Chemistry Reference
In-Depth Information
The plasma density and the mean kinetic electron energy is constant in axial
direction for stable glow discharge mode. At the discharge axis the plasma density
reach values in the order of between 10
15
and 10
17
m
−
3
at mean electron energies of
few eV or electron temperatures of few 10
4
K.
Thephysicsofthestableandstriatedpositivecolumnwasextensivelyinvestigated
over several decades in close interaction between experiments and numerical calcu-
lation involving kinetic calculations by the Boltzmann equation and particle-in-cell
(PIC) simulations as well as drift/diffusions equations, see [45-49].
3.7.4 A
RC
D
ISCHARGES
With increasing discharge current the ion bombardment results usually in the strong
heating of the cathode surface and the thermal electron emission becomes significant.
The glow discharge changes in an arc discharge. In that case the
thermalarcdischarge
is generated which is characterized by high discharge current in the order of few 10 A
and discharge voltage much lower than 100 V. Beside the thermal electron emission,
the electric field emission may be also important for secondary electrons. Here, the
voltage drop over the thin cathode sheath at higher pressure is connected with strong
electric field in front of the cathode surface and the secondary electrons may be
emitted by the quantum mechanical tunneling of electrons over the thin potential
barrier at the cathode surface. In that case the electric field arc is distinguished from
the thermal arc.
The arc discharges at atmospheric or higher pressure produce a plasma in local
thermodynamic equilibrium, see Section 3.2.1. That means in a local region of
the plasma the transport processes represents a small disturbance only, and the
plasma is described by the same translational temperature of the plasmas particles
T
e
=
T
+
=
T
gas
=
∼
−
T
5
10
4
K, but this temperature and the plasma density vary in
the position space.
Because of the focus on nonthermal plasmas the thermal plasma of arc discharges
will not be discussed in detail.
3.7.5 H
IGH
V
OLTAGE
B
REAKDOWN AT
A
TMOSPHERIC
P
RESSURE
,C
ORONA AND
B
ARRIER
D
ISCHARGE
Generally, these types of discharges operate nonstationary despite of the application
of DC power supply in the case of a corona discharge. They need high voltages (high
electric field strength) and they appear typically at higher (atmospheric) pressure.
The classical Townsend breakdown mechanism by charge carrier multiplication due
to electron avalanches and secondary electron emission at the cathode for about
p
cm is not more valid. At higher pressure the electric breakdown
and discharge development have to be partially replaced and extended by more
complicated processes which involve the formation of thin short-lived
discharge
filaments
or
streamers
.
The streamer can grow in one or both directions toward the electrodes (cath-
ode directed, anode directed streamers). Similar to the electrical breakdown at low
pressure the starting point is a free electron and the formation of a primary electron
avalanche and the amplification of the external electric field
E
ext
due to the electric
·
d
E
<
10
4
−
10
5
Pa
·
