Pulse Discharge Characteristics of Composite Electrode Systems

NAOKI SAKAMOTO, YOSHITO KUNINAKA, HIDEKI UENO, and HIROSHI NAKAYAMAHimeji Institute of Technology, Japan

SUMMARY

This paper describes the discharge characteristics of

a needle/dielectric composite electrode in N2 gas produced

by a rectangular pulse voltage. Glass or ZnO was placed in

contact with the needle for the needle�plane electrode

configuration. In the case of glass in contact with the needle,

the discharge voltage increased with increasing L (the dis-

tance from the tip of the needle to the dielectric). In the case

of ZnO in contact with the needle, the discharge voltage

increased with increasing L from 0 mm to 2 mm, but it

gradually decreased with increasing L from 2 mm to 10 mm.

It is considered that these differences in the discharge

voltage characteristics originate from the corona extension

process and discharge path. © 2001 Scripta Technica,

Electr Eng Jpn, 137(1): 1�8, 2001

Key words: Composite electrode; ZnO; local co-

rona; triple junction

1. Introduction

SF6 gas has excellent dielectric insulating properties,

and its application as an insulating medium in electric

power apparatus such as gas-insulated switchgear (GIS)

and gas-insulated transmission line (GIL) has contributed

to miniaturization and improvement of reliability [1]. How-

ever, while leakage of SF6 gas to the air was limited in this

stage, recently it has been regulated just like CO2 gas,

because its contribution to the greenhouse effect is esti-

mated to approximately 20,000 times as great as that of CO2

gas [2], so that the reduction of leakage to the air is neces-

sary. Furthermore, the development of a new insulating

method is required from environmental considerations.

Thus, studies on a new insulating medium in place of SF6,

for example a mixed gas system with N2 as a buffer gas,

have been carried out. As a result of intensive research, a

N2/SF6 mixed gas-insulating system or a high-pressure N2

gas system is expected to be applied to power apparatus [3].

On the other hand, in general, a composite insulating

configuration consisting of a solid dielectric and an insulat-

ing gas has appeared in power apparatus such as GIS,

because of the support of an HV conductor or a winding

strand by a solid dielectric spacer, and molding or coating

of conductors by an insulator in order to improve the

dielectric strength and reliability. In such components, a

small gap between the HV conductor and the solid insulator

has been induced by factors such as contraction of the solid

dielectric material in long-term use. In this case, a local

corona is induced by electric field concentration, causing a

difference of permittivity between the solid and the gas

dielectrics, and it affects the dielectric insulating properties

of the apparatus. Therefore, clarification of the properties

of the corona generated at the contact of the conductor and

the insulator in composite insulation is a prerequisite for

improvement of the withstand voltage and miniaturization

of the apparatus, and for appropriate response to the envi-

ronmental problem.

The properties of the corona generated between HV

and grounded metal electrodes have been studied inten-

sively [4�6]. However, the corona in a small gap between

metal and its effect on the dielectric insulating properties

have not been elucidated in detail. Therefore, the study of

such a corona is important for both improvement of the

reliability of the SF6 gas insulation and the development of

a new insulating technology in place of the SF6 gas insulat-

ing system from the environmental viewpoint.

We have investigated the insulating characteristics of

a dielectric insulating configuration with a small gap and

the creeping flashover characteristics of a composite insu-

lating system in the presence of a nonuniform electric field

by using a steep front impulse voltage [7�10]. In addition,

studies of the impulse discharge characteristics of a met-

al/ZnO composite electrode and of the exterior corona

effect based on a composite electrode system with a secon-

dary electrode have also been carried out. Usually, a triple

junction of a metal conductor, gas, and solid dielectric

© 2001 Scripta Technica

Electrical Engineering in Japan, Vol. 137, No. 1, 2001Translated from Denki Gakkai Ronbunshi, Vol. 120-A, No. 8/9, August/September 2000, pp. 804�809

1

appears in the internal structure of power apparatus such as

GIS. The coexistence of the triple junction and a protrusion

or a metal particle as a local high-field source in the appa-

ratus is modeled by a needle�plane configuration with a

solid dielectric disk in contact with the needle electrode

(needle/dielectric composite system). The local corona and

the insulating characteristics for the system in N2 gas have

been investigated in the present work. The effect of the

dielectric which is in contact with the needle on the dis-

charge process has also been studied by using two kinds of

dielectric material (glass and ZnO), which have different

physical properties such as resistivity and work function. In

this paper, we discuss the corona generation, corona exten-

sion process, and discharge voltage characteristics of the

above composite electrode system.

