22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Bactericidal components in an atmospheric pressure air plasma jet at different
current regimes
A.A. Kirillov, A.V. Paulava and L.V. Simonchik
B.I. Stepanov Institute of Physics of NAS of Belarus, pr. Nezavisimosti 68, BL-220072 Minsk, Belarus
Abstract: Atmospheric pressure air plasma jets at DC, ripple and self-oscillatory current
regimes are realized. Emission and absorption spectroscopy are used to determine the
concentrations of chemical active component of the plasma jets at different electrical
powers of forming discharge.
Keywords: glow discharge, plasma jet, absorption spectra, chemical active component
1. Introduction
One of the promising areas of application of nonequilibrium plasma at atmospheric pressure is medicine
[1]. A basic requirement for non-destructive medical
applications is to provide "cold" plasma source at
atmospheric pressure, wherein the gas temperature does
not exceed 45°C. Among a variety of plasma sources
particular attention is focused on the development and
application of plasma jets [2] due to their ability of
surface treatment outside of a closed discharge volume. In
[3], plasma jets at the dc discharge current of 30 mA were
comparatively investigated for different working gas
mixtures: 95% He + 5% O 2 , 95% Ar + 5% O 2 , N 2 and
air. It was established that an air plasma jet has more
effective inactivation ability. This article focuses on the
implementation of various current modes of plasma jets
and determining concentrations of bactericidal
component.
2. Experimental setup
The discharge chamber is composed of a cylindrical
quartz tube with internal diameter of 8 mm, inside of
which a rod-shaped copper cathode of 6 mm in diameter
is disposed coaxially. Flat copper anode (thickness 4 mm)
with a central hole (1.5 mm) is located at the tube tip.
Interelectrode gap is fixed at 0.7 mm. Air flow of 3-10
l/min is provided into discharge chamber through a hole
of 1.5 mm in diameter drilled along an axis of rod-shaped
cathode.
The schematic of electric circuit and diagnostics
system of the discharge is shown in Fig. 1. A glow
discharge is powered by two power supplies U1 and U2.
One of them (U1) has a DC output high voltage of about 3
kV. Obviously, this voltage value is larger than the
breakdown voltage for a gap of 0.7 mm in atmospheric
pressure air. However, the task of this power supply is to
provide the ignition and continuous maintenance of
microdischarge at any air flow in the range of 1-7 l/min.
Ballast resistance R1 = 300 kΩ is chosen in such a
manner so that the discharge current is in the range of 510 mA.
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The second power supply U2 has AC output voltage in
the range of 1.5-4 kV at the network frequency or at
frequency of 400 Hz. It allows to increase discharge
current up to a few hundreds of milliampere and to
change a current waveform (DC, ripple, self-pulsing etc.)
by a variation of electrical circuit parameters. These two
power supplies are electrically isolated with two diodes
D1 and D2. For realization of a ripple current regime we
used AC high-voltage from source U2. For realization of
self-oscillatory regime we used RC-circuit with variable
capacity C1 and C2, which ranged from 300 pF to 50,000
pF. In accordance with the capacity values the resistance
R2 is adjusted from 0 to 300 kΩ. Ballast resistor R3 is
about 1 kΩ. Resistors R4 and R5 are used as voltage
dividers at a ratio 100:1 for the voltage waveform
registration with oscilloscope O (C8-40, Belvar). Resistor
R6 = 50 Ω is used to register current waveforms.
Fig.1. Scheme of generation and diagnostics of plasma
jet.
The image of air plasma jet at air flow of 5 l/min is
presented in Fig. 1 (at the bottom). Its length (the glowing
area) is about 3 cm and can be altered by means of
variations in discharge current, air flow, discharge gap
etc.).
The bactericidal components in plasma jet are detected
by optical emission and infrared absorption spectroscopy.
A high resolution scanning monochromator MDD-500x2
(two diffraction gratings) with a photoelectric registration
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of the emission spectra of plasma jet is used. The inverse
linear dispersion of the monochromator is 0.5 nm/mm.
We used a photoelectric multiplier FEU-171 as converter
of light intensity into electric signal.
The absorption spectra are registered using a Fourier
IR spectrometer Nexus (Thermo-Nicolet) with a gas cell
186-0305 (Perkin-Elmer), controlled optical path from 11
cm up to 10 m, and germanium windows. Spectrum
registration is carried out using a DTGS detector at
spectral range of 600-4000 cm-1 with achieved resolution
of 2 cm-1 after 128 scans. Usually in the experiment,
optical path of the gas cell is 135 cm. Gas collection into
the cell is carried out using a tube with a diameter of 3.5
mm. The tube is placed on the jet axis in parallel to the
gas flow direction (Fig.1). The absorption spectra
simulations are performed using HITRAN database of
spectral data [4].
