2.2. Atmospheric pressure plasmas: LTE or non-LTE?
The Local Thermodynamic Equilibrium notion [3] is really important, especially for a spectroscopic study of the plasma, since the determination of the plasma parameters (particles distribution functions; electron, excitation, vibration temperatures...) is based on relationships which differ for plasmas in LTE or not.
2.2.1. LTE plasmas LTE plasma requires that transitions and chemical reactions are governed by collisions and not by radiative processes. Moreover, collision phenomena have to be micro-reversible. It means that each kind of collision must be balanced by its inverse (excitation/deexcitation; ionization/recombination; kinetic balance) [4].
Moreover LTE requires that local gradients of plasma properties (temperature, density, thermal conductivity) are low enough to let a particle in the plasma reach the equilibrium: diffusion time must be similar or higher than the time the particle need to reach the equilibrium [5]. For LTE plasma, the heavy particles temperature is closed to the electrons temperature (ex: fusion plasmas).
According to the Griem criterion [6], an optically thin homogeneous plasma is LTE if the electron density fulfills: ne ¼ 9:1023 E21 EHþ 3 kT EHþ m3 where ˝ E21 represents the energy gap between the ground state and the first excited level,
EH+ = 13.58 eV is the ionization energy of the hydrogen atom
T is the plasma temperature.
This criterion shows the strong link that exists between the required electron density for LTE and the energy of the first excited state.
Those rules for LTE are very strict. Thus most of the plasmas deviate from LTE, especially all types of low density plasma in laboratories.
2.2.2. Non-LTE plasmas
Departure from Boltzmann distribution for the density of excited atoms can explain the deviation from LTE. Indeed, for low-lying levels, the electron-induced deexcitation rate of the atom is generally lower than the corresponding electroninduced excitation rate because of a significant radiative deexcitation rate [4].
Another deviation from LTE is induced by the mass difference between electrons and heavy particles. Electrons move very fast whereas heavy particles can be considered static: electrons are thus likely to govern collisions and transitions phenomena. Deviations from LTE are also due to strong gradients in the plasma and the associated diffusion effects.
It has been shown that the LTE distribution can be partial. For example, LTE can be verified for the levels close to ionization threshold [7] (e.g., 5p levels and higher, in argon plasma): such plasmas are pLTE (partial LTE).
The non-LTE plasmas can be described by a twotemperature model: an electron temperature (Te) and a heavy particle temperature (Th). Regarding the huge mass difference between electrons and heavy particles, the plasma temperature (or gas temperature) is fixed by Th. The higher the departure from LTE, the higher the difference between Te and Th is.
Table 1 sums up the main characteristics of LTE and nonLTE plasmas. More details on LTE and deviations from LTE are developed in the books by Huddlestone and Leonard [8], Griem [9], Lochte-Holtgreven [10] and Mitchner and Kruger [11].