EXPERIMENTAL AND COMPUTATIONAL INVESTIGATION OF PULSED LASER ENERGY TRANSMISSION THROUGH THE ATMOSPHERE

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EXPERIMENTAL AND COMPUTATIONAL

INVESTIGATION OF PULSED LASER ENERGY

TRANSMISSION THROUGH THE ATMOSPHERE

M. Autric, J. Caressa, P. Vigliano, Ph. Bournot, D. Dufresne, G. Inglesakis

To cite this version:

JOURNAL DE PHYSIQUE

CoZloque

C9,

supplbment au n o l l , Tome 41, novembre

1980,

page

C9-129

EXPERIMENTAL AND COMPUTATIONAL INVESTIGATlQN

O F

PULSED LASER ENERGY

TRANSMISSION THROUGH THE ATMOSPHERE

M. A u t r i c , J . P . C a r e s s a , P. V i g l i a n o , Ph. Bournot, D. D u f r e s n e and G. I n g l e s a k i s .

I n s t i t u t e of Fluid Mechanics,

2,

rue Honnorat, 13003 Marseille, France.

A b s t r a c t . - An e x p e r i m e n t a l s t u d y h a s been conducted and compared w i t h a computed n u m e r i c a l model i n o r d e r t o d e t e r m i n e t h e l i m i t a t i o n s i n d u c e d by t h e a i r breakdown o n t h e t r a n s m i s s i o n o f a h i g h e n e r g y l a s e r p u l s e t h r o u g h t h e s t a n d a r d a t m o s p h e r e . The 10.6 um C02 l a s e r d i s p l a y s up t o 300 J i n a 2.5 p s e c p u l s e w i t h a h i g h power peak ( t y p i c a l l y 5 x 1 0 ~ W ; 50 n s F

W H

M) f o l l o w e d by a h i g h e n e r g y t a i l . The l a s e r beam i s f o c u s e d by means o f a 5 X t e l e s c o p e a t 67.5 m ( f / D = 135) o u t s i d e t h e l a b o r a t o r y . En- v e l o p e of t h e beam and e n e r g y d e n s i t y d i s t r i b u t i o n i n t h e f o c a l p l a n e a r e p r e s e n t e d . A e r o s o l concen- t r a t i o n and s i z e d i s t r i b u t i o n measurements a r e o b t a i n e d by means of a p a r t i c l e o p t i c a l c o u n t e r

( K n o l l e n b e r g a x i a l l y s c a t t e r i n g s p e c t r o m e t e r p r o b e : 0.5 um < d < 45 pm). Energy and f l u e n c e t r a n s - m i t t e d t h r o u g h t h e a i r breakdown

-

i n d u c e d plasma f o r g i v e n a t m o s p h e r i c a l c o n d i t i o n s a r e measured a s a f u n c t i o n of t i m e and i n c i d e n t f l u e n c e and t h e n compared w i t h r e s u l t s of a computed n u m e r i c a l hydro- dynamic b i - d i m e n s i o n a l model.

I . INTRODUCTION

Among phenomena which l i m i t t h e propaga- t i o n of a h i g h i n t e n s i t y i a s e r beam through t h e a t m o s p h e r e , a i r breakdown 11-2

(

i s t h e most impor- t a n t f o r t h e s h o r t - d u r a t i o n h i g h power p u l s e s . I n d e e d , l i q u i d and s o l i d a e r o s o l s h a n g i n g i n t h e atmosphere c o n s t i t u t e s m a l l s i z e t a r g e t s c a p a b l e t o i n i t i a t e a i r breakdown. Hydrodynamical p e r t u r - b a t i o n r e s u l t i n g of a i r i o n i z a t i o n makes, t h e n , medium l o c a l l y a b s o r b a n t f o r i n c i d e n t r a d i a t i o n . Then, t h e r e i s an i m p o r t a n t l i m i t a t i o n o f 1 0 , 6 ym t r a n s m i s s i o n t h r o u g h t h e plasma. It i s o f i n t e r e s t f o r p r o p a g a t i o n of h i g h e n e r g y p u l s e d l a s e r t h r o u g h atmosphere t o d e t e r - mine t h e a t t e n u a t i o n of t h i s e n e r g y by t h e breakdown plasma. E x p e r i m e n t a l r e s u l t s of t r a n s - m i t t e d e n e r g y and f l u e n c e a s a f u n c t i o n of t i m e and i n f i d e n t l a s e r f l u e n c e f o r measured meteoro-

