Download Mass spectrometry and iccd analysis of coupled and uncoupled mode in a gatling-gun like plasma source

22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Mass spectrometry and iCCD analysis of coupled and uncoupled mode in a
Gatling-gun like plasma source
V. Colombo1, M. Gherardi1, Z.Lj. Petrović2, N. Puač2, N. Selaković2 and A. Stancampiano1
1
2
Alma Mater Studiorum – Università di Bologna, viale Risorgimento 2, IT-40136, Bologna, Italy
Institute of Physics Belgrade, University of Belgrade, Pegrevica 118, RS-11080 Belgrade, Serbia
Abstract: In the present study, the jet-to-jet coupling phenomenon occurring in a novel
type of plasma source, composed of an array of seven plasma jets arranged adjacent to one
another (Gatling-gun like) was investigated by using mass spectrometry, iCCD imaging and
electric measurements. Results shows that jet-to-jet coupling induces emission intensities
and ion concentrations at least one order of magnitude higher than uncoupled jets for
similar operating conditions.
Keywords: jet-to-jet coupling, mass spectrometry, multijet, ions spectra, iCCD
1. Introduction
A novel type of plasma source, composed of an array of
seven plasma jets arranged adjacent to one another,
similar in shape to a Gatling machine gun, was recently
developed to take advantage of the jet-to-jet coupling
phenomenon and to generate atmospheric pressure cold
plasmas with higher intensity and energy with respect to
singular plasma jets [1]. This source can be operated
either in “uncoupled” mode, where seven plasma jets are
independently produced, or in “coupled” mode, where
plasma jets merge in a single combined very intense jet.
Recent studies reported on the use of the coupled mode of
operation to overcome some of the limitations of
atmospheric cold plasmas as the ability to achieve etching
[2]. Previous experiments by some of the authors also
demonstrated a higher antibacterial potential and surface
activation efficacy of this source when operated in the
coupled mode than in uncoupled one [3].
In the present study, the coupling phenomenon
occurring in a Gatling plasma source was investigated by
using mass spectrometry, iCCD imaging and electric
measurements. Results show that in the coupled mode ion
concentrations are at least one order of magnitude higher
as compared to the uncoupled mode for similar operating
conditions. Consistently, ICCD acquisitions shows higher
emission intensity in coupled than uncoupled mode and
differences in plasma front propagation.
2. Gatling plasma source
The Gatling source adopted in this paper is an array of
seven PTFE capillaries (Ø ext 1.6 mm, Ø int 1 mm)
arranged adjacent to one another in an axisymmetric
structure with one in the centre and six surrounding it. A
schematic of the source is reported in Fig.1. The Gatling
source was driven by high voltage sinusoidal waveforms
(80 KHz, up to 6.3 kVpeak-peak). Helium was used as
working gas and different modes of operation were
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achieved by varying the gas flow between 2 slm (coupled
mode) and 4.5 slm (uncoupled mode).
To isolate the effect of the presence of the surrounding
jets on the ions composition of the central jet discharge in
coupled mode, a comparison with only one jet with the
same gas mass flow and excitation was performed. These
tests were achieved using a modified plasma source with
only one central jet ignited while the other jets had no
electrodes and were directed away from the substrate.
Fig.1 Schematic views of the plasma source and
spectrometer set up
3. Mass spectrometry setup
A molecular beam mass spectrometer (HIDEN HPR60)
was used to detect mass spectra of plasmas. The
secondary ion mass spectrometry mode (SIMS+/-) was
used to investigate the mass spectra of positive and
negative ions respectively. In all measurements the
Gatling gun was placed in front of the HPR60 5 mm and
9 mm away from the detection orifice (Ø 0.3 mm). The
pressures in all three stages (P 1 = 6.5x100 Torr;
P 2 = 2.4x10-5 Torr; P 3 = 4.1x10-7 Torr;) were kept constant
1
during the measurements. A photo of the Gatling plasma
source during spectrometry analysis is shown in Fig. 2.
