Analyses of nonequilibrium transport in atmospheric-pressure direct-current argon discharge under different modes

The key plasma parameters under different discharge modes, such as heavy-particle and electron temperatures, electron number density, and nonequilibrium volume of plasmas, play important roles in various applications of gas discharge plasmas. In this study, a self-consistent two-dimensional nonequilibrium fluid model coupled with an external circuit model is established to reveal the mechanisms related to the discharge modes, including the normal glow, abnormal glow,arc, and glow-to-arc transition modes, with an atmospheric-pressure direct-current(DC) argon discharge as a model plasma system. The modeling results show that, under different discharge modes, the most significant difference between the preceding four discharge modes lies in the current and energy transfer processes on the cathode side. On one hand, the current to the cathode surface is mainly delivered by the ions coming from the plasma column under the glow discharge mode due to the low temperature of the solid cathode, whereas the thermionic and secondary electrons emitted from the hot cathode surface play a very important role under the arc mode with a higher cathode surface temperature and higher ion flux toward the cathode. On the other hand, the energy transfer channel on the cathode side changes from mainly heating the solid cathode under the glow mode to simultaneously heating both the solid cathode and plasma column under the arc mode with an increase in the discharge current. Consequently, the power density in the cathode sheath(P_c) was used as a key parameter for judging different discharge modes, and the range of(0.28–1.2) × 10^(12) W m^(-3) was determined as a critical window of P_c corresponding to the glow-to-arc-mode transition for the atmospheric-pressure DC argon discharge, which was also verified by comparison with the experimental results in this study and the data in the previous literature.

Study on characteristics of acoustic signals generated by different DC discharge modes

Acoustic signals contain rich discharge information.In this study,the acoustic signal characteristics of transient glow,spark,and glow discharges generated through DC pin–pin discharge were investigated.The signals were analyzed in the time,frequency,and time–frequency domains,and the correlation between the electric and the acoustic signal was studied statistically.The results show that glow discharge does not produce measurable sound signals.For the other modes,with a decrease in the discharge gap,the amplitude of the acoustic signal increases sharply with mode transformation,the short-time average energy becomes higher,and the frequency components are more abundant.Meanwhile,the current pulse and sound pressure pulse have a one-to-one relationship in the transient glow and spark regimes,and they are positively correlated in amplitude.A brief theoretical analysis of the mechanism of plasma sound and the trends of signals in different modes is presented.Essentially,the change in the discharge energy is closely related to the sound generation of the plasma.

Numerical study of the self-pulsing of DC discharge:from corona to parallel-plate configurations

We present here an investigation of the self-pulsing phenomenon of negative corona and parallel-plate discharge in argon within one frame of a one-dimensional fluid model in cylinder–cylinder electrode geometry.The transition from corona to parallel-plate discharge is obtained by changing the inner and outer radii of the electrodes.The model reproduces the self-pulsing waveform well and provides the spatiotemporal behaviors of the charged particles and electric field during the pulse.The self-pulsing shows a common feature that occurs in various configurations and that does not depend on a specific electrode structure.The self-pulsing is the transformation between a weak-current Townsend mode and a large-current normal glow mode.The behavior of the positive ions is the dominant factor in the formation of the pulse.

Time-frequency analysis of Li solid-phase diffusion in spherical active particles under typical discharge modes

Li transient concentration distribution in spherical active material particles can affect the maximum power density and the safe operating regime of the electric vehicles(EVs). On one hand, the quasiexact/exact solution obtained in the time/frequency domain is time-consuming and just as a reference value for approximate solutions;on the other hand, calculation errors and application range of approximate solutions not only rely on approximate algorithms but also on discharge modes. For the purpose to track the transient dynamics for Li solid-phase diffusion in spherical active particles with a tolerable error range and for a wide applicable range, it is necessary to choose optimal approximate algorithms in terms of discharge modes and the nature of active material particles. In this study, approximation methods,such as diffusion length method, polynomial profile approximation method, Padé approximation method,pseudo steady state method, eigenfunction-based Galerkin collocation method, and separation of variables method for solving Li solid-phase diffusion in spherical active particles are compared from calculation fundamentals to algorithm implementation. Furthermore, these approximate solutions are quantitatively compared to the quasi-exact/exact solution in the time/frequency domain under typical discharge modes, i.e., start-up, slow-down, and speed-up. The results obtained from the viewpoint of time-frequency analysis offer a theoretical foundation on how to track Li transient concentration profile in spherical active particles with a high precision and for a wide application range. In turn, optimal solutions of Li solid diffusion equations for spherical active particles can improve the reliability in predicting safe operating regime and estimating maximum power for automotive batteries.

