22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Precise calorimetric measurements in dielectric barrier discharges (DBD) at
atmospheric pressure
M. Archambault-Caron, H. Gagnon, B. Nisol, K. Piyakis and M.R. Wertheimer
Polytechnique Montréal, Groupe des Couches Minces (GCM) and Department of Engineering Physics, P.O. 6079,
Station Centre-Ville, Québec, H3C 3A7, Canada
Abstract: A special dielectric barrier discharge (DBD) cell has been used to carry out
precise measurements of electrical energy, 𝐸g , dissipated per a.c. voltage cycle in helium
atmospheric pressure glow discharges (APGD). Aided by multiple thermometric probes, a
surprisingly accurate energy balance has been achieved, to our knowledge, for the first
time.
Keywords: helium APGD, discharge energy, thermometry, energy balance
1. Introduction
The cold nature of atmospheric-pressure (AP) dielectric
barrier discharge (DBD) plasmas is of key importance
when they are put to use in many of their current
applications, for example in biomedical (therapeutic)
contexts [1, 2]. We refer to recent work by Wertheimer
et al.[3], where they confronted their own and other
workers’ gas temperature (𝑇) measurements carried out
by optical emission spectroscopy (OES) with 𝑇
measurements based on the use of fiber-optic
thermometers. The latter were developed for use in
conditions involving high voltages and/or intense
electromagnetic fields, where conventional (e.g.,
thermocouple) instrumentation should be disqualified.
Surprisingly, even to this day, only a few workers in
plasma processing science use fiber-optic thermometers,
even though it was clearly shown that 𝑇 based on OES
invariably overestimates the true gas temperature, often
very grossly [3, 4]. Beside knowledge of 𝑇, it is also
clearly important for the operator of a (AP) plasma
system to know the exact amount of electrical power, 𝑃,
delivered to the plasma and hence to a substrate. In this
present work, a specially designed dielectric barrier
discharge (DBD) cell has been used to carry out precise
measurements of electrical energy, 𝐸g , dissipated per
discharge cycle of the applied a.c. voltage, V a over the
frequency range 5 ≤ 𝑓 ≤ 50 kHz. Twin pairs of several
different dielectric materials (2.54 cm diameter discs,
thicknesses = 2.0 or ca. 0.1 mm with relative
permittivities between 2.1 ≤ 𝐾 ′ ≤ 9.5) were used as
dielectric barriers in DBDs of four different gases: He,
Ne, Ar and N 2 . Much of this research, the part that is
reported in this presentation, relates to the study of
atmospheric pressure glow discharge (APGD) plasma in
flowing helium; five separate thermometers (including
two fiber-optic probes in sensitive locations) have enabled
us to perform a detailed calorimetric (heat balance)
investigation in He APGD, believed to be the first of its
kind.
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2. Experimental Methodology
Fig. 1 shows a schematic diagram of the DBD cell,
thermometric and electrical probes, and other associated
components. The discharge chamber is an hermetic Pyrex
glass cylinder (inner diameter = 32 mm) with metallic end
plates, connected to a gas supply and flow-control unit
with which gases can be maintained at a constant flow
rate, 𝐹 (in liters per minute, L/min), slightly above
atmospheric pressure. Small holes in the Pyrex vessel
wall permit insertion of thermometric probes at
appropriate locations, namely in the discharge gap
(𝑇plasma ) and in the effluent gas stream (𝑇gas ). The two
fiber-optic thermometric probes (OpSens Inc., model
OTG-F, Quebec City, Canada) in these two critical
locations were based on the T-dependence of the optical
band gap of GaAs. The reader is referred to a recent
article by Gagnon et al. [5] for information about the
electrical measurement system, and some of the details
about the refined algorithms that permit rapid data
acquisition and processing.
Ttop
Gas out
HV probe
Oscilloscope
Gap
LV probe
PC
Tgas
Tglass
Tplasma
Telec
50 Ω
Gas in
Metal
Dielectric
Micrometer
T Thermocouple
screw
T Optical Fiber Probe
Fig. 1. Schematic diagram of the DBD cell, thermometric
and electrical probes, and other associated components.
1
3. Results, Discussion and Conclusions
Calorimetric calculations have been carried out on the
basis of T measurements accumulated at ½-minute
intervals by the five thermometric probes during a
He APGD experiment at f = 50 kHz, V a = 780 V rms that
lasted for 20 minutes, and during which E g was found to
remain rigorously constant at 18.3 ± 0.2 µJ per period of
the applied a.c. voltage. For these experiments, the gas
entered at the bottom of the reactor at a flow rate, 𝐹, of
3.1 L/min and ambient (laboratory) temperature,
𝑇 = 295 K. The plasma temperature, T plasma , was found to
be a maximum of 5.5 K above ambient, i.e., merely
300 K. This stands in striking contrast with OES-based
values that are reported to be at least 360 K, several tens
of K higher [4].
