Characteristic studies on positive and negative streamers of double-sided pulsed surface dielectric barrier discharge

The mechanisms of streamer generation and propagation in double-sided pulsed surface dielectric barrier discharge (SDBD) on both sides have been analyzed and investigated by experiment and numerical simulation. The fully exposed asymmetric SDBD has two discharge processes located on the high voltage electrode (HVE) side and the ground electrode (GE) side. Discharge images of the HVE side and GE side are taken by a digital camera under continuous pulse and ICCD (Intensified Charge Coupled Device) is utilized to diagnose the generation and propagation of streamers in single pulse discharge. In order to understand the physical mechanisms of streamer evolution more deeply, we establish a 2D simulation model and analyze it from the aspects of electron density, ion density, reduced electric field and electron impact ionization source term. The results show that the primary and secondary discharges on the HVE side and the GE side of the double-sided SDBD are composed of positive streamer and negative streamer, respectively. On the HVE side, the accumulation of positive charges on the dielectric surface causes the direction of the electric field to reverse, which is the principal factor for the polarity reversal of the streamer. On the GE side, both the negative charges accumulated on the dielectric surface and the falling voltage are the key factors for the streamer polarity switch.     

Effects of temperature on creepage discharge characteristics in oil-impregnated pressboard insulation under combined AC–DC voltage

Due to the complexity of the valve side winding voltage of the converter transformer, the insulation characteristics of the oil-impregnated pressboard (OIP) of the converter transformer are different from those of the traditional AC transformer. The study on effect of temperature on the creeping discharge characteristics of OIP under combined AC–DC voltage is seriously inadequate. Therefore, this paper investigates the characteristics of OIP creepage discharge under combined AC–DC voltage and discusses the influence of temperature on creepage discharge characteristics under different temperatures from 70°C to 110 °C. The experimental results show that the partial discharge inception voltage and flashover voltage decrease with increasing temperature. The times of low amplitude discharge (LAD) decrease and amplitude of LAD increases. Simultaneously, the times of high amplitude discharge (HAD) gradually increase at each stage of creepage discharge with higher temperature. The analysis indicates that the charge carriers easily accumulate and quickly migrate directional movement along the electric field ahead of discharging. The residual charge carriers are more easily dissipated after discharging. The ‘hump’ region of LAD moves to the direction of higher discharge magnitude. The interval time between two continuous discharges is shortened obviously. The concentration of HAD accelerates the development of OIP insulation creepage discharge. The temperature had an accelerating effect on the development of discharge in the OIP under applying voltage.     

Square grid pattern with direction-selective surface discharges in dielectric barrier discharge

A new phenomenon that a filament discharged only once instead of twice in a cycle of the applied voltage is observed in a square grid pattern in a dielectric barrier discharge(DBD) with a larger gas gap, which is named intermittent discharge. Its spatiotemporal dynamics and the formation mechanism are studied by the multiple photomultiplier tubes and an intensified charge-coupled device.Corresponding to the positions of spots in the picture with an exposure time of 40 ms, there are some bright s...     

Numerical simulation of RF discharges for plasma processing

In this paper, we present kinetic models for radio-frequency glow discharges. Discussed are models for bulk electrons and positive ions based on the solution of the Boltzmann equation in a one-dimensional geometry. The resulting spatiotemporal behaviour of the electrons is used to compute the plasma emission, which is compared with experimental data. The model for the ions is used to solve the coupled dynamics of the electric field and the ion density in the whole discharge as well as the ion velocity distribution at the electrodes. The results are compared with experimental data and with the results of a Monte Carlo code.     

