EFFECT OF PULSE SHAPING ON INTERACTION PROCESSES IN LASER PRODUCED PLASMAS

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Submitted on 1 Jan 1979

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EFFECT OF PULSE SHAPING ON INTERACTION PROCESSES IN LASER PRODUCED PLASMAS

François Amiranoff, R. Fabbro, E. Fabre, C. Garban, J. Virmont

To cite this version:

François Amiranoff, R. Fabbro, E. Fabre, C. Garban, J. Virmont. EFFECT OF PULSE SHAPING ON

INTERACTION PROCESSES IN LASER PRODUCED PLASMAS. Journal de Physique Colloques,

1979, 40 (C7), pp.C7-729-C7-730. �10.1051/jphyscol:19797352�. �jpa-00219347�

JOURNAL DE PHYSIQUE CoZZoque C7, suppte'ment au n07, Tome 40, JuiZZet 1979, page C7- 729

EFFECT OF PWSE SHAPING ON INTERACTION PROCESSES IN LASER PRODUCED PLASMAS

F. Amiranoff, R. Fabbro, E. F a b r e , C. G a r b a n and J. Virmont.

Laboratoire de Physique des Milieux Ionise's, EcoZe Potytechnique, 91 128 PaZaiseau Cedex, France.

I n o r d e r t o u n d e r s t a n d t h e e f f e c t o f l o n g s h a p e d p u l s e s which must b e used t o r e a c h t h e a b l a - t i v e c o m p r e s s i o n mode i n l a s e r f u s i o n e x p e r i m e n t s , we have s e t a n e x p e r i m e n t i n which we h a v e s t u d i e d t h e e f f e c t o f t h e p r e p u l s e s h a p e , added t o t h e main p u l s e , on t h e i n t e r a c t i o n p r o c e s s e s , and p a r - t i c u l a r l y on t h e s t i m u l a t e d B r i l l o u i n b a c k s c a t t e - r i n g and on t h e g e n e r a t i o n of h o t e l e c t r o n s .

The e x p e r i m e n t s were made w i t h a Neodymium g i a s s l a s e r which d e l i v e r s p u l s e s o f 100 GV peak power w i t h a d u r a t i o n o f 100 p s FWHM. A p u l s e s t a - k e r p e r m i t s t o g e n e r a t e two t y p e s o f p u l s e s h a p e : t y p e I i s a s i n g l e 1 0 0 p s p u l s e l e a d i n g t h e main p u l s e by 500 p s and c o n t a i n i n g 1 0 L o r 30 :A o f t h e main p u l s e e n e r g y , t h e t y p e I 1 i s a 500 p s p l a - t e a u l i k e p r e p u l s e i n f r o n t o f t h e main p u l s e and c o n t a i n i n g 30 Y6 o f i t s e n e r g y . Beam was f o c u s e d o n f l a t aluminium t a r g e t s a t i n t e n s i t i e s of

2.1015 w/cmZ.

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The major r e s u l t s a r e g i v e n i n T a b l e I f o r b a c k s c a t t e r i n g and o p t i c a l e n e r g y b a l a n c e ; we o b s e r v e t h a t t h e b a c k s c a t t e r i n g i s e f f e c t i v e l y i n c r e a s e d by t h e p r e p u l s e , m o s t l y i n t h e c a s e o f t h e d i s c r e t e p r e p u l s e , b u t i t a l w a y s r e m a i n s l o w e r t h a n 20 5. The a b s o r p t i o n i s s u r p r i s i n g l y c o n s t a n t .

The f i g u r e 1 shows t h e e f f e c t o f p r e p u l s e o n f a s t i o n s and h a r d X r a y s e m i s s i o n s . From t h e s e r e s u l t s and e l e c t r o n t e m p e r a t u r e measurements we h a v e a l s o c o n c l u d e d t h a t : s o f t X r a y e m i s s i o n a n d t h e r m a l i o n p r o d u c t i o n a r e i n c r e a s e d ; h a r d X r a y e m i s s i o n , f a s t i o n s , and T h o t e l e c t r o n tempera- H t u r e a r e d e c r e a s e d , d e p e n d i n g somehow o n t h e p u l s e s t r u c t u r e .

