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Corona Discharge
****************
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 (the strength of the electric field exceeds a certain
value, but conditions are insufficient to cause complete electrical breakdown
or arcing.
Contents:
1.- Introduction
2.- Applications of corona discharge
3.- Problems caused by corona discharges
4.- Mechanism of corona discharge
5.- Electrical properties
6.- Positive coronas
6.1.- Properties
6.2.- Mechanism
7.- Negative coronas
7.1.- Properties
7.2.- Mechanism
8.- Examples
1.- Introduction:
A corona is a process by which a current, perhaps sustained, develops
from an electrode with a high potential in a neutral fluid, usually air, by
ionizing that fluid so as to create a plasma around the electrode. The ions
generated eventually pass charge to nearby areas of lower potential, or
recombine to form neutral gas molecules.
When the potential gradient is large enough at a point in the fluid,
the fluid at that point ionizes and it becomes conductive. If a charged
object has a sharp point, the air around that point will be at a much higher
gradient than elsewhere. Air near the electrode can become ionized (partially
conductive), while regions more distant do not. When the air near the point
becomes conductive, it has the effect of increasing the apparent size of the
conductor. Since the new conductive region is less sharp, the ionization may
not extend past this local region. Outside of this region of ionization and
conductivity, the charged particles slowly find their way to an oppositely
charged object and are neutralized.
If the geometry and gradient are such that the ionized region continues
to grow instead of stopping at a certain radius, a completely conductive path
may be formed, resulting in a momentary spark, or a continuous arc.
Corona discharge usually involves two asymmetric electrodes; one highly
curved (such as the tip of a needle, or a small diameter wire) and one of low
curvature (such as a plate, or the ground). The high curvature ensures a high
potential gradient around one electrode </wiki/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. (See below for more details.)
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, with 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.
2.- Applications of corona discharge
Corona discharge has a number of commercial and industrial applications.
Removal of unwanted electric charges from the surface of aircraft in flight
and thus avoiding the detrimental effect of uncontrolled electrical discharge
pulses on the performance of avionic systems.
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.
Surface Treatment of polymer films to improve compatibility with
adhesives or printing inks.
Photocopying.
Air ionisers perhaps benefiting health
Kirlian photography uses photons produced by the discharge to expose
photographic film.
EHD thrusters, Lifters, and other ionic wind devices.
Nitrogen laser.
Surface Treatment for Tissue Culture Polystyrene.
Coronas can be used to generate charged surfaces, which is an effect
used in electrostatic copying. 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 onto oppositely charged plates.
The free-radicals and ions generated in corona reactions can be used to
scrub the air of certain noxious products, through chemical reactions, and
can be used to produce ozone.
3.- Problems caused by corona discharges.
Coronas can generate audible and radio-frequency noise, particularly
near electric power transmission lines. They also represent a power loss, and
their action on atmospheric particulates, along with associated ozone and NOx
production, can also be disadvantageous to human health where power lines run
through built-up areas. Therefore, power transmission equipment is designed
to minimise the formation of corona discharge. Corona discharge is generally
undesirable in:
Electric power transmission, where it causes:
Power loss
Audible noise
Electromagnetic interference
Purple glow
Ozone production
Insulation damage
Inside electrical components such as transformers, capacitors, electric
motors and generators. Corona progressively damages the insulation
inside these devices, leading to premature equipment failure.
Situations where high voltages are in use, but ozone production is to
be minimised.
4.- Mechanism of corona discharge.
Corona discharge of both the positive and negative variety have certain
mechanisms in common.
A neutral atom or molecule of the medium, in a region of strong
electric field (such as the 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 a free electron.
The electric 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 to a higher velocity),
further electron/positive-ion pairs may be created by collision with neutral
atoms. These then undergo the same separating process creating an electron
avalanche. Both positive and negative coronas rely on electron avalanches.
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.
The onset voltage of corona or Corona Inception Voltage (CIV) can be
found with Peek's law (1929), formulated from empirical observations. Later
papers derived more accurate formulas.
5.- Electrical properties
The current carried by the corona is determined by integrating the
current density over the surface of the conductor. The power loss is
determined by multiplying the current and the voltage.
6.- Positive coronas
6.1.- 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 homogeneous 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 total number of electrons may be
lower, the number of a very high energy electrons 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.
6.2.- 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 begins 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 re-accelerated into the plasma. The inner region is known as the plasma
region, the outer as the unipolar region.
7.- Negative coronas
7.1.- 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.
7.2.- 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 travelling 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 non-ionising plasma. The outer
region is known as the unipolar region
Extracted from Wikipedia.com.
Compiled and translated to ASCCI by LW1DSE Osvaldo
Almirante Brown
Buenos Aires
Argentina
28/10/07
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