ELECTRO-OPTICAL MEASUREMENTS OF INSULATOR SURFACE FLASHOVER IN VACUUM

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ELECTRO-OPTICAL MEASUREMENTS OF INSULATOR SURFACE FLASHOVER IN VACUUM

K. Mikkelson, M. Kristiansen, Jiali Lin, J. Thompson

To cite this version:

K. Mikkelson, M. Kristiansen, Jiali Lin, J. Thompson. ELECTRO-OPTICAL MEASUREMENTS

OF INSULATOR SURFACE FLASHOVER IN VACUUM. Journal de Physique Colloques, 1979, 40

(C7), pp.C7-401-C7-402. �10.1051/jphyscol:19797196�. �jpa-00219175�

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JOURUAL DE PHYSIQUE CoZZoque C7, suppZbment au n07, Tome 40, JuiZZet 1979, page C7- 401

K. Mikkelson, M. Kristiansen, and J.

in*,

J. Thompson X

.

xDepariment EZect. Eng., Texas [PechnicaZ University, Lubbock, TX 79409 U.S.

A.

Department EZect. Eng., University South CaroZina, CoZumbia, SC 29208, U.S.A.

A b s t r a c t e l e c t r i c f i e l d s t r e n g t h becomes

E l e c t r o - o p t i c a l measurements o f t h e e l e c t r i c

E

( x , t ) = const. x ( h y ( x , t ) / s y ) f o r t h e Pockels 1 P

f i e l d along i n s u l a t o r surfaces i n vacuum d u r i n g t h e e f f e c t and EIk(x,t) = const. x [ ~ ~ ( x . t ) / 6 ~ ] ' f o r few nanoseconds p r i o r t o i n s u l a t o r f l a s h o v e r have the Kerr e f f e c t , where n y l 6 y = g/2a w i t h 6y being been made. The Pockels and t h e K e r r e f f e c t a r e t h e uniform, o r background, f r i n g e spacing and Ay used i n c o n j u n c t i o n w i t h a p o l a r i z a t i o n i n t e r f e r o - the amount o f measured f r i n g e bending 4

.

meter t o measure t h e i n t e r f a c i a l f i e l d s which have

Experimental arrangement a r i s e t i m e o f a few ns. I n s u l a t o r surface charging

The experimental arrangement i s shown i n F i g . and cathode f i e l d enhancement occurs, followed by

3. The ruby laser is used to probe the test cell plasma formation near t h e cathode which propagates

and also trigger the FX-15, thereby reducing the toward t h e anode a t approximately one t e n t h t h e

t i m i n g problems. The o n l y system j i t t e r i s t h a t o f speed o f l i g h t . Voltage c o l l a p s e across t h e insu-

t h e FX-15 gap, approximately 2 ns. The o p e r a t i n g l a t o r occurs a f t e r t h e plasma f o r m a t i o n has

t e s t c e l l pressure i s 5 x 1 0 - ~ T o r r . reached t h e anode.

I n t r o d u c t i o n

Surface charging o f t h e i n s u l a t o r by secondary e l e c t r o n emission i s w e l l documented '2y3. Regions o f f i e l d i n t e n s i f i c a t i o n near t h e cathode-vacuum- i n s u l a t o r t r i p l e j u n c t i o n causes f i e l d emission o f e l e c t r o n s . T h i s causes r e g e n e r a t i v e surface charg- i n g and e l e c t r o n m u l t i p l i c a t i o n w i t h t h e r e s u l t i n g e l e c t r o n avalanche proceeding towards t h e anode.

E l e c t r o - O p t i c a l Measurements o f E l e c t r i c F i e l d s For a p a r a l l e l e l e c t r o d e c o n f i g u r a t i o n , shown i n Fig. 1, t h e phase d i f f e r e n c e g(x,y) a f,E(x,y) f o r t h e Pockels e f f e c t i n KDP and g(x,y) a aE (x,y) 2 f o r t h e K e r r e f f e c t i n nitrobenzene, where

x

i s the p a t h l e n g t h through t h e o p t i c a l , a c t i v e r e g i o n ( i - e . between t h e e l e c t r o d e s ) and E i s t h e average ap- p l i e d f i e l d . The phase s h i f t g i s measured u s i n g a p o l a r i z a t i o n analyzer (see Fig.

Z),

which pro- duces a f i n i t e f r i n g e i n t e r f e r e n c e p a t t e r n i n d i c a - t i v e o f t h e phase d i f f e r e n c e between E,, and E ~ ,

which i s a f u n c t i o n o f t h e e l e c t r i c f i e l d . A s l i t i s p o s i t i o n e d a t t h e vacuum-insulator i n t e r f a c e and t h e f r i n g e

att tern

streaked w i t h an image c o n v e r t e r camera. The s p e c i f i c r e l a t i o n - ships between t h e f r i n g e motion and i n t e r f a c i a l

*

T h i s work was supported by Sandia Laboratories.

