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Submitted on 1 Jan 1979
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INTERFEROMETRIC MEASUREMENTS OF DENSITY PROFILES IN LASER-TARGET
INTERACTION
R. Benattar, C. Popovics, R. Sigel, J. Virmont
To cite this version:
R. Benattar, C. Popovics, R. Sigel, J. Virmont. INTERFEROMETRIC MEASUREMENTS OF
DENSITY PROFILES IN LASER-TARGET INTERACTION. Journal de Physique Colloques, 1979,
40 (C7), pp.C7-767-C7-768. �10.1051/jphyscol:19797371�. �jpa-00219368�
JOURNAL DE PHYSIQUE CoZZoque C7, suppZdment au e07, Tome 4 0 , JuiZZet 1979, page C7- 767
INTERFEROMETRIC MEASUREMENTS OF DENSITY PROFILES IN LASER-TARGET INTERACTION
R.
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C. ~ o ~ o v i c s " , R. sigelxi and J. virrnontx.XsLaboratoire de Physique d e s f M i Z i e m Ionise's, EcoZe PoZy technique, 91 128 PaZaiseau Ce'dex, France.
On Zeave from Projektgruppe fiir Zaserforschung, ?Qax-PZanck-GeseZZschaft z u r farderung der wissen- schaften E. V . , 0-9046 a r c h i n g b e i Miinchen, R. F. A .
D e n s i t y p r o f i l e s o f t h e p l a s m a c r e a t e d i n t h e w i t h a CO - T E A - l a s e r i n c o m b i n a t i o n w i t h a r u b y 2
i n t e r a c t i o n o f a h i g h p o w e r l a s e r w i t h a s o l i d t a r - d i a g n o s t i c l a s e r ( A = 0.694 o r w i t h a N d - g l a s s - g e t g i v e a b e t t e r k n o w l e d g e o f e n e r g y a b s o r p t i o n l a s e r i n c o m b i n a t i o n w i t h a f r e q u e n c y q u a d r u p l e d a n d t r a n s p o r t m e c h a n i s m s i n l a s e r f u s i o n e x p e r i - beam ( X = 0.265 pm) f o r i n t e r f e r o m e t e r i l l u m i n a t i o n . m e n t s .
F o r t h e f i r s t t i m e i n t h i s t y p e o f e x p e r i m e n t , we u s e d a p o l a r i z e d l i g h t i n t e r f e r o m e t e r b a s e d on a W o l l a s t o n p r i s m w h i c h , b y i t s r e l a t i v e s i m p l i c i t y a n d a b s e n c e of a l i g n e m e n t p r o b l e m s
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i s v e r y c o n v e n i e n t .L e t u s r e c a l l t h e p r i n c i p l e o f t h i s i n t e r f e - r o m e t e r ( F i g . 1 ) . T h e o b j e c t 0 i s d e p i c t e d by a h i g h r e s o l u t i o n l e n s L o n t o t h e f i l m , w i t h a Wol- l a s t o n p r i s m W c r e a t i n g t w o o r t h o g o n a l l y p o l a r i z e d i m a g e s . I n t e r f e r e n c e b e t w e e n t h e s e t w o i m a g e s i s a c h i e v e d by a p a i r o f p o l a r i z e r s P I , P2, e i t h e r p a r a l l e l o r o r t h o g o n a l t o e a c h o t h e r . Two i m a g e s o f e q u a l i n t e n s i t y a n d p o l a r i z a t i o n a r e p r o d u c e d i n t h e i m a g e p l a n e w h i c h h e n c e c a n i n t e r f e r e i n t h e b v e r l a p p i n g r e g i o n .
T a r g e t s w e r e m i c r o s p h e r e s w i t h d i a m e t e r s i n t h e r a n g e o f 4 0 bm t o s e v e r a l h u n d r e d m i c r o m e t e r s .
I n b o t h e x p e r i m e n t s t h e s a m e W o l l a s t o n p r i s m o f 3 0 x 3 0 mm c r o s s s e c t i o n ,
w i t h a s u r f a c e f l a t n e s s o f 1 / 2 0 , p r i s m h a l v e s i n o p t i c a l c o n t a c t
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made o f n a t u r a l q u a r t z , w i t h-
3@ = 5 x 1 0 r a d w a s u s e d . T h e s p a t i a l r e s o l u t i o n o b t a i n e d i s 5 t o 6 pm.
INTERACTION WITH LO2 LASER EXPERIMENT (A = 1 0 . 6 pm) A CO l a s e r ( 4 0 J o u l e i n 4 0 p s ) i l l u m i n a t e s 2
2
1 7 6 pm g l a s s m i c r o s p h e r e .
P i c t u r e 2 i n a n i n t e r f e r o g r a m s h o w i n g p l a s m a e x p a n s i o n 2 0 0 n s a f t e r t h e t o p of t h e CO p u l s e .
