LASER INTERACTION : THERMAL AND MECHANICAL COUPLING TO TARGETS

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LASER INTERACTION : THERMAL AND

MECHANICAL COUPLING TO TARGETS

R. Root

To cite this version:

JOURNAL DE PHYSIQUE CoZZoque C9, supptdment

au

n022, Tome 41, novernbre 2980,

page

~ 9 - 5 9

LASER I N T E R A C T I O N

:

THERMAL AND MECHANICAL COUPLING TO TARGETS

R.G. Root

PhysicaZ

Sciences Inc. Wobm, MA 01801 U.S.A.

A b s t r a c t . - The p h y s i c a l phenomena which i n f l u e n c e s thermal and mechanical coupling of i n f r a r e d l a s e r

--

r a d i a t i o n t o m a t e r i a l s a r e reviewed. Both pulsed and CW i n t e r a c t i o n s a r e c o n s i d e r e d , b u t t h e i n t e r a c -

t i o n of pulsed l a s e r s w i t h m e t a l s i n an a i r environment i s emphasized. S e l e c t e d examples of vacuum in-

t e r a c t i o n s and coupling t o non-metals a r e a l s o i n c l u d e d .

1. INTRODUCTION

When a p u l s e d l a s e r beam i r r a d i a t e s a s u r f a c e , t h e f r a c t i o n of t h e i n c i d e n t energy coupled l o c a l l y i n t o t h e t a r g e t and t h e impulse imparted t o t h e t a r g e t vary s t r o n g l y according t o which p h y s i c a l phenomena dominate t h e i n t e r a c t i o n s with t h e t a r - g e t . F o r example, a t low l a s e r i n t e n s i t i e s t h e thermal energy d e p o s i t e d i n a m e t a l s u r f a c e i s con- t r o l l e d by t h e i n t r i n s i c a b s o r p t i v i t y of t h e metal. However, a t h i g h e r i n t e n s i t i e s , where an a i r plasma i s i g n i t e d , t h e f r a c t i o n of t h e l a s e r energy t r a n s - f e r r e d t o t h e s u r f a c e l o c a l l y can be d r a m a t i c a l l y

increased.''' T h i s e f f e c t i s c a l l e d t h e enhanced

thermal coupling. A t even h i g h e r i n t e n s i t i e s , t h e

l o c a l c o u p l i n g d e c r e a s e s and may f a l l below t h e

i n t r i n s i c absorptance." Thus, t h e f r a c t i o n of

energy d e p o s i t e d i n a m a t e r i a l i s a s e n s i t i v e func- t i o n of t h e l a s e r p a r a m e t e r s - i n t e n s i t y , p u l s e

time, and s p o t s i z e . The purpose of t h i s paper

i s t o review t h e p h y s i c a l phenomena which i n f l u e n c e t h e thermal and mechanical coupling of l a s e r r a d i - a t i o n t o m a t e r i a l s .

The approach which i s followed is: (1) t o

choose a few important i n t e r a c t i o n s , (2) t o de- s c r i b e them b r i e f l y , and (3) t o i l l u s t r a t e t h e i m -

p o r t a n t e f f e c t with s e l e c t e d experimental r e s u l t s . Because of t h e number o f l a s e r wavelengthes, t a r g e t

m a t e r i a l s , ambient c o n d i t i o n s and l a s e r p u l s e i n - t e n s i t i e s is overwhelmingly l a r g e , t h e scope of t h i s review i s l i m i t e d t o i n f r a r e d l a s e r s (10.6 pm 8 2 and 3.8 pm) and t o i n t e n s i t i e s b e h w 10 ~ / c m

.

Metals a r e t h e primary m a t e r i a l s c o n s i d e r e d , b u t s e l e c t e d non-met.als a r e i n c l u d e d i n some i n t e r - a c t i o n regimes. The i n t e r a c t i o n s g e n e r a l l y occur i n a i r a t s t a n d a r d c o n d i t i o n s , e x c e p t f o r a few

examples of vacuum i n t e r a c t i o n s . A n a l y s i s of t h e

m a t e r i a l response i s l i m i t e d t o t h e changes which o c c u r d u r i n g t h e l a s e r p u l s e time which a f f e c t t h e l a s e r / m a t e r i a l s u r f a c e i n t e r a c t i o n . The s e p a r a t i o n of CW i n t e r a c t i o n s from p u l s e d i n t e r a c t i o n s i s based on t h e following a r b i t r a r y c r i t e r i o n : a CW i n t e r - a c t i o n i s i n s e n s i t i v e t o temporal v a r i a t i o n s i n l a s e r i n t e n s i t y ; c o n v e r s e l y , a p u l s e d l a s e r i n t e r - a c t i o n depends n o t o n l y on average i n t e n s i t y b u t a l s o on temporal v a r i a t i o n s .

2. CW INTERACTIONS: THERMAL COUPLING

A t low l a s e r i n t e n s i t y , l a s e r r a d i a t i o n i n t e r - a c t s with a m a t e r i a l by d i r e c t a b s o r p t i o n ; t h e f r a c - t i o n of energy coupled t o t h e s u r f a c e ( h e r e a f t e r

c a l l e d t h e thermal coupling c o e f f i c i e n t ) i s given

by t h e i n t r i n s i c a b s o r p t i v i t y of t h e m a t e r i a l . I n t h i s coupling regime, t h e t h e r m a l c o u p l i n g c o e f f i - c i e n t depends only on t h e l a s e r wavelength and t h e

C9-60 JOURNAL DE PHYSIQUE

t a r g e t m a t e r i a l ; l a s e r parameters such a s i n t e n s i t y , f l u e n c e and s p o t s i z e a r e i r r e l e v a n t . Thus, t h e coupling c o e f f i c i e n t can be determined w i t h any s e t of l a s e r parameters, provided t h e y f a l l w i t h i n t h e i n t r i n s i c coupling regime.

However, a s t h e l a s e r i n t e n s i t y i s i n c r e a s e d , new phenomena o c c u r , such a s t a r g e t h e a t i n g , ' t a r g e t mass removal, v a p o r i z a t i o n and plasma formation, which modify t h e coupling and i n t r o d u c e a depend-

ence on l a s e r parameters. If t h e i n t r i n s i c absorp-

t i v i t y i s a f u n c t i o n of t h e s u r f a c e temperature,

t i o n , m e l t removal and p y r o l y s i s . Even when t h e

mass removal mechanisms do n o t a f f e c t t h e l o c a l i n s t a n t a n e o u s c o u p l i n g , t h e y may s t i l l a f f e c t d a t a

i n t e r p r e t a t i o n s . Experimental measurements of t h e

fluerice r e q u i r e d t o v a p o r i z e t h i n t i t a n i u m f o i l s showed a s h a r p i n c r e a s e i n f l u e n c e a s t h e l a s e r i n t e n s i t y i n c r e a s e d ; t h i s was i n t e r p r e t e d a s a t r a n - s i t i o n from a regime i n which m e l t removal dominated t h e mass removal p r o c e s s b u t was accompanied by v a p o r i z a t i o n of melted d r o p l e t s t o a regime i n which complete v a p o r i z a t i o n o c c u r r e d . 3

t h e i n s t a n t a n e o u s l o c a l absorbed energy f l u x i s The most dramatic a l t e r a t i o n of t h e thermal

s t i l l r e p r e s e n t e d by t h e product of t h e i n t r i n s i c c o u p l i n g o c c u r s when a plasma i s c r e a t e d o v e r a a b s o r p t i v i t y of t h e s u r f a c e a t t h e l o c a l tempera-

t u r e and t h e i n s t a n t a n e o u s l o c a l i n c i d e n t i n t e n s i t y , b u t t h e l o c a l temperature depends upon t h e h i s t o r y of absorbed energy f l u x over t h e e n t i r e l a s e r beam

i n t e r a c t i o n a r e a . Thus, t h e thermal coupling co-

e f f i c i e n t i n g e n e r a l depends on a l l t h e l a s e r para- meters, and experimental d a t a can b e understood only by s o l v i n g t h e coupled problem of t h e t a r g e t thermal response t o t h e absorbed energy f l u x and t h e change of absorbed energy f l u x w i t h t a r g e t tem- p e r a t u r e . T h i s e f f e c t o c c u r s , f o r example, i n a l u -

minum. ~ ( l o s t e r m a n ~ observed an e f f e c t i v e coupling

c o e f f i c i e n t of .079 f o r v a p o r i z i n g t h i n aluminum f o i l s with 10.6 pm l a s e r r a d i a t i o n , whereas t h e

i n t r i n s i c room temperature a b s o r p t i v i t y i s only .03.

