Optical Diagnostic Of Low-pressure Plasma-liquid System With Secondary Discharge Supplied By Plasma Flow

I. Prysiazhnevych, V. Chernyak, Yu. Voevoda, E. Safonov, T. Lisitchenko
Plasma Lab, Dept. of Physical Electronics, Faculty of Radiophysics,
Taras Shevchenko Kyiv National University
Prospect Acad. Glushkova 2/5, 03127 Kyiv, Ukraine, e-mail: priv@ukr.net, chern@univ.kiev.ua
Abstract. The probe and spectroscopy studies are made of the low-pressure gas-liquid plasma system with a secondary
electric discharge. The existence of the potential jump near the plasma-liquid boundary and its dependence on the
secondary discharge voltage are demonstrated. The emission spectra of plasma of the secondary discharge in such system
were analyzed. It was shown that the most intensive spectral lines and bands belong to H, O, OH. The electronic
excitation temperature Te
* was determined from relative intensity of Hα and Hβ lines for different sections of the plasmaliquid
system. Its value depends on the treatment time t and polarity of the “liquid” electrode. It was found that the
behavior of the Hα line intensity and the distribution of Te
* over the time of plasma treatment are different near the liquid
surface and near the source of the auxiliary discharge.
Keywords: plasma-liquid system, secondary discharge, liquid electrode, and potential jump.
PACS: 52, 52.80.Wq
INTRODUCTION
Plasma chemistry has been developed commonly considering plasma as a chemical active medium, which
activity is provided by high temperatures and high concentrations of reactive components: ions, electrons, radicals,
excited particles, and photons. The price for such high activity of plasma is a low selectivity of plasma-chemical
transformations, i.e. the multi-channel passing of chemical reactions and weak control of this process. It was
generally supposed that the main way to increase a plasma-chemical selectivity is to transfer from thermal to nonequilibrium
plasma. However, gas-discharge sources of non-equilibrium plasma can guarantee only plasma nonisothermality
(when the temperature of electrons differs from characteristic temperatures of heavy particles) under
average electron’s energy lower than energies corresponding to the maxima of the cross-sections of excitation for
inelastic processes. Furthermore, the electron temperature varies only in the narrow range even under considerable
changes of external parameters of the gas discharge (current, voltage). The last remark is pertinent to the volume of
plasma, but not to the contact region of plasma with liquid or solid substance. That is why the two-phase gas-liquid
plasma systems, based on self-maintained discharge with one or two electrodes immersed into the liquid [1], are of
great interest today. This paper is dedicated to the study of physical processes in such plasma-liquid systems at lowpressure.
EXPERIMENT
The scheme of the experimental device used for studies of the low-pressure plasma-liquid system is shown on
Fig. 1. It consists of the cylindrical vacuum chamber –1, partially filled by liquid, the auxiliary source of the plasma
flow – 2 and stainless steal funnel-shaped electrode –3 of the secondary discharge. The funnel-shaped construction
of the electrode –3 (with diameter of cone base near 6 cm) decreases instability of liquid surface caused by gaseous
products of electrolysis occurring on this electrode. Used vacuum chamber was a quartz tube of 80 mm diameter and
160 mm of height, which ending covered by flat ebonite flanges – 4 with rubber edges – 5. Exhaustion of the
chamber was made by oil fore pump. The coaxial-end discharger 2 supported plasma flow along to the axis of the
vacuum chamber.

The voltage for the secondary discharge was applied between the metallic electrode 3 immersed into liquid and
anode 7 of the auxiliary discharge. The auxiliary discharge was powered by DC source.
Distilled water was used as working liquid. The distance between the source of the plasma flow and the liquid
surface did not exceed 4 cm, while the height of the liquid column above the immersed electrode was near 2 cm. The
probe technique was used to measure the spatial distributions of the flow potential in the plasma-liquid system. The
molybdenic probe (with diameter 300 μm and length 1 mm) was connected via the divider (MOM/ 2,000 kΩ)
directly to the oscilloscope. The probe was moving along to the axis of the vacuum chamber.
Method of optical emission spectroscopy was applied for optical diagnostics. The emission spectra of plasma
were observed in the range of wavelength 220-1100 nm (with spectral resolution 0.3 nm) by the portable, rapid
(7⋅10-3÷7 sec) PC-operated CCD-based, multi-channel UV-VIS-NIR optical spectra analyzer.
RESULTS
After vacuum degassing of the liquid, the auxiliary and the secondary discharges were switched on. The both
modes (two different polarities of the immersed into the liquid electrode: negative -“liquid” cathode, and positive –
“liquid” anode) were investigated [2]. It was observed that plasma column of the secondary discharge has a truncated
cone-shape with extension at the liquid surface in the case of “liquid” cathode.
Current-voltage characteristic (VACH) of the secondary discharge in plasma-liquid system was measured. The
VACH is given on Fig. 2a. From Fig.2a can be seen, that volt-ampere characteristic (VACH) of the secondary
discharge in plasma-liquid system consists of several typical zones: positive exponential branch and negative branch,
which contains the line section and the range corresponding to the abrupt current increasing. Thus, its behavior
(exponential positive branch and linear range of negative branch) is similar to the VACH of Langmuir probe in the
range of low currents. At the same time investigated VACH has a sharp growth character in the range of high
currents, as typical for the secondary gas discharge in strong electric fields.
The axial distributions of the floating potential of the secondary discharge in plasma-liquid system with a
“liquid” anode (when distance between electrodes is 30 mm, water column is near 15 mm, pressure in the system is
near 12 torr) are represented on Fig. 2b. Measurements were carried out from anode of the auxiliary discharge in
“liquid electrode” direction (z=0 mm − correspond to the plasma-liquid boundary). As can been seen from Fig. 2b,
there is a region near the liquid surface where the axial gradient of the floating potential increases sharply. Such
voltage increasing at the plasma-liquid contact can be interpreted as result of electroconductivity decreasing due to
the process of dissociative attachment of electrons to H2O molecules (e+H2O=OH-+H). It is known that mobility of
negative ions OH- near the liquid surface is lower than mobility of positive ions H+. At the same time, the potential
jump near plasma-liquid boundary can be associated with the basic peculiarity of the potential distribution surround
electrode immersed into the plasma. The behaviour of VACH of the secondary discharge at the range of low
currents, which is similar to the VACH of the Lengmuir probe, justifies this conclusion. From Fig.2b seen that it is
possible to vary the voltage jump value near the liquid surface by changing the secondary discharge voltage Ud.

CONCLUSIONS
From results of our research it was concluded that VACH of the secondary discharge with a liquid electrode
(namely its exponential positive and linear area of the negative branches) is similar to the VACH of Langmuir probe
at the low currents range. The negative branch of the investigated VACH has a sharp growing character in the range
of large currents, that is typical for the secondary gas discharge in strong electric fields. The possibility of varying
the amplitude of the potential jump in the low-pressure plasma-liquid system by changing the potential on the
secondary discharge is shown. It was found that the behavior of the Hα line intensity and the distribution of Te
* over
the time of plasma treatment are different near the liquid surface and near the source of the auxiliary discharge.
REFERENCES
1. V.Ya Chernyak, S.V. Olshevskii Ukr. J. Phys. 50, (3), 242-247 (2005) (in Ukr)
2. Chernyak V.Ya., Naumov V.V., Babich I.L., Voevoda Yu.V. Axial distributions of the potential in plasma of
secondary discharges with a liquid electrode // Electronic Proc. 17th International Symposium on Plasma
Chemistry (ISPC17), Toronto (Canada), August 7-12th, (2005)

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