Study of the Discharge Mode in Micro-Hollow Cathode

In this study, micro-hollow cathode discharge （MHCD） is investigated by a fluid model with drift-diffusion approximation. The MHC device is a cathode/dielectric/anode sandwich structure with one hole of a diameter D=200 um. The gas is a Ne/Xe mixture at a pressure p=50-500 Torr. The evolutions of the discharge show that there are two different discharge modes. At larger pD the discharge plasma and high density excited species expand along the cathode surface and, a ringed discharge mode is formed. At smaller pD, the discharge plasma and the excited species expand along the axis of the cathode aperture to form a columnar discharge.

Simulation of transition from Townsend mode to glow discharge mode in a helium dielectric barrier discharge at atmospheric pressure

The dielectric barrier discharge characteristics in helium at atmospheric pressure are simulated based on a one- dimensional fluid model. Under some discharge conditions, the results show that one discharge pulse per half voltage cycle usually appears when the amplitude of external voltage is low, while a glow-like discharge occurs at high voltage. For the one discharge pulse per half voltage cycle, the maximum of electron density appears near the anode at the beginning of the discharge, which corresponds to a Townsend discharge mode. The maxima of the electron density and the intensity of electric field appear in the vicinity of the cathode when the discharge current increases to some extent, which indicates the formation of a cathode-fall region. Therefore, the discharge has a transition from the Townsend mode to the glow discharge mode during one discharge pulse, which is consistent with previous experimental results.

Operation Mode on Pulse Modulation in Atmospheric Radio Frequency Glow Discharges

The discharge operation regime of pulse modulated atmospheric radio frequency （RF） glow discharge in helium is investigated on the duty cycle and frequency of modulation pulses. The characteristics of radio frequency discharge burst in terms of breakdown voltage, alpha（s）- gamma（γ） mode transition voltage and current are demonstrated by the discharge current voltage characteristics. The minimum breakdown voltage of RF discharge burst was obtained at the duty cycle of 20% and frequency of 400 kHz, respectively. The α-γ mode transition of RF discharge burst occurs at higher voltage and current by reducing the duty cycle and elevating the modulation frequency before the RF discharge burst evolving into the ignition phase, in which the RF discharge burst can operate stably in the γ mode. It proposes that the intensity and stability of RF discharge burst can be improved by manipulating the duty cycle and modulation frequency in pulse modulated atmospheric RF glow discharge.

Investigation on pulsed discharge mode in SF6-C2H6 mixtures

The non-chain chemical HF(DF)laser is one of the most powerful electrically-driven lasers operating in mid-infrared,in which SF6-C2H6 mixtures are often used as lasering media.Due to the electronegativity of SF6,the discharge in SF6-C2H6 presents a complicated discharge mode.To achieve reproducible pulsed laser output,pulsed discharge in SF6-C2H6 mixtures is investigated for discharge mode using plane electrodes assisted by array pre-ionization spark pins in cathode surface.Firstly,two modes can be distinguished.One mode is called the selfsustained volume discharge(SSVD),which is characterized by spatial uniformity in the discharge gap and pulse to pulse repeatability.On the contrary,another mode includes random arc passages in the discharge gap and therefore cannot conduct lasering.By varying discharge conditions(gap voltage,gas pressure,etc)two discharge modes are observed.Secondly,the holding scope of the SSVD mode is analyzed for the optimal mixture ratio of 20:1,and the boundary tend of the holding scope of SSVD indicates there exists maximum gas pressure and maximum charging voltage for SSVD.Finally,the peak current of SSVD relates positively to charging voltage,while negatively to gas pressure,from which it is drawn that synchronous electron avalanches initiated by the sliding array overlap spatially into SSVD and thus SSVD is essentially an α ionization avalanche.

Simulation of mode transitions in capacitively coupled Ar/O_(2) plasmas

In this work,the effects of the frequency,pressure,gas composition,and secondary-electron emission coefficient on the discharge mode in capacitively coupled Ar/O_(2) plasmas were carefully studied through simulations.Three discharge modes,i.e.,α,γ,and drift-ambipolar(DA),were considered in this study.The results show that a mode transition from theγ-DA hybrid mode dominated by theγmode to the DA-αhybrid mode dominated by the DA mode is induced by increasing the frequency from 100 k Hz to 40 MHz.Furthermore,the electron temperature decreases with increasing frequency,while the plasma density first decreases and then increases.It was found that the electronegativity increases slightly with increasing pressure in the lowfrequency region,and it increases notably with increasing pressure in the high-frequency region.It was also observed that the frequency corresponding to the mode transition fromγto DA decreased when the secondary-electron emission coefficient was decreased.Finally,it was found that increasing the oxygen content weakens theγmode and enhances the DA mode.More importantly,the density of oxygen atoms and ozone will increase greatly with increasing oxygen content,which is of great significance for industrial applications.

