22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Spatio-temporal diagnostics of dielectric barrier discharges
R. Brandenburg1, H. Höft1, H. Toder2, M. Kettlitz1, A. Sarani1, H.-E. Wagner3 and K.-D. Weltmann1
1
Leibniz Institute for Plasma Science and Technology (INP Greifswald), Greifswald, Germany
2
Masaryk University Brno, Brno, Czech Republic
3
University of Greifswald, Greifswald, Germany
Abstract: The principles and limitations of methods for the spatio-temporal spectroscopy
and the recording of discharge luminosity are summarized. Selected examples on the
application of such methods for dielectric barrier discharges in nitrogen-oxygen gas
mixtures are discussed. In particular the capabilities of a novel Time-Correlated Single
Photon Counting set-up with automatic scanning of spatial discharge coordinate are
presented.
Keywords: dielectric barrier discharge, streak recording, time-correlated single photon
counting, microdischarge, microplasma
1. Introduction
Many plasmas, in particular those operated at
atmospheric pressure are often transient, small scale and
of short duration [1]. Dielectric barrier discharges
(DBDs), corona discharges and plasma jets are typical
examples of filamentary plasmas with irregular appearing
distinct discharge channels. They obtain strong gradients
in local plasma parameters that can be affected by
instabilities. Its key parameters and important species
must be studied with sufficient spatial and/or temporal
resolution, accordingly. Depending on the plasma source
and the operating conditions a resolution down to the submm and sub-ns range is desired. Special techniques are
required which will be summarized and d emonstrated on
selected examples in this contribution. In particular a
novel time-correlated single photon counting system will
be presented.
2. Methods and methodology
The principal approach to study filamentary plasmas is
to study the most fundamental element, the single
filament or microdischarge. To generate localized and
repetitive filaments special electrode arrangements with
pin-electrodes or semi-spherical dielectric bodies can be
used. To study the morphology and development of
single discharge channels in principle three methods are
available: gated intensified charge coupled device (ICCD)
cameras, streak cameras and time-correlated single
photon counting (TC-SPC) based cross-correlation
spectroscopy (CCS).
ICCD cameras allow photography by means of image
intensification using a micro-channel plate before CCD
detection. For temporally resolved studies the microchannel plate can be gated down to 1 ns, (some modern
ICCD cameras even reach 100 ps). To resolve the spatial
structure of individual discharge channels it can be
magnified by lens systems, far-field microscopes or
classical microscope. ICCD cameras are limited by their
read-out times. Temporally resolved studies must be
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done by investigation of subsequent discharge events with
variable gating with respect to a defined trigger event.
Thus, spatio-temporally resolved studies by ICCD
cameras are typically limited to discharges with a defined
inception in time such as pulsed driven discharges.
Multichannel or framing ICCD cameras enable the
recording of few (usually up to eight) images for one
discharge cycle, but a limited number of recordings is
often not sufficient to describe the discharge development
as a whole in detail. ICCD cameras which can be
operated in stroboscopic regime overcome read-out time
limitations and reach time resolutions of about 80 ps
[2, 3]. Stroboscopic images are well suited if nonreproducible discharges that propagate in multiple
directions (e.g., branching streamer in corona discharges)
are studied.
Streak cameras enable to resolve the spatio-temporally
development of the luminosity of discharges along a
chosen dimension (usually the discharge axis).
Optoelectronic streak cameras transform the temporal
profile of a light signal into a spatial profile on the CCD
array by causing a time-varying deflection of the
photoelectrons which are generated at a photocathode and
intensified by a microchannel plate before CCD detection.
In principal streak camera imaging is the only method
which enables the spatio-temporally resolved study of
single or individual discharges with high resolution (down
to 50 ps) in one exposure [4]. Streak cameras need to be
triggered similar to ICCD cameras. If the discharge event
is well localized and reproducible, the accumulation over
many events can be performed with a reasonable
accumulation time. Even irregular appearing discharge
events can be accumulated by using the so-called jitter
correction function.
For the temporally resolved optical emission
spectroscopy of pulsed operated discharges with low
inception jitter (e.g., nanosecond pulsed discharges
between two steel pin electrodes operated with a high
voltage with rise/fall times of 5 ns) a triggered
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photomultiplier with sufficient rise time at the
monochromator exit can be used [5]. In this case a trigger
generator is controlling the high voltage device and the
photomultiplier. However, the synchronization between
the discharge event and the detection system (ICCD
camera, spectrometer) cannot be provided by an external
trigger generator in case of irregular or self-pulsing
discharges. In such cases the current pulse of the
discharge can be used to trigger the detection unit [6].
Such methods are limited due to delay and transmission
times of cables, electronics and detection system, which
can be overcome by an optical delay of the emission.
However, this results in a complicated optical system and
a weak signal.
