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SOME APPLICATIONS OF PHOTON
CORRELATION SPECTROSCOPY IN PLASMA DIAGNOSTICS
P. Serapinas
To cite this version:
P. Serapinas. SOME APPLICATIONS OF PHOTON CORRELATION SPECTROSCOPY IN PLASMA DIAGNOSTICS. Journal de Physique Colloques, 1988, 49 (C1), pp.C1-315-C1-317.
�10.1051/jphyscol:1988166�. �jpa-00227580�
JOURNAL DE PHYSIQUE
Colloque C1, Supplement au n03, Tome 49, Mars 1988
SOME APPLICATIONS OF PHOTON CORRELATION SPECTROSCOPY IN PLASMA DIAGNOSTICS
P. SERAPINAS
Institute of Physics, Academy of Sciences of the Lithuanian SSR, Vilnius 232600, USSR
A b s t r a c t . - E x c e s s r a d i a t i o n n o i s e s p e c t r a o f a p u l s e d i s c h a r g e d u e t o t h e c a t h o d e s p o t m i c r o s t r u c t u r e a n d t h a t o f a h i g h f r e q u e n c y d i s c h a r g e d u e t o t h e a e r o s o l p a r t i c l e s w e r e o b s e r v e d a n d a n a l y z e d . D e n s i t i e s a n d r e l a x a t i o n t i m e s o f t h e c a t h o d e m i c r o s p o t s a n d t h e a e r o s o l p a r t i c l e s w e r e m e a s u r e d .
P l a s m a d i a g n o s t i c s u s u a l l y i s u n d e r s t o o d as m e a s u r e m e n t s o f t h e mean o r i n s t a n t a n e o u s p a r t i c l e d e n s i t i e s a n d t h e i r e n e r g y d i s t r i b u t i o n f u n c t i o n s . M e a s u r e m e n t s o f p h o t o n c o r r e l a t i o n c o n t a i n i n f o r m a t i o n a b o u t c o r r e l a t i o n l e n g t h s a n d c o r r e l a t i o n t i m e s i n t h e r a d i a t i n g s y s t e m , i t s e v o l u t i o n a t t h e m i c r o s c o p i c l e v e l . Two e x a m p l e s i n p l a s m a d i a g n o s t i c s a r e d i s c u s s e d as f o l l o w s : p h o t o n e x c e s s n o i s e d u e t o t h e h i g h c u r r e n t d i s c h a r g e c a t h o d e s p o t m i c r o s t r u c t u r e a n d t h a t o f t h e h i g h f r e q u e n c y d i s c h a r g e w i t h a e r o s o l p a r t i c l e s .
The l i f e t i m e o f t h e c a t h o d e m i c r o s p o t s i n vacuum a r c s , u n i p o l a r a r c s o f t h e t h e r m o n u c l e a r d e v i c e s i s o f t h e o r d e r o f m a g n i t u d e 10-100 n s , t h e i r d i a m e t e r a b o u t 0 , l - 1 0 pm / 2 / . S t u d y o f t h e m e c h a n i s m o f t h e s p o t f o r m a t i o n a n d t h e p a r a m e t e r s o f s u c h s m a l l a n d s h o r t - l i v i n g s i t e s i s a c o m p l i c a t e d p r o b l e m . The c u r r e n t d e n s i t y i n s u c h m i c r o s p o t s a n d t h e e n e r g y o f t h e p a r t i c l e s l e a v i n g t h e s u r f a c e r e m a i n s a s u b j e c t o f i n t e n s i v e d i s c u s s i o n / 2 / .
