Chemistry Reference
In-Depth Information
distribution in the gap. Particularly in the glow mode the production and presence of
metastable excited particles is essential. Therefore, these discharges operate mostly
with helium. The metastables represent a reservoir of internal energy and have influ-
ence on the charged particle production (Penning ionization, step-like ionization)
and/or on the secondary electron emission at the cathode (exoemission) which may
contribute to the discharge sustaining.
The corona and barrier discharges have found increasing technical interest again
for plasma chemical gas cleaning and surface treatment because of their operation at
atmospheric pressure (no vacuum equipment is necessary) and their good upscaling
from laboratory to industrial devices. Furthermore, the barrier discharges are applied
for efficient UV and VUV radiation sources, e.g., excimer lamps [54] and plasma
display panels [55].
3.7.6 G
LOW
D
ISCHARGES AT
A
LTERNATING
E
LECTRIC
F
IELD
,RF
AND
M
ICROWAVE
P
LASMAS
By use of a harmonic electric field (circular frequency ω
2πν) over the discharge
gap, the charged particles start to oscillate (polarization drift). Taking into account
the solution for the polarization drift
=
·
ω
i
ν
e
,
+
e
−
z
(
t
)
=
ν
e
,
+
)
·
E
0
·
exp
(
−
i
ω
t
)
(3.301)
m
e
,
+
·
ω
·
(
ω
2
+
of charged species in an electric field with constant amplitude
E
0
and the elastic
collision frequency ν
e
,
+
of electrons and positive ions with neutrals see Section 3.1.4,
the corresponding oscillation amplitude
z
A
results in
e
m
e
·
1
ω
2
E
0
·
b
e
,
+
ω
1
z
A
=
E
0
·
ω
·
ν
e
=
·
√
1
+
+
(
ω
/
ν
e
)
2
E
0
p
·
C
e
,
ω
·
1
=
√
1
(3.302)
+
(
ω
/
ν
e
)
2
with
b
e
,
+
=
e
/(
m
e
,
+
·
ν
e
,
+
)
and
b
e
,
+
·
p
=
C
e
,
+
=
const.
1, the charged particles (electrons or positive ions) from
the discharge centre can arrive the electrode at the condition
z
A
=
Assuming
(
ω
/
v
e
,
+
)
2
d
E
/
2.
E
0
p
.
d
E
·
ω
∝
2
·
C
e
,
+
·
(3.303)
Forexample,atthedischargegapof
d
E
=
2.5 cmandthereducedelectricfieldstrength
of
E
cm) in Figure 3.38 the limit for ion and electron trapping in the
gap center corresponds to frequencies ν
/
p
=
10 V/(Pa
·
≥
250 kHz and ν
≥
25.5 MHz, respectively.
