22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Design and preliminary characterization of an atmospheric pressure glow
discharge stamp
F.P. Sainct1, N.Y. Mendoza-Gonzalez1, N. Hordy1 and S. Coulombe1
1
Plasma Processing Laboratory - PPL, Department of Chemical Engineering, McGill University, Montréal,
Québec, Canada
Abstract: The design of a 1.5 cm2 low-power atmospheric pressure glow discharge stamp
(APDG-s) and the results of its preliminary fluid dynamic and spectroscopic
characterization are presented. Operation of the APGD-s was stable and produced an
uniform flowing afterglow over a range of continuous RF (13.56 MHz) powers (10-20 W)
and He flow rates (6-24 L/min). Computational fluid dynamics and optical emission
spectroscopy analyses showed that the He flow entrains and mixes surrounding air,
resulting in the large production of radicals.
Keywords: non-thermal plasma, flowing atmospheric pressure afterglow, computational
fluid dynamics, optical emission spectroscopy
1. Introduction
Nonthermal atmospheric pressure plasma sources are of
increasing interest for a wide range of technological
applications including plasma medicine, nanoparticle
synthesis, plasma chemical functionalization and material
deposition [1-3]. A number of configurations have been
adopted for sustaining a plasma at atmospheric pressure,
and miniature jets are amongst the most popular due to
their applicability to a number of treatments [4]. These
plasma jets feature a localized zone of treatment often not
exceeding a few mm2, and significant gas velocities. In
biomedical and surface modification applications, in
particular, the treatment of larger surfaces with low-gas
flows is often desirable. In an attempt to fill this gap, we
designed, built and tested a novel source that produces a
low-power flowing afterglow compatible with such
application constraints.
We used computational fluid dynamics (CFD)
simulations and optical emission spectrometry (OES) to
assist the design and to perform a preliminary
characterization of the Atmospheric Pressure Glow
Discharge stamp (APGD-s) operating with He as the main
plasma gas. CFD is used to reveal the main flow patterns
under cold conditions (no plasma) while spatiallyresolved relative OES is used to reveal the spatial
structure of the air entrainment downstream to the
source’s exit, as well as the distribution of the excitation
temperature in the discharge.
2. Design and characterization methods
A picture of the APGD-s is shown in Fig. 1, where it is
positioned above a sample for plasma functionalization.
The device consists of a plane-to-plane geometry
featuring a RF-powered grid-electrode located inside a
grounded aluminum support structure, and facing a
grounded grid-electrode (grid bar diameter and spacing
are 0.3 mm and 0.5 mm, respectively). The main plasma
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gas is injected above in a narrower channel (see Fig. 2).
This configuration showed to be efficient at producing
radicals and metastable species in the plasma-forming
region and transporting the species downstream. The
electrode spacing, a critical parameter in this design as it
directly impacts the applied electric field, was set to
0.8 mm. We found that an applied continuous wave RF
(13.56 MHz) power ranging from 10 to 20 W and total
gas flow rates ranging from 6 to 24 L/min provide stable
and suitable operating conditions for the intended
applications. To ensure compatibility with temperaturesensitive applications, the temperature of the gas at the
outlet was measured with a 1 mm RF-shielded
thermocouple.
Fig. 1. Downstream end of the APGD-s positioned for
surface functionalization (helium, outside diameter of the
outlet is 14 mm).
In order to support the design of the source and to help
understand the OES characterization results, the fluid
flow in the APGD-s was analysed with the commercial
CFD code ANSYS Fluent v14.5 [5]. Because of the
cylindrical symmetry of the APGD-s (Fig. 2), a
1
two-dimensional r-z computational geometry was used.
The simulations were done with a glass plate facing the
exit of the APGD-s in order to mimic plasma
functionalization conditions (plate-to-APGD-s distance of
10 mm). An extended area (10 mm in z, and 30 mm in r)
was included for the analysis of the effect of surrounded
air. The plasma formation and effect of the grid
electrodes on the flow pattern were neglected in this
preliminary study.
From the CFD modelling results, streamlines, which
indicate the direction of the fluid flow and existence of
recirculation zones, are presented to analyse the flow
pattern. Fig. 2 shows the streamlines at 20 L/min (He).
Fig. 3. Schematics of the APGD-s and OES setup.
Fig. 2. Model geometry of the APDG-s and resulting He
streamlines. He flow is set at 20 L/min. The APDG-s is
placed 10 mm away from the glass, and the simulation
domain radius is 30 mm wide.
The combined Navier-Stokes and non-reactive species
transport equations were solved in the 2D computational
domain composed of 80 000 control volumes. As
turbulence can be present due to the two geometry
expansions adopted for this design, a standard k-epsilon
model was used. The density of the helium-air mixture
was calculated with the incompressible ideal gas equation.
The boundary conditions are the imposed mass flow rate
of helium at the inlet, the non-slip conditions at the wall,
and atmospheric static pressure at the outlet. A constant
temperature of 300 K was used for the material properties.