2. Experimental

Figure 1 shows the electrode configuration with a

needle/dielectric composite electrode used in the present

work. A steel needle with a tip radius of curvature of 35 Pm

and a 50 u 50 mm2 brass plane were used as the upper

electrode and lower electrode, respectively. A needle/di-

electric composite electrode was formed by placing a semi-

circular dielectric disk in contact with the needle. The

dielectric disk was made of borosilicate glass (volume

resistivity Uv = 1.1 u 1012 : � m, permittivity er = 7) or ZnO

(volume resistivity Uv = 6 u 106 : � m, permittivity er = 8).

The distance L from the needle tip to the bottom of the

dielectric disk was varied from L = 0 mm to L = 10 mm and

the junction point between the needle and the disk was set

at 5 mm above the needle tip. The distance M between the

bottom of the disk and the plane electrode was adjusted to

a constant value of M = 1 mm. This needle/dielectric

composite electrode system was set up in a brass chamber

(3 u 103 cm3), and the chamber was evacuated to about 10�1

Pa, then was filled with nitrogen gas at a pressure of p = 0.1

to 0.3 MPa.

A rectangular pulse voltage with negative polarity

(wavefront duration Tf = 1.5 Ps, pulse width W = 50 Ps)

was applied to the needle�plane electrode system. The

discharge voltage and corona onset voltage were defined by

the instantaneous value at breakdown and the onset of

corona light emission on the wavefront of the applied pulse.

Following the application of the pulse voltage between the

electrodes, the waveform of the voltage and the light emis-

sion by the corona were traced with an oscilloscope (Yok-

ogawa Co. Ltd., DL-1540, 150 MHz), a high-voltage probe

(Iwatsu Co. Ltd., HV-P30, DC-50 MHz), and a photomul-

tiplier (Hamamatsu Photonics Co. Ltd., 931A, wavelength

300 to 650 nm). The discharge extension process was

captured through an optical window (silica glass) of the

brass chamber by using a high-speed digital framing cam-

era (Hadland Photonics Ltd., Imacon 468, wavelength 385

to 900 nm) and a still camera with an image intensifier

(Philips Ltd., XX-1500). The location of corona generation

was checked by comparison with the captured images of

the light emission and of the electrode configuration under

the same conditions.

In general, the time constant of surface potential

decay is expressed by e0erU for the insulator [14], and the

estimated value for the borosilicate glass is approximately

1 minute. Actually, no residual potential was detected by

surface potential measurement after a 1-minute application

of the pulse voltage, and therefore, the influence of the

residual charge on the corona generation and breakdown in

repeated voltage applications with a 1-minute interval can

be neglected.

3. Results and Discussion

3.1 Discharge voltage

Figure 2 shows the relationship between the distance

L from the needle tip to the bottom of the glass disk and the

discharge voltage for the negative needle in the needle/glass

composite electrode system. The discharge voltage meas-

urements were carried out ten times under the same condi-

tions; the average, maximum, and minimum values are

indicated, and the arrow and affixed number represent the

frequency of nonbreakdown at the wavefront of the pulse.

The discharge voltage under the conditions of L = 0 mm

and p = 0.1 MPa was about 8 kV and increased with

increasing L and p. Similar results were obtained for the

positive needle.

The relationship between the distance L from the

needle tip to the bottom of the ZnO disk and the discharge

voltage for the negative needle in the needle/ZnO compos-

ite electrode system is shown in Fig. 3. In this case, the

dependence was quite different from that obtained for the

needle/glass system. The discharge voltage increased byFig. 1. Electrode configuration.

