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LASER INTERACTION : THERMAL ANDMECHANICAL COUPLING TO TARGETS
R. Root
To cite this version:R. Root. LASER INTERACTION : THERMAL AND MECHANICAL COUPLING TO TARGETS.Journal de Physique Colloques, 1980, 41 (C9), pp.C9-59-C9-73. �10.1051/jphyscol:1980909�. �jpa-00220563�
JOURNAL DE PHYSIQUE CoZZoque C9, supptdment au n022, Tome 41, novernbre 2980, page ~ 9 - 5 9
LASER INTERACTION : THERMAL AND MECHANICAL COUPLING TO TARGETS
R.G. Root
PhysicaZ Sciences Inc. Wobm, MA 01801 U.S.A.
Abstract.- The physical phenomena which inf luences thermal and mechanical coupling of in f ra red l a s e r -- r a d i a t i o n t o mate r ia 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 considered, but the interac- t i o n of pulsed l a s e r s with metals i n an a i r environment i s emphasized. Selected 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 included.
1. INTRODUCTION
When a pulsed l a s e r beam i r r a d i a t e s a surface,
t h e f r a c t i o n of t h e incident energy coupled l o c a l l y
i n t o t h e t a r g e t and the impulse imparted t o t h e
t a r g e t vary s t rongly according t o which physical
phenomena dominate t h e i n t e r a c t i o n s with t h e t a r -
ge t . For example, a t low l a s e r i n t e n s i t i e s the
thermal energy deposited i n a metal surface is con-
t r o l l e d by t h e i n t r i n s i c absorp t iv i ty of t h e metal.
However, a t higher i n t e n s i t i e s , where an a i r plasma
i s ign i ted , the f r a c t i o n of t h e l a s e r energy t rans -
f e r r e d t o t h e surface l o c a l l y can be dramatical ly
increased.''' This e f f e c t i s c a l l e d the enhanced
thermal coupling. A t even higher i n t e n s i t i e s , t h e
l o c a l coupling decreases 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 deposited i n a mate r ia 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 parameters - in tens i ty , pu lse
time, and spot s ize . The purpose of t h i s paper
i s t o review the physical phenomena which inf luence
t h e thermal and mechanical coupling of l a s e r rad i -
a t i o n t o mater ials .
The approach which is followed is: (1) t o
choose a few important in te rac t ions , (2) t o de-
sc r ibe 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 -
por tan t e f f e c t with se lec ted 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
mate r ia l s , ambient condit ions and l a s e r pulse in-
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 is l imi ted t o in f ra red 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 behw 10 ~ / c m .
Metals a r e t h e primary mate r ia l s considered, but
se lec ted non-met.als a r e included i n some i n t e r -
ac t ion regimes. The i n t e r a c t i o n s general ly occur
i n a i r a t s tandard condit ions, except f o r a few
examples of vacuum in te rac t ions . Analysis of t h e
mater ial response i s l imi ted t o the changes which
occur during t h e l a s e r pulse time which a f f e c t t h e
l ase r /mate r ia l surface in te rac t ion . The separat ion
of CW i n t e r a c t i o n s from pulsed i n t e r a c t i o n s is 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 -
ac t ion is 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 ; conversely, a pulsed l a s e r in te r -
a c t i o n depends no t only on average i n t e n s i t y b u t
a l s o on temporal var ia t ions .
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 rad ia t ion i n t e r -
a c t s with a mate r ia l by d i r e c t absorption; t h e frac-
t i o n of energy coupled t o t h e surface (hereaf te r
c a l l e d t h e thermal coupling c o e f f i c i e n t ) is given
by t h e i n t r i n s i c absorp t iv i ty of t h e mate r ia l . I n
t h i s coupling regime, t h e thermal coupling coef f i -
c i e n t depends only on t h e l a s e r wavelength and t h e
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980909
C9-60 JOURNAL DE PHYSIQUE
t a r g e t mate r ia l ; l a s e r parameters such a s i n t e n s i t y ,
f luence and spot 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 with any
s e t of l a s e r parameters, provided they f a l l within
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 increased,
new phenomena occur, such a s t a r g e t h e a t i n g , ' t a r g e t
mass removal, vaporizat ion and plasma formation,
which modify the coupling and introduce a depend-
ence on l a s e r parameters. If the i n t r i n s i c absorp-
t i v i t y is a funct ion of t h e sur face temperature,
t i o n , melt removal and pyro lys i s . Even when t h e
mass removal mechanisms do no t a f f e c t t h e l o c a l
instantaneous coupling, they may still a f f e c t da ta
i n t e r p r e t a t i o n s . Experimental measurements of t h e
fluerice required t o vaporize t h i n t i tanium f o i l s
showed a sharp increase i n f luence a s t h e l a s e r
i n t e n s i t y increased; t h i s was i n t e r p r e t e d a s a t ran-
s i t i o n from a regime i n which melt removal dominated
t h e mass removal process b u t was accompanied by
vaporizat ion of melted d r o p l e t s t o a regime i n which
complete vaporizat ion occurred. 3
t h e instantaneous l o c a l absorbed energy f lux is The most dramatic a l t e r a t i o n of t h e thermal
s t i l l represented by t h e product of the i n t r i n s i c coupling occurs when a plasma is c rea ted over a
absorp t iv i ty of t h e surface a t t h e l o c a l tempera-
t u r e and t h e instantaneous l o c a l inc iden t i n t e n s i t y ,
but the 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
in te rac t ion area. Thus, the thermal coupling co-
e f f i c i e n t i n general depends on a l l the l a s e r para-
meters, and experimental da ta can be understood
only by solving 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 with t a r g e t tem-
perature. This e f f e c t occurs, f o r example, i n alu-
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 vaporizing t h i n aluminum
f o i l s with 10.6 pm l a s e r rad ia t ion , whereas t h e
i n t r i n s i c room temperature absorp t iv i ty i s only .03.
Theoret ical ca lcu la t ions4 p r e d i c t an e f f e c t i v e cou-
p l ing of -11 which is in reasonable agreement with
t h e da ta .
