Streamer discharge

Streamer discharges into the air from the high voltage terminal of a large Tesla coil. The streamers form at the end of a pointed rod projecting from the terminal. The high electric field at the pointed end causes the air to ionize there.

A streamer discharge, also known as filamentary discharge, is a type of transient electrical discharge. Streamer discharges can form when an insulating medium (for example air) is exposed to a large potential difference. When the electric field created by the applied voltage is sufficiently large, accelerated electrons strike air molecules with enough energy to knock other electrons off them, ionizing them, and the freed electrons go on to strike more molecules in a chain reaction. These electron avalanches (Townsend discharges) create ionized, electrically conductive regions in the air near the electrode creating the electric field. The space charge created by the electron avalanches gives rise to an additional electric field. This field can enhance the growth of new avalanches in a particular direction. Then the ionized region grows quickly in that direction, 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. They are used in applications such as ozone production, air purification or plasma medicine. Streamers pave the way for arcs and lightning leaders, in which the ionized paths created by streamers are heated by large currents. Streamers can also be observed as sprites in the upper atmosphere. Due to the low pressure, sprites are much larger than streamers at ground pressure, see the similarity laws below.

This time exposure of streamers from a Tesla coil in a glass box shows their filamentous nature.

History

The theory of streamer discharges was preceded by John Sealy Townsend's discharge theory^[1] from around 1900. However, it became clear that this theory was sometimes inconsistent with observations. This was especially true for discharges that were longer or at higher pressure. In 1939, Loeb^[2][3] and Raether^[4] independently described a new type of discharge, based on their experimental observations. Shortly thereafter, in 1940, Meek presented the theory of spark discharge,^[5] which quantitatively explained the formation of a self-propagating streamer. This new theory of streamer discharges successfully explained the experimental observations.

Applications

Streamers are used in applications such as ozone generation, air purification and plasma-assisted combustion. An important property is that the plasma they generate is strongly non-equilibrium: the electrons have much higher energies than the ions. Therefore, chemical reactions can be triggered in a gas without heating it. This is important for plasma medicine, where "plasma bullets", or guided streamers, can be used for wound treatment, although this is still experimental.

Streamer physics

Streamers can emerge when a strong electric field is applied to an insulating material, typically a gas. Streamers can only form in areas where the electric field exceeds the dielectric strength (breakdown field, disruptive field) of the medium. For air at atmospheric pressure, this is roughly 30 kV per centimeter. 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. Ions are much heavier, so they move very slowly compared to electrons. As the electrons move through the medium, they collide with the neutral molecules or atoms. Important collisions are:

• Elastic collisions, which change the direction of motion of the electrons.
• Excitations, where the neutral particle is excited, and the electron loses the corresponding energy.
• Impact ionization, where the neutral particle becomes ionized, with the incident electron losing the energy.
• Attachment, where the electron attaches to the neutral to form a negative ion.

When the electric field approaches the breakdown field, the electrons gain enough energy between collisions to ionize the gas atoms, knocking an electron off the atom. At the breakdown field, there is a balance between the production of new electrons (due to impact ionization) and the loss of electrons (due to attachment). Above the breakdown field, the number of electrons starts to grow exponentially, and an electron avalanche (Townsend avalanche) forms.

The electron avalanches leave behind positive ions, so in time more and more space charge is building up. (Of course, the ions move away in time, but this a relatively slow process compared to the avalanche generation). Eventually, the electric field from all the space charge becomes comparable to the background electric field. This is sometimes referred to as the "avalanche to streamer transition". In some regions the total electric field will be smaller than before, but in other regions it will get larger, which is called electric field enhancement. New avalanches predominantly grow in the high-field regions, so a self-propagating structure can emerge: a streamer.

Positive and negative streamers

There are positive and negative streamers. Negative streamers propagate against the direction of the electric field, that is, in the same direction as the electrons drift velocity. Positive streamers propagate in the opposite direction. In both cases, the streamer channel is electrically neutral, and it is shielded by a thin space charge layer. This leads to an enhanced electric field at the end of the channel, the "head" of the streamer. Both positive and negative streamers grow by impact ionization in this high-field region, but the source of electrons is very different.

For negative streamers, free electrons are accelerated from the channel to the head region. However, for positive streamers, these free electrons have to come from farther away, as they accelerate into the streamer channel. Therefore, negative streamers grow in a more diffuse way than positive streamers. Because a diffuse streamer has less field enhancement, negative streamers require higher electric fields than positive streamers. In nature and in applications, positive streamers are therefore much more common.

As noted above, an important difference is also that positive streamers need a source of free electrons for their propagation. In many cases photoionization is believed to be this source.^[6]

Similarity laws

Most processes in a streamer discharge are two-body processes, where an electron collides with a neutral molecule. An important example is impact ionization, where an electron ionizes a neutral molecule. Therefore the mean free path is inversely proportional to the gas number density. If the electric field is changed linearly with the gas number density, then electrons gain on average the same energy between collisions. In other words, if the ratio between electric field $E$ and number density $N$ is constant, we expect similar dynamics. Typical lengths scale as $1/N$, as they are related to the mean free path.

This also motivates the Townsend unit, which is a physical unit of the $E/N$ ratio.

See also

• electrical discharge
• sprite (lightning)
• corona discharge
• Townsend discharge
• avalanche breakdown

References

1. Townsend, J. S. (1900). "The Conductivity produced in Gases by the Motion of Negatively–charged Ions". Nature 62 (1606): 340–341. Bibcode:1900Natur..62..340T. doi:10.1038/062340b0. ISSN 0028-0836.
2. Leonard Benedict Loeb (1939). Fundamental processes of electrical discharge in gases. J. Wiley & Sons, inc. Retrieved 22 August 2012.
3. Loeb, Leonard B.; Kip, Arthur F. (1939). "Electrical Discharges in Air at Atmospheric Pressure The Nature of the Positive and Negative Point-to-Plane Coronas and the Mechanism of Spark Propagation". Journal of Applied Physics 10 (3): 142. Bibcode:1939JAP....10..142L. doi:10.1063/1.1707290. ISSN 0021-8979.
4. Raether, H. (1939). "Die Entwicklung der Elektronenlawine in den Funkenkanal". Zeitschrift für Physik 112 (7-8): 464–489. Bibcode:1939ZPhy..112..464R. doi:10.1007/BF01340229. ISSN 1434-6001.
5. Meek, J. (1940). "A Theory of Spark Discharge". Physical Review 57 (8): 722–728. Bibcode:1940PhRv...57..722M. doi:10.1103/PhysRev.57.722. ISSN 0031-899X.
6. Nijdam, S; van de Wetering, F M J H; Blanc, R; van Veldhuizen, E M; Ebert, U (2010). "Probing photo-ionization: experiments on positive streamers in pure gases and mixtures". Journal of Physics D: Applied Physics 43 (14): 145204. arXiv:0912.0894. Bibcode:2010JPhD...43n5204N. doi:10.1088/0022-3727/43/14/145204. ISSN 0022-3727.