Quasi-neutrality

From (The Plasma Universe Wikipedia-like Encyclopedia)

M87's "jet" violates quasi-neutrality. It was discovered by Heber D. Curtis in 1918,^[1], is a "sheet beam",^[2] that extends at least 5000 light-years.^[3].

The Heliospheric current circuit extends hundreds of millions of miles across the Solar System, even though the Debye length (measure of charge separation) is of the order of just 10 meters.

Quasi-neutrality (quasi, from the Latin, "as if", "resembling") describes the apparent charge neutrality of a plasma overall, while at smaller scales, the positive and negative charges making up the plasma, may give rise to charged regions and electric fields.

Since electrons are very mobile, plasmas are excellent conductors of electricity, and any charges that develop are readily neutralized, and in many cases, plasmas can be treated as being electrically neutral. The term is sometimes written without the hyphen, ie. quasineutrality, and even sometimes as two words, ie. quasi neutrality.

Quasi-neutrality and Debye length

The distance over which quasi-neutrality becomes apparent depends on factors such as the density and temperature of a plasma. For example, the higher the density of a plasma, the smaller the region of quasi-neutrality because it will contain nearly equal numbers of negative and positively charged particle.

This distance over which quasi-neutrality may break down, is often described by the Debye length (or Debye sphere), and varies according to the physical characteristics of the plasma. The Debye length is typically less than a millimetre (ie. charged regions will not exceed a millimetre), in plasmas found in fluorescent light tubes, tokamaks (used in fusion research), and the ionosphere. However, the Debye length may reach about 10m in the interplanetary medium (solar wind) and interstellar medium (between the star), and up to 10,000m (10km) in intergalactic space.

Violation of quasi-neutrality

Quasi-neutrality is violated by, for example, charged particle beams (jets) and double layers, though the containing plasma as a whole will still maintain charge neutrality, but local regions may not.

The magnitude of the size of the violation of quasi-neutrality is typically a few 10s times the Debye length. In other words, if the Debye length for a particular plasma is about 1cm, then charge separation regions around 10 - 20 cm may occur.

But note that while the distance between two charge separation regions may be quite small, the overall size of each each region may be enormous. For example, the jet emerging from the galaxy M87, has been estimated to be 5400 light-years long.

History

The word "Quasi-neutrality" appears to have been first used in a chemistry context in 1907,^[5]. In a plasma context, the word appears to have been first used by Irving Langmuir and Lewi Tonks, in 1929.^[6]

Notes