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.
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.
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.
However, free electrons are easily captured by neutral oxygen or water vapor molecules (so-called electronegative gases), forming negative ions.
Captured electrons are effectively removed from play — they can no longer contribute to the avalanche process.
If electrons are being created at a rate greater than they are being lost to capture, their number rapidly multiplies, a process characterized by exponential growth.
Avalanche sustenance requires a reservoir of charge to sustain the applied voltage, as well as a continual source of triggering events.
In each case, the energy emitted as photons by the initial avalanche is used to ionise a nearby gas molecule creating another accelerable electron.