Quasi-uniform planar plasma jet generated by using an atmospheric
pressure DBD configuration
Qing Li 1,2, Hidemasa Takana 1, Yi-Kang Pu 2, and Hideya Nishiyama 1
1
Institute of Fluid Science, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai, 980-8577, Japan
2
Department of Engineering Physics, Tsinghua University, Beijing, 100084, China
Abstract: A stable non-thermal quasi-uniform planar plasma jet, originating from
a planar dielectric duct with a rectangular exit and injecting into ambient air at
atmospheric pressure of room temperature, is reported in the present work.
Current-voltage characteristics, one discharge current pulse per sinusoidal half
voltage cycle, show that the discharge is not filamentary. Its spatial uniformity in
the transverse direction is shown to be excellent by monitoring optical emission
spectra in the jet core region other than jet boundaries. These observations are a
challenge to the explanation of the traditional single streamer model for the
propagation of non-thermal plasma jets.
Keywords: atmospheric pressure plasma jets, dielectric barrier discharge, nonthermal plasma, planar plasma jets, uniform discharge
1. Introduction
Homogeneous plasmas with large scale and low
temperature have a great potential for applications,
such as the uniform biological treatment and large
industrial scale process by using the configuration of
dielectric barrier discharge (DBD) at atmospheric
pressure [1, 2]. This kind discharge has been
observed in atmospheric pressure chambers [3, 4],
but has not been obtained as atmospheric pressure
plasma jets (APPJs), which are free of the vacuum
system. Recently the non-thermal APPJ of several
centimeter length, originating from a DBD tube
wrapped with power electrode and injecting into
ambient air, has made it possible for the plasma
direct treatment in the discharge region [5 - 8].
However, the APPJ cross sectional emission pattern
typically covers only a few mm2, which is too small
for large scale applications. To generate large scale
plasmas, arraying up of many DBD APPJs in
parallel was designed [9]. Its uncontrolled
interaction between plasma jets resulted in unstable
and non-uniform discharge [9]. In this work, we
report an attempt study of plasma jets generated in a
planar DBD duct with a rectangular exit, which can
generate a large scale quasi-uniform discharge in the
transverse direction in the jet core region.
Fig. 1 (a) Schematic of the experimental setup; and (b) photo of
the planar DBD plasma jet in the case of 7.3 kV applied voltage.
2. Experimental setup
Figure 1(a) shows the schematic of the experimental
setup. The origin is located at the exit of the
rectangular duct on central axis. The helium gas
flows through a planar quartz duct, 3.1 mm × 50.0
mm × 300 mm (inner) and 6.1 mm × 53.0 mm × 300
mm (outer). A copper strip of 30 mm × 312 mm
worked as a power electrode is wrapped around the
duct and the separator is made of Teflon block. A
copper plate of 40 mm × 150 mm as a grounded
electrode is attached only one side of the duct. The
two electrodes were fixed at z = 7.0 cm and z = 1.0
cm upstream from the rectangular exit, respectively.
In experiment, the applied sinusoidal voltage of 10.0
kHz and the discharge current were recorded by a
digital oscilloscope (Tektronix TDS 2024C,
bandwidth 200 MHz) with a high voltage probe and
a high current probe, respectively. Gas flow rate was
fixed at 20.0±0.2 L/min, controlled by a mass flow
controller. Optical emission spectra, used to
investigate the discharge processes and to monitor
the discharge uniformity, were recorded at different
locations along z-axis using a spectrometer (Ocean
optics MAYA 11127). Its fiber probe was placed at
2 cm and perpendicular to the duct outer surface.
The discharge photo was taken by a digital camera,
as shown in Fig. 1(b). The luminescent jet length is
about 4 cm in the case of 7.3 kV applied voltage.
3. Experimental Results
Figure 2 shows typical waveforms of the
instantaneous applied voltage on the electrode and
the discharge current, i.e., the total current
subtracted the displacement current. The discharge is
characterized by one current pulse per half cycle of
the applied voltage. The amplitudes of the sinusoidal
applied voltage presented in Fig. 2(a) and (b) are 6.5
kV and 7.3 kV, respectively. The current pulse peak
values presented in Fig. 2 are 7.6 mA and 7.9 mA in
the positive half cycles, while they are 5.2 mA and
5.5 mA in the negative cycles, respectively. The first
current pulses increase to their peak values at about
3 μs and 7 μs as shown in Fig. 2(a) and (b),
respectively. These characteristics suggest that the
Fig. 2 Typical waveforms of the applied voltage (red dashed line)
and the discharge current (blue solid line). The amplitudes of the
applied voltages on the gas flow are (a) 6.5 kV and (b) 7.3 kV,
respectively.
plasma tends to be a continuous DBD discharge [4],
instead of a filamentary discharge whose current is
characterized by multiple current pulses of a series
of microdischarges per half cycle of the applied
voltage with a typical microdischarge current pulse
width of 100 ns [11]. Comparative analysis of Fig.
