Stepped form of negative corona current pulses in hydrogen

Current wave forms of initial stages of discharge formation in a short negative point-to-plane gap have been measured with a nanosecond time resolution in hydrogen at pressures (12.5–76) kPa and for various overvoltages and cathode point radii. The measurements revealed the existence of a stepped form of negative corona current pulses in hydrogen. To test existing models for negative corona pulse formation, effects of changing cathode secondary electron emission were studied using copper and brass cathodes coated by CuI and graphite. It is concluded that a negative corona pulse is associated with the ignition of a cathode-directed streamer in the vicinity of the cathode. We report what we believe are the first experimental observations of non-Trichel oscillations of negative corona current with a frequency of (1–10) MHz. This work was supported by the Scientific Grant Agency of the Ministry of Education and Academy of Sciences of Slovak Republic (Project No. 1/5190/98).     

Bifurcation analysis of the corona discharge on a negative electrode

A bifurcation analysis of the cylindrically symmetric solution obtained in the framework of the steady-state model of the negative corona discharge in air at atmospheric pressure has been carried out. No bifurcations have been found, which indicates that current contraction probably does not occur in the considered model. It follows that the current-free stripes on the corona cathode surface, observed by previous authors in two- and three-dimensional computer simulation and in experiment, are due to the specific discharge geometry rather than current contraction.     

Needle Growth Induced by Corona Discharge

It is shown that the growth of a dense array of chromium microneedles is induced on a field cathode tip interacting with a negative corona discharge in ambient Cr(CO)₆ of 1 – 5 × 10^?2 Torr. The growth begins with the initiation of a corona discharge surrounding the tip, and it proceeds as long as the discharge is sustained. The needles emit field electrons during the growth, thereby sustaining the discharge, while the discharge lengthens the needles by supplying metallic particles to their tips. The morphological structure of the needles is strongly affected by an external magnetic field, leading us to hypothesize that the needle grow axially as the electron avalanches, originating at the cathode tip, propagate outward from tip.     

Negative Corona Current Pulses in SF6 – Air Mixtures

Current waveforms of first negative corona pulses have been measured in dry air + SF₆ mixtures at a pressure of 50 kPa and various overvoltages. Effects of changing cathode secondary electron emission were studied using a copper cathode coated by CuI and graphite. It is concluded that in the mixtures containing less than 10 % of SF₆ the negative corona pulse is associated with the formation of a cathode‐directed streamer in the immediate vicinity of the cathode. In the mixtures containing more than 20 % of SF₆ the streamer is quenched and, consequently, the discharge is governed by the Townsend ionisation mechanism fed by cathode photoemission processes.     

Negative coronas: Low current mode – pulse mode transition

In the paper the results of experimental study of negative corona in a point – plane gap in a dust-free air flow at atmospheric pressure are presented. Both the low current mode (LCM) of the discharge and Trichel pulses are externally triggered by UV light. Close to the inception voltage of Trichel pulses short-duration current spikes appear besides the steady component of the LCM which indicates to the switch-on of Fowler-Nordheim type emission mechanism. At the inception voltage spike – Trichel pulse transition is recorded. The influence of dielectric layers at the cathode on the discharge parameters is discussed.     

Verification of positive streamer mechanism for negative corona Trichel pulses in O2/H2 mixtures

Current waveforms of the first negative corona pulses have been measured in O₂ + H₂ mixtures at a pressure range from 27 kPa to 50 kPa. It was observed that the hydrogen admixtures less than 4% do not change significantly the pulse current waveforms. Effects of changing cathode secondary electron emission were studied using a copper cathode coated by CuI and graphite. The results obtained support the theory of the cathode-directed streamer formation during the negative corona pulse rise.     

Role of Streamers in the Formation of a Corona Discharge in a Highly Nonuniform Electric Field

JETP Letters - The ignition of a corona discharge on a tip electrode with a small radius of curvature in air at atmospheric pressure is studied. It is established that the initiation of the corona...     

DC corona discharge ozone production enhanced by magnetic field

We have studied the effect of a stationary magnetic field on the production of ozone from air at atmospheric pressure by a negative corona discharge in a cylindrical electrode configuration. We used a stainless steel hollow needle placed at the axis of the cylindrical discharge chamber as a cathode. The outer wall of the cylinder was used as an anode. The vector of magnetic induction was perpendicular to the vector of current density. We found that: (a) the magnetic field extends the current voltage range of the discharge; (b) for the discharge in the Trichel pulses regime and in the pulseless glow regime, the magnetic field has no substantial effect on the discharge voltage or on the concentration of ozone that is produced; (c) for the discharge in the filamentary streamer regime for a particular current, the magnetic field increases the discharge voltage and consequently an approximately 30% higher ozone concentration can be obtained; (d) the magnetic field does not substantially increase the maximum ozone production yield. A major advantage of using a magnetic field is that the increase in ozone concentration produced by the discharge can be obtained without additional energy requirements.     

