Environmental Engineering Reference
In-Depth Information
1
General Concepts in Physics of Excited and Ionized Gases
1.1
Ideal Plasma
1.1.1
Plasma as a State of Matter
The word “plasma” was introduced into science by the Czech physiologist J.E. Purk-
inje in the middle of the nineteenth century to denote the uniform blood fluid that
is released from particles and corpuscles. This term was suggested for a uniform
ionized gas of the positive column of a gas discharge by Langmuir [1-3] and now
this term denotes any system with electrons and ions where charged particles de-
termine the properties of this system. The most widespread form of plasma is an
ionized gas which consists of atoms or molecules with an admixture of charged
particles, electrons and ions. Such a plasma is the subject of this topic.
To understand the conditions required for the existence of such a plasma under
equilibrium conditions, we compare it with an identical chemical system. Let us
consider, for example, atmospheric air consisting basically of nitrogen and oxygen
molecules. At high temperatures, along with the nitrogen and oxygen, nitrogen
oxides can be formed. The following chemical equilibrium is maintained in air:
N
2
C
O
2
$
2NO
41.5 kcal/mol
(1.1)
Here and below, the sign
means that the process can proceed either in the
forward direction or in the reverse direction. According to the Le Chatelier prin-
ciple [4, 5], an increase in the temperature of the air leads to an increase in the
concentration of the NO molecules.
A similar situation exists in the case of formation of charged particles in a gas,
but this process requires a higher temperature. For example, the ionization equi-
librium for nitrogen molecules has the form
$
N
2
N
2
$
C
e
360 kcal/mol
(1.2)
Thus, the chemical and ionization equilibria are analogous, but ionization of atoms
or molecules proceeds at higher temperatures than chemical transformations. To