As the mass spectrometer detection orifice has a
diameter of only 0.3 mm and is therefore smaller than the
area affected by the plasma generated by the source, the
detection of only one jet at a time in uncoupled mode is
possible. The coupled mode presenting only one jet may
be more easily detected by the mass spectrometer.
Nevertheless the uncoupled mode ions signal even
multiplied by a factor of 7 (to take into account the
undetected jets) results lower than the coupled mode
signal.
107
Fig.2 Photo of the Gatling plasma source in operation
during mass spectrometry analysis
Coupled 2 slm He
Uncoupled 4.5 slm He
Positive ions
6.08 kV, 4.1 Vpp, 9 mm
106
5
Counts [c/s]
4. iCCD and electrical measurements setup
The plasma structure in the two modes of operations
was investigated by means of an ICCD camera (Andor
iStar DH734I) with 25 ns exposure time synchronized
with the sinusoidal excitation waveform. The temporal
evolution of the discharge was scanned for an entire
period (12.5 µs) with a time step of 0.5 µs. A wide range
of operating conditions that granted coupled mode of
operation was also investigated by means of iCCD
acquisitions of 12.5 µs (one period) exposure time.
10
N+
O+
+
H
NO+ +
O2
+
OH
H2O+
N2+
H3O+
104
103
102
0
5
10
15
20
25
30
35
40
45
50
Mass [amu]
For this analysis the Gatling plasma source was
positioned in front of a copper plate grounded through a
100 kΩ resistance. The recorded voltage waveform was
achieved by means of a high voltage probe (Agilent
N2771B) in contact with the high voltage electrode while
the current measurement were obtained measuring the
voltage drop at the 100 kΩ resistance with a differential
voltage probe (Agilent 10076A).
Fig.3 Positive ions spectra for Gatling plasma source in
coupled and uncoupled mode of operation
107
Coupled 2slm He
Uncoupled 4.5 slm He
Negative ions
F6.08 kV, 4.1 Vpp, OH9 mm
-
106
O
-
H
From negative ions spectra it can be noticed that the
most abundant species are F-, OH-, O-, for both operating
modes. The presence of F- shows significant material
release from the PTFE capillaries wall. As for positive
ions spectra, the composition of negative ions does not
change with the change of the Gatling plasma source
operation mode. For negative ions the amount of detected
species is in some cases several orders of magnitude
higher in coupled mode than for uncoupled one. A
significant presence of heavy molecules (<35 amu) was
also detected in negative ion spectra for coupled mode.
2
Counts [c/s]
5. Mass spectrometry results
A selection of obtained positive and negative ions mass
spectra for the Gatling source during coupled and
uncoupled mode of operation is shown in Fig. 3 and 4.
For positive ions the most abundant species are N and O
ions and relative compounds. In the two modes of
operation the composition of positive ions is similar,
nevertheless the amount of positive ions detected is nearly
one order of magnitude higher for coupled mode than for
uncoupled one.
NO2-
O2- H3O
5
10
NO4
10
103
102
0
5
10
15
20
25
30
35
40
45
50
Mass [amu]
Fig.4 Negative ions spectra for Gatling plasma source in
coupled and uncoupled mode of operation
In Fig. 5 and 6 are reported the positive and negative
ions spectra for case where the Gatling source in coupled
mode was compared with the modified plasma source
presenting only the central jet ignited. During these test
the distance between the plasma source and the detection
orifice was reduced to 5 mm.
Similarly to what was previously discussed also in this
comparison the coupled mode presents higher positive
ions concentrations than the single jet source even if
multiplied by a factor of 7 to take into account the
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missing surrounding jets. For negative ions this trend is
even more accentuated as the difference in the recorded
signal is of several orders of magnitude. The presence of
heavy molecules in particular is observed only for the
coupled case. As expected both for negative and positive
ions the coupled mode signal results higher than that
previously shown as the distance from the orifice was
reduced from 9 mm to 5 mm.
