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Produced by the NASA Center for Aerospace Information (CASI)
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AN EXPERIMENTAL INVESTIGATION OF ELECTRIC

FLASHOVER ACROSS SOLID INSULATORS IN VACUUM

Final Contractors Report

NASA Grant NAG 1-332

March 1983 - February 1984

Vu Al e/t

Virginia Associated Research Campus

12070 Jefferson Avenue, Newport News, VA 23606

(NASA- CR -175375) AN LXFERlMINTAL N84-17478

INVESTIGATION OF ELECTRIC ILASHOVER ACROSS

SOLID INSULATORS IN VACUUM Fi.nai a

Contractors Report, Mar. 1983 - Feb. 1584 Unclas

(College of jta.11iam and Mary, Newport News, G3/33 18245

..e.. .<.

.:,.....^., •ice`...-_..,.^+'^`es^.1 S'IR.':nrt!J:c.^k'^x"A ....__ ._,_.._ .yr^
I

Virginia Associated Research Campus

12070 Jefferson Avenue

Newport News, V;A 23606

An Experimental Investigation of Electric

Flashover Across Solid Insulators in Vacuum

Final Contractor Report

NASA grant NAG 1-332

March 1983 - February 1984

Hans C. von Baeyer, Principal Investigator

Hiromu Isa, Senior Research Physicist

Ja H. Lee, Consultant

Dr. (Nelson W. Jalufka

The NASA Technical Officer

NASA Langley Research Center

Hampton, VA 23665
Table of Contents

page

I. Introduction . . . 2

IL. Experimental Arrangement . . . . . . . . . . . . . . . . . . 2

ill. Experimental Results . . . . . . . . . . . . . . . 3

(1) Flashover voltage characteristics , . . . . . . . . . . 3

(2) ;Streak phot^graphs . . . . . . . . . . . . . . . . . . . 5

IV. Discussion . . . . . . . . . . . . .. . . . . . . . . . . . . 6

V. Summary w . . . . . . . . . . 8

VI. References . . . . . . . . ,. . . . . . . . . . 11

VII. Inventory of Residual Equipment and Property . . . . . . 12

VIII. Inventions . . . . . . 13

IX. Publ i cati ens . . . . . . . . . . . . . . . . . . . . . . . . 14

X. List of Figures . . . . . . . . . . . . . . . . . . . . . 15

XI List of Tables . , , , 24

1
I. Introduction

The insulation of high voltage conductors often employs solid

insulators for many applications. In such applications, an unexpected

electric flashover may occur along the insulator surface. Under conditions

of high vacuum, the flashover voltage across the insulator is observed to

be lower compared with that of the same electrode separation without an

insulator. The reason for such an extreme reduction of flashover voltage

is not well understood. Although several models, based on the secondary

electron emission, have been proposed to explain the onset of the surface

flashover, precise experimental and theorectical investigations of

Flashover phenomenon have not been carried out.

In this paper, we report on an investigation to determine the starting

point and the developing velocity of the surface flashover. The study was

carried out using an intensified image converter camera to observe the

initial stage of electrical flashover along the insulator surface parallel

to the electric field. We also used several different insulator materials

as test pieces to determine the effect of the dielectric constant on the

flashover voltage characteristics.

II. Experimental Arrangement

Fig. 1 shows a schematic diagram of the experimental arrangement. The

test gap was placed in a bell jar, made of stainless steel, 17 inches in

diameter and 28 inches in height. The bell jar was evacuated by an oil

2
diffusion and mechanical pumping system. The lowest pressure of 6 x 10-5

Torr was obtained with the vacuum system. A negative high voltage pulse

from a 6-stage iiioi-Marx generator was applied to the test gap through a

Teflon high voltage bushing mounted on a window of the bell jar to initiate

r

r"

a flashover. The flashover phenomena were observed by using an IMACON 600

image converter coupled with an EMI type C 141 image intesifier. The

streak or framing photograph was recorded on Polaroid film. The entire

camera system was placed inside of an aluminum shielding cabinet to reduce

the noise pickup of the system. An oscilloscope, Tektronix type 7844, was

used to observe the wave form of the high voltage pulse. The image

converter was triggered by a signal from the Mini-Marx generator using a

coaxial cable as a delay line for synchronization between the flashover and

the image converter. Test pieces, made of Plexiglass, TiO2 and BaTiO3,

were used. 'These materials were chosen for the large differences in their

dielectric constants. The shape of the test piece was a right cylinder,

3 cm in diameter and 1 cm in length, Each test piece was cleaned in an

ultrasonic cleaner with ethanol, and then rinsed with deionized water. The

test piece and the electrode system is shown Fig, 2.

