Environmental Engineering Reference
In-Depth Information
13 Td, using the values of the first Townsend coefficient and the electron drift veloc-
ity [104]. If attachment of electrons and ions to particles gives the same contribution
to plasma destruction as that due to attachment of electrons and ions to particles,
the reduced electric field strength of the positive column becomes
E
/
N
D
16 Td.
Taking the particle radius as
r
0
D
1
μ
m and the electron temperature as
T
e
D
e
2
/
r
0
T
e
1 eV, we obtain in the case of a rare ionized gas according to (6.21)
j
Z
j
D
2.6
10
3
e
.Next,for
p
and
Z
D
1.8
D
10 Torr we have the number density of argon
10
17
cm
3
, the diffusion coefficient for atomic argon ions at this
atoms
N
a
D
2.4
5.2 cm
2
/s, and
p
T
i
/
m
i
10
4
cm/s. We obtain
pressure is
D
i
780,
which is the efficiency of attachment of electrons and ions to particles with respect
to their attachment to the walls of the gas discharge tube. In addition, we obtain
the number density of particles
N
p
D
D
2.9
D
10
4
cm
3
at which the
rates of plasma destruction as a result of attachment to particles and walls are equal,
so if these particles form a crystal with close-packed particles that corresponds to
the face-centered cubic or hexagonal structure, the distance between nearest par-
ticles is 0.4mm. If the mass density of particles corresponds to a glass, we obtain
from this for the mass density
r
0
R
0
D
1/(2
π
)
D
2.0
10
7
g/cm
3
.For
comparison, we have the mass density of argon atoms under the above conditions,
N
a
of particles in space
D
1.8
p
10
5
g/cm
3
. As seen in this example, the particles influence the pro-
cesses in this gas discharge plasma if the mass density of particles is less than that
of gas atoms by two orders of magnitude.
D
1.6
6.2.6
Particle Structures in Dusty Plasma
We have that a micrometer-sized particle in an ionized gas is surrounded by an ion
coat,andionsscreentheparticlechargethatisestablishedduetoattachmentof
electrons and ions to the particle surface. Electrons of a rare ionized gas give a small
contribution to the particle field shielding. We characterize the region of action of
the particle field by size
l
, the values of which are given in Figure 6.7. The character
of interaction of two particles in an ionized gas is represented in Figure 6.12. At
large distances between two particles (Figure 6.12) the particles do not interact and
they are independent particles. In contrast, at small separations (Figure 6.12b) the
particles are repulsed owing to interaction of the partially screened charges. From
this it follows that under the given conditions the number density of particles in
an ionized gas is restricted. Next, interaction of particles in an ionized gas has a
short-range character because at separations
R
2
l
two particles do not interact.
If this separation is below 2
l
, the charges of the particles are partially screened, and
the particles are strongly repulsed. Note that screening of two interacting particles
differs from that for individual particles, and hence it is not correct to apply to the
particle screened charges that relate to individual particles.
One can confirm the general character of particle interaction in an ionized gas
given in Figure 6.12 and based on measurements of interaction of two particles
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