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Characterization of double dielectric barrier discharge

22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Characterization of double dielectric barrier discharge and microwave plasma
jets in argon at atmospheric pressure for biomedical applications
F. Judée1,2, N. Merbahi1,2, G. Wattieaux1,2, J.-M. Plewa1,2 and M. Yousfi1,2
Université de Toulouse; LAPLACE-UMR5213, 31062 Toulouse, France
CNRS; LAPLACE-UMR5213, 31062 Toulouse, France
Abstract: A microwave and a double DBD plasma jets are compared by using optical
emission diagnostics to determine their rotational and excitation temperatures, electron
density, UV-C irradiance and atomic O concentration.
Keywords: plasma devices, optical spectroscopy, density measurement, UV-C irradiance
1. Introduction
Low temperature plasmas ejected in open air are
nowadays successfully used in many biomedical
applications due to their ability to produce various active
species at quasi-ambient plasma temperature.
It is noteworthy that many plasma jet setups use helium
carrier gas because it is easier to ignite helium plasma that
requires less energy. However, in order to generate
plasma jet with different active species, it is interesting to
use other carrier gases such as argon which is renewable
and less expensive than helium. This is why the present
work deals with low temperature plasma jets using argon
carrier gas. More precisely, two different setups will be
considered. The first one is a double DBD induced plasma
jet whereas the second one is a microwave (MW) induced
plasma jet. Both jets are optically diagnosed in order to
determined their plasma parameters, UV-C irradiance and
atomic O concentration.
2. Experimental set-up
a. Double DBD plasma jet
The double DBD plasma jet is provided through a
double cylindrical dielectric barrier configuration (see
Fig. 1.a). This arrangement is useful to prevent arcing
between the (inner) powered electrode and a plasma
exposed sample which is particularly unwanted for
biomedical applications. The device can operate at
atmospheric pressure in argon. The (inner) high voltage
(HV) electrode is made of a 2 mm diameter copper rod
encapsulated in a quartz tube having an external diameter
of 4 mm. This HV electrode is set in the axis of a quartz
tube having 8 mm outer diameter and 1 mm thickness.
There is a 1 mm gap between the dielectric of the HV
electrode and the quartz tube. A piece of aluminum foil
(17 mm wide) is ground connected and wrapped around
the external quartz tube at about 6 mm from the tube
Ar flow (4.5 purity) is injected into the tube through a
mass flow controller at a rate of 1 l/min. The inner
electrode is powered by a high voltage pulse generator
delivering a 1 µs length voltage pulse at 9 kV with a
repetition rate of 9.69 kHz. The power consumed by the
double DBD device under these conditions is estimated at
about 5.1W from current/voltage waveforms.
b. Microwave Setup
A Sairem Surfatron set-up is used to create the argon
plasma jet (see fig. 1.b). It consists on injecting
microwave energy (40 W, 2.45 GHz) in a cavity
surrounding a cylindrical quartz tube with an internal
diameter of 4 mm. Argon with a purity grade of 4.5 flows
inside the tube. The flow is monitored by a mass flow
controller at 1 l/min. The gas is partially ionized in the
cavity leading to the formation of a plasma and to the
propagation of surface microwaves at the interface
between the Ar plasma and the quartz tube. These waves
ionize the gas when propagating leading to the formation
of a plasma plume or a plasma jet at the tube outlet. A
second mass flow controller allows compressed air to
flow along the external surface of the tube at 22 l/min in
order to cool down the Surfatron and the plasma jet when
mixes with it after the tube outlet. More information can
be found elsewhere [12].
Ar flow
(1 l/min)
quartz tube
µwave generator
(40W, 2.45 GHz)
compressed air
(22 l/min)
plasma jet
Fig. 1. Schematic of the experimental setups.
c. Optical emission Spectrum and UV-C irradiance
Optical emission spectroscopy is carried out by using a
0.75 m focal length imaging spectrophotometer (Acton
Spectra SP 2750, in the Czerny Turner configuration)
equipped with a 2400 grooves per meter grating for the
UV region of the electromagnetic spectrum and a 1800
grooves per meter grating for the visible and near infrared
region of the electromagnetic spectrum. The detecting
device is a CCD camera (PIXIS - 100, 1340 x 100
imaging array of 20 µm x20 µm pixels). The plasma
radiations are guided through an optical fiber (UV-silicon
LG- 455-020-3) onto the entrance slit (200 µm wide) of
the spectrometer. The spectral domain lies within 200 nm
and 920 nm. Lateral radiations of the plasma jets are
collected by a 2x magnification optical system offering a
1 mm spatial resolution. It is mounted on a sliding stand
to perform a scan along the plasma jet axis (see fig.1.a).