I f t h e t a r g e t reaches high enough temperature
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 mater ial must be properly accounted
f o r i n order .to determine the absorbed energy from
experimental data 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 mater ial . Mass can be re-
moved by several processes; f o r example, vaporiza-
surface. The dynamics of a laser-produced plasma
above a sur face w i l l depend upon t h e i n t e n s i t y of
t h e inc iden t l a s e r pu lse and t h e pu lse durat ion.
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 than t h e
plasma threshold i n t e n s i t y , a laser-supported com-
bust ion (LSC) wave5 is usua l ly ign i ted . 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 10 w/cm2 - 6 2
10 W/cm f o r 10.6 pm r a d i a t i o n with both pulsed
and CW l a s e r beams. The i g n i t i o n of an LSC wave
i n i t i a l l y t a k e s place i n t h e t a r g e t ~ a ~ o r . ~ * ~ The
heated t a r g e t vapor subsequently t r a n s f e r s i ts
energy t o t h e surrounding a i r . Once t h e a i r begins
t o absorb 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 propagates i n t o t h e a i r along t h e
beam path.
The nature of t h e coupling i n t h e plasma re -
gime depends on i n t e n s i t y , spo t s i z e , pu lse time,
ambient a i r pressure and t h e t a r g e t mater ial . For
long pu lse times and low i n t e n s i t y t h e c rea t ion
of a LSC wave a t 10.6 Um usua l ly r e s u l t s i n cur-
t a i l i n g t h e thermal coupling a s the LSC wave pro-
pagates toward t h e l a s e r f lux . Thermal coupling
f o r sho t pu lse t imes and high inc iden t i n t e n s i t y
is t r e a t e d a s a pulsed in te rac t ion .
I t remains to'determine t h e time a t which va-
por plasma ign i t ion occurs. The observed i g n i t i o n Experiments a t 5.0 pm ind ica te t h a t t h e vapor
time is t h e sum of t h e time required t o produce t h e
vapor and t h e time required t o breakdown t h e vapor.
The vapor production time is determined from t h e
t a r g e t thermal q s p o n s e t o the d i r e c t absorpt ion
of l a s e r rad ia t ion . The vapor breakdown time is
determined from t h e heat ing of t h e vapor by inverse
Bremsstrahlung absorpt ion of l a s e r rad ia t ion . The
dependence of the breakdown time on l a s e r i n t e n s i t y
and spo t s i z e is i l l u s t r a t e d i n Fig. 1 which com-
pares t h e t h e o r e t i c a l p red ic t ions of P i r r i 6 t o t h e
experimental data of Klosterman. Theoret ical pre-
d i c t i o n s of breakdown times a r e shown f o r 1-D
planar vapor dynamics, and two-dimensional (axisym-
metr ic) vapor dynamics f o r a l a s e r spo t rad ius 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 spot s ize . Thus, a t low
in tens i ty , where the spot , r ad ius i n 1 cm, t h e da ta
agrees with t h e one-dimensional p red ic t ion , whereas
a t higher i n t e n s i t i e s , a s t h e spo t s i z e shr inks t o
0.25 cm and t h e vapor dynamics becomes two-dimen-
s iona l , t h e data tends towards the 2-D predict ion.
A ALUMINUM DATA (KLOSTERMAN) FOR 0.25 5 r s I 1 CM
- THEORY (PIRRI)
- - - -
NO IGNITION -
SUPERSONIC -
Fig. 1 Time t o i g n i t e laser-supported 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 inverse Bremsstrahlung absorption. 3,6
3. CW INTERACTIONS - MECHANICAL COUPLING
Bulk vaporizat ion of t a r g e t mate r ia l generates
sur face pressure and impulse. The vapor pressure
on t h e surface depends on t h e ambient pressure, t h e
l a s e r i n t e n s i t y I ( t ) , l a s e r spo t rad ius 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 igh in-
t e n s i t y , where t h e background pressure i s i r r e l e -
vant , o r i n vacuum, o r f o r t imes s h o r t enough f o r
p lanar vapor dynamics t o be v a l i d t h e surface pres-
sure p can be adequately with
a n a l y t i c models. Even when a steady s t a t e pressure
regime is achieved, .'the observed coupling may show
a time dependence. Calculat ions of t h e ins tan ta -
neous mechanical coupling, p ( t ) /I ( t ) and t h e in te -
g ra ted coupling c o e f f i c i e n t C ( t ) , defined a s
a r e shown i n Fig. 2 a s a funct ion of time. Vapor-
i z a t i o n begins a t time Tv. These calculat ionS 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 radiation. ' It takes t e n times a s long
t o approach t h e steady s t a t e pressure a s it does
t o i n i t i a t e vaporizat ion; and t h e integrated ' cou-
p l ing c o e f f i c i e n t increases even more slowly.
cornon Pnenallc I - 106 w/d o - 0.81
- lnstonto eous caualinp caefflclen Intenrot d counllng coefflclent
IYU ond Nebolrlne)
Fig. 2 Mechanical coupling c o e f f i c i e n t f o r carbon phenolic i r r a d i a t e d by 10.6 pm rad ia t ion , from Ref. 9.
JOURNAL DE PHYSIQUE
J u s t a s i g n i t i o n of a vapor plasma modifies
thermal coupling t o a surface, 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 surface i n t e r a c t i o n proceeds v i a LSC waves
10 o r laser-supported detonat ion (LSD) waves ; they
a r e discussed under pulsed coupling in te rac t ions .
I n a vacuum, however, t h e plasma is confined e n t i r e -
l y t o the vapor. The mechanical coupling of l a s e r s
t o sur faces through vapor plasma can be modelled
by r e l a t i n g t h e th ickness of t h e plasma t o t h e ab-
sorpt ion 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 steady s t a t e coupling c o e f f i c i e n t f o r
a vacuum plasma decreases with i n t e n s i t y , whereas
t h e steady s t a t e coupling from vaporizat ion in-
c reases with 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
pulsed 10.6 v m pu lse with surfaces depends not only
on t h e average i n t e n s i t y of t h e pulse, b u t a l s o on
t h e temporal var ia t ions . A t y p i c a l temporal pu lse
shape is sketched i n Fig. 3. A t t h e leading edge
of t h e pulse there is a gain switched spike follow-
ed by a lower i n t e n s i t y t a i l . The spike l a s t s only
100-400 n s bu t i ts peak i n t e n s i t y i s usua l ly two
t o e i g h t times l a r g e r than 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 the
energy, t y p i c a l l y h a s a durat ion of 3-40 ps.