2
I n i t i a l s u r f a c e ( s o l i d c i r c l e ) a n d t h e c r i t i c a l e l e c t r o n d e n s i t y N = 1 019 cm-3 ( d a s h e d c i r c l e ) a r e i n d i c a t e d .
T h e t w o i n t e r f e r i n g beams a r e t w o p a r t s o f t h e
p r o b e beam ; t h i s r e q u i r e s a r a t h e r g o o d s p a t i a l T h e d e n s i t y p r o f i l e d e d u c e d b y A b e l i n v e r s i o n c o h e r e n c e i n t h e i l l u m i n a t i o n f i e l d . We h a v e a p p l i e d h a s t w o m a i n f e a t u r e s :
t h i s i n t e r f e r o m e t e r i n t w o d i f f e r e n t l a s e r p l a s m a
e x p e r i m e n t s t o s t u d y t h e c o r o n a e x p a n s i o n o f l a s e r
-
F i r s t we c a n m e a s u r e d e n s i t y a s h i g h a s 1 0 t i m e s h e a t e d m i c r o s p h e r e s . P l a s m a p r o d u c t i o n w a s e i t h e r t h e CO c r i t i c a l d e n s i t y i . e . 1 0cm-3 a n d p r o b e ~ ~2
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797371
t h e p l a s m a i n t h e t r a n s p o r t r e g i o n v e r y n e a r t h e t a r g e t s u r f a c e .
-
M o r e o v e r , t h e d e n s i t y g r a d i e n t s c a l e l e n g t h i s e v a l u a t e d a s 3 7 pm ( b e t w e e n n a n d n,/e) t h a t i s o n l y 4 t i m e s t h e C02 l a s e r w a v e l e n g t h .INTERACTION WITH NEODYMIUM LASER ( A = 1.06 pm)
We s t u d y t h e i n t e r a c t i o n o f a neodymium l a s e r beam ( 5 J o u l e s i n a 1 0 0 p s p u l s e ) w i t h g l a s s m i c r o - s p h e r e s 70 pm d i a m e t e r . T h e i n t e r f e r o g r a m s a n d t h e d e d u c e d d e n s i t y p r o f i l e s ( F i g . 3 ) s h o w t h a t s p h e - r i c a l s y m e t r y i s r a t h e r g o o d , f o r a o n e beam i l l u - m i n a t i o n .
T h e d e n s i t y p r o f i l e o b t a i n e d 80 p s a f t e r t h e t o p o f t h e l a s e r p u l s e h a s b e e n m e a s u r e d i n t h e r a n g e 2 x I 0' < n < 4 x I O 2 0 cm-3
a n d t h e d e n s i t y g r a d i e n t s c a l e l e n g t h i s e s t i m a t e d t o b e 2 0 pm i n t h i s r e g i o n . T h e p r o f i l e i s c o m p a r e d t o a n u m e r i c a l s i m u l a t i o n ( F i g . 3 b ) d o n e w i t h a f l u i d , 2 t e m p e r a t u r e , m o n o d i m e n s i o n a l c o d e i n w h i c h t h e h e a t f l u x e l e c t r o n i s l i m i t e d . As t h e m e c h a n i s m o f l i m i t a t i o n of h e a t f l u x i s n o t known, t h e l i m i t a - t i o n i s d o n e p a r a m e t r i c a l l y w i t h a f a c t o r f d e f i n e d by P = Min
C
XVT, 0.6 x f x n e x v x ke ~ ~ )T h e t h e o r e t i c a l c u r v e ( t = 150 p s , f = 0.05) i s t h e b e s t f i t w h i c h c a n b e f o u n d . C u r v e s t r a c e d f o r f
<
0.05 a l s o f i t w e l l t h e e x p e r i m e n t a l r e s u l t sF i g . 1 - S c h e m a t i c r e p r e s e n t a t i o n o f t h e p o l a r i z e d l i g h t i n t e r f e r o m e t e r .
F i g . 2
-
I n t e r f e r o g r a m o f a g l a s s m i c r o s p h e r e 1 7 6 pm d i a m e t e r i l l u m i n a t e d b y C02 l a s e r ( ' t h e p l a i n c i r c l e i s t h e i n i t i a l s p h e r e a n d t h e d a s h e d l i n e i s t h e c r i t i c a l d e n s i t y ) a n d d e d u c e d d e n s i t y p r o f i l e .s o , we c o n c l u d e t h a t t h e h e a t f l u x i s s t r o n g l y li-
F i g . 3 - 7 0 pm d i a m e t e r g l a s s m i c r o s p h e r e i l l u m i - m i t e d p r o d u c i n g a p l a s m a c o r o n a s t r o n g l y e x p a n d e d . n a t e d b y a Neodymium g l a s s l a s e r .
a - I n t e r f e r o g r a m m e a s u r e d a t t i m e t = 1 6 0 p s , i . e . 8 0 p s a f t e r t h e t o p o f t h e l a s e r p u l s e .
b
