Corona discharge

A corona discharge is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor carrying a high voltage. The discharge appears in cases where the voltage exceeds a critical value, but an electric arc cannot form. Instead, the discharge appears as a colored glow around an object.
The corona discharge represents a local region where the air has undergone electrical breakdown and become conductive, allowing charge to continuously leak off the conductor into the air. A corona discharge occurs at locations where the strength of the electric field around a conductor exceeds the dielectric strength of the air. It is often seen as a bluish glow in the air adjacent to pointed metal conductors carrying high voltages, and emits light by the same mechanism as in a gas discharge lamp and in glow discharge, namely, via a combination of bremsstrahlung radiation and changes in electronic state that produce discrete spectral lines. Corona discharges can also happen in thunderstorms or other electrically-active weather, where objects like ship masts or airplane wings have a charge significantly different from the air around them.
In many high-voltage applications, corona is an unwanted side effect. Corona discharge from high-voltage electric power transmission lines constitutes an economically significant waste of energy for utilities. In high-voltage equipment like cathode-ray-tube televisions, radio transmitters, X-ray machines, and particle accelerators, the current leakage caused by coronas can constitute an unwanted load on the circuit. In the air, coronas generate gases such as ozone and nitric oxide, and in turn, nitrogen dioxide, and thus nitric acid if water vapor is present. These gases are corrosive and can degrade and embrittle nearby materials, and are also toxic to humans and the environment.
Corona discharges can often be suppressed by improved insulation, corona rings, and making high-voltage electrodes in smooth rounded shapes.
Corona discharge can also be useful. Applications for controlled corona discharges include air filtration machines, photocopiers, and ozone generators.

Introduction

A corona discharge is a process by which a current flows from an electrode with a high potential into a neutral fluid, usually air, by ionizing that fluid so as to create a region of plasma around the electrode. The ions generated eventually pass the 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 electric field strength around that point will be much higher than elsewhere. Air near the electrode can become ionized, 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 this region of ionization and conductivity, the charged particles slowly find their way to an oppositely charged object and are neutralized.
Along with the similar brush discharge, the corona is often called a "single-electrode discharge", as opposed to a "two-electrode discharge"—an electric arc. A corona forms only when the conductor is separated widely enough from conductors at the opposite potential that an arc cannot jump between the two. If the geometry and gradient are such that the ionized region continues to grow until it reaches another conductor at a lower potential, a low resistance conductive path between the two will be formed, resulting in an electric spark or electric arc, depending upon the source of the electric field. If the source continues to supply current, a spark will evolve into a continuous discharge called an arc.
Corona discharge forms only when the electric field at the surface of the conductor exceeds a critical value, the dielectric strength or disruptive potential gradient of the fluid. In air at sea level pressure of 101 kPa, the critical value is roughly 30 kV/cm, but this decreases with pressure, therefore, corona discharge is more of a problem at high altitudes. Corona discharge usually forms at highly curved regions on electrodes, such as sharp corners, projecting points, edges of metal surfaces, or small diameter wires. The high curvature causes a high potential gradient at these locations so that the air breaks down and forms plasma there first. On sharp points in the air, corona can start at potentials of 2–6 kV. In order to suppress corona formation, terminals on high-voltage equipment are frequently designed with smooth large-diameter rounded shapes like balls or toruses. Corona rings are often added to insulators of high-voltage transmission lines.
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, it has a [|positive corona]; if it is negative, it has 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, with only the electron having the ability to undergo a significant degree of ionizing inelastic collision at common temperatures and pressures.
An important reason for considering coronas is the production of ozone around conductors undergoing corona processes in air. A negative corona generates much more ozone than the corresponding positive corona.

Applications

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
Sanitization of pool water
In an electrostatic precipitator, removal of solid pollutants from a waste gas stream, or scrubbing particles from the air in air-conditioning systems
Photocopying
Air ionisers
Production of photons for Kirlian photography to expose photographic film
EHD thrusters, lifters, and other ionic wind devices
Nitrogen laser
Ionization of a gaseous sample for subsequent analysis in a mass spectrometer or an ion mobility spectrometer
Static charge neutralization, as applied through antistatic devices like ionizing bars
Refrigeration of electronic devices by forced convection

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.

Problems

Coronas can generate audible and radio-frequency noise, particularly near electric power transmission lines. Therefore, power transmission equipment is designed to minimize 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
* Possible distress in animals that are sensitive to ultraviolet light
Electrical components such as transformers, capacitors, electric motors, and generators:
* Corona can progressively damage the insulation inside these devices, leading to equipment failure.
* Elastomer items such as O-rings can suffer ozone cracking.
* Plastic film capacitors operating at mains voltage can suffer progressive loss of capacitance as corona discharges cause local vaporization of the metallization.

In many cases, coronas can be suppressed by corona rings, toroidal devices that serve to spread the electric field over larger areas and decrease the field gradient to below the corona threshold.

Mechanism

Corona discharge occurs when the electric field is strong enough to create a chain reaction; electrons in the air collide with atoms hard enough to ionize them, creating more free electrons that ionize more atoms. The diagrams below illustrate at a microscopic scale the process which creates a corona in the air next to a pointed electrode carrying a high negative voltage with respect to ground. The process is:

A neutral atom or molecule, in a region of the strong electric field, is ionized by a natural environmental event, to create a positive ion and a free electron.
The electric field accelerates these oppositely charged particles in opposite directions, separating them, preventing their recombination, and imparting kinetic energy to each of them.
The electron has a much higher charge/mass ratio and so is accelerated to a higher velocity than the positive ion. It gains enough energy from the field that when it strikes another atom it ionizes it, knocking out another electron, and creating another positive ion. These electrons are accelerated and collide with other atoms, creating further electron/positive-ion pairs, and these electrons collide with more atoms, in a chain reaction process called an electron avalanche. Both positive and negative coronas rely on electron avalanches. In a positive corona, all the electrons are attracted inward toward the nearby positive electrode and the ions are repelled outwards. In a negative corona, the ions are attracted inward and the electrons are repelled outwards.
The glow of the corona is caused by electrons recombining with positive ions to form neutral atoms. When the electron falls back to its original energy level, it releases a photon of light. The photons serve to ionize other atoms, maintaining the creation of electron avalanches.
At a certain distance from the electrode, the electric field becomes low enough that it no longer imparts enough energy to the electrons to ionize atoms when they collide. This is the outer edge of the corona. Outside this, the ions move through the air without creating new ions. The outward moving ions are attracted to the opposite electrode and eventually reach it and combine with electrons from the electrode to become neutral atoms again, completing the circuit.

Thermodynamically, a corona is a very nonequilibrium process, creating a non-thermal plasma. The avalanche mechanism does not release enough energy to heat the gas in the corona region generally and ionize it, as occurs in an electric arc or spark. Only a small number of gas molecules take part in the electron avalanches and are ionized, having energies close to the ionization energy of 1–3 ev, the rest of the surrounding gas is close to ambient temperature.
The onset voltage of corona or corona inception voltage can be found with Peek's law, formulated from empirical observations. Later papers derived more accurate formulas.