T h e o r e t i c a l c a l c u l a t i o n s 4 p r e d i c t an e f f e c t i v e cou-

p l i n g of -11 which i s i n r e a s o n a b l e agreement w i t h

t h e d a t a .

I f t h e t a r g e t r e a c h e s h i g h enough t e m p e r a t u r e t o induce mass l o s s , t h e energy c a r r i e d away by

t h e removed m a t e r i a l must be p r o p e r l y accounted f o r i n o r d e r .to determine t h e absorbed energy from experimental d a t a o r t o p r e d i c t t h e o r e t i c a l l y t h e

energy remaining i n t h e m a t e r i a l . Mass can b e r e -

moved by s e v e r a l p r o c e s s e s ; f o r example, vaporiza-

s u r f a c e . The dynamics of a laser-produced plasma

above a s u r f a c e w i l l depend upon t h e i n t e n s i t y of t h e i n c i d e n t l a s e r p u l s e and t h e p u l s e d u r a t i o n . A t l a s e r i n t e n s i t i e s s l i g h t l y g r e a t e r t h a n t h e plasma t h r e s h o l d i n t e n s i t y , a l a s e r - s u p p o r t e d com- b u s t i o n (LSC) wave5 i s u s u a l l y i g n i t e d . LSC waves 4 a r e o f t e n seen a t i n t e n s i t i e s from 2 x 1 0 w/cm2

-

6 2 10 W/cm f o r 10.6 pm r a d i a t i o n w i t h b o t h p u l s e d

and CW l a s e r beams. The i g n i t i o n of a n LSC wave

i n i t i a l l y t a k e s p l a c e i n t h e t a r g e t ~ a ~ o r . ~ * ~ The

h e a t e d t a r g e t vapor subsequently t r a n s f e r s i t s

energy t o t h e surrounding a i r . Once t h e a i r b e g i n s

t o a b s o r b a s i g n i f i c a n t f r a c t i o n of t h e l a s e r ener- gy, t h e LSC wave p r o p a g a t e s i n t o t h e a i r a l o n g t h e beam p a t h .

The n a t u r e of t h e c o u p l i n g i n t h e plasma r e - gime depends on i n t e n s i t y , s p o t s i z e , p u l s e time, ambient a i r p r e s s u r e and t h e t a r g e t m a t e r i a l . For l o n g p u l s e t i m e s and low i n t e n s i t y t h e c r e a t i o n

of a LSC wave a t 10.6 Um u s u a l l y r e s u l t s i n cur-

t a i l i n g t h e thermal c o u p l i n g a s t h e LSC wave pro- p a g a t e s toward t h e l a s e r f l u x . Thermal c o u p l i n g f o r s h o t p u l s e t i m e s and h i g h i n c i d e n t i n t e n s i t y

i s t r e a t e d a s a p u l s e d i n t e r a c t i o n .

p o r plasma i g n i t i o n occurs. The observed i g n i t i o n Experiments a t 5.0 pm i n d i c a t e t h a t t h e vapor

time i s t h e sum of t h e time r e q u i r e d t o produce t h e

vapor and t h e time r e q u i r e d t o breakdown t h e vapor. The vapor p r o d u c t i o n time i s determined from t h e t a r g e t thermal q s p o n s e t o t h e d i r e c t a b s o r p t i o n

of l a s e r r a d i a t i o n . The vapor breakdown time i s

determined from t h e h e a t i n g of t h e vapor by i n v e r s e

Bremsstrahlung a b s o r p t i o n of l a s e r r a d i a t i o n . The

dependence of t h e breakdown time on l a s e r i n t e n s i t y and s p o t s i z e i s i l l u s t r a t e d i n F i g . 1 which com- p a r e s t h e t h e o r e t i c a l p r e d i c t i o n s of P i r r i 6 t o t h e

experimental d a t a o f Klosterman. T h e o r e t i c a l p r e -

d i c t i o n s of breakdown t i m e s a r e shown f o r 1-D

p l a n a r vapor dynamics, and two-dimensional (axisym- m e t r i c ) vapor dynamics f o r a l a s e r s p o t r a d i u s of

- 5 cm. I n t h e experiments t h e l a s e r i n t e n s i t y was

changed by changing t h e s p o t s i z e . Thus, a t low

i n t e n s i t y , where t h e s p o t , r a d i u s i n 1 cm, t h e d a t a a g r e e s w i t h t h e one-dimensional p r e d i c t i o n , whereas a t h i g h e r i n t e n s i t i e s , a s t h e s p o t s i z e s h r i n k s t o 0.25 cm and t h e vapor dynamics becomes two-dimen- s i o n a l , t h e d a t a t e n d s towards t h e 2-D p r e d i c t i o n .

A

ALUMINUM DATA (KLOSTERMAN) FOR 0.25 5 r s I 1 CM

-

THEORY

(PIRRI)

-

-

-

-

NO IGNITION

-

SUPERSONIC

-

F i g . 1 Time t o i g n i t e l a s e r - s u p p o r t e d combustion wave vs. l a s e r i n t e n s i t y , from Ref. 6.

-2

breakdown time s c a l e s a s (wavelength)

,

a s expected

from i n v e r s e Bremsstrahlung a b s o r p t i o n . 3 , 6

3. CW INTERACTIONS - MECHANICAL COUPLING

Bulk v a p o r i z a t i o n of t a r g e t m a t e r i a l g e n e r a t e s

s u r f a c e p r e s s u r e and impulse. The vapor p r e s s u r e

on t h e s u r f a c e depends on t h e ambient p r e s s u r e , t h e l a s e r i n t e n s i t y I ( t ) , l a s e r s p o t r a d i u s and t h e de-

t a i l e d thermal response on t h e t a r g e t . A t h i g h in-

t e n s i t y , where t h e background p r e s s u r e i s i r r e l e - v a n t , o r i n vacuum, o r f o r t i m e s s h o r t enough f o r p l a n a r vapor dynamics t o b e v a l i d t h e s u r f a c e p r e s -

s u r e p can b e a d e q u a t e l y with

a n a l y t i c models. Even when a s t e a d y s t a t e p r e s s u r e regime i s achieved, .'the observed coupling may show

a time dependence. C a l c u l a t i o n s of t h e i n s t a n t a -

neous mechanical c o u p l i n g , p ( t ) / I ( t ) and t h e i n t e - g r a t e d coupling c o e f f i c i e n t C ( t ) , d e f i n e d a s

a r e shown i n Fig. 2 a s a f u n c t i o n of time. Vapor-

i z a t i o n b e g i n s a t time Tv. These c a l c u l a t i o n S a r e f o r carbon phe?olic t a r g e t s i r r a d i a t e d by 1 &/cm 2

of 10.6

u m

r a d i a t i o n . ' It t a k e s t e n t i m e s a s l o n g t o approach t h e s t e a d y s t a t e p r e s s u r e a s it does t o i n i t i a t e v a p o r i z a t i o n ; and t h e i n t e g r a t e d ' cou- p l i n g c o e f f i c i e n t i n c r e a s e s even more slowly.

cornon Pnenallc I

-

106 w/d

o

-

0.81

-

lnstonto eous caualinp caefflclen Intenrot d counllng coefflclent IYU ond Nebolrlne)

F i g . 2 Mechanical coupling c o e f f i c i e n t f o r carbon p h e n o l i c i r r a d i a t e d by 10.6 pm r a d i a t i o n ,

JOURNAL DE PHYSIQUE

J u s t a s i g n i t i o n of a vapor plasma m o d i f i e s

thermal coupling t o a s u r f a c e , it a l s o a l t e r s t h e

mechanical coupling. I n an a i r environment, t h e

l a s e r s u r f a c e i n t e r a c t i o n proceeds v i a LSC waves 10 o r l a s e r - s u p p o r t e d d e t o n a t i o n (LSD) waves ; t h e y

a r e d i s c u s s e d under p u l s e d c o u p l i n g i n t e r a c t i o n s .

