21st International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4th August – 9th August 2013
Cairns Convention Centre, Queensland, Australia
Time Resolved Spectroscopy of a Nanosecond Pulsed Discharge in Water
Y. Seepersad1, 2, A. Fridman1, 3, D. Dobrynin
1
Drexel Plasma Institute, Camden, NJ, 08103, USA
Department of Electrical and Computer Engineering, Drexel University, Philadelphia , PA, 19104, USA
2
Department of Mechanics and Mechanical Engineering, Drexel University, Philadelphia , PA, 19104, USA
2
Abstract: A time revolved spectroscopic study of the emission phases occurring throughout a
nanosecond pulsed discharge in water is presented. Discharged are ignited in deionized water
via a submerged pin-t-plane electrode configuration by application of 24kV pulses with a rise
time of 4ns and a duration of 10ns. Optical emission spectroscopy of the plasma radiation is
performed, revealing a very complex nature of the excited species created.
Keywords: Optical emission spectroscopy, nanosecond discharges, plasma in liquids
1.
optical emission spectroscopy analysis. Optical emission
spectroscopy still remains one of the most useful noninvasive tools used to study plasma, however many of the
techniques that have been developed for gas phase plasmas
are not easily ported to use when treating with plasma in
liquids. Previous studies performed on similar phenomena
can be found in [9, 10].
Introduction
Plasma produced in liquids continues to be a heavily
researched area amongst many groups. The growing interest
in this field is easily justified by the wide range of
applications plasmas in liquids are being employed for. A
recent review by Graham and Stalder [1] (and the extensive
list of references within) provides a good flavour of the
many applications such discharges have found over the last
few decades.
A universal mechanism describing the formation of
plasma in liquids has yet to be settled upon within the
scientific community, and as research in the field progresses
it seems more unlikely that such an overarching theory can
exist. In [2-5], one can find the definitive experimental
evidence for different mechanisms of plasma initiation and
development in liquids which are a consequence of
variations in the experimental conditions. While [3]
discussed evidence of a clear bubble-assisted initiation
phase associated with microsecond pulsed discharges, [5]
interestingly suggests direct ionization as the mechanism for
ultrafast (sub-nanosecond) pulses.
In this work, we shall focus our attention on nanosecond
pulsed discharges generated in water. In this regime, the
bulk fluid lacks time to expand (no bubbles), however, there
is enough time for dipole reorientation and the possibility of
an “electrostrictive mechanism” of initiation [6-8]. The
emission phase of nanosecond pulsed discharges has been
studied previously in [7, 8]. These discharges evolve over
the duration of the pulse in 3-stages: initiation on the rising
edge of the pulse; plasma quenching during the plateau of
the pulse; and, re-ignition on the falling edge of the pulse.
The spectral content of the emission throughout each
phase is studied via a nanosecond resolution, time-resolved
2.
Experimental Setup
The layout of the experimental setup is depicted in Figure
1 below. Similar to the work in [6, 7] the discharges studied
in these experiments were generated via a point-to-plane
electrode setup. The anode was a 50mm long, 106μm
diameter, Iridium needle, with an electrochemically etched
tip of radius of curvature
(IR20030.1A4,
MicroProbes for Life Sciences). The entire needle was
insulated in Teflon and epoxy except for the last 2mm
which was acted as the effective anode in contact with the
liquid. An 18mm diameter copper plate served as the
cathode, and the electrode gap was 3mm throughout the
experiments. The chamber held ~50ml of liquid when
completely filled and was left open to atmospheric pressure,
with all the experiments being performed in deionized
(Type II) water with conductivity
.
Nanosecond pulse generators from FIDTechnology were
used to produce high-voltage pulses
in amplitude,
with 4ns rise time (10 – 90%) and 10ns duration, with a fall
time of 5ns. Pulses were delivered to the electrodes over
50ft of RG 393/U coaxial cable allowing a 146ns time delay
between the incident pulse and the twice reflected wave on
the discharge gap. A back current shunt (BCS) as described
1
in [8] was used for waveform monitoring and pulse
synchronization.
Normalized Calibration Irradiance
Measured Spectrum
1.2
1
0.8
0.6
0.4
0.2
0
-0.2
200
400
Measured Spectrum Intensity
Manuf. Calibrated Points
Manuf. Calibration Formula
600
800
1000
1200
(nm)
Figure 2: Spectra used for detector calibration. Both curves
normalized for comparison purposes. Y-axis units are
arbitrary.