III. Experimental Results

(1) Flashover voltage cha:-acteristics

Fig. 3 shows an example of a series of flashover voltage

measurements. After the first flashover occurs, the flashover voltage

decreases rapidly as shown in the figure. The characteristic was

consistently observed in our experiment. 'Nis result strongly contradicts

those obtained by other researchers.( 1 ' 2 ) In this experiment, the

conta;,,ts between the upper and lower surfaces of the test piece and

3
elec,,.rode surfaces are very important. However, we did not control the

contact precisely. For example, 2 of the 3 Plexiglass test pieces produced

no flashover for applied voltages upto the maximum applied voltage of

102 kV (See Table 1.), Similar characteristics were also observed for the

Ti02 test pieces. Table 1 shows the highest, the lowest, and 50% flashover

voltages measured. Where, 50% flashover voltage means the voltage at which

50% of the voltage applications produced flashover. In the case of

Plexiglass test piece of No. 1, the first 30 shots were neglected from the

data. As shown in the table, the flashover voltage decreases when the

dielectric constant is large. However, the relation between these

parameters is not inversely proportional. In the case of BaT10 3 , a partial

dishcharge was frequently observed. It is thought that the extremely large

dielectric constant produces a large electric field enhancement at the

triple junction and a large field reduction at mid-gap simultaneously,

caut>ing such a partial discharge. Furthermore, for BaTiO 3 , a large 

hysteresis phenomenon was observed during the measurement of flashover 

voltage. As a result, widely scattered flashover voltages were observed 

for BaTiO 3 as shown in Table 1.

Fig. 4 shows another example of a series of flashover voltage

measurements. In this case, a straight line, parallel to the electric

field, was drawn on the Plexiglass test piece using a pencil lead to

stabilize the position of flashover on the test piece. This test piece

also had a small scratch at the end of the cathode side superposing the

pencil lead line to reduce the flashover voltage. As . shown in Fig. 4, the 

flashover voltage increases with the repetition of flashover at first, then

decreases in the same way as shown in Fig. 3. In the first phase, it can

be 'thought that the flashover initiates at some weak points (dust

4

•
particles, irregularity on the electrode surface, etc.). If the flashover

occurs, these weak points disappear one by one, and the flashover voltage

becomes high. In the second phase, some damage will occur at the triple

junction by an extremely high temperature of flashover arc. This damage

causes the reduction of the flashover voltage as shown in Fig. 3 and the

second phase in Fig. 4.

(2) Streak photograph

At a point where the test piece touches the electrode, a

conductor-dielectric-vacuum junction, called a "triple junction", is'

formed. At the point between the electrode and the test piece, the

electric field becomes so high that electrons can be emitted from the

cathode to the vacuum space by field emission. In a surface flashover in

high vacuum, the microscopic structure of the triple junction can affect

the flahsover characteristics. For example, in the case of p lexiglass test

piece No. 1, the contact between the test piece and the electrode was not

as good as that for No. 2 and No. 3 and consequently the flashover voltag

was lower. Furthermore, as shown in Fig. 5, the bright spots appear at the

end of both the anode and the cathode where a small clearance exists. 

Fig. 6 is an example of a l ow speed (1.2 mn/ns) streak photograph

taken under the same conditions as Fig. 5. From this photograph, it can be

seen that the brightness along the flashover channel changes periodically. 