Power delivered by the plasma, 𝑃1 , at 5 × 104 periods
per second, was
𝑃1 = 18.3 × 10−6 × 5 × 104 = 0.915 W.
The energy budget corresponding to an arbitrarily chosen
time, 𝑡 = 1000 s (16.7 min) is represented by the vertical
line in Fig. 2, where the cumulative value of electrical
energy dissipated was 𝐸1 = 𝑃1 × 𝑡 = 915 J. The five
thermometers permitted the various energy components
E 2 to E 5 to be calculated and this, in turn, allowed the
following overall energy balance to be performed:
∑ 𝐸𝑖 = 𝐸2 + 𝐸3 + 𝐸4 + 𝐸5 = 66.7 + 415.6 + 230 +
190 = 902.3 J;
∆𝐸 = 𝐸1 − ∑ 𝐸𝑖 = 12.7 J.
In Fig. 2, the lowest (Energy Balance) curve
representing ∆E reveals that the astonishing (+1.5%)
numerical agreement was coincidental. Nevertheless,
even the largest deviation (∆𝐸/𝐸1 ~ -20%) was small
enough to confirm that the procedure and results were on
sound footing. We do not have the space here to go into
further details regarding Fig. 2, but these will be reported
in detail elsewhere [6]. In that same pending report, we
show that E g depends on the energy stored in the DBD
cell’s capacitance, C, where
Fig. 2. Plot showing the various contributions to the
overall energy balance, ∆𝐸 = 𝐸1 − ∑ 𝐸𝑖 , versus time.
The lowest curve represents ∆𝐸.
4. Acknowledgements
The authors are grateful for financial support from the
Natural Sciences and Engineering Research Council of
Canada (NSERC), namely research grants to M.R.W. and
an undergraduate research scholarship to M. A.-C.
Financial support was also provided by the Fonds de
recherche du Québec – Nature et technologies (FRQNT)
via Plasma Québec. Skilled technical help by Yves
Leblanc and Francis Boutet is also gratefully
acknowledged. The authors thank Dr. Claude Hudon
(Hydro Quebec Research Institute, IREQ) for loan of the
OpSens thermometers.
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𝐶=
and
𝐶die 𝐶gap
𝐶die +𝐶gap
=
𝐸 = ∫ 𝑃𝑃𝑃 =
𝜀0 𝐴𝐾 ′𝐾1′
(1)
𝐾 ′ 𝑑+𝐾1′ 𝐷
1 𝜀0 𝐴𝐾 ′𝐾1′
�
2 𝐾 ′𝑑+𝐾1′ 𝐷
� 𝑉2
(2)
Eqs. (1) and (2) pertain to a planar DBD cell of area 𝐴,
consisting of a dielectric (thickness 𝐷, relative
permittivity 𝐾′) and a gas gap (spacing distance 𝑑, relative
permittivity 𝐾1′ ̴ 1.0). We further demonstrate that eq. (2)
permits inter-laboratory comparisons to be made, which
have revealed good agreement with power data presented
by other authors for He APGD [7, 8], and which enable
scale-up of reactors.
2
5. References
[1] G. Fridman, G. Friedman, A. Gutsol, A.B. Shekhter,
V.N. Vasilets, A. Fridman, Plasma Processes and
Polymers, 5, 503 (2008)
[2] K. Landsberg, C. Scharf, K. Darm, K. Wende, G.
Daeschlein, E. Kindel, et al., Plasma Medicine, 1,
55 (2011)
[3] M.R. Wertheimer, M. Ahlawat, B. Saoudi, R.
Kashyap, Applied Physics Letters, 100, 201112
(2012)
[4] A. Berchtikou, J. Lavoie, V. Poenariu, B. Saoudi, R.
Kashyap, M.R. Wertheimer, IEEE Transactions on
Dielectrics and Electrical Insulation, 18, 24 (2011)
[5] H. Gagnon, K. Piyakis, M.R. Wertheimer., Plasma
Processes and Polymers, 11, 106 (2014)
[6] M. Archambault-Caron, H. Gagnon, B. Nisol, K.
Piyakis, M.R. Wertheimer, Plasma Sources Sci.
Technol. (submitted)
[7] P. Decomps, F. Massines, C. Mayoux, Acta Physica
Universitatis Comenianae, 35, 47 (1994)
[8] G. Nersisyan, W.G. Graham, Plasma Sources
Science and Technology, 13, 582 (2004)
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