Figure 1: Schematic of experimental setup.
It was previously shown [5, 7] that the emission phase of
these discharges lasts only for a few nanoseconds after the
duration of the applied pulse, and thus we only need focus
our attention on the
over which the voltage pulse
lasts. Time resolved spectra are captured with
gating of
the camera in steps of
in order to trace the development
of the spectrum.
Light from the emission stages of the discharge was
collected using a fiberoptic bundle and coupled to a
Princeton Instruments, Acton Research SP2500i
monochromator with a
focal length and a
grating. The effective entrance slit size was
as
determined by the core size of the fibre-optic cable. The
detector was a Princeton Instruments, PIMAX-III ICCD
camera, connected to an ST133A controller and interfaced
to a PC via WinSpec32 software. This camera controller
contained a built-in PTG module used for nanosecond gate
synchronization and triggering. Both the power supply and
camera controller were triggered from an external function
generator set to trigger discharge events at a frequency
of
. The signal delays were accounted for triggering
delays imposed through WinSpec32 software. Assurance of
accurate synchronization was monitored by observing the
camera gate signal and the BCS signal through the
Tektronix DPO4104B digital oscilloscope.
The variation in spectral response of the detector was
accounted for and used to correct the intensity profiles of all
of the spectra obtained. The detector response to a 1000W
mercury calibration lamp (Newport) was measured from
200 to 900nm using the same fibre-optic cable setup
mounted to the monochromator and camera, the result of
which is shown in Figure 2. The emission spectrum of the
lamp was supplied by the manufacturer, and is overlaid on
the same graph in Figure 2. The ratio the measured
spectrum to the incident irradiance was used to produce an
intensity calibration curve to correct the spectrum collected.
3.
Results
Firstly we remark that voltage ringing on the highvoltage (HV) line created multiple emission phases on the
electrode gap corresponding to each time the twice reflected
pulse reached the electrodes; the ringing transients lasted a
few microseconds and reflections were isolation by
.
The time-resolved spectrum over the emission phase during
the first incident pulse is shown in Figure 3 and Figure 4.
We observed an almost broadband emission intensity
profile throughout the duration of the entire emission phase,
with the absence of any well-defined peaks as seen with
other underwater-initiated discharges. For instance, with
DC excited discharges in distilled water as reported in [9],
the spectrum reveals distinct peaks corresponding to atomic
hydrogen excitation (Balmer series), the hydroxyl OH(A-X)
vibrational band near
and, and atomic oxygen at
and
. These characteristic spectral features
are however seen when the detector exposure is increased to
include emission phases from transient reflected pulses. The
camera was gated for a longer exposure and spectrum is
shown in Figure 5, highlighting the distinct presence of the
peak at
and the
peak at
.
2
x 10
Corrected Absolute Intensity
-4
0.9
20
0.8
15
0.7 Intensity
Corrected
10
0.6
5
0.5
0
0.4
-5
30
0.3
800
20
time ns
600
400
10
0
200
0.2
Wavelenght
nm
0.1
300
-4
0ns
3ns
6ns
9ns
12ns
15ns
18ns
21ns
24ns
27ns
10
700
800
-4
18
x 10
H Peak
16
Max Emission Peak
OH at 306nm
14
12
Intensity (a.u.)
Intensity
12
600
Figure 5: Long exposure spectrum including emission
phases occurring as a consequence of voltage reflections on
the HV line. Camera exposure: 2s, 100 accumulations.
x 10
14
500
Wavelength (nm)
Figure 3: Development of emission spectrum from 200 –
800 nm over duration of pulse. Camera exposure: 3ns, 100
accumulations.
16
400
8
6
10
8
6
4
4
2
2
0
0
-2
300
350
400
450
500
550
Wavelength
600
650
700
750
800
0
5
10
15
20
25
30
Time ns
nm
Figure 6:
Figure 4: Overlay plot showing development of
spectrum over 27ns, including before and after the pulse.
Camera exposure: 3ns, 100 accumulations.
At each of these wavelengths, we observe a small reduction
in the intensity occurring between
, corresponding
to the time of the voltage plateau and the expected dark
phase in the discharge development.