Driving frequency effects on the mode transition in capacitively coupled argon discharges

A one-dimensional fluid model is employed to investigate the discharge sustaining mechanisms in the capacitively coupled argon plasmas, by modulating the driving frequency in the range of 40 kHz-613 MHz. The model incorporates the density and flux balance of electron and ion, electron energy balance, as well as Poisson＇s equation. In our simulation, the discharge experiences mode transition as the driving frequency increases, from the γ regime in which the discharge is maintained by the secondary electrons emitted from the electrodes under ion bombardment, to the a regime in which sheath oscillation is responsible for most of the electron heating in the discharge sustaining. The electron density and electron temperature at the centre of the discharge, as well as the ion flux on the electrode are figured out as a function of the driving frequency, to confirm the two regimes and transition between them. The effects of gas pressure, secondary electron emission coefficient and applied voltage on the discharge are also discussed.

Development of Numerical Model for Radio Frequency Atmospheric Pressure Glow Discharges in Argon

A one-dimensional,self-consistent fluid model is developed for a computational investigation on discharge characteristics and dynamics of radio frequency(RF) glow discharges in atmospheric argon,which are demonstrated through the spatial and temporal profiles of plasma species,electric field,and mean electron energy.Furthermore,in the discharge current density range from 7.1 mA/cm2 to 119.5 mA/cm2,different discharge operation modes of α and γ are indicated by changing differential conductivity of voltage-current characteristics and sheath dynamics in terms of sheath voltage and sheath thickness.

Establishment of a Kind of Three-dimensional Thunderstorm Cloud Electrification-Discharge Mode

[Objective] The research aimed to study the establishment of a kind of three-dimensional thunderstorm cloud electrification-discharge mode.[Method] Based on a kind of three-dimensional strong convective cloud mode,several parameterization schemes of electrification-discharge mechanism which were mature and had experimental basis were coupled into the strong convective cloud mode.The three-dimensional strong convective mode could simulate electrification-discharge process,and the three-dimensional thunderstorm cloud electrification-discharge mode was established.[Result] By numerical simulation method,a kind of three-dimensional thunderstorm cloud electrification-discharge mode was established.The mode not only could assess the relative importance of various kinds of electrification mechanisms under different environments,but also described formation of charge center and electrical field within the thunderstorm cloud,occurrence position,propagation process and influence on charge distribution of lightning.The most important thing was that it could quantitatively describe the complicated relative relationships between power,microphysics and electric process in the evolution process of thunderstorm cloud.[Conclusion] The research provided powerful foundation for further studying the lightning characteristics of different thunderstorm clouds,the relationships between lightning characteristics and power,microphysical process,and deeply discussing the early-warning forecasting technology of thunderbolt activity.

Optical detection method of discharge mode transition of inductively coupled plasma in an atmosphere-breathing electric propulsion system

Plasma discharge stability is an important problem in atmosphere-breathing electric propulsion system when maintaining long-term missions at ultra-low earth orbit.This paper designed an inductively coupled plasma source to imitate the ionization section.The effect of inflow rate and Radio Frequency(RF)power on the plasma discharge mode transition is experimentally studied.A discharge mode detection method is proposed,which determines the discharge mode by identifying the morphology of the plasma core.By using the method,the discharge mode transition is quantified and a control model based on the parameter sensitivity is constructed.To verify the method,the spectra are measured and the electron temperature spatial distribution is calculated.And the method has been proven effective.The results show that the inductively coupled discharge contains capacitive components affected by the mass flow rate and the radio frequency power.The plasma characteristics can be maintained stably by controlling the radio frequency power when the mass flow rate randomly changes in a certain range.It is demonstrated that the application of detection method effectively identifies the discharge mode,which is a promising active control method for the plasma discharge mode.

Temporal Variations of Water Discharge and Sediment Load of Huanghe River,China

Based on the data from gauging stations, the changes in water discharge and sediment load of the Huanghe （Yellow） River were analyzed by using the empirical mode decomposition （EMD） method. The results show that the periodic oscillation of water discharge and sediment load of the Huanghe River occurs at the interannual, decadal, and multi-decadal scales, caused by the periodic oscillations of precipitation, and E1 Nifio/Southern Oscillation （ENSO） af- fects water discharge by influencing precipitation distribution and contributes to periodic varations in precipitation and water discharge at interannual timescale. The water discharge and sediment load of the Huanghe River have decreased since the 1960s under the influence of precipitation and huamn activities, and human activities attribute more than precipitation to the reduction in the water discharge and sediment load, furthermore, water abstraction and water-soil conservation practices are the main causes of the decrease in water discharge and sediment load, respectively. The reduction in sediment load has directly impacted on the lower reaches of the Huanghe River and the river delta, causing considerable erosion of the river channel in the lower reaches since the 1970s along with River Delta changing siltation into erosion around 2000.

