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.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.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.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.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.
Long-exposure photograph of corona discharge on an insulator string of a 500 kV overhead power line . Corona discharges represent a significant power loss for electric utilities .
The corona discharge around a high-voltage coil
Corona discharge from a spoon attached to the high-voltage terminal of a Tesla coil
Large corona discharges (white) around conductors energized by a 1.05-million-volt transformer in a U.S. NIST laboratory in 1941
A variety of forms of corona discharge, from various metal objects. Notice, especially in the last two pictures, how the discharge is concentrated at the points on the objects.
Corona discharges on the 380kV overhead power line over the Albula Pass ( Switzerland ) in foggy weather conditions (30 second long exposure)
Corona discharge on a Wartenberg wheel
overhead power lineelectric utilitiesTesla coilelectrical dischargeionizationconductorvoltageelectrical breakdownelectric fieldpotential gradientdielectric strengthgas discharge lampchemiluminescenceSt. Elmo's firetransmission linescathode-ray-tube televisionsradio transmittersX-ray machinesparticle acceleratorsnitric oxidenitrogen dioxidenitric acidwater vaporcorona ringsphotocopiersozone generatorspotentialionizingplasmachargebrush dischargeelectric arcelectric sparkelectroninelastic collisionelectrostatic precipitatorair-conditioning systemsPhotocopyingAir ionisersKirlian photographyEHD thrustersliftersionic windNitrogen lasermass spectrometerion mobility spectrometerantistatic devicesAlbula PassSwitzerlandelectric power transmissiontransformerscapacitorselectric motorsgeneratorsElastomerO-ringsozone crackingphotoncosmic rayelectron avalanchenonequilibriumPeek's lawphotoelectric effectwork functionionization energyelectronegativemoleculesoxygenWartenberg wheelelectrical windspace chargeAlternating currentAtmospheric pressure chemical ionizationCrookes tubeDielectric barrier dischargeCRC PressDavidson, Jane H.BibcodeTheGuardian.comMcGraw-HillUniversity of California Press