atmospheric-pressure plasma jets: effect of gas flow, active...
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Atmospheric-pressure plasma jets: Effect of gas flow, active species, and snake-likebullet propagationS. Wu, Z. Wang, Q. Huang, X. Tan, X. Lu, and K. Ostrikov
Citation: Physics of Plasmas (1994-present) 20, 023503 (2013); doi: 10.1063/1.4791652 View online: http://dx.doi.org/10.1063/1.4791652 View Table of Contents: http://scitation.aip.org/content/aip/journal/pop/20/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Characteristics of atmospheric-pressure non-thermal N2 and N2/O2 gas mixture plasma jet J. Appl. Phys. 115, 033303 (2014); 10.1063/1.4862304 Investigation on streamers propagating into a helium jet in air at atmospheric pressure: Electrical and opticalemission analysis J. Appl. Phys. 114, 103304 (2013); 10.1063/1.4820570 Atmospheric pressure He-air plasma jet: Breakdown process and propagation phenomenon AIP Advances 3, 062117 (2013); 10.1063/1.4811464 Spectroscopic measurement of electric field in atmospheric-pressure plasma jet operating in bullet mode Appl. Phys. Lett. 99, 161502 (2011); 10.1063/1.3653474 A simple cold Ar plasma jet generated with a floating electrode at atmospheric pressure Appl. Phys. Lett. 93, 011503 (2008); 10.1063/1.2956411
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Atmospheric-pressure plasma jets: Effect of gas flow, active species, andsnake-like bullet propagation
S. Wu,1 Z. Wang,1 Q. Huang,1 X. Tan,1 X. Lu,1,a) and K. Ostrikov2,3,1
1State Key Laboratory of Advanced Electromagnetic Engineering and Technology,Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China2CSIRO Materials Science and Engineering, PO Box 218, Lindfield NSW 2070, Australia3School of Physics, The University of Sydney, Sydney NSW 2006, Australia
(Received 19 November 2012; accepted 22 January 2013; published online 11 February 2013)
Cold atmospheric-pressure plasma jets have recently attracted enormous interest owing to numerous
applications in plasma biology, health care, medicine, and nanotechnology. A dedicated study of the
interaction between the upstream and downstream plasma plumes revealed that the active species
(electrons, ions, excited OH, metastable Ar, and nitrogen-related species) generated by the upstream
plasma plume enhance the propagation of the downstream plasma plume. At gas flows exceeding
2 l/min, the downstream plasma plume is longer than the upstream plasma plume. Detailed plasma
diagnostics and discharge species analysis suggest that this effect is due to the electrons and ions that
are generated by the upstream plasma and flow into the downstream plume. This in turn leads to the
relatively higher electron density in the downstream plasma. Moreover, high-speed photography
reveals a highly unusual behavior of the plasma bullets, which propagate in snake-like motions, very
differently from the previous reports. This behavior is related to the hydrodynamic instability of the
gas flow, which results in non-uniform distributions of long-lifetime active species in the discharge
tube and of surface charges on the inner surface of the tube. VC 2013 American Institute of Physics.
[http://dx.doi.org/10.1063/1.4791652]
I. INTRODUCTION
Cold atmospheric pressure plasma jets (C-APPJs) have
recently attracted a lot of attention due to their numerous appli-
cations in plasma biology, health care, and medicine, as well
as surface and materials processing and nanotechnology.1–12
C-APPJs usually generate plasmas in surrounding air rather
than in confined space.13–16 Hence, they can be used for direct
treatment and there is no limitation on the size of the objects
that can be treated. Studies on the propagation mechanism of
C-APPJs reveal that the plasma jets are formed by fast-moving
plasma bullets rather than a continuous discharge glow as com-
monly perceived by a human eye.17–28
Further investigations on the dynamics of C-APPJs in
noble gases reveal that the plasma plumes propagate at a
speed of 104–105 m/s,17–23 which is several orders of magni-
tude higher than the gas flow velocity. This is why it is gener-
ally accepted that the propagation of the plasma plumes is
driven electrically rather than by the gas flow.
It was recently revealed that the gas flow rates affect the
length and the modes of C-APPJ operation.23,25 However, this
effect is rather indirect because diffusion of the surrounding
air into the working gas stream strongly affects the gas com-
position and eventually the propagation of the plasma plume
in open air.