2

about 4 kV as L was increased from 0 mm to 2 mm, and

then it gradually decreased by 4 kV as L was increased from

2 mm to 10 mm. Thus, the discharge voltage for the nee-

dle/glass composite electrode system increased monotoni-

cally with increasing distance L, while in contrast, the

discharge voltage reached its maximum value at L = 2 mm

for the needle/ZnO composite system. However, on in-

creasing the gas pressure to p = 0.2, 0.3 MPa, the variation

of the discharge voltage on the distance L from the needle

tip to the bottom of the ZnO disk became small, and

furthermore, the pressure dependence of the discharge volt-

age at L = 2 mm was quite small, so that no obvious peak

of the discharge voltage at L = 2 mm was observed for p =

0.3 MPa. Such characteristics were also found for the

positive needle, but were more clearly observed for the

negative needle than for the positive needle.

Figure 4 shows the relationship between the distance

L from the needle tip to the bottom of the disk in the

needle/ZnO system and that in the needle/glass system for

a negative needle at a pressure of p = 0.3 MPa, in which

condition the peculiar dependence of the distance L on the

discharge voltage has been most clearly observed for the

needle/ZnO system. At L = 0 mm, the discharge voltages

were 6 to 8 kV for both electrode systems, and these values

were almost equal to those for the configuration without

any dielectric disk at the same needle�plane distance. Fur-

thermore, in the range of L = 0 to 2 mm, the discharge

voltages increased with increasing distance L for both sys-

tems, and no significant difference in the values was found.

On the other hand, in the range of L > 2 mm, the discharge

voltage for the needle/ZnO system decreased monotoni-

cally with increasing distance L, in contrast to the mono-

tonic increase for the needle/glass system. The differences

in the discharge voltage between these systems were ap-

proximately 18 and 22 kV at L = 7 and 10 mm, respectively.

3.2 Corona onset voltage

The dependence of the distance L from the needle tip

to the bottom of the disk on the discharge voltage is strongly

affected by the nature of the dielectric material in the

needle/dielectric composite electrode system, that is, glass

or ZnO. To investigate the effect of the corona generated at

the wavefront of the applied pulse, the relationship between

the corona onset voltage and the distance L to the bottom

has been examined.

Fig. 2. Discharge voltage on needle/glass composite

electrode.

Fig. 3. Discharge voltage on needle/ZnO composite

electrode.

Fig. 4. Dependence of discharge voltage on L for

composite electrodes.

3

The dependence of the distance L on the corona onset

voltage in the negative needle is indicated in Fig. 5. For the

case of the needle/glass system seen in Fig. 5(a), the corona

onset voltage at L = 0 mm at p = 0.1 MPa was about 4 kV.

Furthermore, the corona onset voltage was increased not

only by increasing L, namely, elongation of the needle�

plane gap length, but also by increasing the gas pressure.

This result is consistent with dependence of the discharge

voltage on the distance L.

In contrast, for the case of the needle/ZnO composite

electrode system shown in Fig. 5(b), at p = 0.1 MPa, the

corona onset voltage increased from 2 kV at L = 0 mm to 3

kV at L = 2 mm; however, the corona onset voltage de-

creased slightly to 2.5 kV beyond L = 5 mm. At p = 0.2 and

0.3 MPa, the corona onset voltage increased in the range of

L = 0 to 2 mm and did not change beyond L = 2 mm. This

fact suggests that corona generation at the needle tip should

be suppressed by increasing L, that is, by elongation of the

needle�plane electrode distance, in the range of L = 0 to 2

mm. On the other hand, it is considered that some factors

facilitate corona generation in spite of the reduction of the

electric field at the needle tip by elongation of the needle�

plane electrode distance beyond L = 2 mm. In addition, the

decrease of the discharge voltage by about 4 kV by increas-

ing L = 2 mm to 5 mm at p = 0.1 MPa is significantly larger

than the corona onset voltage (about 0.5 kV). Thus, the

reduction of the discharge voltage for L = 2 to 5 mm would

be associated not only with the decrease of the corona onset

voltage but also with the corona extension process.

