SHOCK WAVES IN SPARK CHANNELS, PART 2

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SHOCK WAVES IN SPARK CHANNELS, PART 2

M. Kekez, G. Lougheed, P. Savic

To cite this version:

M. Kekez, G. Lougheed, P. Savic. SHOCK WAVES IN SPARK CHANNELS, PART 2. Journal de

Physique Colloques, 1979, 40 (C7), pp.C7-257-C7-258. �10.1051/jphyscol:19797126�. �jpa-00219098�

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JOURNAL DE PHYSIQUE CoZZoque C7, suppZ6ment au n07, Tome 40, ~ u i Z Z e t 2979, page C7- 257

SHOCK WAVES IN SPARK CHANNELS, PART 2

M.M. Kekez, G.D. Lougheed and P. Savic.

NationaZ Research Comci 2, Ottawa, Canada, KIA Experimental Evidence

I n t h i s p a r t we attempt experimental v e r i f i c a - t i o n of t h e shock theory developed i n Part 1

[ r e f . 11. The o b j e c t of t h e s e experiments i s t o record t h e progress of t h e shock and sound waves associated with t h e spark channel f o r ( a ) t h e chan- n e l i n process of formation and ( b ) t h e completed channel, when t h e shock theory i s expected t o apply [ r e f . 21.

A conventional Schlieren system was used com- p r i s i n g of a CW argon l a s e r (Model 556A of Control

~ o r p . ) , a 10 times expander, a Schlieren l e n s , k n i f e edge ( p a r a l l e l t o t h e d i r e c t i o n of t h e spark channel) and a TRW ( ~ u a n t r a d ) image converter camera. The discharge chamber was placed behind t h e expander. The r a d i a l expansion of t h e shock wave was observed through a narrow s l i t placed i n f r o n t of t h e camera and perpendicular t o t h e spark channel a x i s . A master t r i g g e r spark gap a c t i v a t e d t h e camera v i a an o p t i c a l pickup, while i t s elec- t r i c a l component i n i t i a t e d t h e voltage pulse t o be applied t o t h e discharge chamber. I n t h i s manner, t h e j i t t e r was kept below 20 n s , and permitted good synchronisation between t h e s t r e a k record of t h e developing channel, Fig. l a , and t h e voltage cur- r e n t wave forms, Fig. l b , a s well a s between t h e l a t t e r and t h e s t r e a k record o f th; r a d i a l l y ex- panding shock wave, Fig. l c . I n t h e f i r s t s e t of experiments, t h e viewing s l i t was positioned about 1 mm above t h e cathode, whereas i n t h e second s e t it was placed i n t h e c e n t r e between two pointed e l e c t r o d e s made of 1 mm diameter tungsten.

For t h e f i r s t s e t of experiments using t h e e l e c t r o d e geometry and conditions of Fig. 6 i n r e f . 4, t h e energy per u n i t l e n g t h of t h e developing spark channel was determined i n two ways.

F i r s t , it was c a l c u l a t e d by d i v i d i n g t h e average power, P, derived from t h e voltage and c u r r e n t wave form, Fig. l b , by t h e average v e l o c i t y of t h e chan- n e l (1.2 cm/us), and secondly i t was obtained by d i v i d i n g t h e energy, E, determined by i n t e g r a t i o n s of t h e VI product by t h e l e n g t h t h a t t h e spark channel has traversed. These r e s u l t s a r e depicted i n Fig. 2 by squares ( t h e c r o s s e s denote e r r o r b a r s ) . While t h e instantaneous p o s i t i o n of t h e

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shock can be determined with g r e a t accuracy, velo- c i t y measurements a r e s u b j e c t t o considerable e r r o r . I n Fig. 2, t h e o r e t i c a l r e s u l t s a r e depicted by s t r a i g h t l i n e s ; a f u l l l i n e f o r 20°C and t h e broken l i n e f o r 10o°C n e u t r a l gas temperature. Sound v e l o c i t i e s measured from t h e s t r e a k records indi- c a t e t h a t t h e temperature r a r e l y exceeds 100°C, j u s t i f y i n g t h e assumption of low n e u t r a l gas tem- p e r a t u r e i n t h e glow, a s u t i l i s e d i n our t h e o r e t i - c a l work. This i s f u r t h e r strengthened by t h e con- c l u s i o n s of r e f . 3. In Fig. l c , only t h e shock f r o n t i s v i s i b l e ; t h e boundary of t h e channel, termed "leader" i n r e f . 3, corresponds t o our con- t a c t surface.

