Chemistry Reference
In-Depth Information
•
Energy relaxation, e.g., translation-translation τ
TT
, vibration-vibration τ
VV
,
rotation-rotation τ
RR
, translation-rotation τ
TR
, translation-vibration τ
TV
,
vibration-rotation τ
VR
•
Recombination/extraction and heterogeneous chemical reactions of plasma
particles at plasma boundaries (electrodes, walls) due to field drift and/or
diffusion
Different electrical gas discharge plasmas can be generated in dependence on the
neutral gas pressure
, from low pressure to atmospheric or higher pressure, the
spe-
cific boundary conditions
at discharge electrodes and walls, and the applied
electric
power supplies
running over wide frequency range in continuous or pulsed mode
operation, e.g.,
•
Direct current (DC)
•
Standard line frequency (ν
=
50 Hz)
ω
p
+
, ω
pe
•
Middle frequency (ν
ω
=
2
·
π
·
ν
=
50 kHz
...
500 kHz)
ω
=
2πν
<
ω
p
+
,
ω
pe
•
Radio frequency (ν
=
10 MHz
...
100 MHz
...
)
ω
pe
•
Microwave frequency (ν
ω
p
+
<
ω
=
2
·
π
·
ν
<
=
2 GHz
...
10 GHz
...
)
ω
=
2
·
π
·
ν
ω
p
+
,
≥
ω
pe
Additionally, the external magnetic field may be applied for electron confinement
in DC or RF magnetron discharges as well as for the wave heating of electrons like
electron cyclotron resonance heating (ECRH) in microwave plasmas or helicon waves
in RF plasmas.
The different kinds of gas discharges can be classified according to their current-
voltage characteristics, the specific coupling to the electric power supply, and transfer
of energy into the plasma as well as the elementary processes for production of
charged particles in the gas volume and/or at the electrodes/walls. Thereby, the most
important internal parameters of nonthermal plasmas are the electron density
n
e
,
electron temperature
T
e
, or the electron energy distribution function (EEDF).
In connection to the characteristic time behavior of internal parameters the elec-
tric gas discharges can be classified in stationary and nonstationary or transient
discharges. The nonstationarity of the plasma may have their origin from intrinsic
plasma instabilities and relaxations processes or from the time-depending energy
input by the external electric power supply.
Firstly, the
electric breakdown
in a gas is described as an initial process for the
discharge ignition. Secondly, the generation of
nonthermalplasmas
is discussed from
the plasma physics point of view taking into consideration the electric power supplies
running from DC to microwave frequencies. In that way the different gas discharges
or plasmas are classified and characterized, such as the Townsend discharge, the
glow discharge, the arc discharge, the corona discharge, the spark discharge, the
barrier discharge, the radio frequency, and microwave plasmas, see the overview in
Figure 3.31.
