Environmental Engineering Reference
In-Depth Information
For the portion of clusters
which attach to the walls near the orifice this gives
D
cl
,
w
0
0
tan
α
where
w
0
is the drift velocity of clusters near the orifice,
0
istheorificeradius,
and
is the cone angle (Figure 6.1). If we use (4.114) for the diffusion coefficient
and (6.2) for the drift velocity of clusters, we obtain the following dependence for
the potion of attachment clusters on a typical cluster size
n
, the number density
N
a
of buffer gas atoms, and the orifice radius:
α
1
n
4/9
N
5/3
.
5/3
0
a
From this it follows that attachment of clusters to the walls near the orifice becomes
important at low pressures of the buffer gas, a small radius of the orifice, and not
large cluster sizes.
Let us consider one more plasma effect that can influence transport of clusters to
the walls. If a plasma consists of electrons and positive ions, clusters are negatively
charged because of the higher mobility of electrons. As this plasma relaxes in the
course of its drift toward the orifice, electrons and ions go to the walls. As a result,
this plasma becomes charged, and its charge may be connected with clusters. This
charge creates an electric field that accelerates attachment of clusters to the walls.
This effect may be of importance for a certain cluster plasma [60].
6.4
Plasma Processes in the Earth's Atmosphere
6.4.1
Processes in Atmospheric Plasmas
In the foregoing discussion, we examined general principles and concepts relating
to plasmas. We shall now apply general plasma principles and concepts to atmo-
spheric plasmas, the properties of which depend strongly on the altitude above the
Earth's surface and have specifics for each altitude. Plasmas in the upper atmo-
sphere are generated by the absorption of solar radiation that ionizes oxygen atoms
and nitrogen molecules. Plasmas in the lower layers of the atmosphere have prop-
erties depending on transport processes of charged and excited atoms, and also on
the heat balance of these low-lying atmospheric layers.
Atmospheric plasmas can be used to illustrate the elementary collision process-
es that can occur in a weakly ionized plasma. Table 6.6 contains a list of basic
atmospheric plasma processes, and we will be referring to this in the analysis of
atmospheric phenomena. These processes are an explicit illustration of the data
in Tables 2.16-2.19. We can use photoprocesses as the first example. Processes 1-
3 in Table 6.6 are responsible for formation of charged particles in the upper at-
mosphere, and process 4 is associated with the production of atomic oxygen. The
