Corona discharge

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 a gas discharge lamp, chemiluminescence.

Corona discharges can also happen in weather, such as thunderstorms, where objects like ship masts or airplane wings have a charge significantly different from the air around them (St. Elmo's fire).

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.

However, controlled corona discharges are used in a variety of processes such as air filtration, photocopiers, and ozone generators.

The ions generated eventually pass the charge to nearby areas of lower potential, or recombine to form neutral gas molecules.

[4] Corona discharge usually forms at highly curved regions on electrodes, such as sharp corners, projecting points, edges of metal surfaces, or small diameter wires.

[2] In order to suppress corona formation, terminals on high-voltage equipment are frequently designed with smooth large-diameter rounded shapes like balls or toruses.

Coronas can generate audible and radio-frequency noise, particularly near electric power transmission lines.

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 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.

The uniformity of the plasma is caused by the homogeneous source of secondary avalanche electrons described in the mechanism section, below.

The electrons resulting from the ionization are attracted toward the curved electrode, and the positive ions repelled from it.

It appears a little larger than the corresponding positive corona, as electrons are allowed to drift out of the ionizing region, and so the plasma continues some distance beyond it.

For negative corona, instead, the dominant process generating secondary electrons is the photoelectric effect, from the surface of the electrode itself.

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 ionized neutral gas as a source of ionization is further diminished in a negative corona by the high-concentration of positive ions clustering around the curved electrode.

A pinwheel, with radial metal spokes and pointed tips bent to point along the circumference of a circle, can be made to rotate if energized by a corona discharge; the rotation is due to the differential electric attraction between the metal spokes and the space charge shield region that surrounds the tips.