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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 43, NO. 7, JULY 2015
Nanosecond-Resolved Discharge Processes
Revealing Detailed Mechanisms of
Nonequilibrium Atmospheric-Pressure
Plasma Jet of Helium
Nan Jiang, Xianjun Shao, Guan-Jun Zhang, and Zexian Cao
Abstract— Nanosecond-resolved photographs of the nonequilibrium atmospheric-pressure plasma jet of helium, generated
with the conventional dielectric barrier discharge device, were
obtained at different sections of the experimental setup and
at different development stages of the discharges, showing that
various distinct mechanisms are simultaneous in operation. The
streamer from the outer edge of active electrode sitting at
downstream side forms a jet in air, which only turns out to
be hollow when approaching the orifice of the gas conduct. The
streamer from the inner edge temporally lags behind, and it
propagates along a helical path and initiates the glow discharge
when arriving at the ground electrode. The electron deposit
beneath the active electrode expands inward from both sides,
displaying a soliton-like behavior; while the very compact ion
deposit beneath the ground electrode, a typical ionic streamer,
extends outward from the inner edge of the electrode. The
velocities of the jet in air and of the streamer between electrodes
are much larger than those at other parts of the device. The
resolution of these particular processes and features can improve
the implementation of this valuable cold plasma source.
Index Terms— Atmospheric-pressure plasma jet (APPJ),
dielectric barrier discharge (DBD), helium, streamer.
I. I NTRODUCTION
T THE beginning of 1990s, Koinuma et al. [1]
devised an equipment that could generate a microbeam
plasma, and thus initiated the subsequent research of the
nonequilibrium atmospheric-pressure plasma jet (APPJ). Many
similar devices have since then been designed, aiming mostly
at the implementation of this intriguing cold plasma source [2].
Teschke et al. [3] found with the help of an intensified
charge-coupled device (ICCD) that the plasma plumes,
A
Manuscript received July 12, 2014; revised October 2, 2014; accepted
April 1, 2015. Date of publication June 23, 2015; date of current version
July 7, 2015. This work was supported in part by the National Natural
Science Foundation of China under Grant 10904165, Grant 11290161, and
Grant 51172272, in part by the National Basic Research Program of China
under Grant 2012CB933002, in part by the Knowledge Innovation Project
through the Chinese Academy of Sciences, and in part by the China National
Fund for Distinguished Young Scientists under Grant 51125029.
N. Jiang and Z. Cao are with the Institute of Physics, Chinese
Academy of Sciences, Beijing 100190, China (e-mail: jiangnan@iphy.ac.cn;
zxcao@iphy.ac.cn).
X. Shao and G.-J. Zhang are with the State Key Laboratory of Electrical
Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an 710049,
China (e-mail: shaodbd@gmail.com; gjzhang@mail.xjtu.edu.cn).
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TPS.2015.2419639
there comprise a train of bullets. Since then, many reports
on the characterization of such plasma bullets, essentially
as guided ionization waves, have been published [4]–[13].
In 2009, Jiang et al. [14] pointed out that the nonequilibrium
APPJs generated with the apparatus used by Teschke et al. [3]
actually comprise three, instead of two, distinct zones and that
the plasma in each of the three zones arises from a different
mechanism. At that time, the observation was made using two
photoelectron multiplier tubes to measure the optical emission
from different positions on the path of gas flow, and the
conclusion was arrived by comparing the initial moments of
light emission from some specific positions [14]. In due time,
a temporally resolved photography was successfully applied to
investigate the formation and development phases of plasma
bullets, for instance, forward bullet and backward bullet, the
latter is also known as return stroke, were identified in a
helium atmospheric pressure needle-to-plane discharge [15].
The influence of voltage waveform, pulse polarity, and pulse
repetition rate has been studied using nanosecond
ICCD imaging and plasma-front velocity measurement [16].
In this paper, using an ICCD, the observations in our
previous publication [14] are confirmed in a straightforward
manner, and more details concerning the nonequilibrium APPJ
of helium, especially the seeding process revealed to a resolution of nanosecond, are provided. These results can help
illuminate the true mechanisms governing the discharges, and
also the dynamics involved, in distinct parts of this very useful
cold plasma source.
