22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Development and energy distribution of nanosecond surface dielectric barrier
discharge in air
S.A. Shcherbanev, S.A. Stepanyan and S.M. Starikovskaia
Laboratoire de Physique des Plasmas (CNRS, Ecole Polytechnique, Sorbonne Universities, University of Pierre and
Marie Curie-Paris 6, University Paris-Sud), FR-91128 Palaiseau Cedex, France
Abstract: This paper presents the experimental study of nanosecond surface dielectric
barrier discharge (SDBD) in air at atmospheric and elevated pressures and ambient
temperature. The emission spectroscopy study of rotational and vibrational structure of 2+
system of N 2 molecules has been performed for classical airflow control SDBD
configuration. The energy deposition into the discharge has been measured. Strongly nonequilibrium distribution of the rotational spectra were observed at the leading and trailing
edges of high-voltage pulse.
Keywords: nanosecond plasma, barrier discharge, energy release
1. Introduction
Transient plasma of nanosecond discharges is widely
used for studying plasma assisted ignition/combustion
(PAI/PAC) of hydrocarbon–containing mixtures [1, 2] as
well as in plasma assisted hydrodynamics for airflow
control to provide turbulent-to-laminar transition [3, 4].
One of the most considerable advantages related to PAI is
the possibility to get quasi-uniformly distributed energy
release over the entire surface of electrode [5], whereas
the spark discharge that is currently used in IC-engines
provides the local gas heating. The ability of multipoint
ignition allows significant reduction of induction time
providing high efficiency of the flame initiation with
relatively low energy deposition.
Nanosecond surface barrier discharge develops as a set
of synchronously propagated streamers.
Surface
streamers represent a complex 3D structure with strong
gradient of electric field and electron density in the
direction perpendicular to the surface. Despite the fact
that the integral characteristics, like velocity of
propagation on ICCD imaging, are widely presented in
the literature, a little is known about energy deposition,
heating or spectral characteristics with a sub-nanosecond
resolution. Another important issue is a modification of
the nSDBD geometry with pressure increase.
The aim of the present work is to study, in
sub-nanosecond and sub-millimeter scale, a distribution
of energy in SDBD; and to analyze the nSDBD;
modification with pressure.
2. Experimental setup
Two configurations of electrode systems for
nanosecond SDBD initiation are considered in the paper
(see Fig. 1). The flat electrode system (a,b) that is widely
used for airflow control [3, 4] and system in cylindrical
symmetry (c,d) that is a SDBD configuration adapted for
plasma assisted ignition in rapid compression machine [7]
were studied. The electrode system was connected to the
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generator via a 30 m coaxial 50 Ω cable. The high
voltage (HV) pulse generator (FID Technology,
FPG20-03NM) used in the experiments provided the
following parameters: 2 ns front rise time, 20 ns pulse
duration on the half-height and ±(12 - 30) kV voltage
amplitude in the cable. All experiments were performed
in a single shot regime.
A
A
A
Dielectric
material
High-voltage
electrode
Grounded
electrode
Cable
0.3 mm
PVC film
a)
b)
High-voltage
electrode
0.3 mm
PVC film
Cable
Dielectric
material
c)
Grounded
electrode
d)
Fig. 1. Cylindrical (a,b) and flat (c,d) electrode
configurations.
3. Results
The classical airflow configuration of SDBD was used
to study the morphology and the structure of the
discharge. The discharge propagates like a set of parallel
streamers along the surface of dielectric layer in the
direction perpendicular to the high-voltage electrode. The
dimensions of the streamers are different in the case of
positive and negative polarities (Fig. 2). For positive
polarity, the optical diameter of the streamers varies
between 0.7 - 1.2 mm, whereas for negative polarity it is
1
in the range 0.3 - 0.8 mm.
Fig. 2. ICCD imaging of the discharge in air for both
polarities in flat geometry. P = 1 atm, |U| = 37 kV,
T = 300 K. Camera gate is 0.5 ns.
