Download Experimental and numerical study of modulation of ionization front propagation velocity in ?s helium plasma gun discharge with nitrogen admixture

22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Experimental and numerical study of modulation of ionization front
propagation velocity in µs helium plasma gun discharge with nitrogen
admixture
T. Darny1, E. Robert1, F. Pechereau2, P. Viegas3, S. Dozias1, A.Bourdon3 and J.-M. Pouvesle1
1
GREMI UMR 7344 CNRS / Université d’Orléans, 45067 Orleans, France
2
CERFACS, 42 Av Coriolis, 31057 Toulouse, France
3
LPP UMR 7648 CNRS / Ecole Polytechnique, Palaiseau / Université Pierre et Marie Curie, Paris, France
Abstract: This work reports on experimental and numerical analysis of the effect of
nitrogen admixture on the development of helium based atmospheric pressure discharge
produced by the Plasma Gun in long dielectric tubes. Besides, the drastic extension of
discharge propagation length, reactive species generation can be tailored to specific
applications while keeping electric field properties quite constant. This might afford unique
opportunities to study specific action of the different plasma components for biomedical
applications. Good agreement was found with recently developed model calculations,
emphasising the key role of plasma kinetics on the ionization level of the plasma stream
connecting the ionization front with the powered electrode of the DBD reactor.
Keywords: plasma jets, ionization wave, 2D fluid model
1. Introduction
Atmospheric Pressure Plasma Jets (APPJ) are actually
finding an increasing number of applications in biology,
medicine, surface treatments and are rapidly extending
towards liquid processing. APPJ, usually fed with rare
gases are also now demonstrated to work with ambient air
using microcapillaries [1]. Among APPJ, Plasma Gun
(PG) device, used in this work, has great potentialities for
biomedical applications, not only for its ability to produce
large amount of reactive species in the target environment
but also for the many ways to allow the production of
plasmas propagating in long, flexible capillaries [2], of
various diameters. This last feature is a unique advantage
for some in situ loco-regional biomedical treatments. PG
can be supplied with various noble gases (Helium, Neon,
Argon...) with or without admixture (Oxygen, Nitrogen,
air...) upstream or downstream the DBD reactor.
We have previously investigated in [3] the effect of
nitrogen admixture in the helium buffer for a PG
equipped with a 40 cm long capillary. It was
demonstrated that only very small nitrogen admixture in
the helium buffer (0,1 %) led to an increase of the plasma
propagation length inside the capillary, with more
homogenous discharge and tailored reactive species
production. This occurs together with a velocity
modification of the ionization front propagation inside the
capillary, depending on the amount of nitrogen admixture
in the helium buffer.
In the present work, to get more insight in the
understanding of these phenomena, we compare our
experimental results with 2D fluid simulations of the
discharge. In [4] a kinetic scheme has been derived from a
detailed experimental study of an atmospheric pressure
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discharge in helium with a small admixture of N 2 . In this
kinetic scheme, the importance of two body and three
body Penning ionization and charge transfer reactions was
shown. Then in this work, we use this kinetic scheme to
investigate the role of both reactions on the discharge
dynamics.
Comparisons between experiments and simulations are
carried out on the propagation velocity of the ionization
front of the discharge and the optical emission of the
discharge. Furthermore, first comparisons are made on the
time and space evolution of the longitudinal and radial
component of the electric field with or without nitrogen
admixture.
2. Experimental setup
Briefly, the PG is a coaxial dielectric barrier discharge
reactor with a quartz capillary, flushed with helium and
powered by μs voltage pulses in the kHz regime. The
figure 1 shows the experimental setup. A 12 cm long
dielectric quartz capillary with a 4 mm inner diameter and
6 mm outer diameter is used. The inner electrode, 2cm
long, is set inside the capillary. The helium buffer is
injected through the inner hollowed electrode (0,8 mm
inner diameter). The nitrogen is mixed in the helium
buffer. Three optical fibers are regularly disposed along
the capillary, 3 cm apart of each other, and connected to a
photomultiplier tube (PMT). This setup allows to follow
the ionization front propagation and to calculate its
velocity. Time integrated (200ms) emission spectra of the
discharge are obtained by a 2000 maya pro spectrometer.
