Propagation of two symmetrical pulsed atmospheric plasma
streams generated by a pulsed plasma gun
V. Sarron1, E. Robert1, S. Dozias1, D. Ries1, M. Vandamme1,2,3 and J.M. Pouvesle1
1
Groupe de Recherche sur l’Energétique des Milieux Ionisés, GREMI, UMR 6606
Université d’Orléans/CNRS, BP 6744, 45067 Orléans cedex, France
2
TAAM-CIPA, UPS44 CNRS, 3B rue de la Ferollerie, 45071 Orléans Cedex 2, France
3
Germitech, Clichy, France
Abstract: A new plasma source, named Plasma Gun, has been developed in GREMI
within the frame of the Plasmed project dedicated to plasma medicine applications
The plasma gun generates pulsed atmospheric plasma streams (PAPS) which
propagate at high velocity (>108 cm.s-1) in various dielectric tubes, including
flexible catheters. In this work, we performed time resolved ICCD imaging of the
plasma gun allowing measurement of the light emission intensity, the plasma
velocity and the evolution of the plasma streams topography during their
propagation. It is seen that the so-called plasma “bullet” is in fact the PAPS head
which corresponds to a zone of higher intensity in the front of the plasma stream. We
demonstrated PAPS splitting in two or multiple branches. For example, two
symmetrical PAPS can be generated from a single one when reaching T-shape or
ring structures. Each of the two streams ensuing from the splitting of the original
one present very similar intensity and propagation features , also similar to those of
a single PAPS at the same distance from the primary DBD discharge reactor. The
mixing of the two PAPS was studied with the circular assembly and reveals that a
constructive mixing occurs leading to the generation of a new single plasma stream
which light emission is roughly the addition of that of the two colliding ones.
1. Introduction
Cold atmospheric pressure plasma jets (CAPPJs) have recently attracted lots of attention
and especially in the new field of Plasma
Medicine. C-APPJs have been used as a new
tool for decontamination, therapeutic treatment
[1,2].
At GREMI, a new source has been developed,
named Plasma Gun [3]. The plasma gun
basically consists in a dielectric barrier
discharge reactor flushed with rare gas at low
flow rate, a few tens of cm3.min-1, at
atmospheric pressure. This device generates
pulsed atmospheric plasma streams (PAPS)
which propagate at high velocity (>108 cm.s-1)
through high aspect ratio capillaries that could
allow endoscopic treatments. To ensure plasma
treatments in organs such as colon, lungs,
plasma has to propagate in very complex
geometry inducing splitting, shrinking or
expansion of the propagating streams. Splitting
and mixing of PAPS in simple geometries in
under the scope of this paper.
2. Experimental setup
The plasma gun is based on a HV pulsed
dielectric barrier discharge. As shown in
Figure 1, a tungsten electrode, 1 mm in
diameter, is inserted on the axis of a 4 mm
inside diameter dielectric glass tube having
1mm thick wall. The external electrode consists
of a flat ring surrounding the tip of the internal
electrode. The high voltage pulses applied to
this DBD reactor are supplied by a pulsed
generator delivering 30-40 kV. In this work, the
injected gas was neon flowing at 200 cm3 min-1
through dielectric tubes of different shapes.
Figure 1: Experimental setup of a pulsed plasma gun
Bullet propagation was observed with a fast
ICCD camera (512 × 512 pixels) with
integration time as low as 2 ns. Voltages across
the DBD reactor were measured with the use of
Tektronix P6015A 75 MHz probe connected to a
Tektronix TDS 3054 oscilloscope. Light emitted
by the PAPS were detected by fast PIN
photodiode, with rising time of about 5 ns,
connected to a Tektronix TDS 3014
oscilloscope. Set of photodiodes also allowed
study of the PAPS propagation in complement
of the ICCD measurements.
Two dielectric guides flushed with Neon have
been used. The first one is a straight capillary,
and the second one has a ring shape.
Propagation in straight capillary will be detailed
in the section 3. In the section 4, splitting effects
will be illustrated, while mixing effects will be
described in the section 5.
3. PAPS propagation
As mentioned before, PAPS propagation and
topography were monitored by ICCD imaging
while velocity was measured using either ICCD
imaging or time measurements between signals
delivered by optical fiber coupled photodiodes.
Figure 3: a) Profile extracted from ten nanoseconds gated
ICCD images. b) Profile extracted from an accumulation of
100 images with ten nanoseconds exposure time.
The evolution of the velocity PAPS is directly
coupled with the topography of PAPS head. This
is illustrated in Figure 4 where the typical
velocity evolution as a function of the distance
from inner electrode tip is represented. At short
distances, we observed a rapid decrease
corresponding to the “filamentary mode”. At
long distance, after about 20 cm, the velocity
decreases much slower, corresponding to
apparent bullet mode.
Figure 2: PAPS shape in a 30 cm long straight capillary at
various distances from the tip of the inner electrode (gate
aperture is 2 ns)
ICCD imaging has been performed on a 30 cm
long straight capillary and 15 cm diameter ring
shape capillary. In both cases, the inner diameter
of the tubes was 4 mm. PAPS topography
evolution in a straight capillary is illustrated in
Figure 2. At a first glance, two main
topographies can be depicted along the PAPS
propagation. At short distances of the plasma
reactor, plasma propagates along the guide with
a filamentary nature (Figure 2a), high intensity
light expending over few centimeters. At long
distances from inner electrode, the so called
“plasma bullet” shape is observed (Figure 2c),
high intensity plasma being shrinked.