The cross-correlation spectroscopy (CCS) is the most
sensitive technique for the spatio-temporal and spectral
resolved recording of irregular single discharge channels.
It was first applied to corona discharges and
independently on these studies Kozlov proposed this
method for the investigation of single DBDs. The CCS
method substitutes the real-time measurement of
discharge events by the statistically averaged
determination of a cross-correlation function between two
optical signals, both originating from the same source [7].
One of these is used to define a relative time scale
(synchronizing signal) while the second one is consisting
of single photons that are spatially and spectrally resolved
(main signal). The measured quantity is the time delay
between these two signals, which is determined by the
TC-SPC module. Consequently, a time histogram of
counted photons for all positions is accumulated
(technical time resolution about 12 ps), which is the
probability density function for the light pulse luminosity
evolution. If the repetitive light pulses reproduce each
other sufficiently exact, and if the synchronization signal
detection is adjusted in such a way that it occurs always at
the same moment of a single light pulse evolution, then
the recorded probability density function is proportional
to the light intensity of the source. CCS provides a
method for the accumulation and thus averaging over a
great number of elementary events by the measurement
procedure itself.
Therefore it requires a high
reproducibility and long-time spatial stability of the
repetitive discharge events and is therefore very time
consuming.
A similar method was realized in [8] by means of
photomultipliers connected to an analogue–digital
recorder and digital oscilloscopes. The oscilloscopes and
recorders were triggered by current pulses measured by
fast current probes.
Within recent years we used ICCD imaging, streak
camera and CCS simultaneously together with electrical
diagnostics to the same discharge configuration. The
developed methodology is shown in Fig. 1.
The current pulse measurements and the ICCD camera
imaging were used to provide information about the
current duration and amplitude, mean current density,
discharge morphology but also on the reproducibility and
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Fig. 1. Used methodology of the investigation of single
discharges in DBD arrangements.
spatial localization of discharge channels. If this criteria
was fulfilled the accumulation of streak camera images
could be performed. The streak recording was used to
yield only information along the discharge channel but
has the advantage of short accumulation times (usually in
the range of several seconds to minutes) which are
favourable if systematic measurements for variation of
several process parameters are necessary. CCS is even
more sensitive, offers a larger dynamic range and enables
higher spectral resolution. However, the accumulation of
single photons for many different positions can be very
time consuming. Therefore, CCS is more appropriate for
a more detailed investigation of specific aspects of
discharge development (see section 3) for selected
conditions. Furthermore, CCS is mandatory in case of
erratically appearing discharge events with a large jitter of
discharge inception (e.g., sinusoidal operated DBDs). To
get a two dimensional development the discharge can be
scanned in spatial coordinates (radial and axial along the
discharge channel).
3. Selected examples
The first example demonstrates the prospects of the
methods and the methodology in Fig. 1 itself. The
discharge being investigated is shown in Fig. 2a. Single
repetitive discharge channels are generated in a
symmetric electrode arrangement with semi-spherical
electrodes [9]. The dielectric is alumina (Al 2 O 3 ) with a
thickness of about 0.5 mm. The minimum distance
between the electrodes amounts to 1 mm. The electrodes
can be flushed with defined gas mixtures in a discharge
cell.
Electrical measurements (current and applied voltage)
are performed with fast probes and recorded with a digital
sampling oscilloscope [9]. The current probe is directly
embedded in one of the electrodes which reduce stray
capacitances and inductances. The measured current and
voltage are shown in Fig. 2c. A rectangular voltage with
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Fig. 2.
Investigation of rectangular pulsed single
discharges in N 2 with 0.1 vol.% O 2 admixture (a) by
means of ICCD camera (b), electrical diagnostics (c),
individual (d), accumulated streak camera recording (e),
and CCS (wavelength of 337 nm (f)).
a rise time of 250 V/ns and amplitude of 10 kV (10 kHz
frequency) is applied and the measured current I tot
consists of the displacement I dis (which can be obtained
from the derivation of applied voltage or measured, when
no discharge is ignited) and the plasma induced current
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(obtained by subtraction of I dis from I tot and
multiplication with a factors including barrier and total
capacitance of the arrangement) [9]. As outlined above
the discharges are either investigated simultaneously by
ICCD camera and streak camera system by using a beam
splitter or by means of CCS.
The ICCD camera photo in Fig. 2b is for an individual
discharge channel. It expands in the gas gap (diameter
about 70 µm) towards the electrodes and spread into
several discharge channels on the dielectric surfaces. The
streak photo of an individual discharge event (axial
coordinate is ordinate, abscissa is relative time scale and
number of counted photons/luminosity is colour coded in
logarithmic scale) is shown in Fig. 2d. The formation of
the bulk plasma in front of the anode and a weak emission
at the cathode can be observed, but details of discharge
development are obtained only by the accumulation of
streak photos over about 103 events (see Fig. 2e). The
recording of multiple discharges could be excluded and
sufficient reproducibility as well as stability of spatial
localization could be confirmed due to current and ICCD
measurements. Furthermore, the streak camera system
handles a temporal jitter of the discharges in the ns-range
(jitter correction function) [10].
