Axial light emission profile of a parallel plate dc micro discharge
in steady state and during oscillations
Thomas Kuschel1 , Ilija Stefanović1 , Nikola Škoro2 ,
Dragana Marić2 , Zoran Lj Petrović2 and Jörg Winter1
1 Ruhr-Universität
Bochum, Institute for Experimental Physics II, 44780 Bochum, Germany, [email protected]
2 Institute of Physics Belgrade, 11080 Belgrade, Serbia
Abstract:
The self-pulsing regime (current-voltage oscillations)
in a parallel plate dc micro discharge (feedgas: argon; discharge gap
d = 1 mm; pressure p = 10 Torr) was studied by means of time resolved
imaging with a fast ICCD camera and by measuring Volt-Ampere
(V -A) characteristics. Throughout the voltage and current oscillations,
ionized gas oscillates from low current Townsend regime to high current
normal glow. Axial emission profiles are similar to corresponding
profiles in standard size discharges (d ≈ 1 cm, p ≈ 1 Torr).
1 Introduction
Microplasmas have recently become in the focus
of research due to the wide range of their possible
applications [1, 2]. Reproducible and stable discharge conditions are of high importance to realize reliable applications. However, these conditions
are usually not achievable over the full operation
range. Observations of self-pulsing regimes were
amongst others reported in micro hollow cathode
discharges [3], micro plasma jets [4] and recently in
parallel plate micro discharges [5]. Numerous experiments in standard size (d ≈ 1 cm, p ≈ 1 Torr)
parallel plate dc discharges have shown that different instabilities can occur [6, 7, 8, 9] and the
discharge cannot run stable but runs through the
transient regime, switching repetitively from low
to high current mode. With the development of
ICCD cameras time resolved measurements of discharge transients became possible [10, 11]. Very
few studies of parallel plate micro discharges exist at all [12, 13, 14]. In a previous work we have
shown first 2D time integrated recordings of the axial light emission in parallel plate micro discharge
[15].
In this contribution we continue our studies and
show time resolved 2D recordings of parallel plate
dc micro discharge (d = 1 mm, p = 10 Torr) during relaxation oscillations. ICCD camera images
are correlated with current and voltage measurements to gain a better understanding of the formation of space charge effects and the cathode fall
formation. Axial light distribution under steady
state conditions (static V -A characteristics) have
been measured and used to compare with discharge
transients.
2 Experimental setup
A schematic of the dc micro discharge chamber is
shown in figure 1a. The plane parallel stainless
steel electrodes are mounted within a tight fitting
Plexiglas tube to avoid long-path breakdown [12].
The gas inlet and outlet are mounted on opposite
sides of the discharge tube and controlled by two
high precision leak valves. A small flux of Argon
is used as feed gas to minimize the influence of
impurities. The outer and inner walls of the Plexiglas tube are polished to gain optical access to the
plasma volume. The area between each electrode
end and the Plexiglas tube is shrouded by a Teflon
insulator as indicated in figure 1b. The discharge
gap can be changed by a micro positioning linear
stage, but was fixed at d = 1 mm during the experiments. Electrodes with a diameter of 8 mm were
used. The experiments were performed at the point
close to the Paschen minimum pd = 1 Torr cm.
The electrical circuit is similar to the one presented in [16]. The voltage is monitored with a high
voltage probe. The current is determined from the
voltage drop over a monitoring resistor and corrected for the displacement current.
Prior to each experiment the discharge is sustained at low current (roughly 10 µA) mode for
(b)
(a)
Plexiglas tube
anode
gas inlet
linear
stage
electrode
area
10
cathode
abnormal
glow
Teflon
insulator
electrical contact
(ground level)
Figure 1: (a) Schematic of the micro discharge
chamber. The discharge gap is d =
1 mm.
(b) Schematic of one electrode end.
The active electrode area has a diameter of 8 mm.
around 15 minutes until stable discharge conditions
are achieved. During the experiments the discharge
is first ignited in low current (a few µAs) Townsendlike mode. Additionally, short voltage pulses (usually < 3 ms) are applied to change the discharge
working point (intersection between a loading curve
and micro discharge V -A characteristics) to the
higher currents, as described in [7]. Due to the
short pulse length the discharge is running only for
a short time in high current mode, therefore significant gas heating and conditioning of the electrodes
is avoided.
3 Results and discussion
3.1 Steady state Volt-Ampere
characteristics and axial light emission
Figure 2 shows a V -A characteristics recorded under steady state discharge conditions. The V -A
characteristic shows a negative differential resistance as reported for standard size discharges [7, 9].
Due to the negative differential resistance different
instabilities can form, such as current oscillations
[17]. The region of current oscillations is indicated
in figure 2. In this region no steady state can be
reached for given discharge conditions. The analysis of the axial emission and V -A characteristic
during the oscillations will be given in the next
chapter.
2D images of the axial light emission profile at
selected positions on the static V -A characteristics
(label a-d) are presented in figure 3. In the low
current diffuse (Townsend-like) mode (label a) the
peak of emission is located close to the anode and
the discharge spreads over almost the full electrode
D V = V - Vb (V)
0
discharge
gap
gas outlet
electrical contact
(high voltage)
(d)
(a)
-10
oscillations
Townsend
mode
-20
(b) normal
glow
(c)
-30
1
10
100
current (m A)
1000
Figure 2: Volt-Ampere characteristics for steady
state discharge (pd = 1 Torr cm). Labels (a)-(d) indicate the conditions for
the 2D images shown in figure 3.
area. Near the minimum of the V -A characteristic
the discharge is running in the normal glow (label
b) and is highly constricted. The peak of emission
moves closer to the cathode and the constriction to
the electrode edge. As current is increasing (from b
to c) the current density stays constant and the discharge is spreading radially. The discharge is still
operating in the normal glow, similar to the low
pressure, large scale discharges [18]. At point (c)
the discharge occupies the whole electrode surface,
which marks the ending point of the normal glow.