Surface charge accumulation behavior and its influence on surface flashover performance of Al2O3-filled epoxy resin insulators under DC voltages

Surface charge accumulation on insulator surface can have great influence on surface flashover performance. An experimental system is established to investigate surface charge accumulation and decay characteristics ofAl₂O₃-filled epoxy resin insulators in 0.1 MPa SF₆ under DC voltages. Surface potential is recorded by a Kelvin vibrating probe connected to an electrostatic voltmeter. By pre-depositing charges on insulator surface, the influence of surface charges on surface flashover performance is studied. The results reveal that surface charge distribution appearance is the combined effect of electrode injection, back discharge and gas ionization. Surface charge distribution has obvious polarity effect. It is concentrated near the HV electrode under positive voltages and dispersed under negative voltages. The difference in positive and negative surface flashover voltage is attributed to the difference in surface charge distribution under DC voltages of different polarities. Surface charge decay contains two stages, which satisfies the law of double exponential function. At first stage, surface charge decays fast, which corresponds to charges escaping from shallower traps. While it decays slowly at the second stage, which corresponds to charge escaping from deeper traps. Surface charge decay process is dominated by surface conductivity mechanism. The pre-deposited charges on insulation surface have great influence on surface flashover performance. The deposited positive charges can increase positive flashover voltage but decrease negative flashover voltage.     

Ion-induced electrostatic charging of ice

We studied electrostatic charging on amorphous ice films induced by the impact of 100 keV Ar⁺ ions at 45° incidence. We derived the positive surface electrostatic potential from the kinetic energy of sputtered molecular ions. Measurements were performed as a function of film thickness, ion flux and accumulated fluence. The main results are (a) films charge up to a saturation value, following an exponential time dependence. (b) The time constant for charging is approximately proportional to the reciprocal of the ion flux. (c) The maximum surface voltage depends on film thickness and ion flux. (d) Charging does not occur for films thinner than the maximum range of projectile. (e) Dielectric breakdown is observed for surface potentials above ∼100 V. We explain the measurements with a model in which charges can drift into the substrate or be trapped temporarily near the ionization range of the projectiles. A charge can be released from the trap by the electric field produced by a nearby charge injected by subsequent projectiles.     

Evolution of ionization waves in a multi-pulsed plasma jet: the role of memory charges

In this paper, we discuss the properties of ionization waves(IWs) in a multi-pulsed plasma jet while using the two-dimensional computational approach. The IWs are generated by application of three short negative pulses with a repetition frequency 12.5 MHz. The simulations are performed continuously during a single run while accounting for charges accumulated inside(surface charges)and outside(space charges) the tube. The plasma forming gas mixture(He/O2 = 99.8%/0.2%) is injected through the disc...     

Numerical study on peak current in pulse-modulated radio-frequency discharges with atmospheric helium–oxygen admixtures

Atmospheric pressure pulse-modulated radio-frequency (rf) plasmas have drawn growing attention due to their potential in applications. By selecting appropriate modulation parameters, the diffused and large-volume plasma can be generated in the pulse-modulated rf plasma with plenty of reactive oxygen species, which is essential for the biomedical application of helium–oxygen plasmas. In this paper, by means of a fluid model, the formation of the peak current in the first period (PCFP) in a pulse-modulated rf helium–oxygen discharge driven by a sinusoidal voltage is discussed, the existence of a reverse field near the anode caused by the negative and positive charges contributes greatly to the mechanism of PCFP. In the simulation, as oxygen admixture increases, the negative ions of O− and ${{\rm{O}}}_{2}^{-}$ become dominative anions in the sheath region, which can't be driven to the anode very quickly to build a reverse field, thus the PCFP eventually disappears. This study can effectively enhance the understanding of different transportation behavior of heavy negative ions and electrons, and further optimize pulse-modulated rf discharges with helium–oxygen mixtures in various applications.     

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.     

Exploration of a MgO cathode for improving the intensity of pulsed discharge plasma at atmosphere

With regard to the lower density and energy of electrons in pulsed discharge plasma (PDP) at atmosphere, leading to the lower energy utilization of plasma, we propose a MgO cathode to enhance the plasma intensity according to field emission principle. The MgO cathode is prepared by an electro-depositing MgO film on a stainless steel plate. This way, the positive charges come to the cathode and accumulate on the surface of the MgO film, leading to the enhancement of the electric field intensity between the cathode and MgO film, and result in the strong emission of secondary electrons from the MgO cathode. As a result, the intensity of plasma can be enhanced. Herein, the effect of the MgO cathode on the intensity of PDP is investigated. It was shown that the discharge peak current was improved by 20% compared with that of without the MgO cathode. With increasing the MgO film thickness, discharge intensity, including the peak current, transforming charge and spectrum intensity first increased and then decreased. Higher enhancement of peak current, transforming charge and spectrum intensity were acquired with a higher peak voltage. Compared to a cathode without MgO film, the ozone production is higher with MgO cathode employed. The research proposes a novel approach for improving the intensity of discharge plasma, and also provides a reference for further application of PDP.     

