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TEMPORAL SPACE-CHARGE FIELD-CHANGES AT THE CATHODE IN NEGATIVE POLARITY
DISCHARGE
R. Waters, E. Selim
To cite this version:
R. Waters, E. Selim. TEMPORAL SPACE-CHARGE FIELD-CHANGES AT THE CATHODE IN NEGATIVE POLARITY DISCHARGE. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-245- C7-246. �10.1051/jphyscol:19797120�. �jpa-00219092�
JOURNAL DE PRYSIQUE CoZZoque C7, s u p p t 6 m a t a u n07, Tome 40, J u i Z Z e t 1979, page C7- 245
TEMFQRAL SPACE-CHARGE FIELD-CHANGES AT THE CATHODE IN NEGATIVE POLARITY DISCHARGE
R.T. Waters and E.O. Selim
Department af Physics, E Z s c t r o n i c s & E Z e c t r i c a Z E n g i n e e r i n g , UWISY, C a r d i f f CFI 3NU, Walks, UK.
I n t r o d u c t i o n
A l t h o u g h n e g a t i v e - p o l a r i t y c o r o n a p u l s e s h a v e b e e n s t u d i e d e x t e n s i v e l y by many a u t h o r s , ( I ) t h e a v a i l a b l e d a t a h a v e b e e n c o n f i n e d t o c u r r e n t a n d l i g h t o u t p u t m e a s u r e m e n t s . T h e o r e t i c a l m o d e l s a n d p o s s i b l e d i s c h a r g e mechanisms h a v e t h e r e f o r e had t o b e b a s e d o n t h e s e p a r a m e t e r s . No i n f o r m a t i o n i s a v a i l a b l e a b o u t f i e l d - c h a n g e s n e a r t h e c a t h o d e w h i c h a r e i m p o r t a n t t o e n a b l e a r e a l i s t i c model f o r t h e d i s c h a r g e t o b e d e v e l o p e d . T h i s l a c k o f d a t a i s due t o d i f f i c u l t i e s i n m e a s u r i n g f i e l d a t t h e h i g h - v o l t a g e e l e c t r o d e b e c a u s e o f t h e p r e s e n c e of c o n d u c t i o n c u r r e n t . The p r e s e n t p a p e r i s a b r i e f i n t r o d u c t i o n t o a new t e c h n i q u e - t h e f i e l d f i l t e r
- f o r f i e l d m e a s u r e m e n t s a t h i g h v o l t a g e e l e c t r o d e s , a n d d e m o n s t r a t e s some p r e l i m i n a r y r e s u l t s o f m e a s u r e d s p a c e - c h a r g e f i e l d - c h a n g e s i n n e g a t i v e c o r o n a .
E x p e r i m e n t a l A r r a n g e m e n t
An i n v e r t e d g a p o f s e p a r a t i o n 8 3 mm was f o r m e d b e t w e e n a 1 2 . 6 mm d i a m e t e r h e m i s p h e r i c a l l y - c a p p e d r o d c a t h o d e a n d a 2 5 0 mm d i a m e t e r s p h e r e a n o d e , a s shown i n F i g . 1. The f i e l d f i l t e r was i n s e r t e d i n t o t h e c a t h o d e . I n t h i s a p p l i c a t i o n t h e f i l t e r t o o k t h e f o r m o f a n a p e r t u r e o f 2 mm d i a m e t e r a t t h e c a t h o d e t i p , a n d a 6 mm d i a m e t e r m e t a l l i c d i s c w h i c h l a y 2 mm b e n e a t h t h e a p e r t u r e . T h i s d i s c r e p r e s e n t s t h e f i e l d - s e n s i n g e l e c t r o d e w h i c h i s c o n n e c t e d t o t h e o u t p u t c i r c u i t a n d i s b i a s e d w i t h r e s p e c t t o t h e c a t h o d e ( w h i c h i s a t e a r t h
p o t e n t i a l ) u s i n g a s t a b i l i z e d DC s u p p l y + 2 . 5 k V
maximum. A p h o t o m u l t i p l i e r was u s e d t o r e g i s t e r t h e t o t a l l i g h t o u t p u t o r t h e l i g h t f r o m a d e f i n e d s p a t i a l r e g i o n i n t h e g a p , u s i n g a t e l e s c o p i c a r r a n g e m e n t a n d a slit 0.1 mm w i d e .