With further increase of the current the discharge
switches to the abnormal glow (label d) which leads
to a high increase of the light intensity as well as
a shift closer to the cathode. All of these observations in our micro discharge are also typical for
standard size discharges [10, 18].
3.2 Time resolved development of the axial
light emission during oscillations
Figure 4 shows the discharge voltage V normalized to the breakdown voltage Vb as well as the
current as a function of time. After applying the
voltage pulse, the discharge voltage increases. The
discharge current starts to rapidly increase slightly
before the discharge voltage reaches the maximum.
With passing time space charge is building up and
the circuit capacitance looses charge leading to a
drop of the voltage and the increase of the current.
Thus the discharge is not able to reach steady state
condition, the relaxation oscillations follow and the
discharge runs from low current to high current
mode repetitively. The first pulse is somewhat different and follows the shape of the voltage as ex-
5
0
1600
0
0
-2
D V = V - Vb (V)
radial position (mm)
(c)
-10
-20
(d)
-30
(c)
600
450
(b)
(d)
300
(a)
0
2
0
-2
907 µA
-0.5
-0.25
0
0.25
axial position (mm)
-4
0.5
3440 µA
-0.5
-0.25
0
0.25
axial position (mm)
-4
0.5
3000
0
10000
0
1500
(a)
150
4
(d)
cathode
anode
cathode
2
anode
-4
0.5
800
(b)
0
750
248 µA
-0.5
-0.25
0
0.25
axial position (mm)
4
(c)
radial position (mm)
10
-4
0.5
10
current (m A)
2 µA
-0.5
-0.25
0
0.25
axial position (mm)
0
-2
anode
-2
2
cathode
0
radial position (mm)
anode
cathode
2
4
(b)
5000
radial position (mm)
4
(a)
Figure 3: 2D images of the axial light emission
profile of steady state discharge. Labels
(a)-(d) correspond to the conditions indicated in figure 2. Doted lines mark
the central axes of the discharge chamber, while solid lines mark the position
of the peak of emission. The discharge
current is shown in the bottom left corner of each image. For better visualization of the axial distributions the images are un-proportionally enlarged in
horizontal direction.
(a) Townsend-like discharge
(b)-(c) Normal glow discharge
(d) Abnormal glow
pected in vacuum [10]. Later, the shape of voltage
and current pulses are slightly different because the
discharge has switched to self sustained relaxation
oscillations.
Figure 5 shows 2D images of the axial light emission recorded by the ICCD camera at different discharge voltage and current values, as indicated by
the labels (a)-(d) in figure 4. During the oscillations the discharge develops from low current diffuse mode to the high current glow discharge.
In Townsend-like mode (label a) the discharge
is diffuse and the peak of emission is close to the
anode. The discharge occupies almost the full electrode surface. The light emission increases exponentially from the cathode to the anode, which is
characteristics for the homogeneous electric field
with negligible space charge effect. As space charge
builds up the current rises, the light emission of the
0
20
40
time (m s)
60
80
100
Figure 4: Discharge voltage (with substracted
breakdown voltage Vb = 224 V) and
current as a function of time during oscillations (pd = 1 Torr cm). The dots
(a)-(d) indicate the positions of the 2D
images shown in figure 5. Dashed lines
mark the positions of the maximum of
each current peak.
discharge increases and the peak of emission moves
away from the anode (label b). The discharge is
highly constricted. Comparing the current values
and the emission profile with the steady state conditions (figure 3b) we conclude that the discharge
is operating at the start of the normal glow. At the
current maximum (label c) the light emission has
reached its highest value and the peak of emission
is located at around the middle of the discharge
gap. The discharge is broadened in radial direction, again characteristics for the normal glow. As
the current is dropping (label d) the peak of emission moves away from the center, closer to the anode while the profile becomes more Bessel-like. Afterwards, this process repeats.
4 Summary
We have shown time resolved axial light 2D images of parallel plate dc micro discharge in steady
state as well as during discharge transient behavior. The static V -A characteristics is similar to
the large scale, low pressure discharges, with low
current diffuse mode, normal and abnormal glow.
The measured axial distributions support this similarity. Between the low current mode and normal
glow the region of oscillations has been found. During the relaxation oscillations the discharge develops from the low current mode (several µA) to the
high current normal glow mode (≈ 600 µA) repetitively. With increasing current the discharge in-
3 µA
4
-2
-0.25
0
0.25
anode
-2
409 µA
-4
0.5
-0.5
axial position (mm)
30k
0
4
anode
cathode
2
0
-2
-0.5
-0.25
0
0.25
25k
0
4
(d)
0
-2
315 µA
-4
0.5
-0.5
-0.25
0
0.25
-4
0.5
axial position (mm)
0
30k
15k
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0.5
2
axial position (mm)
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A
radial position (mm)
(c)
628 µA
0.25
anode
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0
cathode
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-0.25
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radial position (mm)
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Figure 5: 2D images of the time development of
the axial light emission during oscillations. Labels (a)-(d) correspond to the
positions indicated in figure 4. Doted
lines mark the central axes of the discharge chamber, while solid lines mark
the position of the peak of emission.
The discharge current is shown in the
bottom left corner of each image. The
curves at the top and on the left side
of the image show the radial and axial
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with reported results for standard size discharges.
5 Acknowledgment
This project is supported by DFG (German Research Foundation) within the framework of the
Research Group FOR1123, the Research Department ’Plasmas with Complex Interactions’ at RuhrUniversity Bochum, DAAD Grant project 50430267
and Ministry of Science (MNTRS) of Republic of
Serbia project 171037.
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