Environmental Engineering Reference
In-Depth Information
1.1.3
Methods of Plasma Generation
There are various methods to create a plasma as a result of action on matter, and we
consider them briefly. A gas discharge plasma is the most widespread form of plas-
ma and can have a variety of characteristics. It can be either stationary or pulsed,
depending on the character of the external fields. An external electric field may
cause electric breakdown of gas, which then generates different forms of plasma
depending on the conditions of the process. In the first stage of breakdown, a uni-
form current of electrons and ions may arise. If the electric field is not uniform, an
ionization wave can propagate in the form of an electron-avalanche streamer. In the
next stage of the breakdown process, the electric current establishes a distribution
of charged particles in space. This is one form of gas discharge.
After the external electric field is switched off, the plasma decays as a result of
recombination processes of electrons with ions, and spatial diffusion of the plasma
occurs. This plasma is called an afterglow plasma, and is used to study recombina-
tion and diffusion processes involving ions and excited atoms. A convenient way
to generate plasma uses resonant radiation, that is, radiation whose wavelength
corresponds to the energy of an atomic transition in the atoms constituting the ex-
cited gas. As a result of the excitation of the gas, a high density of excited atoms
is attained, and collision of these atoms leads to formation of free electrons. Thus,
the atomic excitation in the gas leads to its ionization and to plasma generation.
This plasma is called a photoresonant plasma. The possibility of generating such
a plasma has improved with the development of laser techniques. In contrast to a
gas discharge plasma, a photoresonant plasma is formed as a result of excitations
of atoms and therefore there are specific requirements for its formation. In particu-
lar, the temperature of the excited atoms can be somewhat in excess of the electron
temperature. This plasma may be used for generation of multicharged ions, as a
source of acoustic waves, and so on.
A laser plasma is created by laser irradiation of a surface and is described by some
parameters such as the laser power and the duration of the process. In particular, if
a short (nanosecond) laser pulse is focused onto a surface, material evaporates from
thesurfaceintheformofaplasma.Ifthenumberdensityofelectronsexceedsthe
critical density (in the case of a neodymium laser, where the radiation wavelength
is 1.06
m, this value is 10
21
cm
3
), the evolving plasma screens the radiation, and
subsequent laser radiation heats this plasma. As a result, the temperature of the
plasma reaches tens of electronvolts, and this plasma can be used as a source of
X-ray radiation or as the source of an X-ray laser. Laser pulses can be compressed
and shortened up to about 2
μ
10
14
s. This makes possible the generation of a
plasma in very short times, and it permits the study of fast plasma processes.
If the laser power is relatively low, the evaporating material is a weakly ionized
vapor. Then, if the duration of the laser pulse is not too short (more than 10
6
s),
there is a critical laser power (10
7
10
8
W/cm
2
) beyond which laser radiation is
absorbed by the plasma electrons, and laser breakdown of the plasma takes place.
For values of the laser power smaller than the critical value, laser irradiation of a
