Download Time resolved images of an atmospheric pressure plasma bullet

Time resolved images of an atmospheric pressure plasma bullet
Dejan Maletić1,3, Saša Lazović1,3, Nevena Puač1,3, Gordana Malović1,3,
Antonije Đorđević2 and Zoran Lj. Petrović1,3
1
2
Institute of Physics, Pregrevica 118, 11080 Belgrade, Serbia
School of Electrical Engineering, University of Belgrade, Bulevar kralja Aleksandra 73, 11000 Belgrade, Serbia
3
University of Belgrade, Studentski trg 1, 11000 Belgrade, Serbia
e-mail: [email protected]
Abstract: Plasma bullet is a relatively new plasma source with a large field of
potential applications, from biomedical to material processing and surface
activation. Our plasma source was made of Pyrex glass tube with inner diameter
of 4 mm and outer diameter of 6 mm. Electrodes were made of thin copper foil
(13 mm wide) and the gap between the electrodes was 10 mm. The power supply
was a waveform generator connected to an HF amplifier and custom-made HV
transformer. The frequency that we used was 80 kHz and the applied voltage was
in the range of 6-10 kVpeak-to-peak. In this paper, we will show time-resolved ICCD
images of our plasma bullet device and how the emission changes with the
applied power and flow of working gas. The power transmitted to the plasma was
calculated and it was lower than 10 W.
Keywords: plasma, atmospheric, bullet, ICCD camera, time resolved
1. Introduction
In the last few decades, there has been a huge
advance in plasma research; many atmospheric
pressure plasma devices have been constructed and
analyzed using various diagnostic techniques [1,
2, 3]. There is large potential use of atmospheric
pressure plasmas in surface modification, plasma
etching, thin film deposition, medicine and
cosmetology. The low gas temperature is suitable for
the treatment of thermo-sensitive samples like
polymer materials and biological samples.
Dimensions of plasma can be very different, from a
few millimeters, suitable for microsurgery and
stomatology, to large plasmas appropriate for
treatment of large surfaces like wounds, textile and
plant seeds [4, 5, 6].
Some of the well-known small-size plasma
sources are: plasma needle [3, 7], APPJ [8], plasma
bullet [9], plasma torch [10] and floating electrode
dielectric barrier discharge plasma [11]. Their
electrode configuration, voltages and excitation
frequencies are very different; some of them work at
13.56 MHz and other at 5-120 kHz in sine or pulse
regime. Some authors recently reported that the
plasma jet that is formed with low excitation
frequency is not continuous, but instead consisted of
small plasma packages that are formed in positive
and negative half cycle of the period [12]. The
velocity of these packages is much larger than the
speed of the flowing feed gas. In this paper, we will
present our results of time-resolved images of
plasma bullet obtained by using ICCD camera.
2. Experimental setup
The atmospheric pressure plasma jet that we used
is made of a Pyrex glass tube (I.D. 4 mm and O.D.
6 mm). Electrodes were made of a thin copper foil
wrapped around the glass tube. The distance
between the powered and the grounded electrode
was 10 mm. The width of both electrodes was
13 mm. The experimental scheme is given in figure
1. The left electrode was grounded and the second
electrode, closer to the end of the glass tube, was
powered. The distance between the powered
electrode and the end of the glass tube was 5.6 mm.
The feeding gas was helium and the flow rates used
in this work were 2, 3, 4 and 5 slm. The flow rate is
adjusted with a mass flow controller (Omega
FMA5400/5500).
3
Voltage no plasma
6
Voltage plasma
Current no plasma
Current plasma
2
4
2
0
0
U[kV]
I[mA]
1
-2
-1
-4
-2
5slm
3.5Vpp
Figure 1. Experimental setup.
Pa = 3 W
55
For the current and voltage measurements, we
used two commercial probes and two oscilloscopes
(Agilent DSO3202A). The first probe, a highvoltage probe (Agilent N2771A), was connected to
the HV-output and used for obtaining voltage
waveforms. For the current waveforms we used the
second probe (Agilent 10076A) which measured the
voltage drop on a 100 kΩ resistor placed in the
grounded branch of the electrical circuit (see Fig. 1).