The optical fiber collecting light to the spectrometer is set
5 cm downstream from the grounded electrode. A
PIMAX 3 fast ICCD camera is used to image the
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discharge. Voltage pulses with peak amplitude set to 16
kV and delivered at 1 kHz have been used in this work.
coefficient is the highest for 1000ppm of N 2 and
decreases significantly for admixtures with more than 1%
of N 2 . It is interesting to note that the excitation
coefficient of He* is the highest also for small amounts of
N 2 and starts decreasing for admixtures with more than
0.1% of N2. As initial condition, we consider a uniform
preionization of 104cm-3 in the He-N 2 mixture.
Fig. 2. Setup used in the 2D simulations. Powered
electrode is the black rectangle, discharge propagation
from right to left is considered.
Fig.1. Experimental setup
3. Numerical model
The geometry considered in the simulations is shown I
n Figure 2. The length, radius and the thickness of the
quartz tube (ε r =4) are the same as in the experiment. A
high voltage electrode is wrapped around the dielectric
tube and the whole discharge set-up is set in a grounded
metallic cylinder to ensure that the potential decreases
down to zero at some distance of the discharge set-up.The
applied voltage has a rise time of 2µs as in experiments.
In this work, we use a 2-D axisymmetric fluid code with
drift-diffusion equations of charged species coupled to
Poisson’s equation [5]. We take into account 5 positive
ions (He+, He 2 +, N 2 +, N 2 +(B) and N 4 +), 1 excited species
(He*) and electrons and 32 reactions, with the key
reactions proposed in [4]. On the inner surface of the
dielectric tube, we take into account the deposition of
surface charges during the discharge propagation and we
assume a secondary emission coefficient of 0.1 at the
dielectric interface. In this work, we take into account a
simple photoionization model as proposed in [6]. It is
assumed that the ionizing radiation emitted by excited
helium species is absorbed by nitrogen molecules of the
admixture and ionizes them. Here the ionizing radiation is
assumed to be proportional to the excitation rate of
helium atoms by electron impact. To compare with
experiments, we simulate the discharge dynamics for
different quantities of admixtures of N 2 in helium from
10ppm to 10 000ppm (1%). Then we have pre-calculated
with BOLSIG+[6] the electron transport coefficients and
the electron impact rate coefficients for He-N 2 mixtures
with 10, 1000 and 10000 ppm of N 2 . We have observed
that electron transport coefficients depend very weakly on
the amount of N2 admixture. Conversely, the electron
impact ionization coefficient and the excitation coefficient
of He* depend more significantly on the mixture
composition. In particular, we note that the ionization
2
4. Results
Figure 3 shows the velocity of the ionization front
propagation inside the capillary, between the two
downstream positions 5 cm and 8cm, versus the
N 2 admixtures.
Fig. 3.
Average velocity of the discharge versus
N 2 admixture. The average velocity is calculated here
between 5 cm and 8 cm downstream positions.
It is observed that a significant ionization front velocity
increase is obtained up to 0.7 % N 2 admixture range in
comparison with that measured for helium only fed PG,
while for higher amount of N 2 , the velocity is gradually
decreasing. Both the separate or combined roles of
electric field at the ionization front tip and ionization level
of the plasma column connecting this front with the
powered electrode of PG have been considered and
questioned to be responsible for the plasma properties
modification versus the N2 admixture. The model
calculations, experimental spectra and electric field
characterizations have been key elements to clarify the
role of each of the two plasma properties likely to
influence discharge behaviour.
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Figure 4 shows the distributions of the He* density at t
=1.5μs for 10, 1000 and 10 000ppm of N 2 . We have
checked that the ignition time of the discharge in the tube
depends very weakly on the amount of N2 in the He-N2
mixtures studied in this work.
Fig. 4. Cross-sectional views of the He* density at t
=1.5μs for 10, 1000 and 10 000 ppm of N2 for an applied
voltage of 16kV.
Figure 4, shows that for the same time, the location of
the ionization front and the He* structure shape depends
on the amount of N2. These results clearly confirm the
influence of the discharge kinetics on the discharge
dynamics. It was deduced from model calculations that
main variation originates from plasma column ionization
degree rather than from very minor EF amplitude
difference with respect with the N2 amount in gas
mixture.