OES measurements were performed using a 25 cm
focal length spectrometer (Acton SpectraPro 2750)
equipped with a PiMax camera (Princeton Instruments PIMAX 1024x256). The 1200 grooves/mm grating, blazed
at 300 nm, provided a spectral resolution of 0.22 nm. The
light emitted by the discharge was collected with a system
of two lenses with focal lengths of 50 mm and 100 mm,
respectively, as presented on Fig. 3. With this optical
setup, the magnification is 0.5. A 200 µm diameter
optical fiber, located at the focal point of the imaging
lens, was connected to the entrance slit of the
spectrometer providing a spatial resolution of 400 µm in
the radial direction.
3. Results
2
A first recirculation zone is observed as the gas enters
the powered electrode zone because of the geometry
expansion. A second recirculation zone is observed in the
open area between the APGD-s and glass substrate. This
recirculation pattern is responsible for the entrainment of
surrounding air into the inter-electrode zone. Fig. 4
shows the radial distribution of the air mass fraction at the
grounded electrode location. Obviously, a larger amount
of air is observed near the edges but a significant amount
finds its way to the centerline (~30%). However it is
important to note that the magnitude of the air mass
fraction is likely to change with the addition of the wire
grid electrodes to the model, as the presence of the two
electrode grids breaks up the main gas flow into smaller
cells, which may promote mixing with the entrained air.
From experimental testing of the APGD-s with a
combined He/O 2 flow we can estimate the maximum
mass fraction of oxygen to be less than 0.5 %, since the
injection of 0.5 % of O 2 with helium significantly
changes the plasma behaviour.
Fig. 4. Simulated mass fraction of air at the grounded
electrode location (z = 13.6 mm). The symmetry of the
profile is a consequence of the cylindrical symmetry
chosen for the simulation. The full profile is provided to
simplify the comparison with Fig. 2.
The OES emission spectrum presented in Fig. 5, acquired
4 mm from the center of the discharge, clearly shows that
surrounding air is entrained and mixes with the main He
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stream to produce emission bands from OH, N 2 , N 2 + and
emission lines from atomic O and He.
Fig. 5. Emission spectrum of the He plasma, 4 mm from the center, integrated over 100 ms and 100 accumulations.
Results presented in Fig. 6 reveal a rapid decrease of
the gas temperature (measured with the thermocouple)
with increasing gas flow and decreasing RF power. The
observed gas temperature range is compatible with most
heat-sensible applications.
Fig. 6. Gas temperature evolution with He flow rate and
applied RF power.
The gas temperature obtained with the thermocouple
was compared to the plasma temperature estimated by
OES, using the OH, N 2 and N 2 + rotational band emissions
over the 300-400 nm range. The Specair software [6, 7]
was used to model the experimental OES results and to
determine the corresponding rotational and vibrational
temperatures. The OH emission spectrum presented in
Fig. 6 was well reproduced by the simulation using a
rotational temperature relatively close to the gas
temperature (400 K), and a high vibrational temperature
(5000 K).
This high non-equilibrium temperature
situation could result from the collision with high-energy
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helium metastables [8]. The same situation is observed
with the N 2 (C) and N 2 + emission bands presented in
Fig. 8, with a low rotational temperature and a high
vibrational temperature.
Fig. 7. OH molecular emission spectrum in the center of
the discharge at 20 L/min He and 20 W, along with the
best fit obtained with Specair.
The radial distribution of the optical emission of each
species has been studied from the centerline of the
APGD-s to its side. The results presented in Fig. 9 show
a nearly constant emission intensity for helium across the
discharge, whereas the optical emission from air-related
species features a strong gradient, with a weaker intensity
in the center and stronger emission on the sides. The
optical emission from the air-related species increases in
the same proportion for all species compared to He
emission, which observation suggests a greater air
fraction on the side of the APGD-s than in the center.
These results are in qualitative agreement with the
simulated mass fraction of air shown in Fig. 4.
3
The APGD-s source has been analyzed using CFD and
OES diagnostics. CFD was used to better understand the
basic flow pattern in the outlet area as well as the mixing
of the plasma-forming gas with surrounding air. The
simulations showed that a recirculation pattern forms due
to the sudden expansion of the geometry and presence of
a substrate. This recirculation introduces a small amount
of surrounding air in the inter-electrode zone, thus leading
to the production of radicals from air. These results are in
qualitative agreement with the OES results, which
revealed air-related species emission in the plasmaforming zone. A more detailed model considering the
plasma-forming zone and a realistic representation of the
grid electrodes will be developed to optimize the APGD-s
design for specific applications.
Fig. 8. Emission spectrum in the center of the discharge
at 20 L/min He and 20 W, with the Specair fit of N 2 and
N 2 + emission.
Fig. 9. Radial distribution of the emission intensity of
selected at 20 L/min and 20 W.
4. Conclusion
4
5. Acknowledgements
The authors wish to thank C. Szalacsi for his technical
assistance. The project was funded by the National
Sciences and Engineering Research Council of Canada
(NSERC), the Fonds de recherche du Québec – Nature et
technologies (FRQNT) and McGill University.
6. References
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(2012)
[4] J.S. Sousa, et al. J. Appl. Phys., 109, 1-8 (2011)
[5] ANSYS FLUENT 14.5, Theory Guide. (Ansys,
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[6] C.O. Laux, T.G. Spence, C.H. Kruger and
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[7] www.specair-radiation.net
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