L

l o g i c a l c o n d i t i o n s ( p a r t i c l e o p t i c a l . c o u n t e r ) a r e t h e n compared w i t h t h o s e o b t a i n e d by a computed n u m e r i c a l hydrodynamic b i - d i m e n s i o n a l model. The

s i m u l a t i o n method employs two d i f f e r e n t develop- ment models f o r a i r breakdown plasma : one i s c h a r a c t e r i s t i c of a l a s e r - s u p p o r t e d d e t o n a t i o n wave d e s c r i b e d by R a i z e r 1 3 ) ( R a d i a l v e l o c i t y =

0 . 5 V a x i a l ) . The o t h e r i s an e x t r a p o l a t i o n of

D

new p u b l i s h e d r e s u l t s o f Edwards-Fleck J r ) 4 y (Ra- d i a l v e l o c i t y = 0 . 3 8 V a x i a l R a i z e r ) .

D

11. EXPERIMENTAL SET UP

A 1 0 . 6 C02 l a s e r f u r n i s h i n g . u p t o 200 J i n

a 2 . 5 Usec p u l s e is u s e d t o s t u d y a e r o s o l i n d u c e d a i r breakdown phenomena and i t s consequences on t h e h i g h e n e r g y t r a n s p o r t t h r o u g h a t m o s p h e r e . The l a s e r p u l s e i s c o n s t i ' t u t e d w i t h a h i g h power peak

P

( t y p i c a l l y 5 . 1 0 W ; 5 0 n s r i s e t i m e ; 5 0 ns

' 8

iF W H M ) and w i t h a lower powered t a i l ( 1 0 W).

The r a t i o o f e n e r g y between t h e peak and t h e t a i l

is a b o u t 0 . 2 5 . The l a s e r beam is focused o u t s i d e t h e l a b o r a t o r y a t 6 7 . 5 m by means of a "Casse- g r a i n " o p t i c a l s y s t e m . The o p t i c a l system i s

C9-130 JOURNAL DE PHYSIQUE composed by a n on-axis p a r a b o l i c m i r r o r f / 1 . 5

( 0

= 650 mm ; f = 1000 mm) c o u p l e d w i t h a n hyper- b o l i c m i r r o r

( 0

= 120 mm ; f = 200 mm) t h e diame- t e r of t h e l a s e r beam on t h e e x i t p u p i l o f t h e 5X t e l e s c o p i c s y s t e m i s D = 500 mm r e s u l t i n g i n a

Fnumber o f 135. The u n s t a b l e l a s e r c a v i t y produces a n a n n u l a r beam (100 mm e x t e r n a l d i a m e t e r ) w h i c h , when f o c u s e d w i t h t h e a p p r o p r i a t e t e l e s c o p e ( f = 67.5 m ; f / D = 1 3 5 ) , g i v e s a non-uniform i n t e n s i t y d i s t r i b u t i o n . T h i s l a s e r r e s o n a t o r h a s b e e n chosen f o r i t s h i g h F r e s n e l number (N = 8 8 ) . T h a t a l l o w s t o c o n s i d e r a n e a r l y d i f f r a c t i o n l i m i t e d beam ( 2 . 3 X d i f f r a c t i o n

-

l i m i t e d beam). A

article

o p t i c a l c o u n t e r i s u s e d t o know t h e a e r o s o l c o n c e n t r a t i o n and s i z e d i s t r i b u t i o n i n t h e i n t e r a c t i o n zone d u r i n g e x p e r i m e n t s . Measurements a r e made o f t h e t r a n s m i t t e d l a s e r f l u e n c e by means of germanium photon-drag d e t e c t o r on which t h e r a d i a t i o n i s r e f o c u s e d . A scheme o f t h e experimen- t a l s e t u p used f o r a i r breakdown e x p e r i m e n t s and e n e r g y b a l a n c e i s shown i n F i g . 1.

