22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Effect of surface charge on the propagation of plasma bullet in tubes
X.P. Lu, H. Xu, Y. Xian and S. Wu
State Key Laboratory of Advanced Electromagnetic Engineering and Technology, Huazhong University of Science and
Technology, Wuhan, Hubei 430074, P. R. China
Abstract: In this work, to better understand the propagation mechanism of plasma bullets
in capillary tubes passing through a curved or narrow passage for some biomedical or
material applications, the propagation of plasma streams in a U-shape tube or a tube
covered by a floating conductor is investigated. For a plasma stream propagating in a
U-shape tube, it’s observed that the smaller the distance d between the bended tubes is, the
shorter the total length of the plasma stream is. Moreover, high-speed photographs show
that the plasma bullet starts to accelerate as soon as the secondary discharge is ignited.
Such behavior is totally different with the propagation of plasma streams in straight tube.
Furthermore, for a plasma stream propagating in a tube covered by a conductor, the plasma
stream is suppressed and becomes shorter, and a secondary stream is generated at the
downstream end of the conductor. According to these results, we can conclude that the
surface discharge plays an important role in the propagation of the plasma bullet, which is
important for applications of plasma jets.
Keywords:
plasma jet, surface charge, surface discharge, nonequilibrium plasma
1. Introduction
Atmospheric pressure non-equilibrium plasma jets
(APNP-Js) are of interests for many applications, such as
materials processing [1], nanotechnology [2], and plasma
medicine [3-9]. The plasma jets can expand into
ambient air with lengths from few millimeters to more
than ten centimeters [10-12].
APNP-Js generate
plasmas in open space rather than in confined space,
which is favorable for various applications because they
can be used for direct treatment and there is no limitation
on the size of objects to be treated.
Most of the APNP-Js generate plasmas inside
dielectric tubes first before the plasmas expand into the
surrounding air. The physical characteristics of the
APNP-Js could be affected by the plasma behavior when
they are propagating inside the dielectric tubes. More
importantly, in a practical application, if the sample to be
treated is behind a curved and narrow passage, a
dielectric tube will be used to transport plasma to the
sample. Most probably, the tube should be curved and
contact with the objects around it. And what's worse,
the object could be a conductor. For example, in
biomedical applications, plasma is viewed as a promised
cancer therapy. If the tumor is in the abdominal cavity,
when using a plasma jet to treat the tumor, the situation
mentioned
above
will
happen.
Therefore,
understanding the plasma behavior when they are
propagating inside a curved or conductor surrounded
dielectric tube is very crucial for plasma applications.
In this paper, to better understand the plasma bullet
behavior in dielectric tubes, two independent
experiments are carried out. In the first one, the
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propagation of plasma streams in a specially designed
U-shape tube is investigated. In the second one, the
propagation of plasma streams in a tube with a part of it
covered by a conductor is investigated.
2. Experiment 1
2.1 Experimental setup
Figure 1 shows the schematic of the discharge device.
Atmospheric-pressure plasma streams are generated in
the U-shapeglass tube. The inner diameter of the tube
is 1 mm. The wall thickness is 1 mm. The high
voltage electrode is made of steel needle with radius of
100 µm, which is inserted into the end of the upper tube.
The distance between the tip of the steel needle and the
bending point is 40 mm. To minimum the ambient air
contamination, another 3 m long tube is connected to the
end of the bottom tube. When high pulsed-dc voltage
(Amplitude: 8 kV, frequency: 8 kHz, pulse width: 1 µs)
is applied to the HV electrode and helium gas is fed into
the U-shape tube with flow rate of 1 L/min, a plasma
stream is generated inside the tube.
Fig. 1.
Schematic of the discharge device.
2.2 Experimental results
Fig. 2 shows that the plasma streams can pass through
the curve of the tubes and keep propagating in the
opposite direction in the bottom part of the tube. As
1
can be seen from Fig. 2, the smaller the d is, the shorter
the total length of the plasma stream is. To have a
detail discussion about the length of the plasma stream,
the length of the plasma stream vs. d is plotted in Fig. 3.
When d increases from 1 mm to 30 mm, the stream
length increases from 70 mm to 130 mm. It is worthy
to point out that, in Fig. 2c and 2d, the upper inner
surface propagation of plasma stream is observed after U
turn rather than all along the whole tube uniformly.
velocity is.
Fig. 4. The velocities of plasma bullet vs distances for
straight tube and U-shape tube with d = 30 mm, 15 mm,
6 mm, and 1 mm. The discharge parameters are the
same as that in Fig. 3.
Fig. 2. The photos of the plasmas in U-shape tubes for
different distances (d). (a) d = 30 mm, (b) d = 15 mm,
(c) d = 6 mm, (d) d = 1 mm. Applied voltage: 8 kV,
pulse repetition frequency: 8 kHz, pulse width: 1 μs,
helium gas flow rate: 1 L/min.