3. DC plasma jet
Images of air plasma jet are shown in Fig. 2, a and b at
DC discharge current of 30 mA, and at air flow of 3 l/min
and 10 l/min correspondingly. In the first case, air flow is
laminar; in the second case it is turbulent. A transverse
dimension of laminar jet is thinner, but its length is larger
in comparison with the turbulent jet.
a
b
c
a
b
Fig. 3. End-on (a) and side-on (b) emission spectra of air
plasma jet.
In the absorption spectrum of jet registered at the
distance of 4 cm from the edge of the anode the
vibrational-rotational bands of H 2 O and CO 2 are
observed. These gas components are present in the
plasma-forming gas, since a stream of ambient laboratory
air is used. The presence of jet components causing death
of the microorganisms is more interesting. In order to
observe clearly that these are present in spectrum, the
more intensive bands of CO 2 and H 2 O of the absorption
spectrum are removed by subtraction. The resulting
absorption spectrum of bactericidal components is
presented in Fig. 4.
d
Fig. 2. Spatial temperature profiles in laminar (a) and
turbulent (b) air jets.
Spatial temperature profiles in both the laminar and
turbulent jets are shown in figure 2, c and d. The
temperature was determined using a typical thermometer.
As it can be seen, a laminar jet is hot at the distance up to
4 cm, while turbulent jet occupies larger space. So, at a
distance of 15 mm from the anode, temperature varies
from 80 ºC to 35 ºC, while a gas flow changes in the
range of 3–10 l/min.
Air plasma jet spectra are registered in two directions,
namely, along the jet axis (end-on) and at a side-on
observation. The vibration-rotation bands of NO γ , OH (AX) and N 2 are present in an end-on spectrum of the jet
(Fig. 3, a). A side-on jet emission spectrum is radically
different, and its intensity is several orders lower (Fig. 3,
2
b). In bactericidal range, the bands NO and OH are
observed. Broad NO2 band is observed as well, probably,
due to the chemiluminescent three-body reaction [5].
Fig. 4. Absorption spectrum of bactericidal components.
Fig.5. Mole concentration of active chemical components
of air plasma jet.
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As it can be seen in Fig. 4, the bands of nitric oxide,
nitrogen dioxide and nitrous acid appear clearly in the
spectrum of the air jet after the fulfilled subtraction. The
absorption spectra are obtained at several distances from
the outlet hole of the jet (a few millimeters from it and at
a distance of 10 mm). Using these spectra, mole fractions
of NO, NO 2 and HNO 2 are determined. Their dependence
on distance is shown in Fig. 5. Mole fractions of NO,
NO 2 and HNO 2 monotonically decrease from 400 ppm,
350 ppm and 100 ppm close to the anode down to 40
ppm, 20 ppm and 10 ppm at a distance of 4 cm from the
anode.
Unfortunately, we cannot determine the concentration
of OH radicals using absorption spectra. This happens due
to the high values of reaction rate constants for OH and
products of plasmachemical reactions that are present in
the jets [6] as well. The concentration of OH radicals
decreases drastically as soon as its forming stops, i.e.
outside the interelectrode gap. Therefore, OH practically
is absent in the spectra registered at a distance of 4 cm.
This is also the reason why hydroxyl cannot be detected
using our IR-absorption spectroscopy method. Absorption
spectrum is usually recorded not directly from the jet, but
some time after the collection of gas into the cell and its
installation into Fourier spectrometer.
In order to increase bactericidal components
concentration, the increase in electrical power deposited
into plasma is needed, but not in the gas temperature of
the jet. Pulse current discharges can be useful for this
purpose.
intensity fluctuations in active discharge region are in
phase with current fluctuation, the jet intensity
fluctuations have a phase delay of 100-200 µs. This time
is defined by the air flow velocity (40-50 m/s) in the
anode hole, the distance (6-8 mm) between active
discharge region and observation point in the jet.
The discharge regime when DC and ripple voltages
are applied to electrodes can be more preferable (Fig. 7).
In this case, a DC discharge at a current of 30 mA burns
continuously, but the discharge current increase from
additional ripple voltage occurs without the breakdown
effects, which accompanies every voltage half-wave in
the previous case and can lead to the electrode sputtering.
