Corona discharge
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In electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor, which occurs when the potential gradient exceeds a certain value, in situations where sparking is not favoured.
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Introduction
A corona is a process by which a current, perhaps sustained, develops between two high-potential electrodes in a neutral fluid, usually air, by ionising that fluid so as to create a plasma around one electrode, and by using the ions generated in plasma-processes as the charge carriers to the other electrode.
Corona discharge usually involves two asymmetric electrodes, one highly curved (such as the tip of a needle, or a narrow wire) and one of low curvature (such as a plate, or the ground). The high curvature ensures a high potential gradient around one electrode, for the generation of a plasma.
Coronas may be positive, or negative. This is determined by the polarity of the voltage on the highly-curved electrode. If the curved electrode is positive with respect to the flat electrode we say we have a positive corona, if negative we say we have a negative corona. The physics of positive and negative coronas are strikingly different. This asymmetry is a result of the great difference in mass between electrons and positively charged ions, and so only the electron having the ability to undergo a significant degree of ionising inelastic collision at common temperatures and pressures.
An important reason for considering coronas is the production of ozone around conductors undergoing corona processes. A negative corona generates much more ozone than the corresponding positive corona.
Applications of Corona Discharge
Corona discharge has a number of commercial and industrial applications.
- Manufacture of ozone
- Scrubbing particles from air in air-conditioning systems
- Removal of unwanted volatile organics, such as chemical pesticides, solvents, chemical weapons agents, from the atmosphere
- Photocopying
- Air ionisers perhaps benefiting health
- Kirlian photography is believed, by some, to be of use in visualising auras.
- Electrostatic levitation
Corona discharge is generally to be avoided in
- Electric power transmission, where it is sometimes called Partial Discharge, owing to loss of power in corona processes, and noise
- Inside electrical components such as transformers, capacitors, electric motors and generators. Partial discharges and corona progressively damage the insulation inside these devices, leading to premature failure.
- Situations where high voltages are in use, but ozone production is to be minimised
Coronas can be used to generate charged surfaces, which is an effect used in electrostatic copying (photocopying). They can also be used to remove particulate matter from air streams by first charging the air, and then passing the charged stream through a comb of alternating polarity, to deposit the charged particles on the oppositely charged plates.
The free-radicals and ions generated in corona reactions can be used to scrub the air of certain noxious products, through free-radical and ion reactions, and can be used to produce ozone.
Coronas can generate audible and radio-frequency noise, particularly in AC Power Transmission lines. They also represent a power loss to ground and can indicate equipment about to fail. Their action on atmospheric particulates, and their ozone and NOx production, can also be disadvantageous to the health where power lines run through built-up areas. Therefore, power transmission equipment is designed to minimise the likelihood of a corona forming.
Mechanism of Corona Discharge
Corona discharge of both the positive and negative variety have certain mechanisms in common.
- A neutral atom in the medium, in a region of strong field (high potential gradient, near the curved electrode) is ionized by an exogenous environmental event (for example, as the result of a photon interaction), to create a positive ion and an electron.
- The strong field then operates on these charged particles, separating them, and preventing their recombination, and also accelerating them, imparting each of them with kinetic energy.
- As a result of the energisation of the electrons (which have a much higher charge/mass ratio and so are accelerated more acutely), further electron/positive-ion pairs are created by collision with neutral atoms. These then undergo the same separating process creating an electron avalanche.
- In processes which differ between positive and negative coronas, the energy of these plasma processes is converted into further initial electron dissociations to seed further avalanches.
- An ion species created in this series of avalanches (which differs between positive and negative coronas) is attracted to the uncurved electrode, completing the circuit, and sustaining the current flow.
Electron Avalanches
Both positive and negative coronas rely on a process known as the electron avalanche.
A corona begins with a rare natural 'background' ionisation event of a neutral air molecule, perhaps as the result of photo-excitation or background radiation. This ionisation creates a positive ion, and a free electron. If this event occurs in an area with a high potential gradient, the positive ion will be strongly attracted toward, or repelled away from, the curved electrode (depending on the polarity of the corona), whereas the electron will be attracted in the opposite direction. This will, occasionally, prevent the recombination of electron and positive ion.
These high-energy electrons, accelerated by the field, (whichever their direction of travel) often collide with neutral atoms inelastically, potentially ionizing those atoms. In a chain-reaction - or 'electron avalanche' - those additional electrons are also separated from their positive ions by the strong potential gradient, causing a large cloud of electrons and positive ions to be momentarily generated by just a single initial event.
A number of mechanisms can sustain this process, creating avalanche after avalanche, to create a constant corona current. A secondary source of corona electrons is required as the electrons are always accelerated by the field in one direction, meaning that avalanches always proceed linearly toward or away from an electrode. The dominant mechanism for the creation of secondary electrons depends on the polarity of the corona. In each case, the energy emitted as photons by the initial avalanche is used to ionise a molecule creating another accelerable electron. What differs is the source of this electron.
Positive Coronas
Properties
A positive corona is manifested as a uniform plasma across the length of a conductor. It can often be seen glowing blue/white, though much of the emissions are in the ultraviolet. The uniformity of the plasma owes itself to the homogenous source of secondary avalanche electrons described in the mechanism section, below. With the same geometry and voltages, it appears a little smaller than the corresponding negative corona, owing to the lack of a non-ionising plasma region between the inner and outer regions. There are many fewer free electrons in a positive corona, when compared to a negative corona, except very close to the curved electrode: perhaps a thousandth of the electron density, and a hundredth of the total number of electrons.