3.3 Observation of discharge development

process using high-speed imaging system

From the investigations of the effect of the distance

L from the needle tip to the bottom of the dielectric disk on

the discharge voltage and the corona onset voltage in N2

gas, it is clear that the characteristics of the discharge

voltage and corona onset voltage are strongly dependent on

the material of the dielectric disk. The observations of the

discharge extension process were also made using a high-

speed digital imaging system during negative pulse voltage

application at p = 0.1 MPa, L = 7 mm, and M = 1 mm.

Figure 6 shows the discharge development images for

the needle/dielectric composite electrode systems viewed

diagonally from the top. The white semicircular curve and

the vertical and horizontal lines seen in the image represent

the outer edge of the disk, the needle, and the forward edge

of the plane electrode, respectively. Although the radii of

the disk between the glass in Fig. 6(a) and for ZnO in Fig.

6(b) seem different at a first glance, this results from the

different included angle and magnification. In Fig. 6(a) for

the needle/glass composite electrode system, corona light

emission was first detected near the needle tip seen in frame

2, and thereafter the discharge development was observed

on the glass surface and in the gas gap between the elec-

trodes, and finally the breakdown was induced.

The discharge development behavior for the nee-

dle/ZnO system is shown in Fig. 6(b). In this case, the

discharge images ware taken from a more horizontal direc-

tion than those for the needle/glass system. In frame 2 and

subsequent frames, corona emissions were observed at the

gas gap between the bottom of the ZnO and the plane

electrode, and at the needle tip. Next, dielectric breakdown

occurred between the ZnO and the plane. In this process,

creeping corona extension was not observed on the ZnO

surface, in contrast to that for the glass system, so that the

corona extension process is quite different in the two sys-

tems. The difference in the effect of the distance L from the

needle tip to the bottom of the dielectric disk on the dis-

charge voltage thus appears to be associated with those

discharge development processes.

As mentioned above, the discharge voltage at p = 0.1

MPa for the needle/ZnO system had its maximum value at

L = 2 mm. To clarify the reason, the corona extensionFig. 5. Corona onset voltage for composite electrodes.

4

process at L = 2 mm was investigated similarly to that at

L = 7 mm by means of the high-speed imaging system.

However, no creeping corona extension on the ZnO surface

was observed and the discharge development behavior was

similar to that obtained at L = 7 mm, namely, dielectric

breakdown occurred between the bottom of the disk and the

plane electrode just after the detection of the corona at the

needle tip.

3.4 Discussion for corona generation and

discharge development process

The discharge development behavior is dependent on

the dielectric material (glass or ZnO) which is in contact

with the needle. To clarify the relationship between the

discharge development process and the characteristics of

the discharge voltage, corona generation and its extension

at the needle has been investigated mainly by using a still

camera with an image intensifier.

Figure 7 shows the corona light emission viewed

from the right side of the composite electrode and their

schematic representations for the needle/glass system (p =

0.1 MPa, L = 7 mm) at various negative applied voltages.

The position of corona generation was identified by com-

paring the electrode configuration image captured under the

same conditions. At the applied peak voltage value of Vp =

14 kV, negative corona emission was observed. Here, the

voltage detected at corona emission was higher than the

corona onset voltage under the same conditions as in Fig.

5(a), because the sensitivity of the image intensifier is lower

than that of the photomultiplier and more light exposure is

needed to capture the image. At Vp = 17.1 kV, the corona

near the needle tip became intense and extends slightly on

the glass surface. At the same time, a positive corona begins

to extend from the plane electrode. Thereafter the creeping

corona hardly extends and the positive corona from the

plane develops markedly. Thus, the discharge development

process is characterized by the association of the positive

corona with a pronounced extension from the plane and the

negative corona near the needle tip of the composite elec-

trode system.

