Chemistry Reference
In-Depth Information
TABLE 3.11
Important Similarity Parameters
in Gas Discharge Physics
α
/
p
=
Const.
E
/
p
=
Const.
p
·
d
E
=
Const.
j
/
p
2
=
Const.
Summarized, the simple analytical description of the electric breakdown and the
transition to stationary gas discharges provide important similarity parameters, see
Table 3.11, which have
generalimportance
for comparison of gas discharges running
at different processing parameters and geometry.
3.7.3.2 Glow Discharges
The glow discharges at low pressure (1-10
4
Pa) are characterized by typical discharge
currents between 1 mA and a few 100 mA at voltages between 100 V and few kV.
According to the U/I discharge characteristics the glow discharges are distinguished
between
subnormal, normal
, and
anomalous glow discharges
.The
subnormal glow
discharge
with negative U/I-characteristics represents a transition regime from the
Townsend discharge to the normal glow discharge mode. The
normal glow discharge
mode is achieved if the voltage and current density at the cathode remains constant,
even if the discharge current is varied. That means the cathode surface area con-
tributes partially as discharge electrode, only. The effective cathode area increases or
decreases corresponding to the total discharge current. The normal glow discharge is
well characterized by the corresponding values for the normal cathode sheath voltage
U
N
and thickness for the considered kind of gas and pressure
p
d
CN
. If the complete
cathode surface is involved the normal glow discharge changes in the
anomalous
glow discharge
mode which is characterized by a positive U/I-characteristics.
Inaglowdischargethewellseparateddischargeregionsinaxialdirectionbetween
the cathode and anode are observed concerning the light emission, electric potential
and field strength, as well as current and charge carrier densities. The following
discharge regions are distinguished, well known described in many textbooks [3,36,
40,44]:
·
1. The
cathode sheath
with Aston dark space, the cathode glow, and Crookes
dark space
2. The
negative glow region
3. The
Faraday dark space
4. The
positive column
5. The
anode sheath
with anode dark space and anode glow
The external applied discharge voltage drops mainly over the cathode sheaths which
is characterized by an inhomogeneous axial electric field strength of positive space