I n a vacuum, however, t h e plasma is c o n f i n e d e n t i r e -

l y t o t h e vapor. The mechanical coupling of l a s e r s

t o s u r f a c e s through vapor plasma can b e modelled by r e l a t i n g t h e t h i c k n e s s of t h e plasma t o t h e ab-

s o r p t i o n depth of t h e l a s e r r a d i a t i o n i n t h e va-

por. l1 The s t e a d y s t a t e coupling c o e f f i c i e n t f o r

a vacuum plasma d e c r e a s e s w i t h i n t e n s i t y , whereas t h e s t e a d y s t a t e c o u p l i n g from v a p o r i z a t i o n in- c r e a s e s w i t h i n t e n s i t y .

4. PULSED INTERACTION PHENOMENOLOGY

The phenomenology of t h e i n t e r a c t i o n of a p u l s e d 10.6 v m p u l s e w i t h s u r f a c e s depends n o t only on t h e average i n t e n s i t y o f t h e p u l s e , b u t a l s o on

t h e temporal v a r i a t i o n s . A t y p i c a l temporal p u l s e

shape i s sketched i n F i g . 3. A t t h e l e a d i n g edge

of t h e p u l s e t h e r e i s a g a i n switched s p i k e follow- ed by a lower i n t e n s i t y t a i l . The s p i k e l a s t s o n l y 100-400 n s b u t i t s peak i n t e n s i t y i s u s u a l l y two t o e i g h t t i m e s l a r g e r t h a n t h e average i n t e n s i t y

of t h e t a i l . The t a i l , which c a r r i e s most of t h e

energy, t y p i c a l l y h a s a d u r a t i o n of 3-40 ps.

Time

-

A i r plasmas can b e c r e a t e d above s u r f a c e s by t h e s p i k e . T h i s p r o c e s s i s c a l l e d prompt i g n i t i o n t o d i s t i n g u i s h it from t h e breakdown of t h e p r o d u c t s of b u l k v a p o r i z a t i o n which was d i s c u s s e d e a r l i e r . Bulk vapor breakdown cannot occur u n t i l enough time h a s e l a p s e d f o r t h e s u r f a c e t o r e a c h t h e vaporiza- t i o n t e m p e r a t u r e . , The s p i k e c o n t a i n s i n s u f f i c i e n t energy t o c a u s e bulk v a p o r i z a t i o n , i n s t e a d , i g n i t i o n t a k e s p l a c e r a p i d l y from l o c a l i z e d + f e c t s which

v a p o r i z e and break down. 12113 The s p i k e , then, con-

t r o l s whether o r n o t an a i r plasma i s formed. The

t h r e s h o l d f o r prompt i g n i t i o n from aluminum i s e s - 2 t i m a t e d t o b e a s p i k e f l u e n c e 1 2 of 1.7 J/cm

,

and

2

a s p i k e i n t e n s i t y 1 4 of 10-30 MW/cm

.

T y p i c a l h i g h energy p u l s e s meet t h e s e requirements when t h e aver-

2

age i n t e n s i t y i n t h e t a i l i s approximately 1 MW/cm

.

I f no plasma i s i g n i t e d d u r i n g t h e s p i k e , t h e t a i l of t h e p u l s e i s absorbed d i r e c t l y by t h e s u r - f a c e , and t h e thermal c o u p l i n g c o e f f i c i e n t i s given by t h e i n t r i n s i c a b s o r p t i v i t y . I f a plasma i s i n i - t i a t e d , t h e subsequent i n t e r a c t i o n proceeds v i a LSC

wave f o r low average i n t e n s i t y ( l e s s t h a n 8 MW/

15'16 and an LSD nave a t high i n t e n s i t y . A s

cm )

a consequence of t h e i g n i t i o n p r o c e s s and t h e pre- sence of t h e t a r g e t s u r f a c e , a p r e c u r s o r shock pre- c e d e s t h e LSC wave e x c e p t a t t h e lowest i n t e n s i t i e s .

When t h e LSC wave h a s propagated f a r enough f o r two-

dimensional e f f e c t s t o dominate t h e plasma flow i n t h e v i c i n i t y of t h e s u r f a c e , t h e plasma c o n f i g u r a - t i o n resembles t h a t shown s c h e m a t i c a l l y i n F i g . 4a. The LSC wave p r o p a g a t i n g i n t o t h e a i r behind t h e

p r e c u r s o r shock induces a flow towqrd t h e t a r g e t . C l o s e t o t h e s u r f a c e t h e flow resembles a stagna- t i o n p o i n t flow. A s t a g n a t i o n p o i n t boundary l a y e r a n a l y s i s must b e matched t o a c o r r e c t model f o r t h e LSC wave p r o p a g a t i n g away from t h e s u r f a c e i n o r d e r

Fig. 3 s k e t c h of 10.6

u m

temporal p u l s e shape. t o o b t a i n t h e temperature and p r e s s u r e d i s t r i b u t i o n

g e t and t h e conductive energy t r a n s f e r .

A t high l a s e r i n t e n s i t i e s , g r e a t e r than 8 MW/

cmL f o r 10.6 pm l a s e r r a d i a t i o n , 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 (LSD) wave i s i g n i t e d . A s i m p l i f i e d model of t h e plasma c o n f i g u r a t i o n r e s u l t i n g from

LSD wave i g n i t i o n i s shown i n Fig. 4b. The l a s e r

beam a b s o r p t i o n t a k e s p l a c e i n a t h i n zone of h o t , h i g h p r e s s u r a i r behind t h e d e t o n a t i o n wave. S i n c e

t h e d e t o n a t i o n wave d r a g s a i r away from t h e s u r f a c e , expansion f a n s form t o s a t i s f y t h e boundary condi- t i o n s of z e r o p a r t i c l e v e l o c i t y a t t h e t a r g e t s u r - f a c e . One-dimensional g a s dynamics can b e matched t o d e t o n a t i o n and p l a n a r b l a s t wave t h e o r y t o de- s c r i b e t h i s a s p e c t of t h e flow f i e l d , and c y l i n d r i - c a l blast-wave t h e o r y can be u t i l i z e d t o p a r t i c a l l y

account f o r two-dimensional e f f e c t s .I6 An unsteady

LASER BEAM

M

SHOCK a) CONDUCTION LASER BEAM BOUNDARY LAYER CONDUCTION

Fig. 4 Sketch of l a s e r a b s o r p t i o n wave plasma

dynamics ( a ) LSC wave, (b) LSD wave. From Ref. 19.

boundary l a y e r forms on t h e s u r f a c e ; it resembles

t h e boundary l a y e r behind a p r o p a g a t i n g shock wave a s t h e c y l i n d r i c a l b l a s t wave s p r e a d s o u t over t h e t a r g e t . Energy i s t r a n s f e r r e d from t h e plasma t o t h e t a r g e t through t h i s boundary l a y e r by r a d i a t i o n and conduction.

5. LSD WAVE COUPLING: RADIAL EXPANSION EFFECTS

I n one-dimension t h e thermodynamic p r o p e r t i e s behind a t r u e LSD wave i s p r e d i c t e d by t h e Raizer

theory.10 The a n a l y s i s of p i r r i 1 6 g i v e s t h e condi-

t i o n s above t h e s u r f a c e and t h e time h i s t o r y of t h e plasma p r o p e r t i e s a s t h e LSD wave p r o p a g a t e s away from t h e s u r f a c e . A t 10.6 pm f o r an i n t e n s i t y of

2

20 MW/cm

,

t h e temperature and p r e s s u r e above t h e

s u r f a c e a r e p r e d i c t e d t o be 9000°K and 53 atm., r e s p e c t i v e l y . Under t h e s e c o n d i t i o n s t h e r a t i o of t h e energy t r a n s f e r r e d from t h e plasma t o t h e t a r - g e t by r a d i a t i o n and conduction t o t h e l a s e r ener- gy w i l l b e l e s s t h a n 1% f o r l a s e r p u l s e t i m e s of

t h e o r d e r of t e n s of microseconds. 14

I n o r d e r t o c a l c u l a t e t h e t o t a l c o u p l i n g coef-

f i c i e n t , plasma s p r e a d i n g must b e included. The

plasma remains approximately one-dimensional u n t i l t h e expansion f a n s from t h e edge of t h e s p o t reach

t o a x i s of symmetry. T h i s time i s approximated by

t h e beam r a d i u s d i v i d e d by t h e speed of sound i n

t h e plasma. The p r e s s u r e decay can be approximated

1 6

by c y l i n d r i c a l b l a s t wave t h e o r y f o r time s c a l e s

g r e a t e r t h a n two-dimensional time s c a l e . I n t h e

v i c i n i t y o f t h e s u r f a c e t h e r e i s no l a s e r absorp- t i o n ; t h e plasma p r o p e r t i e s a r e determined from

i s e n t r o p i c expansion r e l a t i o n s . The energy t r a n s -

f e r a t any i n s t a n t of time i s t h e sum of t h e boun-

dary l a y e r h e a t t r a n s f e r and r a d i a t i o n c o n t r i b u - t i o n s .

