22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Dielectric barrier discharges: evidence for mobile surface charge
F.J.J. Peeters1,2, R.F. Rumphorst2 and M.C.M. van de Sanden1,2
1
Dutch Institute for Fundamental Energy Research (DIFFER), De Zaale 20, 5612 AJ Eindhoven, the Netherlands
2
Eindhoven University of Technology, Department of Applied Physics, P.O. Box 513, 5600 MB Eindhoven,
the Netherlands
Abstract: Filamentary DBDs can be described by an equivalent circuit which assumes
discharging occurs uniformly across the surface. This is counter-intuitive, since DBDs
actually consist of many spatially and temporally separated, transient microdischarges.
Studying the electrical characteristics of DBDs more closely, we find that individual
filaments are always roughly equivalent; irrespective of the phase or amplitude of the
applied voltage. A mechanism consistent with all these observations is suggested; namely
the constant redistribution of surface charge on the dielectric.
Keywords: dielectric barrier discharge, surface charge, electrical diagnostics
1. Introduction
Dielectric Barrier Discharges (DBDs) have been
studied for over a century, with increasing interest in
recent years in the areas of materials processing, plasma
medicine and gas conversion. DBDs in filamentary mode
consist of many small, transient microdischarges with
diameters of ~ 0.1 mm and durations on the order of
several 10’s of nanoseconds, distributed over the
dielectric surface. Studies of filamentary discharges tend
to focus either on single filaments, experimentally or via
numerical modeling, while the link with the collective
behavior of many filaments spread across the electrode
area is rarely treated in literature [1]. T.C. Manley,
however, already pointed out in 1943 that the plasma in a
DBD can be described as having a constant burning
voltage, U b , throughout a discharge half-cycle [2]. This is
shown schematically in Fig. 1 for sinusoidal applied
voltage. In the work presented here, we take a closer look
at these electrical characteristics of DBD and their
relation to charges accumulated on the dielectric surface.
2. Experimental details
We investigate filamentary discharges under sinusoidal
excitation in detail using a miniature planar DBD in air
using alumina as a dielectric on one of the electrodes, as
shown in Fig. 1. This DBD has an electrode area of
approximately 7 mm2, which is only sufficiently large to
accommodate up to ~ 20 filaments per discharge cycle.
This has the advantage that individual filaments can be
easily distinguished in e.g. current measurements, which
is impossible with larger area electrodes. Both standard
Q-V diagrams of the discharge and statistical analysis of
individual current pulses is performed for different DBD
configurations and applied voltage amplitudes. This data
is evaluated as a function of filament density using our
method for analyzing Q-V diagrams for partial surface
discharging in DBDs, presented in [3].
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3. Results
The most striking result is that the Q-V diagrams for the
miniature DBD have the expected parallelogram shape,
but with clearly distinguishable steps when there are
filamentary discharges, see Fig. 2. The slope ζ diel , which
can be thought of as the effective capacitance of the
dielectric exposed to discharges, is built up out of discrete
Fig. 1. Schematic of the electrical characteristics of
filamentary DBD and a cross-section of the DBD reactor.
1
Fig. 2. Q-V plot for one period of the applied voltage
with ‘staircase’ behavior during one of the half-cycles.
vertical steps q, followed by horizontal steps ∆V. Note
that the left-hand-side of the Q-V diagram in Fig. 2 is
smooth because the discharge is not filamentary but
Townsend-like during this half-cycle; a result of the
asymmetric configuration of the DBD. Focusing on the
filamentary half-cycle on the right-hand-side, we can
obtain statistical distributions of the vertical steps q as a
function of applied voltage amplitude. Results are
depicted in Fig. 3. We find that, while the number of
filaments per half-cycle increases dramatically with
applied voltage amplitude (inset of Fig. 3), the general
shape and weighted average charge per current pulse
barely changes. Furthermore, the burning voltage U b is
similarly unaffected by filament number density and
applied voltage amplitude, as seen in Fig. 4.
4. Discussion
While it is commonly assumed that the dielectric layer
in a DBD can be treated as a single capacitance in the
electrical analysis of discharges, no explanation is ever
offered for this remarkable behavior. As we show above,
Fig. 4. Burning voltage U b as a function of filament
number density.
filaments ignite one-by-one, in different locations, and
maintain the same charge/pulse and burning voltage over
a wide range of applied voltages and filament number
densities. This results in an effective slope ζ diel in Q-V
diagrams and allows treatment of the discharge as though
it were uniform in space and time. We hypothesize that
the only consistent explanation for this behavior is that a
substantial proportion of the charge deposited by the
plasma on the dielectric surface is able to redistribute
between filament ignitions. Our hypothesis can
consistently account for the following observations:
(1) After each filament transfers a charge q through
the gap, the external voltage has to be increased
by ∆V for the next filament to ignite. A filament
in one location affects the moment of ignition of
a filament in another location.
(2) Burning voltage, which can be interpreted as the
gap voltage at which a filament ignites, is
practically a constant, but is also independent of
the number of filaments per cycle or the phase of
the applied voltage.
Both points imply a constant redistribution of surface
charge, or filamentary discharges would be far more
erratic. As we will show, however, redistribution of
surface charge also limits the control over DBD in terms
of electron energy distribution function: a constant
burning voltage will always establish itself in steady-state
operation, resulting in similar dissociation efficiencies for
different DBD configurations and applied voltage
amplitudes.
Fig. 3. Histograms of integrated charge per filamentary
current pulse, q, with increasing applied voltage
amplitude.
2
5. References
[1]
Stollenwerk L, Amiranashvili S, Boeuf J-P and
Purwins H-G, 2006, Phys. Rev. Lett. 96 255001
[2]
Manley T C, 1943, Trans. Electrochem. Soc. 84
83–96
[3]
Peeters F J J and van de Sanden M C M, 2015,
Plasma Sources Sci. Technol. 24 015016
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