The F i e l d F i l t e r P r i n c i p l e
t o p r e v e n t c o n d u c t i o n c u r r e n t f r o m r e a c h i n g t h e s e n s o r . T h i s r e v e r s e f i e l d [ F i g 2 ( b l ) d o e s n o t a f f e c t t h e c h a r g e i n d u c t i o n p r o c e s s s o t h a t t h e e f f e c t i v e a c t i o n of t h e d e v i c e i s t o f i l t e r t h e f i e l d c h a n g e s ( i . e , d i s p l a c e m e n t c u r r e n t ) f r o m t h e c o n d u c t i o n c u r r e n t c o m p o n e n t s . The method i s w e l l s u i t e d f o r r e c o r d i n g of t h e t r a n s i e n t f i e l d - c h a n g e s a t t h e c a t h o d e ; a b i a s e d p r o b e f o r s t a t i c f i e l d s h a s b e e n d e s c r i b e d i n a n e a r l i e r p a p e r ( ' ) .
The s e n s i t i v i t y o f t h e f i e l d f i l t e r was d e t e r m i n e d a s f o l l o w s : t h e c o r o n a i n c e p t i o n f i e l d a t t h e c a t h o d e was c a l c u l a t e d t o b e 43 kvccm-I o n t h e o r e t i c a l g r o u n d , w h i c h c o r r e s p o n d e d t o a n o b s e r v e d i n c e p t i o n v o l t a g e o f 75 kV. The f i e l d - f i l t e r s i g n a l a t j u s t below c o r o n a i n c e p t i o n t h e n showed t h e s e n s i t i v i t y o f t h e f i e l d f i l t e r t o b e 0 . 0 5 6 pC(kV c m - l l - l .
R e s u l t s --
The o s c i l l o g r a m s of F i g s . 3 ( a , b , c ) i l l u s t r a t e t h e f u n c t i o n of t h e p r o b e i n t h e p r e s e n c e o f c o r o n a a t t h e r o d e l e c t r o d e . I n t h e a b s e n c e o f t h e b i a s v o l t a g e , t h e r e c o r d e d s i g n a l r e p r e s e n t s t h e sum o f t h e i n t e g r a t e d c o r o n a c u r r e n t a n d t h e t r a n s i e n t f i e l d - c h a n g e s o c c u r r i n g i n t h e r e g i o n o f t h e a p e r t u r e ( F i g . 3 ( b ] ) . When a s u f f i c i e n t r e v e r s e b i a s was a p p l i e d , no c o n d u c t i o n c u r r e n t was r e c o r d e d , a n d t h e f i e l d c h a n g e s c o u l d b e seaen
( F i g . 3 ( c l ) . I f a n e g a t i v e b i a s v o l t a g e was a p p l i e d , t h e c o n d u c t i o n c u r r e n t was.much i n c r e a s e d a n d t h e f i e l d c h a n g e s a r e masked ( F i g . 3 c a ) ) .
