Initial comparison of SSPM ground test results and flight data to NASCAP simulations

The s a t e l l i t e s u r f a c e p o t e n t i a l m o n i t o r (SSPM) h a s been deve loped f o r t h e P78-2 SCATHA ( S p a c e c r a f t Charg ing At The High A l t i t u d e s ) s a t e l l i t e t o d e t e r m i n e t h e r e s p o n s e o f s e l e c t e d s p a c e c r a f t m a t e r i a l s t o c h a r g e d p a r t i c l e e n v i r o n * o menta l f l u x e s . S i n c e t h e m o n i t o r i n f e r s t o t a l 0 I s u r f a c e v o l t a g e s from a s i n g l e p o i n t on t h e i n ir] t e r i o r s i d e of i n s u l a t o r s , a ground s i m u l a t i o n program was u n d e r t a k e n t o d e v e l o p a n a l y t i c a l t e c h n i q u e s t o model t h e m o n i t o r s and t o o b t a i n a n e x p e r i m e n t a l c a l i b r a t i o n o f t h e r e l a t i o n s h i p between t h e f l i g h t measurement t e c h n i q u e s and a c t u a l measurements. The e x p e r i m e n t a l t e s t i n g was conduc ted u s i n g monoenerget i c e l e c t r o n beams irr a d i a t i n g samples i n t h e da rk . An a n a l y t i c a l computer model was deve loped i n t h e NASCAP (NASA Charging Ana lyzer Program) code. The a n a l y t i c a l model m a t e r i a l p r o p e r t i e s f o r Kapton t h a t cont r o l l e d b a c k s c a t t e r and secondary y i e l d were adj u s t e d t o o b t a i n a s i n g l e s e t of v a l u e s t h a t produced r e a s o n a b l e f i t s f o r b o t h v o l t a g e s and c u r r e n t s . The a n a l y t i c a l t e c h n i q u e s deve loped i n t h e ground t e c h n o l o g y i n v e s t i g a t i o n have b e e n a p p l i e d t o s p a c e f l i g h t c o n d i t i o n s . P r e d i c t i o n s were compared w i t h l i m i t e d f l i g h t d a t a . The agreement i s ve ry good i n d i c a t i n g t h a t t h e t e c h n i q u e , and NASCAP, c a n be used t o p r e d i c t spacec r a f t m a t e r i a l c h a r g i n g behav ior . D e t a i l s of t h e t e s t i n g , t h e a n a l y t i c a l m o d e l l i n g t e c h n i q u e and f l i g h t d a t a compar i sons a r e p r e s e n t e d . I n t r o d u c t i o n S p a c e c r a f t c h a r g i n g a n o m a l i e s t h a t have occ u r r e d i n geosynchronous s a t e l l i t e s a r e b e l i e v e d t o be c a u s e d by d i s c h a r g e s i n i t i a t e d by d i f f e r e n t i a l c h a r g i n g of s p a c e c r a f t d i e l e c t r i c s u r f a c e s by geomagnet ic subs to rm env i ronments . l T h i s b e l i e f i s based upon t h e d e t e r m i n a t i o n of spacec r a f t ground p o t e n t i a l s from ATS-5 and -6 p a r t i c l e d e t e c t o r s measurements , 2-4 t r a n s i e n t p u l s e measurements ,5 h a r n e s s n o i s e c o u n t i n g o n e l e c t r i c a l h a r n e ~ s e s , ~ ground t e ~ t i n ~ ~ ~ and anal y t i c a l m o d e l l i n g of d i e l e c t r i c m a t e r i a l responses t o c h a r g i n g environment^.^^-^^ The space f l i g h t d a t a were o b t a i n e d on d i f f e r e n t s a t e l l i t e s and c o u l d be used a s i n d i c a t o r s of i n t e r a c t i o n s between c h a r g e d p a r t i c l e env i ronment s and s p a c e c r a f t sys tem1 s response . C l e a r l y , what was needed t o v a l i d a t e s p a c e c r a f t c h a r g i n g c o n c e p t s i s s p a c e f l i g h t d a t a t h a t would s i m u l t a n e o u s l y p r o v i d e e n v i r o n m e n t a l p a r t i c l e f l u x e s a l o n g w i t h s p a c e c r a f t s u r f a c e c h a r g i n g and sys tems response . T h i s i n f o r m a t i o n i s be ing prov ided by t h e AF P78-2 s a t e l l i t e which h a s been c a l l e d SCATHA a n acronym s t a n d i n g f o r Spacec r a f t Charg ing At The High ~ l t i t u d e s l , 12913 and which was l aunched i n t o o r b i t i n January 1979. One of t h e e x p e r i m e n t s on t h i s s a t e l l i t e i s t h e S a t e l l i t e S u r f a c e P o t e n t i a l Moni to r o r S S P M . ' > ~ ~ T h i s m o n i t o r p r o v i d e s s u r f a c e v o l t a g e and l e a k a g e c u r r e n t measurements o n s e l e c t e d s p a c e c r a f t m a t e r i a l s i n r e s p o n s e t o t h e chargedp a r t i c l e environement