INTENSITY AND SHAPE OF SPECTRAL LINES FROM LASER-PRODUCED PLASMAS

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INTENSITY AND SHAPE OF SPECTRAL LINES FROM LASER-PRODUCED PLASMAS

G. Jamelot, P. Jaeglé, A. Carillon, C. Wehenkel

To cite this version:

G. Jamelot, P. Jaeglé, A. Carillon, C. Wehenkel. INTENSITY AND SHAPE OF SPECTRAL LINES

FROM LASER-PRODUCED PLASMAS. Journal de Physique Colloques, 1979, 40 (C1), pp.C1-91-

C1-97. �10.1051/jphyscol:1979117�. �jpa-00218396�

JOURNAL DE PHYSIQUE

Colloque

C1,

supplkment au n o

2 ,

Tome 40, fkvrier 1979, page

C1-91

INTENSITY AND SHAPE OF SPECTRAL LINES FROM LASER-PRODUCED PLASMAS

G. Jamelot

-

P. JaeglC - A . C a r i l l o n

-

^C. Wehenkel

Laboratoire de Spectroscopie Atomique e t Ionique du C.N.R.S., Universite Paris-Sud Bat. 350? 91405 Orsay

GRECO " I n t e r a c t i o n Laser-MatiPre", Ecole Polytechnique, 911 20 Palaiseau, France

Resume

-

P a r t a n t d'observations exp6rimentales de s p e c t r e s d t i o n s multfchargCs dans des plasmas p r o d u i t s par l a s e r , on dCcrit l e s principawc f a c t e u r s qui conditionnent l t i n t e n s i t 6 e t l e pro- f i l des r a i e s s p e c t r a l e s dans l e domaine X-W. On souligne l e r81e du t r a n s p o r t de rayonnement.

On s ' i n t e r e s s e aux anomalies d t i n t e n s i t e qui peuvent dbnoter l a presence d'inversions de popula- t i o n e t on d e c r i t une experience recente d e s t i n e e Ctudier ces anomalies.

Abstract

-

I n s t a r t i n g from s p e c t r a l s t u d i e s of multicharged ions i n dense laser-produced plasmas we decribe t h e main processes which determine t h e i n t e n s i t y and t h e shape of l i n e s i n t h e X-W range. The r o l e of r a d i a t i o n t r a n s f e r i s underlined. I n t e n s i t y anomalies r e s u l t i n g from occurence of population inversions a r e considered and a recent experiment performed f o r i n v e s t i g a t i n g such anomalies i s described.

I n t h i s paper, we w i l l p r e s e n t various as- p e c t s of t h e study of s p e c t r a l l i n e i n t e n s i t i e s f o r multiply charged ions i n dense hot plasmas, produ- ced by powerful l a s e r s . Q u a n t i t a t i v e i n t e r p r e t a t i o n of s p e c t r a l i n t e n s i t i e s i s Of a g r e a t i n t e r e s t , on t h e one hand, f o r knowing space and time v a r i a t i o n of plasma d e n s i t y and temperature; i t provides valu- a b l e informations f o r understanding plasma hydrody- namics. On t h e o t h e r hand, spectroscpy of multiply charged ions i n plasmas i s t h e only way f o r i n v e s t i - g a t i n g atomic processes which c o n t r o l ion and elec- t r o n populations.

For such q u a n t i t a t i v e i n t e r p r e t a t i o n , it i s n o t enough t o have r a d i a t i v e t r a n s i t i o n probabi- l i t i e s , because one must take i n t o account s p e c i f i c f e a t u r e s of l a s e r produced plasmas, c o n s i s t i n g i n very high d e n s i t y and s t r o n g l y unhomogeneous s t r u c - t u r e

-

^5). That i s why, we w i l l consider b r i e f l y t h r e e a s p e c t s of s p e c t r a l i n t e n s i t i e s :

ISt: t h e processes which a r e of importance f o r populating t h e e x c i t e d l e v e l s of t h e ions;

2nd: t h e r o l e of r a d i a t i o n reabsorption through

ging s p h e r i c a l wave w i l l f a l l upon De-Li t a r g e t . The t r a n s f e r of energy from the beam t o t h e t a r g e t i s expected t o bring f o r t h an extremely l a r g e compres- s i o n of t h e t h e matter, not owing t o r a d i a t i o n pres- s u r e , but because a shock wave propagates toward t h e c e n t e r of t h e sphere, a s a consequence of momentwn t r a n s f e r from f a s t ions leaving the coronal zone.