Corona images near the needle tip and their sche-

matic illustrations for the needle/glass system ( p = 0.1 MPa,

negative applied voltage Vp = 6 kV) at L = 2 and 7 mm are

shown in Fig. 8. The corona was generated at the triple

junction of the needle, ZnO, and gas dielectrics, and its

emission was weak in the case of L = 2 mm indicated in Fig.

Fig. 6. Discharge images for composite electrodes

(L = mm, M = 1 mm, p = 0.1 MPa).

Fig. 7. Corona images for needle/glass composite

electrode.

5

8(a). In addition, no creeping corona extension on the ZnO

surface was observed. The corona for L = 7 mm seen in Fig.

8(b) occurred at a triple junction similar to that obtained for

L = 2 mm, and in this case the corona emission was intense

and extended along the surface to the bottom.

Figure 9 shows the applied voltage dependence of the

corona light emission intensity measured using the photo-

multiplier for the needle/ZnO combination at distances L =

1.2, 2, and 7 mm. The intensity at an applied voltage slightly

higher than the corona onset voltage was about 2 to 5 a.u.

regardless of L. In the case of L = 1.2 mm, the intensity was

enhanced by one order of magnitude. Similarly, in the case

of L = 7 mm, the corona light emission increased markedly

from 5 a.u. to about 200 a.u. as a result of an increase from

2.5 kV to 3.0 kV. In contrast, no remarkable change in

corona light emission intensity at L = 2 mm was found in

the range of the applied voltages investigated (3.5 to 5.0

kV). Therefore, the peculiar behavior of the discharge volt-

age, which has its maximum value at L = 2 mm, should

result from easier corona extension at a longer distance L

from the needle tip to the bottom of the ZnO.

The discharge development process for the negative

needle in the present composite electrode system has been

elucidated as follows, based on the above experimental

results. In the case of the needle/glass composite electrode

system, first, the negative corona occurs near the needle tip

due to the locally high electric field. At this time, it is

considered that charges accumulate on the glass surface and

the creeping corona extension is suppressed. Then, the

positive corona is subsequently generated on the plane

electrode and its extension is faster than that of the former

corona. Thus, the positive corona extends markedly upward

with archlike paths, which connect with the negative corona

from the needle tip.

On the other hand, in the case of the needle/ZnO

electrode system, the corona extension process is quite

different from that for the needle/glass system. Namely, a

localized negative corona is generated at the triple junction

of the metallic needle electrode, the solid insulator, and the

dielectric gas above the needle tip. In general, it is known

that a creeping discharge is induced on a solid insulator

surface by electric field enhancement near such a triple

junction, and thus the discharge and its development proc-

ess have also been well investigated [15�18]. However, in

the present needle/ZnO composite electrode system, no

marked extension of the creeping discharge is observed.

The reasons why the corona generation and its extension

process between the glass and the ZnO systems are quite

different are as follows. The volume resistivity Uv and

permittivity Hr are Uv 6 u 106 :�m, Hr = 8 for ZnO and

Uv 1.1 u 1012 :�m, Hr = 7 for the borosilicate glass, respec-

tively. There is no significant difference in the permittivity,

although the volume resistivity of ZnO is 6 orders of

magnitude lower than that of the glass and shows nonlinear

characteristics with respect to the applied voltage. In addi-

tion, electron emission is relatively easy, because the work

function of ZnO is small compared with those for metals

such as Fe and Ni (4.4 to 4.5 eV) [19]. Taking these facts

into consideration, the electrons liberated by the corona will

readily drift in the ZnO disk or on the surface and will be

readily emitted at the bottom of the ZnO disk to the gaseous

gap. Thus, the corona near the triple junction and the

subsequent discharge between the ZnO disk and the plane

electrode should be observed without creeping corona ex-

tension on the ZnO surface.

We now discuss the peculiar relation between the

distance L from the needle tip to the lower ZnO edge and

the discharge voltage. It is considered that the corona at

L = 0 mm is also induced near the triple junction by field

intensification due to its uniqueness. In this case, the dis-

tance between the needle tip and the plane electrode is equal

to the distance M = 1 mm from the bottom of the ZnO to

Fig. 8. Corona images for needle/ZnO composite

electrode.

