Chemistry Reference
In-Depth Information
Townsend discharge
Glow discharge
Arc discharge
Corona discharge
Spark discharge
Barrier discharge
Streamer mechanism
microdischarges
Townsend mechanism
Radio frequency plasmas—CCP, ICP, single/two frequency
Microwave plasmas, surface wave plasmas
Electrons in high frequency electric field
Magnetized plasmas
Magnetron, microwave—ECR, radio frequency—Helicon wave
Cyclotron motion, ExB drift, wave heating, resonances
FIGURE 3.31
Overview and classification of electric gas discharges (CCP/ICP:
Capacitively/inductively coupled plasma; ECR: electron cyclotron resonance).
3.7.2 E
LECTRIC
B
REAKDOWN IN
G
ASES AND
T
OWNSEND
D
ISCHARGE
The classical Townsend mechanism describes the electric breakdown in a gas by
electron avalanches without space charge effects. The electric breakdown means
the transition of the gas from the insulator into the conductor which depends on
the pressure and the external electric field strength. We consider a discharge gap
consisting of two parallel metal plates as electrodes (cathode and anode) with the
diameter
R
and separation
d
E
(
R
d
E
)
filled with neutral gas at the pressure
p
,see
Figure 3.32.
The uniform electric field strength
E
=
d
E
is generated by the applied volt-
age
U
E
from the external DC power supply over the electrode distance
d
E
.The
starting point is the production of free electrons at the cathode surface by energetic
photons resulting from an external source (e.g., cosmic radiation or natural radioac-
tivity). These electrons are accelerated in the uniform electric field toward the anode.
Thereby, they ionize neutral gas atoms in the gap due to electron impact ionization if
their kinetic energy is above the threshold for ionization.
The
firstTownsendcoefficient
α
U
E
/
dz
describes the number of produced
electron-ion pairs per length unit in
z
-direction by electrons moving from the cathode
to the anode.
For a given gas, the total number of produced electron-ion pairs depends on the
electrode separation
d
E
and the total gas pressure
p
. The ionization probability
P
ion
of electrons can be calculated taking into account the distribution law of the mean
free path length of electrons for ionizing collisions λ
ion
and the ionization length
z
ion
which corresponds to the path length of electrons in the electric field to achieve the
=
1
/
n
e
·
dn
e
/