II. E XPERIMENT
The setup for the generation of nonequilibrium APPJ is
similar to that used in [3], and has been described in our
previous publications [14], [17]. Briefly, high-purity (5N) gas
of He was introduced at a flow rate of 3.0 L/min into a quartz
tube with an outer diameter of 4.0 mm and an inner diameter
of 2.0 mm. The two electrodes, made of either a 50 μ-thick
copper foil or a net of metal nickel with a transparency
of 90% wrapping the quartz tube, are both 2.0-cm wide and
separated for 2.0 cm, with the outer edge of the electrode
at the downstream side being 1.0 cm away from the orifice
of the quartz tube. The electrode at the upstream side was
grounded, and the one at the downstream side was connected
0093-3813 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
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JIANG et al.: NANOSECOND-RESOLVED DISCHARGE PROCESSES REVEALING MECHANISMS OF NONEQUILIBRIUM APPJ OF He
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Fig. 1. Characteristic curve of discharge current of the nonequilibrium APPJ
of He generated with the electrode configuration of the DBD drawn against
the applied voltage. Applied voltage: 3.2 kV.
to a power supply operated at 17 KHz (the period measures
∼58.82 μs). The discharge current and the applied voltage
were monitored using an oscilloscope (Tek4104): the voltage
was directly measured using a HV-probe (Tek, P6015A), while
the discharge current was obtained by measuring the voltage
on a resistor Ri = 100 connecting the ground electrode
to the ground [14, Fig. 2]. The device was photographed
using a digital camera (Canon 5DII) and the discharges, both
inside the quartz tube and outside in the air, were dynamically resolved using an ICCD system (PI-Max II, Princeton
Instruments). For taking the side-view photographs for the
discharge inside quartz tube, the gate time of ICCD was
set, with deliberately chosen delay times, with regard to the
zero time that corresponds to the moment of appearance
for the onrising front of the discharge current (Fig. 1). The
ICCD has a low resolution (512 × 512 pixels), and thus
to raise the resolution of the photographs, the whole setup
was photographed in five sections, and the results were then
synthesized into one picture using Photoshop without introducing any other alteration. The sectional images of the plasma
jet in the ambient were taken with the ICCD positioned on
the line through the axis of the quartz tube. The discussions
here concern with discharges obtained at, but not limited to,
an applied voltage of 3.2 kV under which the discharge is very
stable. Other working parameters would be specified at proper
places.
III. R ESULTS AND D ISCUSSION
Fig. 1 shows the characteristic curve of discharge current for
the nonequilibrium APPJ of helium drawn against the applied
voltage, which shows a pronounced peak in the positive half
period. The zero time for one period of the applied voltage
in the following discussion is set at the onrising front of
this positive pulse of discharge current. The gate time for
ICCD photographs was also settled this way. In fact, given
other parameters, the discharge currents obtained with those
voltages that can sustain the discharge are usually unstable.
The moment for the appearance of the first pulse (referring to
Fig. 2.
(a) Six ICCD photographs (pseudo color) in series taken at
the moments specified in (b), overlapping the photograph of the device.
(b) Characteristic curve of the discharge current in a time interval of 4 μs, with
labels showing the gate-ON times for the corresponding ICCD photographs
in (a). Exposure times: 10×10 = 100 ns for the ICCD photographs and 0.125 s
for the photograph of the device.
the phase of the applied voltage) and its intensity are quite
random and irregular [18]. This instability at microscopic
scale is probably related to some nonlinear processes in
this particular kind of discharge; the precise reason for this
phenomenon needs be clarified, but it is beyond the scope
of this paper. Under some distinct voltages, however, the
discharge current can be very stable and the characteristic
curve of the discharge current is well reproducible, as the
one obtained at 3.2 kV, which is shown in Fig. 1. Thanks to
this property, we could obtain ICCD photographs of a stable
discharge, which furnishes the basis for the discussion here.
Fig. 2(a) shows six ICCD photographs in series showing the
plasma bullet moving from the active electrode at downstream
side (left) to the ground electrode, overlying on a conventional
digital photograph of the whole device. As the discharge
process is well reproducible, the taking of ICCD photographs
was triggered by the onrising front of a previous positive
current pulse, and the first shot occurred at ∼58 μs later.
Then with the help of a timing controller (ST-133, Princeton
Instruments) a time delay of 20 ns was set to make each next
photograph. In the time interval between 58–62 μs corresponding to the development phase of the positive pulse, 200 such
photographs had been taken. To achieve a better quality of
the photographs, what is presented here is the accumulated
record of the ICCD in 10 periods of applied voltage, and
thus the total exposure time amounts to 100 ns. We choose
six ICCD photographs to present in Fig. 2(a), in which the
corresponding triggering moments are labeled with letters a–f
on the curve of the discharge current [Fig. 2(b)].