Loser

\ /

.

Target

Fig. 1

-

Sketch of implosion experiment f o r produ- unhomogeneous plasma;

cing thermonuclear r e a c t i o n s by l a s e r . grd: t h e p o s s i b l e occurence of population inver-

s i o n s leading t o i n t e n s i t y anomalies by amplifica- This process i s kown a s i n e r t i a l confinement. Com-

t i o n of spontaneous emission. pression being followed by heating t h e plasma,

conditions f o r g e t t i n g thermonuclear r e a c t i o n s can A motive f o r extending t h i s research i s

be s a t i s f i e d . i t s c l o s e r e l a t i o n with c o n t r o l l e d thermonuclear

f u s i o n experiments. A simple sketch of such an ex- A view of f e a t u r e s e x h i b i t e d , b y a plasma periment i s shown on f i g . 1 . Many l a s e r beams a r e near the t a r g e t s u r f a c e can be seen on f i g . 2. It i s o p t i c a l l y s e t t l e d i n such a way t h a t a quasi-conver- assumed t h a t t h e massive t a r g e t has a plane surface.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979117

JOURNAL DE PHYSIQUE

,,ATo ^*

sitometer t r a c e s , showing t h e s o f t X-ray s p e c t r a of

~ 1and ~~+1ions i n a Nd-laser produced plasma, ~ + presented on f i g . 3. The lower t r a c e s correspond t o

- ^-

a plasma l a y e r s i t u a t e d a$ a d i s t a n c e of 1 nun from

"1.J T o ,

./

t h e t a r g e t surface: t h e d e n s i t y , of t h e order of

O/'

^X 1018 c ~ n - ~ , i s l e s s than t h e c r i t i c a l density; t h i s /.

2%- .-. -.-."

l a y e r shows sharp l i n e s and i n t e n s i t y r a t i o s very Fig,

-

Interaction between the laser beam

(4)

and s i m i l a r t o what can be observed i n d i l u t e d plasmas

a plane massive t a r g e t ; t h r e e zones of densi- (except f o r some i n c r e a s i n g of 111.8 and 117.4 t Y (P's), of temperature ( ~ ' s ) , of ion v e l o c i t y ( v % )

;; -

^{lines). w i t h this, the upper traces}

a r e shown.

Three regions can be distinguished: i ) t h e unpertur- bed s o l i d m a t e r i a l of d e n s i t y p o , temperature To and ion v e l o c i t y v =O; i i ) a region where t h e compres- s i o n occurs, making t h e d e n s i t y e 1 7 e , and where t h e p a r t i c l e v e l o c i t y has t h e same d i r e c t i o n a s t h e propagation of t h e l a s e r beam; i i i ) t h e region of plasma expansion, where t h e v e l o c i t y vg i s of oppo- s i t e s i g n ; g r a d i e n t s of d e n s i t y a n d of temperatu- r e T2 take p l a c e here.

The plasma l a y e r of c r i t i c a l d e n s i t y

P C

plays an important r o l e . Since t h e l a s e r beam cannot propagate beyond t h e p o i n t of c r i t i c a l d e n s i t y , t h a t i s t o say about 1 02' f o r Nd-laser, i n t e r a c t i o n s between high i n t e n s i t y r a d i a t i o n and plasma a r e es- p e c i a l l y i n t e r e s t i n g t o i n v e s t i g a t e i n t h i s region.

Most of t h e s p e c t r a l s t u d i e s discussed below a r e c l o s e l y r e l a t e d t o t h e p r o p e r t i e s of t h e plasma i n t h e neighbourhood of c r i t i c a l density.

As a start p o i n t f o r d i s c u s s i n g t h e inten- s i t y of i o n s p e c t r a l l i n e s , l e t us consider t h e den-

show t h a t t h e spectrum coming from a high d e n s i t y l a y e r , t a k i n g place between t h e t a r g e t surface and t h e c r i t i c a l d e n s i t y p o i n t , i s q u i t e a l t e r e d . One observes l a r g e modifications of i n t e n s i t y r a t i o s , l a r g e broadening of l i n e s , changes i n l i n e shape, going s o f a r a s t o s p l i t some l i n e s i n two. Stark e f f e c t , due t o t h e p e r t u r b a t i o n of atomic p o t e n t i a l by t h e next charged p a r t i c l e s , i s involved i n mecha- nisms leading t o l i n e broadening. The two o t b e r f e a - t u r e s which have t o be considered f o r explaining t h e experimental observations a r e t h e r a t e of population of every e x c i t e d l e v e l of t h e ions and t h e a c t i o n of l i n e r a d i a t i o n t r a n s f e r through t h e plasma.