Improvement of Spatial Uniformity of Nanosecond-Pulse Diffuse Discharges in a Multi-Needle-to-Plane Gap

Large-scale non-thermal plasmas generated by nanosecond-pulse discharges have been used in various applications, including surface treatment, biomedical treatment, flow con?trol etc. In this paper, atmospheric-pressure diffuse discharge was produced by a homemade nanosecond-pulse generator with a full width at half maximum of 100 ns and a rise time of 70 ns. In order to increase the discharge area, multi-needle electrodes with a 3×3 array were designed. The electrical characteristics of the diffuse discharge array and optical images were investigated by the voltage-current waveforms and discharge images. The experimental results showed that the intensity of diffuse discharges in the center was significantly weaker than those at the margins, resulting in an inhomogeneous spatial uniformity in the diffuse discharge array. Simulation of the electric field showed that the inhomogeneous spatial uniformity was caused by the non-uniform distribution of the electric field in the diffuse discharge array. Moreover, the spatial uniformity of the diffuse discharge array could be improved by increasing the length of the needle in the centre of the array. Finally, the experimental results confirmed the simulation results, and the spatial uniformity of the nanosecond-pulse diffuse discharge array was signi?cantly improved.     

The enhanced aerosol deposition by bipolar corona discharge arrays

The corona discharges provide an efficient way to induce precipitation or eliminate fog by increasing ion density in the open air.In this paper,one bipolar corona discharge array (positive and negative high voltage coupled simultaneously) which can generate high densities of positive and negative ions is developed.The comparison between bipolar corona discharge array and unipolar corona discharge array (positive or negative coupled only) indicates that bipolar corona discharge array can generate ～3 times higher ion density than unipolar corona discharge array.More charged aerosols are produced through collisions between ions and aerosols.The collision rate between aerosols is increased substantially by the attractive forces between positively and negatively charged aerosols.The deposition of aerosols induced by bipolar discharges is 25.7％higher than that of unipolar discharges at the humidity super-saturation condition.Therefore,the bipolar corona discharge system is a new option for the large scale ion sources used for artificial weather modification.     

Modelling of electronegative collisional warm plasma for plasma-surface interaction process

An electronegative collisional plasma having warm and massive positive ions, non-extensive distributed electrons and Boltzmann distributed negative ions is modelled for the plasma-surface interaction process that is used for the surface nitriding. Specifically the sheath formation is evaluated through the Bohm's criterion, which is found to be modified, and the variation of the sheath thickness and profiles of the density of plasma species and the net space charge density in the sheath region in addition to the electric potential. The effect of ion temperature, nonextensivity and collisional parameter is examined in greater detail considering the collisional cross-section to obey power-law dependency on the positive ion velocity. The positive ions are found to enter in the sheath region at lower velocities in the collisional plasma compared to the case of collision-less plasma; this velocity sees minuscule reduction with increasing nonextensivity. The increasing ion temperature and collisional parameter lead to the formation of sheath with smaller thickness.     

Electrical Characteristics of Pseudoglow Discharges in Helium under Atmospheric Pressure

A pseudoglow discharge behaviour is achieved at a 2.0-mm dielectric-dielectric electrode gap in pure helium under atmospheric pressure. An experimental study of the pseudoglow discharges is presented. The electrical characteristics and the discharge photos of the pseudoglow discharges are analyzed and discussed. The current-voltage parameters of the pseudoglow dis- charges are considered in regard to the influence on their behaviour.     