calculation^^^

f o r a two-dimensional LSD wave

2

plasma with a l a s e r i n t e n s i t y of 1 5 MW/cm

,

a s p o t

C9-64 JOURNAL DE PHYSIQUE

t h a t : (I) r a d i a t i v e energy t r a n s f e r i s minimal, (2) boundary l a y e r energy t r a n s f e r dominates, a s t h e plasma s p r e a d s o u t over t h e t a r g e t , f o r t i m e s up t o 1000 psec, ( 3 ) f o r t h e 1 psec p u l s e t h e t o t a l

coupling c o e f f i c i e n t is approximately 25% b u t i s

a r e s u l t of energy s p r e a d o u t over a l a r g e a r e a com- pared t o t h e s p o t a r e a , and ( 4 ) a s t h e l a s e r i n t e n - s i t y i s i n c r e a s e d , t h e t o t a l c o u p l i n g c o e f f i c i e n t t e n d s t o remain c o n s t a n t , b u t t h e energy i s s p r e a d o u t over a l a r g e r agea. T h e r e f o r e , f o r s h o r t l a s e r p u l s e s t h e t o t a l c o u p l i n g c o e f f i c i e n t i s s i g n i f i - c a n t ; however, t h e l o c a i energy t r a n s f e r r e d i n t o t h e t a r g e t i s n o t g r e a t e r t h a n would b e o b t a i n e d

i f no plasma was formed. F i n a l l y , f o r t h i s calcu-

l a t i o n t h e c o u p l i n g c o e f f i c i e n t v a r i e d i n v e r s e l y with p u l s e time.

The e f f e c t of plasma spreading on energy t r a n s - f e r i n t h e LSD regime h a s been e x p e r i m e n t a l l y ob- s e r v e d , 17'18 and t h e thermal coupling i s indepen- d e n t of i n t e n s i t y f o r l a r g e t a r g e t s . 1 7 T o t a l t h e r - mal coupling of a p u l s e d 10.6 pm l a s e r t o n i c k e l

a s measured by H a l l e t a1.,18 is shown i n F i g . 5.

The nominal l a s e r p u l s e t i m e is 6 psec. The t o t a l

c o u p l i n g i n c r e a s e s d r a m a t i c a l l y a f t e r a plasma i s i g n i t e d , b u t t h e c o u p l i n g d e c r e a s e s a t h i g h e r i n c i -

d e n t f l u e n c e because the plasma expands beyond t h e

t a r g e t .

Peok lncldent Fluence [ J / c ~

36 ,25

-

* .20- '2 P

-

₂

.15

-

f

L

1

. l o F i g . 5 Thermal coupling of 10.6 pm r a d i a t i o n t o

.: n i c k e l . Data from Ref. 18.

6. LSC WAVE REGIME: ENHANCED LOCAL COUPLING

I g n i t i o n of an LSC wave can l e a d t o enhanced

1

l o c a l thermal coupling f o r aluminum s u r f a c e s . I t

h a s been shown t h a t enhanced t h e r m a l coupling t o aluminum i s a r e s u l t of energy t r a n s f e r r e d by r a d i - a t i o n from t h e h o t , h i g h p r e s s u r e , l a s e r - s u p p o r t e d plasma a d j a c e n t t o t h e t a r g e t . 5,19,20,21 A s k e t c h

of t h e one-dimensional LSC wave plasma c o n f i g u r a -

t i o n i s shown i n Fig. 6. The l a s e r i s i n c i d e n t

from t h e r i g h t hand s i d e . The g a i n switched s p i k e

i g n i t e s an a i r plasma n e x t t o t h e t a r g e t s u r f a c e , and t h e expansion of t h e plasma d r i v e s a p r e c u r s o r shock i n t o t h e a i r . The l a s e r a b s o r p t i o n zone pro- p a g a t e s i n t o t h e shocked a i r ; t h e a b s o r p t i o n o c c u r s e s s e n t i a l l y a t c o n s t a n t p r e s s u r e and t h e propaga- t i o n of t h e a b s o r p t i o n zone i s c o n t r o l l e d by con- 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 from t h e h o t plasma.

0 .

I

I N1 Doto 1Holl et 01.) d - 1 0 . 6 p r n Rs = . I 6 cm RT

-

1.6 cm AIR LSC WAVE (LASER ABSORPTION ZONE)

l

I l i l l A PRECURSOR SHOCK I 1 I 1 1 1 1 1 ~

0

FLUX

0

0

{;: ::ctl Qnl --•

0

Direct Absorntlon---*

-

F i g . 6 One-dimensional LSC wave c o n f i g u r a t i o n . t-

The LSC wave p r o p a g a t e s i n t o t h e shocked a i r a t

slow speed, and a l a r g e f r a c t i o n of t h e energy i s

used t o h e a t t h e plasma t o a temperature of approx-

i m a t e l y 2000OoK. T h i s h o t plasma i s capable of

0

r a d i a t i n g i n s p e c t r a l r e g i o n s l e s s t h a n 1250 A

which a r e w e l l absorbed by t h e aluminum t a r -

g e t s . 20'21 The expansion of t h e a i r a s it i s h e a t -

e d a c t s a s a p i s t o n which m a i n t a i n s t h e p r e c u r s o r shock.

plasma must b e i g n i t e d a d j a c e n t t o t h e s u r f a c e ; (2)

t h e plasma must b e an LSC wave, and ( 3 ) t h e one- dimensional c o n f i g u r a t i o n i l l u s t r a t e d i n F i g . 6 must b e maintained throughout t h e p u l s e . These r e q u i r e - ments a r e s u f f i c i e n t t o i d e n t i f y t h e range of l a s e r p a r a m e t e r s corresponding t o t h e enhanced c o u p l i n g r e g i o n . The e?hanced c o u p l i n g r e g i o n i s i l l u s t r a t - e d i n F i g . 7. The c o o r d i n a t e s of t h e p l o t a r e l a s e r i n t e n s i t y , I , and ?, which i s t h e la,ser p u l s e time T normalized by t h e time, T Z D , a t which r a d i a l

P

expansion of t h e high p r e s s u r e plasma a f f e c t s t h e c e n t e r of t h e l a s e r s p o t .

(r2D

i s d e f i n e d a s R/a

P where R i s t h e r a d i u s o f t h e l a s e r s p o t and a i s

P

F i g . 7 Region of enhanced l o c a l thermal coupling.

t h e sound speed i n t h e plasma, approximately 4.5 x

5

10 cm/s.) Once r a d i a l expansion b e g i n s , t h e dy- namics of t h e plasma no l o n g e r m a i n t a i n s t h e one- dimensional c h a r a c t e r i l l u s t r a t e d i n F i g . 6; t h e p r e s s u r e d r o p s and s o does t h e plasma temperature. A s a r e s u l t , t h e e f f e c t i v e n e s s of t h e plasma i n t r a n s p o r t i n g energy v i a r a d i a t i o n i s r a p i d l y dimi- nished. Thus, t h e r e g i o n of ? > 1 i s e l i m i n a t e d from t h e enhanced coupling r e g i o n because r a d i a l expansion c u r t a i l s t h e r a d i a t i v e t r a n s p o r t b e f o r e t h e l a s e r p u l s e i s term-inated. The h i g h i n t e n s i t y r e g i o n i s e l i m i n a t e d because, above t h e LSD wave

2

t r a n s i t i o n t h r e q h o l d of 8 W c m

,

an LSD wave i s

produced which h a s p o o r coupling. The low i n t e n -

s i t y r e g i o n , below t h e plasma i g n i t i o n t h r e s h o l d of 1 MW/cm2, i s e l i m i n a t e d because t h e g a i n s w i t c h s p i k e corresponding t o t h i s l a s e r i n t e n s i t y i s n o t s t r o n g enough t o c r e a t e a plasma o v e r aluminum. The remaining a r e a i s t h e enhanced c o u p l i n g regime.