4. Ripple current plasma jets
In this experiment, AC voltage from power supply U2
at frequency of 400 Hz is supplied to diode bridge D3-D4
(Fig. 1), while power supply U1 is turned off. After that,
ripple voltage at frequency of 800 Hz is fed to the
electrodes. Fig. 6 shows the registered waveforms of
voltage and the corresponding discharge current and jet
light intensity waveforms.
5. Self-oscillatory current plasma jet
A self-oscillatory mode of plasma jet can be realized
due to the falling current-voltage characteristic of air glow
discharge [7]. Parallel connection of a variable capacitor
~102–105 pF (Fig. 1) to the interelectrode gap induces
relaxation oscillations with frequency up to 100 kHz. Fig.
8, a shows the current and jet glow intensity waveforms
for the case of the large capacity of 0.1 mF. It is a spark
discharge regime with infrequent pulses with duration of
5-10 µs. As it can be seen, the jet glow appears about
100 µs later, while glow in the interelectrode gap follows
the discharge current. However, this discharge regime
cannot probably be applied for processing of biological
samples because of electrodes sputtering.
Fig. 7. Waveforms of the discharge current (top),
interelectrode voltage (middle) and jet intensity (bottom)
for a gas flow of 5 l/min for DC (30 mA, dashed lines) +
ripple voltage.
Fig. 6. Waveforms of the discharge current (top),
interelectrode voltage (middle) and jet intensity (bottom)
for a gas flow of 5 l/min.
As it can be seen, a glow discharge is ignited when
voltage reaches the breakdown values for a given gap in
air. Light intensity of the jet follows the current at air flow
rate higher than 5 l/min. At lower gas flow (3 l/min), the
jet is not stable. It should be noticed that while light
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Fig.8. Single pulse current and light intensity
waveforms.
A self-pulsing regime in combination with DC
discharge is more preferable for this purpose. Current,
voltage and light intensity values for DC discharge at
current of 30 mA are shown by dashed lines in Fig. 9.
When an additional capacitor of about 330 pF and ballast
resistor R1 ~ 50 kΩ are connected to the electrodes, the
periodic current pulses of ~200 mA and ~ 0.5 µs long are
observed. They are accompanied by the voltage drop from
600 V to 300 V. While current and voltage are pulsing,
the jet glow is continuous.
Fig. 9. Waveforms (b) of current (top), voltage and
light intensity of jet (bottom) in DC (dashed lines) +
self-oscillating regimes.
6. Concentrations of bactericidal components
In the experiments, the temperature (Fig. 10) and the
concentrations of bactericidally active components (Fig.
11) are determined for each of the resulting plasma jets
with the corresponding current waveforms. The applied
average electrical powers to plasma are naturally different
for all these cases. The lowest temperature is observed in
the plasma jet in a self-oscillatory current mode with a
power of about 9 W. The maximal temperature is
observed for the DC current glow discharge at 30 mA
with additional ripple voltage at frequency of 800 Hz,
resulting in average power of about 52 W. It should be
noted that the temperature in the area of interaction with
microorganisms (3-4 cm) is not very different for all
current modes of plasma jets and it is about 45 °C
(Fig. 10).
Fig. 10. Axial profiles of temperature air of the
plasma jet at different discharge current regimes.
For each of the discharge current regimes, the
concentrations of bactericidal components are determined.
Their dependences on applied average electrical power
are presented in Fig. 11. As it can be seen, the
concentration of nitrous acid changes only slightly with
the power increase. On the other hand, the concentration
of nitrogen dioxide increases almost by one order of
magnitude when the power increases from 10 to 50 W. At
the same time the nitrogen oxide concentration increases
more than 2 times.
Fig. 11. Concentrations of bactericidal components
against average power.
Due to the difference in concentration growth rate of
NO and NO2, their ratio changes according to the power
of discharge from 3 to 1. Тhe same values of
concentrations are achieved in the plasma jet when larger
power of 52 W is applied to discharge plasma. In the selfoscillating current mode, when an average electrical
power applied to discharge is about 9 W, concentrations
of these bactericidal components differ significantly.
Thus, a self-oscillating DC jet mode is promising for
the biomedical applications, because the short current
pulses do not lead to a substantial heating of jet plasma,
while a production of reactive oxygen- and nitrogencontaining particles is rather effective in this case. By
changing the discharge power we may control a
quantitative composition of bactericidal components in
the plasma jets. This may be of interest for NO-treatment.
7. Acknowledgement
This work is partially supported by BRFFR under the
grant F14SRB-001.
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8. References
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[4] http://www.cfa.harvard.edu/HITRAN/.
[5] K. H. Becker, W. Groth, D. Thran, Chemical Physics
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