Corona Discharge

In electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized. The discharge will occur when the strength (potential gradient) of the electric field around the conductor is high enough to form a conductive region, but not high enough to cause electrical breakdown or arcing to nearby objects.

 

Problems caused by corona discharge

Corona discharge is generally undesirable. It represents 

1. Power loss
2. Audible noise
3. Electromagnetic interference
4. Purple glow
5. Ozone production
6. Insulation damage

Also Coronas can generate audible and radio-frequency noise.

Mechanism of corona discharge

Corona discharge results 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 electrons which ionize more atoms. The process is: 

1.   A neutral atom or molecule, in a region of strong electric field (such as the high potential gradient near the curved electrode) is ionized by a natural environmental event (for example, being struck by an ultraviolet photon or cosmic ray particle), to create a positive ion and a free electron.

2.   The electric field accelerates these oppositely charged particles in opposite directions, separating them, and preventing their recombination, and imparting to each of them kinetic energy.

3.   The electron has a much higher charge/mass ratio and so is accelerated to a higher velocity than the 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.  

 

4.   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.

 

 5.   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 non equilibrium 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 (CIV) can be found with Peek's law (1929), formulated from empirical observations. Later papers derived more accurate formulas.