st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Sub-microsecond Pulsed Atmospheric Pressure Glow Discharges
J. J. Shi, S. T. Song, Y. R. Zhang, J. Zhang, J. Zhang
1
College of Science, Donghua University, Shanghai 201620, P. R. China
Abstract: The discharge mechanism of dielectric barrier glow discharges in atmospheric helium excited by repetitive voltage pulses was numerically studied by a one-dimensional
self-consistent fluid model. The discharge characteristics in terms of discharge current density
waveforms are presented with dependence on the voltage pulse duration, the pulse voltage
amplitude, the rising and falling time of voltage pulse, respectively.
Keywords: Sub-microsecond pulse; Dielectric barrier discharge; Discharge mechanism
1. Introduction
Recently, the Dielectric Barrier Discharges (DBD) at
atmospheric pressure are attracted much attention for their
wide applications scope on etching, surface modification,
deposition of thin film and biological manipulation et al
[1-3]. Compared with the traditional DBD, pulsed voltage
excited DBD shows higher efficiency of power consumption and electron generation [4]. Furthermore, the
sub-microsecond pulsed voltage can also be efficiently
used to produce the homogenous atmospheric pressure
glow discharges(APGDs) without dielectric barriers at
kilohertz frequencies [5], which are desirable for
room-temperature processing such as food decontamination, industrial-surface treatment [6,7]. It has been
demonstrated that in pulsed APGDs with bare electrodes,
one discharge event occurred at the pulse voltage falling
phase [8,9] and there were two discharge events happened
with dielectric insulated electrodes at the voltage rising
and falling phase [5,10,11], respectively. Despite significant interest, the understanding of discharge mechanism
and characteristics of APGDs excited by pulsed voltage
under sub-microsecond remain incomplete.
2. Numerical model
The one-dimensional self-consistent numerical model
was developed to simulate the parallel-plate DBD reactor
with double dielectric barrier of 0.1 cm in thickness and
the plasma region of gap distance is 0.2 cm. The dielectric
barrier is alumina with a relative permittivity of 9.0. The
gas temperature is kept constant to be 300K. Five species
of electrons (e), helium metastable atom (He*), helium
metastable molecule (He2*), helium ion (He+), helium
molecular ion (He2+) are considered in the model. The
rate coefficients of elementary reaction between plasma
species are obtained using the Boltzmann solver [12] and
the other reaction rate coefficients follow the data in literature [13]. The electron transport coefficients are calculated as a function of the mean electron energy using a
Boltzmann solver [12] in advance. The transport properties of the positive ions and metastables are from literature.
The governing equations consist of the continuity equation with the drift-diffusion approximation, the electron
energy conservation equation and Poisson’s equation.
Boundary conditions at the plasma-barrier interfaces are
determined as follows. The electron flux to the border is
given by the sum of thermal flux minus the rate of secondary-electron release. The ions are assumed to be mobility limited at the boundaries and their boundary fluxes
are given by the sum of the mobility and thermal fluxes.
The boundary fluxes of metastables are assumed to be
thermally limited. For simplicity, mean electron energy at
both electrodes is fixed at 1 eV, the secondary electron
emission probability of ion bombardment is assumed to
be 0.02.
3. Results and discussions
Fig. 1 presents the waveforms of discharge current density with the voltage pulse duration of 200 ns, 400 ns, 600
ns and 800 ns, with the amplitude, repetition frequency,
rising and falling time of 2 kV, 10 kHz and 100 ns, respectively.
Fig.1 Waveforms of discharge current density at voltage
pulse duration of 200 ns, 400 ns, 600 ns and 800 ns.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
It shows that the waveforms of first discharge keep the
same on the time duration and amplitude of 0.12 A/cm2
with different voltage pulse durations. On the other hand,
the time instant of second discharge moves with the voltage pulse duration and the discharge current density amplitude of second discharge is lower than that of the first
discharge, which can be explained by the reverse of gas
voltage at the voltage falling phase. It is also found that
the discharge current density amplitude of second discharge grows monotonously from 0.09 A/cm2 to 0.1
A/cm2 with the voltage pulse duration expanding from
200 ns to 800 ns. It can be attributed to the accumulation
of the space charges on the dielectric surface inducing the
high localized electric field, which helps the ignition of
second discharge with the revering of gas voltage at the
voltage falling phase. The discharge characteristics of first
and second discharge event happened at the voltage rising
and falling phase are consistent with the experimental
findings.