The periodic length in the figure is approximately 0.5 mm. Fig. 7 shows an

example of a streak photograph taken at a higher speed (2.8 mn/ns) and a

higher sensitivity than Fig. 6. In this photograph, it can be seen that

two dark spaces appear at the- initial stage of flashover at points 3 mm and 

5 mm from the cathode. Those dark spaces really are not dark but are less

5
bright than the adjacent part, and they become bright a few nanoseconds

later. The position of these dark spots varies from shot to shot. Fig. 8

shows examples of streak photographs at various applied -ioltages, where the

streak speed (15 mm/ns) is much faster than that of Fig. G and 7. From

those photographs, it can be observed that two bright portions, which

probably correspond to two cathode spots shown in Fig. 5, appear at the

cathode and move toward mid-gap at a speed of approximately 3 x 10 6 m/s.

These bright portions travel only 2 mm in the gap space. From Fig. 8, ' the

speed and distance traveled of these bright portions are almost independent

from the applied voltage. Except for these bright portions, the entire

flashover channel is `formed within 0.2 ns. This time it is almost the same

as that in which a free electron travels from the cathode to the anode

under the force of the applied electric field. We took many streak

photographs at various pressures (4 x 10-5

4 x 10- 3 Torr), but no large

difference could be observed in these data.

IV Discussion

Various models have been proposed by many researchers to explain the

mechanism of surface flashover in vacuum( 3 - 5 ). All the models have

considered the existence of electron emission from the triple junction. In

our experiment, the flashover usually occured at a fixed position. And

after several thousand shots the cylindrical surface of the plexiglass text

piece became very smooth compared with the original surface. It can be

assumed that electrons came out from the triple junction and hit the entire

surface.

The cathode and anode spots shown in Fig. 5 were formed because of a

rather large clearance at the contact between the test piece and the

6

x
electrode. This clearance was made by the flashover itself, i.e., the heat

produced by the flashover melted and evaporated a small amount of the test

piece material. To eliminate the formation of such cathode and anode

spots, we replaced the test piece with a new one, the surface of which had

a thin straight scratch to make the flashover path straight and parallel to

the electric field. Table 2 shows the flashover voltage of the scratched

test piece together with that of Nc. 1 Plexiglass test piece. Fig. 9

shows a still photograph and a streak photograph taken with this new test

piece. These photographs show that no cathode and anode spots were formed

when the contact between the test piece and the electrode is good. In this

case, the flashover, initiates at a point of 2.5 mm from the cathode and

spreads toward both the cathode and the anode with a speed of approximately

107 m/s as shown if Fig. 9 (c). Futhermore, a fine periodic structure can

be observed. If we assume that this periodic length corresponds to the

step length of electron-hopping on the insulator surface, we obtain the

value of 2 x 10 7 m/s for the averege electron velocity under the condition

of our experiment. This value is in good agreement with the expansion

velocity of the bright portion in Fig. 9.

The fine peiod?cal structure along the flashover channel observed in

our streak photographs is a direct result of the periodical irregularity

formed on the test piece surface. In fact, we can observe such an

irregularity along the flashover path on the test piece surface after the

experiment. The periodical length for test piece No. 6 is approximately

0.5 mm, which is the same as those obtained in Fig. 6 and 9 (b). From

these data, we can conclude that these periodical irregularity were formed

by periodical electron bombardment on the insulator surface as proposed by

Boersch et al.(r)

9

7

0,
a

i

M

Fig. 10 shows an example of still photographs of the surface flashover

in atmospheric air and in vacuum for the same test piece?. The brightness

of the flashover in atmospheric air is at least four times higher than that

in vacu, im. The diameter of the flashover chawiel in air is also larger

than that in vacuum as shown in these photographs. Futhermore, in the case

of vacuum flashover, a luminous plasma extends from both the cathode and•

the anode triple junctions to the vacuum space more than 5 cm from the test

piece surface. These phenomena mean that there exist some mechanisms to

confine the flashover channel in a narrow region. The Coulomb force

between the electrons and the surface charge produced at the test piece

surface by the secondary electron emission can be one of such mechanisms.

