EXTINCTION OF A VACUUM ARC BY APPLICATION OF A TRANSVERSE MAGNETIC FIELD

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EXTINCTION OF A VACUUM ARC BY

APPLICATION OF A TRANSVERSE MAGNETIC FIELD

C. Kimblin, F. Holmes, J. Gorman, P.G. Slade

To cite this version:

C. Kimblin, F. Holmes, J. Gorman, P.G. Slade. EXTINCTION OF A VACUUM ARC BY APPLI-

CATION OF A TRANSVERSE MAGNETIC FIELD. Journal de Physique Colloques, 1979, 40 (C7),

pp.C7-413-C7-414. �10.1051/jphyscol:19797202�. �jpa-00219182�

JOURML DE PHYSIQUE CoZZoque C7, suppZ6ment au n07, Tome 40, JuiZZet 2979, page C7- 413

EXTINCTION (X A VACUUM ARC BY APPLICATION M A TRANSVERSE MAGNETIC FIELD

C.W. Kimblin, F.A. Holrnes, J.G. Gorman and P.G. Slade.

Westinghouse R&D Center Pittsbmgh, PennsyZvania 15235, U.S. A.

INTRODUCTION: It h a s p r e v i o u s l y been shown ^{s 2} t h a t a r a p i d l y r i s i n g magnetic f i e l d a p p l i e d p e r - p e n d i c u l a r t o t h e e l e c t r o d e a x i s of a d i f f u s e vacuum a r c c r e a t e s a H a l l e l e c t r i c f i e l d and sub- s e q u e n t anode s p a c e charge r e g i o n . With a c a p a c i - t a n c e of 50 t o 200 pF connected i n p a r a l l e l w i t h t h e a r c i n g e l e c t r o d e s , and w i t h t h e a r c v o l t a g e r i s i n g t o s e v e r a l k i l o v o l t s , t h e a r c c u r r e n t i s reduced w i t h p o s s i b l e a r c e x t i n c t i o n . The p r e s e n t paper* i s concerned w i t h e x p e r i m e n t a l o b s e r v a t i o n s of f o r c e d a r c e x t i n c t i o n by t r a n s v e r s e magnetic f i e l d s a t c u r r e n t l e v e l s t o 1 5 kA.

EXPERIMENTAL OBSERVATION WITH AN ESSENTIALLY

LINEARLY RISING MAGNETIC FIELD: The 1 4 cm d i a m e t e r d i s k shaped e l e c t r o d e s of a n e x p e r i m e n t a l vacuum d e v i c e were s e p a r a t e d a d i s t a n c e o f 2.5 cm d u r i n g t h e c u r r e n t r i s e of a 60 Hz h a l f c y c l e . At c u r r e n t c r e s t , a t r a n s v e r s e m a g n e t i c f i e l d w i t h a n i n i t i a l r a t e of r i s e o f 7400 T / s was a p p l i e d t o t h e vacuum a r c . T h i s f i e l d waveform had a s i n u s o i d a l rise i n 50 p s t o a c o n s t a n t v a l u e . For c u r r e n t l e v e l s

< 5 kA, f i e l d magnitudes >0.03 T, and a p a r a l l e l

-

c a p a c i t a n c e of 50 pF, t h e a r c e x t i n g u i s h e d i n less t h a n 20 ps. We e x p l a i n e d 1 ^{y 2} t h e s e o b s e r v a t i o n s i n t e r n s of t h e t h r e s h o l d f i e l d r e q u i r e d t o d e t a c h t h e plasma from t h e anode, s p a c e c h a r g e f o r m a t i o n i n t h e anode r e g i o n , and consequent c u r r e n t d i v e r - s i o n i n t o t h e p a r a l l e l c i r c u i t . s t e f a n i a k 3 h a s a l s o observed t h a t t h e a r c v o l t a g e i n c r e a s e s when f i e l d s ' 0 . 0 3 T a r e a p p l i e d t o low c u r r e n t vacuum a r c s . However, S t e f a n i a k p r i m a r i l y a t t r i b u t e s t h e v o l t a g e i n c r e a s e t o cathode r a t h e r t h a n anode s h e a t h phenomena.