The 1-D plasma c o n f i g u r a t i o n shown i n F i g . 6

h a s been s t u d i e d t h e o r e t i c a l l y by many i n v e s t i g a t o r s and a summary of t h e i r c o n t r i b u t i o n s i s p r e s e n t e d i n Refs. 14 and 15. To p r e d i c t thermal c o u p l i n g t h e c o n t r i b u t i o n o f t h e r a d i a l l y expanding plasma shown i n F i g . 4a must b e included. To make q u a n t i - t a t i v e p r e d i c t i o n s , a model was s y n t h e s i z e d 19,22 i n which t h e e a r l y time plasma cynamics was de- s c r i b e d by t h e one-dimensional c o n f i g u r a t i o n o f F i g .

6 and t h e l a t e t i m e plasma dynamics was r e p r e s e n t e d by b l a s t wave decay laws of t h e a p p r o p r i a t e geomet- r y . The boundary between e a r l y time and l a t e time was determined t o be t h e s m a l l e r of

r

and

r

P 2D'

P r e d i c t i o n s made w i t h t h i s model agreed w i t h t h e d a t a w i t h i n 30% o v e r most of t h e range o f i n t e r e s t .

A complementary approach which was an a x i a l l y sym-

m e t r i c numerical s i m u l a t i o n h a s a l s o been develop- ed,'' and t h e r e is good agreement between t h e pre- d i c t i o n s of t h e two models.

A comparison of t h e a n a l y t i c a l model p r $ d i c -

t i o n s and experimental d a t a of Rudder and Augus- t ~ n i , ~ ~ and McKay e t a l .

,'

a r e shown i n F i g . 8.

JOURNAL DE PHYSIQUE S i n c e t h e experiments i n v o l v e a range of i n t e n s i - t i e s , t h e t h e o r e t i c a l c u r v e s were c a l c u l a t e d f o r two l i m i t i n g i n t e n s i t i e s . The d a t a i s g e n e r a l l y i n agreement w i t h t h e t h e o r y ; i n p a r t i c u l a r t h e p r e d i c t e d d e c r e a s e i n t h e l o c a l thermal coupling

with i n c r e a s i n g

4

i s observed. A more d e t a i l e d

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

d i s c u s s e d l a t e r f o r o b l i q u e a n g l e s of incidence. The amount of energy t r a n s f e r r e d t o t h e zur- f a c e depends n o t only on t h e r a d i a t i v e p r o p e r t i e s o f t h e plasma, b u t a l s o on t h e s p e c t r a l a b s o r p t i o n c h a r a c t e r i s t i c s of t h e t a r g e t . The coupling of metal s u r f a c e s o t h e r t h a n aluminum i s determined by u s i n g t h e a p p r o p r i a t e s p e c t r a l a b s o r p t i v i t y .

LSC wave plasmas t e n d t o r a d i a t e s t r o n g l y i n

0

t h e s p e c t r a l r e g i o n

1

< 1250 A. For s h o r t p u l s e t i m e s and small s p o t s , and a t low i n t e n s i t y most of t h e r a d i a t i o n i s e m i t t e d a t wavelengths l e s s

0

t h a n 1250 A, which a l l t h e m e t a l s a b s o r b w e l l . There i s l i t t l e d i f f e r e n c e between t h e f l u e n c e ab-

sorbed by v a r i o u s m e t a l s f o r t h e s e l a s e r parameters. ~t lo n g e r p u l s e times, o r h i g h e r i n t e n s i t y , t h e r a d i a t i o n i n t h e s h o r t wavelength regime becomes s a t u r a t e d and it is c o n t r o l l e d by t h e plasma tem- p e r a t u r e c l o s e t o t h e t a r g e t , whereas t h e r a d i -

D

a t i o n i n t h e band A > 1250 A, which i s t r a n s p a r e n t ,

i s c o n t i n u a l l y i n c r e a s i n g . Even w i t h aluminum, which a b s o r b s l o n g wavelenghs p o o r l y , t h e l o n g wave-

i n terms of t h e s p e c t r a l a b s o r p t i v i t y . Thus, f o r example, copper and s i l v e r , which a r e b e t t e r r e f l e c - t o r s of 10.6 ym r a d i a t i o n t h a n aluminum, a r e p r e - d i c t e d t o absorb plasma r a d i a t i o n more s t r o n g l y t h a n aluminum. For a l l o y s , t h e s t r o n g e s t a b s o r b e r i s t i t a n i u m , followed by s t e e l and aluminum.

7. PULSED LASER MECHANICAL COUPLING

I g n i t i o n of LSC and LSD waves c r e a t e s h i g h p r e s s u r e plasma o v e r t h e t a r g e t s u r f a c e . The r e - s u l t i n g impulse d e l i v e r e d t o t h e s u r f a c e r e c e i v e s a c o n t r i b u t i o n from b o t h t h e e a r l y time p l a n a r waves

( i l l u s t r a t e d i n F i g . 6 ) and t h e l a t e time two-di-

mensional waves, shown i n F i g . 4. The impulse de-

l i v e r e d by an LSD wave was f i r s t modelled by P i r r i , 6

u s i n g a n a l y t i c methods. More e x t e n s i v e modelling,

i n c l u d i n g numerical s i m u a l t i o n s , w& performed by

F e r r i t e r e t a1.24 The impulse from LSC waves h a s 25,26

a l s o been c a l c u l a t e d based upon t h e p r e s s u r e

s c a l i n g laws used i n t h e thermal coupling c a l c u l a - t i o n s . 19

A convenient method of p r e s e n t i n g t h e r e s u l t s

i s t o g i v e t h e r a t i o of t h e p r e d i c t e d impulse over a g i v e n a r e a d i v i d e d by t h e impulse determined from t h e p l a n a r s u r f a c e p r e s s u r e , ps, a c t i n g on t h e l a s e r

beam a r e a

n~:

f o r t h e p u l s e time T Ths r e s u l t s

P'

of Bouche e t a 1

.,

26 f o r t h e LSC wave models a r e

summarized i n F i g . 9 a s a f u n c t i o n of ?. Calcula-

t i o n s a r e made f o r one-dimensional plasma p r e s s u r e s

l e n g t h band makes an i n p o r t a n t c o n t r i b u t i o n f o r f o r 10 and 30 atm, which a r e t h e l i m i t s t y p i c a l l y

h i g h i n t e n s i t y , l o n g p u l s e t i m e s and l a r g e s p o t s . observed e x p e r i m e n t a l l y f o r LSC waves, and t h e a r e a I n t h i s regime t h e coupling t o m e t a l s o t h e r t h a n

aluminum, which absorb t h e l o n g wavelength band b e t t e r , i s enhanced r e l a t i v e t o t h e c o u p l i n g t o

A 1 2024.

T h e o r e t i c a l p r e d i c t i o n s of t h e f l u e n c e absorb- e d by v a r i o u s a l l o y s 2 2 i n d i c a t e t h a t t h e metalis c a n b e a r r a n g e d i n t o a h i e r a r c h y based upon ab-

s o r b e d f l u e n c e ; t h e h i e r a r c h y c a n b e understood

between them i s shaded f o r a l l b u t t h e i n f i n i t e

p l a n e c a l c u l a t i o n s . The f o u r c a l c u l a t i o n s a r e :

(1) c e n t r a l coupling which u s e s t h e p r e s s u r e p r e -

i n g r a d i a l shock b e f o r e t h e end of t h e p u l s e , and a t e a r l y time; t h e r e f o r e , t h e plasma dynamics i s

( 4 ) coupling t o a i n f i n i t e p l a n e which i n c l u d e s one-dimensional p e r p e n d i c u l a r t o t h e t a r g e t . The

t h e t o t a l impulse d e l i v e r e d by t h e expanding plasma plasma c o n f i g u r a t i o n i s i d e n t i c a l t o F i g . 6

-

a

a f t e r t h e p u l s e t e r m i n a t e s . p r e c u r s o r shock followed by a l a s e r a b s o r p t i o n zone

_-

-

e x c e p t t h a t t h e l a s e r beam i s i n c i d e n t a t a d i f - f e r e n t a n g l e . The e a r l y t i m e b e h a v i o r can s t i l l b e modelled a s a p l a n a r LSC wave i f t h e l a s e r i n t e n - s i t y I i s changed t o t h e p r o j e c t e d i n t e n s i t y I c o s 8, and t h e l a s e r a b s o r p t i o n c o e f f i c i e n t kL, i s r e - TARGET at Normal Incidence

F i g . 9 P r e d i c t i o n s of mechanical coupling by LSC F i g . 1 0 Angle of i n c i d e n c e geometry. ( a ) Cross-

wave plasmas. From Ref. 26. s e c t i o n a l view, (b) t a r g e t p l a n e view.