Time -
A i r plasmas can be c rea ted above sur faces by
t h e spike. This process is c a l l e d prompt i g n i t i o n
t o d i s t inguish it from the breakdown of t h e products
of bulk vaporizat ion which was discussed e a r l i e r .
Bulk vapor breakdown cannot occur u n t i l enough time
has elapsed f o r the surface t o reach t h e vaporiza-
t i o n temperature., The spike contains i n s u f f i c i e n t
energy t o cause bulk vaporizat ion, ins tead , i g n i t i o n
takes place rap id ly from loca l ized + f e c t s which
vaporize and break down. 12113 The spike, then, con-
t r o l s whether o r no t an a i r plasma is formed. The
threshold f o r prompt i g n i t i o n from aluminum is es-
2 t imated t o be a spike fluence12 of 1.7 J/cm , and
2 a spike in tens i ty14 of 10-30 MW/cm . Typical high
energy pu lses meet these requirements when the aver-
2 age i n t e n s i t y i n t h e t a i l is approximately 1 MW/cm .
I f no plasma is i g n i t e d during t h e spike, the
t a i l of t h e pulse is absorbed d i r e c t l y by t h e sur-
face , and t h e thermal coupling c o e f f i c i e n t is given
by t h e i n t r i n s i c absorp t iv i ty . I f a plasma is i n i -
t i a t e d , t h e subsequent in te rac t ion proceeds v i a LSC
wave f o r low average i n t e n s i t y ( l e s s than 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 process and t h e pre-
sence of t h e t a r g e t surface, a precursor shock pre-
cedes t h e LSC wave except a t t h e lowest i n t e n s i t i e s .
When t h e LSC wave has propagated f a r enough f o r two-
dimensional e f f e c t s t o dominate the plasma flow i n
t h e v i c i n i t y of t h e surface, t h e plasma configura-
t i o n resembles t h a t shown schematically i n Fig. 4a.
The LSC wave propagating i n t o t h e a i r behind t h e
precursor shock induces a flow towqrd t h e t a r g e t .
Close t o t h e surface t h e flow resembles a stagna-
t i o n po in t flow. A s tagnat ion po in t boundary layer
a n a l y s i s must be matched t o a c o r r e c t model f o r t h e
LSC wave propagating away from t h e surface i n order
Fig. 3 sketch of 10.6 u m temporal pulse shape. t o ob ta in t h e temperature and pressure d i s t r i b u t i o n
i n the plasma, t h e r a d i a t i v e t ranspor t t o t h e t a r -
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 rad ia t ion , a laser-supported
detonation (LSD) wave i s igni ted. A s impl i f i ed
model of t h e plasma configurat ion r e s u l t i n g from
LSD wave i g n i t i o n is shown i n Fig. 4b. The l a s e r
beam absorption takes place i n a t h i n zone of h o t ,
high pressur a i r behind t h e detonat ion wave. Since
t h e detonation wave drags a i r away from t h e surface,
expansion fans form t o s a t i s f y the boundary condi-
t i o n s of zero p a r t i c l e ve loc i ty a t t h e t a r g e t sur-
face . One-dimensional gas dynamics can be matched
t o detonation and planar b l a s t wave theory t o de-
s c r i b e t h i s aspec t of t h e flow f i e l d , and cy l indr i -
c a l blast-wave theory 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 absorption wave plasma dynamics (a) LSC wave, (b) LSD wave. From Ref. 19.
boundary layer forms on the sur face ; it resembles
t h e boundary layer behind a propagating shock wave
a s t h e c y l i n d r i c a l b l a s t wave spreads out over the
t a r g e t . Energy is t rans fe r red from t h e plasma t o
the t a r g e t through t h i s boundary layer by rad ia t ion
and conduction.
5. LSD WAVE COUPLING: RADIAL EXPANSION EFFECTS
I n one-dimension t h e thermodynamic proper t i es
behind a t r u e LSD wave i s pred ic ted by t h e Raizer
theory.10 The a n a l y s i s of p i r r i 1 6 gives t h e condi-
t i o n s above t h e surface and t h e time h i s t o r y of the
plasma proper t ies a s t h e LSD wave propagates away
from t h e surface. A t 10.6 pm f o r an i n t e n s i t y of
2 20 MW/cm , the temperature and pressure above the
surface a r e predicted t o be 9000°K and 53 atm.,
respect ively. Under these condit ions t h e r a t i o of
the energy t rans fe r red from the plasma t o t h e t a r -
g e t by rad ia t ion and conduction t o t h e l a s e r ener-
gy w i l l be l e s s than 1% f o r l a s e r pulse times of
t h e order of t e n s of microseconds. 14
I n o rder t o c a l c u l a t e t h e t o t a l coupling coef-
f i c i e n t , plasma spreading must be included. The
plasma remains approximately one-dimensional u n t i l
t h e expansion fans from t h e edge of the spo t reach
t o a x i s of symmetry. This time is approximated by
t h e beam rad ius divided by t h e speed of sound i n
the plasma. The pressure decay can be approximated
16 by cy l indr ica l b l a s t wave theory f o r time s c a l e s
g r e a t e r than two-dimensional time scale . I n t h e
v i c i n i t y of the surface t h e r e is no l a s e r absorp-
t i o n ; t h e plasma proper t ies a r e determined from
i sen t rop ic expansion r e l a t i o n s . The energy t rans-
f e r a t any i n s t a n t of time is t h e sum of t h e boun-
dary layer hea t t r a n s f e r and rad ia t ion contribu-
t ions .
