The formation mechanism of a laser-produced plasma

Volume26A, number 6

THE

PHYSICS LETTERS

FORMATION

MECHANISM

OF

A

12 February 1968

LASER-PRODUCED

PLASMA

W . G. GRIFFIN and J. S C H L t J T E R *

Culham Laboratory, United Kingdom Atomic Energy Authority, Abingdon, Berkshire, UK Received 4 December 1967

By observation of the time-dependent absorption of laser light at a focus on thin films of polythene the velocity of the absorbing layer in the solid has been measured to be 2.5 × 105 - 7.5 × 105 cm/sec for flux densities of 3 × 1010 - 80 × 1010 W/cm2.

When r ad i at i o n f r o m a h i g h - p o w e r l a s e r is f o c u s e d on to a solid s u r f a c e , the e f f e c t of high a b s o r p t i o n in the r ap id ly i o n i s e d m a t e r i a l in c o n junction with low t h e r m a l conductivity in the sol i d l ead s to s t r o n g s u r f a c e heating. As the p l a s m a p r o d u c e d by the heating expands away f r o m the s u r f a c e the a b s o r b i n g l a y e r p r o g r e s s e s into the m a t e r i a l [1,2]. We have a t t e m p t e d to i n v e s t i g a t e the f o r m a t i o n of the a b s o r b i n g l a y e r p r o d u c e d by a ruby l a s e r on polythene f i l m and to a s s e s s i t s v e l o c i t y by m e a s u r i n g the v a r i a t i o n of the a b s o r p t i o n during the l a s e r pulse. The l a s e r beam was f o c u s e d on to the f i l m with a lens. A f t e r t r a n s m i s s i o n o r r e f l e c t i o n at the film the l a s e r light was c o l l e c t e d , o v e r an angle of 2~ s t e r a d i a n s , by a p h o t o m e t r i c s p h e r e [3] and d e t e c t e d by a f a s t photodiode. An o v e r a l l t i m e r e s o l u t i o n f o r the a p p a r a t u s of about 1 nanos e c was a c h i e v e d . Quantitative m e a s u r e m e n t s of t r a n s m i s s i o n and r e f l e c t i o n w e r e c a r r i e d out on the r i s i n g fr ont of the l a s e r p u l s e when the flux Po i n c r e a s e s exponentially with t i m e up to m a x i m u m flux of Po = 200 MW. Fig. l a shows the c h a r a c t e r i s t i c v a r i a t i o n of the t r a n s m i t t e d flux P T r o b s e r v e d , showing two distinct k n e e s at t 1 and t 2. S i m i l a r b e h a v i o u r was o b s e r v e d with d i f f e r e n t f i l m t h i c k n e s s e s and with l a s e r p u l s e s of d i f f e r e n t r i s e t i m e 7. F o r t < t l , when the incident flux density qo < 1010W/cm, the f o i l s r e m a i n t r a n s p a r e n t . At t 1 the t r a n s m i s s i o n f a c t o r P T r / P o f a l l s to a s m a l l value, t y p i cal ly a few p e r c e n t and as no si gni f i can t r e f l e c t e d flux i s o b s e r v e d it is evident that an a b s o r b i n g l a y e r has been produced. The a b s o r p t i o n r e m a i n s e f f e c t i v e until t 2 when the * Present address: Institut ffir Plasmaphysik der Kernforschungsanlage Jfilieh, Germany.

t2 - t I

4- /

~0

/+

p~ Figlo:

o

,o ,,

;o

Fig. 1. Time of passage of the absorbing layer through the film as a function of film thickness do a) Incident and transmitted power (Po and PTr respectively) as a function of time, (70 ~m polythene, T = = 3.5 nanosec). t r a n s m i s s i o n i n c r e a s e s s h a r p l y . It is found that t 1 is independent of the foil thickness w h e r e a s t 2 i n c r e a s e s m o n o t o n i c a l l y with it. It is concluded that the o b s e r v a t i o n s a r e of an a b s o r b i n g l a y e r p r o g r e s s i n g through the film with en t r y and exit o c c u r r i n g at t 1 and t 2 r e s p e c t i v e l y . The v e l o c i t y of the l a y e r m ay thus be deduced f r o m the t i m e of p a s s a g e through the f i l m (t2-t 1) f o r v a r i o u s t h i c k n e s s e s (d) of the f i l m (fig. 1). The d e r i v e d v a r i a t i o n of the v e l o c i t y with l a s e r flux density i s shown in fig. 2. The r e a s o n that any s i g n i f i cant t r a n s m i t t e d flux i s o b s e r v e d during the 241

Volume26A, number 6

PHYSICS

106J ¢

-

5/0 s

.,.+

+ J 2.10

J

240"

5"10 '°

10 n

2,10 ~'

5"~0"