The CCS image (Fig. 2f) is for the most intensive
emission in the optical emission spectrum at 337 nm
(0-0 transition of molecular bands of second positive
system of nitrogen). The advantage of CCS is the high
sensitivity which is beneficial if studying the weak
pre-phase of discharge development or in case of
spectrally resolved measurements of weak signals. The
example in Fig. 2f shows a pre-phase, which is located at
the anode. More precise measurements show that it is
about 35 ns long [10] which correlated with the time
window between the start of the voltage slope and the
inception of the discharge. For sinusoidal generation of
DBDs more than 100 ns long pre-phase has been obtained
[7]. It is clearly seen in CCS image and streak photo that
the cathode directed ionization wave is starting at the
anode and reaching a maximum velocity of about
2 106 m/s in front of the cathode. The differences in both
plots are most likely due to the fact that the streak image
is recorded along the central line of the discharge channel
while CCS accumulated over the full width of the
discharge channel in this case.
In order to reduce accumulation times of CCS
measurements a new set-up has recently been developed
(see Fig. 3).
The recording is based on a multi-dimensional TC-SPC
process that records the photon distribution over the time
in the discharge emission pulses, the applied voltage, and
the position along the discharge gap. Scanning over the
gap is performed by a galvanometer mirror and controlled
by a scan controller which generates both the scan ramp
for the mirror and a sinusoidal signal that is amplified to
generate the discharge (sinusoidal operation at about
12 kV pp amplitude and 7 kHz frequency). The photon
distribution is built up by the TC-SPC module.
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Fig. 5 shows the discharge development (resolved along
position in the gap and fine time scale) for 337 nm,
consisting of pre-phase, streamer phase, bulk plasma and
decay. The origin of the weak discharges several 10 ns
before the main discharge are still unclear but most
probably artefacts of the discharge accumulation caused b
several discharge channels per half-period.
Fig. 3. Novel TC-SPC set-up with automatic spatial
scanning along discharge axis.
Fig. 4 shows the integral intensity over the phase of the
applied voltage and the position along the gap for a DBD
in air. The figure should be interpreted as the mean
distribution of photons (and thus discharge events) over
the phase and the position. It shows that for sinusoidal
operated DBD in symmetric electrode arrangement the
discharges are distributed over about one quarter of the
voltage period. Each pixel of this image consists of a
curve with the temporally resolved emission signal (fine
time scale of 50 ns duration). These data are used to
compose Fig. 5.
Fig. 4. Integral intensity versus phase in sinusoidal high
voltage and position along the gap (1 mm) in logarithmic
intensity scale.
4. Conclusions
The techniques for spatio-temporally resolved imaging
and/or spectroscopy of transient and short-living
discharge events (e.g., single DBDs) have been presented.
Different techniques in simultaneous application on the
same discharge configuration offer the possibility to get a
complete
picture
on
discharge
morphology,
reproducibility, localization and development. A new
CCS set-up with first results was presented. The set-up
provides automatic scanning over phase of the applied
voltage and the position along the discharge gap.
5. Acknowledgement
Partly supported by the European Union's 7th
Framework Programme
(Project “PlasmaShape”,
3162169). We are grateful to Dr Kirill V. Kozlov
(Moscow State University) as well as Dr W. Becker and
H. Netz (Becker & Hickl GmbH Berlin).
6. References
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Appl. Phys, 399, 26 (2013)
[2] S. Pancheshnyi, et al. Phys. Rev. E, 71, 16407
(2005)
[3] D. Trienekens, et al. IEEE Trans. Plasma Sci., 99
(2014)
[4] V. Gibalov and G. Pietsch. J. Phys. D: Appl. Phys.,
33, 2618-2636 (2000)
[5] D. Pai, et al. J. Appl. Phys., 107, 093303 (2010)
[6] M. Janda, et al. in: Proc. 20th Symp. Applications
of Plasma Processes (SAPP). (Tatranska Lomnica)
IL-06 (2015)
[7] K. Kozlov, et al. J. Phys. D: Appl. Phys., 34, 3164
(2001)
[8] Yu. Shcherbakov and R. Sigmond. J. Phys. D:
Appl. Phys., 40, 460-473 (2007)
[9] M. Kettlitz, et al. J. Phys. D: Appl. Phys., 45,
245201 (2012)
[10] R. Brandenburg, et al. J. Phys. D: Appl. Phys., 46,
464015 (2013)
Fig. 5.
Spatio-temporally resolved discharge
development of a single DBD in air (at 337 nm). Cathode
is top electrode, logarithmic intensity scale.
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