Environmental Engineering Reference
In-Depth Information
obtained by an individual electron after passage of one period subsequently goes
into atom ionization. Equation (5.171) holds in the absence of energy exchange be-
tween electrons and atoms as a result of elastic collisions, and in the absence of
electron-electron collisions. Evidently, the energy
is the
electric voltage on one striation given in Table 5.1 for inert gases. For example, one
can expect
Δ
ε
is equal to
e
'
,where
'
to be equal to or close to the atomic excitation energy for striations of
a certain type. Then collision of two excited atoms formed after this striation leads
to ionization. Unfortunately, we do not have a direct connection between the val-
ues in Table 5.1 and the atomic excitation energies or atomic ionization potentials.
This testifies also to the role of metastable atoms in ionization processes involving
striations.
Nevertheless, if we overlook this contradiction, we can come to an agreement
about the character of variation of striation parameters in this model with the
Pekarek theory [86, 104-106]. Indeed, at the beginning of some striation, where
the electrons are slow, the number density of electrons is higher than that in a re-
gion of fast electrons because their flux in the longitudinal direction is conserved.
Hence, the regions of the maximum number density and the maximum energy
(temperature) of electrons (or glowing maximum) are shifted in accordance with
the above-mentioned theory. Note that along with ionization processes, the loss of
electrons and ions takes place as a result of their ambipolar diffusion to walls. We
assume there are long striations in a discharge tube if the tube radius is small com-
pared with the striation wavelength, so the loss process proceeds uniformly along
the tube.
In conclusion of this analysis, we note that the striation mechanism under con-
sideration corresponds to low pressures in atomic gases, which allows us to ignore
the energy loss of electrons in elastic collisions with atoms. Next, we ignore elec-
tron-electron collisions, so the discharge currents for this mechanism of striations
are limited. Under such conditions, the energy transferred to electrons from the
electric field of the discharge is consumed in excitation and ionization of atoms.
Because striations exist for a nonlinear dependence of the ionization rate on the
number density of electrons, the ionization process in a striated plasma has a
stepwise character. From this it follows that metastable atoms are of importance
for generation of striations in inert gases, and along with electron collisions with
metastable atoms, diffusion of metastable atoms in a plasma may be important for
formation of striations. The latter process leads to washing out of the striation wave
in contrast to the above-mentioned bunching mechanism.
This model shows that two regimes of the plasma can exist. In the first, the plas-
ma is homogeneous, and ionization of atoms by electron impact occurs throughout
the plasma. In the other regime, ionization occurs in narrow ionization zones dis-
tributed periodically over the length of the discharge tube. That is, striations exist
in this regime. The regime in which the discharge plasma finds itself is determined
by the energy available to the system, so the characteristics of an external source of
electricity are of great importance for the plasma behavior.
Δ
ε
