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.