Environmental Engineering Reference
In-Depth Information
illustrate this, Table 1.1 contains examples of chemical and ionization equilibria.
This table gives the temperatures at which 0.1% of molecules are dissociated in
the case of a chemical (dissociation) equilibrium or 0.1% of atoms are ionized for
an ionization equilibrium at a gas pressure of 1 atm. Thus, a weakly ionized gas,
which we shall call a plasma, has an analogy with a chemically active gas. Therefore,
although a plasma has characteristic properties, which we shall describe, it is not
really a new form or state of matter as is often stated. An ideal plasma is a form
of a gas, whereas a dense nonideal plasma (a plasma with strong coupling) is an
analogue of a condensed atomic system.
In most cases a plasma is a weakly ionized gas with a small degree of ionization.
Table1.2givessomeexamplesofrealplasmasandtheirparameters-thenumber
densities of electrons
N
e
and of atoms
N
a
, the temperature (or the average energy)
of electrons
T
e
, and the gas temperature
T
. In addition, some types of plasma sys-
tems are given in Figures 1.1 and 1.2. We note also that an equilibrium between
charged particles and atoms or molecules may be violated if the plasma is located
in an external field. In particular, electric energy is introduced in a gas discharge
plasma and it is transferred to electrons in a first stage, and then as a result of
collisions it is transferred from electrons to atoms or molecules. This form of in-
jection of energy may lead to a higher electron energy compared with the thermal
energy of atoms or molecules, and the plasma becomes a nonequilibrium plasma.
Figure 1.3 gives some examples of equilibrium and nonequilibrium plasmas.
Ta b l e 1 . 1
Temperatures corresponding to dissociation of 0.1% of molecules or ionization of
0.1% of atoms at a pressure of 1 atm.
Chemical equilibrium
T
, K
Ionization equilibrium
T
,K
H
C
C
2CO
2
$
2CO
C
O
2
1 550 H
$
e
7 500
He
C
C
H
2
$
2H
1 900 He
$
e
12 000
Cs
C
C
O
2
$
2O
2 050
Cs
$
e
2 500
N
2
$
2N
4 500
2H
2
O
$
2H
2
C
O
2
1 800
Ta b l e 1 . 2
Parameters of some plasmas.
N
e
and
N
are the number densities of electrons and
neutral atomic particles, respectively, and
T
e
and
T
are their temperatures.
N
e
,cm
3
N
,cm
3
Type of plasma
T
e
,K
T
,K
10
13
10
17
Sun's photosphere
6000
6000
10
5
10
13
E layer of ionosphere
250
250
10
11
10
16
10
4
He-Ne laser
3
2
3
400
10
13
10
14
10
5
10
3
Ar laser
