22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Diagnostic of underwater discharge based on pin-hole configuration
V. Mazankova1, F. Krcma1 and B. Obradovic2
1
Faculty of Chemistry, Brno University of Technology, Purkynova 118, Brno 612 00, Czech Republic
2
University of Belgrade, Faculty of Physics, Studentski trg 12, 11000 Belgrade, Serbia
Abstract: A novel plasma jet based on generation of so-called pin-hole discharge is
presented. The DC jet of both polarities in NaCl solutions of various conductivities was
used for the presented study. The plasma rotational temperature, electron temperature and
electron concentration were calculated from the discharge emission spectra in axial
resolution.
Keywords: underwater discharge, plasma jet, optical emission spectroscopy
1. Introduction
Electrical discharges in liquids have been subject of
many studies during the last years mainly focused on the
formation of a conductive channel in the discharge gap
filled with liquid, its diagnostic and various potential
applications [1-4]. Nowadays, discharges in liquids are
being studied extensively also for advanced chemical,
biotechnology as well as medical applications. The new
promising direction in application fields ranges chemical
synthesis [5], surface activation and cleaning [6] and
medical applications and sterilizations [7].
The creation of electric discharges in liquids is very
complex and it is not fully understood, because they are
operating under extreme conditions. Generally, there are
two groups of theories describing the discharge ignition in
liquids, electron and thermal (also called as bubble)
theory [8]. The electron theory is based on the fact that
water molecules are ionized and dissociated by the
applied very high electric field, and plasma creation is
more or less analogical to the Townsend’s theory of
electron avalanches in gases. According to the thermal
theory, liquid is heated by passing current which leads to
its evaporation and bubble (or more exactly micro bubble)
formation. Subsequently, the discharge is ignited in the
gaseous phase inside the bubbles due to the potential
gradient over the bubble size.
Another specificity of underwater discharges is their
configuration. The most studied configuration is a pointto-plate electrode geometry [9] where DC high voltage up
to tens of kV is applied on the small tip immersed into the
grounded liquid. The coaxial configuration [10] is a
modification of the previous one, and it is more suitable
for water treatment applications in a flowing regime. The
pin-hole systems where the discharge is created inside a
small orifice connecting two chambers filled by any
conductive solution (each chamber contains one of the
electrodes) were intensively studied, too [11-13]. Besides
the DC pulsed high voltage, also AC, high frequency,
microwave or DC non-pulsed voltage regimes can be used
for the generation of this kind of the under liquid
discharge.
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In the work presented here, we investigate the operation
of an underwater discharge jet based on pin-hole
configuration that was very recently developed by our
research team. The electrical measurement, fast camera
imaging and the optical emission spectroscopy were
performed.
2. Experimental set up
The plasma jet based on the DC pin-hole configuration
of underwater discharge was used for this experimental
study. A simplified schematic drawing of the used
experimental setup and its photo are given in Fig. 1.
Fig. 1. Schematic drawing of the plasma jet (left) and
reactor with installed plasma jet (right). 1 – ceramic body,
2 - inner tungsten wire electrode (diameter of 0.6 mm),
3 – space with increased current density, 4 – outer copper
electrode, 5 – silicon insulation.
A specially constructed discharge reactor (total volume
of 250 ml) was used in the study of the pin-hole discharge
formation, see in Fig. 1 (right). There are two quartz
glasses (diameter of 40 mm) in opposite walls of
polycarbonate reactor chamber. The plasma jet consists of
copper envelope as an outer grounded electrode (diameter
of 12 mm), dielectric body from Macor ceramics and
tungsten high voltage inner electrode (diameter of
0.6 mm). Distance between the inner electrode tip and the
1
nozzle end in the ceramics is adjustable. The DC jet of
both polarities (mean voltage of 650 V and mean current
0.12 A) in NaCl solutions of various conductivities (1501200 μS/cm) was applied. The jet position was vertically
adjustable using micro screws, with accuracy better than
0.1 mm.
The optical spectra were recorded by Jobin Yvon
monochromator TRIAX 550 with CCD detector. A
300 g/mm grating was used for overview spectra
acquisition in the range of 400–850 nm; the 3600 gr/mm
grating was applied for H β line profile acquisition as well
as for the well resolved OH radical A-X transition. The
light emitted from the discharge was focused by quartz
lens (f=100 mm) on the small orifice (diameter of 0.5
mm) mounted just at the front of the optical multimode
quartz optical fibre leading light on the monochromator
entrance slit. The emission spectra were recorded in axial
resolution better than 0.5 mm. The atomic hydrogen (H α ,
H ß , H γ ), oxygen and sodium spectral lines and OH bands
were recorded in all spectra (Fig. 2). The plasma
rotational temperature was calculated according
Boltzmann plot technique using well resolved OH radical
spectra. Electron temperature was calculated from H β and
H γ line intensities. The electron concentration was
calculated using Stark broadening of H β line profile and
simple deconvolution procedure. The mean energy
dissipated in the system was calculated using integration
of current and voltage over 0.1 s to eliminate discharge
instabilities and its self-pulsing character (see Fig. 3).
generation. The energy supplied into plasma during these
peaks also results in high drop of the discharge voltage.