The ICCD photograghs in Fig. 2(a) clearly show the process
of the plasma bullet developing on the way from the active
electrode to the ground electrode. One sees that the discharge
in between the two electrodes is essentially a bulk discharge
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instead of a surface discharge [14], [19] as we speculated
previously based on the consideration of the device
configuration. This is a typical streamer discharge process
in helium [16], starting with an almost invisible seed
discharge and developing into a discharge channel connecting
the two electrodes at the final stage. That this essentially
streamer-type discharge appears in average as a bright
spot-bullet is termed bullet-streamer dualism [20].
From Fig. 2(a), one can conclude that in between the
electrodes, the streamer is initiated at the outer edge of the
active electrode, in the neighborhood of the central axis of
the glass tube. Here, the discharge is very weak. On the way
propagating toward the ground electrode, the streamer head
steadily becomes larger and brighter. The clear photographing
of this very important, rather the universal process for helium
gas discharge has not been reported, to the best knowledge of
the authors. From the pictures available in the literature, it is
usually impossible to tell assuredly whether the plasma bullet
arises from a surfacial discharge or from a bulk discharge [16].
Here, we see that at the initial stage of the discharge in a
period of applied voltage, the discharge, being a streamer, is
initiated at the inner edge of the active electrode; it develops
toward the ground electrode and grows in intensity all the way.
The bullet, or strictly speaking, the streamer head, does not
propagate along the axis of the quartz tube, rather it whirls
around the axis, i.e., the path is helical. This helical path
maybe results from the irregularity of the gas flow inside the
tube. The disturbance to the gas flow is probably induced by
the dielectric barrier discharge (DBD), which essentially is
an electrohydrodynamic effect [21]. Fig. 2 reveals only the
process of the streamer-triggered discharge in a very short
time interval within a period of the applied voltage. When the
plasma bullet arrives at the inner edge of the ground electrode,
a conducting channel is established between the electrodes, the
discharge between the electrodes turns into a glow discharge,
and a large discharge current is measured. The DBD is generally more violent than the streamer, which, besides causing
a turbulent flow, has an effect on the subsequent discharge in
different aspects [22]. The correlation of the streamer path to
the turbulent flow pattern is a factor of practical importance
for atmospheric-pressure plasma sources [23].
In Fig. 2, the bullet grows larger on the way forward to
the ground electrode. This is related to the generation of seed
electrons around the bullet initiated by metastable He∗ atoms
(see [22] for detailed discussion). The impurity gases lurked
into the quartz tube, mainly O2 and N2 , provide seed electrons
when ionized by metastable He∗ atoms via the Penning
process, since their ionization energies, 12.2 and 15.58 eV,
respectively, are much lower those of He [24]. As the
He∗ atoms are neutral, their distribution is indifferent to the
electrical field, and thus the range of discharge can become
larger, and an enlarged diameter of the streamer head results
from a weakened electrical field. Consequently, the plasma
bullet, whether in helium confined inside the quartz tube or in
He/air mixture outside is much slower than the streamers generated in air, of which the typical velocities fall in the range of
106 –107 m/s. This velocity of bullet is a few times lower
than the reported values in the literature since here only a
IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 43, NO. 7, JULY 2015
Fig. 3.
Synthesized picture from the ICCD photographs for the whole
nonequilibrium APPJ taken on five zones. In particular, zone C is the active
electrode on the downstream side and zone E is the ground electrode.
Insets: magnified ICCD pictures for the plasma jet in zone B, for the
approaching electron deposits beneath the active electrode, and for the ionic
wave front beneath the ground electrode. The discharge current and the applied
voltage are presented for reference.
smaller voltage was applied [16]. The diameter of the streamer
in air under a pressure of one atmosphere is estimated to
be 0.1 mm [25], but that of the steamer in helium in Fig. 2 is
definitely larger than 1.0 mm. The reason for this difference
lies in the presence of metastable He∗ atoms in the discharge
of helium.
To reveal the generation, development, and propagation
of the discharges at different parts of the setup,
ICCD photographs of the whole device in operation
should be obtained. Since our ICCD has only a resolution of
512 ×512, we divided the device into five zones, labeled A–E,
namely, zone A is the part outside in the ambient air, zone B
is the part from the outer edge of the active electrode to the
orifice, zone C is the part over active electrode, zone D is
the region between the electrodes, and zone E is the part
over ground electrode. The zone-specific photographs were
then digitally synthesized (Fig. 3). Under a low voltage of
only 3.2 kV, no charge overflow beyond the ground electrode
could be observed [21].
