AN UNAMBIGUOUS METHOD OF ELECTRON DENSITY MEASUREMENT IN A SHOCK-GENERATED PLASMA, USNG A LASER INTERFEROMETER

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AN UNAMBIGUOUS METHOD OF ELECTRON DENSITY MEASUREMENT IN A

SHOCK-GENERATED PLASMA, USNG A LASER INTERFEROMETER

A. Lesage, J. Richou, P. Charil, M. Combier

To cite this version:

A. Lesage, J. Richou, P. Charil, M. Combier. AN UNAMBIGUOUS METHOD OF ELECTRON DENSITY MEASUREMENT IN A SHOCK-GENERATED PLASMA, USNG A LASER INTERFEROMETER. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-805-C7-806.

�10.1051/jphyscol:19797388�. �jpa-00219386�

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JOURlu'AL DE PHYSIQUE CoZZoque C7, suppZ6ment a u n07, Tome 40, J u i Z Z e t 1979, page C7- 805

AN UNAMBIGUOUS METHOD OF ELECTRON DENSITY MEASUREMENT

M

A SHOCK-GENERATED PLASMA, USNG A LASER INTERFEROMETER

A. Lesage, J. Richou, P. Charil and M. Cornbier.

D.E.P.E.G. O b s e r v a t o i r e de Meudon, 92190 Meudon France.

INTRODUCTION

L i Nb O 3 e l e c t r o - o p t i c c r y s t a l r e g u l a r l y Degree of i o n i s a t i o n p r o f i l e s have p r e - modulated by P o c k e l e f f e c t ( 2 ) . A v o l t a g e v i o u s l y been measured i n shock waves by p r o p o r t i o n a l t o t h e d r i v i n g v o l t a g e l a s e r i n t e r f e r o m e t r y i 1 ) . The t e c h n i q u e was (phase s h i f t of t h e l a s e r beam) of t h e however c o n f i n e d t o i n c i d e n t shocks becau- is fed to t h e y-axis of t h e o s c i l - s e o f t h e d i f f i c u l t y i n i n t e r p r e t i n g i n t e r - loscope ( ~ i ~ . l ) . ~ o d u l a t i o n o f t h e b r i g h t - ferograms. A s s p e c t r a l l i n e s t e n d t o have n e s s of t h e o s c i l l o s c o p e t r a c e ( z - a x i s ) b e t t e r b r i g h t n e s s behind f i r s t - a n d m u l t i - produces one b r i g h t p u l s e f o r e v e r y r e f l e c t e d shock waves, it i s d e s i r a b l e t o _fringe_{( 3 ) ,}The chronology of evens i s e x t e n d t h i s powerful method o f e l e c t r o n - r e c o v e r e d from t h e o s c i l l o s c o p e t i m e b a s e d e n s i t y measurement t o t h e s e more h i g h l y i n t h e u s a l way.

compressed r e g i o n s .

Li NC 0 Plorrna Inlerfcrcntfel

THEORY OF OPERATION

A l l c o n s t i t u e n t s o f a plasma : n e u t r a l atoms ( s u s c r i p t 1)

,

i o n s (+)

,

e l e c t r o n s ( e ) c o n t r i b u t e t o i t s r e f r a c t i v i t y :

(n-1) = ( n - l I l

+

(n-1)+

+

( n - l ) ,

-

( n - l ) o ; where ( n - l ) o i s t h e r e f r a c t i v e i n d e x o f t h e g a s b e f o r e i o n i s a t i o n . I n t e r m s of s p e c i f i c r e f r a c t i v i t y K , and by n o t i n g t h a t i o n s have n e a r l y t h e same s u s c e p t i b i - l i t y a s atoms :

(n-1) = K1 (N1

+

^{N + )}

+

Ke Ne

-

KO No.

The l a s e r produces a maximum o u t p u t f o r : Because No << N1 a c r o s s a shock f r o n t , and

L = mX (where m i s a c o n s t a n t , L t h e d o e s n o t v a r y u n t i l a r r i v a l of t h e n e x t

i n i t i a l o p t i c a l - p a t h between t h e two m i r - f r o n t :

r o r s M1, M 2 , and X t h e o p e r a t i n g wave- ( n

-

¹⁾⁼^K~ (N1

+

N+)

+

K~ N ~ . l e n g t h ) . The c o n d i t i o n f o r a b l i p on t h e T h i s r e l a t i o n s h i p makes i t e a s y t o compare 0 ~ ~ i l l o s c o p e i s : L + F P ( t ) + f ( t ) e l e c t r o n and atom c o n t r i b u t i o n t o t h e r e - = m X (where F ( t ) and f (t) a r e t h e chan-

P

f r a c t i v e i n d e x

.