Between c o r o n a i n c e p t i o n a t 75 kV a n d 9 0 kV, f i e l d - c h a n g e s f r o m t h e mean f i e l d w e r e a l w a y s n e g a t i v e ; a r a p i d r e d u c t i o n o f f i e l d a c c o m p a n i e d t h e d i s c h a r g e d e v e l o p m e n t , t h e n t h e f i e l d g q a d u a l l y r e c o v e r e d t o t h e mean f i e l d w h i c h t h e n a l l o w e d f u r t h e r d i s c h a r g e t o s t a r t . The r e g u l a r i t y o f t h e s e s u c c e s s i v e f i e l d r e d u c t i o n s c o r r e s p o n d s w i t h The e x t e r n a l f i e l d p e n e t r a t i n g t h e a p e r t u r e
t h e well-known r e g u l a r i t y o f t h e T r i e h e l c u r r e n t w i l l b e a t t e n u a t e d by t h e g e o m e t r y o f t h e d e v i c e
p u l s e s e q u e n c e . A p l a t e a u i n t h e f i e l d was s o t h a t t h e f i e l d a t t h e s u r f a c e o f t h e s e n s o r
s o m e t i m e s o b s e r v e d b e f o r e t h e f i e l d r e c o v e r y . [ t h e r e d u c e d f i e l d ) i s p r o p o r t i o n a l t o t h e e x t e r n a l
f i e l d ( F i g . Z c a l l . A t h i g h e r v o l t a g e s , a r e m a r k a b l e r e s u l t was
A r e v e r s e f i e l d i s a p p l i e d b e t w e e n t h e s e n s o r o b s e r v e d . B e s i d e s c o n t i n u e d n e g a t i v e f i e l d - c h a n g e s a n d t h e e x t e r n a l e l e c t r o d e [ t h e c a t h o d e 1 i n o r d e r [ F i g . 4 ( a ) l , p o s i t i v e f i e l d - c h a n g e s w e r e a l s o s e e n ,
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797120
( F i g . 4 ( b ) I . T h e s e w e r e much l a r g e r t h a n t h e
T I
n e g a t i v e f i e l d - c h a n g e s , b u t s i m u l t a n e o u s
p h o t o m u l t i p l i e r m e a s u r e m e n t s showed t h a t t h e r e was no a p p r e c i a b l e d i f f e r e n c e i n t o t a l l i g h t o u t p u t accompanying e i t h e r p o l a r i t y o f f i e l d - c h a n g e . N e g a t i v e f i e l d - c h a n g e showed a p l a t e a u i n t h e l i g h t o u t p u t w h i l e p o s i t i v e c h a n g e showed o n l y a s p i k e . Between p o s i t i v e a n d n e g a t i v e f i e l d - c h a n g e s t h e r e was a l w a y s a t r a n s i t i o n mode
( F i g . 4 [ c ) ) w h e r e t h e n e g a t i v e f i e l d - c h a n g e s e q u e n c e was p r e c e d e d by o n e o r more s m a l l e r p o s i t i v e f i e l d - c h a n g e s o f s h o r t e r d u r a t i o n .
With a v e r t i c a l s l i t a l i g n e d on t h e a x i s o f t h e c a t h o d e ( F i g . 4 ( d l l , t h e p h o t o m u l t i p l i e r showed no d i f f e r e n c e i n t h e l i g h t a m p l i t u d e between p o s i t i v e a n d n e g a t i v e f i e l d - c h a n g e s . W i t h a v e r t i c a l s l i t a l i g n e d 3 mm o f f a x i s , t h e r e d u c e d l i g h t a m p l i t u d e s o f F i g . 4 ( e ) w e r e f o u n d . With t h e s l i t a l i g n e d h o r i z o n t a l l y 7 . 5 mm a b o v e t h e t i p , p o s i t i v e f i e l d - c h a n g e s accompany l i g h t a m p l i t u d e s l a r g e r t h a n t h o s e f o r n e g a t i v e f i e l d - c h a n g e s ( F i g . 4 ( f ) ) .
The c a l i b r a t i o n f a c t o r i n d i c a t e s t h a t t h e maximum p o s i t i v e a n d n e g a t i v e f i e l d - c h a n g e s w e r e o f t h e o r d e r of 20 MV m-' a n d 4 MV m - l r e s p e c t i v e l y . .
R e f e r e n c e s
( 1 1 LOEB, L. 0 . : ' E l e c t r i c a l c o r o n a s ' [ U n i v . C a l i f . P r e s s l , 1 9 6 5 .
( 2 ) SELIR, E. 0. a n d WATERS, R . T.: ' S t a t i c p r o b e f o r e l e c t r o s t a t i c f i e l d measurement i n t h e p r e s e n c e o f s p a c e c h a r g e ' , IEEE-IAS C o n f e r e n c e R e c o r d 78CH 1346-61A, 136-141, 1 9 7 8 .
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F i g . 4 F i e l d - c h a n g e s ( u p p e r t r a c e s ) a n d o p t i c a l s i g n a l s [ l o w e r t r a c e s )
Sweep time 800 us ( e x c e p t ( e l , 2 0 0 u s ] B a r s i n d i c a t e f i e l d c h a n g e o f 1 0 HV m-1