, and it appears as the branching characteristic at the initial stage. With the extension of the streamer, the branching and tortuosity phenomena become gradually obvious, but the branching is suppressed when the streamer crosses the gap. The low-pressure condition has little influence on the tortuosity length and the tortuosity number of the streamer, but affect the diameter of streamer obviously.     

E-beam generation in discharges initiated by voltage pulses with a rise time of 200 ns at an air pressure of 12.5–100 kPa

The effect of air pressure (12.5, 25, 50, and 100 kPa) on the generation of runaway electron beams in a non-uniform electric field when applying voltage pulses (≈35 kV) with a rise time of ≈200 ns has been studied. The results show that the discharge has various stages: streamer, diffuse, and spark. Initially, a wide streamer develops in the gap and a diffuse discharge is formed. A spark is formed ≈100 ns after the breakdown. The current pulse of a supershort avalanche electron beam (SAEB) was measured with a collector at various pressures of air. Experiments show that there are two modes of generation of runaway electrons. At an air pressure of 25–100 kPa, a single SAEB current pulse with a full width at half-maximum (FWHM) of 120–140 ps is observed. At the air pressure of 12.5 kPa, two current pulses of the electron beam are observed. FWHM of the first and second current pulses are ≈140 ps and ≈300 ps, respectively. The current pulse amplitude of the second electron beam is higher than that of the first one, but the electron energy is less.     

Investigation on the streamer propagation in atmospheric pressure helium plasma jet by the capacitive probe

In this letter, the streamer propagation in the atmospheric pressure helium plasma jet with afloating electrode nozzle driven by the kHz AC power supply is investigated. The current signal induced by the space charges and the mean propagation velocity of the guided ionization waves are measured by the capacitive probe method in the discharge region. The space charges in the guided ionization waves are found to increase with the applied voltage, which enhances both the electric field near the streamer head and the propagation velocity. The applicability of the streamer mechanism to the propagation of the guided ionization waves is validated by this electrical diagnostic method.     

Experimental Study on Branch and Diffuse Type of Streamers in Leader Restrike of Long Air Gap Discharge

One of the main problems in the Ultra High Voltage(UHV) transmission project is to choose the external insulation distance,which requires a deep understanding of the long air gap discharge mechanism.The leader-streamer propagation is one of most important stages in long air gap discharge.In the conductor-tower lattice configuration,we have measured the voltage,the current on the high voltage side and the electric field in the gap.While the streamer in the leader-streamer system presented a conical or hyperboloid diffuse shape,the clear branch structure streamer in front of the leader was firstly observed by a high speed camera in the experiment.Besides,it is found that the leader velocity,width and injected charge for the branch type streamer are greater than those of a diffuse type.We propose that the phenomenon results from the high humidity,which was 15.5-16.5 g/m~3 in our experiment.     

Quantitative relationship between the maximum streamer length and discharge voltage of a pulsed positive streamer discharge in water

A linear relationship has been realized between the maximum streamer length and discharge voltage of a pulsed positive streamer discharge by measuring the streamer length in water with conductivity of 100 μS cm-1using high-speed photography. Based on Ohm's law, a quantitative equation has been derived for the dependence of the maximum streamer length on the discharge voltage of a pulsed positive streamer discharge in water. According to the equation, our results suggest that the streamers spontaneously stop propagating through water due to the voltage at the streamer head dropping below the ignition voltage of a pulsed positive streamer discharge.     

A two-dimensional air streamer discharge modified model based on artificial stability term under non-uniform electric field at low temperature and sub-atmospheric pressure

In this paper, an improved air discharge fluid model under non-uniform electric field is constructed based on the plasma module COMSOL Multiphysics with artificial stability term, and the boundary conditions developed in the previous paper are applied to the calculation of photoionization rate. Based on the modified model, the characteristics of low temperature subatmospheric air discharge under 13 kV direct current voltage are discussed, including needle-plate and needle-needle electrode structures. Firstly, in order to verify the reliability of the model, a numerical example and an experimental verification were carried out for the modified model respectively. Both verification results show that the model can ensure the accuracy and repeatability of the calculation. Secondly, according to the calculation results of the modified model, under the same voltage and spacing, the reduced electric field under low temperature subatmosphere pressure is larger than that under normal temperature and atmospheric pressure. The high electric field leads to the air discharge at low temperature and sub atmospheric pressure entering the streamer initiation stage earlier, and has a faster propagation speed in the streamer development stage, which shortens the overall discharge time. Finally, the discharge characteristics of the two electrode structures are compared, and it is found that the biggest difference between them is that there is a pre-ionization region near the cathode in the needle-needle electrode structure. When the pre-ionization level reaches 10~(13) cm~(-3), the propagation speed of the positive streamer remains unchanged throughout the discharge process, and is no longer affected by the negative streamer. The peak value of electric field decreases with the increase of pre-ionization level, and tends to be constant during streamer propagation. Based on the previous paper, this paper constructs the air discharge model under non-uniform electric field, complements with the previous paper, and forms a relatively complete set of air discharge simulation system under low temperature and sub atmospheric pressure, which provides a certain reference for future research.     

