Quasi-neutrality

The quasi-neutrality is one of the basic characteristics of physical plasmas . It says that the densities of positive and negative charges in a plasma are roughly equal. The term was first used by the physicist Irving Langmuir in 1929.

A plasma is a gas whose atoms or molecules have partially or completely lost their electrons through ionization . The electrons are then no longer bound to the atoms or molecules, but are freed from them. The negative charges are in a plasma in the form of electrons and in certain cases, which u. a. depend on the type of gas, also in the form of negative ions , while the positive charges are in the form of positive ions.

definition

Quasi-neutrality exists if

{\ displaystyle \ left | \ sum _ {i} (Z_ {i} \ cdot n_ {i}) - n_ {e} \ right | \ ll n_ {e}.}

It is

${\ displaystyle Z_ {i}}$ the number of charges of the i-th, here positively charged ion species
${\ displaystyle n_ {i}}$ the density of the i-th, here positively charged ion species (the sum is formed over all ion species)
${\ displaystyle n_ {e}}$ the density of free electrons .

Strictly speaking, both sides of the equation must be multiplied by the elementary charge in order to obtain the charge densities in coulombs per volume . ${\ displaystyle e}$

In extreme cases, in which the density of negative ions can no longer be neglected compared to the density of positive ions, it must also be taken into account in the equation:

{\ displaystyle \ sum _ {i} (Z_ {i} \ cdot n_ {i}) \ approx n_ {e}.}

This formulation is also known as a plasma approximation .

Injury from electric fields

Electric fields can locally disturb the neutrality of a plasma, e.g. B. in the vicinity of the plasma boundary layer or in plasma double layers. The resulting separation of positive and negative charge carriers creates electrical voltages . In a quasi-neutral plasma, neutrality is only given over areas which are large compared to the Debye length . In the case of plasmas in gas discharge tubes and in tokamaks , this is typically in the range from 0.1 to 1 mm, in the solar wind around 10 m and in the interstellar medium 10 km. In plasma double layers that form between plasmas of different types - such as between the magnetosphere and the ionosphere of the earth - and which can extend over great distances in space (expansions of over 5000 light years have already been observed in the jet of the galaxy M87 ), neutrality is typically violated over distances of ten times the Debye length.

There are also charged plasmas such as B. in the charged particle beams of particle accelerators or in ion traps .

Further information

Quasi-neutrality on plasma-universe.com (engl.)
Ulrich Stroth: Plasma Physics . 1st edition. Vieweg + Teubner, Wiesbaden 2011, ISBN 978-3-8348-1615-3 .

Individual evidence

↑ Irving Langmuir and Lewi Tonks, "General Theory of the plasma of an Arc" , Phys. Rev. 34, 876-922 (1929)

[1] Irving Langmuir and Lewi Tonks, "General Theory of the plasma of an Arc" , Phys. Rev. 34, 876-922 (1929)