From Fig. 3, one notices that in zone A, i.e., in the ambient
air, the plasma jet propagates as a bullet with an attenuating
intensity and a shrinking size. This is right the plasma jet of
interest for application, thus it has been intensively investigated [2]–[6]. Plasma dynamics in a helium jet are probably
very similar to the streamer propagation in a dielectric tube
filled with helium [26], and the attenuating helium jet in
air leads to a shrinking size of the plasma bullet [27].
Jiang et al. [14] pointed out that the plasmas on the two sides
of the active electrode are independent from each other, and
this will be confirmed in Fig. 3. In the first half period of
the applied voltage, it is the outer edge of the active electrode
that first triggers a streamer, a feature of the corona discharge.
From zones B and C in Fig. 3, one sees that it becomes visible
at ∼50 μs. Following Raizer [24], the head of a positive
streamer comprises ions. At the same time, the electrons
are accelerated toward the temporal anode, now the active
electrode, and then deposit on the inner surface of the quartz
tube beneath that electrode. The electrons first landed at the
outer edge of the electrode and then proceeded forward.
JIANG et al.: NANOSECOND-RESOLVED DISCHARGE PROCESSES REVEALING MECHANISMS OF NONEQUILIBRIUM APPJ OF He
As electron propagation can induce light emission via
excitation and/or ionization of the surrounding gas atoms, we
see that the illuminated part develops inward gradually from
the outer edge of the active electrode (see zone C in Fig. 3),
indicating on the process of electron deposition.
The discharge in between the electrodes, i.e, in the so-called
DBD zone (zone D in Fig. 3), begins at the inner edge of the
active electrode. Its triggering time, a little earlier before the
moment of 55 μs, lags behind that initiated at the outer edge of
the active electrode for ∼5 μs. This observation is consistent
with the simulation [26] and our experimental demonstration
that the streamer can be initiated from a single electrode. The
streamer from there also induces electron deposition beneath
the active electrode, which propagates inward from the inner
edge. Thus, we have the chance to observe a very interesting
phenomenon in zone C: the deposited electrons beneath the
active electrode expand inward from both sides that at some
moment later, they finally meet at a position nearer to the
inner edge, since the streamer at the inner edge of the active
electrode occurs ∼5 μs later. The deposited electrons from
the two edges do not merge into a whole; rather they cross
each other to continue to propagate forward with their own
profiles well preserved, a behavior similar to the interaction of
solitary waves of water. More experimental evidences, in conjunction with theoretical considerations [28]–[31], are required
to elucidate this phenomenon. Naidis’s simulation [31] of the
interaction between two counter-porpagating streamers may
provide some hints for further simulation of the situation
in Fig. 3.
When the streamer from the inner edge of the active
electrode arrives at the ground electrode, a conducting channel
is formed, and the discharge changes from a streamer to a
glow discharge, and the discharge current suddenly becomes
large, resulting in a sharp peak (Fig. 1). At the same time,
more charges are generated that they deposit beneath the active
electrode. From Fig. 3, one observes that the luminescence
caused by the charge deposit from the inner edge of the active
electrode suddenly becomes very bright and expands toward
the outer side very fast. Following this, the jet in the ambient
air comes to the farthest end and the electron deposit from the
outer edge of the active electrode then becomes weakened and
gets slowed down. Clearly, the transition of the discharge in
the DBD zone into a glow discharge casts no influence on the
jet outside in ambient, or we can say that the streamers from
the two edges of the active electrode are mutually independent.
First, a few ICCD shots for zone D reflects the process
discussed above before the readers are referring to Fig. 2(a).
When the head of the streamer arrives at the ground electrode,
a spike appears on the discharge current curve. After that, the
region near the active electrode immediately turns brighter.
Now, a conducting channel is formed between the electrodes
and the discharge has turned into a glow discharge. As mentioned before, the negative charges, dominantly electrons, flow
to the temporal anode. The positive ions deposit beneath
the temporal cathode, currently the ground electrode, and
expand inward. At sufficiently large applied voltages, the
charge deposition may expand beyond the ground electrode
to form a jet outside, which we called an overflow jet [21].
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Fig. 4. Sectional images of the plasma jet in air taken at different distances
from the orifice of the quartz tube. The plasma jet shows a hollow structure
in the neighborhood of the orifice of the quartz tube. Exposure time: 100 ns.
