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Experiments about Discharge in Water with
Changing Conductivity and Applied Voltages
Masatoshi Fue, Ogata Ryoma, Takuma Oikawa, Masahiro Akiyama
Electrical Engineering
University of Iwate
Japan
Abstract—Discharge plasma generated by pulsed high voltage
applied to water is used in industry for such purposes as
improving water quality. High voltages have been considered
necessary for discharge plasma in water. However, discharge
plasma in water was here generated using a low applied voltage
of 3 kV by capacitor discharge circuit. This report discusses two
experimental methods: one using an MPC (Magnetic Pulse
Compression) [1] circuit, the other using a capacitor discharge
circuit. In these experiments, parameters are conductivity of
water and applied voltage. From these experiments, waveforms
were measured and pictures of the discharge plasma were taken.
110 and 3000 μS/cm to analyze plasma formation with voltage
waveforms. We took pictures of discharge plasma using a
single-lens reflex camera and a high speed camera.
Keywords—Plasma; Discharge in water; Pulsed power;
Capacitor; Conductivity; MPC circuit
I.
INTRODUCTION
Applying a high voltage pulse to an electrode in water
generates a discharge plasma with streamers around the
electrode tip [1]. Many researchers have studied discharge in
water [2,3]. While discharges have multiple industrial
applications, such as improving water quality [4,5], some
aspects of the water discharge mechanism remain unclear.
Typically, water discharges are generated by a pulsed high
voltage. For example, discharge plasma in water by a magnetic
pulse compression (MPC) circuit is generated at the applied
voltages greater than approximately 15 kV, while applied
voltages under 15 kV seem unable to generate discharge
plasma. In this manuscript, we generate discharge plasma in
water at approximately 3 kV using a capacitor discharge circuit.
We show experimental results as a function of conductivity
and applied voltage using an MPC circuit and a capacitor
discharge circuit.
II.
EXPERIMENTAL SETUPS
Fig. 1 shows the experiment setup. Discharge plasma is
generated by either an MPC circuit or capacitor discharge
circuit. resulting in discharge plasma in water generated
around a 0.8 mm diameter copper electrode tip. Discharge
voltage waveforms were measured by a high voltage probe
(Tektronix Model P6015A). Conductivity was varied between
Fig. 1. Experimental setup.
III.
EXPERIMENTAL RESULTS BY MPC
MPC circuits are a common pulsed power generator. In our
experiment, voltage waveforms were measured under varopis
conductivity and applied voltage. Images of discharge plasma
formation were taken by a single-lens reflex camera.
A. Varied conductivity under the MPC circuit
Discharge plasma was generated by an MPC circuit and
measured under reactor water with conductivity between 110
μS/cm and 1500 μS/cm. The applied voltage was 30 kV. Fig. 2
shows voltage waveforms, and Fig. 3 shows discharge plasma
formations at three conductivity states (a) 110 μS/cm, (b) 500
μS/cm, and (c) 1500 μS/cm. Discharge plasma formations
changed depending on conductivity. Increasing conductivity
reduced discharge plasma streamer length while incrasing
plasma brightness.
(a)
35
30
25
Voltage (kV)
(b)
110 μS/cm
20
Fig. 5. The formations of discharge plasma of each applied voltage.
(a) 30 kV, (b) 15 kV
500 μS/cm
15
10
1500 μS/cm
5
0
IV.
-5
-10
-15
The capacitor discharge circuit used in our experiment
was unable to generate as high a voltage as that generated by
the MPC circuit, with a maximum applied voltage of about 5
kV. An applied voltage of 5 kV was insufficient to generate
discharge plasma under an MPC circuit, while it was
possible at that voltage using the capacitor discharge circuit.
Fig. 6 shows the capacitor discharge circuit used in our
experiment. The capacitance was 0.56 μS/cm. V0 was a DC
power supply variable between 0 kV and 5 kV. The switch
was a gap switch which determined applied voltage by gap
distance. The load in this experiment was the reactor.
time (μs)
Fig. 2. Voltage waveforms
(a)
EXPERIMENTAL RESULTS BY CAPACITOR DISCHARGE
(b)
(c)
Switch
Fig. 3. Discharge plasma formation at each conductivity:
(a) 110 μS/cm, (b) 500 μS/cm (c), 1500 μS/cm
V
B. Varying the applied voltage using MPC circuit
We measured the discharge plasma for voltages from 15
kV to 30 kV. Discharge plasma was not generated at 15 kV.
Fig. 4 shows waveforms of applied voltage, and Fig. 5 shows
discharge plasma formation at (a) 30 kV and (b) 15 kV.
Reducing applied voltage reduced plasma size and brightness.