The l o c a l coupling t o t h e s p o t o r t h e c e n t e r p l a c e d by t h e c o e f f i c i e n t k L ,/cos

0

a p p r o p r i a t e f o r

of t h e spot is enhanced a t low b u t is diminished d e s c r i b i n g l a s e r beam a t t e n u a t i o n p e r p e n d i c u i a r

a t l a r g e

?;

however, t h e t o t a l coupling i s always t o t h e t a r g e t .

enhanced. The o n s e t of l a t e r a l expansion a t t h e c e n t e r

8. EXTENSION ANGLE OF INCIDENT AND

AMBIENT PRESSURE

The i n t e r a c t i o n of p u l s e d 10.6 pm l a s e r p u l s e s w i t h aluminum s u r f a c e s , which a r e a t on o b l i q u e a n g l e t o t h e l a s e r beam, h a s been s t u d i e d b o t h t h e o r e t i c a l l y 2 2 ~ 2 7 and e x p e r i m e n t a l l y . 28 The goe- metry of t h e i n t e r a c t i o n a t o b l i q u e a n g l e s of in- c i d e n c e i s s k e t c h e d in F i g . 10. The e a r l y time c o n f i g u r a t i o n , a s viewed from a p l a n e d e f i n e d by t h e i n c i d e n t l a s e r beam d i r e c t i o n and t h e t a r g e t

normal, is shown i n F i g . 10A. The plasma a t t h e

c e n t e r of the t a r g e t h a s no knowledge of t h e edges

of t h e s p o t i s determined by a competition between

t h e time f o r a r a r e f a c t i o n wave g e n e r a t e d a t t h e edge of t h e t a r g e t s p o t t o r e a c h t h e ' c e n t e r and t h e time f o r t h e l a s e r - s u p p o r t e d a b s o r p t i o n wave a t t h e c e n t e r of t h e t a r g e t t o p r o p a g a t e v e r t i c a l l y

[ i n F i g . 10a) t o t h e edge of t h e l a s e r beam. A

8

view of t h e l a s e r beam in t a r g e t p l a n e i s shown

i n F i g . l o b . The c h a r a c t e r i s t i c time f o r l a t e r a l

expansion a l o n g t h e semi-minor a x i s i s R/a = T

p 2 ~ '

where a i s t h e speed of sound i n t h e plasma; t h i s

P

C9-68 JOURNAL DE PHYSIQUE

expansion f o r normal i n c i d e n c e . Only a t a l a t e r n a i v e l y s c a l i n g t h e f l u e n c e absorbed f o r 8 = O 0

t i m e , c h a r a c t e r i z e d by T3D, d o e s motion a l o n g t h e by t h e f a c t o r c o s 8 . The LSC wave thermal cou-

semi-major a x i s b e g i n . The b l a s t wave decay laws _{p l i n g c o e f f i c i e n t i n c r e a s e s a s t h e p r o j e c t e d i n t e n -}

f o r time t a r e chosen t o r e p r e s e n t two-dimensional sity is reduced. ~h~ data also shows that plasma

motion f o r T3D > t > T2D and three-dimer?sional mo- _{i g n i t i o n t h r e s h o l d is c o n t r o l l e d by t h e beam i n t e n -}

t i o n f o r t > T3D. The expansion i s r e p r e s e n t e d by powered o r unpowered b l a s t wave decay laws accord- i n g t o whether o r n o t t h e l a s e r i s s t i l l on.

A comparison between experimental d a t a , 2 8 and

t h e o r e t i c a l p r e d i c t i o n s 22'27 f o r t h e f l u e n c e ab-

sorbed by A1 2024 t a r g e t s a r e shown i n Fig. 11, f o r

2

a nominal l a s e r beam i n t e n s i t y of 1.5 MW/cm

.

The

2

beam a r e a i s 40 cm and t h e p u l s e time i s 10 ps.

The agreement between d a t a and t h e o r y i s q u i t e good

e x c e p t f o r a few d a t a p o i n t s a t normal incidence which a r e marked by l i n e s t o i n d i c a t e t h a t t h e r e

was poor plasma i g n i t i o n . T h i s good agreement sup-

p o r t s t h e o r i g i n a l model a s w e l l a s t h e e x t e n s i o n t o a n g l e of i n c i d e n c e .

8

Angle of Incidence F i g . 11 Fluence d e p o s i t e d i n A 1 2024 by 10.6 pm ! r a d i a t i o n i n c i d e n t a t an a n g l e .

Both t h e d a t a and t h e t h e o r y show l a r g e oou- p l i n g a t l a r g e a n g l e s of i n c i d e n c e (8 > 74O). The

s i t y , n o t t h e p r o j e c t e d i n t e n s i t y . 28

The t h e o r e t i c a l p r e d i c t i o n f o r t h e s u r f a c e p r e s s u r e g e n e r a t e d by t h e LSC wave is p l o t t e d a s a f u n c t i o n of a n g l e of i n c i d e n c e i n F i g . 12. The experimental data28 a g r e e q u i t e w e l l w i t h t h e theo- r e t i c a l p r e d i c t i o n s ( e x c e p t f o r t h e d a t a p o i n t which

i s f l a g g e d because of p o o r i g n i t i o n ) . The theore- t i c a l c a l c u l a t i o n s of impulse imparted t o a s u r f a c e

Angle of Incidence

F i g . 12 S u r f a c e p r e s s u r e f o r 10.6 pm r a d i a t i o n i n c i d e n t a t an a n g l e .

p r e d i c t t h a t t h e r e is l i t t l e d e g r a d a t i o n i n p r e -

d i c t e d impulse between 8 = O 0 and 8 = 6 0 ° ; t h e d r o p

t h e slower p r e s s u r e decay a s 0 i s i n c r e a s e d . A t

6 = 7 5 O , t h e p r o j e c t e d f l u e n c e i s only 1 / 4 of t h e

normal f l u e n c e , b u t t h e d e l i v e r e d impulse f o r ? = 1

i s 70% of t h e v a l u e f o r normal i n c i d e n c e .

The plasma impulse c o u p l i n g i s p r e d i c t e d t o 10,14,15 vary a s t h e ambient p r e s s u r e t o t h e 1 / 3 power. The d e c r e a s e h a s been observed e x p e r i m e n t a l l y i n t h e LSD wave regime, although t h e peak p r e s s u r e s measured f a l l below 30 p e r c e n t below t h e p r e d i c t e d v a l u e s . 2 g Experimental d a t a i n d i c a t e t h a t t h e t h e r - mal coupling by r a d i a t i v e t r a n s p o r t from LSC wave plasma remains approximately c o n s t a n t a s t h e pres-

s u r e i s reduced, a s long a s a well-developed plasma

i s formed. However, a s t h e ambient p r e s s u r e drops

a t h r e s h o l d i s reached below which an a i r plasma

cannot be c r e a t e d . The thermal coupling a t p r e s -

s u r e s below t h e a i r t h r e s h o l d i s accomplished by d i r e c t a b s o r p t i o n ; a t l e a s t f o r low l a s e r i n t e n -

s i t i e s . However, it h a s been observed by McKay

and schriempf31 t h a t a t h i g h i n t e n s i t y it i s pos- s i b l e t o i g n i t e a vacuum plasma which enhances t h e t h e r m a l c o u p l i n g . T h i s plasma is

not

t h e r e s u l t of

bulk

vaporization,31 i n s t e a d it a p p a r e n t l y is c r e a t e d by d e f e c t v a p o r i z a t i o n and i s , t h e r e f o r e ,

d i f f e r e n t t h a n t h e CW vapor plasma mentioned i n

S e c t i o n 3.

9. PULSED 3.8 pm COUPLING CONSIDERATIONS

Although b o t h 10.6 pm and 3.8 pm p u l s e d l a s e r r a d i a t i o n e x h i b i t enhanced thermal coupling t o high- l y r e f l e c t i v e m e t a l t a r g e t s , t h e mechanisms by which

t h e coupling i s achieved may b e d i f f e r e n t .

The DF l a s e r p u l s e , from t h e Boeing photoly- t i c a l l y i n i t i a t e d l a s e r , 3 2 which i s shown i n F i g .