Negative corona discharge from a water droplet under the pulsating DC field

Corona discharge from a fine water droplet always involves deformation of the droplet shape or Taylor-cone formation, emission of fine water jets or disruption of droplet. Therefore, corona discharge from a water droplet always manifests complicated aspects. In addition, disruption of Taylor cone simultaneously affects not only discharge current but also motion of water droplet. To confirm corona discharge phenomena from a water droplet protruded from a tip of a metal capillary tube with a diameter of 1 mm, negative corona discharge was investigated by using a water droplet located at a tip of grounded rod electrode facing a ring electrode with positive dc voltage superimposed by ac one. Since the droplet has inherent resonant vibrating frequency defined by the size or volume, the volume of water droplet was adjusted at 20 nL where the corresponding resonant frequency was 500 Hz. The period of the event of successive corona discharge is exactly consistent with resonant frequency defined by the size of the water droplet. As a result, corona pulse trains with a definite duration appeared intermittently corresponding to its resonant vibration. When dc voltage superimposed by ac voltage with resonant frequency of 500 Hz was applied to the water droplet, corona pulse trains appeared at the period corresponding to the frequency. The maximum value of corona current reasonably increased with the applied voltage. Even when the frequency of ac field superimposed on dc field was varied from the resonant frequency, corona pulse trains occur corresponding to not only the superimposed field frequency but also resonant frequency.     

On the initiation of a spark discharge upon the breakdown of nitrogen and air in a nonuniform electric field

Switchover from a runaway-electromnduced volume discharge to a spark is studied when a nanosecond discharge is initiated in high-pressure nitrogen an d air at a voltage of 50–250 kV. In the case of a cathode with a small radius of curvature and a flat anode and in the presence of cathode spots, the leader of the spark channel may propagate from the flat cathode. When the rate of rise of the voltage across centimeterwide gaps is high (dU/dt ∼ 10¹⁵ V/s or higher), cathode spots in the case of a corona discharge emerge within 200 ps.     

Current analysis of DC negative corona discharge in a wire-cylinder configuration at high ambient temperatures

To study the characteristics of DC negative corona discharge in a wire-cylinder configuration at an ambient temperature range of 350–850 °C, the I–V characteristics and the current composition are analyzed under different conditions. A simple method is proposed to determine the DC corona onset threshold voltage. At high ambient temperatures, in the DC negative corona discharge gap, some electrons are not attached to the electronegative gas molecules and move to the anode tube. Thus, these electrons form an electron current, which may account for most of the total discharging current. The ratio of the electron current to the total discharging current increases with increasing temperature. In a mixture of O₂ and N₂ and a mixture of CO₂ and N₂, the ratio of electron current increases with increasing N₂ content in the mixtures. The cathode material has little influence on the corona discharge characteristics at high ambient temperatures.     

Experimental and modelling study of the dependence of corona discharge on electrode geometry and ambient electric field

The results of experimental observations and mathematical modeling of corona formation on the tips of grounded rods are presented as a function of their tip height, curvature radius, the magnitude and polarity of the applied electric field producing corona. The investigations demonstrate that corona current depends on the active volume of zone in which electric field strength exceeds the breakdown criteria for air. The mathematical model was verified with the experimental data, enabling dependence of corona current on rod tip height, tip radius and applied electric field strength to be quantified with the need for a plethora of experiments.     

Impulse breakdown characteristics of the plane-to-plane electrode system with a needle-shaped protrusion in SF6

This paper presents the impulse pre-breakdown and breakdown characteristics of the plane-to-plane electrode system with a needle-shaped protrusion in SF₆ gas. The breakdown voltage–time (V–t) characteristics and the breakdown voltage–gas pressure (V–p) characteristics of a highly non-uniform SF₆ gas gap under positive and negative lightning impulse voltages are investigated in the pressure range between 0.1 and 0.5 MPa. The pre-breakdown developments are examined by the corona current and light emission measurements with high time resolution. As a result, the dielectric strengths versus time-to-breakdown of SF₆ gas gap under positive lightning impulse voltages were nearly independent of the gas pressure. The first streamer corona was initiated at the tip of the needle electrode, and the streamer corona pulses developed with a stepwise propagation. The discharge paths were zigzag, and the branches of the discharge channel for positive polarity were created. On the other hand, the leader channel in the negative polarity was thicker and brighter than that in the positive polarity.     

