Environmental Engineering Reference
In-Depth Information
surface is a method for generating beams of weakly ionized vapor. This vapor can
be used for formation and deposition of atomic clusters.
A widely used method of plasma generation is based on passage of electron
beams through a gas. Secondary electrons can be used further for certain pro-
cesses. For example, in excimer lasers, secondary electrons are accelerated by an
external electric field for generation of excited molecules with short lifetimes. The
electron beam as a source of ionization is convenient for excimer and chemical
lasers because the ionization process lasts such a short time.
A chemical method of plasma generation occurs in flames. The chemical energy
of reagents is expended on formation of radicals or excited particles, and chemion-
ization processes with participation of active particles generate charged particles.
Transformation of the chemical energy into the energy of ionized particles is not
efficient, so the degree of ionization in flames is low. Electrons in a hot gas or vapor
can be generated by small particles. Such a process takes place in the products of
combustion of solid fuels.
Introduction of small particles and clusters into a weakly ionized gas can change
its electric properties because these particle can absorb charged particles, that is,
electrons and positive ions or negative ions and positive ions recombine on these
particles by attachment to them. This process occurs in an aerosol plasma, that is,
an atmospheric plasma which contains aerosols. In contrast, in hot gases small
particles or clusters can generate electrons.
A plasma can be created under the action of fluxes of ions or neutrons when
they pass through a gas. Ionization near the Earth's surface results from the decay
of radioactive elements which are found in the Earth's crust. Ionization processes
and formation of an ionized gas in the upper atmosphere of the Earth are caused
by UV radiation from the Sun. There are various methods of plasma generation
that lead to the formation of different types of plasmas. Some methods of plasma
generation and the types of plasmas resulting from them are given in Table 1.3.
Ta b l e 1 . 3
Methods of plasma generation.
Character of action
Matter
Type of plasma
Electric field
Gas
Stationary gas discharge plasma
Electromagnetic wave
Gas
Alternative gas discharge plasma
Resonant radiation
Atomic vapor
Photoresonant plasma
Excitation from chemical reactions
Chemically active mixture
Chemical (flame) plasma
Laser
Surface or particles
Laser plasma
Injection of electrons or ions
Surface or ionized gas
Beam plasma
Injection of nucleating vapor
Ionized gas
Cluster plasma
Injection of dust particles
Ionized gas
Dusty plasma
Ionization by hard radiation
Gas (air) with aerosols
Aerosol plasma
