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Submitted on 1 Jan 1979
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PLASMA TARGET COUPLING IN THE CASE OF TEA-CO2 LASER PRODUCED BREAKDOWN IN
FRONT OF A SOLID TARGET
I. Apostol, E. Cojocaru, M. Dinescu, V. Drǎgǎnescu, Th. Julea, I. Morjan, M.
Moldovan, I. Mihǎilescu, V. Konov
To cite this version:
I. Apostol, E. Cojocaru, M. Dinescu, V. Drǎgǎnescu, Th. Julea, et al.. PLASMA TARGET COUPLING IN THE CASE OF TEA-CO2 LASER PRODUCED BREAKDOWN IN FRONT OF A SOLID TARGET. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-765-C7-766.
�10.1051/jphyscol:19797370�. �jpa-00219367�
JOURNAL DE PHYSIQUE CoZZoque C7, suppZBment au n07, Tome 4 0 , JuiZZet 1979, page C7- 765
PLASMA TARGET COUPLING IN THE CASE OF TEA-COZ LASER PRODUCED BREAKDOWN IN FRONT ff A SOLID TARGET
I. Apostol, E. Cojocaru, M. Dinescu, V. ~rtig&nescu, Th. Julea, I. Morjan, M. Moldovan, I.N. Mihgilescu and V.I. ~onov'.
*Lasers Department, I.F.T.A.R., Central I n s t i t u t e of Physics, Bucharest, Romania.
Physical I n s t i t u t e "P.N. Lebedev", Moscow, U.S.S.R.
The e x p e r i m e n t a l r e s u l t s of h i g h l y a b s o r b a n t ( m e t a l l i c ) t a r g e t s i r r a d i a t i o n by a f o c u s e d TEA-CO, l a s e r i n a i r a t a t - mospheric p r e s s u r e , f o r i n c i d e n t power i n - t e n s i t i e s , I , o f
( l o 6 +
10') w/cm2, a r e r e - p o r t e d . P a r t i c u l a r l y , t h e c o u p l i n g c o e f f i - c i e n t between t h e breakdown plasma and t a r g e t , d e f i n e d a s t h e r a t i o of t h e t h e r - mal e n e r g y t r a n s f e r r e d t o t h e t a r g e t t o t h e l a s e r energy, was i n v e s t i g a t e d a s a f u n c t i o n of i n c i d e n t i n t e n s i t y ( l a s e r f l u - e n c e ) , t h e l a s e r s p o t s i z e and t h e geome- t r y of t h e e x p e r i m e n t .A s it i s known [1,2] when i n c i d e n t l a s e r i n t e n s i t y on t a r g e t e x c e e d s n o t o n l y t h e breakdown t h r e s h o l d i n t e n s i t y , b u t e q u a l l y a n o t h e r c h a r a c t e r i s t i c v a l u e
-
t h el a s e r t h r e s h o l d i n t e n s i t y f o r i n d u c i n g a d e t o n a t i o n wave
-
t h e breakdown plasma de- t a c h e s from t h e t a r g e t and t h e i o n i s a t i o n f r o n t p r o p a g a t e s i n a d i r e c t i o n o p p o s i t e t o t h a t o f t h e l a s e r beam. I n t h i s c a s e , t h e plasma dimension a l o n g t h e l a s e r beam e x c e e d s c o n s i d e r a b l y i t s t r a n s v e r s e s i z e . On t h e o t h e r hand, a t lower i n t e n s i t i e s t h e r e i s no detachement from t h e t a r g e t and t h e plasma expands s p h e r i c a l l y i n t o t h e s u r r o u n d i n g g a s . I n [3] t h e momentum p e r u n i t e n e r g y , p/E, a c t i n g backward on t h e t a r g e t was d e t e r m i n e d from t h e d e f l e c - t i o n o f a b a l l i s t i c pendulum. Maximum va- l u e s o f ( 6 + 7 ) and ( 1 3 t 1 4 ) dyn. s/J were o b t a i n e d c o r r e s p o n d i n g t o c y l i n d r i c a l and s p h e r i c a l plasma e x p a n s i o n s , r e s p e c t i v e l y . Here E was t a k e n t o be t h e l a s e r e n e r g y s t o r e d i n t o t h e plasma s o u r c e , Eo(l-R), where Eo i s t h e i n c i d e n t l a s e r e n e r g y and R i s t h e t i m e i n t e g r a t e d l o s s c o e f f i c i e n t . On