Study on the Microsecond Pulse Homogeneous Dielectric Barrier Discharges in Atmospheric Air and Its Influencing Factors

The homogeneous dielectric barrier discharge （DBD） in atmospheric air between two symmetric-columnar copper electrodes with epoxy plates as the dielectric barriers is generated using a us pulse high voltage power supply. The discharge characteristics are studied by measurement of its electrical discharge parameters and observation of its light emission phenom- ena, and the main discharge parameters of the homogenous DBD, such as discharge current and average discharge power, are calculated. Results show that the discharge generated is a homogeneous one with one larger single current pulse of about 2 #s duration appearing in each voltage pulse, and its light emission is radially homogeneous and covers the entire surface of the two elec- trodes. The influences of applied voltage amplitude, air gap distance and barrier thickness on the transition of discharge modes are studied. With the increase of air gap distance, the discharge will transit from homogeneous mode to filamentary mode. The higher the thickness of dielectric barriers, the larger the air gap distance for generating the homogeneous discharge mode. The average discharge power increases non-linearly with increasing applied voltage amplitude, and decreases non-linearly with the increase of air gap distance and barrier thickness. In order to generate stable and homogeneous DBD with high discharge power, thin barriers distance should be used, and higher applied voltage amplitude should be applied to small air gap.

Simulation Study on the Self-Sustained Oscillations in DC Driven Glow Discharges at Atmospheric Pressure Under Different Gas Gaps

In this paper, a one-dimensional plasma fluid model is employed to study the self- sustained oscillations in DC-driven helium glow discharges at atmospheric pressure under different gas gaps. Our simulation results indicate that a harmonic current oscillation with tiny amplitude always occur at the onset of instability and transits into a relaxation one as the conductivity of the semiconductor is decreased. It is found that the dynamics of the oscillations are dependent on the gas gaps. The discharge can only exhibit a simple oscillation with unique amplitude and frequency at smaller gas gaps （〈2 mm） while it can exhibit a more complex oscillation with several different amplitudes and frequencies at larger gas gaps （〉2 mm）. The discharge modes in these current oscillations have also been analyzed.

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 2.31×10^(19)m^(-3),as well as 790.13 Td and 3.58×10^(19)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.

The Characteristics of Dielectric Barrier Discharge and its Influence on the Excimer XeCl Emission

In this work, the influence of discharge modes on the excimer XeCl emission (308 nm) has been studied by adding helium gas into the xenon and chlorine mixture. It is found that the transition from filament discharge to glow /filament - combined discharge leads to the decrease in excimer emission. We are the first one to use a flowing water film as an outer transparent electrode, and achieve a higher UV intensity, compared with the case by using a metal mesh as the outer electrode. The influence of the gas temperature both in the reactor Tg and in a discharge channel Tc on the excimer emission has been analyzed preliminarily. Finally, it has been expected that the replacement of chlorine gas Cl2 by another chlorine gas may reduce the heat generated in the discharge processes and give rise to the excimer XeCl* radiation.

Simulation Study on Multi-Pulse Phenomena of Atmospheric Pressure Argon Dielectric Barrier Discharge

In this paper, a one-dimensional discharge model is employed to study multi-pulse phenomena in Ar dielectric barrier discharge （DBD） under atmospheric pressure. The finiteelement method is employed to solve the model. The influences of applied voltage amplitude and frequency as well as gas gap distance on the variation of multi discharge pulses are investigated and discussed. The simulation results show that, both the intensity of discharge current and the number of discharge pulses increase with the amplitude of applied voltage, and narrower gas gap is more favorable for the formation of multi pulses. It is revealed that Ar DBDs behave in glow discharge mode when the applied voltage and gas gap distance vary from 2 kV to 6 kV and from 1 mm to 3 mm, respectively. With the frequency decreasing from 250 Hz to 125 Hz, the intensity of discharge current weakens and the number of discharge pulses increases, and the discharges behave in the typical Townsend discharge mode.

Effects of Gas Flow Rate on the Discharge Characteristics of a DC Excited Plasma Jet

A direct current（DC） source excited plasma jet consisting of a hollow needle anode and a plate cathode has been developed to form a diffuse discharge plume in ambient air with flowing argon as the working gas.Using optical and electrical methods,the discharge characteristics are investigated for the diffuse plasma plume.Results indicate that the discharge has a pulse characteristic,under the excitation of a DC voltage.The discharge pulse corresponds to the propagation process of a plasma bullet travelling from the anode to the cathode.It is found that,with an increment of the gas flow rate,both the discharge plume length and the current peak value of the pulsed discharge decrease in the laminar flow mode,reach their minima at about1.5 L/min,and then slightly increase in the turbulent mode.However,the frequency of the pulsed discharge increases in the laminar mode with increasing the argon flow rate until the argon flow rate equals to about 1.5 L/min,and then slightly decreases in the turbulent mode.