pDe--"-

F i g . 1 E x p e r i m e n t a l a p p a r a t u s

L1 Lg NaCl windows ; PD Photon d r a g d e t e c t o r ; C C a l o r i m e t e r ; PM P h o t o m u l t i p l i e r ; C' Camdra ; A A t t e n u a t o r s ; Re Ne L a s e r p r o b e 6328

1

; CK K n o l l e n b e r g s e n s o r ; Z Break- down r e g i o n ; M M M M P l a n e m i r r o r s ; M g Hyiperbolic m i r r b r 2 ; 5 ~ < ~ P a r a b b l i c m i r r o r . I n o r d e r t o measure and c a l c u l a t e t h e t r a n s - m i t t e d f l u e n c e t h r o u g h t h e plasma, i t i s n e c e s s a r y 1 ) t h e c r o s s - s e c t i o n o f t h e beam a l o n g t h e a x i s ( e n v e l o p e and energy d e n s i t y v a r i a t i o n s ) . 2 ) energy d e n s i t y and i n t e n s i t y d i s t r i b u t i o n s i n t h e b e s t f o c u s p l a n e . 3) t h e a e r o s o l c o n c e n t r a t i o n s and s i z e d i s t r i b u - t i o n s i n t h e f o c a l volume d u r i n g e x p e r i m e n t s . 4 ) t h e t i m e e v o l u t i o n o f t h e i n c i d e n t and t r a n s - m i t t e d l a s e r power.

The beam e n v e l o p e i s o b t a i n e d by measuring b u r n t s p o t s d i a m e t e r on p i e c e of p h o t o g r a p h i c p a p e r

l o c a t e d i n d i f f e r e n t p o s i t i o n s a l o n g t h e d i r e c t i o n o f p r o p a g a t i o n . The f i g u r e 2 g i v e s t h e beam impact when t h e l a s e r beam i s f o c u s e d a t 67.5 m .

F i g . 2 I s o - i n t e n s i t y c o u n t o u r i n t h e f o c a l r e g i o n . f / d = 1 3 5 . I t shows t h e two m a r g i n a l focused

t e m p o r a l power e v o l u t i o n of t h e l a s e r p u l s e W ( t ) = a t , i t i s p o s s i b l e t o d e t e r m i n e peak f l u e n c e F and maximal i n t e n s i t y $p r e a c h e d a t t

_P

=

50 n s . The dashed zone c o r r e s p o n d s t o a v e r a g e d peak f l u e n - c e and i n t e n s i t y v a l u e s

Fp

and

qp

( t = 50 n s ) on each d i f f r a f t i o n r i n g 15

1 .

F i g . 3 I n t e n s i t y and e n e r g y d e n s i t y s p a t i a l d i s t r i - b u t i o n i n t h e b e s t f o c u s p l a n e a t F/D=135. = t o t a l f l u e n c e ( t = 2 . 5 u s ) $,$, = peak f l u e n c e an4 i n t e n s i t y ( t = 5 O c s )

a

Average peak f l u e n c e Fp and i n t e n s i t y

qp

a t t = 5 0 n s . P r e v i o u s l y e x p e r i m e n t s have shown t h a t t h e a i r breakdown p r o c e s s i s i n i t i a t e d by t h e p r e s e n c e o f m i c r o s c o p i c p a r t i c l e s suspended i n a i r ( a e r o - s o l s ) 16-71

.

A e r o s o l a b s o r b s i n c i d e n t l a s e r e n e r - gy and i n c r e a s e s i t s t e m p e r a t u r e , ( h e a t i n g ) , and c a n r e a c h t h e v a p o r i z a t i o n t e m p e r a t u r e ( v a p o r i z a t i o n p r o c e s s ) . Vapor formed by t h e e v a p o r a t i n g p a r t i c l e a b s o r b s l a s e r energy and h e a t s ( h e a t i n g of t h e v a p o r c l o u d ) . Energy i s t r a n s f e r r e d from t h e h o t v a p o r c l o u d t o t h e s u r r o u n d i n g a i r by c o n d u c t i o n and r a d i a t i v e t r a n s p o r t . Then, t h e r e a r e a i r i o n i - z a t i o n and a i r plasma f o r m a t i o n (supposed a t l o c a l

thermodynamic e q u i l i b r i u m ) .