The f i g u r e 2 shows a n example o f t h e s p e c t r a o f t h e s p e c u l a r l y r e f l e c t e d l i g h t i n t h e case o f t h e 2Z05 o b l i q u e l y i n c i d e n c e , o b t a i n e d i n d i f f e - r e n t e x p e r i m e n t a l c o n d i t i o n s . I t a p p e a r s t h a t t h e s p e c t r u m is b l u e s h i f t e d w i t h o u t p r e p u l s e , i s u n s h i f t e d w i t h t h e c o n t i n u o u s p r e p u l s e , r e d s h i f - t e d a n d very b r o a d w i t h t h e d i s c t e p r e p u l s e .

The e x p e r i m e n t a l r e s u l t s h a v e been a n a l y s e d by c o m p a r i s o n w i t h t h e d a t a o b t a i n e d f r o m a ID L a g r a n g i a n n u m e r i c a l s i m u l a t i o n . F i g u r e 3 g i v e s t h e e l e c t r o n d e n s i t y p r o f i l e s f o r t h e t i m e c o r r e s - p o n d i n g t o t h e maximum o f t h e main p u l s e . I t ap- p e a r s t h a t t h e p r e p u l s e i n t r o d u c e s s t r o n g m o d i f i - c a t i o n i n d e n s i t y p r o f i l e . I n t h e two c a s e s o f p r e p u l s e a g e n t l e d e n s i t y g r a d i e n t is g e n e r a t e d i n t h e u n d e r d e n s e p l a s m a a t 0.6 t o 0.8 n

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Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797352

T h i s c o n d i t i o n i s v e r y f a v o r a b l e f o r t h e B r i l l o u i n i n s t a b i l i t y and t h u s c a n e x p l a i n t h e i n c r e a s e d o f t h e b a c k r e f l e c t i o n . I t i s i n t e r e s t i n g t o n o t e t h a t t h e t o t a l l e v e l o f s c a t t e r e d l i g h t i s a l m o s t i n d e - p e n d a n t o f p u l s e s t r u c t u r e . T h i s seems t o i n d i c a t e t h a t when t h e p r e p u l s e i s p r e s e n t t h e s c a t t e r i n g is o n l y more c o l l i m a t e d a s o n e c a n e x p e c t f o r a l e s s inhomogeneous p l a s m a w i t h more g e n t l e e l e c t r o n den- s i t y g r a d i e n t .

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The s e c o n d p o i n t o f i n t e r e s t c o n c e r n t h e r e d u c - t i o n o f t h e e f f e c t o f h o t e l e c t r o n s p r o d u c t i o n a s o b s e r v e d by t h e r e d u c t i o n o f h a r d X r a y a n d f a s t i o n e m i s s i o n a n d t h e d e c r e a s e o f h o t e l e c t r o n t e m p e r a - t u r e . T h i s i s n o t c l e a r l y u n d e r s t o o d b u t however t h e s m a l l e r v a l u e o f t h e h o t e l e c t r o n t e m p e r a t u r e i n d i - c a t e s t h a t p r o b a b l y r e s o n a n c e a b s o r p t i o n i s l e s s e f f e c t i v e b e c a u s e o f t h e l o n g e r d e n s i t y g r a d i e n t . However, t h e s e l a r g e r d e n s i t y g r a d i e n t s w i l l . , i n t u r n f a v o u r c o l l i s i o n n a l a b s o r p t i o n a n d m a i n t a i n o r e v e n i n c r e a s e t h e f r a c t i o n n a l a b s o r p t i o n a s shown o n t a b l e I.

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,Table I : Fraction of backscattered l i g h t : B.S.

Total absorption : Abs.

Pulse shape

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