High speed imaging characterization of a Dielectric Barrier Discharge Roller
plasma source
Marco Boselli2, Vittorio Colombo1,2, Emanuele Ghedini1,2, Matteo Gherardi1,
Romolo Laurita1, Anna Liguori1, Paolo Sanibondi1 and Augusto Stancampiano1
Alma Mater Studiorum – Università of Bologna
1
Department of industrial engineering (DIN)
2
Industrial Research Center for Advanced Mechanics and Materials (C.I.R.I.-M.A.M.)
Via Saragozza 8-10, 40123 Bologna, Italy
Abstract: The structure of the plasma generated by a Dielectric Barrier Discharge (DBD)
roller plasma source operating at atmospheric pressure in ambient air is investigated by mean
of an intensified Charge Couple Device (iCCD) camera with exposure time down to 3 ns. The
goal is to determine which geometrical and operating conditions result in a filamentary
structure possibly causing non-uniformities and material damage, with the final aim of
process optimization.
Keywords: Atmospheric pressure plasma, iCCD camera, DBD roller, diffuse condition
1. Introduction
Chemical and physical properties of the surface
have great influence on the final characteristic
of materials.
Atmospheric non-thermal plasma
treatment is a new promising inexpensive technology able
to achieve surface modification of the top layer (1-10 nm)
of polymeric materials (for example for biomedical or
packaging applications) without altering their bulk
properties [1]. Furthermore plasma treatment proves to be
an environmental friendly process, solvent free and with
good energy efficiency [2].
However too aggressive treatments may induce
damages on the processed material due to localized
temperature peaks generally linked to a filamentary
condition [3]. Therefore an accurate investigation and
selection of the operating conditions necessary to obtain a
diffuse discharge is generally desirable in order to attain a
uniform treatment.
iCCD imaging techniques are generally regarded as a
reliable method to investigate plasma discharges behavior
in their nanosecond time scale evolution [4][5][6].
2. DBD roller plasma source
DBD roller plasma source object of this study has been
specifically designed for the treatment of films and can be
easily scaled up to support the treatment of larger foils.
The source is composed of two electrodes made of
metallic rods covered by a nylon (PA 6) dielectric layer
and separated by a gap of a few millimeters (Fig. 1). In
order to generate a non-thermal plasma in the gap, one of
the electrodes is grounded while the other is electrically
connected to a high voltage generator able to produce
nano-second pulses, that have been proven to be more
effective in achieving a diffuse plasma condition than
micro-second ones [7].
The plasma generated between the two rollers has a
long and narrow geometry, suitable to treat large foils.
This type of source is particularly fit for treating heat
sensitive materials processed on rotating shafts (for
example polymers for biomedical or packaging
applications), however too aggressive treatments may
induce damage on the processed material.
Fig.1 DBD roller: (a) schematic of the plasma source
treating a film, (b) photograph of the actual source, (c) 50
ms exposure time photograph of the inter-electrode region
during operation.
3. Experimental setup
In order to characterize the plasma source in different
geometrical and operating conditions an iCCD camera
(Princeton Instruments PIMAX3) with exposure time
down to 3 ns, is adopted. A schematic of the experimental
setup is reported in Fig.2. A synchronous pulses generator
(BNC 575 digital pulse/delay generator) is used to time
the nano-second rise time pulse generator and the
oscilloscope that in turn triggered the iCCD camera. A
high voltage probe (Tektronix P6015A) is adopted to
verify the relative time position of the iCCD acquisition
gate with respect to the voltage pulse waveform. An
explanatory schematic of the waveforms visualized by the
oscilloscope is reported in Fig.3.
Table 1.
Table 1
Fig.2 Schematic of the acquisition setup apparatus
Fig.3 Voltage waveform and acquisition gate.
For every case considered we obtain multiple iCCD
acquisitions in order to scan the whole pulse waveform.
The iCCD camera doses not allow consecutive acquisition
within the same nanosecond voltage pulse. Therefor it has
been decided to take a single acquisition for each voltage
pulse and to increase the time delay by 250 ps from each
exposure to the subsequent one. In this way a series of
acquisition scanning the whole waveform with a time step
of 250 ps are obtained.
4. Results
In this paragraph a small selection of the acquired
images for each operating conditions set is shown in Tab.
1 to 4. In order to achieve a good characterization of the
plasma source we have varied some of the operating
conditions accordingly to the following value:
Peak voltage: 26-35 kV
Repetition frequency: 500-1000Hz
Nylon dielectric layer thickness: 1-4 mm
Interelectrod gap width: 1-2 mm
For each case the corresponding voltage waveform and
time positions of the presented acquisitions are reported
The acquisition are numbered in sequential order starting
from the first one that is took at the beginning of the
voltage ramp. The first image reported is always the first
acquisition in which a light emission is visible since the
pulse start. At the bottom of each table are reported few
comments that both describe the discharge evolution and
suggest comparisons between the various cases.
Table 3
Table 2
Table 4
In general plasma appears more intense in proximity of
the dielectric surface of both electrodes, where it seems to
form a layer of plasma, than near the center of the gap
where the light emission is less powerful. The intensity
and uniformity of these plasma layers increase with the
peak voltage and the repetition rate. The plasma behavior
seems to be greatly influenced by the voltage waveform
characteristic following the initial voltage ramp. In fact, in
the cases with a thin dielectric layer where the waveform
greatly varies with the repetition frequency the plasma
behavior too is influenced by the frequency while in the
cases with a thicker dielectric layer where the pulse
waveform is more constant in shape the behavior is much
more influenced by the peak voltage than the repetition
frequency.
5. Conclusions
In all the presented condition plasma appears diffuse
and uniform to the naked eye, but always presents at least
a partial filamentary structure if observed in a
nano-second time scale. Furthermore the presented cases
show a great variety in the microdischarges behavior, both
concerning shape and time evolution. As expected a
diffuse plasma condition is more easily achieved in
presence of a minimum gap and a thin dielectric layer.
iCCD imaging appears to be an indispensable method
to accurately evaluate the discharge behavior and identify
the operating conditions in order to optimize the process
and to avoid localized temperature peaks.
The DBD roller plasma source has proven to be able to
generate a uniform discharge if operated with a 1mm
dielectric thickness. Further future investigations will
point to test the feasibility of reducing the dielectric
thickness adopting different materials with a higher
dielectric strength in order to achieve a diffuse plasma
condition for a wider set of operating conditions.
References
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(2011)
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