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 15 MW/cm , a spot
rad ius of 1 cm and a pulse time of 1 ps ind ica te
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 is minimal, (2)
boundary l a y e r energy t r a n s f e r dominates, a s the
plasma spreads ou t over t h e t a r g e t , f o r times up
t o 1000 psec, ( 3 ) f o r t h e 1 psec pulse 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 is
a r e s u l t of energy spread o u t over a l a r g e a rea com-
pared t o t h e spo t a rea , and (4) a s t h e l a s e r inten-
s i t y i s increased, the t o t a l coupling c o e f f i c i e n t
tends t o remain constant , b u t t h e energy is spread
out over a l a r g e r agea. Therefore, f o r s h o r t l a s e r
pulses the t o t a l coupling c o e f f i c i e n t is s i g n i f i -
can 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
the t a r g e t is no t g r e a t e r than would be obtained
i f no plasma was formed. F ina l ly , f o r t h i s calcu-
l a t i o n the coupling c o e f f i c i e n t var ied inversely
with pulse time.
The e f f e c t of plasma spreading on energy t rans -
f e r i n the LSD regime has been experimentally ob-
served, 17'18 and t h e thermal coupling is indepen-
dent of i n t e n s i t y f o r l a r g e targets .17 Total ther-
mal coupling of a pulsed 10.6 pm l a s e r t o nickel
a s measured by Hall e t a1.,18 is shown i n Fig. 5.
The nominal l a s e r pulse time is 6 psec. The t o t a l
coupling increases dramatical ly a f t e r a plasma is
ign i ted , bu t t h e coupling decreases a t higher inc i -
dent fluence 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
Fig. 5 Thermal coupling of 10.6 pm r a d i a t i o n t o .: nickel . 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 surfaces. It
has been shown t h a t enhanced thermal coupling t o
aluminum is a r e s u l t of energy t rans fe r red by rad i -
a t i o n from t h e ho t , high pressure, laser-supported
plasma adjacent t o t h e t a r g e t . 5,19,20,21 A sketch
of t h e one-dimensional LSC wave plasma configura-
t i o n is shown i n Fig. 6. The l a s e r is inc iden t
from t h e r i g h t hand s ide. The gain switched spike
i g n i t e s an a i r plasma next t o t h e t a r g e t surface,
and t h e expansion of t h e plasma d r i v e s a precursor
shock i n t o t h e a i r . The l a s e r absorpt ion zone pro-
pagates i n t o t h e shocked a i r ; t h e absorpt ion occurs
e s s e n t i a l l y a t constant pressure and t h e propaga-
t i o n of t h e absorpt ion zone is cont ro l led by con-
duction and r a d i a t i v e t r a n s p o r t from t h e ho t plasma.
0 .
I I N1 Doto 1Holl et 01.) d -10 .6prn 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
{;:::ctlQnl --• 0 Direct Absorntlon-----* -
Fig. 6 One-dimensional LSC wave configurat ion.
t-
The LSC wave propagates 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 is
used t o h e a t t h e plasma t o a temperature of approx-
imately 2000OoK. This ho t plasma is capable of 0
r a d i a t i n g i n s p e c t r a l regions l e s s than 1250 A
which a r e wel l absorbed by the aluminum t a r -
ge t s . 20'21 The expansion of t h e a i r a s it is heat-
ed a c t s a s a p i s t o n which maintains t h e precursor
shock.
E f f i c i e n t l o c a l thermal coupling t o t h e sur-
face requ i res t h r e e c r i t e r i a t o be met: (1) a
plasma must be ign i ted adjacent t o t h e surface; (2)
t h e plasma must be an LSC wave, and (3) t h e one-
dimensional configurat ion i l l u s t r a t e d i n Fig. 6 must
be maintained throughout t h e pulse. These require-
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
parameters corresponding t o t h e enhanced coupling
region. The e?hanced coupling region is i l l u s t r a t -
ed i n Fig. 7. The coordinates 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 the la,ser pu lse
time T normalized by t h e time, TZD, a t which r a d i a l P
expansion of t h e high pressure plasma a f f e c t s t h e
cen te r of t h e l a s e r spot. (r2D is defined a s R/a P
where R is t h e rad ius of the l a s e r spo t and a is P
Fig. 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 begins, t h e dy-
namics of t h e plasma no longer maintains t h e one-
dimensional character i l l u s t r a t e d i n Fig. 6; t h e
pressure drops and so does t h e plasma temperature.
A s a r e s u l t , t h e e f fec t iveness of t h e plasma i n
t ranspor t ing energy v i a rad ia t ion is rap id ly dimi-
nished. Thus, t h e region of ? > 1 is el iminated
from the enhanced coupling region 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 before
t h e l a s e r pu lse is term-inated. The high i n t e n s i t y
region is el iminated because, above t h e LSD wave
2 t r a n s i t i o n threqhold of 8 W c m , an LSD wave i s
produced which h a s poor coupling. The low inten-
s i t y region, below t h e plasma i g n i t i o n threshold
of 1 MW/cm2, is el iminated because t h e gain switch
spike corresponding t o t h i s l a s e r i n t e n s i t y is not
s t rong enough t o c r e a t e a plasma over aluminum. The
remaining a rea is t h e enhanced coupling regime.
The 1-D plasma configurat ion shown i n Fig. 6
has been s tud ied 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 con t r ibu t ions is presented
i n Refs. 14 and 15. To p r e d i c t thermal coupling
the contr ibut ion of t h e r a d i a l l y expanding plasma
shown i n Fig. 4a must be included. To make quanti-
t a t i v e p red ic t ions , a model was synthesized 19,22
i n which t h e e a r l y time plasma cynamics was de-
scr ibed by t h e one-dimensional configurat ion of Fig.
6 and t h e l a t e time plasma dynamics was represented
by b l a s t wave decay laws of t h e appropriate geomet-
ry. The boundary between e a r l y time and l a t e time
was determined t o be t h e smaller of r and r P 2D'
Pred ic t ions made with t h i s model agreed with t h e
data within 30% over most of t h e range of i n t e r e s t .
A complementary approach which was an a x i a l l y sym-
metr ic numerical simulation has 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 pr$dic-
t i o n s and experimental data of Rudder and Augus-
t ~ n i , ~ ~ and McKay e t a l . ,' a r e shown i n Fig. 8.