Fig. 9. Applied voltage dependence of photon emission.

6

the plane, and therefore, the electric field at the needle tip

is also high and the corona should be generated at the needle

tip. In fact, corona emission at that point has been detected

by high-speed camera observation. In addition, the dis-

charge voltage at L = 0 mm is almost the same as that for

the needle�plane configuration (electrode distance 1 mm)

without the dielectric disk. The breakdown at the needle�

plane gap would be induced by the corona from the needle

tip rather than that of the triple junction.

For both L = 2 and 7 mm, the breakdown occurs at

the gas gap between ZnO and the plane without creeping

corona extension in spite of the generation of a corona at

the triple junction. The electrons can readily drift in the ZnO

disk or on its surface, because ZnO has a volume resistivity

6 orders of magnitude lower than that of glass and also has

a nonlinear voltage�current characteristic. For the above

reasons, the electrons released by the corona discharge can

be transferred to the bottom of the ZnO disk and are readily

emitted at that point due to the relatively small work func-

tion. Thus, the breakdown will be induced at the gas gap

between the bottom of the ZnO and the plane. However, the

corona emission intensity was very weak and its extension

toward the plane electrode was not found for L = 2 mm, in

contrast to the case of L = 7 mm. In this condition, the

discharge voltage was relatively high and was insensitive

to the gas pressure. Hama�s group reports a correlation

between the discharge development process after the co-

rona onset and the pressure dependence of the discharge

voltage for the corona induced near the triple junction and the

subsequent creeping discharge [16, 17]. Further investigation

is now in progress to clarify the detailed mechanism of dis-

charge development for the composite electrode system.

4. Conclusions

The discharge characteristics of a needle�plane elec-

trode with the needle in contact with a glass or a ZnO

dielectric disk have been investigated in N2 gas by using a

rectangular pulse voltage. The results of the present work

can be summarized as follows.

(1) For the needle/glass composite electrode system,

at a distance M = 1 mm from the bottom of the glass to the

plane electrode, the discharge voltage increased with in-

creasing distance L from the needle tip to the bottom of the

glass. On the other hand, for the needle/ZnO composite

electrode system, peculiar behavior was found, especially

at p = 0.1 MPa. Namely, the discharge voltage increased

with increasing distance L in the range of L = 0 to 2 mm,

although the discharge voltage was reduced beyond L = 2

mm.

(2) From the observation of the corona generation and

the discharge development, in the case of the needle/glass

system, the negative corona was generated at the needle tip.

In addition, the positive corona occurred on the plane

surface and its extension was faster than that of the negative

corona, after which the positive corona connected with the

negative one near the needle tip, resulting in dielectric

breakdown.

(3) In the case of the needle/ZnO system, the dielec-

tric breakdown was induced between the bottom of the ZnO

disk and the plane after the onset of the negative corona at

the triple junction. The intensity of corona light emission

near the triple junction at L = 2 mm was weak in comparison

with that at L = 7 mm and corona extension from the

junction was suppressed. From these results, the discharge

voltage at L = 7 mm should be reduced from that at L = 2

mm.

REFERENCES

1. Inui A, Teranishi T, Murase H, Yanabu S. Charac-

teristics of partial discharge in SF6 gas wedge gap

against oscillating impulse voltage. Trans IEE Japan

1990;110A:846�852.

2. Christophorou LG, Van Brunt RJ. SF6/N2 mixture

basic and HV insulation properties. IEEE Trans

1995;DEI-2:952�1003.

3. Takuma T. Gas insulation and greenhouse effect. J

IEE Japan 1999;119:232�235.

4. Hosokawa T, Kondo Y, Miyoshi Y. Pre-breakdown-

phenomena of negative point to plane gap in air. J IEE

Japan 1969;89:1823�1832.

5. Hosokawa T, Miyoshi Y. Characteristics of positive

corona and its pre-corona pulse in air. Trans IEE

Japan 1973;93A:420�426.

