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21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Uniform dielectric barrier discharge generation in atmospheric air using nanosecond pulses: fast imaging, spectroscopy, and production of active species
Chong Liu1,2, Dayonna Park1, Gary Friedman1,2, Alexander Fridman1,3 and Danil Dobrynin1
1
A. J. Drexel Plasma Institute, Drexel University, Camden NJ 08103
Electrical and Computer Engineering Department, Drexel University, Philadelphia PA 19104
3
Mechanical Engineering and Mechanics Department, Drexel University, Philadelphia PA 19104
2
Abstract: Currently, there is still little understanding of the mechanism of the DBD transition
from the filamentary mode to uniform mode. Here we have been able to perform imaging of
the discharge development in nanosecond time scales, and show that DBD uniformity strongly depends on applied electric field in the discharge gap. We show that in the case of strong
overvoltage on the discharge gap, there is transition from filamentary to uniform DBD mode
which may be fundamentally explained by transition from cathode- to anode-directed streamers.
Keywords: nanosecond DBD in air, uniformity, imaging, streamers
1. Introduction
One of the most promising and exciting application of
atmospheric air plasmas is medicine. In the area of plasma
medicine, a variety of different atmospheric pressure
non-equilibrium plasma systems have been employed by
researchers. They differ in uniformity of plasma produced,
degree of ionization, plasma temperature, gasses employed, and ultimately in the amount and identity of active species and charged particles that can influence biological interactions. Plasma itself can be influenced significantly by living tissues or biological medium when it
is produced in direct contact with them. While plasma
produced in plasma jet systems can sometimes be better
controlled, the flux of active species from it depends on
distances from biological media and flow rates. In order
to study the mechanism of interaction of plasma with biological systems it is critically important to be able to
produce plasma that, on the one hand, is capable of delivering all possible active agents (radicals, electric fields,
charges) to the biological targets and, on the other hand,
can be well-characterized and controlled during the
treatment process. Nanosecond-pulsed Dielectric Barrier
Discharge (DBD) is uniquely suited for that purpose because, on the one hand, it can be applied directly to the
biological target delivering all active species that
non-equilibrium plasma can produce and, on the other, it
produces highly uniform plasma independently of the
features of the biological target which permits effective
characterization and control of the plasma.
The DBD is a common source of non-thermal plasma at
atmospheric pressure [1]. In the standard DBD system the
discharge is ignited by applying a variable voltage between two metal electrodes, one or both of them being
covered by a solid dielectric layer. The purpose of the
dielectric layer is to limit transferred charge and conduction current value, avoiding transition to an arc discharge.
Dielectric barrier discharges can be realized in two major
operating modes namely the filamentary mode and the
homogeneous mode at atmospheric pressure, under certain conditions. Filamentary DBDs are characterized by
transient microdischarges which are usually stochastically
distributed in space and in time. These discharges result in
inhomogeneous treatment and are less suitable for applications like surface treatments where uniformity is the
main aspect. A homogeneous discharge is still advantageous than a filamentary discharge due to the uniformity
rendered by the former. Unlike filamentary discharge, the
discharge parameters of a uniform DBD can be controlled
both spatially and temporally in number of ways [2-4].
Besides the application prospects, studies are also focused
on the phenomenal aspects of barrier discharges and the
physics governing them. Unfortunately, the most common
type of high-pressure DBD is non-uniform and filamentary. The existence of the uniform diffuse modes of DBD
is usually strongly limited by the use of a specific gas
mixture composition, dissipated power, operation frequency, etc. The transitions between discharge modes in
the same experimental conditions have been profoundly
investigated in nitrogen, rare gases and their mixtures
with air and other gases [5-9].
