Environmental Engineering Reference
In-Depth Information
Thus, there are two regimes for electron kinetics in a photoresonant plasma. At
low radiation fluxes the average electron energy is determined by processes (3.109),
and the electron temperature T e coincides with the excitation temperature T .In
the regime of high photon fluxes, the balance (3.114) for the electron temperature
is determined by processes (3.113), and the electron temperature does not depend
on the photon flux, whereas the excitation temperature increases with increasing
photon flux according to (3.107). Evidently, in the second regime of electron kinet-
ics, the loss of electron energy results from ionization of resonantly excited atoms
rather than excitation of atoms in the ground state, and corresponds to the criterion
exp
T e
exp
D
1 .
(3.115)
T
The values of this parameter for photon flux j ω D
j 0 are given in Table 3.4. As is
seen, the second regime of electron evolution is realized for photon flux j 0 defined
by (3.105).
Table 3.4 also contains the values of the equilibrium constant K ( T e )atthemaxi-
mum electron temperature T e according to the relation K ion
N e / N ,where N e
and N are the equilibrium values of the number densities of electrons and excit-
ed atoms at the ionization equilibrium. Note that the above evaluations relate to a
weakly ionized plasma where the electron number densities are small compared
with those of atoms, whereas remarkable ionization takes place when ionization
equilibrium is attained. For example, in the sodium case at a typical number densi-
ty of atoms N
D
10 16 cm 3 we have at j ω D
10 15 cm 3 ,and
D
1
j 0 that N D
6
10 16 cm 3 at ionization equilibrium. This means that ionization process-
es destroy atoms, and then the character of interaction of resonant radiation with a
plasma is changed.
Thus, we obtain strong action on a plasma in the regime of high radiation flux-
es, and the sum of the interaction processes between photoresonant radiation and
plasma leads to strong plasma ionization. Note that this interaction between pho-
toresonant radiation and plasma proceeds in a narrow plasma region of length
1/ k ω ( k ω is the absorption coefficient). Blooming of this region results from this
interaction, and subsequently excitation of atoms proceeds in deeper plasma layers.
Because of this character of interaction, the times for individual processes are of
interest, and we determine them at j ω D
N e
1
j 0 assuming that the radiation frequency
corresponds to the center of the spectral line. We have that a strong interaction pro-
ceeds in a region of length of approximately 1/ k 0 , that is, of the order of 10 5 cm
for alkali metal vapors. A typical time for equilibrium establishment between the
number densities of atoms in the ground and resonantly excited state is
N
j ω k ω
τ D
,
(3.116)
and the values of this time are given in Table 3.4 along with the values for the
establishment of the electron temperature (according to (3.112)). We also give in
 
Search WWH ::




Custom Search