Degree of ionizati
Degree of ionization
For plasma to exist, ionization is necessary. The term "plasma density" by itself usually refers to the
"electron density", that is, the number of free electrons per unit
volume. The degree of ionization of a plasma is the proportion of atoms that have lost or gained
electrons, and is controlled mostly by the temperature. Even a partially
ionized gas in which as little as 1% of the particles are ionized can
have the characteristics of a plasma (i.e., response to magnetic fields
and high electrical conductivity). The degree of ionization, 
, is defined as 
, where 
is the number density of ions and 
is the number density of neutral atoms. The electron density is related to this by the average charge state 
of the ions through 
, where 
is the number density of electrons.
