Multipactor electron gun for millimeter wave tubes

Abstract

Army m i l l i m e t e r wave radar systems designers i m p o s e e v e r -i n c r e a s i n g r e s t r i c t i o n s o n t u b e volume, weight, and cost while at the same time r e q u i r i n g i m p r o v e d t u b e l i f e and r e l i a b i l i t y . The b u l k i e s t p o r t i o n o f a m i l l i m e t e r wave tube i s comprised o f t h e t h e r m i o n i c e l e c t r o n gun and t h e beam focusing magnet. The t h e r m i o n i c e l e c t r o n gun i s o f t e n t h e s o u r c e o f t u b e f a i l u r e . The magnet accounts f o r a b o u t 30% o f t h e c o s t and 60% o f t h e t u b e volume and weight. An RF a c t i v a t e d , n o n -t h e r m i o n i c , e l e c t r o n gun which employs the principle of secondary electron resonance (mu1 t i p a c t o r ) t o p h a s e -f o c u s e l e c t r o n s i n t o t i g h t , h i g h d e n s i t y e l e c t r o n bunches i s discussed. Since electrons are prebunched w i t h i n t h e gun i t s e l f , l o n g d r i f t l e n g t h s and the attendant need for magnetic focusing are eliminated or minimized, thereby decreasing tube s i z e , w e i g h t , a n d c o s t . R e l i a b t l l t y and l i f e a r e s u b s t a n t i a l l y i n c r e a s e d b y v i r t u e o f t h e f a c t t h a t t h e r m i o n i c e m i s s i o n i s n o t employed. C a p a b i l i t y o f m u l t i p a c t o r guns t o produce s i g n i f i c a n t beam c u r r e n t s has been established by numerous i n v e s t i g a t o r s . The p r a c t i c a b i l i t y and f e a s i b i l i t y o f such guns f o r u s e a t m l l l i m e t e r wavelengths, however, depends not only on their e l e c t r o n c u r r e n t c a p a b i l i t y , b u t a l s o o n t h e i r a b i l i t y t o be d r i v e n by l o w power, compact, RF sources. T e s t v e h i c l e s ( s c a l e d t o 5 GHz) t o demon? s t r a t e r e q u i r e d d r i v e l e v e l s have been b u i l t and e v a l u a t e d a t ET&DL, ERADCOM. Experimental res u l t s show t h a t d r i v e l e v e l s o n t h e o r d e r o f 10 m i l l i w a t t s a r e s u f f i c i e n t , d e m o n s t r a t i n g t h a t s m a l l , l i g h t w e i g h t , s o l i d -s t a t e RF drive sources a r e p r a c t i c a l . T h e o r e t i c a l e v i d e n c e .Is presented showing t h a t t h e l e v e l of d r i v e i s t h e same f o r millimeter frequencies (e.g. 95, 140, 220 GHz) as i t i s f o r t h e 5 GHz s c a l e d t e s t v e h i c l e s .