22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
An atmospheric argon plasma jet and its interaction with a dielectric surface
G. Bauville1, S. Pasquiers1, N. Blin-Simiand1, B. Bournonville1, Et. Es-sebbar1, M. Fleury1, P. Jeanney1 and
J. Santos-Sousa1
1
Laboratoire de Physique des Gaz et des Plasmas, CNRS, Univ. Paris-Sud, FR-91405 Orsay, France
Abstract: The propagation of a DC-pulsed argon plasma jet in the surrounding wet air, and
its interaction with a glass plate placed perpendicular to the jet trajectory, is studied by
means of fast imaging. We observe that the dielectric surface plays an important role in the
spatio-temporal characteristics of the discharge, allowing the creation of a diffuse plasma
instead of a filamentary one for a short distance between the nozzle and the plate.
Keywords: plasma jet, argon, dielectric surface
1. Introduction
Low temperature atmospheric plasma jets using rare
gases (He, Ar) are under study for several years [1], and
for different types of applications (e.g., material
processing, biomedicine). In particular, these microplasmas can be used for chemical analysis with mass
spectrometry, for example in the detection of low volatile
organic molecules adsorbed on surfaces [2].
The
description of the interaction between the plasma jet and
the surface is an important issue in the study of such
applications. In the present experiment, an argon jet
propagates in ambient air and meets a glass surface . The
effect of the solid obstacle on the spatio-temporal
characteristics of the discharge is described.
2. Experimental set-up and electrical parameters
The plasma jet is generated from a dielectric barrier
discharge (DBD) made of a quartz tube with inner and
outer diameters of 1.7 mm and 4.3 mm, respectively.
A grounded copper foil electrode, 1 cm long and 100 µm
thick, is wrapped around the external side of the dielectric
at a distance of 5 mm from the nozzle. A capillary
electrode, with an inner diameter of 1.2 mm, is glued
inside the dielectric tube at a distance of 10 mm from the
nozzle.
The DBD is driven by high-voltage pulses produced by
a homemade power supply, and the discharge electrical
parameters are measured using adapted probes connected
to a digital oscilloscope. The spatio-temporal evolution
of the plasma emission is investigated using an ICCD
camera (Princeton Instrument PIMAX-3) equipped with a
UV-macro lens (EADS Sodern CERCO2178 F/2.8,
spectral range 220-900 nm). The optical axis of the
camera is placed perpendicular to the axis of the
discharge tube.
An example of an electrical recording is given in Fig. 1,
for a peak value of the applied voltage to the DBD of
6.0 kV at a repetition rate of 20 kHz, and for an argon
flow of 0.7 l/mn NTP; the HV-pulse duration is 215 ns
(FWHM). For these conditions, the electrical energy
deposited in the discharge is 15.6 µJ/pulse. The moment
of the discharge breakdown is marked by the arrow “A”
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in Fig. 1; the measured current before this instant is due to
the circuit capacitance (2.3 pF).
Fig. 1. Time evolution of current and voltage for an
applied HV-pulse of 6.0 kV at 20 kHz. Arrows A, B,
C: see text.
3. The free jet case
In Fig.1, the arrow “B” indicates the moment when a
streamer begins to propagate outside the capillary tube.
Typically, the propagation speed is 1.5x105 m/s at 200 ns
after the discharge peak current. Without an obstacle on
the path of the jet, streamers are able to propagate at a
maximum distance of 30 mm from the nozzle.
Fig. 2 shows an ICCD camera recording at instant
marked by the arrow “C” in Fig. 1, taken with an
exposure time of 20 ns.
The presented picture
corresponds to the accumulation of plasma emission over
10 successive HV-pulses. A group of high intensity
streamer heads is seen at 1.75 cm, and very small
emission intensity is detected behind them, up to the
discharge tube. A more detailed study of the spatiotemporal distribution of the plasma shows that, for one
HV-pulse and only after 2 or 3 mm of propagation, the
discharge outside the tube appears filamentary with some
branching phenomena.
1
times: first, case a/, when the streamer just reached the
surface, and second, case b/, 40 ns after the case a/.
Logically, the surface plasma appears circular to the
eye. For the chosen experimental parameters, the diameter
of this plasma reaches about twice the external diameter
of the capillary (see Fig. 4b). Also, ICCD measurements
emphasize that the plasma emission is much more intense
at the surface than in the gas volume separating the nozzle
and the surface, though less homogeneous. This may be
interesting for desorption of heavy organic molecules that
could be adsorbed on the surface. Work is in progress on
this subject.
Fig. 2. Plasma emission captured by the ICCD
camera (exposure time 20 ns) at the instant indicated
by the arrow “C” in Fig. 1 (grey level intensity
converted into false colours). Hachured area: quartz
capillary.
4. Effect of a dielectric obstacle
When a glass plate (1 mm thick, not grounded) is
placed perpendicularly to the trajectory of the argon jet,
the streamer impacts the surface and the plasma spreads
over it. This can be seen in Fig. 3, which shows an ICCD
image for a single HV-pulse and for a distance between
the nozzle and the plate of 15 mm (with the same
parameters as those of Figs. 1 and 2). For such a distance,
the discharge characteristics are similar to those measured
for the free jet, except for the development of the plasma
on the surface. Especially, branching phenomena appear.
Fig. 3. Plasma emission for a glass plate at 15 mm
from the nozzle. Same parameters as those of Figs. 1
and 2.
If the distance between the tube exit and the plate is
reduced to less than 10 mm, with all other things being
equal, the spatial distribution of the plasma appears more
homogeneous in the small gaseous gap; branching
phenomenon disappears. This effect is illustrated in Fig.
4, showing camera recordings (integrated over
10 HV-pulses) for a distance of 5 mm, at two different
2
Fig. 4. Plasma emission for a plate positioned at 5
mm, measured shortly after the discharge breakdown.
Same parameters as those of Fig. 3. Cases a/ and b/:
see text.
A plasma emission is still measured well after the
discharge breakdown, both on the surface and in the gas
volume. Fig. 5 presents ICCD measurements at 350 ns
after the peak current, case a/, and at 750 ns, b/.
Fig. 5. Plasma emission well after the discharge
breakdown. Cases a/ and b/: see text.
This emission should be related to the long life time of
Ar electronic metastable states created by the discharge,
these states being lost through quenching by molecules of
the surrounding gases (N 2 , O 2 , H 2 O).
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5. Acknowledgments
Authors are grateful to V. Puech for valuable
discussions. This work is supported by the French
“Agence Nationale de la Recherche” under the
PLASPAMS project (grant No ANR-2013-SECU-000203).
6. References
[1] X. Lu, M. Laroussi and V. Puech. Plasma Sources
Sci. Technol., 21, 034005 (2012)
[2] G. Harris, A. Galhena and F. Fernandez. Anal.
Chem., 83, 4508 (2011)
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