Though we first coined this term, the charge overflow process
has already been simulated in [19]. Unlike the jets from edges
of the active electrode, here it involves a typical surfacial
discharge process. The ICCD photographs in Fig. 3 help us
inspect this process more closely. In comparison with those in
zone C, the deposition of ions in zone E looks more compact,
and the head of the ionic wave can be seen clearly. This is just
the difference between the positive streamer and the negative
streamer [13], [16], [19].
Fig. 3 shows that the discharges in different zones of
the APPJ setup occur at variable moment within a period
of the applied voltage. By measuring the positions of the
brighter parts, i.e., the streamer heads and charge deposits,
the velocities for the discharges at different zones can be
roughly estimated. In the ambient air, i.e., in zone A, the
maximum velocity, under given conditions, grows to ∼25 km/s
at 58 μs and drops to zero at 59.4 μs. The plasma bullet
visible in zone B develops to the moment of ∼56.5 μs with
the velocity growing to ∼4.4 km/s. This difference is larger
than only by twice as observed in a similar device [32].
In zone C, i.e., beneath the active electrode, the situation is
a little complicated. The velocity of the deposited charges
starting from the outer edge of the active electrode, visible
from the moment of ∼50 μs, grows to 4.5 km/s at ∼59.5 μs
and then drops to zero within 2 μs, whereas that from the
inner edge, measurable at about 58 μs, grows steadily to a
value of 2.4 km/s. In zone D, the streamer flies from the
active electrode to the ground electrode within 58.1–59.1 μs
with a velocity steadily growing to ∼40 km/s. In zone E, the
charge overflow is detectable within the time interval between
59.4–64 μs, and the velocity grows to a maximum value of
only 8.3 km/s at 60.6 μs and then drops to zero.
It should be emphasized again that at the outer edge of the
active electrode, the initiation and development of the streamer
are quite the same as at the inner edge. The plasma bullet takes
form in the neighborhood of the axis of the glass tube, and it
flies along the axial line toward the orifice of the tube. It turns
into a hollow structure only when it approaches the orifice of
the tube (Fig. 4). The plasma jet maintains its hollow structure
for a while in the ambient air (within a distance of 18 mm
in Fig. 4), and it merges into a solid structure further into the
air because of the narrowing helium channel. Evidence of the
hollow structure of the jet and its contraction was also acquired
by spatially resolved optical emission measurements [27].
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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 43, NO. 7, JULY 2015
IV. C ONCLUSION
In summary, the discharge processes occurring in the generation of nonequilibrium APPJ with a DBD device, also in
regions beneath the two electrodes, have been resolved to
nanosecond and recorded with the aid of the ICCD, which
reveals very interesting phenomena concerning the discharge
details that can help understand the true mechanisms underlying this intriguing cold plasma source. At both edges of the
active electrode, under given conditions, the streamer starts as
a weak bulk discharge at the neighborhood of the axis of the
quartz tube. The streamer from the outer edge of the active
electrode starts earlier, and in approaching the orifice of the
glass tube, it turns into a hollow structure due to the invasion
of air. After leaving the orifice of the quartz tube, it attenuates
in intensity and shrinks its size, but its velocity grows to a
maximum value a few times larger than that measured within
the quartz tube. The streamer from the inner edge temporally
lags behind that from the outer edge, and it propagates along
the axis of the gas conduct, growing larger and brighter on
the way to the ground electrode to eventually launch the glow
discharge. At that moment, a large discharge current peak
appears. Electron deposition beneath the active electrode, the
temporal anode, occurs from both edges, and the fronts of
the electron deposit run across each other like solitary waves
of water. Ionic charge deposit beneath the ground electrode,
the temporal cathode, looks more compact, and can expand
beyond the electrode to form an overflow jet when the applied
voltage is sufficiently large. The velocities of the plasma bullet
running from the active electrode to the ground electrode,
and of the plasma jet running from the active electrode to
protrude into the ambient air, are an order of magnitude
larger than the propagation velocities of deposited charges
beneath the electrodes. These observations provide a deep
understanding of this very useful cold plasma source, which
may be helpful for its successful implementation. For instance,
by precise control of the launching moment and duration of
glow discharge that causes turbulence in gas flow, which can
be realized by varying the electrode separation and voltage
frequency, a longer plasma plume can be obtained under other
given conditions. At the same time, knowledge over the profile
evolution of a plasma bullet in the ambient air is critical for
the optimization of discharge density in conjunction with the
plume length.
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Authors’ photographs and biographies not available at the time of publication.