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corona discharge[kə′rō·nə ′dis‚chärj]
a type of glow discharge. It arises in highly inhomogeneous electric fields in the vicinity of one or both electrodes. Fields of this type are formed at electrodes with very large curvature of surface, such as points or thin conductors. During corona discharge these electrodes are surrounded by a characteristic glow, which is also called a corona or a corona discharge zone. The nonluminous (”dark”) zone of the gap between electrodes adjacent to the corona is called the outer zone. A corona frequently appears on tall pointed objects (Saint Elmo’s fire), around the conductors of electrical transmission lines, and the like.
Corona discharge may occur at various pressures of the gas in the discharge gap; however, it is observed with maximum clarity at pressures that are not lower than atmospheric pressure. The discharge commences when the potential U between electrodes attains the “initial potential” of the corona U0 (typical values are thousands and tens of thousands of volts). The current in a corona discharge is proportional to the difference U — U0 and to the mobility of the gaseous ions formed by the discharge. The current is usually fairly small, being equal to a fraction of a milliampere per 1 cm length of electrode displaying corona discharge. An increase in U causes an increase in the intensity and thickness of the corona discharge zone. When U attains the “spark flashover” potential, the corona becomes a spark discharge.
The corona is positive when only the anode displays a corona discharge. In this case, the primary electrons are released at the exterior boundary of the corona zone as a result of the photoionization of the gas by photons emitted within the corona. Upon acceleration in the field of the anode, these electrons excite gaseous atoms and ions by collision and generate electron avalanches in the process of collision ionization events. Positive ions are carriers of the current in the outer zone, and the positive space charge formed by them limits the corona discharge current.
The positive ions in a negative corona, which are accelerated by the strong field in the vicinity of the cathode displaying a corona discharge, dislodge electrons from the latter (secondary electron emission). After being emitted from the cathode, these electrons ionize the gas by collision, generating avalanches and ensuring the regeneration of positive ions. In pure electropositive gases the current in the outer zone is carried by electrons; in the presence of electronegative gases having electron affinity, it is carried by negative ions arising from the capture of electrons by neutral gas molecules. These electrons or ions form a negative space charge in the outer zone, which limits the current of the corona discharge.
In bipolar coronas both electrodes display corona discharge. The processes in the corona zones are analogous to those described above. The current is carried in the outer zone by an opposing flow of positive ions and electrons (or negative ions).
During periodic changes in the electrode polarity (alternating current corona discharge), the low-mobile heavy ions in the outer zone are unable to reach the electrodes during a single half-period, and oscillations of the space charge arise. A corona discharge at frequencies of 100,000 Hz and higher is called high-frequency corona.
In a corona discharge the electrical energy is converted mainly into thermal energy; upon collision the ions impart the energy of their motion to neutral molecules of the gas. This mechanism leads to appreciable losses of energy in high-voltage transmission lines. Corona discharge has been applied in electrical separation processes (electrical filters, for example), in electrical coloration (particularly in powder coating), and in detection of ionizing radiation (Geiger-Muller counters).
REFERENCESKaptsov, N. A. Koronnyi razriad i ego primenenie v elektrofil’trakh. Moscow, 1947.
Loeb, L. Osnovnyeprotsessy elektricheskikh razriadov vgazakh. Moscow-Leningrad, 1950. [Translated from English.]
Granovskii, V. L. Elektricheskii tok v gaze; Neustanovivshiisia tok. Moscow [in press].
A. K. MUSIN