An optical high pass filter is used for the visible range
analysis in order to prevent any perturbation of the visible
range spectra due to the unwanted grating second order of
diffraction of UV radiations. Perpendicular UV-C (190280 nm) irradiance (relatively to the axis of the jet) is
determined by using a photodiode which is only sensitive
to radiations between 225 and 280 nm (SG01S-C18, sglux
GmbH). The photocurrent is determined by short
circuiting the photodiode with a transimpedance amplifier
circuit having a conversion factor of 1.82 V/nA. The
active area of the photodiode is 0.06 mm2 and its quantum
efficiency is about 0.1 A/W at 265 nm.
3. Results
a. Optical emission and plasma temperatures
The optical emissions spectra obtained in both plasma
jet configurations are quite similar thus only the emission
spectrum in the double DBD configuration is given in
figure 2. The optical emission spectra of double DBD
plasma jet is dominated by emission of OH
(A2∑+ u →X2∏ g ), N 2 (C3∏ u →B3∏ g ), Ar, and O species.
This is in accordance with the literature in the case of Ar
plasma jets [12, 15]. NO bands in the UV-C range
between 225 and 245 nm and the H α Balmer line at 656
nm are also present with a weaker intensity. The same
spectrum is observed in the case of the Ar microwave
plasma jet with the H β Balmer line peaking at 486.14 nm
in addition. Rotational temperature (gas temperature) is
estimated by fitting the simulated spectrum of OH(AX)
on the experimental spectrum between 306 and 310 nm.
The rotational temperature of the MW plasma jet is higher
than the rotational temperature of the double DBD plasma
jet (see tab.1). It is due to the fact that in the case of MW
plasma random occurrence of filamentary structures are
observed at the tube outlet. According to Motret et al [19]
the OH rotational temperature is strongly correlated with
the gas temperature in the streamer region. It is worth
noting that the compressed air used to cool down the
Surfatron plasma jet manages to keep the plasma jet
temperature at about 320 K that is to say in the same
range of the double DBD plasma jet temperature.
Excitation temperature Texc is obtained by using neutral
argon emission lines (4d-4p, 6s-4p, 4p-4s) under
Boltzmann approximation [12]. The Ar excitation
temperature with the double DBD is slightly higher than
the Ar excitation temperature obtained with the
microwave plasma jet (see tab.1).
b. Electron density
The electron density was estimated from the Stark
broadening of hydrogen lines. The estimation of the Stark
broadening of the lines has been obtained by comparing
the observed full width at high maximum (FWHM) of the
lines with their theoretical FWHM which have been
computed by the convolution of the instrument
broadening, the Doppler broadening, the Stark broadening
and the Van der Waals broadening assuming a gas
temperature of 380 K and a pressure of 105 Pa. More
information about the computation of these broadenings
can be found in [12] and [21].
Fig. 2. Optical emission spectra of the double DBD
plasma jet between 200 and 440 nm (a) or 680 and 880
nm (b) in the case of Ar flow of 1 l/min, repetition rate of
9.69 kHz, pulse duration of 1 µs, voltage magnitude of 9
kV, and the viewing position of the spectrometer
corresponding to the tube outlet.
Table 1. Rotational and excitation temperatures in the
axis of the Ar plasma jets measured in tube outlets.
MW Plasma jet
DBD plasma jet
Trot (K)
800 ±100
380 ±30
Texc (K)
4200 ±200
4780 ±200
Table 2. Electron density in the axis of the plasma jets up
at a distance of about 15 mm from the gas carrier tube
outlet. Estimation from the Stark broadening of H α and
H β lines.
MW jet
DBD jet
H β (λ=486 nm)
H α (λ=656 nm)
H β (λ=486 nm)
H α (λ=656 nm)
Stark Broadening (nm)
n e (cm-3)
1.1 x1014
1.4 x1014
2.5 x1015
c. UV-C radiation measurement
A thick aluminum foil was wrapped around the MW
plasma jet to act as a Faraday shield in order not to disturb
the UV-C detector circuit during the analysis. This
arrangement prevented measurements at distance shorter
than 30 mm from the gas carrier tube outlet of the MW
plasma jet. A quartz plate protected the detector from
potential arcing during the diagnostic of the double DBD
plasma jet. The occurrence of random arcing between the
body of the detector and the double DBD setup prevented
measurements at distance shorter than 15 mm from the
gas carrier tube outlet of the double DBD plasma jet.