R a d i a t i o n a n d c u r r e n t n o i s e s p e c t r a o f t h e p u l s e d i s c h a r g e ( C = 6 0 p F , U=300 V , L = 8 pH, d i s t a n c e b e t w e e n e l e c t r o d e s 0 . 4 cm) w e r e m e a s u r e d . A c a t h o d e , d i a m e t e r 1 . 4 cm, made f r o m t h e m a t e r i a l u n d e r s t u d y , a n d a t u n g s t e n a n o d e w e r e u s e d . T h e m a x i m a l d i s c h a r g e c u r r e n t ( 1 kA) was a c h i e v e d a t t h e m i d d l e o f t h e s i n u s o i d a l p u l s e o f t h e d u r a t i o n a b o u t 6 0 p s . The r a d i a t i o n p u l s e l a s t e d f o r 4 0 vs f o r t h e s p e c t r a l l i n e A 1 111 4 5 1 . 3 nm a n d 8 0 p s f o r A 1 I 394.4 nm.
The c u r r e n t n o i s e a n d t h e r a d i a t i o n n o i s e o f t h e s e p a r a t e s p e c t r a l l i n e s w e r e m e a s u r e d . The m e a s u r e m e n t s w e r e s t a r t e d 2 5 p s a f t e r t h e p u l s e i g n i t i o n . The c u r r e n t n o i s e was m e a s u r e d o n a t h i n p r o b e i n s t a l l e d i n t h e c a t h o d e . The r a d i a t i o n w a s d e t e c t e d w i t h a p h o t o - m u l t i p l i e r , T h e d i s c h a r g e w a s f o c u s e d on t h e m o n o c h r o m a t o r e n t r a n c e s l i t - a n d o n l y t h e r a d i e t i o n o f t!le p a r t c l o s e t o t h e c a t h o d e was m e a s u r e d . The s i g n a l s w e r e a n a l y z e d by a c o m p u t e r w i t h t h e a i d o f t h e f a s t F o u r i e r t r a n s f o r m a t i o n . The s p e c t r a w e r e m e a s u r e d i n t h e f r e q u e n c y r e g i o n f r o m 3 kIlz t o 60 !.!Hz. A t t h e h i g h e s t f r e q u e n c i e s t h e d u r a t i o n o f t h e s e t s w a s 2 p s , t i m e r e s o l u t i o n 8 n s . U s u a l l y t h e mean v a l u e f r o m 1 0 s e t s w a s c a l c u l a t e d .
The i n t e n s i v e e x c e s s n o i s e a t t h e f r e q u e n c i e s l o w e r t h a n 20 MHz w a s o b s e r v e d ( F i g . 1 ) . The c u r r e n t n o i s e s p e c t r u m w a s s i m i l a r t o t h a t o f t h e r a d i a t i o n o f t h e a t o m i c c o m p o n e n t o f t h e s a m p l e . R a d i a t i o n n o i s e o f t h e i o n s was a t l o w e r f r e q u e n c i e s , s i m i l a r t o t h a t o f t h e a t o m i c c o m p o n e n t f a r f r o m t h e c a t h o d e . From t h o s e r e s u l t s it f o l l o w s t h a t t h e n e u t r a l c o m p o n e n t p r e v a i l s i n t h e s a m p l e l e a v i n g t h e c a t h o d e . The c a t h o d e m i c r o s p o t l i f e t i m e s on A 1 a n d Cu c a t h o d e s w e r e f o u n d t o be a b o u t 1 0 0 n s . The o r i g i n o f t h e e x c e s s n o i s e c a n be d i s c u s s e d as f o l l o w s ( s e e e . g . / 3 / ) . I f t h e s i g n a l c o n s i s t s o f n s t a t i s t i c a l l y d i s t r i b u t e d p u l s e s o f t h e i n t e g r a l i n t e n s i t y m a n d t h e n o r m a l i z e d F o u r i e r s p e c t r u m F ' , t h e n t h e c u r r e n t
I = e n m g ( 1
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Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988166
JOURNAL DE PHYSIQUE
F i g . 1 . The r a d i a t i o n n o i s e s p e c t r a o f t h e p u l s e d i s c h a r g e . 1- A 1 I11 451.3nm r z d i a t i o n n o i s e , 2
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c u r r e n t n o i s e .a n d t h e s p e c t r a l d e n s i t y o f t h e c u r r e n t n o i s e
S = 2 e 2 n m 2 g 2 F 2
,
( 2 )w h e r e e i s t h e c h a r g e of t h e e l e c t r o n , g i s t h e g a i n f a c t o r . The s p e c t r a l d e n s i t y o f t h e c u r r e n t s h o t n o i s e
s = 2 e 2 n m g 2 . ( 3 )
C o n s e q u e n t l y ,
S / s = m F 2
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S / 2 1 Z = F 2 / n.
