Basic Parameters

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

$$T_s \equiv \frac{1}{3} \,m_s\, \langle \,v_s^{\,2}\rangle$$ (1)

to denote a kinetic temperature measured in energy units (i.e., joules). Here, $v$ is a particle speed, and the angular brackets denote an ensemble average. The kinetic temperature of species $s$ is essentially the average kinetic energy of particles of this species. In plasma physics, kinetic temperature is invariably measured in electron-volts (1 joule is equivalent to $6.24\times 10^{18}$eV).

Quasi-neutrality demands that

$$n_i \simeq n_e \equiv n,$$ (2)

where $n_s$ is the number density (i.e., the number of particles per cubic meter) of species $s$.

Assuming that both ions and electrons are characterized by the same $T$ (which is, by no means, always the case in plasmas), we can estimate typical particle speeds via the so-called thermal speed,

$$v_{ts} \equiv \sqrt{2\,T/m_s}.$$ (3)

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

$$v_{ti} \sim \sqrt{m_e/m_i}\,\,v_{te}.$$ (4)

Of course, $n$ and $T$ are generally functions of position in a plasma.