The self-pinch effect of the flashover channel can probably be the other

mechanism. In our experiment, we mainly applied the voltage of 81 kV. We

also used a 6 KR resistor as a current limiter, so the peak current flown

through the flashover channel was approximately 14 A. If we assume the

diameter of flashover channel is 2 mm, and the current flows in the channel

entirely, the force acting on the electron at the boundary of the channel

is 9 x 10- 13 N, were we assumed that the electron velocity is 2 x 107 m/s,.

The distance at which the repulsive force between two electrons corresponds

to 9 x 10 -13 N is 1.5 x 10-7 m. These data mean that an electro-magnetic

force produced by the flashover current is strong enough , to confine the

vacuum flashover current in a narrow channel as observed.

V Summary

In this research, we observed the characteristics of the onset of

surface flashover in a vacuum for three kinds of test pieces. We also

observed the development of surface flashover with good time and space

8
resolution by using an image converter camera. The results obtained may be

summarized as follows:

(1) in many cases, the flashover voltage sharply decreases after the first

flashover.

(2) The flashover voltage varies as a function of contact between the test

piece and the electrode.

(3) The flashover voltage decreases when the dielectric constant of the

test piece is increased.

(4) The shat-to-shot flashover voltage varies widely when the dielectric

constant is large.

(5) The brightness of the flashover channel varies periodically along the

channel. The periodic length is approximately 0.5 mm for our

experiment.

(6) The cathode spot appears when a large clearance exists between the

test piece and the cathode. On the streak photographs, the bright,

part initiates at the cathode and moves toward mid-gap at a speed of

10 7 m/s, 

(7) The speed of the bright front is almost independent of pressure and

applied voltage.

(8) If the clearance between the test piece and the electrode is small,

the cathode spot does not appear.

(9) The flashover was drastically reduced and the flashover path 

restricted by scratching the surface of the test piece.

(10) The flashover initiates at a point about 3 mm from the cathode, and

expands toward both the cathode and the anode with a speed of

approximately 1.2 x 107 m/s.

9

r
(11) The calculated velocity of electron hopping o n,, the surface is in good

agreement with the expansiun velocity of the bright portion.

(12) The surface flashover channel in a vacuum consists of a narrow cl-annel

on the surface and a widely diffused plasma in the vacuum Space. 

.t

S • t

10

•
VI References

(1) J. P. Shannon, S. F. Philp, and J. C. Trump: J. Vac. Sci. Technol.

Vol. 2, 234 (1365).

(2) Y. Ohki, and K. Yahagi: J. Appl. Phys. Vol. 46, 3695 (1975).

(3) T. S. Sudarshan, J. R. Cross, and K. 0. Srivastava: IEEE Trans.

Electr. Insul. Vol. EI-12, No. 3, 200 (1977).

(4) K. D. Bergeron: J. Appl. Phys. Vol. 48, 3073 (1977).

(5) R. A. Anderson, and J. P. Brainard: J. Appl. Phys. vol. 51, 1414

(1980).

(6) V. N. Boersch, 11. Hamisc', and W. Ehrlich: Z. fur angewandte Phys,

Vol. 15, 518 (1963).

11
^ a

VII Inventory of residual equipment rind propt_

No residual equipment or property has been acquired with grant funds having

acquisition cost of over $1,000 for the entire grant period.

12
VIII Inventions

NONE

0
IX Publications

NONE

I

14
Y

X List of Figures

Fig. 1. Schematic diagram of experimental arrangement

Fig. 2. Test piece and electrode system

Fig. 3. Example of a series of flashover voltage measurement

Fig. 4. Example of a series of flashover voltage measurement

Fig 5. Still photograph of surface flashover

Fig. 6. Low speed streak photograph of su.rface flashover

Fig. 7. High speed streak photograph

Fig. U. High speed streak photograph at various voltage

Fig. 9. Ultra-high speed streak photograph

Fig. 10,, Photographs of surface flashover in air and vacuum

15
ORIGINAL PAGE IS

OF POOR QOALITY

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17

ORIGINAL. PACE 19

OF POOR QUALITY

INSULATOR (TEFLON) ELECTRODE (BRASS)

WINDOW (QUARTZ) TEST PIECE (DIELECTRIC

SPECIMEN)

,

Fig. 2. Test piece and electrode system.