The velocities are different as well. For positive
polarity, the discharge front starts from the high–voltage
electrode with a velocity of 2.7 mm/ns, and slows down to
about 0.7 mm/ns in approximately 10 ns. For negative
polarity, discharge starts with a velocity equal to the
velocity of a positive discharge; slows down to 1 mm/ns
in 3 - 4 ns, and slows down further to about 0.15 mm/ns at
about 10 ns from the start of the discharge. Fig. 3
presents the x-t diagrams for positive and negative
polarity discharge synchronized with the voltage pulse on
the HV electrode.
Fig. 4. Power and energy deposited into the discharge of
positive polarity in air. P = 1 atm, U = +37 kV,
T = 300 K.
To study the SDBD at different voltages and gas
densities, the cylindrical electrode configuration has been
used (Fig. 5). In this case, the discharge develops radially
from the edge of the metal disk (20 mm in diameter)
served as a high-voltage electrode. The ground electrode
is covered by the dielectric layer (PVC) of thickness
0.3 mm (see Fig. 1).
Fig. 5. ICCD imaging of the discharge in cylindrical
geometry. Camera gate is 2 ns.
Fig. 3. X-t diagrams of the discharge front for both
polarities. P = 1 atm, |U| = 37 kV, T = 300 K.
The total power deposited in the discharge is shown in
Fig. 4 for pulses of positive polarity with the voltage
amplitude 37 kV on the electrode.
During the discharge, 32 mJ is deposited into gas for
positive and 22 mJ - for negative polarity. The most
considerable power dissipates in the gas during first 5 ns
of the pulse, corresponding to a fast propagation of the
streamers along the dielectric and charging a “plasma”
capacitance. After about 7 ns, the streamers slow down,
and the energy provided by high–voltage electrode
decreases.
2
This modification of the SDBD discharge was
successfully used to ignite stoichiometric C 2 H 6 :O 2
mixture at ambient initial pressure and temperature in a
constant volume chamber [6]. Fast ICCD imaging proved
that multiple combustion waves develop, merging, from
the high-voltage electrode. It should be noted that no
acceleration of the combustion wave along the streamers
has been recorded [6]. Presumably, the energy release in
each of the streamers is small to get a significant gradient
of density of active species along the streamer. At the
same time, stable non–detonative combustion initiation is
achieved with the distributed nanosecond SDBD system.
The ICCD images of three typical cases are given in
Fig. 2. The first (1 atm, +20 kV) case corresponds to
"traditional" morphology of nanosecond surface discharge
with the only difference that the geometry is coaxial. It
should be noted that no visible changes were detected for
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Emission intensity from the vibrational level
obtained (i) at the trailing edge of the pulse; (ii) in the
region 5 - 6 mm from the HV electrode; and (iii) both for
negative and positive polarity pulses.
0,5 ns
1 ns
1,5 ns
2 ns
2,5 ns
3 ns
4 ns
5 ns
1
0,1
0
1
Vibrational level
2
Fig. 6. Peak intensities of (0 - 2), (1 - 3) and (2 - 4) lines
of second positive system at different instances of time.
T = 0 correspond to the beginning of the emission from
the discharge. Discharge in ambient air. U = -37 kV.
0.5 ns
2 ns
1,0
Intensity, a.u.
different tested edges of the high-voltage electrode, so its
sharpness does not seem to be an important parameter.
The thickness of the discharge in any of the considered
experimental conditions did not exceed 0.5 mm.
The second case (3 atm, -18 kV, reflected) correspond
to the image of nSDBD in the reflected pulse appearing in
the system 250 ns later than the incident signal (20 kV) of
the positive polarity. The filamentation, clearly seen in
the figure, has been obtained by different authors in
repetitive [8] pulse mode. The emission from the
filaments is uniform along the filament, with a wellpronounced streamer “brush” in the head of each
filament.