1 3 , does n o t have a l e a d i n g edge s p i k e and it h a s

a r e l a t i v e l y l o n g r i s e time t o approximately -85

psec. Plasma i g n i t i o n o c c u r s i n t h e middle of t h e

p u l s e . 2132r33 A s a r e s u l t , t h e p h y s i c s o f t h e i n - be c a t e g o r i z e d f o r t h e d u r a t i o n of t h e whole p u l s e , a s e i t h e r d i r e c t l a s e r a b s o r p t i o n o r plasma r a d i - a t i v e t r a n s f e r . 1 ime tpsl F i g . 13 Sketch of DF l a s e r p u l s e shape.JL Nor can d i r e c t . a b s o r p t i o n of DF l a s e r r a d i - a t i o n b e simply c h a r a c t e r i z e d by t h e i n t r i n s i c room temperature a b s o r p t i v i t y a s it can be f o r 10.6 pm p u l s e d r a d i a t i o n . For 3.8

urn

r a d i a t i o n t h e i n i t i a l a b s o r p t i v i t y of A l 2 0 2 4 i s l a r g e r , namely .05, and t h e peak i n t e n s i t y i s u s u a l l y g r e a t e r t h a n 10 MW/

cmL. The absorbed h e a t f l u x i n t h e d i r e c t absorp-

t i o n regime i s more t h a n an o r d e r of magnitude

l a r g e r t h a n a t 10.6 pm. T h i s r a p i d h e a t t r a n s f e r can r a i s e t h e t a r g e t s u r f a c e temperature s i g n i f i - c a n t l y d u r i n g t h e p u l s e . To understand t h e cou- p l i n g when no plasmas a r e formed, t h e e f f e c t s of t a r g e t h e a t i n g and mass l o s s must b e c o n s i d e r e d j u s t a s t h e y a r e i n CW i n t e r a c t i o n s .

I t i s n o t known a p r i o r i whether plasma i g n i - t i o n by p u l s e d 3.8 pm r a d i a t i o n i s a s s o c i a t e d w i t h l o c a l i z e d d e f e c t s , a s it i s f o r 10.6 um r a d i a t i o n ,

o r whether it r e s u l t s from breakdown of t h e vapor

produced by bulk e v a p o r a t i o n of t h e s u r f a c e a s it does i n CW i n t e r a c t i o n s . S i n c e i n v e r s e Bremsstrah-

lung a b s o r p t i o n by e l e c t r o n s s c a l e s a s wavelength squared, t h e t h r e s h o l d i n t e n s i t y f o r d e f e c t i n i t i - a t i o n of plasma i s h i g h e r f o r DF l a s e r p u l s e s . HOW-

JOURNAL DE PHYSIQUE

t h e m e t a l under 3.8 Um i r r a d i a t i o n may l e a d t o bulk v a p o r i z a t i o n of t h e t a r g e t d u r i n g t h e p u l s e ; t h e n plasma i n i t i a t i o n can proceed by breakdown of t h e b u l k vapor a s i t does i n CW i n t e r a c t i o n s .

The t h r e s h o l d f o r LSD waves f o r s h o r t p u l s e s of 3.8 u m r a d i a t i o n i s e s t i m a t e d t o be 40-50 MW/

cm

.

_{14'15 The LSC wave plasmas a t 3.8 pm a r e sup-}

p o r t e d by much h i g h e r i n t e n s i t i e s than a t 10.6 pm; r e p r e s e n t s t h e range of v a l u e s observed f o r t h e f i r s t s h o t on a f r e s h aluminum s u r f a c e . The c i r - c l e s w i t h b a r s r e p r e s e n t t h e d a t a range observed f o r t h e e i g h t s h o t on t h e s u r f a c e . The m u l t i p l e p u l s e e f f e c t was i n v e s t i g a t e d a t o n l y t h r e e f l u e n c e s . Plasma i g n i t i o n o c c u r s between a t an average f l u e n c e of 40-50 J/cm2 ( t h e s p a t i a l peak 2 i n t h e f l u e n c e i s about 70 ~ / c m ) . F o r f l u e n c e s

i n consequence t h e plasmas a r e h o t t e r , a t h i g h e r l e s s t h a n 40 ~ / c m ~ , t h e i n t e r a c t i o n proceeds by

2

p r e s s u r e , and t h e r a d i a t i v e t r a n s f e r t o t h e s u r f a c e d i r e c t a b s o r p t i o n . Above 40-45 J/cm

,

d i r e c t ab-

i s a f a c t o r of t e n l a r g e r . _{s o r p t i o n o c c u r s a t t h e beginning of t h e p u l s e , b u t}

A l a y e r of metal vapor may b e c r e a t e d between

t h e plasma and t h e t a r g e t e i t h e r a s a r e s u l t of t h e i g n i t i o n p r o c e s s o r a s a consequence of t h e h i g h h e a t f l u x . The vapor a b s o r b s t h e plasma r a d i a t i o n , t h u s it i n t e r f e r e s w i t h energy t r a n s p o r t t o t h e s u r -

f a c e u n t i l i t is r a i s e d t o high temperatures where

i t a l s o w i l l r a d i a t e . However, m e t a l vapors t e n d t o r a d i a t e p r e f e r e n t i a l l y i n t h e l o n g e r wavelength bands, which reduces t h e thermal coupling t o alumi- num.

Recent data3' on t h e f l u e n c e d e p o s i t e d by pul- s e d 3.8 um l a s e r r a d i a t i o n i n t e r a c t i n g with alumi-

num s u r f a c e s i s shown i n F i g . 14. The shaded a r e a

a

l n c f d e r t Pulse Fluence

i n t h e middle of t h e p u l s e a plasma i s i g n i t e d and

t h e subsequent i n t e r a c t i o n i s mediated by an LSC

wave plasma. The d a t a i n d i c a t e s t h a t t h e r e i s

l i t t l e enhancement a s s o c i a t e d w i t h plasma formation f o r t h e f i r s t s h o t , b u t t h e enhancement i s sub- s t a n t i a l on subsequent s h o t s i n t h e plasma regime. The i n c r e a s e of c o u p l i n g w i t h t h e number of s h o t s i s n o t completely understood, b u t it a p p e a r s t o b e r e l a t e d t o t h e i n c r e a s e . i n t h e s u r f a c e absorp- t i v i t y of t h e t a r g e t which o c c u r s a s t h e r e s u l t of s u r f a c e damage by p r i o r i r r a d i a t i o n s . 32 2 A t h i g h f l u e n c e s , say above 80 J/cm

,

t h e t a r - g e t can v a p o r i z e and t h e vapor l a y e r c u r t a i l s r a d i - a t i v e t r a n s p o r t . C r e a t i o n of l o c a l LSD waves c o u l d a l s o cause t h e r e d u c t i o n i n absorbed f l u e n c e , b u t t h e m u l t i p l e p u l s e enhancement a r g u e s a g a i n s t t h e i n t e r p r e t a t i o n . a l t h o u g h t h e b a s i c i n t e r a c t i o n s which govern 3.8 ).impulsed l a s e r i n t e r a c t i o n s w i t h m e t a l s u r - f a c e s appear t o b e a combination of t h o s e observed in CW i n t e r a c t i o n s and t h o s e found i n p u l s e d 10.6

).im i n t e r a c t i o n s , a d e t a i l e d u n d e r s t a n d i n g of t h e d a t a shown i n Fig. 14 i s s t i l l l a c k i n g .

10.. INTERACTION WITH NON-METALS

The t y p e s of i n t e r a c t i o n s which have been ob-

F i g . 14 Fluence d e p o s i t e d i n A 1 2024 by 3.8 !AI s e r v e d i n m e t a l s can a l s o o c c u r f o r non-metals such

r a d i a t i o n . Data from Ref. 32.

i s t h a t the.non-metals can absorb r a d i a t i o n in- e r a t e d by LSC waves over aluminum t a r g e t s ; t h a t

depth. Simple t h e o r e t i c a l models have been used is, t h e p r e s s u r e i s given by t h e LSC wave p r e d i c -

t o a n a l y z e d a t a from s i n g l e p u l s e experimeilts i n 8 f r e s h s u r f a c e s of f i b e r g l a s s (Cordopreg E-glass)

,

s l i p c a s t f u s e d s i l i c a (SCFS) and pyroceram. F i g u r e 1 5 shows t h e p u l s e d a t a f o r t h e maxi- mum v a l u e o f t h e s u r f a c e p r e s s u r e observed d u r i n g t h e i n t e r a c t i o n of p u l s e d 10.6 u m r a d i a t i o n with n ~ n - m e t a l s . ~ ~ Also shown a r e t h e t h e o r e t i c a l p r e - d i c t i o n s f o r t h e p r e s s u r e , based upon t h e s y n t h e s i s of models d e s c r i b e d below. The t h e o r e t i c a l p r e d i c - t i o n s a r e i n good agreement w i t h t h e d a t a , t h u s l e n d i n g credence t o t h e phenomenology u n d e r l y i n g t h e p r e d i c t i o n s .

t i o n s below 4 NW/cmL, by LSD wave p r e d i c t i o n s above 2

8 MW/cm

,

and by a t r a n s i t i o n from LSC wave v a l u e s

2

t o LSD wave v a l u e s between 4 MW/cm2 and 8 MW/cm

.

