Using Magnetic Pulse Shaping to Reduce Spark Gap Losses and Electrode Erosion

Abstract

Spark gap e l e c t r o d e e r o s i o n i s due, i r i p a r t , t o t h e p r o d u c t o f g a p v o l t a g e a n d c u r r e n t . A s t h e s p a r k gap b r e a k s down, t h e i n i t i a l v o l t a g e h a s n o t f u l l y dropped b e f o r e t h e c u r r e n t b e g i n s t o r i s e , and t h e V I p r o d u c t i n t h i s i n i t i a l phase can b e q u i t e h i g h . I f t h e c u r r e n t can be d e l a y e d u n t i l t h e d i s c h a r g e c h a n n e l i s f u l l y formed, and t h e v o l t a g e h a s dropped, t h e V I p r o d u c t can b e s i g n i f i c a n t l y reduced. In p r e v i o u s s t u d i e s on s p a r k gap e l e c t r o d e e r o s i o n , one of t h e p a r a m e t e r s was t o i n c l u d e m a g n e t i c p u l s e s h a p i n g [l] . A s i g n i f i c a n t r e d u c t i o n i n e r o s i o n r e s u l t e d from u s i n g a s a t u r a b l e r e a c t o r t o d e l a y t h e i n i t i a l c u r r e n t through t h e gap. Recently, t h e same method w a s a p p l i e d t o a vacuum s w i t c h , and t h e turn-on and r e c o v e r y c h a r a c t e r i s t i c s of t h e s w i t c h w e r e improved [ 2 1 . e c t e d t o h a r s h o p e r a t i n g c o n d i t i o n s . Every t y p e of s w i t c h has u l t i m a t e o p e r a t i n g p a r a m e t e r s t h a t cannot b e exceeded w i t h o u t r e d u c i n g t h e l i f e t i m e o f , o r c a u s i n g damage t o t h e d e v i c e . s i d e s t h e one mentioned h e r e . F o r t h i s p a r t i c u l a r method, a s a t u r a b l e i n d u c t o r ( a n i n d u c t o r w i t h magnetic m a t e r i a l ) i s p l a c e d i n s e r i e s w i t h a s p a r k gap. The n o n l i n e a r i t y of t h e magnetic m a t e r i a l i s u t i l i z e d t o c r e a t e a s o r t of s w i t c h i n g a c t i o n from a h i g h impedance t o a low impedance. The change t o a low impedance can b e p r e d i c t e d i f t h e t y p e and geometry of t h e magnetic m a t e r i a l i s known, and a l s o t h e s w i t c h h o l d o f f v o l t a g e . T h e r e f o r e , a s a t u r a b l e i n d u c t o r c a n b e d e s i g n e d f o r an i n d i v i d u a l c i r c u i t t o l i m i t t h e c u r r e n t flow u n t i l t h e v o l t a g e a c r o s s t h e s p a r k gap h a s dropped c o n s i d e r a b l y . The concept i s p i c t u r e d i n F i g u r e 1. A F i g u r e l a . V I P r o d u c t w i t h o u t D e l a y F i g u r e lb. V I P r o d u c t w i t h Delay F i g u r e 1. Reduction o f V I Product Due t o Magnetic Delay s a t u r ab1 e Induct o r Desian The d e l a y time i s t h e amount of t i m e it t a k e s f o r t h e magnetic material i n t h e s a t u r a b l e i n d u c t o r t o s a t u r a t e and c a n be c a l c u l a t e d from F a r a d a y ' s Law and Gauss' Law. The r e s u l t i n g s a t u r a t i o n t i m e i s t = N A A B V where AB i s t h e f l u x d e n s i t y swing of t h e c o r e , A i s t h e c r o s s s e c t i o n a l a r e a , N i s t h e number of t u r n s , and V i s t h e i n i...