10 '~

LETTERS

12 February 1968

models a constant flux density which is switched on a t t i m e t = 0. T h e i r r e s u l t s m a y be a p p l i e d in t h e p r e s e n t c a s e of an e x p o n e n t i a l l y i n c r e a s i n g l a s e r f l u x u n d e r the c o n d i t i o n that uOq/Ox > Oq/Ot o r u > LXx/'r = 1 0 - 4 / 3 . 5 x 10 -9 = 3 x 104 c m / s e c ( w h e r e u i s the v e l o c i t y of a p a r t i c l e in the l a y e r r e l a t i v e to the l a y e r ) . T h i s c o n d i t i o n i s f u l f i l l e d f o r n e a r l y a l l r e g i o n s of the a b s o r b i n ~ l a y e r . C a r u s o et al. [2] p r e d i c t v F ~ q o ~ t-t~. N e g l e c t i n g the we~k t i m e d e p e n d e n c e our e x p e r i m e n t a l d a t a f i t s q ~ d e p e n d e n c e and i s c l e a r l y at v a r i a n c e O 1 1 with both qo T and qo ~ d e p e n d e n c e .

Fig. 2. Velocity vF of the absorbing layer as a function of the incident flux density qo- (Focus a r e a 2.5 x 10-4 cm2). p e r i o d t 1 < t < t 2 i s that, light is t r a n s m i t t e d by the f i l m in t h e o u t e r z o n e s of the f o c u s w h e r e the f l u x i s i n s u f f i c i e n t to i o n i s e the m a t e r i a l . T h e e f f e c t i v e t h i c k n e s s of the a b s o r b i n g l a y e r i s d e d u c e d to be ~ x ~< 1 p m a s the k n e e s at t i m e s t 1 and t 2 w e r e c l e a r l y o b s e r v e d f o r d ~ 3 p m . In r e f s . 1 and 2 the a u t h o r s a s s u m e f o r t h e i r

NUCLEAR

SPIN

VERSUS

RELAXATION ELECTRON

References 1. Yu. V.Afanasyev, O.N. Krokhin and G. V. Sklizkov, IEEE J. Quantum Electronics 2 (1966) 483. 2. A. Caruso, B. Bertotti and P. Giupponi, Nuovo Cimento 45B (1966) 176. 3. J . W . T . W a l s h , Photometry (Constable, London, 1958).

AND DYNAMIC POLARIZATION SPIN-SPIN RELAXATION

M. BORGHINI CERN, Geneva, Switzerland Received 8 January 1968

The spin-spin relaxation of paramagnetic centres in dielectric solids may act as a cause of nuclear spin relaxation or dynamic polarization, according to the case.

It h a s b e e n shown by B l o e m b e r g e n , s o m e t i m e ago [1,2], t h a t the m a i n m e c h a n i s m of r e l a x a t i o n f o r n u c l e a r s p i n s b e l o n g i n g to d i e l e c t r i c s o l i d s w a s due to t h e i r d i p o l a r c o u p l i n g with p a r a m a g n e t$c i m p u r i t i e s , and to t h e s p i n - l a t t i c e o r to t h e s p i n - s p i n r e l a x a t i o n of t h e s e i m p u r i t i e s . T h e p u r p o s e of t h i s l e t t e r i s to a n a l y z e the i n f l u e n c e of e l e c t r o n s p i n - s p i n r e l a x a t i o n on n u c l e a r d y n a m i c p o l a r i z a t i o n . We h a v e s i m p l y e x t e n d e d s o m e c a l c u l a t i o n s of P r o v o t o r o v on e l e c t r o n s p i n c r o s s - r e l a x a t i o n [3], i . e . s p i n - s p i n r e l a x a t i o n of e l e c t r o n s p i n s not h a v i n g e x a c t l y t h e s a m e r e s o n a n c e f r e q u e n c i e s . It h a s b e e n s h o w n by B l o e m b e r g e n et al. [4] t h a t , in s u c h a c a s e , s p i n 242

s p i n r e l a x a t i o n m a y not be n e g l i g i b l e c o m p a r e d to s p i n - l a t t i c e r e l a x a t i o n , e v e n f o r two s p i n species having relatively well resolved resonance lines. Provotorov has reanalyzed this mechanism, t a k i n g into a c c o u n t t h a t , w h e n an e l . e c t r o n s p i n Si f l i p s , c h a n g i n g i t s e n e r g y by ~ ) ~ , s i m u l t a n e o u s l y with a n o t h e r one Sj w h i c h c h a n g e s i t s e n e r g y by ~wJe, t h e e n e r g y b a l a n c e ~ i j = ~wie - ~WeY be t a k e n up by the e l e c t r o n i n t e r a c t i o n e n e r g y r e s e r v o i r . A s s u m e t h a t S = ½ and t h a t the h i g h t e m p e r a t u r e a p p r o x i m a t i o n is m a d e , i . e . that the s p i n d e n s i t y m a t r i x is t a k e n as [5] 1 + ~i 2P/eS~ - / 3 ~ S S ' w h e r e /~e~ is t h e p o l a r i z a -