To the best of our knowledge, there is presently no evi-
dence that the gas flow rate has a direct effect on the propaga-
tion of C-APPJs in noble gases. To clarify this issue, here we
report on a dedicated experimental study on the behavior of
two plasma plumes generated upstream and downstream the
gas flow in a quartz tube. The quartz tube is used to avoid the
effect of the diffusion of the surrounding air. It is found that
when the gas flow rate is higher than �2 l/min not only do the
lengths of both plasma jets shorten but an asymmetry devel-
ops such that the downstream plasma plume becomes unex-
pectedly longer than the upstream plasma plume. The higher
is the gas flow rate, the larger is the length difference between
the two plasma plumes. Besides, high-speed photography
reveals that the plasma plumes propagate in snake-like
motions, which is very different from the previous reports.
II. EXPERIMENTAL
The schematic of the experimental setup is shown in
Fig. 1(a). The high-voltage (HV) electrodes are made of two
stainless steel needles, which are inserted into the quartz
tube. The distance between the tips of the two needles of the
same size and shape is 77 mm. The radius of the needles is
about 100 lm. The inner diameter of the quartz tube is about
0.8 mm. The working gas flows into the quartz tube from the
right end. Research-grade Ar gas (99.999% purity) is used in
the experiment. To keep the ambient air contamination in the
discharge region as low as possible, another quartz tube with
a length of 2 m is connected to the left end of the quartz tube
as shown in Fig. 1(a).
Thus, the two plasma plumes are generated in the gas
with the same composition. In other words, the difference
between the gas composition in the two plasma plumes
caused by the air diffusion from the left end of the tube can
be neglected; this is also confirmed by the computational
fluid dynamics (CFD) modeling results obtained using FLUENT
software.29 Moreover, when the gas flow rate is higher thana)Electronic address: [email protected].
1070-664X/2013/20(2)/023503/7/$30.00 VC 2013 American Institute of Physics20, 023503-1
PHYSICS OF PLASMAS 20, 023503 (2013)
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1 l/min, the amount of air around the two plasma plumes is
even lower compared with the amount of impurities in
research-grade argon. Besides, the optical emission spectros-
copy measurements (presented later in this paper) show that
optical emission from nitrogen- or oxygen-related species is
extremely low when the Ar flow rate is higher than 1 l/min.
The two needles are connected to the high-voltage pulsed
dc power supply in parallel (amplitude up to 10 kV, repetition
rates up to 10 kHz, and pulse width variable from 200 ns to
dc). The applied voltage of 8 kV, pulse frequency of 9 kHz,
and pulse width of 400 ns were fixed in this series of experi-
ments. When HV pulsed dc voltage is applied to the two nee-
dles and working gas is injected into the quartz tube with a
gas flow rate of 2 l/min, the plasma plumes are generated
inside the quartz tube as shown in Fig. 1(b). It should be men-
tioned that all images in the paper are spectrally broadband.
III. RESULTS AND DISCUSSION
A. I-V waveforms of the upstream plasma plume
To study the I-V characteristics of the plasma plume, a
P6015 Tektronix HV probe is used to measure the voltage on
the needle, and a Pearson 2877 current probe is used to mea-
sure the plume current. The distance between the current
probe and the tip of the right needle is about 5 mm. The di-
ameter of the probe jaw is about 6.1 mm, which is larger
than the outer diameter of the tube (1.8 mm). It allows us to
measure the current of the plasma plume by letting the
plasma plume across the probe jaw as shown in Fig. 1(a).
Figure 2 shows the applied voltage and plume current wave-
forms of the upstream plasma. Two distinct discharge current
pulses per applied voltage pulse are observed. The mecha-
nism of the second discharge is related to the voltage
induced by the residual charges during the first discharge,
and more explanation can be found elsewhere.30 Moreover,
its peak current reaches as high as 280 mA, which means
that the plasma plumes are very active. However, it should
be mentioned that the current probe perturbs the plasma
plume during the current measurement.