2(a) and (b) can also suggest that the current slowly
increases with applied voltage, and this is consistent
with glow-like discharge [12].
Figure 3 shows the spectra at different plasma
locations from upstream to downstream along z-axis
with the applied voltage of 7.3 kV. Spectra (a)-(d) are
corresponding to z = - 9.0 cm, z = - 4.5 cm, z = 0.5 cm, and z = - 0.5 cm, respectively. Spectra (a)
and (b) show the continuous emissions, the helium
lines, and the bands from residue gases of H2O and
O2 in the upstream region. Spectrum (c) shows weak
emissions from He, H2O, and N2. Spectrum (d) shows
that bands from the N2 (C 3∏u – B 3∏g) bands. These
spectra characteristics are consistent with the optical
emission colors in different locations of Fig.1(b): blue
in the downstream region (mainly from nitrogen) and
dark between the power electrode and the exit. The
high concentration of helium in the upstream region
results in the formation of He2 [13, 14], and the high
electron concentration and temperature, due to the
requirement of the high helium excited energy. These
effects are responsible for the presence of the bands
from OH and He2 in the spectra (a) and (b), and the
continuum radiations, which are possible from the
recombination of electrons and positive ions, such as
H2O+, O+, He+, and He2+.
decreases in the downstream region, possibly since
the high collision cross section between electrons and
Figure 4 shows the distributions of 337 nm
emission intensities from N2 and the gas flow
velocities in the transverse direction (i.e., x
direction) with difference distances downstream the
exit (i.e., different z values). The gas velocities are
uniform in jet core and non-uniform in the jet
boundary due to the development of thick boundary
layers, where the helium flow mixes with entrained
ambient air. The 337 nm emission intensities show
similar characteristic, and the non-uniform in the jet
boundary are possibly due to that the different mixing
ratios of air in helium plasma can result in different
reaction possibilities between helium and entrained
air [15]. The quasi-uniform distribution (in the jet
core region) of the emission 337 nm intensities
(representing the discharge emissions) is consistent
with the distribution of the gas flow velocities
(representing the gas compositions).
4. Discussion
Comparing with single tube plasma jets [14, 16],
our discharge current pulse is several times wider.
This electrical characteristic is consistent with spectra
in Fig. 3. The high intensity discharge between two
electrodes generates high concentrations of charged
particles and metastable helium states, which could
result a high preionization level to ignite a quasiuniform planar jet [4]. The electron temperature
Fig. 3 Relative optical emission spectra (left) from 290 to 385
nm and (right) from 390 to 900 nm. The pink (a), green (b), red
(c), and blue (d) spectra are corresponding to z = - 9.0 cm (the
center of grounded electrode), z = - 4.5 cm (the space center
between two electrodes), z = - 0.5 cm (the space center between
the power electrode and the exit), and z = 0.5 (0.5 cm
downstream away from the exit), respectively. The base lines
are shifted in order to distinguish these spectra. The voltage
amplitude is 7.3 kV.
in-Aid for Challenging
(No.22656004).
Exploratory
Research
References
Fig. 4 Distributions of emission 337 nm intensities and gas flow
velocities along the transverse direction (x direction) with
different z locations. The voltage amplitude is 7.3 kV.
diatom molecules such as entrained N2 and O2.
However, its large scale in the transverse direction
and its rectangular cross-section direction could not
be explained by the traditional single streamer model
[16, 17], which propose that the plasma jet is a
streamer propagation with a cylindrical symmetry
channel. Its optical emission and discharge current
suggest that it is a continuous discharge with large
scale and non-cylindrical channel, and tend to
sustain our previous proposed streamer-coupling
model [18], which explains that the formation of
plasma jets is the propagation of many simultaneous
streamers coupling with each other. Comparing with
small tube plasma jets, the long planar DBD jet is
more important for its applications [19].
5. Summary
A non-thermal planar plasma jet is obtained by
using a planar dielectric duct. Electrical and optical
characterizations of the planar APPJ have been used
to demonstrate its stability and spatial quasiuniformity of the DBD plasma jet in the transverse
direction.
6. Acknowledgements
The authors are grateful to Mr. Tomoki Nakajima
and Mr. Kei Niinuma for the experimental support.
This work was partially supported by the Japan
Society for the Promotion of Science under a Grant-
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