Values a t t w o wavelenghts geS o f p a t h d u e t o t h e c y c l i c modulator have been computed v i a : and t h e plasma r e ~ p e c t i v e l y ) . Because F P ( t ) i s p r o p o r t i o n a l t o t h e s c o p e ' s Y

4.48 k 2 N e

Ke Ne

/

K1 N1 = x

-

d e f l e c t i o n , t h e l o c u s of b r i g h t s p o t s ( A '

+

^B')/ ^{k 2} _N1

w i l l form a b r i g h t band g i v e n by : where A ' and B ' a r e t h e Cauchy formula y = (m X

-

1)

-

^{f ( t )}^;i . e . b r i g h t bands c o n s t a n t s d i v i d e d by t h e Loschmidt number. t r a c i n g o u t d i r e c t l y t h e p a t h l e n g t h Behavior of t h e f r i n g e p a t t e r n , when t h e v e r s u s t i m e p r o f i l e . ~ n t e r - b a n d s s e p a r a - l i g h t p a t h ' s r e f r a c t i v i t y i s c y c l i c a l l y t i o n i s X/2.

modulated, shows how t o i n t e r p r e t i n t e r -

APPARATUS f e r o m e t r i c s i g n a l s from shock-tube p l a s m a s

The c y c l i c change i s induced by a The i n t e r f e r o m e t e r i s t h e same a s p r e - v i o u s l y d e s c r i b e d ( ' ) . A (15 x 5 x 1.3)mm

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797388

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L i Nb

O 3

c r y s t a l i s l o c a t e d i n t h e r e f e r e n c e c a v i t y ( M 2 , M 3 ) of an ASHBY- JEPH COTT i n t e r f e r o m e t e r ; ( 4 ) and i t s o p t i c a l a x i s i s p a r a l l e l t o t h e l a s e r beam p o l a r i z a t i o n . M u l t i r e f l e x i o n i s used t o lower t h e minimum modulation v o l t a g e r e q u i r e d f o r a X/2 phase s h i f t . The m i r r o r M3 c a n b e o s c i l l a t e d f o r t r i a l p u r p o s e s . The c r y s t a l modulation f r e q u e n c y choosen i s a t r a d e - o f f between t h e plasma i n d e x of r e f r a c t i o n , r e s o l u t i o n and i n t e r f e r o m e t e r s i g n a l a m p l i t u d e . A germa- nium wedge i n s e r i e s w i t h i n t e r f e r e n c e f i l t e r i s o l a t e s t h e 3.39um l a s e r band and p r e v e n t s swamping t h e I n A s d e t e c t o r . The s i g n a l of t h e i n t e r f e r o m e t e r i s a m p l i f i e d and shaped t o enhance t h e z - a x i s d i s p l a y . When a t r a n s i e n t (shock t u b e ) plasma

p a s s e s a c r o s s t h e l a s e r beam, i t s e l e c t r o w d e n s i t y v a r i a t i o n p e r t u r b s t h e c y c l i c r e f r a c t i v i t y v a r i a t i o n d r i v e n by t h e c r y s t a l : p h a s e - s e n s i t i v e d e t e c t i o n i s used t o measure t h e s e c o m p a r a t i v e l y s m a l l p e r t u r b a t i o n s . The o u t p u t from a q u a r t z t r a n s d u c e r mounted i n t h e schock t u b e p r o v i d e s a synchronous p r e s s u r e p r o f i l e which f a c i l i t a t e s i n t e r f e r o g r a m i n t e r p r e - t a t i o n .

RESULTS AND CONCLUSIONS

The method was t e s t e d f i r s t w i t h an o s c i l l a t i n g m i r r o r M3, n e x t w i t h i n c i d e n t shock waves, and f i n a l l y w i t h a s u c c e s s i o n

Advantages of t h i s t e c h n i q u e a r e : 1) i t t r a c k s r e v e r s a l s of p a t h r e f r a c t i v i t y i n a n unambiguous way, 2 ) i t d e t e c t s s m a l l f l u c t u a t i o n s n o t r n a d i l y o b s e r v e d by e a r l i e r i n t e r f e r o m e t r i c methods, and 3 ) it p e r m i t s e s t i m a t i o n s of f r a c t i o n of a f r i n g e s h i f t on 'a l i n e a r s c a l e . The m e t h o d ' s l i m i t a t i o n i s t h e i n t e r f e r o m e t e r m o d u l a t i o n c u t - o f f which l i m i t s t h e r e s o - l u t i o n : f o r t r a n s i e n t phenomena, one would e x p e c t t h e d i s p l a y t o b r e a k down i f t h e f r i n g i n g r a t e approached one f r i n g e p e r o s c i l l a t i o n c y c l e . Higher l a s e r wavelenghts and a m p l i t u d e s c a n b e used f o r d e n s e r of b r i g h t e r plasmas. More d e t a i l s a b o u t t h e s e i n s t r u m e n t s c a n be found i n r e f e r e n c e 5.

REFERENCES

(1 ) J. RICHOU and A. MOLITOR C .R. -Acad. SV.

P a r i s , t 271, p.753-756 (12 Octobre 1 9 7 0 ) .

( 2 ) F. POCKELS, Lehrbuch d e r K r i s t a l l o p - t i k (Teubner

,

L e i p z i g 1906)

.

( 3 ) A. GIBSON and G.W. R E I D Appl. Phys L e t t e r s ,

5 ,

1 0 , (1964)

( 4 ) D.E.T.F. ASHBY, D.F. JEPHCOTT, A.

MAZEIN and F.A. RAYNOR, J. Appl. Phys.

36 29 (1965).

-I

( 5 ) A. LESAGE, ThSse D o c t o r a t d ' E t a t U n i v e r s i t 6 P a r i s XI, 1977

( u n p u b l i s h e d ) . of i n c i d e n t and r e f l e c t e d shock waves i n

neon ( F i g . 2 ) . I n t h e l a t t e r c a s e a f r i n g e ( c y c l i c v a r i a t i o n ) c o r r e s p o n d s t o

Ne = 6.6 x 10 16cm-3