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Concept
Electron avalanche
Summary
An electron avalanche is a process in which a number of free electrons in a transmission medium are subjected to strong acceleration by an electric field and subsequently collide with other atoms of the medium, thereby ionizing them (impact ionization). This releases additional electrons which accelerate and collide with further atoms, releasing more electrons—a chain reaction. In a gas, this causes the affected region to become an electrically conductive plasma.
The avalanche effect was discovered by John Sealy Townsend in his work between 1897 and 1901, and is also known as the Townsend discharge.
Electron avalanches are essential to the dielectric breakdown process within gases. The process can culminate in corona discharges, streamers, leaders, or in a spark or continuous arc that completely bridges the gap between the electrical conductors that are applying the voltage. The process extends to huge sparks — streamers in lightning discharges propagate by formation of electron avalanches created in the high potential gradient ahead of the streamers' advancing tips. Once begun, avalanches are often intensified by the creation of photoelectrons as a result of ultraviolet radiation emitted by the excited medium's atoms in the aft-tip region.
The process can also be used to detect ionizing radiation by using the gas multiplication effect of the avalanche process. This is the ionisation mechanism of the Geiger–Müller tube and, to a limited extent, of the proportional counter and is also used in spark chambers and other wire chambers.
A plasma begins with a rare natural 'background' ionization event of a neutral air molecule, perhaps as the result of photoexcitation or background radiation. If this event occurs within an area that has a high potential gradient, the positively charged ion will be strongly attracted toward, or repelled away from, an electrode depending on its polarity, whereas the electron will be accelerated in the opposite direction. Because of the huge mass difference, electrons are accelerated to a much higher velocity than ions.
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