A two-dimensional air streamer discharge model based on the improved Helmholtz equation at low temperature and sub-atmospheric pressure

In this paper, an efficient boundary condition is applied to solve the photoionization rate, and a two-dimensional numerical simulation is carried out for the development and propagation of an air streamer at low temperature and sub-atmospheric pressure. The results show that the new boundary condition improves the calculation accuracy, but the influence of photoionization on the streamer discharge process is not obvious. The discharge current in the development of streamer discharge is defined, and the corresponding expression of the positive and negative streamer discharge current is given. The influence of the electric field exceeding the threshold value on the discharge process is preliminarily introduced. In the process of discharge, only the propagation velocity of the streamer is obviously higher than that of normal temperature and pressure, and the trend of the other parameters is basically the same as that described in the previous paper. The above results give us a deeper understanding of the discharge characteristics under low temperature and sub-atmospheric pressure, which has certain significance for the development of aviation and high voltage engineering.     

Correlation between surface charge and creepage discharge on polymeric dielectrics under high-frequency high-voltage stress

In the present work, creepage discharge characteristics, i.e. amplitudes, phases, and repetitiveness, and surface charge dynamic behaviors under a 20 kHz high-frequency sinusoidal waveform high-voltage electrical stress were captured in a discharge chamber with temperature and humidity control. The results showed that the creepage discharges mostly occurred in the positive half phase, whose maximum amplitude increased with the development of discharge. The inception voltage of the creepage discharge is independent of the frequency of the external electrical stress. Once the discharge occurred, there were a large number of positive and negative particles ionized by a high electric field. Because of the much higher velocity of electrons than positive ions, the energetic discharge-produced electrons are likely to disperse away along the surface and be accumulated through adsorption, collision, and reactions. Moreover, the positive ions join the high-conductive discharge channel and disappear though the ground electrode. Thus, after high-frequency creepage discharge, only negative charges remained on the dielectric surface, as measured. Particularly, the creepage discharges mostly occurred in the positive half phase, owing to the reverse electric field induced by the accumulated negative charges. With the development of creepage discharge, some large-amplitude discharges began to occur in the positive-peak-phase region. The research concluded that the synergistic effect of negative surface charges and large-amplitude discharges eroded the dielectrics and excited the streamer to creep toward the ground electrode until flashover along the surface. Therefore, the correlation between high-frequency creepage discharge and surface charge is preliminarily revealed.     

How bead shapes affect the plasma streamer characteristics in packed-bed dielectric barrier discharges:a kinetic modeling study

Different shapes of dielectric packing beads could affect streamer propagating direction,plasma streamer behavior,and streamer types,such as surface discharge,surface-to-surface discharge,and volume discharge.In this paper,a 2D particle-in-cell/Monte Carlo collision model is used to investigate the effect of the bead shapes on streamer characteristics in packed-bed dielectric barrier discharges.We calculate the electron density,ion density,excitation rate,ionization rate and the electric field with different bead shapes in two cases of seed electron configurations.The results demonstrate that both the configurations of seed electrons and the shape of beads could influence plasma properties.In the case of seed electrons located directly above the beads,the streamer cannot be generated with square beads,while weak surface ionization waves(SIWs)are developed with circle and triangle beads,when the distance between the seed electrons and the upper plate is as close as 0.02 mm.Whereas,the distance between the seed electrons and the upper plate is 0.06 mm,the streamers can be generated with all three bead shapes,but SIWs are still weak.This is because different shapes of beads induce different electric field and surface charging along the dielectric bead surfaces,determining the generation of SIWs.In the case of seed electrons placed between two beads,streamers can propagate in all three bead shape configurations,and the SIWs are enhanced.     

