What is the difference between NEI and CIE plasma models?
An “NEI” plasma is one with “non-equilibrium ionization”, while a “CIE” plasma is in “collisional ionization equilibrium”. Implied in the latter case is that the electron velocity distribution is described by the Maxwell-Boltzmann equation (i.e., it is a thermal plasma), and that the ion population for all atoms is not time-dependent (i.e, the rate of ionization out of any ion is exactly balanced by the rate of recombination. Simply because a plasma is in CIE does not mean that the entire system is in Local Thermodynamic Equilibrium (LTE); this occurs when each level of each ion is also in equilibrium, a much more restrictive statement that requires much higher densities than simple CIE (see the physics page for more details). An NEI plasma is simply defined as not being in CIE; it may not have a thermal distribution of electrons, or it may be undergoing more ionizations than recombinations (in which case it is often called an ‘ionizing’ plasma), or it may be undergoing more recombinat