~ 9 - 1 3 2 JOURNAL DE PHYSIQUE

F i g . 5 shows t h e t i m e h i s t o r y o f t h e i n c i d e n t and t r a n s m i t t e d e n e r g i e s and f l u e n c e s and t h e i r

- -

r a t i o [ E ~ / E ~ and FT/FI ( d a s h e d 1 i n e s ) J i n t h e s p i k e o f t h e l a s e r p u l s e ( 5 0 n s )

.

F i g . 5 I n c i d e n t and t r a n s m i t t e d l a s e r e n e r g y and f l u e n c e v e r s u s time i n t h e s p i k e o f t h e l a - s e r p u l s e ( t = 0

-

200 n s ) . Taking i n t o a c c o u n t t h e s p a t i a l d i s t r i b u t i o n o f t h e i n t e n s i t y and f l u e n c e and t h e r i n g s t r u c - t u r e i n t h e b e s t f o c u s s p o t , breakdown o c c u r s , f i r s t i n t h e c e n t r a l l o b e o f t h e d i f f r a c t i o n p a t - % t e r n ( t

-

22 n s ) and b e c a u s e o f t h e i m p o r t a n t i n c r e a s e o f l a s e r i n t e n s i t y Q ( t ) t i l l t = 5 0 n s , breakdown may t h e n o c c u r on t h e 2nd r i n g ( t = 3 3 n s ) 3 r d r i n g ( t = 3 9 n s ) and 4 t h r i n g ( t S 4 5 n s ) when

F

> 3-4 ~ [ c m ' .

fl

r e p r e s e n t ( ~ i g . 5 ) t h e s e s u c c e s -

I

s i v e i n s t a n t s . T h i s e x p l a i n s , p a r t l y , t h e r a p i d d e c r e a s e o f t h e r a t i o of t r a n s m i s s i o n . When t > 200 n s , t h e r a t i o remains p e r c e p t l y c o n s t a n t i n t h e t a i l o f t h e l a s e r p u l s e (dashed l i n e F i g . 4 ) . A b s o r p t i o n r e s u l t s o f a x i a l and r a d i a l plasma e x p a n s i o n . I n o r d e r t o o b t a i n t h e t r a n s m i t t e d e n e r g y and f l u e n c e t h r o u g h t h e breakdown zone, t h e e n e r g y b a l a n c e ' i s e v a l u a t e d ' b y t e m p o r a l i n t e g r a t i o n of t h e photon-drag r e c o r d s o f t h e i n c i d e n t and t r a n s - m i t t e d l a s e r power. The e x p e r i m e n t a l r e s u l t s ( f o r m e t e o r o l o g i c a l c o n d i t i o n s 4'C < T < 7'C, 7 7 % < RH < 9 0 % ) l e a d i n g t o where t i s t h e p u l s e l e n g t h , a r e summarized i n P F i g . 6 . F i g . 6 Energy b a l a n c e v e r s u s i n c i d e n t e n e r g y and f l u e n c e f/D= 135. + t o t a l t r a n s m i t t e d e n e r g y and f l u e n c e

---

e s t i m a t e d r a t i o i n c a s e breakdown o c c u r s on t h e whole s p o t area:

Counter model ASSP 100) and a p u l s e - h e i g h t a n a l y - s e r (PDS), c a p a b l e of s i z i n g p a r t i c l e s w i t h a use- f u l r a n g e of 0 . 5 pm t o 45 pm d i a m e t e r ( f o u r s i z e r a n g e s ) . F i g . 7 shows e n v i r o n m e n t a l a e r o s o l d i s - t r i b u t i o n f o r t h e f o l l o w i n g m e t e o r o l o g i c a l condi- t i o n s : 4°C 9 0 % ; 7 ° C 7 7 % .

