by Anatol Jaworek, Andrzej Krupa and Tadeusz Czech

CORONA DISCHARGE

Corona discharge is an electrical discharge of relatively low electric energy that takes place at or near atmospheric pressure. The corona discharge can be generated by a strong electric field at an edge of a conductor, i.e., at an electrode of small radius of curvature. The dc corona discharge can not be developed for plane-parallel electrodes. The type of corona discharge depends on the polarity of sharp emitter electrode, and according to that positive or negative coronas can be distinguished.

Schematic diagram of the point-to-plane electrode geometry for corona discharge generation

The current-voltage characteristics of the corona discharge are shown in the following figure:

Corona discharge is one of the most effective methods to generate charged gas molecules of high density. It is also used in cold plasma chemistry, electrophotography, electrostatic separation, air pollution control, decomposition of toxic compounds, aerosol particle charging, improving adhesive properties of dielectric films, static electricity elimination, dielectric charging, electrets production, flowing-gas lasers etc.

One of the electrode configuration of more practical value is the multipoint-to-plate. In this configuration the space charge produced in a given volume can be of higher density than that from a single point. The time-averaged discharge current from a single corona point surrounded by a set of other corona points

In the region A of the current - voltage characteristics of a positive polarity, the current is caused by charges generated by cosmic rays and natural radioactivity, mainly gamma radiation (in each case about 20 electrons/Pa/s/m). At low potentials, the current is in the form of irregular pulses. In the region B, at sufficient high voltages the saturation current is attained because all the charges generated are removed to electrodes without recombination.

At higher voltages the electrons are accelerated by the electric field to energies sufficient to ionize gas molecules, causing the current to rise rapidly.

In the region C the current is in the form of irregular pulses, called burst pulses. The discharge is still not self-sustained, depending on external ionization. Bursts occur in gases containing electronegative molecules, like oxygen. Random bursts develop into nearly regular onset streamers at slightly higher voltages.

The region D is developed with increasing voltage. The discharge is self-sustained in this region without any additional external ionization.

The transition from onset streamers to positive corona discharge (region E) in air at atmospheric pressure starts with a chain of individual electron avalanches, which begin at voltages lower than the corona onset voltage. The mean frequency of steamers first increases with applied voltage, attains a maximum value and then decreases nearly to zero. The steady glow discharge develops in the region E. It occurs primarily near the anode and steadily bridges the gap with increasing voltage. In this region the rate of current increase diminishes with voltage increasing. The current flowing through the discharge gap is nearly steady and is given by the Townsend law

I = k U (U - Uo)

where Uo is the corona onset voltage and k is an experimental constant. This relation has been confirmed by many authors for different electrode geometries and voltages up to 1 MV.

As the voltage is further increasing, the current increases more rapidly, and breakdown streamers occur in region F, reducing the voltage drop across the gap. The positive streamers are in the form of filaments with multiple branches (much more numerous than in negative corona). The current of streamers in air is self limiting even when bridging throughout the whole gap. Only streamers of sufficiently high energy can lead to spark breakdown.

When the voltage exceeds a certain critical value the current still increases while the voltage across the gap decreases rapidly (region G). There is an unstable transition region of negative differential resistance on current voltage characteristics. The current can be stabilised only by positive resistance of sufficient high value in series. The ionisation propagates from both electrodes due to photoionisation and electron emission from the cathode.

The unstable transition region develops into spark or arc (out of plot) depending on the circuit resistance.

In the regions A and B of the discharge of negative polarity the current is caused by charges generated by cosmic rays and natural radiation, and the discharge is similar to that of the positive polarity.

In the region C Townsend ionisation appears that results in a rapid increase in the discharge current. The current is in the form of irregular pulses called Trichel pulses.

With further increase in voltage, the pulses become regular and ionisation is self sustained. The pulses are generated by electron avalanches which are extinguished by electrons attachment to electronegative molecules, like oxygen, forming thus slow negative ions. The current in the pulse diminishes until these ions are removed from the interelectrode gap.

In the region D the current increases slowly with voltage applied. The frequency of Trichel pulses is given by Lama and Gallo formula:

f = K m r d U (U - Ut)

where Ut is the threshold voltage for regular Trichel pulses generation, m is the negative ion mobility, K is a constant, r is the radius of the needle tip, and d is the needle to plane space.

Ferreira et al. found that the electronic component of the current is dominant for short interelectrode distances (<15 mm) and the discharge current in the region D can be approximated by the relation:

I = k (U - Uo)

while for larger distances the Townsend relation better fits experimental data.

For sufficiently high voltages there are many charge clouds present simultaneously in the gap but this space charge is not sufficient to suppress the field and stop ionisation, and the discharge transits into the pulseless glow (region E). The current tends to saturate because of the existing space charge. The pulseless glow is composed of individual electron avalanches which cause successive avalanches at their paths to the anode.

In the region F, for higher voltages, when interelectrode space is sufficiently long, pulsating negative feathers are superimposed on the steady glow. These feathers extend far into the gap, and can develop into streamers, which can bridge entire gap forming an ionisation avalanche. The current still increases, while the voltage across the gap can sometimes decrease.

In the region G, the self-sustained ionisation propagates across the gap from both electrodes simultaneously, which results in negative differential resistance. If the current is not stabilised by an external positive resistance a spark or arc finally develops.

MULTIPOINT-TO-PLANE CORONA DISCHARGE

Similar characteristics are generally observed in multipoint to plane corona discharge, although new phenomena can also take place, because of interaction between the discharge points.