107
Counts [c/s]
Coupled
Single Jet
Positive ions
5.04 kV, 4.14 Vpp
5 mm,
4.5 slm He
106
uncoupled mode, even if not all the jets are detected at the
same time, while only one front is observed for couple
mode.
105
104
103
102
0
5
10
15
20
25
30
35
40
45
50
Mass [amu]
Fig.5 Positive ions spectra for Gatling source in coupled
mode compared with the modified single jet source
107
106
Counts [c/s]
Coupled
Single jet
Negative ions
5.04 kV, 4.14 Vpp
5 mm, 4.5 slm He
105
104
103
102
0
5
10
15
20
25
30
35
40
45
50
Mass [amu]
Fig.6 Positive ions spectra for Gatling source in coupled
mode compared with the modified single jet source
6. iCCD and electrical measurements results
The temporal evolution of the plasma discharge
generated by the Gatling plasma source was investigated
by means of an iCCD camera. In Fig. 7 and 8 time
resolved images for the coupled and uncoupled mode of
operation are shown. In both picture are shown six
acquisition related to different instants of the voltage
pulse (t= 0 µs at the start of the positive half-period of the
voltage pulse). As it can be observed in the pictures
during the positive half period the plasma propagates
from the source outlet toward the grounded target. Light
emission in coupled mode starts later than in uncoupled
one. As expected multiple ionizing fronts are visible in
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Fig.7 Recorded voltage and current waveforms (top) with
iCCD acquisitions (25 ns gate) of the Gatling plasma
source (blue square on the right) in coupled mode
(He 2 slm, 9mm, 6.08 kVpp) at different instant of the
high voltage pulse waveform.
3
analysis the emission intensity of the coupled mode
appears to be always higher than the uncoupled one
(different colour scale in the figures).
Deeper investigations showed that for the coupled mode
of operation there is not always a direct correlation
between the voltage imposed by the pulse generator and
the voltage recorded at the high voltage electrode. In fact,
after a certain value the recorded voltage does not
increase with the imposed voltage but reaches a plateau,
while the current constantly increases. iCCD acquisitions
showed plasma discharges of increasing intensity and
cross section as the imposed voltage was increased and a
significant change in the plasma discharge structure as the
recorded voltage plateau was reached.
Fig.9 Recorded voltage and current values for different
pulse generator imposed voltage during operation in
coupled mode (He 2 slm, 9 mm gap)
7. Conclusion
Results show that in the coupled mode ion
concentrations are higher (at least one order of
magnitude) than in the uncoupled mode for similar
operating conditions. This is more pronounced for
negative ions and may suggest a key role of these species
in the coupling phenomenon. ICCD acquisitions also
show higher emission intensity in coupled than uncoupled
mode and differences in plasma front propagation.
This work presents new insights on the Gatling-like
plasma sources and more in general on the jet-to-jet
coupling phenomenon suggesting that the jet produced by
coupling phenomenon results to be different from simply
being the sum of the single jets.
Fig.8 Recorded voltage and current waveforms (top) with
iCCD acquisitions (25 ns gate) of the Gatling plasma
source (blue square on the right) in uncoupled mode (He
4.5 slm, 9mm, 6.08 kVpp) at different instant of the high
voltage pulse waveform.
In the negative half period, for both cases and with
similar timing, is possible to observe a counter
propagating front (repopulation of the PAPS tail).
Consistently with the mass spectrometry ions mass
4
8. Acknowledgments
Work partially supported by COST Action MP1101
“Biomedical Applications of Atmospheric Pressure
Plasma Technology”
9. References
[1] J. Kim et al, Plasma Process Polymers, 9, (2012)
[2] J. Kim et al, IEEE Transactions on Plasma Science,
39, 2338-2339 (2011)
[3] V. Colombo et al, IEEE Transactions on Plasma
Science, 42, 10 (2014)
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