Thus, the mean electrical power was calculated as a
quantity characterizing the discharge. Also due to this
fact, all the spectroscopic measurements were carried out
with integration times at least 0.1 s to minimize these
strong discharge instabilities. Fig. 4 presents
dependencies of calculated mean electrical power at the
constant mean current of 150 mA (stabile discharge
operation) on solution conductivity for both polarities.
The electrical power is decreasing with the conductivity
increasing at both polarities, as it was expected, and
significantly higher power consumption is observed if
discharge negative polarity is used.
Fig. 5 presents the mean (i.e. average over whole
discharge volume) electron concentrations calculated
from H ß line profile. The Doppler broadening was
supposed nearly the same in all cases because rotational
temperature obtained from OH radical spectra is slightly
dependent of the discharge conditions, only, as it is
demonstrated by Fig. 6. The electron concentration
decreases with conductivity increasing, but the
concentration in positive polarity is significantly lower
mainly at the higher solution conductivities.
Fig. 2. Overview spectrum of the plasma jet in NaCl
solution.
3. Results
Fig. 3 shows typical waveforms of discharge voltage
and the current measured under conditions used in this
work in the both polarities. A positive polarity means that
inner electrode is as anode and water solution was
grounded, the positive high voltage was applied on liquid
and inner electrode was grounded under negative polarity.
In contrary to normal DC non pulsing voltage supplied
pin-hole discharges [14] there are appeared high current
peaks with very short duration. These pulses are also
accompanied by very strong acoustic shock waves
2
Fig. 3. Current and voltage wave forms of the plasma jet
in NaCl solution (500 μS/cm) in the positive polarity (top)
and negative polarity (bottom) of inner electrode.
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The yellow colour in negative polarity is due to much
stronger emission of sodium doublet at 589 nm.
Fig. 4. Dissipated power of the plasma jet in NaCl
solution.
Fig. 7. Photos of plasma jet in positive polarity (left) and
negative polarity (right).
Fig. 5. Mean electron density of the plasma jet in NaCl
solution.
Fig. 6. Axial profiles of plasma rotational temperature at
the selected discharge conditions.
The OES spectra were measured in axial resolution.
The intensity profiles for OH radical integral intensity and
H β line are shown in Figs 8 and 9, respectively for the
selected discharge conditions. These graph clearly
demonstrate that maximum emission intensity is in higher
distance from the nozzle tip in the negative polarity with
nearly no conductivity influence. Also, the OH radical
emission is visible from much larger volume that
emission of atomic hydrogen.
Fig. 8. Axial profile of OH intensity for selected
conductivities.
Besides the global characteristics shown in Figs 3-5, the
axially resolved studies were carried out at the selected
conditions. The discharge appearance strongly depends on
the jet polarity as it is demonstrated by photos shown in
Fig. 7. The discharge dimension is much higher in the
negative polarity and also its overall brightness is higher.
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3
Fig. 9. Axial profile of H β intensity for selected
conductivities.
Probably the most interesting observation is
demonstrated in the Fig. 8 and Fig: 9. While OH radical
emission decreases with the solution conductivity increase
in negative polarity, the opposite and even much stronger
effect is visible in the case of H β line intensity. All these
effects will be studied in more detail very soon because
the spatial distribution of discharge created active species
is critical with respect to future applications.
4. Conclusion
The presented contribution gives the first results of
electrical and OES measurement carried out on the newly
developed plasma jet generated in liquids based on the
pin-hole configuration. The results demonstrate mainly
the strong dependence of the discharge characteristics on
the jet polarity; the influence on the solution conductivity
is pointed, too. The formation of active species like OH
radicals and atomic hydrogen is studied in axial resolution
and shows that maximal presence of these species is
spatially non-uniform and depends on the species kind.
The rotational temperature of plasma is nearly
independent on the discharge conditions. The electron
concentration increases with the solution conductivity
however the dissipated power is decreasing. Much higher
ionization is observed in the negative discharge.
Due to the strongly self-pulsing discharge operation,
more experiments using time resolved imaging are
planned for the near future to be able to understand fully
the discharge creation and propagation as well as to
determine
production
of
the
active
species.
4
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Acknowledgments
This work was done under COST Action TD1208 STSM
and it was also supported by Czech Ministry of
Education, Youth and Sports under project No. LD14014.
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