The p r e s s u r e d a t a f o r i n t e n s i t i e s above 2 m/cm 2 a r e shown i n Fig. 1 5 and they a r e c o n s i s t e n t w i t h t h e t h e o r e t i c a l p r e d i c t i o n s .

2

A t l a s e r i n t e n s i t i e s below 2 MW/cm

,

t h e s u r - f a c e i n t e r a c t i o n of t h e v i r g i n t a r g e t i s dominated by d i r e c t a b s o r p t i o n of t h e l a s e r by t h e t a r g e t

with an in-depth a b s o r p t i o n l e n g t h o f 6 um. Pres-

s u r e i s g e n e r a t e d a s t h e r e s u l t of v a p o r i z a t i o n of t h e g l a s s f i b e r s . The time r e s o l v e d s u r f a c e p r e s s u r e t r a c e s i n d i c a t e t h a t t h e p r e s s u r e b u i l d s

pato ( ~ o l m & I I 1 I

'

l ' l ' l I I I L

_-

up slowly, r a t h e r t h a n promptly a s would b e expect-

A A ASCFS

-

o++PYROCERANI - ed i f a plasma were i g n i t e d . 3 3 The maximum s u r -

0 @

e

CORD0 PREG

D

cr.

E-c (Ass f a c e p r e s s u r e d a t a , shown i n F i g . 1 5 , show an ab-

r u p t f a l l - o f f a s a f u n c t i o n of f l u e n c e . T h i s be-

-

h a v i o r i s i n c o n s i s t e n t w i t h v a p o r i z a t i o n induced by s u r f a c e a b s o r p t i o n of t h e l a s e r , b u t it i s i n good agreement w i t h v a p o r i z a t i o n models based on No Plasma

-

in-depth a b s o r p t i o n with an a b s o r p t i o n l e n g t h of 2

-

_{/ I - Air Plasma 6 Um. A s t h e l a s e r p u l s e f l u e n c e i s i n c r e a s e d be-}

-

_,'

/' Theory

1 1 l l d l ' l 1

'

1 ' 1 ' 1 1 1 1 1 - yond t h e t h r e s h o l d v a l u e s f o r p r e s s u r e g e n e r a t i o n ,

0.1 0.2 0.4 0 . 6 . 8 1 2 4 6 8 1 0

F i g . 1 5 S u r f a c e s p r e s s u r e f o r 10.6

vm

r a d i a t i o n

on non-metals. Data from Ref. 33.

2

A t l a s e r i n t e n s i t i e s above 2 MW/cm

,

t h e s u r -

f a c e i n t e r a c t i o n i s dominated by t h e prompt forma-

t i o n of an a i r plasma above t h e s u r f a c e , and t h e s u r f a c e p r e s s u r e and r a d i a t i v e t r a n s f e r t o t h e s u r - f a c e can b e determined from t h e LSC wave model de- s c r i b e d i n S e c t i o n 6. 23

C9-72 JOURNAL DE PHYSIQUE

2

i s l i m i t e d t o l e s s than 7 ~ / c m

.

However, t h e same

m u l t i p l e p u l s e experiments i n d i c a t e t h a t t h e plasma t h r e s h o l d f o r a p r e v i o u s l y i r r a d i a t e d t a r g e t may

2 d e c r e a s e t o below I MW/cm

.

11. SUMMARY

The thermal and mechanical coupling of l a s e r beams t o m a t e r i a l s v a r i e s s t r o n g l x a s t h e i n t e r a c - t i o n phenomenology changes. For d i r e c t a b s o r p t i o n , t h e i n s t a n t a n e o u s thermal coupling i s given by t h e t a r g e t s u r f a c e a b s o r p t i v i t y , b u t t h e t o t a l thermal coupling i n c l u d e s e f f e c t s from t a r g e t h e a t i n g and mass l o s s . Whenever plasmas a r e c r e a t e d , e i t h e r by bulk vapor breakdown o r by d e f e c t induced break- down, t h e l a s e r energy i s absorbed i n t h e plasma, and mechanical and thermal coupling i s determined by t h e plasma p r o p e r t i e s . For LSD wave plasmas, thermal coupling i s dominated by plasma r a d i a l ex- pansion and enhanced t o t a l thermal coupling can oc- c u r ; f o r LSC wave plasmas, r a d i a t i v e t r a n s p o r t dom- i n a t e s t h e t r a n s p o r t and enhanced l o c a l coupling i s observed. Mechanical coupling r e s u l t s from v a p o r i z a t i o n i n t h e d i r e c t a b s o r p t i o n regime, and from t h e high p r e s s u r e plasma i n t h e plasma medi- a t e d coupling regime. The plasma phenomena which a r e observed f o r CW i n t e r a c t i o n s and p u l s e d 10.6 pm i n t e r a c t i o n s , a l s o occur f o r o b l i q u e a n g l e s of i n c i d e n c e , f o r p u l s e d 3.8 u m l a s e r r a d i a t i o n , and f o r t h e i n t e r a c t i o n with non-metal m a t e r i a l s , how- e v e r , t h e l a s e r parameters which d e l i n e a t e t h e in- t e r a c t i o n regimes, a s w e l l a s t h e magnitude of t h e e f f e c t s , a r e d i f f e r e n t .

11. RESUME

Le couplage thermique e t m6canique d'un f a i s c e a u l a s e r avec d e s matgriaux v a r i e en f o n c t i o n de l ' i n t g r a c t i o n q u i a l i e u . S i l ' a b s o r p t i o n e s t d i r e c t e , l a couplage thermique i n s t a n t a n g e s t dgterming p a r l ' a b s o r p t i v i t g de l a s u r f a c e mais l e couplage i n t g g r 6 comprend l e s e f f e t s de chauffage

de l a c i b l e e t de p e r t e de masse. Quand il y a c r g a t i o n d'un plasma, p a r claquage s o i t dans l e g r o s de l a vapeur s o i t i n i t i g p a r d e s d 6 f a u t s de s u r f a c e , l ' g n e r g i e e s t absorbge p a r l e plasma e t l e s couplages thermique e t m6canique s o n t d e t e r - mings p a r l e s p r o p r i g t g s de l a vapeur. Quand il

y a formatioii d'un onde de' d g t o n a t i o n (LSD), l e couplage thermique e s t doming p a r l ' e x p a n s i o n r a d i a l e du plasma: il p e u t y a v o i r augmentation du couplage thermique g l o b a l . Quand il y a f o r - mation d'une onde de combustion (LSC), l e t r a n s - p o r t thermique e s t p r i n c i p a l e m e n t p a r r a d i a t i o n :

on observe une augmentation du couplage

3.

Le couplage mgcanique p r o v i e n t de l a v a p o r i s a t i o n de

l a s u r f a c e dans l e rggime

2

a b s o r p t i o n d i r e c t e e t d e l a p r e s s i o n 61eveB du plasma dans l e rggime

2

couplage i n d i r e c t e . Les msmes phgnomsnes que l ' o n abserve dans l e s i n t g r a c t i o n s avec l a s e r s c o n t i n u s e t l a s e r s C02

2

impulsion on l i e u a u s s i dans l e c a s d ' i n c i d e n c e o b l i q u e , dans l e c a s d ' u n l a s e r impulsionnel

2

3.8 pm ou dans l e c a s de matgriaux non mgtalliques: l e s param&tres du l a s e r q u i l i m i t e n t l e s d i f f g r e n t s rggimes d 1 i n t 6 r a c t i o n e t l ' o r d r e d e grandeur d e s e f f e t s s o n t cependant d i f f g r e n t s .

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