However, the electrons in a positive corona are concentrated close to the surface of the curved conductor, in a region of high-potential gradient (and therefore the electrons have a high energy), whereas in a negative corona many of the electrons are in the outer, lower-field areas. Therefore, if electrons are to be used in an application which requires a high activation energy, positive coronas may support a greater reaction constants than corresponding negative coronas; though the number of electrons may be lower, the number of a very high energy may be higher.
Coronas are efficient producers of ozone in air. A positive corona generates much less ozone than the corresponding negative corona, as the reactions which produce ozone are relatively low-energy. Therefore, the greater number of electrons of a negative corona leads to an increased production.
Beyond the plasma, in the unipolar region, the flow is of low-energy positive ions toward the flat electrode.
Mechanism
As with a negative corona, a positive corona is initiated by an exogenous ionisation event in a region of high potential gradient. The electrons resulting from the ionisation are attracted toward the curved electrode, and the positive ions repelled from it. By undergoing inelastic collisions closer and closer to the curved electrode, further molecules are ionized in an electron avalanche.
In a positive corona, secondary electrons, for further avalanches, are generated predominantly in the fluid itself, in the region outside the plasma or avalanche region. They are created by ionization caused by the photons emitted from that plasma in the various de-excitation processes occurring within the plasma after electron collisions, the thermal energy liberated in those collisions creating photons which are radiated into the gas. The electrons resulting from the ionisation of a neutral gas molecule are then electrically attracted back toward the curved electrode, attracted into the plasma, and so begin the process of creating further avalanches inside the plasma.
As can be seen, the positive corona is divided into two regions, concentric around the sharp electrode. The inner region contains ionising electrons, and positive ions, acting as a plasma, the electrons avalanche in this region, creating many further ion/electron pairs. The outer region consists almost entirely of the slowly migrating massive positive ions, moving toward the uncurved electrode along with, close to the interface of this region, secondary electrons, liberated by photons leaving the plasma, being reaccelerated into the plasma. The inner region is known as the plasma region, the outer as the unipolar region.
Negative Coronas
Properties
A negative corona is manifested in a non-uniform corona, varying according to the surface features and irregularities of the curved conductor. It often appears as tufts of corona at sharp edges, the number of tufts altering with the strength of the field. The form of negative coronas is a result of its source of secondary avalanche electrons (see below). It appears a little larger than the corresponding positive corona, as electrons are allowed to drift out of the ionising region, and so the plasma continues some distance beyond it. The total number of electrons, and electron density is much greater than in the corresponding positive corona. However, they are of a predominantly lower energy, owing to being in a region of lower potential-gradient. Therefore, whilst for many reactions the increased electron density will increase the reaction rate, the lower energy of the electrons will mean that reactions which require a higher electron energy may take place at a lower rate.
Mechanism
Negative coronas are more complex than positive coronas in construction. As with positive coronas, the establishing of a corona begins with an exogenous ionisation event generating a primary electron, followed by an electron avalanche.
Electrons ionised from the neutral gas are not useful in sustaining the negative corona process by generating secondary electrons for further avalanches, as the general movement of electrons in a negative corona is outward from the curved electrode. For negative corona, instead, the dominant process generating secondary electrons is the photoelectric effect, from the surface of the electrode itself. The work-function of the electrons (the energy required to liberate the electrons from the surface) is considerably lower than the ionisation energy of air at standard temperatures and pressures, making it a more liberal source of secondary electrons under these conditions. Again, the source of energy for the electron-liberation is a high-energy photon from an atom within the plasma body relaxing after excitation from an earlier collision. The use of ionised neutral gas as a source of ionisation is further diminished in a negative corona by the high-concentration of positive ions clustering around the curved electrode.
Under other conditions, the collision of the positive species with the curved electrode can also cause electron liberation.
The difference, then, between positive and negative coronas, in the matter of the generation of secondary electron avalanches, is that in a positive corona they are generated by the gas surrounding the plasma region, the new secondary electrons travelling inward, whereas in a negative corona they are generated by the curved electrode itself, the new secondary electrons traveling outward.
A further feature of the structure of negative coronas is that as the electrons drift outwards, they encounter neutral molecules and, with electronegative molecules (such as Oxygen and Water vapour), combine to produce negative ions. These negative ions are then attracted to the positive uncurved electrode, completing the 'circuit'.
A negative corona can be divided into three radial areas, around the sharp electrode. In the inner area, high-energy electrons inelastically collide with neutral atoms and cause avalanches, whilst outer electrons (usually of a lower energy) combine with neutral atoms to produce negative ions. In the intermediate region, electrons combine to form negative ions, but typically have insufficient energy to cause avalanche ionisation, but remain part of a plasma owing to the different polarities of the species present, and the ability to partake in characteristic plasma reactions. In the outer region, only a flow of negative ions and, to a lesser and radially-decreasing extent, free electrons toward the positive electrode takes place. The inner two regions are known as the corona plasma. The inner region is an ionising plasma, the middle a nonionising plasma. The outer region is known as the unipolar region.
Negative coronas can only be sustained in fluids with electronegative molecules, to capture free electrons. Without the electronegative molecules capturing the free electrons, a simple path of electron flow of ionised gas exists between the two electrodes and an arc, or spark, develops.
The onset voltage of corona or Corona Inception Voltage (CIV) can be found with Peek's law (1929), formulated from empirical observations.
References
- [1] Junhong Chen, "Direct-Current Corona Enhanced Chemical Reactions", Phd Thesis, University of Minnesota, USA. August 2002. http://www.menet.umn.edu/~jhchen/Junhong_dissertation_final.pdfde:Koronaentladung