6. Hosokawa T. Formation mechanism of corona dis-

charges and the recent trend in their researches. Trans

IEE Japan 1991;111A:370�375.

7. Nakayama H, Onoda M, Kuroda S, Amakawa K.

Polarity effect on the corona and breakdown of mix-

ture (SF6/N2) in a needle�plane gap under pulse

voltage. Jpn J Appl Phys 1990;29:1550�1551.

8. Watabe K, Kamatani F, Kobayashi N, Onoda M,

Nakayama H. Effect of a barrier on creeping dis-

charge characteristics in SF6 and N2 gases under

pulse voltages. Trans IEE Japan 1997;117A:1090�

1096.

9. Watabe K, Shinomoiya K, Ueno H, Onoda M,

Nakayama H. Creeping discharge characteristics in

(N2/SF6) gas mixture under pulse voltage. Trans IEE

Japan 1999;119A:6�12.

10. Ueno H, Watabe K, Nakayama H. Creeping discharge

characteristics in N2/SF6 and N2/O2 gas mixtures. J

Inst Electrostat Japan 1999;23:266�271.

7

11. Nakayama H, Onoda M, Amakawa K, Sumino Y.

Breakdown characteristics of small air gap with ce-

ramics composite electrodes. Trans IEE Japan

1995;115A:839�844.

12. Sakamoto N, Kuninaka Y, Ueno H, Nakayama H.

Pulse discharge characteristics on composite elec-

trodes. Rep Fac Eng Himeji Inst Techol 1999;53A:1�

10.

13. Kamatani H, Watabe K, Onoda M, Nakayama H.

Exterior corona effect on sphere�plane discharge in

SF6 gas utilizing optical measurement. Jpn J Appl

Phys 1997;36:L170�L172.

14. Ieda M, Sawa G, Shinohara U. A decaying process of

surface electric charges across polymer films. J IEE

Japan 1968;88:1107�1113.

15. Takuma T. Electric field intensification on composite

dielectrics. J Inst Electrostat Japan 1990;14:40�48.

16. Hama H, Inami K, Fujii H, Oonishi T, Yoshimura M,

Nakanishi K. Leader formation and propagation of

positive surface discharge at impulse voltage in SF6

gas. Trans IEE Japan 1994;114A:397�406.

17. Hama H, Inami K, Yoshimura M, Nakanishi K. Effect

of impulse voltage polarity on surface leader dis-

charge in SF6 gas. Trans IEE Japan 1994;114A:467�

475.

18. Manabe Y, Shimazaki T, Tsuneyasu I, Hara M. For-

mation of surface corona on dielectric plates under

negative impulse voltage in atmospheric air. Trans

IEE Japan 1994;114A:710�717.

19. Sudoh H. Electrodes and their materials. Industrial

Publishing & Consulting, Inc., Tokyo; 1989; p 110.

AUTHORS (from left to right)

Naoki Sakamoto (student member) received his master�s degree from Himeji Institute of Technology in 2000 and is now

in the doctoral program of the School of Engineering.

Yoshito Kuninaka (student member) received his B.E. degree from Himeji Institute of Technology in 2000 and is now in

the M.E. program.

Hideki Ueno (member) received his M.E. degree from Shinshu University in 1985 and his D.Eng. degree from Osaka

University in 1988. His academic appointments include Himeji Institute of Technology, associate professor, 1997. His research

interests include composite insulating systems and creeping discharge phenomena. He is a member of the Institute of

Electrostatics Japan, the Applied Physics Society of Japan, the Physical Society of Japan, and the Carbon Society of Japan.

Hiroshi Nakayama (member) received his B.E. degree from Himeji Institute of Technology in 1965 and his D.Eng. degree

from Osaka University in 1980. His academic appointments include Himeji Institute of Technology, research associate, 1965,

lecturer, associate professor, and professor. His research interests include gas discharge phenomena and dielectric breakdown.

He is a member of the Applied Physics Society of Japan and the Polymer Society.

8