We now r e p o r t t h a t a t c u r r e n t l e v e l s >5 kA, and w i t h t h e above e x p e r i m e n t a l c o n d i t i o n s , t h e a r c c u r r e n t o n l y approaches c u r r e n t z e r o d u r i n g t h e

*This work i s s u p p o r t e d by t h e E l e c t r i c Power Research I n s t i t u t e under c o n t r a c t RP564.

I ,

_I

*n!- Time +

Extinction

ov

0 2 0 4.0 6 0 SO ICLO 120 160

Circuit Current, I, kA

Fig 1 -Measurements of the current reduction with application of a linearly rising magnetic field. 12-5cm eledmde separation. i =7400 Tls. 50 pF parallel capaitance l

f i r s t 25 p s of approximately r i s i n g f i e l d . The a r c c u r r e n t t h e n rises even though t h e f i e l d c o n t i n u e s t o i n c r e a s e . A p l o t of t h e c u r r e n t r e d u c t i o n v e r s u s i n i t i a l c u r r e n t magnitude a p p e a r s i n F i g u r e 1. At f i r s t we c o n s i d e r e d t h a t t h e minimum i n t h e c u r r e n t t r a c e was a s s o c i a t e d w i t h c u r r e n t c o l l e c t - i o n from t h e r e s i d u a l plasma, and t h a t t h e subse- q u e n t r i s e i n a r c c u r r e n t was a s s o c i a t e d w i t h cathode s p o t r e i g n i t i o n . However, a n a l y s i s showed t h a t t h e o r e t i c a l r e s i d u a l c u r r e n t magnitudes were t o o low. Furthermore, t h e r e was no o b s e r v a b l e r e i g n i t i o n peak o n t h e v o l t a g e t r a c e . W e t h e r e f o r e conclude t h a t c a t h o d e s p o t s c o n t i n u e t o b u r n throughout t h e p e r i o d of f i e l d a p p l i c a t i o n .

EXPERIMENTAL OBSERVATIONS WITH AN OSCILLATING

MAGNETIC FIELD: The f i e l d c o i l power supply was modified t o p e r m i t a p p l i c a t i o n of an o s c i l l a t i n g magnetic f i e l d , and a s c h e m a t i c of t h e o v e r a l l c i r c u i t a p p e a r s i n F i g u r e 2. With a magnetic f i e l d frequency of 5 kHz and maximum f i e l d change of 7000 T / s , t h e a r c was f o r c e d u n s t a b l e from h i g h e r c u r r e n t l e v e l s i n t h e range of 1 0 t o 1 5 kA.

The magnetic f i e l d , a r c v o l t a g e , and a r c c u r r e n t were r e c o r d e d w i t h a C.R.O. f o l l o w i n g f i e l d a p p l i -

c a t i o n , and r e p r e s e n t a t i v e t r a c e s a r e shown i n Fig. 3. As e x p e c t e d , t h e a r c v o l t a g e v a r i e s w i t h

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797202

60 Hz Source

I

-

I

I

i

¹

^{' I}

^I

Fig. 2 -Circuit schematic

t h e a b s o l u t e magnitude of t h e magnetic f i e l d . F u r t h e r , i t w i l l b e n o t e d t h a t t h e c u r r e n t i s f o r c e d i n t o o s c i l l a t i o n and t h e a r c i s e x t i n g u i s h - ed when t h e c u r r e n t r e a c h e s z e r o w i t h e x t i n c t i o n o f t h e l a s t c a t h o d e s p o t . The p r o b a b i l i t y o f t h e a r c o s c i l l a t i n g t o c u r r e n t z e r o was o b s e r v e d t o d e c r e a s e when t h e n a t u r a l f r e q u e n c y f C of t h e ca- p a c i t o r c i r c u i t was 5 t w i c e t h e n a t u r a l frequency of t h e magnetic f i e l d f

B'

Voltage

"c*" 0 v

0.21T Magnetic Field

0

Arc Current

8.5 kA 0

-

L

- ^{Time, 50 psldiv}

Fig. 3.- Arc extinction with an oscillating magnetic field from an initial current level of 8.5 .kA

We a r e p r e s e n t l y ~ ~ I & u l e t o c a l c u l a t e t h e d e t a i l e d c u r r e n t e v o l u t i o n based on o u r u n d e r s t a n d i n g of t h e a r c i n t e r a c t i o n w i t h t h e t r a n s v e r s e magnetic f i e l d w i t h due c o n s i d e r a t i o n of t h e p a r a l l e l c i r c u i t p a r a m e t e r s . When, however, t h e a r c v o l t a g e i s r e a d from t h e C.R.O. t r a c e , t h e c u r r e n t p a s s i n g t h r o u g h t h e vacuum a r c can b e c a l c u l a t e d by j u s t c o n s i d e r - i n g t h e l o c a l LC c i r c u i t .