nr'"

F i g . 7 A e r o s o l d i s t r i b u t i o n measurements. --A- t e m p e r a t u r e T=h°C, r e l a t i v e h u m i d i t y ' R H = ~ o % , s p e c i f i c h u m i d i t y ( g r w a t e r v a p o r / m3 d r y a i r ) SH=4.6, s a t u r a t i o n v a l u e ( g r / m 3 ) ~ ~ = 5 . 1 . + ~ = 7 0 ~ , ~ ~ = 7 7 % , ~ ~ = 5 , ~ ~ = 6 . 3 : T h e s e r e s u l t s a r e p r e s e n t e d i n comparison w i t h c h a r a c t e r i s t i c d i s t r i b u t i o n s o f l i q u i d and s o l i d a e r o s o l s f o r d i f f e r e n t g e o g r a p h i c a l s i t u a t i o n s ( c o n t i n e n t a l , normal a e r o s o l , o c e a n , f o g , smog, e t c . . . ) 181. At t h e same t i m e a s e x p e r i m e n t a t i o n , a nume- r i c a l b i - d i m e n s i o n a l method s i m u l a t i n g p u l s e d l a - s e r beam p r o p a g a t i o n t h r o u g h s t a n d a r d a t m o s p h e r e

a s b e e n worked o u t . Such method a l l o w s , f o r v a r i o u s l a s e r e n e r g y and f l u e n c e v a l u e s , v a r i o u s breakdown t h r e s h o l d i n t e n s i t y v a l u e s , v a r i o u s a t m o s p h e r i c a l a e r o s o l c o n c e n t r a t i o n s a n d v a r i o u s plasma develop- m e n t s , t o compute p e r c e n t a g e of e n e r g y t r a n s m i s s i o n on b o t h s i d e s of t h e i n t e r a c t i o n a r e a . The method r e l i e s on t h e f o l l o w i n g e x p e r i m e n t a l d a t a : i n c i - d e n t l a s e r p u l s e (photon-drag d e t e c t o r r e c o r d s ) , l a s e r beam e n v e l o p e a l o n g o p t i c a l a x i s ( p h o t o g r a p h i c p a p e r ) , e n e r g y d e n s i t y d i s t r i b u t i o n i n t h e p l a n e o f b e s t f o c u s ( c a l i b r a t e d K a l v a r f i l m ) , a v e r a g e v a l u e s o f i n t e n s i t y and f l u e n c e . t h r e s h o l d s n e c e s s a r y t o a i r breakdown, c o n c e n t r a t i o n and s i z e d i s t r i b u t i o n of a e r o s o l h a n g i n g on t h e a i r ( a x i a l l y s c a t t e r i n g s p e c t r o m e t e r p r o b e ) . The breakdown zone i s d i v i d e d i n t o e l e m e n t a r y c y l i n d e r s , t h e s e c t i o n of which b e i n g a l s o s p l i t up i n t o c o n c e n t r i c r i n g s ( F i g . P ) . B R E I K D O W H ZONE L 2 3 c.

1

F i g . 8 E l e m e n t a r y a r e a c o n s t r u c t i o n of t h e f o c a l zone u s e d i n t h e s i m u l a t i o n method. A e r o s o l p a r t i c l e s a r e d i s t r i b u t e d u n i f o r m l y i n each e l e m e n t a r y c y l i n d e r s . I n c i d e n t l a s e r p u l s e i s d i s - t r i b u t e d i n each r i n g a c c o r d i n g t o e x p e r i m e n t a l measurements ( K a l v a r f i l m ) . It p r o p a g a t e s a l o n g o p t i c a l a x i s i n s i d e breakdown a r e a from one c y l i n - d e r t o t h e f o l l o w i n g and may i n i t i a t e a i r b r e a k - down ( a f t e r 4-6 pm-diameter a e r o s o l v a p o r i z a t i o n and i o n i z a t i o n o f t h e s u r r o u n d i n g a i r ) w h e n 8 2 8 2 1 . 1 0 W/cm <

qS

< 2 . 1 0 W/cm and 2 3 .3/crn2 <

is

<, 5 J / c m w i t h

qS

a v e r a g e breakdown t h r e s h o l d i n t e n s i t y and

FS

a v e r a g e breakdown t h r e s - h o l d f l u e n c e .