35
Voltage (kV)
30
Max voltage : 30 kV
25
Max voltage : 22.5 kV
20
15
Max voltage : 15 kV
10
5
0
-5
-10
-15
Time (μs)
Fig. 4. Waveforms of applied voltages.
C
load
0
Fig. 6. Capacitor discharge circuit.
A. Comparison of MPC and capacitor discharge circuits
Output waveforms displayed large differences between the
MPC and capacitor discharge circuits. In the waveform of
conductivity that is 110 μS/cm by the MPC circuit, the
maximum voltage is 30 kV and FWHM (full width at half
maximum) is about 2 μs. Fig. 7 shows voltage waveforms of
conductivity that is 110 μS/cm by the capacitor discharge
circuit. The maximum voltage is 3.5 kV and the FWHM is
about 20 ms. Fig. 8 is the formation of discharge plasma taken
by high speed camera. The interval time of these pictures are 1
ms. These pictures indicate that discharge plasma lasted for
about 9 ms under the capacitor discharge circuit but only about
2 μs under the MPC circuit; therefore, the discharge plasma
generated by capacitor discharge is far longer-lasting.
4
110 μS/cm
1500 μS/cm
3
2.5
2
1.5
500 μS/cm
2500 μS/cm
4.5
20 ms
4
1
3.5
0.5
Voltage (kV)
Voltage (kV)
3.5
0
-0.5
time
Fig. 7. Waveforms of voltage of conductivity of 110 μS/cm.
3
2.5
2
1.5
0.4 ms
1
0.5
①
②
③
0
time
Fig. 9. Waveforms of voltages of each conductivity
④
⑤
⑥
⑦
⑧
⑨
(a)
(b)
(c)
Fig. 10. Formations of discharge plasma.
(a) 500 μS/cm, (b) 1500 μS/cm, (c) 2500 μS/cm
⑩
⑪
Fig. 8. Discharge plasma formation taken by the high speed camera.
B. Varying conductivity using the capacitor discharge
circuit
We also assessed the discharge plasma generated by the
capacitor discharge circuit for conductivities ranging from 110
μS/cm to 3000 μS/cm for an applied voltage of approximately
3.5 kV. Fig. 9 shows waveforms of the voltage at each
conductivity. As the conductivity increased, FWHM grew
shorter. Fig. 10 shows the formation of discharge plasma at (a)
500 μS/cm, (b) 1500 μS/cm, and (c) 2500 μS/cm. Discharge
plasma formation changed depending on conductivity.
Increased conductivity influences discharge plasma color, size,
and shape with the plasma becoming larger and more ballshaped. In addition, the discharge plasma also becomes louder.
C. Low applied voltage using capacitor discharge circuit
Discharge plasma was generated using the capacitor
discharge circuit under a low applied voltage, and discharges
were measured under varying conductivities. Fig. 11 shows
waveforms of a low applied voltage. We reduced the applied
voltage as we increased the conductivity. For instance, we
applied 1.5 kV for a conductivity of 3000 μS/cm. Fig. 12
shows the formation discharge plasma generated at low
applied voltage for conductivities of (a) 500 μS/cm, (b) 1500
μS/cm, and (c) 2500 μS/cm. As conductivity increases, the
color of the discharge plasma changes.
Fig. 11. Waveforms of minimum applied voltage.
(a)
(b)
(c)
Fig. 12. Discharge plasma formations generated by minimum applied voltage.
(a) 500 μS/cm, (b) 1500 μS/cm, (c) 2500 μS/cm
V.
CONCULUSION
We generated discharge plasma using both MPC and
capacitor discharge circuits. We measured waveforms and
discharge plasma formation. Changing the conductivity altered
the discharge plasma formed by an MPC circuit with
increasing conductivity shortening the length of the streamer of
the discharge plasma and increasing the plasma brightness.
Decreasing the applied voltage caused the discharge plasma to
grow smaller. Discharge plasma was unable to be generated by
any voltage under 15 kV. In the case of the capacitor discharge
circuit, discharge plasma was generated by a comparatively
low applied discharge of 3.5 kV. In this case, the lifespan of
discharge plasma FWHM was considerably longer than under
the MPC: in the MPC circuit, FWHM was about 2μs, while in
the capacitor discharge circuit, FWHM was about 20 ms.
From the pictures of discharge plasma by high speed camera, it
is considered that the discharge plasma lasts for about 9 ms.
For the capacitor discharge circuit, increasing the conductivity
caused the discharge plasma to change color, grow larger, and
became ball shaped. In the output waveform, as the
conductivity increased, FWHM shortened. Also, increasing
conductivity decreased the applied voltage for discharge
plasma formation. Conductivities below 3000 μS/cm required
an applied voltage of 1.5 kV for plasma generation.
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