phenomena are very widespread in our surroundings. You can see them in
nature, in everyday life, in your laboratory and in industry. Sun and
other stars, aurora, neon lamps in the streets or plasma display of
your TV set, ICP or laser in laboratory and so on. You may focus no
attention on them. But all this is plasma phenomena. And it is very
important and interesting thing to investigate.
The schematics above is from: R. Redmer, Phys. Reports 282, 35 (1997)
very different and by their characteristics can be divided into many
types. You can see a scheme, illustrating types of plasma with their
parameters on the picture below. Left Y axis is logarithm of plasma
temperature, evaluated in K. Right Y axis is also temperature, but it
has energetic meaning (corresponds with particles energies) and
evaluated in eV. High X axis shows so called “plasma frequency”(density
of charges divided on density of total gas ). Low X axis shows logarithm
of ions density (number of ions in a unit of volume). You can see here
not only laboratory plasma (ICP, laser…) parameters, but also
characteristics of your everyday plasma. For example, you can find the
temperature of solar corona or temperature during supernova explosion,
ions densities in flames or in ionosphere. You can compare parameters
of solar corona, atmosphere and sun core. Even metals can be seen here!
It seems strange, isn’t it? But remember, that metals have free
electrons, forming so called “electron gas”.
Glow discharge plasma, the type we interested in most of all, is only the small part of all plasma phenomena.
So what are the typical parameters of GD? Here they are:
||0,01 - 10 Torr
||0,1 – 10 cm
||100 – 2000 V
||0,1 – 100 mA
||300 – 1000 K
|Charged particle density
||106 – 1013 cm-3
|Plasma electrons (energy)
||0,1 – 1 eV
|Ions at cathode
||1 – 1000 eV
|Low ionisation degree
||10-7 – 10-4
Different kinds of particles exist in glow discharge plasma: electrons,
ions, metastables, excited atoms and so on. But if we interested in
electrical effects in glow discharge, we should concern the charged
particles most of all.
The particles move under the influence of electric and magnetic fields
and experience many collisions with the background gas.
- Elastic Total kinetic energy is conserved.
- e + Ar → e + Ar
- Inelastic Part of the kinetic energy is converted to internal energy.
- e + Ar → e + Ar* leading to light emission
e + Ar → e + Ar+ + e leading to charge production
1. All types of collisions have their own characteristics – cross sections (σ)
Hard sphere cross section
But reality is more complicated
sections give the probability of collision processes. Compare to
electrons velocity, atoms are too slow that’s why they assumed stabile
but not like a solid ball. Through strong electrostatic forces there
are interactions between atoms, electrons and ions occur. Atoms have
electrons, ions and also empty space if electrons approaching to
collision. Electrons characteristics like directions, velocities effect
these interactions and also caused probability of collisions with the
gas density per unit volume (n) and effective collisions area of each
If ideal gas atom has 3 Å radius (or 2.8
10-15 cm2 area) and electron radius is 2.8179 × 10-13 cm, collision has
also almost 3 Å radius and this will sweep out a volume of gas as the
atom moves. Total probability of collisions has a value between 0-1
this means there is a collisions or not but there is also different
collisions according to electrons energy so probability of that
interactions depends on, number of the collisions in unit time and
number of the interaction in the same time interval. In hard core
model, collisions diameter is equal to summation of projectile and
target spheres diameters (see figure above).
Cross sections give the probability of collision processes.
this figure you can see the order of magnitude for cross sections in
argon and its depending on energy. But also you can see what processes
can be caused in these conditions. So another parameter that helps us
to describe the motion of particles and processes in plasma is energy,
gained between collisions. It defines not only by cross sections but
also by electrical conditions (electric field intensity E) and density
of particles (n).
2.Energy gain between collisions:
So we can characterize the motion of particles by E/n.
First published on the web: 09.12.2007
Authors: Anna Kravchenko and Hakan Candan. The text is based on a lecture given by Zoltan Donko, RISSP Budapest, at the first
Gladnet training course in Antwerp Sept. 2007