coupling , defined as a ratio of the Coulomb energy to the thermal one:

The Coulomb energy (per particle) is

where for the typical inter-particle distance usually is taken the Wigner-Seitz radius. Therefore,

Clearly, up to a numeric factor of the order of unity,

In general, for multicomponent plasmas one defines the coupling parameter for each species s separately:

Here, s stands for either electrons or (a type of) ions.

The ideal plasma approximation