t h e o t h e r hand, t h e o r e t i c a l e s t i m a t i o n s performed i n t h e same work p r e d i c t e d v a l u e s o f 7.5 and 32 dyn.s/J f o r t h e two c a s e s .A p o s s i b l e r e a s o n o f such a d i s a g r e e m e n t c o u l d b e t h e amount o f e n e r g y t r a n s f e r r e d , d u r i n g and a f t e r t h e l a s e r p u l s e , from plasma t o t h e t a r g e t , which a c c o r d i n g l y a l - t e r s t h e denominator o f t h e r a t i o p/E. W e a l s o remark t h e i m p o r t a n t s i g n i f i c a n c e o f t h i s q u a n t i t y f o r t h e e n e r g y b a l a n c e o f t h e p r o c e s s . Such a t r a n s f e r t a k e s p l a c e
t h r o u g h two d i f f e r e n t c h a n n e l s :
( a ) Absorbing t h e plasma e m i t t e d l i g h t . A s t h e m e t a l s e x h i b i t h i g h r e f l e c t i - v i t y f o r l o n g e r wavelengths t h i s mechanism i s e f f i c i e n t o n l y f o r t i m e s comparable w i t h t h e p u l s e d u r a t i o n when t h e plasma i s h o t enough t o p r e s e n t an e m i t t e d s p e c t r u m w i t h an uv maximum. A s t h e p a t h o f , t h e uv r a d i a - t i o n i n a i r i s o f t h e o r d e r o f l mm t h i s a b s o r p t i o n i n d u c e s o n l y a s m a l l supplemen- t a r y e x p a n s i o n o f plasma and a l l l a s e r en- e r g y goes i n t o a s p o t a l m o s t e q u a l t o t h a t o f t h e l a s e r beam.
( b ) Thermal c o u p l i n g between plasma and t a r g e t . I t i s e a s y t o prove t h a t t h i s p r o c e s s i s d e v e l o p p i n g s e v e r a l o r d e r s o f magnitude more i n t e n s e l y t o t h e t a r g e t d i - r e c t i o n t h a n i n t h e o p p o s i t e one.
T h i s work i s , i n t e n d e d t o b r i n g a deep- er i n s i g h t i n t o t h e t r a n s f e r p r o c e s s mainly r e g a r d i n g t h e c o n t r i b u t i o n o f t h e two a f o r e m e n t i o n e d c o m p e t i t i v e mechanisms i n t h e c a s e o f 10.6 pm l a s e r r a d i a t i o n . To t h i s aim e x p e r i m e n t s were performed u s i n g a d o u b l e d i s c h a r g e TEA-CO, l a s e r which was a b l e t o d e l i v e r o u t p u t e n e r g i e s o f s e v e r a l J i n p u l s e s o f a t y p i c a l shape. I n view o f o b t a i n i n g o n l y monomode l a s e r e m i s s i o n a v a r i a b l e a p e r t u r e diaphragm was i n t r o d u c e d i n s i d e t h e r e s o n a n t c a v i t y . Beam s p l i t t e r s , a p y r o e l e c t r i c d e t e c t o r and a c a l o r i m e t e r w e r e used t o m o n i t o r t h e p u l s e s h a p e and e n e r g y on e a c h s h o t , w h i l e plasma plumes
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797370
e v o l u t i o n w a s r e c o r d e d u s i n a a h i q h s p e e d SFR p h o t o g r a p h i c camera. The t a r q e t s w e r e s m a l l p i e c e s o f c o p p e r , of d i f f e r e n t s i z e s , rear-?3;hermocoupled, yrhich w e r e c a r e f u l l y c l e a n e d a f t e r e v e r y s h o t . F p l o t o f t h e c o u p l i n g c o e f f i c i e n t i s shown i n F i g . 1 .
F i g . 1 The c o u p l i n g c o e f f i c i e n t , A , a s a f u n c t i o n o f l a s e r f l u e n c e , E/S, and i n t e n - s i t y , I. (a) t h e f o c u s was on and a f t e r the t a r g e t w i t h l a s e r s p o t s : *
-
0.8.cm2; o
-
1 . 3 . 1 0 - ~ c m ~ ; A-
4 . 1 ~ 1 0 - ~ c m ~ ;n -
6 . 2 .