( 4 )The o b t a i n e d r e l a t i o n s e n a b l e u s t o e v a l u a t e t h e number o f s i m u l t a n e o u s l y w o r k i n g m i c r o s p o t s , n
,
w h i c h i s o f t h e o r d e r o f m a g n i t u d e 50-100 i n good a g r e e m e n t w i t h t h e r e s u l t s /4/ a t t h e a b o v e d i s c u s s e d c r a t e r m i c r o s p o t l i f e t i m e 1 0 0 n s .A s a n a n o t h e r e x a m p l e t h e r a d i a t i o n n o i s e of t h e h i g h f r e q u e n c y d i s c h a r g e i n a r g o n ( 2 7 . 1 2 M H z , 2 . 5 kW, a r g o n g a s f l o w r a t e a b o u t 20 l / m i n , d i a m e t e r o f t h e t u b e 1 . 8 cm) w a s s t u d i e d . A e r o s o l of t h e w a t e r s o l u t i o n s w a s i n t r o d u c e d t h r o u g h t h e i n t e r n a l t u b e ( d i a m e t e r 0 . 1 5 cm).
The r a d i a t i n g p l a s m a image w a s a l s o f o c u s e d on t h e m o n o c h r o m a t o r e n t r a n c e s l i t w i t h o u t m a g n i f i c a t i o n and t h e r a d i a t i o n d e t e c t e d w i t h a p h o t o m u l t i p l i e r .
S i m i l a r m e a s u r e m e n t s of t h e h i g h f r e q u e n c y d i s c h a r g e r a d i a t i o n n o i s e s p e c t r a w e r e c a r r i e d o u t i n / 5 / . N e v e r t h e l e s s , e x c e p t t h e e x c e s s n o i s e a t t h e n e t w o r k f r e q u e n c y a n d t h o s e o f p l a s m a r o t a t i o n , o n l y t h e w h i t e p h o t o c u r r e n t s h o t n o i s e w a s o b s e r v e d . To i n c r e a s e t h e s e n s i t i v i t y o f t h e m e a s u r i n g s y s t e m t o t h e e x c e s s n o i s e due t o t h e a e r o s o l s t r u c t u r e o f t h e s a m p l e it i s n e e d e d t o i n c r e a s e a s p o s s i b l e t h e mean number ( a b o v e 1 ) o f t h e p h o t o n s d e t e c t a b l e f r o m t h e r a d i a t i n g a t o m s o r i o n s o f t h e same a e r o s o l p a r t i c l e . To p r e s e r v e t h e p h o t o m u l t i p l i e r s i g n a l i n t h e l i n e a r d y n a m i c a 1 r a n g e , as low p h o t o m u l t i p l i e r g a i n s a s 1 0 0 w e r e u s e d . The n o i s e s p e c t r a were m e a s u r e d w i t h a s e l e c t i v e v o l t m e t e r . The p h o t o c u r r e n t s h o t n o i s e was m e a s u r e d w i t h t h e a i d of t h e t h e r m a l r a d i a t i o n o f a t u n g s t e n lamp a t t h e same p h o t o s i g n a l v a l u e ( I = l P A ) .
~t was S = I O - ~ ~ I A ~ S .