Fig. 8 Central thermal coupling t o aluminum by 10.6 pm l a s e r rad ia t ion . Comparison of d a t a and theory.
JOURNAL DE PHYSIQUE
Since the experiments involve a range of in tens i -
t i e s , t h e t h e o r e t i c a l curves were ca lcu la ted f o r
two l i m i t i n g i n t e n s i t i e s . The da ta is general ly
i n agreement with t h e theory; i n p a r t i c u l a r t h e
p red ic ted decrease i n t h e l o c a l thermal coupling
with increasing 4 is observed. A more d e t a i l e d
comparison of thermal coupling da ta and theory i s
discussed l a t e r f o r oblique angles 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 no t only on t h e r a d i a t i v e proper t i es
of t h e plasma, b u t a l s o on t h e s p e c t r a l absorption
c h a r a c t e r i s t i c s of the t a r g e t . The coupling of
metal surfaces o ther than aluminum i s determined
by using t h e appropriate s p e c t r a l absorp t iv i ty .
LSC wave plasmas tend t o r a d i a t e s t rongly i n 0
t h e s p e c t r a l region 1 < 1250 A. For shor t pulse
times and small spots , and a t low i n t e n s i t y most
of t h e rad ia t ion is emit ted a t wavelengths l e s s 0
than 1250 A, which a l l t h e metals absorb well.
There is l i t t l e d i f fe rence between t h e f luence ab-
sorbed by various metals f o r these l a s e r parameters.
~t longer pu lse times, o r higher in tens i ty , the
r a d i a t i o n i n t h e s h o r t wavelength regime becomes
sa tura ted and it is cont ro l led by t h e plasma tem-
pera ture c l o s e t o t h e t a r g e t , whereas t h e radi-
D
a t i o n in t h e band A > 1250 A, which is transparent ,
is cont inual ly increasing. Even with aluminum,
which absorbs long wavelenghs poorly, t h e long wave-
i n terms of t h e s p e c t r a l absorp t iv i ty . 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 than aluminum, a r e pre-
d ic ted t o absorb plasma rad ia t ion more s t rongly than
aluminum. For a l l o y s , t h e s t ronges t absorber i s
t i tanium, 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 high
pressure plasma over t h e t a r g e t surface. The re -
s u l t i n g impulse del ivered t o t h e surface rece ives
a con t r ibu t ion from both t h e e a r l y time p lanar waves
( i l l u s t r a t e d i n Fig. 6) and t h e l a t e time two-di-
mensional waves, shown i n Fig. 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
using a n a l y t i c methods. More extensive modelling,
including numerical s imualt ions, 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 ca lcu la ted based upon t h e pressure
sca l ing laws used i n t h e thermal coupling calcula-
t ions . 19
A convenient method of present ing t h e r e s u l t s
is t o give t h e r a t i o of t h e p red ic ted impulse over
a given a r e a divided by t h e impulse determined from
t h e p lanar surface pressure, 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 pu lse 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 Fig. 9 a s a funct ion of ?. Calcula-
t i o n s a r e made f o r one-dimensional plasma pressures
l eng th band makes an inpor tan t con t r ibu t ion 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
high in tens i ty , long pulse t imes and l a r g e spots . observed experimentally f o r LSC waves, and t h e a rea
I n t h i s regime t h e coupling t o metals o ther than
aluminum, which absorb t h e long wavelength band
b e t t e r , i s enhanced r e l a t i v e t o t h e coupling t o
A 1 2024.
Theoret ical p red ic t ions of t h e f luence absorb-
ed by var ious a l loys22 i n d i c a t e t h a t t h e metalis
can be arranged i n t o a hierarchy based upon ab-
sorbed f luence; t h e hierarchy can be understood
between them is shaded f o r a l l b u t t h e i n f i n i t e
plane ca lcu la t ions . The four ca lcu la t ions are:
(1) c e n t r a l coupling which uses t h e p ressure pre-
d i c t e d a t t h e cen te r of t h e spot , (2) coupling over
t h e l a s e r spo t which includes t h e s p a t i a l v a r i a t i o n
i n pressure over t h e spot , (3) coupling over t h e
l a s e r spo t p l u s shock annulus which includes t h e
contr ibut ion from t h e a rea covered by t h e expand-
ing r a d i a l shock before t h e end of the pulse, and a t e a r l y time; there fore , t h e plasma dynamics is
(4 ) coupling t o a i n f i n i t e plane which includes one-dimensional perpendicular t o t h e t a r g e t . The
t h e t o t a l impulse de l ivered by t h e expanding plasma plasma configurat ion is i d e n t i c a l t o Fig. 6 - a
a f t e r t h e pulse terminates. precursor shock followed by a l a s e r absorpt ion zone - - except t h a t t h e l a s e r beam is inc iden t a t a d i f -
f e r e n t angle. The e a r l y time behavior can still
be modelled a s a p lanar LSC wave i f t h e l a s e r inten-
s i t y I is changed t o t h e p ro jec ted i n t e n s i t y I cos
8, and t h e l a s e r absorpt ion c o e f f i c i e n t kL, is re-
TARGET
at Normal Incidence
Fig. 9 Pred ic t ions of mechanical coupling by LSC Fig. 10 Angle of incidence geometry. (a) Cross- wave plasmas. From Ref. 26. sec t iona l view, (b) t a r g e t plane view.