We note that the minimum gating time used in these
experiments (
) was limited by the acquisition of an
acceptable signal to noise ratio. At this somewhat “coarse”
time resolution, it was difficult to clearly identify the “dark
phase” associated with these nanosecond pulsed discharges
as reported in [5, 11]. The evolution of the maximum
intensity peak, the emission at the hydrogen alpha
wavelength and the
wavelegnth is shown in
Figure 6.
4.
Discussion
Our results reveal very little that allows application of
traditional analytical techniques to study the data. The lack
of distinct peaks in the spectrum inhibits measuring such
characteristics as line broadening (FWHM), and thus
3
methods for measuring parameters such as electron
density . It is expected that both Van der Waals effect as
well as the Stark effect contribute significantly to the
complex spectral broadening observed, as also hypothesized
in [10]. The intensity maximum of the spectra occurs
between the wavelengths for
(
) and
(
). These results could offer new insight into
electronic structure of ionized water molecules still in the
condensed liquid phase. Plasma generated via the bubble
mechanism is associated with clear atomic transitions of
hydrogen and oxygen not seen in these nanosecond
discharges. The observed continuum of emission in these
experiments might be associated with undissociated, ionized
water molecules in an environment very high pressure
(number density) and high degree of ionization.
5.
7.
1.
Graham, W.G. and K.R. Stalder, Plasmas in
liquids and some of their applications in nanoscience.
Journal of Physics D: Applied Physics, 2011. 44(17): p.
174037.
2.
Kolb, J.F., et al., Streamers in water and other
dielectric liquids. Journal of Physics D: Applied Physics,
2008. 41(23): p. 234007.
3.
Bruggeman, P. and C. Leys, Non-thermal plasmas
in and in contact with liquids. Journal of Physics D:
Applied Physics, 2009. 42(5): p. 053001.
4.
Joshi, R.P., et al., Aspects of Plasma in Water:
Streamer Physics and Applications. Plasma Processes and
Polymers, 2009. 6(11): p. 763-777.
5.
Starikovskiy, A., et al., Non-equilibrium plasma in
liquid water: dynamics of generation and quenching.
Plasma Sources Science and Technology, 2011. 20(2): p.
024003.
6.
M. N. Schneider, M.P., A. Fridman, Theoretical
Study of the Initial Stage of Sub-nanosecond Pulsed
Breakdown in Liquid Dielectrics. IEEE Transactions on
Dielectrics and Electrical Insulation, 2012. 19(5): p. 15971582.
7.
Seepersad, Y., et al., On the electrostrictive
mechanism of nanosecond-pulsed breakdown in liquid
phase. Journal of Physics D: Applied Physics, 2013. 46(16):
p. 162001.
8.
Starikovskiy, A., Pulsed nanosecond discharge
development in liquids with various dielectric permittivity
constants. Plasma Sources Science and Technology, 2013.
22(1): p. 012001.
9.
Bruggeman, P., et al., Characterization of a direct
dc-excited discharge in water by optical emission
spectroscopy. Plasma Sources Science and Technology,
2009. 18(2): p. 025017.
10.
I. Marinov, O.G., A. Rousseau, S. Starikovskaia,
Spectroscopic and shadowgraphic investigation of
nanosecond underwater discharge, in ESCAMPIG
XXI2012: Viana do Castelo, Portugal. p. 2.
11.
Dobrynin, D., et al., Non-equilibrium nanosecondpulsed plasma generation in the liquid phase (water,
PDMS) without bubbles: fast imaging, spectroscopy and
leader-type model. Journal of Physics D: Applied Physics,
2013. 46(10): p. 105201.
Conclusion
These results identified the complex nature of the
emission spectrum associated with nanosecond pulsed
discharges in water. As compared with discharges ignited
with slower rising pulses (DC and microsecond pulsing), no
distinct peaks appear in the spectrum, such as the
commonly seen hydrogen Balmer lines. This either suggests
that there is almost little dissociation of the water molecule,
or that the electronic states approach a continuum under
these conditions of high number density with high
ionization degree. Further quantitative analysis of the data is
still in progress, and results will follow in a subsequent
report.
6.
References
Acknowledgements
This work was supported by Defense Advanced Research
Projects Agency (grant #DARPA-BAA-11-31).
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