At the same place the third probe was connected
(Agilent 10076A) for external triggering of the
ICCD camera (Andor iStar DH734I). We have used
camera’s internal delay generator for delaying
camera gating and for external triggering of the
oscilloscope. The working frequency was 80 kHz
and the applied voltage was in the range 610 kVpeak-to peak. The power tranmited to the plasma
did not exceed 8 W during all measurements.
65
70
75
Figure 2. Signals of the current and voltage without
plasma and when the plasma is formed for 5slm and
3.5Vpp at the signal generator (Pa = 3W).
4.0
3.5
3.0
2slm - direct
2slm - reverse
3slm - direct
3slm - reverse
5slm - direct
5slm - reverse
No plasma
2.5
2.0
0.5
1.0
1.5
2.0
2.5
Irms [mA]
Figure 3. Vrms as the function of Irms for different
flows of helium.
No plasma
Plasma 4slm
Difference
10
3. Results and discussion
8
Pa [W]
Current and voltage waveforms when the plasma
is formed and without discharge are shown in figure
2. When the plasma is off, the phase difference
between the current and voltage is close to 90˚. In
this case, we have a capacitive impedance of several
MΩ, corresponding to the capacitance of about
0.5 pF. On the other hand, when plasma is formed,
the current signal is larger, deformed and shifted in
phase towards the voltage signal. The plasma
ignition introduces a parallel nonlinear load into the
electrical circuit and in this case the slopes of the
VRMS–IRMS curves are lower (see Fig. 3.).
60
t[s]
Vrms [kV]
For powering the plasma jet, we used a signal
generator (Peak Tech DDS function generator 4025)
connected to a custom-made amplifier connected to
an additional homemade step-up transformer.
-6
-3
6
4
2
0
0
1
2
3
4
5
Vpp signal generator [V]
Figure 4. Dependence of the average power and the
power difference when the plasma is on and off as
the function of voltage at the signal generator.
Figure 5. Plasma jet at 5 slm of helium, 6 W,
exposure time 6 ms, gate width 5 ms.
We calculated average powers from signal
waveforms when the plasma is on and off and
difference between them (see fig. 4). The mean
power increases with the increase of the applied
voltage. One can see (fig 4.) that power transmitted
to the plasma was in the range from 1 to 8 W.
For the exposure times larger than the cycle
period (12.5 s), the plasma looks continuous, like a
plume (see fig. 5). The length of the plasma plume is
up to five centimeters, depending of the flow rate
and applied voltage.
For the time-resolved images, we have used
integration on the chip because the light emission in
a single shot is not always sufficient to obtain clear
images with gate widths less than 25 ns. In figure 7
we show the propagation of the plasma for the entire
period of excitation signal (12.5 s). All images are
scaled to the same maximum intensity and they can
be compared to each other. We can see that when the
current and voltage signals are close to zero, the
plasma is not visible. In the negative part of the
current and voltage waveforms, the plasma is
confined between the electrodes. During the positive
part of the waveforms, the plasma is first confined
between the electrodes (rising slope) and then, near
the maximum of the curves, it leaves the glass tube
in the form of a bullet. The dimensions of the bullet
are very small, on the order of a few millimeters. We
calculated the speed of the bullet at ~20 km/s,
depending on the position from the end of the glass
tube. The plasma bullet is much faster than the speed
of the buffer gas flow (1 to 7 m/s).
Figure 7. Plasma jet at 5 slm, exposure time 2 ms,
gate width 25 ns and gate delay from 0.4 to 12.4 s.
Fig. 8 shows plasma bullet images obtained for
several different flows of gas. We can see that with
the decrease in the He flow, the plasma bullet starts
to be elongated, deformed and its intensity is much
smaller.
Figure 8. Plasma jet at 1, 2, 3, 4 and 5 slm.
Exposure time 2 ms, gate width 25 ns, gate delay
10.8 s.
4. Conclusion
In this paper we have presented current-voltage
characteristics and ICCD images of the atmospheric
plasma jet. The results show that our plasma source
was not continuous, but it consisted of very small
plasma packages that traveled at high speed. By
varying the plasma parameters, the length and
intensity of the plasma coming out of the tube can be
adjusted.
5. Acknowledgement
This research has been supported by the MNTR,
Serbia, under the contract numbers ON171037 and
III41011.
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