The time-averaged total current in multipoint corona increases with increasing inter-point distances. This effect is more pronounced for negative polarity.

For positive corona, at inter-point distances, m, nearly equal and lower than 5 mm the Townsend relation, derived for the single point-to-plane corona, is not valid. At these inter-point distances the current I due to applied voltage U transits from Townsend square law to near square-root law.

The surprising result is also that at positive polarity, for inter-point distances larger than 15 mm, the discharge current from the central point is only weakly influenced by this distance. The current voltage characteristics for a single point located in the middle of the multipoint electrode are similar to that of a single point operating alone.

For negative polarity, the current emitted from the central point of the multipoint electrode increases with increasing inter-point distances. This is due to the diminishing influence of space charge generated by the adjacent points. For larger inter-point distances (m>20 mm) and interelectrode spacing d<10 mm the discharge current only slightly differs from that from a single point. This is probably because of for short interelectrode distances the dominant role in electrical conductivity play free electrons of high mobility, which can be easily removed from discharge region. An increase in the inter-point distances causes an increase in the electric field at the needle tip, increasing thus the discharge current. Long inter-point distances yield zones between points at which electron space charge decreases practically to zero, and the discharge is like from separate individual points. At positive corona only heavy positive gas ions are present in the discharge gap forming a stable space charge.

In the multipoint configuration the Trichel pulses start with certain frequency and after few seconds the frequency decreases randomly and then the discharge ceases. As a certain time elapses the whole process starts again. As the applied voltage increases the discharge stabilises with regular Trichel pulses.

The corona inception voltage, Uo in the multipoint configuration increases with decreasing interpoint distance m for either polarities. For low inter-point distances (equal and lower than about 5 mm) the inception voltages are nearly the same for both positive and negative corona. But when this distances increase up to infinity (the case of a single point) the inception voltage at the positive corona is higher than at negative.

CORONA DISCHARGE IN FLOWING AIR

It was found that even for low gas velocities (<1 m/s) the current-voltage characteristics of the discharge for positive polarity are modified significantly, and the type of the discharge can be changed by the flow. When the gas velocity is increased from 0 to 4 m/s, the breakdown voltage increases of about 25%.

The time averaged discharge current increases for gas flows from 0 to 0.5m/s, and next decreases again. This phenomenon is not observed for negative polarity of the corona discharge in this gas velocity range.

The air flow in the interval up to about 1 m/s has no effect on the corona onset voltage.

Two negative resistance regimes can be observed in the current - voltage characteristics in flowing air. One is the spark discharge regime, and the second is the arc discharge. Between theses two modes of discharge a narrow range of current exists in which the discharge resistance is positive. The discharge current and the mode of discharge are however unstable in this regime and they can transit easily to each other. The positive resistance occurs for flow velocities lower than 2 m/s, and for positive polarity.

For positive polarity, the voltage at which the corona discharge transits into the spark discharge (breakdown voltage) also increases with the flow velocity increasing. For flow velocity of 4 m/s, this voltage increases of about 25% as compared to the still air. Also the discharge current in the negative resistance mode increases.

The gas flow affects in different way the corona characteristics at negative and positive polarity. At low interelectrode spacing the negative corona current is conducted mainly by free electrons of high mobility, which can be not easily removed by the flowing gas from the discharge space. In the case of positive corona, there are only heavy ions in the interelectrode space, which form a stable space charge. For low values of electrical Reynolds number for ions, which is of the order of magnitude of about 0.1 for the velocity of a few m/s, the ions can be removed by the flowing gas, reducing the space charge and increasing locally the electric field. That cause the increase in the discharge current.

BACK - CORONA DISCHARGE

Back-corona discharge is a type of gaseous discharge that occurs when the corona-counter electrode is covered with a dielectric layer of high resistivity (about 10 GW m). The layer can be made of a porous material or of a solid one with prepared small holes in it. The back-corona discharge can be used as a plasma source for decomposition of gaseous contaminants such as nitrogen oxides and hydrocarbons. The back-corona discharge reactors can be of higher energy efficiency than those operating in the dc streamer corona.

The back-corona discharge is more complex phenomenon than the normal corona. Two types of streamers generated in back corona: surface streamers which propagate along the surface of a dielectric layer, and space streamers developing between the electrodes were distinguished by Masuda and Mizuno. Both of these streamers can under certain conditions occur simultaneously.

The back-corona discharge generates much numerous streamers and the energy transfer to the plasma is also higher than in the case of a normal corona discharge. The back-corona glow discharge seems also to be much uniform. The additional advantage of the back-corona discharge is that it operates at lower voltages than the dc streamer corona in the same geometry, and for the same electrical circuit.

Different forms of back-corona discharge can be observed depending on the voltage applied to the electrodes. Initially many faint breakdown points on the surface of the layer can be observed from which ions of polarity opposite to that of the discharge electrode are emitted. The discharge mode can be recognised as glow discharge. For higher voltages, when the energy of the discharge is sufficiently high, the breakdown streamers concentrate only in a few points which temperature becomes higher and ionisation is easier. At certain discharge current, the inter-electrode voltage drop decreases rapidly and a discharge crater of high temperature is formed in one point of the dielectric layer through which the total current is flowing. In this discharge mode, a short spark discharge or a continuous arc, depending on the ballast and layer resistance, and the interelectrode capacitance occurs.

Schematic diagram of the point-to-plane electrode geometry for back-corona discharge generation