C9- 134 JOURNAL DE PHYSIQUE p r o c e s s : -KR W,(t) = W I ( t )

.

e ( 1 ) w i t h W ( t ) , W ( t ) i n c i d e n t and t r a n s m i t t e d l a s e r I T power (w/cmL),

R

plasma a b s o r b i n g l e n g t h (cm), K a b s o r p t i o n c o e f f i c i e n t (cm-'1. w i t h c s p e e d o f l i g h t e e l e c t r o n c h a r g e r a d i a l l y : V ( t ) = 0 . 7 5 V ( t ) a x i a l ( 7 ) w i t h y = a d i a b a t i c c o n s t a n t = 1 . 2

-

3 po = a i r v o l u m e t r i c mass = 1 . 3 1 0 gr/cm 3 @ = i n c i d e n t l a s e r i n t e n s i t y .

F i g . 9 shows t h e a s p e c t o f t h e breakdown plasma r e s t i l t i n g of t h e s e two d i f f e r e n t development models i n t h e t i m e . i X r ( t ) I g G a u n t ' s f a c t o r t I ...-v-- ..- -..T: -..-. . .- h P l a n c k ' s c o n s t a n t

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t o w r i t e a b s o r p t i o n c o e f f i c i e n t KV a s a f u n c t i o n o f l a s e r i n t e n s i t y )I 7 419

The plasma expands b o t h l o n g i t u d i n a l l y and t r a n s v e r s e l y i n t o t h e l a s e r beam ; t h e s i m u l a t i o n method employs two d i f f e r e n t development models :

one i s c h a r a c t e r i s t i c o f a one-dimensional l a s e r s u p p o r t e d d e t o n a t i o n wave (LSD) d e s c r i b e d by R a i z e r 13

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a x i a l l y v D ( t ) = 12 ( Y - 1 )

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( 4 ) r a d i a l l y v R ( t ) = 0 . 5 v D ( t ) ( 5 ) The o t h e r i s an e x t r a p o l a t i o n o f new r e s u l t s o f Edwards-Fleck J r 141 where t h e dynamics o f a i r breakdown i s d e s c r i b e d u s i n g a t h r e e - t e m p e r a t u r e non e q u i l i b r i u m thermodynamic model o f - t h e a i r plasma and two-dimensional Lagrangaan hydrodyna-

a x i a l l y : V ( t ) = 0 . 5 V ( t ) R a i z e r D ( 6 )

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t h e breakdown a r e a w i t h 0 . 5 p a r t i c l e / c m 3 and 140 J . ( F i g . 4 and F i g . 5 ) .

F i g . 1 0 T r a n s m i s s i o n r a t i o v e r s u s i n c i d e n t e n e r g y ( f l u e n c e ) . Numerical and e x p e r i m e n t a l compa- r i s o n . $ = 1 . 1 0 ~ w/cm2. F l e c k development t y p e . 0 ,

',

, ,

3, show r e s p e c t i v e l y NZ0,1 0 . 5 , 1, 5 , 25 p/cm

.

e x p e r i m e n t a l r e s u l t s f o r measured m e t e o r o l o g i c a l c o n d i t i o n s ( f i g . 7 ) Numerical r e s u l t s show

-

a s l i g h t i n f l u e n c e o f t h e plasma development t y p e ( R a i z e r o r Edwards

-

F l e c k J r ) on t h e t r a n s m i s s i o n r a t i o ( a b o u t 3 % ) ,

-

a s l i g h t i n f l u e n c e of t h e t h r e s h o l d i n t e n s i t y and

e

2 f l u e n c e v a l u e s u s e d i n t h e model (1-2.5 1 0 W/cm ) ( a b o u t 2 %) ( ~ i ~ . l l ) .