Study on Glow Discharge Plasma Used in Polyester Surface Modification

To achieve an atmospheric pressure glow discharge(APGD)in air and modify the surface of polyester thread using plasma,the electric field distribution and discharge characteristics under different conditions were studied.We found that the region with a strong electric field,which was formed in a tiny gap between two electrodes constituting a line-line contact electrode structure,provided the initial electron for the entire discharge process.Thus,the discharge voltage was reduced.The dielectric barrier of the line-line contact electrodes can inhibit the generation of secondary electrons.Thus,the transient current pulse discharge was reduced significantly,and an APGD in air was achieved.We designed double layer line-line contact electrodes,which can generate the APGD on the surface of a material under treatment directly.A noticeable change in the surface morphology of polyester fiber was visualized with the aid of a scanning electron microscope(SEM).Two electrode structures–the multi-row line-line and double-helix line-line contact electrodes–were designed.A large area of the APGD plasma with flat and curved surfaces can be formed in air using these contact electrodes.This can improve the efficiency of surface treatment and is significant for the application of the APGD plasma in industries.     

Mechanisms of Streamer Propagation Affected by Driven Voltage Polarity in a Cold Atmospheric Pressure Plasma Jet

A two-dimensional self-consistent fluid model is used to investigate the effects of DC-voltage polarity in plasma initiation and propagation of helium plasma jet.The simulation results indicate that the difference in initial breakdown for the positive jet and negative jet leads to a difference in the electron density of about 4 orders of magnitude,even with the same initial electric field,which also influences the subsequent propagation.In the propagation process of negative jets,the ionization process exists in a longer gas channel behind the streamer head.In addition,the drift process to the infinite grounded electrode driven by the electric field results in higher energy consumption in the ionization process.However,in the positive jet,the ionization process mainly exists in the streamer head.Therefore,the differences in the initial breakdown and propagation process make the electric field intensity and the ionization weaker in the streamer head of the negative jet,which explains the weaker and shorter appearance of the negative jet compared to the positive jet.Our model can adequately reproduce the experimental results,viz.a bullet-like propagation in the positive jet and a continuous plasma plume in the negative jet.Furthermore,it also indicates that the streamer velocity shows the same variations as the electron drift velocity for both positive and negative jets.     

Experimental Study on Surface Dielectric Barrier Discharge Plasma Actuator with Different Encapsulated Electrode Widths for Airflow Control at Atmospheric Pressure

The surface dielectric barrier discharge(SDBD) plasma actuator has shown great promise as an aerodynamic flow control device. In this paper, the encapsulated electrode width of a SDBD actuator is changed to study the airflow acceleration behavior. The effects of encapsulated electrode width on the actuator performance are experimentally investigated by measuring the dielectric layer surface potential, time-averaged ionic wind velocity and thrust force. Experimental results show that the airflow velocity and thrust force increase with the encapsulated electrode width. The results can be attributed to the distinct plasma distribution at different encapsulated electrode widths.     

Temporal evolution of atmospheric cascade glow discharge with pulsed discharge and radio frequency discharge

Atmospheric cascade discharges with pulsed discharge and radio frequency(RF)discharge were experimentally investigated by the temporal evolution of discharge spatial profile and intensity.The indium tin oxide(ITO)coated glass was employed as the transparent electrode to capture the discharge distribution above the electrode surface.It is demonstrated that in the pulsed discharge with dielectric barrier,the first discharge at the rising edge of pulse voltage is uniformly ignited and then forms an expanding plasma ring on the ITO electrode surface,which shrinks to the same diameter as that of bare stainless steel electrode with the generation of second discharge at the falling edge of pulse voltage.The discharge profiles along the electrode surface and discharge gap of the successive RF discharge are dependent on the intensity and spatial distribution of residual plasma species generated by the pulsed discharge,which is determined by the time interval between the pulsed discharge and RF discharge.It is demonstrated that the residual plasma species before the RF discharge ignition help to achieve the stable operation of RF discharge with elevated intensity.     

Parametric study on the characteristics of a SDBD actuator with a serrated electrode

Active flow control based on surface dielectric barrier discharge (SDBD) has become a focus of research in recent years,due to its unique advantages and diverse potential applications.Compared with the conventional SDBD with straight electrodes,the serrated electrode-based SDBD has a great advantage due to its 3D flow topology.It is believed that the boundary layer separation of moving objects can be controlled more effectively with this new type of SDBD.In SDBD with a serrated electrode,the R (tip sharpness) and N (tip number per unit length) have a great influence on the discharge and induced airflow characteristics.In this paper,a parametric study of the characteristics of SDBD with a serrated electrode has been conducted with different ranges of R and N.Aspects of the power consumption,the steady medium temperature distribution,and the maximum induced airflow velocity have been investigated.The results indicate that there is a critical value of R and N where the maximum power consumption and induced airflow velocity are achieved.The uniformity of the steady temperature distribution of the medium surface is found to be more dependent on N.We found that the accelerating effects of the induced airflow can be evaluated with the Schlieren technique,which agree well with the results from the pitot tube.     