PARAMETERS AFFECTING ARC EXTINCTION WITH AN

OSCILLATING MAGNETIC FIELD: Within t h e c o n s t r a i n t

f > 2 f g , t h e p r o b a b i l i t y of a r c e x t i n c t i o n i n -

C

-

c r e a s e s w i t h i n c r e a s i n g f i e l d a m p l i t u d e and f i e l d frequency , T h i s i s c o n s i s t e n t w i t h p r e v i o u s 1 y 2 o b s e r v a t i o n s on f i e l d magnitude and f i e l d r a t e of r i s e observed w i t h a n e s s e n t i a l l y l i n e a r l y r i s i n g f i e l d . The a r c s h o u l d b e i n a d i f f u s e mode p r i o r t o f i e l d a p p l i c a t i o n , w i t h no anode s p o t s p r e s e n t , and w i t h t h e m u l t i p l e c a t h o d e s p o t s d i s t r i b u t e d o v e r t h e t o t a l e l e c t r o d e s u r f a c e . The e f f e c t of t h e i n i t i a l columnar a r c 4 i s n o t y e t c l e a r . How- e v e r , we have observed a r c e x t i n c t i o n s a t c u r r e n t l e v e l s of 11 kA when t h e e l e c t r o d e s were s e p a r a t e d a t 1 3 kA and t h e f i e l d was s u b s e q u e n t l y a p p l i e d a t a n e l e c t r o d e s p a c i n g of 2 cm w i t h i n 2 . 8 m s of e l e c t r o d e p a r t . With r e s p e c t t o e l e c t r o d e s p a c i n g , i n g e n e r a l t h e p r o b a b i l i t y o f a r c e x t i n c t i o n i n - c r e a s e s w i t h e l e c t r o d e gap. A l o n g e r gap i s a s s o c i a t e d w i t h a h i g h e r d i e l e c t r i c r e c o v e r y v o l t - a g e and b e t t e r a r c / f i e l d i n t e r a c t i o n . However, t h e gap l e n g t h s h o u l d n o t b e g r e a t enough t o pro- mote anode s p o t f o r m a t i o n 5 p r i o r t o magnetic f i e l d a p p l i c a t i o n .

CONCLUSIONS: A p p l i c a t i o n o f a r a p i d l y r i s i n g t r a n s v e r s e magnetic f i e l d t o vacuum a r c s r e s u l t s i n a p p r e c i a b l e anode s p a c e c h a r g e v o l t a g e s . I n t h e p r e s e n c e of a p a r a l l e l c a p a c i t a n c e and a l i n e a r l y r i s i n g f i e l d , t h e a r c c u r r e n t i s d i v e r t e d i n t o t h e p a r a l l e l c a p a c i t o r , and a r c s of 55kA a r e e x t i n g u i s h e d . However, c a t h o d e s p o t s c o n t i n u e t o b u r n a t h i g h e r c u r r e n t l e v e l s . When t h e anode s p a c e c h a r g e h a s a p e r i o d i c n a t u r e due t o t h e ap- p l i c a t i o n of a n o s c i l l a t i n g magnetic f i e l d , t h e a r c

c u r r e n t o s c i l l a t e s w i t h a frequency determined by t h e f i e l d and p a r a l l e l c i r c u i t . We t h e n o b s e r v e a r c e x t i n c t i o n from i n i t i a l c u r r e n t l e v e l s of up t o

1 5 kA. REFERENCES

1.P.R. Emtage e t a l . , 1 3 t h I.C.P.I.G., p.673, 1977.

2.C.W. Kimblin e t a l . , E.P.R.I. Report EL-393,1977.

3 . K . S t e f a n i a k , 3rd I n t . Symp. on S w i t c h i n g Arc Phen., Lodz, pp. 217-221, 1977.

4.J.V.R. H e b e r l e i n e t a l . , 5 t h I n t . Conf. on Gas D i s c h a r g e s , pp. 281-284, 1978.

5.C.W. Kimblin, 1.E.E.ELTrans. PAS-2, 310, 1974.