B. Effect of gas flow rate on the length of the twoplasma plumes
When a high voltage is turned on and Ar flows through
the quartz tube, two plasma plumes are generated in front of
the two needles. Figure 3 shows the photographs of the
plasma plumes taken with a Sony digital camera (DSC-HX1)
at an exposure time of about 100 ms for different Ar flow
rates. With the increase of the Ar flow rate from 0.2 to
1 l/min, both the plasma plumes become longer. The differ-
ence between the lengths of the two plasma plumes is very
small when the Ar flow rate is lower than 0.5 l/min. When
the argon flow rate is increased to 2 l/min, the both plasma
plumes become shorter. Moreover, the length of the
upstream plasma plume Lup becomes even shorter than the
length of the downstream plasma plume Ldown. Figure 4
shows the difference between lengths Lup and Ldown. In the
following, a detailed analysis of the effect of the Ar flow rate
on the length of the plasma plumes is presented.
As the Ar flow rate is increased, the length of the plasma
plumes increases at first. This can be explained as follows.
Due to the diffusion of the ambient air, there are always
some trace amounts of air inside the quartz tube. When the
Ar flow rate is increased from 0.2 to 1 l/min, the concentra-
tion of air in the quartz tube is reduced. Thus, the lower is
the concentration of air in the background gas, the larger is
the length of the plasma plume. The CFD modeling shows
that at Ar flow rates above �1 l/min, the concentration of the
air in the tube becomes too low (�10�5 fraction of the Ar
flow) to affect the discharge behavior.
To explain why both the plasma plumes get shorter
when the flow rate is further increased, the Reynolds num-
bers of the Ar flow inside the quartz tube were calculated for
different gas flow rates. The calculated Reynolds number for
FIG. 1. (a) Schematic of the device and
(b) spectrally broadband photograph of
the discharge. The exposure time is
approximately 100 ms.
FIG. 2. The applied voltage and the plume current waveforms of the upstream
plasma versus time. The Ar flow rate is 3 l/min.
023503-2 Wu et al. Phys. Plasmas 20, 023503 (2013)
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a flow rate of 2 l/min is about 4568. Since the critical Reyn-
olds number is 2320 to distinguish between the laminar and
transition flows in the tube, the flow becomes hydrodynami-
cally unstable when the flow rate is 2 l/min or higher. Hence,
the higher is the flow rate, the stronger is the hydrodynamic
instability, which in turn shortens the plasma plume.
To elucidate why the upstream plume becomes shorter
than the downstream one with the increase of the gas flow
rate, first, we switched the positions of the two needles. The
result was exactly the same, which means that any small dif-
ference in the sizes and shapes of the two needles had no
influence on the observed phenomenon.
Second, for the upstream plasma plume, the gas flows
from the tube with the needle in the centre to the free space
where there is no needle. On the contrary, for the downstream
plasma plume, the gas flows from the free space to the place
where there is a needle. This could cause the flow mode
difference and might result in the observed phenomenon. To
verify if this is the actual reason why Lup is shorter than Ldown,
another experiment was carried out. As shown in Fig. 5, when
a single needle is used as an electrode, the gas flow direction
can either be the same as the plasma plume propagation direc-
tion or the opposite. Figure 5 shows that the plasma plume
with the gas flow direction which is the same as the plasma
plume propagation direction is longer than the plasma plume
when the plasma plume propagation direction and the gas
flow direction are the opposite. This trend is the opposite with
the results in Fig. 2, where Lup is smaller than Ldown. This phe-
nomena may be related to hydrodynamic effects.31,32
The other possibility is the interactions between the
upstream and downstream plasma plumes through the
exchange of active species. Indeed, since the gas flows from
the right (upstream) to the left (downstream), long-living
active species (e.g., electrons, ions, radicals, and excited spe-
cies) generated upstream may flow into the downstream
region of the tube. Thus, the background gas of the down-
stream region may have higher concentrations of the active
species. This could be the reason why Lup is smaller than
Ldown. In the following, further experiments are carried out
to investigate what kind of active species may flow from the
upstream to the downstream regions thereby affecting the
propagation of the plasma plume.
FIG. 3. Photographs of the plasma
plumes at different gas flow rates in Ar
working gas. The photographs are spec-
trally broadband. The exposure time is
approximately 100 ms.
FIG. 4. The length difference of the downstream and the upstream plasma
plumes versus the gas flow rate.