The mechanisms leading to the formation of the uniform DBD mode are still the subject of scientific research
and discussion. Mangolini et al [10] studied the glow-like
discharge development in He by means of fast imaging
and space-resolved current measurements with sectioned
electrodes. It was found that first a relatively large diffuse
glow-like current spot a few centimeters in diameter is
formed, followed by a ring-like expansion. A similar behavior for APGD in He was observed by Luo et al [11],
who also demonstrated that initially a subnormal
glow-like current spot appears by the transition from the
low current Townsend-like discharge. Two-dimensional
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21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Figure 1. High-voltage 10 ns pulse duration (90% amplitude), 2 ns rise time and 3 ns fall time pulses used
in preliminary experiments.
fluid modeling performed by Golubovskii et al [12] for a
DBD in nitrogen indicates that the Townsend–glow transition can occur via a narrow cathode-directed streamer,
which would form an initial glow-like cathode spot. Depending on the external electrical circuit this spot may
expand over larger areas. Rahel et al [13] investigated the
visually uniform DBD discharge in air by registering
time-resolved emission with a photomultiplier and recording discharge current. They concluded that the discharge pattern developed similarly to that described by
Golubovskii’s model [12]. Yet no fast imaging was reported in [13] to directly verify the glow-like discharge
character. It was shown in [14] that at a high density of
non-stationary streamers the discharge can also have a
diffusive appearance.
Our recent works [15, 16] report a uniform mode of dielectric barrier discharge in atmospheric air, where the
discharge was generated by application of high voltage
pulses with nanosecond duration. Below we describe
new preliminary experimental results that allow us to
formulate a hypothesis for the mechanism of generation
of uniform DBD in atmospheric air by application of nanosecond pulses with fast rise time.
2. Experimental setup and results
To initiate the uniform DBD discharge in atmospheric
air we used a pulsed power system. The power supply
generated pulses with +15.5 kV pulse amplitude in 50
Ohm coaxial cable (31 kV on the high-voltage electrode
due to pulse reflection), 10 ns pulse duration (90% amplitude), 2 ns rise time and 3 ns fall time. Maximum pulse
frequency was 5 kHz. Pulse shapes that are applied to the
electrode is presented in Figure 1. The generator was
made by FID Tech Company on the basis of solid-state
switches. The discharge cell had a sphere-to-plate geometry with a plane high-voltage electrode diameter of 2.4 cm
covered with 1 mm thick quartz. The grounded bronze
sphere diameter was 5 cm, and the inter-electrode distance was 1-4 mm.
The discharge visualization measurements were performed using 4Picos ICCD camera from Stanford Computer Optics. The camera had an 18mm diameter multi-alkaline photocathode with a spectral response from
180 to 750 nm. The camera’s typi-cal spectral response
was 250 – 750 nm. The synchronization system for the
experi-ments was built on the basis of a Tektronix
AFG-3252 Arbitrary/Function Generator (Figure 2). The
generator has synchro-output and two adjustable channels
with a signal rise/fall time of less than 2.5 ns and a typical
jitter (RMS) less than 20 ps with a delay time resolution
of 10 ps. Discharge optical emission spectrum was obtained using a fiberoptic bundle (Princeton Instruments-Acton, 10 fibers – 200 μm core) connected to the
spectrometer (Princeton Instruments – Acton Research,
TriVista TR555 spectrometer system with PIMAX digital
ICCD camera, Trenton, NJ).
Figure 2 Experimental setup schematics.
Integral discharge images with exposure time of 20 ns
obtained for different applied electric fields (discharge
gaps) are presented in Figure 3. At high (114 kV/cm)
electric field discharge appears uniform, while at lower
fields discharge develops via formation of cathode directed streamers.
Time resolved imaging of the discharge initiation is
shown in Figure 4. Here, images are taken with 200 ps
exposure time and 100 accumulations after the application
of HV pulse. As in the long exposure images, the discharge appears to be developing via different mechanisms
depending on applied electric field: uniform regime is
observed for 114 kV/cm, while formation of cathode directed streamers is obvious for twice lower field.
Optical emission spectroscopy is performed to measure
the local electric field in the gap as the discharge develops.
Electric field was calculated from the measured ratio of
emission signals of nitrogen, I391.4/I337.1, connected to the
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21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
reduced electric field by the expression [17]:
1 mm
2.5 mm
0.1 ns
The results of OES electric field measurements (integral, 20 ns exposure) for different applied electric fields
are shown in Figure 5. It is shown that the at lower applied electric field discharge develops via cathode-directed streamers, and corresponding electric field in
the gap is high (~500 Td, corresponding to the electric
field in the head of the streamer). At high applied electric
field, discharge develops uniformly, and therefore measured electric field in the gap is almost equal to the applied
electric field (no streamers). This transition is observed
for ~300 Td applied electric field.