Trap distribution of polymeric materials and its effect on surface flashover in vacuum

The surface trap parameter can significantly affect the development of surface flashover in vacuum, but the effective mode and mechanism are not very clear yet. The trap parameters of three polymeric materials were tested and calculated by means of isothermal surface potential decay. The flashover experiment was developed under different applied voltages. The results show a positive correlation between the withstand voltage and the deep trap, i.e., the deeper trap energy level is, the higher flashover voltage is. The dynamics process of charge trapping and detrapping was analyzed based on the charge transport model in dielectrics with a single trap level and two discrete trap levels. The time of charge trapping was compared with that of the discharge development. The results show that the charge trapping time is longer than the flashover development time. The way to influence flashover for a trap is not to decrease the secondary electrons in single discharge development, but to change the electric field distribution on the dielectric surface by charge capture.     

Particle-in-cell/Monte Carlo simulation of filamentary barrier discharges

The plasma behavior of filamentary barrier discharges in helium is simulated using a twodimensional (2D) particle-in-cell/Monte Carlo model.Four different phases have been suggested in terms of the development of the discharge:the Townsend phase;the space-charge dominated phase;the formation of the cathode layer,and the extinguishing phase.The spatialtemporal evolution of the particle densities,velocities of the charged particles,electric fields,and surface charges has been demonstrated.Our simulation provides insights into the underlying mechanism of the discharge and explains many dynamical behaviors of dielectric barrier discharge (DBD) filaments.     

Mechanisms of Charge Buildup in MOS Insulators

The radiation sensitivity of MOS devices has been recognized to be the result of a charge buildup caused by the sweepout of electrons and the trapping of holes following hole-electron pair production by ionizing radiation. Holes have been shown to have a finite, although small, mobility in thermally grown SiO2. Trapping of holes takes place near the Si-SiO2 interface, possibly by oxygen vacancies in the oxide. Normal thermally grown SiO2 possesses only small concentrations of electron traps. Electron traps have been shown to be generated, however, in the oxide by ion implantation, by irradiation with nonpenetrating electrons, and by exposure of the surface of the oxide to negative ions from a corona discharge. Although Na+ and Li+ ions have been shown to be mobile at room temperature in SiO2, contamination can be kept to levels where ionic charge buildup is negligible. The role of contaminants in the formation of hole traps, however, remains to be determined.     

Measurement of surface charges on the dielectric film based on field mills under the HVDC corona wire

The ion flow field on the ground is one of the significant parameters used to evaluate the electromagnetic environment of high voltage direct current (HVDC) power lines. HVDC lines may cross the greenhouses due to the restricted transmission corridors. Under the condition of ion flow field, the dielectric films on the greenhouses will be charged, and the electric fields in the greenhouses may exceed the limit value. Field mills are widely used to measure the groundlevel direct current electric fields under the HVDC power lines. In this paper, the charge inversion method is applied to calculate the surface charges on the dielectric film according to the measured ground-level electric fields. The advantages of hiding the field mill probes in the ground are studied. The charge inversion algorithm is optimized in order to decrease the impact of measurement errors. Based on the experimental results, the surface charge distribution on a piece of quadrate dielectric film under a HVDC corona wire is studied. The enhanced effect of dielectric film on ground-level electric field is obviously weakened with the increase of film height. Compared with the total electric field strengths, the normal components of film-free electric fields at the corresponding film-placed positions have a higher effect on surface charge accumulation.     

Influence of dust particles on spatial distributions of particles and fluxes in positive column of glow discharge

The research herein examined the results of numerical simulations of the positive column of a glow discharge in argon dusty plasma using COMSOL Multiphysics software under conditions similar to the project known as PK-4. Various scenarios dealing with formations of spatial distributions of densities and fluxes for charged particles were studied, and evaluations of the influence of dust particles on the discharge were obtained in a wide range of dust densities. Two extreme cases were distinguished: weak dust influence when the densities, fluxes and electric field profiles are not perturbed, and strong dust influence when all three density profiles(electrons, ions and charged dust) in the dust cloud are similar(parallel) to each other, resulting in all created charges in the dust cloud being lost inside the cloud. In such a case, the ambipolar field and the transport of charged particles are decreased in the dust cloud, and any ambipolar flux is almost absent within the cloud.