s. The m o n i t o r was d e s i g n e d and b u i l t by t h e Aerospace Corpora t ion . I t was t e s t e d i n t h e NASA-Lewis Research C e n t e r subs to rm s i m u l a t i o n f a c i l i t i e s t o c a l i b r a t e f l i g h t measurement t e c h n i q u e s and t o p r o v i d e a d a t a b a s e t o compare t o f l i g h t r e s u l t s . An a n a l y t i c a l model o f t h e m o n i t o r h a s b e e n deve loped f o r compar i sons t o ground s i m u l a t i o n s and s p a c e f l i g h t d a t a . I n t h i s p a p e r a b r i e f d e s c r i p t i o n of t h e SSPM hardware i s g i v e n and t h e ground s i m u l a t i o n t e s t program i s summarized. I n i t i a l compar i sons of t h e s i n g l e Kapton i n s u l a t o r sample (SC1-2) a n a l y t i c a l model p r e d i c t i o n s a g a i n s t t h e e x p e r i menta l r e s u l t s and s e l e c t e d s p a c e f l i g h t d a t a a r e p r e s e n t e d . D e s c r i p t i o n of SSPM The SSPM h a s been d e s c r i b e d i n t h e l i t e r a tu re1 .12 ,14 and i s o n l y b r i e f l y d i s c u s s e d i n t h i s r e p o r t f o r t h e s a k e o f comple teness . The SSPM c o n s i s t s o f t h r e e s e p a r a t e boxes , e a c h 3 3 by 3 3 by 5 cm. These boxes a r e numbered SC1-1, SC1-2, and SC1-3 i n t h e s a t e l l i t e e x p e r i ments l is t . The SC1-1 and SC1-3 boxes e a c h house f o u r t e s t samples , e a c h 13.2 by 13.2 cm s q u a r e , w h i l e t h e SC1-2 box h a s a s i n g l e sample, 29.7 by29.7 cm s q u a r e ( s e e F ig . 1 ) . The m a t e r i a l s used f o r e a c h o f t h e t e s t s u r f a c e s a r e g i v e n i n T a b l e I. Around e a c h o f t h e samples of a l l boxes i s a g o l d f rame connec ted t o s p a c e c r a f t e l e c t r i c a l ground. F o r e a c h of t h e t e s t s u r f a c e s a s u r f a c e v o l t a g e measurement (made a t a s p o t on t h e i n t e r i o r s i d e of t h e m a t e r i a l ) and a t o t a l l e a k a g e c u r r e n t measurement a r e made. A guard r i n g i s used t o i n s u r e t h a t c u r r e n t measurements a r e due t o b u l k p r o p e r t i e s o n l y and d o n o t i n c l u d e s u r f a c e e f f e c t s . I n a d d i t i o n , t h e r e i s a second v o l t a g e measurement t e c h n i q u e ( i n v o l v i n g a h o l e th rough t h e i n s u l a t o r t o s e n s e s u r f a c e e l e c t r i c f i e l d s ) on SC1-2, t e m p e r a t u r e measurements i n s i d e e a c h box and t h e measurement o f t h e v o l t age o f a n e l e c t r i c a l l y i s o l a t e d , g o l d r e f e r e n c e band. A11 v o l t a g e measurements a r e made r e l a t i v e t o s p a c e c r a f t ground. The loca t ion of t he SSPM on the s a t e l l i t e i s a s fo l lows ( s e e Fig. 2) : SC1-1: located on the c y l i n d r i c a l s i d e of t h e s a t e l l i t e above t h e c e n t r a l experiment band opposi te t o SC1-2. SC1-2: located 180° around the s a t e l l i t e from SC1-1 but below t h e c e n t r a l experiment band. SC1-3: located on the nominally shaded "top" of t he s a t e l l i t e . Aluminized Kapton has been chosen a s t he standard t e s t ma te r i a l f o r t h i s experiment and i s used on a l l t h r e e boxes. By comparing t h e res u l t s of t h i s one ma te r i a l information on a r e a e f f e c t s and sun-shade e f f e c t s i s obtained. Ground Simulation Inves t iga t ion The ground s imula t ion i n v e s t i g a t i o n cons i s t e d of an experimental and a n a l y t i c a l study. The ob jec t ives of t h i s i n v e s t i g a t i o n were t o provide a c a l i b r a t i o n between the o v e r a l l f r o n t surf ace vo l t age of t h e va r ious samples w i th t h e f l i g h t vol tage measurement made from the r e a r s i d e a t one spot on t h e sample, t o provide a d a t a base f o r comparing ground s imula t ion r e s u l t s with space ( i n e f f e c t c a l i b r a t i n g ground s imula t ion concepts) , and t o v a l i d a t e t h e a n a l y t i c a l model which would be used t o r e l a t e space measurements t o o v e r a l l sample behavior. I n t h i s s e c t i o n of t he r epo r t , the SC1-2 monitor r e s u l t s a r e d i s cussed i n d e t a i l . Experimental Inves t iga t ion