FIG. 5. Photographs of the discharges produced with a single needle electrode
in two gas flows with the opposite directions. The Ar flow rate is 4 l/min. The
photographs are spectrally broadband. The exposure time is approximately
100 ms.
023503-3 Wu et al. Phys. Plasmas 20, 023503 (2013)
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C. Effect of active species on the propagation of theplasma plume
In order to affect the propagation of the downstream
plasma plume, the active species have to effectively flow
from the upstream to the downstream region. For a flow rate
of 2 l/min, the time required for the active species to flow
from the upstream to the downstream region is about 0.5 ms.
Hence, the lifetimes of the active species that could affect
the propagation of the plasma plume should be �0.5 ms or
longer. Since the plasma-on time is only 400 ns, the active
species generated in the upstream plasma during the voltage
pulse could not affect the downstream plasma during the
same voltage pulse. However, if the lifetime of the active
species is longer than the pulse repetition time, the active
species generated by a voltage pulse in the upstream plasma
could flow to the downstream region and affect the propaga-
tion of the plume generated by the subsequent voltage
pulses.
To confirm this possibility, the pulse frequency was
increased from 2 to 10 kHz. It was found that both lengths
Lup and Ldown did not change. This means that the lifetimes
of the active species which may affect the propagation of the
plasma plume are either much longer than 0.5 ms or shorter
than 0.1 ms. The species with the lifetime shorter than 0.1 ms
do not have enough time to flow between the upstream and
the downstream regions. Therefore, the lifetimes of the rele-
vant active species are most likely longer than 0.5 ms.
1. Excited OH* and Arm* species
To identify the active species generated by the plasma
plumes, optical emission spectra of the two plasma plumes
were measured. The emission spectra were collected at a dis-
tance of 5 mm away from each needle in the upstream and
downstream regions. Figure 6 shows the emission spectra of
the two plasma plumes in the spectral range from 200 to
900 nm. The spectra clearly show that excited OH* and Ar*
exist in the plasma plumes. The intensity of the excited Ar*
in the downstream region is stronger than in the upstream
region. The OH emission likely originates from dissociation
of the H2O impurity of the Ar gas. The lifetime of excited
OH* is less than 1 ls;33 it is thus unlikely that this species
plays any significant role in the observed phenomenon.
The other potentially relevant species are Arm* metasta-
bles. To estimate their lifetimes, we have analyzed the rele-
vant loss processes of these species. The main loss process
of metastable Arm* at atmospheric pressure is the three-body
collision
Ar�m þ Ar þ Ar ¼ Ar�2 þ Ar
with the rate constant k¼ 1.4� 10�32 cm6 s�1.34–36 There-
fore, the characteristic decay time of the metastable Arm* can
be estimated to also be less than 1 ls. This species is quickly
converted to the excited Ar2* through three-body collisions.
Then, due to the short lifetime (less than 3.2 ls) of the
excited Ar2* species,37 it quickly quenches to the ground state
by emitting a photon before the following discharge pulse.
Hence, Arm* species are also not the cause of the observed
phenomenon.
2. Excited N2* species
As mentioned above, due to the very low presence of
the air in the discharge tube, excited N2* species are also not
expected to affect the interaction between the two plasma
plumes. To confirm this, 1% of N2 was added to the Ar gas
flow. Figure 7 depicts the propagation of the two plasma
plumes with and without nitrogen addition. Detailed analysis
shows that the difference between Lup and Ldown remains the
same for the two experimental conditions. Therefore,
nitrogen-related species also have no effect on the length dif-
ference between the upstream and downstream plumes.
3. Charged particles
The remaining active species that are likely to play a
role in the interactions between the two plasma plumes are
charged particles, namely electrons and ions. For the Ar
plasma plumes with the flow rate of more than 1 l/min, Arþ
ions are initially produced by electron impact. However, the
characteristic time of conversion of Arþ into Ar2þ through
three-body collisions involving Ar atoms
Arþ þ Ar þ Ar ¼ Arþ2 þ Ar
is less than 100 ns at atmospheric pressure given the rela-
tively high rate constant of this reaction k¼ 1.46–3.9
� 10�31 cm6 s�1.38 Therefore, the Ar2þ ions should be the main
positive ion in the plasma plumes. At atmospheric pressure, it
is reasonable to assume that the electron-ion recombination is
the dominant loss process of the Ar2þ ions. According to the
FIG. 6. Optical emission spectra of the upstream and downstream plasmas
in the spectral range from 200 to 900 nm.