Fig. 3.
The stream length versus d.
In order to describe the acceleration behavior
quantitatively, the detailed information of bullet
velocities is plotted in Fig. 4. The bullet velocity is
determined by photographs taken by an ICCD (Princeton
Instruments, Model PIMAX2). It is interested to point
out that, firstly, when the plasma bullets propagate in the
first part of the tube (<40 mm), the bullet velocities for
the five cases are almost the same. Secondly, it can be
seen clearly that the acceleration behavior of plasma
bullets always occurs at t f (fall edge of pulse voltage) in
the U-shape tubes no matter how d changes, but no
acceleration behavior is observed in straight tube.
Thirdly, after the plasma bullets pass through the curve
of the tube, the smaller the d is, the lower the bullet
2
3. Experiment 2
3.1 Experimental setup
Fig. 5 shows the schematic of the discharge device.
Atmospheric-pressure plasma streams are generated in a
glass tube. The inner diameter of the tube is 1 mm.
The wall thickness is 0.25 mm. A steel needle with a
radius of 100 µm is inserted into the left end of the tube
to be the HV electrode. A 30 mm section of the tube is
covered by a saline layer of 1 mm thick. The distance
between the tip of the steel needle and the left end of the
saline is 15 mm. The saline layer is floating without
connecting to the ground or a HV power supply. To
minimum the ambient air contamination, another 3 m
long tube is connected to the end of the tube. When
high pulsed-dc voltage (Amplitude: 8 kV, pulse
repetition frequency: 8 kHz, pulse width: 800 ns) is
applied to the HV electrode and helium gas is fed into
the tube with flow rate of 1 L/min, plasma streams are
generated inside the tube.
Fig. 5.
Schematic of the discharge device.
3.2 Experimental results
As can be seen from Fig. 6, when there is a saline layer
out of the tube, the plasma stream is divided into
3 sections: The primary streamer generated from the HV
electrode and enter the region covered by saline for about
5 mm. Then a dark section extends to the right end of
the saline layer. Beyond this section, a secondary
streamer is generated. The total length of these three
sections is much shorter than the plasma stream
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propagating in a tube without a saline layer. It’s clear
that the primary plasma stream is suppressed by the
saline layer.
Fig. 6. The photos of the plasmas in tubes for: (a) with
a saline layer out of the tube, (b) without a saline layer.
Applied voltage: 8 kV, pulse repetition frequency: 8 kHz,
pulse width: 1μs, helium gas flow rate: 1 L/min.
To understand how the plasma stream propagates in
the tube covered by a saline layer and why the primary
stream is suppressed, an ICCD (Princeton Instruments,
Model PIMAX2) is used to capture the dynamics of the
discharges in the tube covered by a 30 mm saline layer.
Fig. 7 shows the dynamics of the discharge in the tube
covered by a 30 mm long saline layer. It can be seen
that the plasma bullet arrived at the area covered by the
saline layer at 110 ns. Then the plasma becomes
weaker and weaker after it enters the saline layer, and
distinguishes at 350 ns. At 415 ns, a secondary plasma
bullet is generated at the right end of the saline layer. It
should be noted that, with some other setups, the
secondary plasma bullet could be ignited before the
primary plasma bullet disappearing.
Fig. 7. High-speed images of the discharges in the tube
covered by a 30 mm long saline layer. The exposure
time is fixed at 5 ns. Each image is an integrated
picture of 20 shots with the same delay time. The time
labeled on each image corresponds to the dc pulse rise
time. The discharge parameters are the same as that in
Fig. 6a.
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4. Discussion and Conclusions
The propagation of plasma streams in a U-shape tube
and a tube covered by a conductor is investigated. For
a plasma stream propagating in a U-shape tube, with d
varying from 1 mm to 30 mm, the length of plasma
stream increases from 70 mm to 130 mm, but it is always
shorter than 150 mm in straight tube with the same
discharge parameters. ICCD images show that when the
secondary discharge is ignited, the propagation of the
plasma bullet accelerates rather than to be quenched, and
the plasma bullet propagates along the upper inner
surface of the lower part of the tube. On the other hand,
for a plasma stream propagating in a tube covered by a
conductor, the plasma stream is suppressed and becomes
shorter, and a secondary stream is generated at the
downstream end of the conductor. These results are
probably caused by the reversion of the electric field
produced by the surface charges accumulating on the
inner surface of the tube.
5. Acknowledgements
This work was partially supported by the National
Natural Science Foundation (Grant No. 51077063,
51277087, 51477066), and Chang Jiang Scholars
Program, Ministry of Education, P.R. China.
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