The population of ion e x c i t e d l e v e l s can be obtained i n solving a system of coupled r a t e equations. Let Ni, N j , Nk... be the population den- s i t i e s of l e v e l s i , j, k

...;

Ne, t h e f r e e e l e c t r o n density; NZ+l, t h e ground s t a t e population d e n s i t y of ions of charge Z+1; t h e A's and t h e B's, Einstein9 c o e f f i c i e n t s f o r emission and absorption respective- l y ; the < r v > 's, t h e c o l l i s i o n v e l o c i t i e s f o r Max- well d i s t r i b u t i o n of f r e e e l e c t r o n s ; Xm, t h e t h r e e

3

-

Difference i n r e l a t i v e i n t e n s i t y and i n l i n e shape f o r aluminium s p e c t r a coming r e s p e c t i v e l y from a low d e n s i t y region (lower densitometer t r a c e s ) and from a high d e n s i t y region (upper densitometer t r a c e s ) .

body recombination c o e f f i c i e n t . The d e s i g n a t i o n o f each p r o c e s s , on t h e r i g h t o f t h e corresponding Wnm, w i l l make t h e meaning of h e r e a f t e r e q u a t i o n q u i t e

t h

c l e a r . For t h e i l e v e l , we have:

r a d i a t i v e decay from upper l e v e l s

r a d i a t i v e decay of i t h l e v e l

+ 'k &i*> p o p u l a t i o n by e l e c t r o n

k impact

-

N e N i E &m.v> depopulation by e l e c - t r o n impact

m

+

NrNZtl

<$

^ph.> r a d i a t i v e recombination

-

^NeNi

4 ">

e l e c t r o n impact i o n i r a - t i o n

photoabsorption from i t h l e v e l

+I

N r T b r i ) Bri p h o t o e x c i t a t i o n of

r <i c ith l e v e l

2

+ N e N ~ *r 7 t h r e e body recombina- t i o n

+ ... ⁺ ... ⁺ ...

o t h e r s

Fig. 4

-

Energy r a d i a t e d by s i x l i n e s of A 1 X I l i t h i u m l i k e i o n s v e r s u s absorbed l a s e r energy, a s a r e s u l t o f computer experiment (from r e f . 6)

t r a n s i t i o n p r o b a b i l i t y r i n g s I s23d

-

I s22p 1 01 2 sec-1

d o t s 3P 2s 3.10'~ Ir

dashes 4d 2p 3.10~'

dashes-dots 3 s 2p 1 011

s o l i d l i n e 2~ 2s 8.1

o8 "

t r i a n g l e s 1 ~ 2 ~ 2

-

^{1 s22p 3.1 013}

The maximum temperature Te i s c o r r e l a t e d with t h e l a s e r energy a s shown by t h e upper s o l i d curve.

Fig. 5

-

C a l c u l a t i o n of r a d i a t i v e t r a n s f e r ; t h e plasma model r e p r e s e n t e d by t h i s curve c o n s i s t e s i n a h o t dense c o r e surrounded by a c o o l e r external s h e l l . T h i s t y p e o f d i s t r i b u t i o n occurs very l i k e l y a t t h e f o c a l s p o t of laser-impact experiments.

S t r i c l y speaking, t h e system t h a t i s t o s o l v e must i n c l u d e a l s o t h e r a d i a t i o n t r a n s f e r and t h e plasma motion. However, f o r many a p p l i c a t i o n s , i t i s p o s s i b l e t o t r e a t s e p a r a t e l y each p o i n t . Fig. 4, from work under r e f . 6 , gives an exemple of l i n e i n t e n s i t y c a l c u l a t i o n f o r plasma d i a g n o s t i c s . I n t e n s i t y r a t i o s a s f u n c t i o n s of temperature can be deduced f r o m t h e s e p l o t s f o r d i f f e r e n t i n i t i a l den- s i t i e s . The f i g u r e show5 t h a t t h e 2p

-

2 s l i n e i s t h e most s e n s i t i v e t o t h e d e n s i t y and t h e s a t e l l i t e l i n e I s 2p2

-

I s 2p i s very s e n s i t i v e t o t h e tempe- 2 r a t u r e . For improving plasma d i a g n o s t i c s , it i s ne- c e s s a r y t o extend such i n t e n s i t y c a l c u l a t i o n s t o many t r a n s i t i o n s of v a r i o u s i o n s and, consequently, it i s n e c e s s a r y t o c o l l e c t a g r e a t d e a l of atomic d a t a on r a d i a t i v e a s w e l l on non-radiative processes.