The l o c a l coupling t o t h e spo t o r t h e cen te r placed by t h e c o e f f i c i e n t k L ,/cos 0 appropriate f o r
of the spot is enhanced a t low but is diminished describing l a s e r beam a t tenua t ion perpendicuiar
a t l a r g e ?; however, the t o t a l coupling is always t o the t a r g e t .
enhanced. The onset of l a t e r a l expansion a t t h e cen te r
8. EXTENSION ANGLE OF INCIDENT AND AMBIENT PRESSURE
The i n t e r a c t i o n of pulsed 10.6 pm l a s e r pu lses
with aluminum surfaces, which a r e a t on oblique
angle t o t h e l a s e r beam, h a s been s tud ied both
t h e o r e t i c a l l y 2 2 ~ 2 7 and experimentally. 28 The goe-
metry of the in te rac t ion a t oblique angles of in-
cidence is sketched in Fig. 10. The e a r l y time
configurat ion, a s viewed from a plane defined by
t h e inc iden 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 Fig. 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 spo t is 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 generated a t t h e
edge of t h e t a r g e t spo t t o reach the' cen te r and
t h e time f o r t h e laser-supported absorpt ion wave
a t t h e cen te r of t h e t a r g e t t o propagate v e r t i c a l l y
[ i n Fig. 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 plane is shown
i n Fig. lob. 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 along t h e semi-minor a x i s i s R/a = T p 2 ~ '
where a is t h e speed of sound i n t h e plasma; t h i s P
i s t h e same time s c a l e t h a t c o n t r o l s t h e r a d i a l
C9-68 JOURNAL DE PHYSIQUE
expansion f o r normal incidence. Only a t a l a t e r naively sca l ing t h e f luence absorbed f o r 8 = O 0
t ime, character ized by T3D, does motion along t h e by t h e f a c t o r cos 8 . The LSC wave thermal cou-
semi-major a x i s begin. 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 increases a s t h e p ro jec ted inten-
f o r time t a r e chosen t o represen 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 threshold is controlled by the beam inten-
t i o n f o r t > T3D. The expansion is represented by
powered o r unpowered b l a s t wave decay laws accord-
ing t o whether o r no t t h e l a s e r is still on.
A comparison between experimental data ,28 and
t h e o r e t i c a l p red ic t ions 22'27 f o r t h e f luence 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 area is 40 cm and t h e pu lse time is 10 ps.
The agreement between data and theory is q u i t e good
except 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 ind ica te t h a t t h e r e
was poor plasma ign i t ion . This good agreement sup-
p o r t s t h e o r i g i n a l model a s well a s t h e extension
t o angle of incidence.
8
Angle of Incidence
Fig. 11 Fluence deposited i n A 1 2024 by 10.6 pm ! rad ia t ion inc iden t a t an angle.
Both t h e d a t a and t h e theory show l a r g e oou-
p l i n g a t l a r g e angles of incidence (8 > 74O). The
s i t y , not t h e p ro jec ted in tens i ty . 28
The t h e o r e t i c a l p red ic t ion f o r t h e surface
pressure generated by t h e LSC wave is p l o t t e d a s
a funct ion of angle of incidence i n Fig. 12. The
experimental data28 agree q u i t e well with t h e theo-
r e t i c a l p red ic t ions (except f o r t h e d a t a p o i n t which
is flagged because of poor 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 surface
Angle of Incidence
Fig. 12 Surface pressure f o r 10.6 pm rad ia t ion inc iden t a t an angle.
p r e d i c t t h a t t h e r e is l i t t l e degradation i n pre-
d i c t e d impulse between 8 = O0 and 8 = 60°; t h e drop
absorbed fluence is twice t h e value pred ic ted by i n surface pressure is p a r t i a l l y compensated by
t h e slower pressure decay a s 0 is increased. A t
6 = 7 5 O , t h e p ro jec ted f luence is only 1 / 4 of t h e
normal fluence, b u t the del ivered impulse f o r ?= 1
is 70% of t h e value f o r normal incidence.
The plasma impulse coupling i s predicted t o
10,14,15 vary a s the ambient pressure t o t h e 1 / 3 power.
The decrease has been observed experimentally i n
t h e LSD wave regime, although t h e peak pressures
measured f a l l below 30 percent below t h e pred ic ted
values.2g Experimental da ta ind ica te t h a t t h e ther -
mal coupling by r a d i a t i v e t ranspor t from LSC wave
plasma remains approximately constant a s t h e pres-
sure is reduced, a s long a s a well-developed plasma
is formed. However, a s t h e ambient pressure drops
a threshold is reached below which an a i r plasma
cannot be created. The thermal coupling a t pres-
s u r e s below t h e a i r threshold is accomplished by
d i r e c t absorption; a t l e a s t f o r low l a s e r inten-
s i t i e s . However, it has been observed by McKay
and schriempf31 t h a t a t high i n t e n s i t y it is pos-
s i b l e t o i g n i t e a vacuum plasma which enhances t h e
thermal coupling. This plasma is not t h e r e s u l t
of bulk vaporization,31 ins tead it apparent ly is
crea ted by defec t vaporizat ion and is, there fore ,
d i f f e r e n t than t h e CW vapor plasma mentioned i n
Sect ion 3.
9. PULSED 3.8 pm COUPLING CONSIDERATIONS
Although both 10.6 pm and 3.8 pm pulsed l a s e r
rad ia t ion 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 metal t a r g e t s , t h e mechanisms by which
t h e coupling i s achieved may be d i f f e r e n t .
The DF l a s e r pulse, from t h e Boeing photoly-
t i c a l l y i n i t i a t e d lase r ,32 which is shown i n Fig.
13, does no t have a leading edge spike and it has
a r e l a t i v e l y long r i s e time t o approximately -85
psec. Plasma i g n i t i o n occurs i n t h e middle of t h e
pulse. 2132r33 A s a r e s u l t , t h e physics of the in-
be categorized f o r t h e durat ion of t h e whole pulse,
a s e i t h e r d i r e c t l a s e r absorption o r plasma radi-
a t i v e t r a n s f e r .
1 ime tpsl
Fig. 13 Sketch of DF l a s e r pulse shape.JL
Nor can d i rec t . absorp t ion of DF l a s e r radi-
a t ion be simply character ized by the i n t r i n s i c room
temperature absorp t iv i ty a s it can be f o r 10.6 pm
pulsed rad ia t ion . For 3.8 urn rad ia t ion t h e i n i t i a l
absorp t iv i ty 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 is usual ly g r e a t e r than 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 is more than an order of magnitude
l a r g e r than a t 10.6 pm. This rap id 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 surface temperature s i g n i f i -
can t ly during the pulse. To understand t h e cou-
p l ing when no plasmas a r e formed, t h e e f f e c t s of
t a r g e t heat ing and mass l o s s must be considered
j u s t a s they a r e i n CW in te rac t ions .