Electrical model and experimental analysis of a double spiral structure surface dielectric barrier discharge

A surface dielectric barrier discharge (SDBD) can discharge at atmospheric pressure and produce a large area of low-temperature plasma. An SDBD plasma reactor based on the double spiral structure is introduced in this paper. To study the discharge mechanism of SDBD, an equivalent circuit model was proposed based on the analysis of the micro-discharge process of SDBD. Matlab/Simulink is used to simulate and compare the voltage–current waves, Lissajous and discharge power with the experimental results. The consistency of the results verifies the validity of the SDBD equivalent circuit model. Maxwell software based on the finite elements method is used to analyze the electrostatic field distribution of the device, which can better explain the relationship between the discharge image and the electrostatic field distribution. The combination of equivalent circuit simulation and electrostatic field simulation can provide better guidance for optimizing a plasma generator. Finally, the device is used to treat PM2.5 and formaldehyde. The test results show that the degradation rate of PM2.5 can reach 78% after 24 min, and formaldehyde is about 31.5% after 10 min of plasma treatment.     

Wire-to-Plate Surface Dielectric Barrier Discharge and Induced Ionic Wind

The electrical and mechanical characteristics of the wire-to-plate surface dielectric barrier discharge and the induced ionic wind are investigated experimentally.The different temporal behaviors in positive and negative half-cycles are studied by time-resolved images.It is shown that the discharge and the light emission are generally stronger in the positive half cycle.The discharge is inhomogeneous and propagates in streamer mode;however,in the negative half-cycle,the discharge appears visually uniformly and operates in the diffuse mode.The surface discharge can produce ionic wind about several m/s above the dielectric surface.There exists an optimal width of the grounded electrode to produce a larger plasma area or active wind region.Increasing of the applied voltage or normalized dielectric constant leads to a larger wind velocity.The performance of ionic wind on flow control is visualized by employing a smoke stream.     

Study of ionic wind based on dielectric barrier discharge of carbon fiber spiral electrode

Based on the idea that a large number of charged particles can be generated by a high-frequency alternating current(AC)dielectric barrier discharge(DBD),and charged particles can be accelerated directionally by a direct current(DC)electric field,a new type of ionic wind formation method is proposed in this paper.To this end,a carbon fiber spiral electrode serves as the generation electrode and a metal rod electrode as the collection electrode,with AC and DC potentials applied respectively to the generation electrode and the collection electrode to form an AC-DC coupled electric field.Under the action of the coupled electric field,a dielectric barrier discharge is formed on the carbon fiber spiral electrode,and the electrons generated by the discharge move from the generation electrode to the collection electrode in the opposite direction of the electric field vectors.During the movement,energy is transferred to the gas molecules by their colliding with neutral gas molecules,thereby forming a directional gas stream movement,i.e.ionic wind.In the research process,it is verified through electric field simulation analysis and discharge experiment that this method can effectively increase the number of charged particles in the discharge process,and the velocity of the ionic wind is nearly doubled.On this basis,the addition of a third electrode forms a distinct discharge region and an electron acceleration region,which further increases its velocity.The experimental result shows that the ionic wind speed reaches up to 2.98 m s^?1.Thanks to the ability of the electrode structure to generate an atmospheric pressure DBD plasma and form an ionic wind,we can create a noise-free air purification device without resorting to a fan,with this device having good application prospects in the field of air purification.     

On-Line Measurement of Ion Density in Atmospheric Nitrogen Discharge Filaments via Radiation Signals from Plasma Oscillation

Diagnosis of the particle number density of plasma plays an important role in the understanding of plasma sources and processing.Regular radiation signals from plasma oscillation in filaments of atmospheric nitrogen discharge,which were excited by the injection of secondary electron beams during the propagation of the streamer,are employed to determine the ion density of plasma and its evolution in the filaments.Results show that the density of N_4~+ in a filament of atmospheric nitrogen discharge is of the order of 10~(13) cm~(-3).It is also found that the recombination processes play a dominant role in plasma decay,and that the ion density decreases non-monotonically with time during streamer propagation.