22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Threshold electric fields for the inception of a primary streamer and a
secondary streamer in water
H. Fujita1, S. Kanazawa2, K. Ohtani1, A. Komiya1, T. Kaneko3 and T. Sato1
1
Institute of Fluid Science, Tohoku University, Sendai, Japan
Department of Electrical and Electronic Engineering, Oita University, Oita, Japan
3
Department of Electronic Engineering, Tohoku University, Sendai, Japan
2
Abstract: The influence of gap distance, capacitance, and water conductivity on the
inception electric fields of a primary streamer and a secondary streamer was investigated
when a single-shot pulsed positive voltage with a duration of 10 µs was applied to a needle
electrode in water. As the inception electric fields were estimated to be constant regardless
of the discharge parameters, it was suggested that there are threshold electric fields for the
inceptions of a primary streamer and a secondary streamer.
Keywords: underwater discharge, electric field, primary streamer, secondary streamer
1. Introduction
Streamers in water are pre-breakdown phenomena,
mainly observed as the development of luminous
filaments from a needle electrode submerged in water.
We have investigated the initiation process [1], the
propagation process of a primary streamer [2] and a
secondary streamer [3] through the highly spatiotemporal
visualization of streamer discharges in ultra-pure water.
Initiation process is the time lag process between the
beginning of voltage application and the streamer
inception. During the initiation process, Joule heating
increases the water temperature at the tip of a needle
electrode and results in the formation of a gaseous region
by evaporation. Weak discharges occur in the gaseous
region. Protrusions are formed on the surface of the
gaseous region and then an electric field is concentrated at
the tip of the protrusion. Streamers are initiated from the
tip of the protrusion. Generally, a primary streamer with a
semi-spherical brush-like structure appears at a lower
voltage level and a secondary streamer with a filamentary
structure appears at a higher voltage level. However, the
influence of discharge parameters on the appearance
tendency of a primary streamer and a secondary streamer
has not been clarified.
In this study, the influence of the discharge parameters
such as gap distance, capacitance, and water conductivity
on the inception electric fields of a primary streamer and a
secondary streamer were investigated.
2. Experimental methods
Figure 1 shows a schematic of the experimental setup.
A single-shot pulsed high voltage with a duration of 10 µs
was generated through a high voltage circuit. A DC power
source charged up the ceramic capacitor, the capacitance
of which was changed in 666 pF, 2 nF, and 6 nF. The
charged electrons were transferred to a discharge part
when a MOS-FET switch was turned on. The discharge
part was composed of a needle-to-wire electrode system,
P-I-3-23
High voltage probe
Pulsed
power
source
Current probe
Electrodes
High speed camera
Water
Fig. 1. Schematic of experimental setup.
the gap distance of which was changed in the range of 2 –
10 mm [1-3]. The pulsed high voltage was applied to the
needle electrode with a tip radius of around 40 µm. The
wire electrode was grounded. Both electrodes were made
of a platinum wire with a diameter of 500 µm and were
covered with insulation tubes except for their tips. The
discharge reactor was filled with 3 ml of ultrapure water
(Milli-Q water) and its typical electrical conductivity was
0.8 µS/cm after exposure to atmospheric air. Water
conductivity was adjusted in the range of 0.8 µS/cm to 1
mS/cm by adding sodium chloride to ultra-pure water.
Inception voltage is defined as the applied voltage level
at which a streamer appears with a probability of 20%.
This is because 20% would be the minimum appearance
rate to eliminate the influence of the individual variability
of the needle electrodes. Actual inception voltage V ai is
the inception voltage considering the voltage decay
during the initiation time. Inception electric field E i is
estimated by the actual inception voltage as follows [4]:
,
(1)
1
electric fields of a primary streamer and a secondary
streamer in water with various conductivities. The water
conductivity also had little influence on the inception
electric fields. The inception fields of a primary streamer
Fig. 2. Inception electric field estimated by the equation
(1) at rtip = 1 µm vs. gap distance. (σ = 0.8 µS/cm, C =
666 pF)
Fig. 5. Threshold electric fields for the inception of a
primary streamer and a secondary streamer in water.
Fig. 3. Inception electric field estimated by the equation
(1) at rtip = 1 µm vs. capacitance. (d = 6 mm, σ = 0.8
µS/cm)
and a secondary streamer were estimated to be constant,
respectively, regardless of the discharge parameters. This
result shows that there are threshold electric fields for the
inceptions of a primary streamer and a secondary streamer.
The threshold field of a primary streamer is estimated to
be 23 MV/cm and that of a secondary streamer is
estimated to be 34 MV/cm. That is, if the local electric
field at the tip of a protrusion exceeds 23 MV/cm, a
primary streamer can propagate from the protrusion. If the
local electric field exceeds 34 MV/cm, there is the
possibility of secondary streamer propagation in addition
to primary streamer propagation as shown in Fig. 5.
4. Conclusion
It was suggested that there are threshold electric fields
for the inceptions of a primary streamer and a secondary
streamer, respectively. This is because the inception
fields were estimated to be almost constant when the gap
distance, the capacitance, and the water conductivity
changed.
Fig. 4. Inception electric field estimated by the equation
(1) at rtip = 1 µm vs. conductivity. (d = 6 mm, C = 666
pF)
where r tip is the tip radius of a protrusion on the gaseous
region, and d is the gap distance. Here, the inception field
is calculated assuming r tip = 1 µm [1, 5].
3. Results and Discussion
Figure 2 shows the inception electric fields of a primary
streamer and a secondary streamer at each gap distance.
The gap distance had little influence on the inception
electric fields. Figure 3 shows the inception electric fields
of a primary streamer and a secondary streamer at each
capacitance. The capacitance had little influence on the
inception electric fields. Figure 4 shows the inception
2
Acknowledgements
This study was partly supported by a Grant-in-Aid for
Scientific Research from JSPS, by a grant from the
Collaborative Research Project of the Institute of Fluid
Science, Tohoku University, and by a grant from Tohoku
University International Advanced Research and
Education Organization. The author wishes to thank
Tomoki Nakajima, Tohoku University, for technical
support.
References
[1] H. Fujita et al., J. Appl. Phys., 116, 213301 (2014).
[2] H. Fujita et al., J. Appl. Phys., 113, 113304 (2013).
[3] H. Fujita et al., EPL, 105, 15003 (2014).
[4] P. Gournay and O. Lesaint, J. Phy. D: Appl. Phys., 26,
P-I-3-23
1966 (1993)
[5] H. Fujita et al., IEEE Trans. Plasma Sci., 42, 2398
(2014)