In fact, when emphasizing low light levels, as
shown on Figure 3, plasma “bullets” appears
still connected to the initial plasma source
through a weaker plasma canal. High intensity
light peak observed is the so-called “plasma
bullet”. Long accumulation evidence a plasma
stream still connected from the source. This is
why we now prefer to call the describe
phenomenon as Pulsed Atmospheric Plasma
Stream: PAPS.
Figure 4: Plasma bullet velocity evolution as a function of the
distance from the inner electrode. Plasma bullets propagate in a
straight capillary.
4. Splitting of PAPS
Splitting has been characterized at the ring
inlet bifurcation as shown in Figure 6. Each
picture corresponds to a different discharge
pulse. In this figure, images have been recorded
with a 2 ns integration time
Initial PAPS propagates in a straight capillary
until the splitting point (t = 40 ns). Once this
point is reached, two secondary PAPS are
produced. As it can be seen on the different
pictures taken at increasing times, these
secondary PAPS propagate at exactly the same
velocity suggesting that the splitting of the
initial PAPS generates two symmetrical
secondary ones with the same characteristics.
For example, one must notice that levels of light
extracted from the pictures in Figure 6, indicate
that the intensity in the PAPS head is divided by
a 2 after branching.
PAPS described above propagate until this
mixing point and then are allowed to exit
through a common new straight capillary part. It
is very surprising to note that the PAPS resulting
from the mixing of the secondary PAPS
propagates at a higher velocity than the ones
measured just before for the PAPS in the two
branches of the ring. We can say that
“constructive” secondary PAPS mixing occurs
leading to the generation of a new single plasma
PAPS which light emission intensity is roughly
the addition of that of the two colliding ones
(Figure 7).
Figure 6: Two nanosecond gated ICCD images of the PAPS
splitting in a ring shape capillary.
Current measurements in a T-shape junction
confirm the symmetric energy distribution
between the secondary PAPS.
Figure 5: Two nanoseconds gated ICCD images of one
secondary PAPS in a ring shape capillary. The distance
mentioned below each picture has been measured from the inner
electrode tip. Each picture presents one part of the 15 cm
diameter ring.
As for PAPS propagating in the straight
capillary, secondary PAPS show different shapes
along their propagation (Figure 5). At 8 cm
from the inner electrode, the “filamentary
mode” is observed. Then, a transient mode is
observed to end at 25 cm in an apparent bullet
shape. Secondary PAPS follow the same
evolution as PAPS in a single capillary. This
means that propagation characteristics are kept
even after splitting.
5. Mixing of secondary PAPS
Mixing of PAPS has been characterized at
the ring exit as shown in Figure 7a. Secondary
Figure 7: a) Two microseconds gated ICCD images of
symmetrical plasma bullets; b) Ten nanoseconds gated ICCD
images of non-symmetrical plasma bullets. Plasma bullets are
propagating in a 4 mm inner diameter capillary.
Minor modifications of potential around one of
the two branches (piece of metal, finger) allow
to modify the PAPS velocity in one branch of
the ring and to induce dissymmetric collisions
as seen on Figure 7b. First arrived secondary
PAPS at the branching of the ring outlet, splits
itself in two new PAPS branches. One of these
then propagate in the straight section at the
outlet of the ring (see Figure 7b). The second
one propagates towards the PAPS coming from
the other branch of the ring. Strong self electric
field influence is observed finally leading these
facing PAPS to vanish. Similar observation was
already reported with AC driven plasma jet
[5,6].
6. Conclusion
Pulsed Atmospheric Plasma Streams, PAPS,
generated by the plasma gun are able to
propagate in long and flexible capillaries of
complex shapes including branching. In the
domain of plasma medicine, these properties are
very interesting when thinking to the treatment
of complex organs such as lungs.
We showed that the so-called plasma “bullets”
generated in plasma jets are in fact the front part
or “head” of the propagating plasma stream
which emits light with much higher intensity
than the plasma volume still connected to the
plasma reactor.
The topography of the PAPS evoluates along its
way in the capillary. At short distances from the
plasma reactor, the PAPS appears with a
filamentary structure which slowly disappears at
longer distances to lead to an apparently much
more homogenous plasmas volume head. PAPS
splitting has been demonstrated in this work.
When one PAPS reached a T-shape junction, it
generates at this point, two secondary PAPS
with same properties, characteristics and
behavior than the primary one. It is possible to
provoke the remixing of secondary PAPS. In
that case, it leads to a new PAPS that propagates
at a higher velocity than the ones measured just
before for the mixing ones. Light intensity
produces by this resulting PAPS is also higher
than the one of each previously converging
ones, letting think to constructive interaction.
All this very interesting properties render the
PAPS very attractive for applications in many
fields such as treatment of materials,
sterilization
and
decontamination,
and
especially in plasma medicine.
Acknowledgements. This work was supported
by Région Centre through the APR program
“Plasmed” and ANR PAMPA. D.R. was
financially supported by CNRS and Région
Centre, M.V. by Germitec and V.S. by Conseil
Général du Loiret.
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