Emission spectra documented in figure 5, clearly
indicate that severe modulations of N2/N2+ ratio is
induced depending on the N 2 admixture. The highest
velocity, obtained for 0.2 % N 2 admixture (fig.3)
coincides with the higher production of N 2 +* lines in the
penning mixtures while for higher N 2 amounts, electron
impact excitation of N 2 second positive system becomes
predominant and consistent with a lower ionization
degree of the plasma. While no detailed analysis has for
yet been performed, one can note that the higher the
nitrogen admixture is, the stronger the nitric oxide gamma
system production occurs. It is worthwhile indicating that
the impact of N 2 admixtures on helium lines production
addressed in section 3, is in good agreement with the
experimental evolution of helium lines in figure 5, where
the 706 nm He line could for instance be followed,
revealing an optimum level for moderate N2 admixtures
(i.e. around 0,2%).
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Fig. 5. Plasma emission spectra for helium only (top),
0.1 % (middle) and 1% (bottom) N 2 admixtures.
The key role of penning reaction on the discharge
features has been checked from model calculations, as
presented in figure 6, comparing electron density patterns
with or without accounting for penning reactions in the
model kinetic scheme. When penning reactions are
artificially suppressed, the plasma velocity is clearly
slower and the mean ionization degree of the plasma
column is strongly decreased. Combining results from fig.
5 and fig.6, and electrostatic calculations [8], there is
strong indication that main contender for specific
properties of He/N 2 discharges as a function of nitrogen
admixtures, is the ionization level of the plasma column,
strongly correlated with the penning reactions
involvement in the plasma kinetics. The hypothetical role
of electric field was investigated from preliminary
measurement using a specially designed Pockels effect
based probe supplied by Kapteos’ company.
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Fig. 6. Cross-sectional views of the electron density at t
=1.5μs for 1000ppm of N2 for an applied voltage of
16kV, with and without Penning reactions.
Figure 7 presents the temporal evolutions of
longitudinal and radial electric field components
measured 3 cm downstream the HV electrode, and 2mm
away from the outer surface of the quartz capillary, i.e. in
a non intrusive, non perturbative setup dealing with
discharge propagation.
Both electric field components present very similar
temporal profile and amplitude for either helium only or
helium with 0.1% N 2 admixture. This indicates the very
minor role, in our experimental conditions, of the electric
field in the evolution of plasma front propagation velocity
with or without nitrogen admixtures. This result was
confirmed from electrostatic calculations, not shown in
this work, performed in the model setup presented in
section 3. More detail on the electric field profile analysis
is given in [8].
penning reactions, on the ionization degree of the plasma
column connecting the ionization wave front with the
powered electrode was shown to induce severe
modification of the plasma front velocity versus the
nitrogen admixture. Balance between the different plasma
excited species, revealed by helium lines, nitrogen neutral
and ionic bands and lines and nitric oxide bands in the
UV range, was shown to be strongly correlated with the
nitrogen admixture and may afford new opportunities for
biomedical applications. Conversely, the minor role of
electric field variation induced by nitrogen amount is
reported from preliminary non intrusive measurement and
confirmed through electrostatic calculation. The
consideration of a reduced kinetic reaction set is shown to
be relevant for helium based plasma jet modeling,
providing consistent analysis of the plasma ignition and
propagation in dielectric tubes.
6. Acknowledgments
This work is supported by ANR BLAN 093003
PAMPA, APR PLASMEDNORM, TD is supported by
MENSR.
7. References
[1] D.A Lacoste et al Plasma Sources Sci. Technol. 23
062006 (2014)
[2] E. Robert et al Plasma Sources Sci. Technol. 21
034017 (2012)
[3] T. Darny et al Proc. XXth Inter. Conf. On Gas
Discharges and their applications, pp 578 (2014)
[4] J.M. Pouvesle et al, J. Chem. Phys. 77 pp 817-825
(1982)
[5] J. Jansky et al Plasma Sources Sci. Technol 23 025001
(2014)
[6] G.J.M. Hagelaar and L.C. Pitchford, Plasma Sources
Sci. Technol. 14 722-733 (2005)
[7] GV. Naidis, J. Phys. D: Appl. Phys. 43 (2010) 402001
[8] T. Darny et al, ISPC22
Fig. 7. Longitudinal (Ex) and radial (Er) electric fields
collected 3cm downstream the PG powered electrode for
helium only and 0,1 nitrogen admixture.
5. Conclusion
Experimental and numerical analysis of plasma
propagation and plasma properties of helium based
atmospheric pressure discharge in long dielectric tubes
with nitrogen admixture has been documented and
discussed. The key role of kinetic processes, especially
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