F'urthermore, measured l i n e i n t e n s i t i e s de- pend a l s o on t h e amount of r e a b s o r p t i o n by t h e p l a s - ma. It can be e s t i m a t e d t h a t r e a b s o r p t i o n , f o r s o f t X-ray resonance l i n e s of i o n s having charge number of t e n o r more, i s s i g n i f i c a n t from a plasma densi- t y v a l u e o f about l o 1 * ~ m ' ~ . I n laser-plasmas, t h e s t r o n g unhomogeneity g e t s r e a b s o r p t i o n e f f e c t r a t h e r complex. I n t h e s i m p l e s t c a s e , it can be assumed t h a t t h e plasma h a s a c o r e a t t h e f o c a l s p o t of l a - se*beam and an e x t e r n a l s h e l l , where temperature and d e n s i t i e s a r e growing s m a l l e r . Fig. 5 shows t h i s type of plasma model which h a s been f r e q u e n t l y used i n c a l c u l a t i o n of r a d i a t i o n t r a n s f e r . I n a d d i t i o n t o t h e v a r i a t i o n of plasma parameters along l i g h t p a t h , a s h i f t o f wavelength, due t o expansion velo- c i t y , must be taken i n t o account.

JOURNAL DE PHYSIQUE

Fig. 6

-

Line shape a s a f u n c t i o n of t h e d i s t r i b u t i o n of i o n d e n s i t y ; e l e c t r o n d e n s i t y and temperature, i o n v e l o c i t y and a p p a r a t u s f u n c t i o n a r e p l o t t e d on t h e t o p of t h e f i g u r e .

Fig. 7

-

E f f e c t of a p p a r a t u s f u n c t i o n on l i n e shape.

Qu

i s t h e o p t i - c a l l y t h i n p r o f i l e , which should be observed i n d i l u t e d plasma; Ic, i s t h e p r o f i l e a s modified by r e a b s o r p t i o n f o r a model of plasma r e p r e - s e n t e d above; Iexp r e s u l t of t h e c o n v o l u t i o n of t h i s p r o f i l e by t h e Gaussian apparatu's f u n c t i o n s shownon t h e top.

The c a l c u l a t i o n of i n t e n s i t y e m i t t e d by For i n t e r p r e t i n g absorbed o r s e l f - r e v e r - t h e plasma i n d i r e c t i o n OX, a t a given frequency JJ

,

sed p r o f i l e s ( f o r i n s t a n c e , a t 111.2 and 125.5 i s performed i n i n t e g r a t i n g .the d i f f e r e n t i a l q u a - on f i g . 3 ) , t h e continuous spectrum due t o f r e e - t i o n oP r a d i a t i o n t r a n s f e r ( s e e f i g . 5 ) : f r e e and bound-free t r a n s i t i o n s must be p r o p e r l y

S I u = ( j , - k O I p dx

where jg _and k , ~ a r e emission and a b s o r p t i o n coef- f i c i e n t s ; A d e t a i l e d d i s c u s s i o n of making u s e of t h i s i n t e g r a t i o n i n laser-produced plasma h a s been p r e s e n t e d elsewher-e(4). Thus we w i l l r e s t r i c t t i e p r e s e n t s t a t e m e n t t o s p e c i f y t h e c o n c r e t e form of ju and ~ J J

.

included i n t h e e x p r e s s i o n s of jd and k U

.

^General-

l y , i t should be d i f f i c u l t t o c a l c u l a t e t h e absolu- t e v a l u e of continuous c o n t r i b u t i o n s , b u t t h e cor- r e c t dependance of j9 and k ~ ) upon e l e c t r o n i c tem- p e r a t u r e T and d e n s i t y N can be e a s i l y e x p r p s s e d . A d j u s t a b l e parameters a r e then i n t r o d u c e d f o r f i t - t i n g c a l c u l a t i o n s with experimental r e s u l t s . Furthermore, i f l i n e s a r e c l o s e each o t h e r , more

t h a n one d i s c r e t e t r a n s i t i o n can c o n t r i b u t e t o i n - (1

).

