Environmental Engineering Reference
In-Depth Information
leads to variation of the gas discharge current because of the subsequent ionization
processes.
Gas discharge is a simple method to create a plasma, and because gas discharge
has many regimes and forms which are increasing with time, the number of appli-
cations of a gas discharge plasma is growing. New applications with old ideas are
being developed when the technology allows these ideas to be used. An example
of this are plasma displays
1)
, the concept of which for television was described in
1936 by Kalman Tihanyi as a system of single transmission points of a grid of cells
arranged in a thin panel display, where these points are excited to different levels
by varying the voltages applied to these point. Nevertheless, production of plasma
displays started in the 1980s when they could compete with electron-beam tubes or
liquid crystal displays. Roughly, a plasma display includes two parallel planes with
parallel bus bars in each plane. Bus bars of each plane are surrounded by dielectric
planes and bus bars of different planes are in perpendicular directions. As a result,
the space inside the bus-bar plane is divided into some cells, and each cell relates
to one intersection of bus bars. The space is filled by a mixture of nitrogen and
neon (or other inert gases), and gas discharges occur in each cell almost indepen-
dently. Behind the cathode each cell is divided into three parts covered by colored
luminophors, so after absorption radiation of local gas discharge, one of subcells
gives a red color, the second one gives a green color, and the third subcell trans-
forms radiation of gas discharge into a blue color. These three colors are joined in
the overall color of this element, which depends on the intensity of local discharge,
and this intensity in turn is created through bus bars. Competing with other types
of displays, plasma displays or plasma panels are favorable for screens of large size
and their applications are determined by the possibilities of new technology.
Various laboratory devices and systems contain a plasma, where it is divided into
a low temperature plasma and a hot plasma, depending on the concentration of
electrons and ions in the plasma or the temperature of the charged particles. In a
hot plasma, a thermal energy of the atomic particles exceeds a characteristic atomic
value (which may be the ionization potential of the plasma atoms), and is much
less than this characteristic value in a low temperature plasma. Correspondingly,
plasma devices containing hot plasmas or low temperature plasmas are different
in principle. An example of a hot plasma is a thermonuclear fusion plasma, that is,
a plasma for a controlled thermonuclear reaction [3]. This reaction proceeds with
participation of nuclei of deuterium or of tritium - isotopes of hydrogen. To achieve
this reaction, it is necessary that during the time of plasma confinement, that is,
during the time when ions of deuterium or tritium are present in the reaction zone,
these ions have a chance to participate in a thermonuclear reaction. There must be
both a high ion temperature (about 10 keV) and a high number density of ions
for the thermonuclear reaction to proceed. The threshold number density of ions
N
i
for the thermonuclear reaction depends on the plasma lifetime
τ
such that the
product of these values
N
i
must exceed a certain value. This condition, the Lawson
criterion, at ion temperatures of several thousand electronvolts corresponds to an
τ
1)
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