lo-'
cm2 ; x-
9 . 6 - 1 0 - ~ c m ~ , r e s p e c t i v e - l y . ( b ) t h e l a s e r s p o t on t a r g e t i n b o t h c a s e s o f 3 - 1 0 - ~ c m ~ b u t t h e f o c a l p l a n e be- f o r e e , and a f t e r-
0 , t h e t a r g e t .A s may b e o b s e r v e d beyond t h e b r e a k - down t h r e s h o l d a s i g n i f i c a n t enhancement o f t h e c o u p l i n g c o e f f i c i e n t i s o b t a i n e d f o l l o w e d by a f u r t h e r s l o w d e c r e a s e . The v a l u e and t h e p o s i t i o n o f t h i s maximum de- p e n d s on t h e s p o t a r e a . The l a r g e s t r e q i s - t e r e d v a l u e o f A was o f 0.6 c o r r e s p o n d i n g t o a l a s e r f l u e n c e o f 10 ~ / c m ~ and a l a - s e r s p o t of 0 . 8 - 1 0 - ~ c m ~ . The d e c r e a s e o f t h e c o u p l i n g c o e f f i c i e n t a s t h e l a s e r f l u - e n c e s u r D a s s e s t h e breakdown t h r e s h o l d i s a r e s u l t of r e l e a s i n q a l a s e r a b s o r p t i o n wave and d e v e l o p s more s l o w l y f o r l a r q e r
l a s e r s p o t s o r t a r g e t .
Such a b e h a v i o u r c l o s e r a g r e e s w i t h t h e f i r s t mechanism. Thus, when t h e d e t o - n a t i o n wave t h r e s h o l d i h t e n s i t y is s u r - p a s s e d t h e p l a s m a d e t a c h e m e n t from t h e t a r g e t d i s p l a c e s t h e uv e m i t t i n g r e g i o n
f u r t h e r and f u r t h e r t o t h e l a s e r beam d i - r e c t i o n . A l a r g e r p a r t o f t h e e m i t t e d r a - d i a t i o n i s t h e n a b s o r b e d by the c o l d e r nlasma zones and o n l y a s m a l l amount i s r e a c h i n g t h e t a r q e t ; t h e c o u p l i n g c o e f f i - c i e n t i s a c c o r d i n q l y f a l l i n g down. The r e - d u c t i o n i s s l o w e r f o r l a r g e r l a s e r s p o t b e c a u s e o f an i n c r e a s e d c o n t r i b u t i o n i n t h i s ' c a s e o f t h e s e c o n d mechanism r e s u l t - i n g from a r a i s e d c o n t a c t s u r f a c e between plasma and t a r q e t .
W e a l s o o u t i n t o e v i d e n c e t h a t f o r a same l a s e r s p o t t h e c o u p l i n g c o e f f i c i e n t i s h i q h e r when t h e f o c u s was s i t u a t e d $f- t e r t h e t a r g e t . Such a phenomenon may b e a l s o e x p l a i n e d by t h e dominance o f t h e r a - d i a t i v e mechanism. T h u s , when t h e f o c a l p l a n e i s p l a c e d b e f o r e t h e t a r g e t t h e h o t - e s t p a r t o f t h e plasma i s formed a t a c e r - t a i n d i s t a n c e i n f r o n t o f t h e t a r g e t and a g r e a t p a r t o f t h e e m i t t e d uv r a d i a t i o n i s a b s o r b e d on i t s p a t h t o t h e t a r g e t .
I n F i a . 2 t h e l a s e r i n t e n s i t i e s c o r - r e s p o n d i n g t o maximum v a l u e s o f A w e r e p l o t t e d a g a i n s t t h e l a s e r s p o t a r e a .
E l S 1 3 l c r n 2 ) 1 ( x l 0 6 W / c r n 2 ) 15
F i g . 2 I n c i d e n t l a s e r i n t e n s i t y c o r r e s - p o n d i n g t o maximum v a l u e s o f t h e c o u p l i n g c o e f f i c i e n t a s a f u n c t i o n of t h e l a s e r s p o t on t h e t a r g e t .
A v e r y s m a l l v a r i a t i o n may b e o b s e r v - e d which is c o n f i r m i n g t h a t i n t h e r a n g e o f i n c i d e n t l a s e r i n t e n s i t y we s t u d i e d b o t h mechanisms have t o b e c o n s i d e r e d b u t t h e most i m p o r t a n t r o l e i s t o b e due t o t h e e n e r g y t r a n s f e r t h r o u g h u v r a d i a t i o n e m i t t e d b y t h e plasma.
BIBLIOGRAPHY
[ I ] I. A p o s t o l e t a l l P r o c . ~ 1I.C.P. 1 ~ ~ I.G. Eindhoven, p . 337 ( 1 9 7 5 )
[2] D. A p o s t o l e t a l , P r o c . ~ 1 1 1 t h I.C.P.
I . G . B e r l i n , p . 1108 (1977)
[ 3 ] V.P. Aqeev e t a l l I z v . Vuz. F i z . 11,
34 ( 1 9 7 7 )
