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Analysis of two-dimensional microdischarge distribution in dielectric-barrier discharges
I
NSTITUTE OF
P
HYSICS
P
UBLISHING
P
LASMA
S
OURCES
S
CIENCE AND
T
ECHNOLOGY
Plasma Sources Sci. Technol. 13 (2004) 623635
PII: S0963-0252(04)85754-2
Analysis of two-dimensional
microdischarge distribution in
dielectric-barrier discharges
A Chirokov
1
, A Gutsol
1
, A Fridman
1
, K D Sieber
2
, J M Grace
2
and K S Robinson
2
1
Department of Mechanical Engineering, Drexel University, Philadelphia, PA, USA
2
Eastman Kodak Company, Rochester, NY, USA
E-mail: chirokov@drexel.edu
Received 2 April 2004
Published 5 October 2004
Online at
stacks.iop.org/PSST/13/623
doi:10.1088/0963-0252/13/4/011
Abstract
The two-dimensional spatial distribution of microdischarges in atmospheric
pressure dielectric-barrier discharges (DBDs) in air was studied.
Experimental images of DBDs (Lichtenberg gures) were obtained using
photostimulable phosphors. The storage phosphor imaging method takes
advantage of the linear response of the phosphor for characterization of
microdischarge intensity and position. A microdischarge interaction model
in DBDs is proposed and a Monte Carlo simulation of microdischarge
interactions in the discharge is presented. Comparison of modelled and
experimental images indicates interactions and short-range structuring of
microdischarge channels.
1. Introduction
Dielectric-barrier discharges (DBDs), or silent discharges,
have a number of industrial applications. For example, in
addition to ozone generation, the DBD in air is commonly
used in the web conversion industry, where it is known com-
mercially as corona discharge treatment. It is used to treat
polymer surfaces in order to promote wettability, printability
and adhesion [1, 2]. This non-equilibrium discharge is espe-
cially advantageous for the web conversion industry because
it operates at atmospheric pressure and ambient temperature.
The use of the so-called corona treatment, as well as other vari-
ous surface modication methods for the manufacture of many
different types of products on moving webs, is extensively
described in the literature.
Recently there has been interest in characterizing and
understanding the diversity of phenomena that can be found
in atmospheric pressure discharges [3]. The nature of the
discharge depends on the gas mixture employed, the dielectric
and the operating conditions. Both diffuse and lamentary
discharges are observed at atmospheric pressure and the
experimental conditions leading to ordering or patterning of
barrier discharges have been reported [3]. The development
of experimental methods, such as imaging techniques,
for quantitative characterization of microdischarges and
associated cooperative phenomena in atmospheric pressure
discharges is lacking.
Furthermore, theoretical models
describing cooperative phenomena in these discharges are not
complete.
In this paper, we report an experimental technique that
employs the inherent linearity of photostimulable phosphors
for quantitative imaging of microdischarges as well as a
novel theoretical model describing microdischarge interactions
and cooperative phenomena in the barrier discharges.
Experimental and simulated images were compared using
two approaches: Voronoi polyhedra and the radial correlation
function. Microdischarge interactions leading to short-range
structuring of microdischarge channels are found.
2. The physical nature of microdischarge interaction
and structuring
In most cases, DBDs are not uniform and consist of numerous
microdischarges distributed in the discharge gap as can be
seen in gures 1 and 2. The physics of microdischarges is
based on an understanding of the formation and propagation
of streamers, and consequent plasma channel degradation.
0963-0252/04/040623+13$30.00
© 2004 IOP Publishing Ltd
Printed in the UK
623 A Chirokov et al
Figure 1. Storage phosphor image of laments in a DBD gap in air
obtained from an experimental set-up using one applied excitation
cycle at 20.4 kHz and a discharge gap of 4.57 mm.
Figure 2. Storage phosphor image of laments in a DBD gap in air
obtained from an experimental set-up using ten excitation cycles at
20.9 kHz and a discharge gap of 0.762 mm.
Streamers are local ionization waves usually moving from
anode to cathode to meet avalanches propagating in opposite
directions.
Streamers move very fast (about 10
8
cm s
1
)
and cover the distance between electrodes in nanoseconds.
The electrons in the conducting plasma channel established
by the streamers dissipate from the gap in about 40 ns, while
the heavy and slowly drifting ions remain in the discharge gap
for several microseconds (table 1). Deposition of electrons
from the conducting channel onto the anode dielectric barrier
results in charge accumulation and prevents new avalanches
Table 1. Calculated microdischarge characteristics for the
experimental system used in this study (with discharge gap 1 mm).
Duration
Charge
time
transferred (C)
Microdischarge (0.2 mm radius)
40 ns
10
9
Electron avalanche
10 ns
10
11
Cathode-directed streamer
1 ns
10
10
Plasma channel
30 ns
10
9
Microdischarge remnant
1 ms
10
9
and streamers nearby until the cathode and anode are reversed
(if the applied voltage is not much higher than the voltage
necessary for breakdown).
The operating frequency of
the discharges used here is around 20 kHz, and therefore the
voltage polarity reversal occurs within 25 µs. After the voltage
polarity reverses, the deposited negative charge facilitates
the formation of new avalanches and streamers in the same
spot. As a result, a many-generation family of streamers is
formed that is macroscopically observed as a bright lament
that appears to be spatially localized.
It is important to
clarify and to distinguish the terms avalanche, streamer and
microdischarge. An initial electron starting from some point
in the discharge gap (or from the cathode or the dielectric
that covers the cathode in the case of a well developed DBD)
produces secondary electrons by direct ionization and develops
an electron avalanche. If the avalanche is big enough (Meek
condition [28]) then a cathode-directed streamer is initiated
(usually from the anode region). A streamer is a very fast
ionization wave that bridges the gap in a few nanoseconds
and forms a conducting channel of weakly ionized plasma
[29, 30]. An intense electron current will ow through this
plasma channel until the local electric eld collapses. Collapse
of the local electric eld is caused by the charges accumulated
on the dielectric surface and ionic space charge (ions are
too slow to leave the gap for the duration of this current
peak). The group of local processes in the discharge gap
initiated by the avalanche and developing until electron current
termination is usually called a microdischarge. After electron
current termination there is no more electronion plasma in
the main part of the microdischarge channel, but a high level
of vibrational and electronic excitation in the channel volume,
along with charges deposited on the surface and ionic charges
in the volume, allows us to separate this region from the
rest of the volume and we call it a microdischarge remnant.
Positive ions (or positive and negative ions in the case of
an electronegative gas) of the remnant slowly move to the
electrodes, resulting in a low and very long (
10 µs for
1 mm gap) falling ion current. A microdischarge remnant will
facilitate formation of a new microdischarge in the same spot
when the polarity of the applied voltage changes. That is why
it is possible to see single laments in DBD. If microdischarges
formed at a new spot each time the polarity changed, the
discharge would appear uniform. Thus the laments in a DBD
is a group of microdischarges that form at the same spot each
time the polarity is changed. The fact that a microdischarge
remnant is not fully dissipated before the formation of the next
microdischarge is called the memory effect [21, 25].
The charge distribution associated with streamers and the
local electric eld in the gap associated with the plasma channel
624 The two-dimensional distribution of microdischarges in DBDs
Figure 3. Streamer formation (left-hand side) plasma channel (and microdischarge remnant) and electric eld distortion (right-hand side)
are due to space charges. The solid curve is the superposition of the electric eld from the microdischarge and the applied electric eld; the