Results

The e x c i t a t i o n p u l s e t o t h e t e s t c e l l has a r i s e t i m e o f 2.5 ns, a 5.5 ns t o p which drops 5%, and a 4 ns f a l l time. The t e s t c e l l a c t s as a ca- p a c i t i v e l o a d and charges t o t w i c e t h e p u l s e v o l t - age.

The streaked f r i n g e s h i f t p a t t e r n f o r t h e KDP t e s t c e l l , shown i n F i g . 4, a r e non-uniform, i n d i - c a t i n g greater, o r enhanced, e l e c t r i c f i e l d s t r e n g t h near t h e cathode. The maximum f i e l d i s reached a t the anode .6 ns b e f o r e the maximum f i e l d a t t h e cathode. T h i s i s due t o continued surface charging a f t e r t h e a p p l i e d f i e l d has peaked. The e l e c t r i c f i e l d as a f u n c t i o n o f anode t o cathode d i s t a n c e shows t h e e l e c t r i c f i e l d enhancement f o r a s p e c i f i c time, ( F i g . 5). Flashover occur as t h e e x c i t a t i o n pulse i s beginning t o f a l l , making anal- y s i s i n t h i s area impossible. A nitrobenzene t e s t c e l l w i t h reduced r i s e t i m e gave t h e t y p i c a l streaked f r i n g e p a t t e r n shown i n F i g . 6. The p r e v i o u s l y discussed surface charging e f f e c t s a r e again pre- s e n t as t h e f r i n g e s r i s e t o t h e i r peaks. Note t h a t t h e f r i n g e s have d i f f e r e n t slopes a f t e r peaking and t h a t they r e t u r n t o t h e a p p l i e d f i e l d value p r i o r t o f l a s h o v e r . The slopes i n d i c a t e d i f f e r e n t surface charging r a t e s along t h e i n s u l a t o r , corresponding t o t h e e l e c t r o n avalanche propagation. The delay

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797196

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between positive surface charging occuring a t the cathode and anode i s - 1.3 ns giving the avalanche a propagation speed of - 7 . 7 ~ 1 0 m/s o r .025c. 6 After positive surface charging begins near the anode the f i e l d a t the cathode begins t o return to t h e applied f i e l d value a t time t3. This phenom- enon propagates across the f r i n g e pattern in . 3 ns.

Sufficient energy has been deposited near the cathode t o induced gas desorption from the sur- face 3 . ^{A t} ^time ^t3 the gas density reaches a point where ionization by f i e l d emitted electrons from the t r i p l e junction causes plasma formation. The number of f r e e electrons increases s i g n i f i c a n t l ~ . These electrons cause a rapid increase in the ion- ization of desorbed gases and a l s o a n n i h i l a t e the positive surface charge adjacent to the plasma by surface recombination. The propagation speed of the plasma formation i s 3 . 3 ~ 1 0 7 m/s or I l c , while the recombination time i s - 500 ps. After a delay of 1 ns, associated with the highly inductive stage of arc formation, the voltage across the t e s t c e l l col lapses.

References

1 . C .

H.

de Tourreil e t a l . , IEEE Trans. Elec.

Insul. EI-8, 17 (1973).

2.

J . P.

Brainard e t a l . , J . Appl. Phys. 45, 3262 (1 974).

3. R. A . Anderson, 1974 Ann. Rep. Conf. Elec.

Insul . and Dielectr. Phenom., 436 (1974).

4. J . E . Thompson e t a l . , IEEE Trans. on I n s t . and Meas. 5, 1 (1976).

F i g . 1. T e s t C e l l Geometry

Half Wave

P o l a r i z e r s

L a s e r T r i g g e r e d Scopes and

Gap

\

^Camera

O u t e r ~ o n d u c t o d

I

To A n a l y z e r and Camera

F i a . 3 . E x ~ e r i m e n t a l A r r a n e ~ m ~ n t

I I i 1 1 1 ' 1

0 t l 10 t ( n s )

r i ~ . 4. F r i n g e P a t t e r n f o r KDP T e s t C e l l

F r i n g e S h i f t A Y / ~ Y

9

-

_Plot

_{f o r}

_{t i m e}_{t l}

6

- +

J

I I 1 I I 1 1 I

0 . 5 1.0

Anode t o Cathode D i s t a n c e (x/d)

F i g . 5 . E l e c t r i c F i e l d S t r e n g t h f o r KDP C e l l

P i g . 2. P o l a r i z a t i o n A n a l y z e r F i g .

6 .

F r i n g e P a t t e r n

f o r

N i t r o - T e s t C e l l