The third case (6 atm, -47 kV) was described for the
first time in [8]. It was observed in air for negative
polarity of the discharge with increase of pressure or of
the high voltage pulse amplitude. The fast transition from
the streamer (or "quasi-uniform" [8]) form to filamentary
discharge occurs at the conditions of a single–shot
regime. The wave of the radially propagated streamers
stops at certain distance (this distance was 2 - 7 mm,
depending upon pressure and amplitude) from the high
voltage electrode, similar to negative polarity streamers at
atmospheric pressure (see Fig. 5 b), the emission from the
streamers abruptly decreases, and a few bright filaments
start from the electrode and propagate in the radial
direction. The filaments are the most bright near the
electrode; emission intensity decreases radially; a weak
streamer "bush" can be identified at the head of each
filament. They travel through the charged region and
continue to move with relatively high velocity up to the
end of the pulse. The number of the filaments increases
during the pulse duration. At long (about 10 mm)
distances from the electrode the filaments can branch into
2 - 3 smaller filaments. In the developed filamentation
mode, one filament is created instead of 4 - 6 streamers,
and the filamentation structure is very regular, as it is
represented in Fig. 5c.
A distinctive feature of nanosecond SDBD which has
never been observed before, is a strong non-equilibrium
of vibrational and rotational spectra in the rising front and
trailing edge of the pulse, that is in the intervals with a
high dU/dt. The results have been obtained in air-flow
geometry at atmospheric pressure.
The evolution of vibrational population of N 2 (C3Π u ,v`)
for v = 0,1,2 is given by Fig. 6. It is clearly seen that the
distribution (i) is non-Boltzmann; (ii) changes
significantly during a few nanosecond.
Fig. 7 demonstrates the rotational spectra of (0-0) lines
of 2+ system of molecular nitrogen on the leading edge of
the pulse for negative polarity discharge. The optical
signal was obtained over the region 0 - 1 mm near
high-voltage electrode. As one can notice, the R-branch
of rotational spectrum is mostly suppressed whereas the
P-branch is clearly seen. In a short time (1.5 - 2 ns) the
spectra transform to a “common” shape corresponding to
Boltzmann distribution over the rotational lines. It should
be noted that similar non-equilibrium spectra have been
0,5
0,0
335
336
337
Wavelength, nm
338
339
Fig. 7. Normalised emission spectra of molecular
nitrogen N 2 (C3Π u (v` = 0)) → N 2 (B3Π g (v`` = 0)). Air,
P = 1 atm, U = -37 kV.
4. Conclusion
Nanosecond surface discharge with DBD configuration
of electrodes was studied in air at ambient pressure and
temperature for two configurations of electrode: classical
airflow configuration and electrode with cylindrical
symmetry.
The power and the energy deposition were measured
for classical airflow configuration; it was shown that the
main energy release takes place at first 5 - 7 ns, during
discharge propagation along the dielectric. Significant
difference was observed between positive and negative
polarity pulses: for the same voltage amplitude and pulse
shape, the deposited energy is 1.5 higher for a positive
polarity discharge.
3
At pressure increase from 1 to 6 atm, morphology of
streamers practically does not change for positive polarity
of the high-voltage electrode. For negative polarity, the
streamers slow down at a few mm from the high-voltage
electrode and the discharge transforms into the
filamentary form.
Time– and space–resolved emission spectra were taken
for the nSDBD discharge in air. It was shown that
ro-vibrational spectra of N 2 (C3Π u ) → N 2 (B3Π g )
transition are strongly non-equilibrium at the rising front
and trailing edge of the high voltage pulse.
5. Acknowledgements
The work was partially supported by French National
Agency, ANR (PLASMAFLAME Project, 2011 BS09
025 01), AOARD AFOSR (FA2386-13-1-4064 grant),
PUF and by Plas@Par Projects.
6. References
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combustion:
nanosecond
discharges
and
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[6] I.N. Kosarev, V.I. Khorunzhenko, E.I. Mintoussov,
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Accepted for publication (2014)
[8] S.A. Stepanyan, A.Yu. Starikovskiy, N.A. Popov
and S.M. Starikovskaia. “A nanosecond surface
dielectric barrier discharge in air at high pressures
and different polarities of applied pulses: transition
to filamentary mode”.
Plasma Sources Sci.
Technol., 23, 045003 (2014)
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