Chemistry Reference
In-Depth Information
charges, decreasing from maximum fields strength at the cathode toward about zero
at the cathode sheath edge at the axial position
z
d
C
.
In the
negative glow
region the electric field tends to zero. Only little negative
space charges are present due to energetic electrons entering from the cathode sheath
into the negative glow. A significant light emission in the negative glow is observed
which results from excitation processes of the relaxing hot electrons from the cathode
sheath and charge carrier recombination. Both, the cathode sheath and the negative
glow, are the essential discharge regions for the maintenance of the glow discharge.
Toward the anode the following
Faradaydarkspace
represents a transition region
between the negative glow and the positive column and is characterized by the
increasing axial electric field strength.
The
positive column
represents a quasi-neutral region with constant and low
axial electric field strength usually in the range of between 1 and 10 V/cm depending
on the kind of gas, pressure, and tube radius [50]. In particular, the cylindrical posi-
tive column in discharge tubes has been widely used as a model system for weakly
ionized nonthermal plasmas [45,46]. In dependence on the total pressure and dis-
charge current, the diffuse and contracted positive column is observed. Furthermore,
instabilities and nonlinear effects generate ionization waves which result in moving
or standing striations of the positive column [47].
The nonthermal plasma of the positive column is mostly applied for low-pressure
plasma light sources, such as fluorescent lamps.
The Faraday dark space and the positive column are only observed at sufficient
high electrode separation and/or higher pressure, and they serve mainly to satisfy the
current continuity in larger discharge gaps.
=
3.7.3.3 Cathode Sheath and Negative Glow in Strongly Anomalous
Glow Discharges
In strong anomalous glow discharges at low pressure and high discharge voltage the
basic physics and discharge mechanisms in the cathode sheath and negative glow
will be discussed using the simple analytical description by means of drift/diffusion
approximation. The boundary between the cathode sheath and the negative glow
at the position
z
d
C
is defined if the electric field strength of the cathode layer
tends to zero. In accordance with experimental investigations the cathode sheath
can be approximated by a matrix sheath (constant positive space charge density)
which means that the electric field strength decreases linearly from the maximum
value at cathode surface (
z
=
d
C
). The
glow discharge is sustained by physical processes in both the cathode sheath and
the negative glow. At the cathode, the secondary electron emission (γ-process) by
impinging energetic positive ions is essential, similar to the Townsend mechanism
of the gas breakdown. The transport of the secondary electrons is determined by
their drift in the inhomogeneous electric field of the cathode layer and the elastic
and inelastic collisions with neutral gas atoms/molecules. At low gas pressure and
strong electric field strength near the cathode, the secondary electrons gain quickly
high kinetic energy which is combined with the decreasing collision cross section of
the electrons (runaway electrons). The electron energy distributions function
f
e
(
=
0) to zero at the cathode sheath edge (
z
=
ε,
z
)