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18
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'-"' 

co 

o ~6 as I ....., 

-

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> 

0) 

c 

- ' 

o 

~ 

12 

° : flashover 

o 

/:1A. 0 0 

/:1 0A.A. 0 

~A. °0 

~ 0 

A °0 

° A.A. ° 

o ~OAOAO ~ 0 A.00 A. 

A A ~ ~ A 0 A AA 

~~ A 

lPA.A. 

, . 

~ : no flashover 

a:s 

.c 

o 86 20 40 

" number of shots 

> 

~ 20r 

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~ ;16" -

o 

> 

0) 12 c' 

-- . C) 

~ 

as 

°AOO 0

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0 

°AA °A.°OAOO OOAAOOO AOO 0 0 ° A.A A. 0AOAA. 

o A A A A. A. 0A. A O~A 

-5 8' a'o 100 ' 

number" of shots' 

Fig. 4. Flashover voltage measurement for Plexiglass test piece w

a pencil mark on the surface. 

.", ..... "":,,, ,,' ~,J\~:; . ..,~~ 
cathode

C

U

anode

OF 

?OCR Q'J FLIP

streak direction

cathode w':+

+'^* anode

.'4

Fig. 5. Still photograph of

surface flashover for Plexiglass

test piece. Va( applied voltage)

=81 kV, P=9 x10 Torr.

Fig. 6. Low speed streak photograph

of I cm spark for Plexiglass. Va=81

kV, P=44 Torr, streak speed= I. 2

mm/ n s.

streak direction

Fig. 7. Streak photograph of I cm spark-for Plexiglass

at higher streak speed than Fig. 6. V0.=?31 kV, P=10-4

Torr, streak speed = 2.8mm/ ns.

20
streak direction

ORIGINAL PAGE tOF POOR QUA L11Y

(a) Voa=98 kV

(b) Va=81 kV

(c) Va=65 kV

Fig. 8. High speed streak photogrph of I cm shark for

Plexiglass at various applied voltage. PAxIO Torr,

streak speed= 15 mm/ ns.

21
sti eak dir ection

(b)

Streak photogrph

(a)

Sti I I photogrph

cathode

EU

anode

,IG1Nh1-

OF PuvR 4-%^^11ti

(c)

Sketch of streak p!iotograph

Fig. 9. Still and ultra-high speed y f.reak photogrphs

of I cm spark for scratched Plexiglass test piece. Va.

70 kV, P = 2,, 10 Torr, streak speed = 27 min! ns.

22

r

(W
ORIGINAL PAGE !S

OF POOR QUAH Y

3 cm

-- vacuum , test piece

e E

front view side view

la! Flashover in I Atm. air. VQ - 56 M

3 cm

Y

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tai

• { I ^y ^ • .«YiYM

front view side view

(b) Flashovu, i n vacuum. Va = 70 k V.

Fig. 10. Comparison of still photographs of surface flashover

in atmospheric air and in vacuum.

23

vp
J

XI Lis;; of Tables

Table 1. Maximum, Minimum and 50, flashover voltage

Table 2. Flashover voltage of normal and scratched test pieces

,

24
i

' UWClINAL 

"Cale

0, .Oft lam

Table 1. Maximum, Minimum, and 50% flashover vol tage

observed for various test pieces. (units 0)t

material plexiglass T102 BaT103

Flo. of piece e -, 3.5) ( e tt 80L_^ ( e M. 300

Max 53.5 44.2 -

1 V50 47.6 38.0 ^--

M-i n 39.6 34.9 2.7

Max > 102 11.6 14.2

2 V50 > 102 9.6 8.5

Min > 102 8.0 1.8

Max > 102. 13.3 14.2

3 V50 > 102 10.3 9.6

Min > 102 8.9 6.2

Table 2. Comparison of flashover voltages between

normal test piece and scratched test piece

p exig ass p exig ass

(normal) (scratched)

first flashover 81.5 kV 65.2 0

V50 47.6 KV 23.3 kV

25

i