直流大气压电晕放电电子密度的光谱研究

利用自制针—板式放电装置,在大气中进行电晕放电实验。用发光区域照片光斑的大小,讨论了电晕层厚度与电源电压的关系。在相同针板间距下,电晕层厚度随着电压的升高而增大;在相同电压下,电晕层厚度随着针板间距的增大而减小。由于高能电子密度能够通过氮分子第二正带系337.1 nm的光谱强度大小反映,因此对氮分子第二正带系337.1 nm谱线的强度用发射光谱法进行了测量。实验结果发现在针尖附近高能电子密度最大,并且高能电子密度随电压的升高而增大;电压一定时,高能电子密度随针板间距的增大而减小。在针板间距和电源电压不变的情况下,高能电子密度随针尖曲率半径的减小而增大。     

Characteristics and underlying physics of ionic wind in dc corona discharge under different polarities

During a dc corona discharge, the ions' momentum will be transferred to the surrounding neutral molecules, inducing an ionic wind.The characteristics of corona discharge and the induced ionic wind are investigated experimentally and numerically under different polarities using a needle-to-ring electrode configuration.The morphology and mechanism of corona discharge, as well as the characteristics and mechanism of the ionic wind, are different when the needle serves as cathode or anode.Under the different polarities of the applied voltage, the ionic wind velocity has a linear relation with the overvoltage.The ionic wind is stronger but has a smaller active region for positive corona compared to that for negative corona under a similar condition.The involved physics are analyzed by theoretical deduction as well as simulation using a fluid model.The ionic wind of negative corona is mainly affected by negative ions.The discharge channel has a dispersed feature due to the dispersed field, and therefore the ionic wind has a larger active area.The ionic wind of positive corona is mainly affected by positive ions.The discharge develops in streamer mode, leading to a stronger ionic wind but a lower active area.     

The velocity of streamer tips in impulse point-to-plane corona in air,using Lichtenberg figure techniques

Positive and negative streamer lengths were measured in point-to-plane impulse corona in air at atmospheric pressure using Lichtenberg figure technique. Applying short time pulses from 9 to 40 nsec duration time-distance plots were obtained, which allowed one to determine the streamer tip velocity. For a 4 cm gap, a 1 mm diameter point, and a 30 kV pulse an average tip velocity of 1.8×10⁸ cm/sec was found. Comparing these data withHudson's photomultiplier measurement, we may identify the Lichtenberg figure with the “primary” streamer ofHudson. The negative streamer tip velocity in the cathode part of the gap was approximately an order of magnitude smaller than the positive streamer velocity.     

Parametric study of the characteristics of Trichel pulses in the needle-plane negative corona discharge in atmospheric air

A numerical study of the impact of different model coefficients on the frequency, DC current, and the rise time of the Trichel pulses in a needle-plane negative corona discharge is presented here. The studied coefficients include the coefficients of the ionization and attachment reactions, the mobilities of the three charged species considered in the model (electrons and positive and negative ions), and the coefficient of the secondary emission of electrons. The characteristics of the pulses were also studied using another set of model coefficients which includes an extended number of reactions such as detachment of the electrons from negative ions.     

Erosion of a copper cathode in a negative corona discharge

The pulsed-periodic regime of a negative corona (Trichel pulses) in atmospheric-pressure air, which leads to explosion emission mechanisms (ecton generation) of pointed cathode erosion, is investigated. The jet erosion process at the copper cathode is discovered, and micrometer dendritelike structures formed by erosion products returning to the cathode are detected.     

On stabilization of multipoint negative corona by using ballast resistances

The I-V characteristics of a negative corona discharge initiated by a multipoint cathode in an argon flow are experimentally studied. It is found that adjustable ballast resistances connected to the corona points provide stable operation and uniform filling of the discharge gap by the plasma.     

Formation Mechanism of the So-Called Corona Figure

A dense radial array of chromium microneedles is formed on a field-emission cathode interacting with a negative corona discharge in ambient Cr(CO)6 of the order of 10-2 torr. The article discusses physical and chemical processes which underlie the formation of this so-called corona figure, based on extensive electron microscope observations. The overall shape of a corona figure is a function of the substrate geometry, indicating that field electrons emitted from the substrate play a vital role in nucleating the figure. A negative electric field curves the growth direction of the needles that compose the figure, and a high magnetic fileld applied externally well aligns them along magnetic-field lines. These facts support strongly the hypothesis that the figure is formed as the electron avalanches, originating at the substrate, propagate outward. The technical term "field-emission corona" is proposed for the present corona discharge, because the discharge is initiated and sustained by field-emitted electrons.