Environmental Engineering Reference
In-Depth Information
coupled plasma. When we apply the term “plasma” to a weakly ionized gas, we con-
centrate on properties that are determined by charged particles. These properties
of an ideal plasma do not depend on the presence of neutral particles even though
the density of neutral particles is greatly in excess of the density of charged parti-
cles. For example, the presence of neutral atoms in the Earth's ionosphere has no
effect on the character of the propagation of electromagnetic waves through it. This
explicitly plasma property is qualitatively distinct from such ionosphere properties
as thermal capacity and thermal conductivity that are determined by the neutral
component only. We can thus use the term “plasma” as a universal description of
a wide variety of systems of weakly ionized gases whose electric or electromagnetic
properties are determined by charged particles only. This allows us to analyze in
a general way the properties of systems that may be very diverse in terms of the
properties arising from their neutral components.
The explanation for this situation is to be found in the character of the interac-
tions in a weakly ionized gas. There is a short-range interaction between neutral
particles - atoms or molecules - and a long-range Coulomb interaction between
charged particles. These interactions produce effects that can be treated indepen-
dently in a gaseous system. Therefore, if a particular property of this system is
determined by a long-range interaction between charged particles, one can ignore
the short-range interactions in this system, That is, the presence of neutral particles
does not affect these properties of the system. It follows from the nature of this de-
duction that such a conclusion can be valid only for systems with weak interactions
among the constituents; that is, it can apply only to gaseous systems. On the other
hand, a strongly coupled plasma is a system with strong interactions among the
particles, and these strong interactions make it impossible to divide interactions
into independent short-range and long-range types. Therefore, the term “strongly
coupled plasma” cannot be employed as a generic description for a wide variety of
systems as can be done in the case of an ideal plasma.
We can illustrate this conclusion with some examples of strongly coupled plas-
mas. Consider a metallic plasma. Taking a metal of a singly-valent element, we
assume that all the valence electrons contribute to the conductivity, and that the
metallic ions form the metal lattice. Then the electron number density is
N
e
D
is the density of the metal and
m
is the atomic mass. For example, in
thecaseofcopper,wehave
N
e
/
m
,where
10
22
cm
3
corresponding to room tempera-
D
8.4
10
7
and
ture, along with the parameters
400. Assuming this plasma
to be classical, we find that it is a strongly coupled plasma. The other example of
a nonideal plasma is an electrolyte, which is a solution with positive and negative
ions. The high density of molecules in the solution makes it a strongly coupled
plasma. These examples refer to stationary strongly coupled plasmas. The strong
action of an intense, short pulse of energy on matter can generate a nonstationary
strongly coupled plasma. Plasmas of this type can result from a strong explosion
that compresses matter. Another example of this type is the plasma associated with
laser fusion, where a target is irradiated by short laser pulses directed onto the tar-
get simultaneously from different directions. The energy transferred to the target
causes extreme heating and compression, with a dense plasma created as a result.
γ
D
1.5
Γ
D