FIG. 7. Photographs of the plasma plumes with the addition of nitrogen to
the Ar flow. The Ar flow rate is fixed at 4 l/min. The photographs are spec-
trally broadband. The exposure time is approximately 100 ms.
023503-4 Wu et al. Phys. Plasmas 20, 023503 (2013)
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measurement results,39–41 during the discharge, the electron
density of the plasma plumes could reach �1013 cm�3.
Therefore, assuming the initial densities of electrons ne0
and ions ni0 to be �1013 cm�3, the rate of variation of the
electron density can be expressed as
@ne
@t¼ �bn2
e ; (1)
where the electron-ion recombination coefficient b is �10�7
cm3/s.38 From Eq. (1), one obtains
neðtÞ ¼1
1
ne0
þ bt
for the temporal variation of the electron density. For the pulse
repetition frequency of 1 kHz, the electron density drops to
about �1010 cm�3 just before the next voltage pulse.
As mentioned above, it takes approximately 0.5 ms for
the gas to flow from the upstream to the downstream region
when the gas flow rate is 2 l/min. Hence, the electrons and
the ions generated by a voltage pulse in the upstream plasma
region could flow to the downstream region and, therefore,
affect the propagation of the downstream plasma plume gen-
erated by the subsequent voltage pulses.
D. Effect of surface charges
The surface charge accumulation plays an important role
in the plasma plume formation and dynamics, as confirmed by
other authors for a variety of atmospheric plasma configura-
tions.42–44 In our experiment, the (small) differences in the
bullet initiation processes between the downstream and
upstream jets may result in different lengths of the surface
charge areas on the inner surfaces of the tube. This difference
may affect the lengths of the plasma plumes. Because of the
limitations of the available equipment, we were not able to
directly measure these charge distributions. However, one can
speculate that such areas may be expected to be slightly larger
for the upstream plume than for the downstream plume when
only one “upstream” or “downstream” needle is used alone.
When the two needles are used, the surface charge distribution
on the tube surface between the needles may be fairly uniform
when the gas flow is slow.
One can also expect that at the place where there is no
plasma plume (around but not exactly at the mid-point
between the two needles), the surface charge density is low.
Moreover, the surface charges are mostly supplied from the
upstream plume. Indeed, the density of charged particles typ-
ically experiences a 3 orders of magnitude drop in about
1 ms. During this short period of time, the charges can travel
�1 cm in the gas flow with a typical velocity of �10 m/s.
When the gas flow becomes faster (and hence turbulent),
the non-uniform impingement of charged species onto the
inner surface of the tube very likely leads to non-uniform
charge distributions. This also affects the dynamics of the
plasma plumes, and in particular, the observed snake-like
behavior. Importantly, because the flow strongly affects the
charge distribution of the surface charges, it is very difficult
(if possible at all) to separate the effects of the gas flow and
the surface charges, as is not only evident from our experi-
ments but also from the results of other authors.42–45 Further
studies involving direct measurements of non-uniform
charge distributions are needed to clarify these important
issues.
E. Propagation dynamics of the plasma plumes
To further investigate the gas flow effect on the propaga-
tion of the plasma plumes, a fast intensified charge-coupled
device (ICCD) camera (Princeton Instruments, Model
PIMAX2) was used to capture the dynamics of the discharge.
Figure 8 shows the ICCD images of the plasma plumes. The
exposure time of the camera was fixed at 4 ns. Each image
represents a single-shot video frame. Figures 8(a)–8(h) cor-
respond to the primary discharge and (i)–(n) correspond to
the secondary discharge ignited during the fall time of the
voltage pulse. It is worth emphasizing that both the plasma
plumes propagate in a snake-like motion, which is very dif-
ferent from the previous reports.17–23
It is noteworthy that the downstream plasma initiates
about 8 ns earlier than the upstream plasma as shown in Figs.