50 ns, which is same as what the experimental results. It
is interesting to note that when the rising and falling time
is further reduced to 25 ns and 10 ns, the discharge current density amplitude of second discharge is higher than
that of the first discharge, although both the amplitudes
grows at shorter rising and falling time. The different dependence of discharge current density on the voltage rising and falling time can be attributed to the different discharge mechanism of first and second discharge. The first
discharge is ignited by the applied voltage with the electron avalanche. The electrons generated in the first discharge travel to the instantaneous cathode, which induces
the space charges in the discharge region. On the other
hand, the second discharge is ignited by reversing of the
gas voltage with the assistance of residual electrons from
the first discharge, which act as the seed electrons. It is
suggested that the amplitude, the rising and falling time of
the voltage pulse decide the both discharge events and the
second discharge is also related to the characteristics of
first discharge. When the rising and falling time is as short
as 25 ns and 10 ns, the fist discharge is ignited quickly
with the rising voltage pulse and the amplitude of discharge current density is limited by the dielectric barriers.
On the other hand, during the voltage falling phase, the
reduction of pulse voltage induces the enhancement of
negative gas voltage and ignites the second discharge and
also drives the movements of space charges, which elevates the discharge current density. It is also worth noting
the displacement current density is also contributed to the
enhancement of discharge current density with reduction
of voltage pulse duration.
Fig.2 Waveforms of discharge current density at voltage
voltage rising and falling time of 10 ns, 25 ns, 50 ns, 75 ns
and 100 ns.
Fig. 2 give the discharge current density waveforms
with the rising and falling time voltage pulse at 10 ns, 25
ns, 50 ns, 75 ns and 100 ns and the voltage amplitude,
repetition frequency, voltage pulse duration of 2 kV, 1
kHz, 400 ns, respectively. It is clearly shown that by the
applying shorter rising and falling time of voltage pulse,
the amplitudes of both first and second discharge current
density are enhanced, the duration of discharge current
density peak is reduced, which suggests the stronger discharge event happened with shorter voltage rising and
falling time. The movement of time instant when the discharge event happened can be considered mostly due to
the temporal evolution of the pulse voltage. It is also
shown that the discharge current density amplitude of first
discharge is higher than that of the second discharge when
the rising and falling time of voltage pulse is higher than
Fig.3 Waveforms of discharge current density at pulse voltage amplitude of 2000 V, 2500 V, 3000 V, 3500 V and 4000
V..
The waveforms of discharge current density with the
pulse voltage amplitudes of 2000 V, 2500 V, 3000 V, 3500
V and 4000 V with the rising time, falling time, duration
and repetition frequency of voltage pulses of 100 ns, 100
ns, 400 ns and 10 kHz, respectively. With elevating the
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
voltage amplitude, the amplitudes of both first and second
discharge current density grow except that when the voltage amplitude reaches 3500 V, the amplitude of discharge
current density keeps around 0.27 A/cm2 because the limitation of discharge current density by the dielectric barriers. The time instant of discharge event is found to move
forwardly because that the applied voltage reaches the gas
breakdown voltage earlier with higher amplitude of pulse
voltage.
4. Summary
The numerical simulation of pulsed dielectric barrier
atmospheric glow discharge has been performed to study
the characteristic and mechanism of the two discharge
events at the voltage rising and falling phases. It is
demonstrated that with expanding the voltage pulse duration, the first discharge keeps its characteristics in terms
of duration and amplitude of discharge current density.
The amplitude of second discharge current density grows
and the time instant moves with that of voltage falling
phase. On the other hand, the discharge characteristics
depends severely on the voltage rising and falling time.
The amplitudes of both first and second discharge current
density increases with reducing the voltage rising and
falling time and the amplitude of second discharge current
density can be even higher than that of first discharge
current density when the voltage rising and falling time
reduces to 25ns and 10 ns. It is also found that the amplitude of discharge current density elevates by applying
higher pulse voltage except that when the amplitude of
pulse voltage reaches 3500 V, the discharge current density keeps its magnitude with even higher pulse voltage.
5. References
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6. Acknowledgement
This work was funded by the Natural Science Foundation of China (Grant No. 10835004 and 10905010) and
sponsored by Shanghai Shuguang Program (Grant No.
08SG31).