It is no t known a p r i o r i whether plasma ign i -
t i o n by pulsed 3.8 pm rad ia t ion i s assoc ia ted with
loca l ized defec t s , a s it is 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 evaporation of t h e surface a s it
does i n CW in te rac t ions . Since inverse Bremsstrah-
lung absorpt ion by e lec t rons s c a l e s a s wavelength
squared, t h e threshold i n t e n s i t y f o r defec t i n i t i -
a t i o n of plasma is higher f o r DF l a s e r pulses . HOW-
t e r a c t i o n between t h e l a s e r and t h e t a r g e t cannot ever, t h e l a r g e r bulk hea t ing r a t e experienced by
JOURNAL DE PHYSIQUE
t h e metal under 3.8 Um i r r a d i a t i o n may lead t o bulk
vaporizat ion of t h e t a r g e t during t h e pulse; then
plasma i n i t i a t i o n can proceed by breakdown of t h e
bulk vapor a s it does i n CW in te rac t ions .
The threshold f o r LSD waves f o r s h o r t pu lses
of 3.8 u m rad ia t ion is estimated t o be 40-50 MW/
cm . 14'15 The LSC wave plasmas a t 3.8 pm a r e sup-
ported by much higher i n t e n s i t i e s than a t 10.6 pm;
represen ts t h e range of values observed f o r t h e
f i r s t shot on a f r e s h aluminum surface. The c i r -
c l e s with b a r s represen t t h e da ta range observed
f o r t h e e i g h t sho t on t h e surface. The mult iple
pu lse e f f e c t was inves t iga ted a t only t h r e e
fluences. Plasma i g n i t i o n occurs between a t an
average f luence of 40-50 J/cm2 ( the s p a t i a l peak
2 i n t h e fluence is about 70 ~ / c m ) . For f luences
i n consequence t h e plasmas a r e h o t t e r , a t higher l e s s than 40 ~ / c m ~ , t h e i n t e r a c t i o n proceeds by
2 pressure, 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 surface d i r e c t absorption. Above 40-45 J/cm , d i r e c t ab-
is a f a c t o r of t en la rger . sorpt ion occurs a t t h e beginning of t h e pulse, bu t
A l ayer of metal vapor may be created 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 process o r a s a consequence of the high
h e a t f lux. The vapor absorbs t h e plasma rad ia t ion ,
t h u s it i n t e r f e r e s with energy t ranspor t t o t h e sur-
face u n t i l it is r a i s e d t o high temperatures where
it a l s o w i l l r ad ia te . However, metal vapors tend
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 longer wavelength
bands, which reduces t h e thermal coupling t o alumi-
num.
Recent data3' on t h e f luence deposited by pul-
sed 3.8 um l a s e r rad ia t ion i n t e r a c t i n g with alumi-
num surfaces i s shown i n Fig. 14. The shaded a rea
a l nc fder t Pulse Fluence
i n t h e middle of t h e pu lse a plasma is ign i ted and
t h e subsequent i n t e r a c t i o n is mediated by an LSC
wave plasma. The data i n d i c a t e s t h a t t h e r e is
l i t t l e enhancement assoc ia ted with plasma formation
f o r t h e f i r s t shot, b u t t h e enhancement is sub-
s t a n t i a l on subsequent sho ts i n t h e plasma regime.
The increase of coupling with t h e number of sho ts
is not completely understood, bu t it appears t o
be 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 surface absorp-
t i v i t y of t h e t a r g e t which occurs a s t h e r e s u l t
of sur face damage by p r i o r i r r a d i a t i o n s . 32
2 A t high f luences, say above 80 J/cm , the t a r -
g e t can vaporize and t h e vapor l ayer c u r t a i l s radi-
a t i v e t ransport . Creation of l o c a l LSD waves could
a l s o cause t h e reduction i n absorbed fluence, bu t
t h e mult iple pulse enhancement argues a g a i n s t t h e
i n t e r p r e t a t i o n .
although 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 with metal sur-
faces appear t o be a combination of those observed
in CW i n t e r a c t i o n s and those found i n pulsed 10.6
).im i n t e r a c t i o n s , a d e t a i l e d understanding of t h e
d a t a shown i n Fig. 14 is s t i l l lacking.
10.. INTERACTION WITH NON-METALS
The types of i n t e r a c t i o n s which have been ob-
Fig. 14 Fluence deposi ted i n A 1 2024 by 3.8 !AI served i n metals can a l s o occur f o r non-metals such rad ia t ion . Data from Ref. 32.
a s ceramics and f iberg lass . The major d i f fe rence
is t h a t the.non-metals can absorb r a d i a t i o n in- e ra ted 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 pressure is given by t h e LSC wave predic-
t o analyze data from s i n g l e pu lse experimeilts i n
8 f resh surfaces of f i b e r g l a s s (Cordopreg E-glass) ,
s l i p c a s t fused s i l i c a (SCFS) and pyroceram.
Figure 15 shows t h e pu lse data f o r t h e maxi-
mum value of t h e surface pressure observed during
t h e in te rac t ion of pulsed 10.6 u m rad ia t ion with
n ~ n - m e t a l s . ~ ~ Also shown a r e the t h e o r e t i c a l pre-
d i c t i o n s f o r the pressure, based upon t h e synthesis
of models described below. The t h e o r e t i c a l predic-
t i o n s a r e i n good agreement with t h e da ta , thus
lending credence t o t h e phenomenology underlying
t h e predict ions.
t i o n s below 4 NW/cmL, by LSD wave pred ic t ions above
2 8 MW/cm , and by a t r a n s i t i o n from LSC wave values
2 t o LSD wave values between 4 MW/cm2 and 8 MW/cm .
The pressure data 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. 15 and they a r e cons i s ten t with
the t h e o r e t i c a l p red ic t ions .