The r a d i a t i o n e x c e s s n o i s e , e x c e e u i n g by a b o u t two o r d e r s o f m a g n i t u d e t h e s p e c t r a l d e n s i t y o f t h e s h o t n o i s e , w a s f o u n d a t t h e f r e q u e n c i e s o f some kHz a n d l o w e r ( F i g . 2 ) . The s p e c t r a l d e n s i t y o f t h a t n o i s e was p r o p o r t i o n a l t o t h e s e c o n d power o f t h e p h o t o s i g n a l u p t o t h e p h o t o m u l t i p l i e r e n t r a n c e s l i t w i d t h a s l a r g e a s 3 mm e n c l o s i n g a l m o s t f u l l d i a m e t e r of t h e r a d i a t i n g s a m p l e . The v a l u e of t h e s p e c t r a l d e n s i t y was S = I O - ~ 12 A 2 s , what e n a b l e s t o c a l c u l a t e ( a c c o r d i n g t o ( 4 ) ) t h e number ( a n d d e n s i t y ) o f t h e a e r o s o l p a r t i c l e s . I f t h e s a m p l e f l o w r a t e i s known ( i n o u r c a s e i t was 1 0 c m 3 / s ) t h e a e r o s o l p a r t i c l e d i a m e t e r c a n be f o u n d a l s o . The a e r o s o l d i a m e t e r o f 5 pm a n d t h e e v a p o r a t i o n t i m e a b o u t 1 0 0 p s w e r e o b t a i n e d i n g o o d a g r e e m e n t w i t h t h e e a r l i e r s t u d i e s .
The s u b m i t t e d r e s u l t s , b e s i d e s t h e d i r e c t i n t e r e s t , show t h e p o s s i b i l i t y t o u s e t h e r a d i a t i o n n o i s e m e a s u r e m e n t s f o r t h e s t u d y of t h e s t r u c t u r e a n d e v o l u t i o n o f t h e r a d i a t i n g s a m p l e .
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F i g . 2 . The r a d i a t i o n n o i s e s p e c t r a o f t h e h i g h f r e q u e n c y d i s c h a r g e . 1
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1 . G u i l e A.E., P r o c e e d i n g s of t h e XV I n t e r n a t i o n a l C o n f e r e n c e on Phenomena i n I o n i z e d G a s e s , I n v i t e d p a p e r s ( M i n s k , 1 9 8 1 ) ; J i i t t n e r B . , P l a s m a P h y s i c s a n d C o n t r o l l e d F u s i o n
26
( 1 9 8 4 ) 2 4 9 .2 . Z e k c e r M.P. a n d Rakchovsky V . I . , S o v . P h y s . D o k l a d y 2 7 8 ( 1 9 8 4 ) 8 6 ; M a r c i n o v s k i i A . M . a n d Yuryev V . G . , S o v . P h y s . P i s m a JETP a m 8 2 1 1 3 1 2 .
3. v a n d e r Z i e l A . , N o i s e . S o u r c e s , C h a r a c t e r i z a t i o n , Measurement ( P r e n t i c e - H a l l . I n c .
,
N . J . , 1 3 7 0 ) .4 . U r s u I . , A p o s t o l I . , M i h a i l e s c u I . N . , Popa A l . , S e r a p i n a s P.D.
a n d B u t e i k i s R.Yu., P r o c e e d i n g s o f t h e X V I I I n t e r n a t i o n a l C o n f e r e n c e on Phenomena i n I o n i z e d G a s e s , C o n t r i b u t e d P a p e r s ( B u d a p e s t , 1 9 8 5 ) .
5 . Walden G.L., Bower J . N . , N i k d e l S . , B o l t o n D . L . a n d W i n e f o r d n e r J . D . , S p e c t r o c h i m i c a A c t a 35B ( 1 9 8 0 ) 5 3 5 ; B e l c h a m b e r R . M . a n d H o r l i c k G . , S p e c t r o c h i m i c a A c t a ~ ( 1 9 8 2 ) 1 7 .