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Consider an idealized plasma consisting of an equal number of electrons, with
mass and charge (here, denotes the magnitude of the electron
charge), and ions, with mass and charge . We do not necessarily
demand that the system has attained thermal equilibrium, but nevertheless use
the symbol

(1) 
to denote a kinetic temperature measured in energy units (i.e., joules).
Here, is a particle speed, and the angular brackets denote an
ensemble average. The kinetic temperature of species is essentially
the average kinetic energy of particles of
this species. In plasma physics, kinetic temperature
is invariably measured in electronvolts (1 joule is equivalent to
eV).
Quasineutrality demands that

(2) 
where is the number density (i.e., the number of particles
per cubic meter) of species .
Assuming that both ions and electrons are characterized by the
same (which is, by no means, always the case in plasmas), we can
estimate typical particle speeds via the socalled thermal speed,

(3) 
Note that the ion thermal speed is usually far smaller than the
electron thermal speed:

(4) 
Of course, and are generally functions of position in a plasma.
Next: Plasma Frequency
Up: Introduction
Previous: Brief History of Plasma
Richard Fitzpatrick
20110331