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 sur-
face in te rac t ion of t h e v i r g i n t a r g e t is dominated
by d i r e c t absorption of t h e l a s e r by t h e t a r g e t
with an in-depth absorption length of 6 um. Pres-
sure is generated a s t h e r e s u l t of vaporizat ion
of t h e g l a s s f i b e r s . The time resolved surface
pressure t r a c e s ind ica te t h a t t h e pressure b u i l d s
pato (~o lm& I I 1 I ' l ' l ' l I I I L up slowly, r a t h e r than promptly a s would be expect- -
A A ASCFS - o++PYROCERANI
- ed i f a plasma were ignited.33 The maximum sur-
0 @ e CORD0 PREG D cr. E-c (Ass
face pressure da ta , shown i n Fig. 15, show an ab-
r u p t fa l l -o f f a s a funct ion of fluence. This be-
- havior is incons i s ten t with vaporizat ion induced
by surface absorption of the l a s e r , bu t it is i n
good agreement with vaporizat ion models based on
No Plasma - in-depth absorpt ion with an absorpt ion length of
2 - / I - Air Plasma 6 Um. A s t h e l a s e r pulse fluence i s increased be- - ,' /' Theory
1 1 l l d l ' l 1 ' 1 ' 1 ' 1 1 1 1 1 - yond t h e threshold values f o r pressure generation, 0.1 0.2 0.4 0 . 6 . 8 1 2 4 6 8 1 0
Fig. 15 Surfaces pressure f o r 10.6 vm rad ia t ion 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 sur-
face in te rac t ion is dominated by t h e prompt forma-
t i o n of an a i r plasma above t h e surface, and t h e
surface pressure and r a d i a t i v e t r a n s f e r t o the sur-
face can be determined from t h e LSC wave model de-
scr ibed i n Sect ion 6. 23
Theoret ical ca lcu la t ions p r e d i c t t h a t vapori-
zat ion induced by plasma re rad ia t ion has only only
a neg l ig ib le e f f e c t upon t h e surface pressure gen-
the maximum surface pressure is cons i s ten t with
t h e pred ic t ions based on steady s t a t e t a r g e t v a p o r -
i za t ion .
A t i n t e n s i t i e s below plasma threshold, t h e
energy remaining i n t h e t a r g e t a f t e r the pulse,
c a l l e d t h e res idua l energy, is l imi ted by t h e on-
34 s e t of r a p i d vaporization. Theoret ical ana lys i s ,
p r e d i c t a maximum res idua l energy of 6 ~ / c m ~ f o r
a 6 pm absorption depth and t a r g e t i n i t i a l l y a t
room temperature.
The response of f i b e r g l a s s t o
by pulsed l a s e r s ind ica tes t h a t t h e usefu l energy
f o r bulk mate r ia l heat ing on a mul t ip le pu lse b a s i s
6
C9-72 JOURNAL DE PHYSIQUE
2 is l imi ted t o l e s s than 7 ~ / c m . However, t h e same
mult iple pulse experiments ind ica te t h a t t h e plasma
threshold f o r a previously i r r a d i a t e d t a r g e t may
2 decrease t o below I MW/cm .
11. SUMMARY
The thermal and mechanical coupling of l a s e r
beams t o mate r ia l s var ies s t ronglx a s the in te rac-
t i o n phenomenology changes. For d i r e c t absorption,
t h e instantaneous thermal coupling is given by t h e
t a r g e t surface absorp t iv i ty , b u t the t o t a l thermal
coupling includes e f f e c t s from t a r g e t heat ing and
mass loss . Whenever plasmas a r e created, e i t h e r
by bulk vapor breakdown o r by defec t induced break-
down, t h e l a s e r energy is absorbed i n t h e plasma,
and mechanical and thermal coupling is determined
by t h e plasma propert ies . For LSD wave plasmas,
thermal coupling is dominated by plasma r a d i a l ex-
pansion and enhanced t o t a l thermal coupling can oc-
cur ; f o r LSC wave plasmas, r a d i a t i v e t ranspor t dom-
i n a t e s t h e t ranspor t and enhanced l o c a l coupling
i s observed. Mechanical coupling r e s u l t s from
vaporizat ion i n t h e d i r e c t absorption regime, and
from t h e high pressure plasma i n the 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 pulsed 10.6
pm in te rac t ions , a l s o occur f o r oblique angles of
incidence, f o r pulsed 3.8 u m l a s e r rad ia t ion , and
f o r t h e in te rac t ion with non-metal mate r ia l s , how-
ever , t h e l a s e r parameters which de l inea te t h e in-
t e r a c t i o n regimes, a s well a s t h e magnitude of the
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
fa i sceau l a s e r avec des matgriaux var ie en fonct ion
de l ' i n t g r a c t i o n qu i a l i e u . S i l ' absorp t ion e s t
d i r e c t e , l a couplage thermique instantang e s t
dgterming par l ' a b s o r p t i v i t g de l a surface mais l e
couplage intggr6 comprend l e s e f f e t s de chauffage
de l a c i b l e e t de per te de masse. Quand il y a
crgat ion d'un plasma, par claquage soit dans l e
gros de l a vapeur s o i t i n i t i g par des d6fauts de
surface, l ' gnerg ie e s t absorbge par l e plasma e t
l e s couplages thermique e t m6canique sont deter-
mings par l e s proprigtgs de l a vapeur. Quand il
y a formatioii d'un onde de' dgtonation (LSD), l e
couplage thermique e s t doming par l 'expansion
r a d i a l e du plasma: il peut y avo i r augmentation
du couplage thermique global . Quand il y a for-
mation d'une onde de combustion (LSC), l e t rans-
p o r t thermique e s t principalement par radiat ion:
on observe une augmentation du couplage 3. Le
couplage mgcanique provient de l a vaporisat ion de
l a surface dans l e rggime 2 absorption d i r e c t e e t
de l a pression 61eveB du plasma dans l e rggime 2
couplage ind i rec te . Les msmes phgnomsnes que
l ' o n abserve dans l e s in tgrac t ions avec l a s e r s
continus e t l a s e r s C02 2 impulsion on l i e u a u s s i
dans l e cas d'incidence oblique, dans l e cas d'un
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
d1 in t6rac t ion e t l ' o r d r e de grandeur des e f f e t s
sont cependant d i f fg ren ts .
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