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].
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