1 mm, 114 kV/cm
0.2 ns
0.3 ns
0.4 ns
2 mm, 67 kV/cm
Figure 4 Discharge development. Top electrode –
grounded metal sphere (anode), bottom – powered
2.5 mm,
55 kV/cm
barrier electrode (cathode). Exposure time 200 ps, 100
accumulations.
Figure 3 Integral discharge images at different applied
electric fields. Top electrode – grounded metal sphere
(anode), bottom – powered barrier electrode (cathode).
Exposure time 20 ns, single exposure.
In our preliminary experiments we have also studied
production of biologically important active species by
plasma. Delivery of hydrogen peroxide into liquid was
measured using fluorescent dyes (Amplex UltraRed).
When comparing uniform and non-uniform nanosecond
DBD plasmas we have found, that even at about the same
discharge energy, production rate of hydrogen peroxide in
liquid is about two times lower in the case of uniform
nanosecond-pulsed DBD operated at high overvoltage.
This effect may be explained by the higher rate of production of OH radicals in the discharge volume in the
case of streamer DBD. Indeed, at the same power,
G-factor of OH radical production depends on the electric
field: the higher the electric field, the greater the G-factor.
The results of our experiments are shown in Figure 6.
Figure 5. Calculated electric field in the discharge form
the emission spectrum. Exposure time 20 ns, 10 accumulations
3. Discussion
Let us now analyze the cause of non-uniformity of DBD
discharge in the case of longer – microsecond – pulses.
After the ignition of the discharge, it’s conductivity increases. Therefore, even if the amplitude of the microsecond pulse provided by the power supply is higher than
breakdown voltage of the air gap, as soon as the break-
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21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
down voltage of the air gap is achieved, the discharge is
ignited and it’s resistance decreases, the rest of the potential drop occurs on the dielectric barrier. This means, that
only traditional cathode-directed streamers may be generated. These streamers appear as conductive channels with
thicknesses of ~0.1-1 mm, and are the reason of
non-uniformity of the DBD. In contrast, anode-directed
streamers that are generated in the case of high overvoltage may provide DBD uniformity. Indeed, avalanches and
subsequent streamers occur over very short time. This
time can be roughly estimated on the basis of avalanche
development. Given the applied voltage of about 15 kV
over a 1mm gap, the reduced electric fields (E/n, where n
is the gas concentration) is about ∼4×10−15 V cm2. This
gives an electron drift velocity (νd) in air of ∼107 cm s−1
and a time of ∼10−8 s = 10 ns to bridge a 1mm gap. This
time is the characteristic time of build-up of possible local
non-uniformities in the electric field within the discharge
gap.
If the voltage rise time is shorter than the time needed to
bridge the gap and the maximum voltage significantly
exceeds the critical breakdown voltage (discharge develops under high overvoltage conditions) the discharge develops uniformly due to the high electric field in front of
the ionization wave. The high value of the reduced electric field E/n means: (1) suppression of the instabilities by
saturation of the ionization coefficient; (2) fast expansion
of the plasma channels and their overlapping; (3) generation of VUV radiation and photoionization of the gas
ahead of the ionization front; (4) generation of run-away
electrons and pre-ionization of the gas.
Figure 6. Hydrogen peroxide generation in liquid by
nanosecond DBD plasma.
Thus the criteria of the uniform discharge development
could be formulated as simple relations:
(1)
and
(2)
where U is the pulse voltage, n - the gas density, d - the
discharge gap length, rise - the pulse rise time and vd the electron’s drift velocity in critical electric field.
The uniform discharge, if initial conditions are satisfied,
with time will develop non-uniformities. This is because
of increase of conductivity, and therefore current through
the discharge and therefore eventual filamentation. This
gives another criteria for a uniform DBD – short pulse
duration.
4. Acknowledgment
This work was supported by W.M. Keck foundation.
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