t e n s i t y a t a given frequency That i s d i s r e g a r - ded i n t h e f o l l o w i n g e x p r e s s i o n s f o r t h e sake of s i m p l i c i t y . Let A21 and B12 be E i n s t e i n ' s c o e f f i - c i e n t s of spontaneous emission and a b s o r p t i o n f o r t r a n s i t i o n s between l e v e l s 1 and 2, N1 and N2, t h e r e s p e c t i v e p o p u l a t i o n d e n s i t i e s o f t h e l e v e l s , gl and g2, t h e i r s t a t i s t i c a l w e i g h t s , p and q , two a d j u s t a b l e parameters. We c a n w r i t e :

where

@(u)

i s a p r o f i l e f u n c t i o n whose t h e form i s t o be f i x e d according t o l i n e braodening mechanisms p l a y i n g i n t h e plasma. For t h e examples given i n t h i s paper ( f i g . 6 and 7) L o r e n t z i a n p r o f i l e , of width depending on i n t e n s i t y and temperature a s

1

N,TLY has been used; t h e c e n t e r of t h e p r o f i l e was Doppler-shifted i n o r d e r t o account f o r an i o n velo- c i t y p r o p o r t i o n a l t o t h e d i s t a n c e from plasma c e n t e r ; moreover, p o p u l a t i o n r a t i o N1/N2 was deduced from Boltzmann's law.

Fig. 8

-

P r o f i l e of t h e

La

l i n e of B ~ * i o n s a t v a r i o u s d i s t a n c e s from t h e t a r g e t (from r e f . , 7 ) . S o l i d l i n e s r e p r e s e n t t h e experimental p r o f i l e s and dashed l i n e s , t h e computed p r o f i l e s .

1 s 3d

'D, )

^Predicted

' \

i n w r sion

1s 2~ 3 F

Fig. 9

-

Diagram showing one p o s s i b l e mechanism f o r y i e l d i n g p o p u l a t i o n i n v e r s i o n i n dense plasmas ( r e f . 1 4 ) . I n h e l i u m l i k e i o n s t h e c o l l i s i o n a l e x c i t a t i o n r a t e s a r e of t h e same magnitude f o r s i n g l e t and t r i - p l e t l e v e l s , whereas t h e r a d i a t i v e decay i s much f a s t e r f o r a s i n g l e t t h a n f o r a t r i p l e t , l e a d i n g t o d e p o p u l a t i o n o f s i n g l e t lower l e v e l .

.. diclectronic . inclostic rccpmblmtion

.

couision

a

.

(autoionizing Levd)

. .

Fig. 1 0

-

Role of a u t o i o n i z i n g l e v e l i n t r a p p i n g f r e e e l e c t r o n s of plasma and i n o v e r p o p u l a t i n g a c c o r d i n g l y p e c u l i a r e x c i t e d l e v e l s by r e s o n a n t recombination.

A p p l i c a t i o n s of t h e s e c a l c u l a t i o n s have been performed p r e v i o u s l y f o r l o o k i n g i n t o t h e i n f - l u e n c e of i o n d e n s i t y , on t h e one hand, of c o l l i s i o - n a l broadening, on t h e o t h e r hand, upon t h e i n t e n - s i t y and t h e shape o f l i n e s t 4 ) . A s r e g a r d s t h e r o l e of d e n s i t y , it h a s been proven a s l i a b l e f o r s e l f - r e v e r s i n g of l i n e s ; i n c r e a s i n g of d e n s i t y above a given v a l u e does n o t e n t a i l enhancement of l i n e - t o p i n t e n s i t y b u t emphasizes s e l f - r e v e r s i n g and broade- ning. Here, we have chosen t o p r e s e n t , on f i g . 6 , an e x t e n s i o n o f t h i s s t u d y t o a more complex plasma model. A s t o f i g . 7, it shows t h e consequence o f a p p a r a t u s f u n c t i o n widening. A t l a s t , a s an example of u s i n g t h i s t y p e of c a l c u l a t i o n f o r plasma d i a g - n o s t i c s , f i g . 8 gim?s a r e s u l t o b t a i n e d by J a n n i t i e t a l . ( 7 ) i n comparing observed p r o f i l e s of B 4+

l i n e s , a t d i f f e r e n t d i s t a n c e s from a massive t a r g e t , w i t h r e s u l t s of p o p u l a t i o n and r a d i a t i o n t r a n s f e r computation.