UV-C irradiance measurements are displayed in fig. 3.
Production of UV-C irradiance by the MW plasma jet is
about twice that produced by the double DBD plasma jet.
In both cases, the UV-C irradiance level is high enough to
obtain a significant bactericidal effect after a plasma
exposure of bacteria during several minutes [24].
Fig. 4. Evolution of the argon UV-C irradiance in the
axis of the plasma jets with the distance from the gas
carrier tube outlet.
d. Atomic oxygen concentration
Oxygen atoms are well known to play an important role in
plasma biological applications [13]. Th variation of the
atomic oxygen concentration can be estimated by optical
emission spectroscopy based on the comparison of the
intensity of the argon line at 852 nm and the oxygen line
at 844 nm [12, 27]. Comparative measurements have
shown that the concentration of atomic oxygen
determined by this spectroscopic method is in the same
order of magnitude as what is given by TALIF
measurement [29].
Figure 5 shows the evolution of the order of magnitude
of atomic oxygen concentration along the plasma jets
after the tube outlet.
We observe that the atomic oxygen concentration in the
double DBD plasma jet (770 ppm) is one order of
magnitude higher than in the MW plasma jet (80 ppm) at
the tube outlet and that this difference decreases when one
moves away from the tube outlet of the jets.
Atomic oxygen concentration (ppm)
This diagnostic is usually performed with H β line (λ =
486 nm) but the intensity of this line was not observable
in the case of the double DBD plasma jet. Consequently,
the electron density in the DBD plasma jet was
determined according to the Stark broadening of H α line
(λ = 656 nm) which was easily observed. In the
meantime, as both lines were easily observed in the MW
plasma jet, we checked that the determination of the
electron density of this jet provides similar values either
from H α or H β lines (see tab. 2) and is in accordance with
Hofmann's work on helium and argon RF plasma jets [22]
and with Thomson scattering diagnostic [23].
We observed that the electron density in the double
DBD plasma jet (2.5 x1015cm-3) is one order of magnitude
higher than in the MW plasma jet (1.1 x1014cm-3). This
finding explains why H β line does not appear on the
spectrum of the double DBD plasma jet. In fact, the
FWHM of this line is about 0.4 nm according to an
electron density of 2.5 x1015cm-3. In this condition the
spontaneous emission of H β line is spread over a large
portion of the electromagnetic spectrum and the line does
not clearly emerge from the spectrum baseline of the
double DBD plasma jet.
MW plasma jet
DBD plasma jet
Distance from the gas carrier tube outlet (mm)
Fig. 5. Evolution of the atomic oxygen concentration in
the axis of the plasma jets.
4. Summaries
In this work, cold argon plasma jets are generated with
both double DBD and MW configurations at atmospheric
pressure. Several interesting information on active species
and thermodynamic plasma properties have been
obtained. Table 4 summarizes these data. The main
discrepancies between the two jets characteristics are the
electron density and the atomic oxygen concentration
which are higher by about one order of magnitude for the
double DBD plasma jet configuration at the tube outlet in
comparison to the MW plasma jet. This is probably due to
the fact that high energetic electrons are more present in
the double DBD plasma jet since an electron temperature
higher than about 7 eV [27] is generally required to selftransport the ionization wave generated in a DBD setup
whereas the electron temperature due to the action of the
electromagnetic field does not overpass about 5eV inside
the tube of the MW plasma jet and fall down to about 1eV
after the tube outlet [7]. It is also interesting to underline
the fact that the MW plasma jet produces twice as much
as UV-C radiations than the double DBD plasma jet
whereas this last contains about twice as much as O atoms
for distances greater than 1 cm away from the tube outlet.
Finally it is interesting to emphasize that the gas
temperature measured by a thermocouple is much lower
than the rotational temperature for both plasma jets and
reaches about 320 K at a distance of 1.7 cm from the tube
outlet. Consequently these two plasma jets are suitable to
be used in biomedical applications without significant
thermal effects.
Malley. Technical report, U.S. Environmental Protection
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Table 3. MW and double DBD plasma jets comparison.
Measurement taken 1.7 cm after the tube outlet.
Trot (K)
Texc (K)
Ne (x 1014 cm-3)
[O] (ppm)
UV-C (µW/cm2)
MW jet
800 ± 100
4200 ± 200
DBD jet
380 ± 30
4780 ±200
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