A corona discharge is an electrical discharge that occurs when the fluid around a conductor ionizes. The strength of the electrical field must be great enough to cause the ionization but insufficient to cause actual arcing. This process requires an electrode with a high electrical potential within an electrically neutral fluid — usually air. A fluid in this context includes gases as well as liquids.
The electrical current creates charged particles known as ions from the neutral molecules in the fluid if the potential gradient is great enough. This is most likely to occur when the conductor has a sharp point. When the air around the conductor begins to conduct electricity, it effectively becomes part of the conductor. This makes the conductor less sharp and might prevent the ionization from extending beyond a certain distance from the conductor.
The ions eventually travel beyond this conductive region and become neutral molecules once again. The conditions around the electrode might also allow the area of ionization to continue growing and form a completely conductive path. This will result in continuous arcing, or sparking, rather than a corona discharge.
The ideal conditions for a corona discharge generally require a pair of electrodes. One electrode should be highly curved and typically is a small wire. The other electrode should be flat, such as a plate. This difference in curvature between the electrodes ensures a high potential around the curved electrode.
The polarity of the curved electrode determines the polarity of the corona discharge. This means that a curved electrode with a positive charge produces a positive corona, and a curved electrode with a negative charge produces a negative corona. These two types of coronas have very different behavior because of the difference in mass between positive ions and electrons. Positive ions do not collide with each other nearly as often as electrons do. This means that only negative coronas produce the ozone, violet glow and hissing associated with coronas.
There are a variety of applications for a corona discharge. It can remove an electrical charge from the surface of a flying aircraft. This can prevent an electrical discharge from damaging the aircraft’s electronic systems.
A corona discharge can also remove particulate matter from the air. It ionizes the air, which charges the particulate matter. The air then passes over a comb with the opposite charge. The comb attracts the charged particles, thus removing them from the air.