8(a) and 8(b). Besides, Fig. 9 shows the propagation veloc-
ities of the two plasma plumes versus time. It clearly shows
that both the primary and the secondary downstream plasma
plumes have higher propagation velocities than the upstream
plasmas. These two phenomena could be due to the gas flow
effect, which results in the higher electron density in the
downstream plasma region, as discussed in Sec. III D. It
should be emphasized that the propagation velocity of the
plume reaches as high as 3� 105 m/s, which means the dis-
charge is streamer-like.20
Regarding the snake-like propagation behavior of the
plasma plumes, it takes place when the gas flow rate is higher
FIG. 8. ICCD single-shot images of the plasma plumes. The exposure time
is fixed at 4 ns. The time labeled on each image corresponds to the dc pulse
rise time in Fig. 2. The working gas is Ar and the gas flow rate is 5 l/min.
The images are spectrally broadband.
023503-5 Wu et al. Phys. Plasmas 20, 023503 (2013)
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than 2 l/min. As discussed above, under such conditions:
(i) the effect of air diffusion from the open end of the tube is
negligible; (ii) the gas flow is turbulent. Over many pulse
discharge cycles, the argon gas is not “pure” anymore in the
discharge region. Besides Ar (ground state), some long rela-
tive lifetime species such as residual charges exist in the back-
ground gas. For example, as mentioned in Sec. III B, the
density of residual charges just before the next voltage pulse
could be as high as �1010 cm�3 for the initial density of the
charge of �1013 cm�3. Therefore, the density of residual
charges is relatively high just before the next voltage pulse.
Within the timescale of the charge decay phase (the time
between consecutive pulses, which is �1 ms), the turbulence
fluctuations may significantly redistribute the residual charges
just before the next voltage pulse. This non-uniformity of the
residual charges may play a significant role in the formation
of the snake-like bullet patterns.
According to the analysis above, one can conclude that
because of the turbulence fluctuations, the densities of the
long-lifetime species such as charges are not uniform within
the tube and on the inner surface of the tube just before
the next voltage pulse. When the discharge is initiated, the
plasma plume propagates towards the place where (1) the
electric field is high (part of it is due to the non-uniform dis-
tribution of surface charges) and (2) the concentration of
background reactive species (such as charged particles) is
high. These factors contribute to the observed snake-like
propagation behavior. We emphasize that if the gas flow had
no direct effect on the propagation of the plasma plume, the
plasma would propagate like in a laminar gas flow. In other
words, the plume propagation would be uniform and stream-
lined, as in the previous reports.17–23
IV. CONCLUSION
The effects of the gas flow on the propagation of atmos-
pheric pressure plasma plumes have been studied. Within the
range of the Ar flow rates from 2 to 6 l/min, the length of the
downstream plasma plume was found surprisingly larger
compared to the upstream plume. Moreover, the length dif-
ference Ldown � Lup became larger with the increase of the
gas flow rate. Detailed investigations show that this phenom-
enon is due to the effects of the electrons and ions (rather
than excited OH, Ar metastables, or nitrogen-related species)
generated by the upstream plasma, which flow from the
upstream to the downstream regions thereby increasing the
background density of the charged species in the down-
stream plasma region. Besides, the ICCD images revealed
the highly unusual and previously unreported snake-like
propagation of the plasma plumes at gas flow rates above
�2 l/min. The-snake like plasma bullet behavior is probably
due to (1) the non-uniform distribution of the surface charges
on the inner surface of the tube and (2) the non-uniform dis-
tribution of active species such as charged particles inside
the tube. Both these effects are affected by the turbulent flow
of the gas.
A detailed analysis involving CFD modeling showed that
this kind of propagation was caused by the hydrodynamic
instability of the Ar flow. It was also shown that the down-
stream plasma plume is ignited earlier and propagates faster
than the upstream plume. This is also probably due to the
higher density of the charge species in the downstream gas
flow. These results have thus clarified the issue of the role of
the gas flow and active discharge species in atmospheric pres-
sure plasma jets and can be used in improving control of this
type of non-equilibrium plasmas in plasma biology, health
care, medicine, and nanotechnology applications.
ACKNOWLEDGMENTS
This work was partially supported by the National Natu-
ral Science Foundation (Grant Nos. 51077063 and 51277087),
Research Fund for the Doctoral Program of Higher Education
of China (20100142110005), and Chang Jiang Scholars Pro-
gram, Ministry of Education, People’s Republic of China, as
well as the Australian Research Council and CSIRO’s OCE
Science Leadership Scheme.
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