Streamer discharge
It occurs when the electric field at the surface of a conductor exceeds the dielectric strength of air, around 30 kilovolts per centimeter.
These electron avalanches (Townsend discharges) create ionized, electrically conductive regions in the air near the electrode.
The space charge created by the electron avalanches gives rise to an additional electric field, causing the ionized region to grow at its ends, forming a finger-like discharge called a streamer.
Streamers are transient (exist only for a short time) and filamentary, which makes them different from corona discharges.
[citation needed] If a streamer reaches the opposite polarity conductor it creates an ionized conductive path through which a large current can flow, releasing a large amount of heat, resulting in an electric arc; this is the process through which lightning leaders create a path for lightning bolts.
Shortly thereafter, in 1940, Meek presented the theory of spark discharge,[5] which quantitatively explained the formation of a self-propagating streamer.
An important property is that the plasma they generate is strongly non-equilibrium: the electrons have much higher energies than the ions.
Streamers can emerge when a strong electric field is applied to an insulating material, typically a gas.
The electric field accelerates the few electrons and ions that are always present in air, due to natural processes such as cosmic rays, radioactive decay, or photoionization.
The electron avalanches leave behind positive ions, so in time more and more space charge is building up.
In both cases, the streamer channel is electrically neutral, and it is shielded by a thin space charge layer.
Both positive and negative streamers grow by impact ionization in this high-field region, but the source of electrons is very different.
As noted above, an important difference is also that positive streamers need a source of free electrons for their propagation.
[10] The electric streamer, strictly speaking, is an ionization front in the shape of a growing filament.
If the electric field is changed linearly with the gas number density, then electrons gain on average the same energy between collisions.
[22] Recent simulations have shown that such perturbations are even capable to facilitate the production of X-rays (with energies of several tens of keV) from such streamer discharges, which are produced by run-away electrons through the Bremsstrahlung process.