In t h e second s e t of experiments, we s t u d i e d t h e r a d i a l expansions of t h e channel f o r a com- p l e t e d discharge i n a gap of 3.2 IMU length. The v o l t a g e was varied between 11-13 kV, and t h e energy was s t o r e d i n various condensers ( 8 . 5

-

2400 pF).

(The lowest point i n Fig. 2 corresponds t o a capac- i t y of 8.5 pF from a 10 cm long Cable of RG 8 / ~ type.* Such sparks a r e u s e f u l f o r t e s t i n g photo- m u l t i p l i e r s . ) The discharge c i r c u i t c o n s i s t s of a condenser, a t r i g g e r gap and t h e gap under observa- t i o n . An attempt was made t o minimize t h e l e n g t h of l e a d s and t h e c i r c u i t inductance. In t h e calcu- l a t i o n of El (energy per u n i t l e n g t h and d e n s i t y ) we assume t h a t t h i s energy i s a constant i n t h e two gaps and t h a t no energy i s consumed i n t h e l e a d s .

I n conclusion, we would point out t h a t t h e t r a n s i t i o n from a shock t o a sonic wave i n t h e v i c i n i t y of t h e spark channel proceeds a s predicted by theory, t h u s t h e concept of t h e hypersonic model f o r spark channel development i s f u r t h e r

stengthened.

References : [ l ] As i n P a r t 1.

[ 2 ] Freeman, R.A. and Craggs, J.D., 1969, J. Phys.

D: Appl. Phys

.

²⁴²¹

[ 3 ] Kurimoto, A . , F a r i s h , 0. and Tedford, D . J . , 1978, Proc. IEE,

125,

767.

[b] Kekez, M.M. and Savic, P., 1978, Proc. IEE _5th I n t . Conf. Gas Discharges, Liverpool, 336.

*

I n t h i s case, t h e gap was reduced t o 1 . 2 mm.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797126

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F i g . 1 ( a ) S t r e a k p h o t o g r a p h o f a r r e s t e d d i s c h a r g e i n a i r a t 2 5 0 T o r r g a s p r e s s u r e a n d 2 . 1 5 cm g a p l e n g t h . T h e d e v e l o p i n g s p a r k c h a n n e l s t a r t i n g a t t h e c a t h o d e w a s d e p i c t e d a s t h e b r i g h t a r e a o n t h e p h o t o - g r a p h .

( b ) V - v o l t a g e w a v e f o r m ( 2 O K V l d i v ) I - c u r r e n t w a v e f o r m ( l O O A / d i v ) P - c a l c u l a t e d p o w e r ( 1 0 6 W/di v ) E - e n e r g y ( 2 x 1

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e r g s / d i v ) One h o r i z o n t a l d i v i s i o n e q u a l s .5 p s . The w a v e f o r m s a r e s y n c h r o n i z e d w i t h t h e s t r e a k p h o t o g r a p h .

F i g . 2 T i m e o f t r a n s i t i o n f r o m h y p e r s o n i c t o s o n i c m o t i o n , to v s ( t h e e n e r g y p e r u n i t l e n g t h a n d d e n s i t y s u p p l i e d t o t h e b l a s t w a v e , El ) % . T h e o r e t i c a l r e s u l t s a r e d e p i c t e d b y s t r a i g h t l i n e s .

F i g . 1 ( c ) R a d i a l e x p a n s i o n o f s h o c k w a v e d u e t o d e v e l o p i n g s p a r k c h a n n e l .