-

E,.140 J F i g . 1 1 T r a n s m i s s i o n r a t i o v e r s u s t h r e s h o l d i n t e n s i - t y v a l u e s f o r E,=140 J and v a r i o y s a e r o s o l c o n c e n t r a t i o n ( 0 . 1 up t o 25 p/cm-') (Edwards- F l e c k J r development t y p e ) . I n f i r s t a p p r o x i m a t i o n , t h i s model permits t o e s t i m a t e , f o r a g i v e n f o c a l zone, t h e t r a n s m i t t e d e n e r g y ( f l u e n c e ) a s a f u n c t i o n o f t h e i n c i d e n t e n e r g y ( f l u e n c e ) f o r v a r i o u s a e r o s o l c o n c e n t r a t i o n . The F i g . 1 2 shows n u m e r i c a l r e s u l t s o b t a i n e d f o r 2 f/D=135, i n c i d e n t e n e r g y 0-1000 J (0-330 J/cm )and

F i g . 1 2 Computed t r a n s m i s s i o n r a t i o f/D=135,E =O.lKJ a e r o s o l c o n c e n t r a t i o n 0.1-10 p/cm3. I CONCLUSION

The n u m e r i c a l method h a s shown t h a t o n l y t h e p u l s e s h a p e and t h e a e r o s o l c o n c e n t r a t i o n l i a b l e t o i n d u c e a i r breakdown a r e i m p o r t a n t . Experiments now i n p r o g r e s s i n t h e l a b o r a t o r y c o n c e r n f i s t l y , t h e m o d i f i c a t i o n o f t h e l a s e r p u l s e by d e c r e a s i n g on s u p p r e s s i n g t h e peak i n t e n s i t y o f t h e s p i k e (which i n i t i a t e s t h e breakdown) s e c o n d l y , t h e m o d i f i c a t i o n of t h e a e r o s o l r e p a r t i t i o n by c l e a n i n g t h e l a s e r beam p a t h by means o f a p r e c u r - s o r p u l s e ( 1 K J - 1 0 y s e c ) . ACKEOWLEDGKENTS

The a u t h o r s would l i k e t o t h a n k J . P . FUGASSI and M. SARAZIN f o r t h e i r a s s i s t a n c e t h r o u g h o u t t h e c o u r s e o f t h i s work. T h i s s t u d y i s s p o n s o r e d by D i r e c t i o n d e s Recherches e t Etudes T e c h n i q u e s . REFERENCES

( 1 ) J.P.CARESSA,M. AUTRI-C, D . DUFRESNE,Ph. BOURNOT J . A p p l . % y s . 50 ( l l ) , p p . 6 8 2 2 - 6 8 2 5 ( 1 9 7 9 ) .

( 2 ) M . AUTRIC, J . P . CARESSA, D . DUFRESNE,Ph.BOURNOT

C.R.Acad.Sc. P a r i s , t . 2 8 8 , S 6 r i e B,pp.237-240 ( 1 9 7 9 ) .

( 3 ) Yu.P. RAIZER, Sov.Phys. J.E.T.P. , 2 l , p p . l 0 0 9 -

1017 ( 1 9 6 5 ) .

( 4 ) A.L. EDWARDS, J . A . FLECK J r . , J . Appl.Phys., 50 ( 6 ) , pp.4307-4313 ( 1 9 7 9 ) .

( 5 ) M . AUTRIC, J . P . CARESSA, Ph. BOURNOT, D . DUFRESNE, M . SARAZIN, A.I.A.A. P a p e r n o 8 0 1379,

1 3 t h F l u i d and Plasmadynamics Conf. ( 1 9 8 0 ) . ( 6 ) D.E. LENCIONI, A p p l . P h y s . L e t t . 2 3 , 1 2 ( 1 9 7 3 ) . ( 7 ) D . C . SMITH, R.T. BROWN, J . A p p l . P h y s . , 4 6 , p . 1 1 4 6 ,

( 1 9 7 5 ) .

( 8 ) J . P . REILLY, M . DELICHATSIOS, S.L. GLICKLER,

D . KORFF, P . I . SINGH, G . M . VEYL, I n t e r i m R e p o r t N o 1 7 3 76C 0059, ( 1 9 7 6 ) .