Excited l e v e l p o p u l a t i o n s d e p a r t i n g Prom e q u i l i b r i u m a r e i n t e r e s t i n g t o i n v e s t i g a t e , e s p e c i a l - l y because t h e y can o r i g i n a t e , i n some c a s e s , ampli- f i c a t i o n of r a d i a t i o n ; t h i s i s suggested by i n t e n - s i t y anomalies observed i n s o f t X-ray s p e c t r a (8-1 3 ) A t f i r s t s i g h t , t h e h i g h d e n s i t y of laser-produced

C

1-96 JOURNAL DE PHYSIQUE

plasmas i s an a d v e r s e circumstance f o r g e t t i n g i n - v e r s i o n between p o p u l a t i o n s , s i n c e i t i s expected t o promote s t a t i s t i c d i s t r i b u t i o n , o n a c c o u n t on t h e b i g r a t e of c o l l i s i o n s . It may e x p l a i n t h a t t h e v e r y f i r s t works i n t h i s a r e a have been s h a r p l y q u e s t i o n e d But, a t t h e p r e s e n t time, a f a i r l y l a r g e nwnber of mechanisms has been t h e o r e t i c a l l y brought o u t a s s u i - t a b l e f o r y i e l d i n g p o p u l a t i o n ^4-1 7 ) .

A common f e a t u r e of t h e s e mechanisms i s t h a t t h e y d o N O T r e q u i r e a n y o p t i c a 1 pumping f o r gene- r a t i n g i n v e r s i o n . I n f a c t , plasma d i f f e r s from u s u a l l a s e r m a t e r i a l by f r e e e l e c t r o n s , rambling above a g r e a t d e a l of empty d i s c r e t e l e v e l s . D i s t u r b a n c e of plasma s t a t e w i l l b r i n g on t h e f i l l i n g o f some o f them, which a r e n o t n e c e s s a r i l y t h e lowest ones.

F u r t h e r , t h e very l a r g e energy flow, f a s t v a r y i n g , from t h e l a s e r beam t o t h e plasma a l l o w e s e a s i l y t h e occurence of such d i s t u r b a n c e s which hold plasma f a r from complete e q u i l i b r i u m . For i n s t a n c e , t h e f a s t recombination of plasma, a f t e r t h e end o f l a s e r i m - p u l s e , can d r i v e p o p u l a t i o n i n v e r s i o n . D e t a i l e d c a l - c u l a t i o n o f t h i s p r o c e s s h a s been made f o r hydroge- n i c ions(16). A d i f f e r e n t s i t u a t i o n has been c o n s i - d e r e d , a s shown on f i g . 9, i n a c a s e i n v o l v i n g he-

I ,

^/

^Target

/ /

Fig. 11 - A s t i g m a t i c f o c u s i n g of l a s e r beam f o r making e l o n g a t e d plasma. Comparison of emission between a x i a l and t r a n s v e r s e d i r e c t i o n s w i l l d i s p l a y t h e a c t i o n of plasma l e n g t h upon l i n e i n t e n s i t y .

l i w n l i k e i o n s , t h e i n v e r s i o n being now p r e d i c t e d i n t h e c o u r s e of l a s e r impulse(14). A s f o r f i g . 1 0 , i t r e p r e s e n t s t h e growing o f a l e v e l p o p u l a t i o n due t o a resonance i n recombination produced by a u t o i o n i z i n g l e v e l , l y i n g w i t h i n t h e gas of f r e e

electron^(^'^'.

The achievement of experiments f o r s e e k i n g

transverse spectrud(l) Fig. 1 2

-

^A r e s u l t of experiment shown on f i g , 11, f o r a plasma l e n g t h of about 4 mm. Three s h o t s have been used f o r t h e a x i a l spectrum and six, f o r t h e t r a n s v e r s e spectrum. Energy p e r shot: 4 0 J; p u l s e width: 25 ns.

The l i n e s a t 117.4

1

and a t 105.7

1

a r e among t h e most s e n s i t i v e t o t h e change of plasma l e n g t h .

p o p u l a t i o n i n v e r s i o n i n laser-produced plasmas en- of 15O. A d i r e c t comparison of r e s u l t s obtained f o r c o u n t e r d i f f i c u l t i e s because t h e o p t i c a l p r o p e r t i e s both l e n g t h s i s n o t p o s s i b l e because t h e power den- i n t h e s o f t X-ray range oppose till now t o c a r r y o u t s i t y s u p p l i e d o n t h e t a r g e t i s modified by changing a r e s o n a n t c a v i t y , o r even t o double t h e p a t h of r a - t h e angle. But one can compare each t o each t h e spec- d i a t i o n by s p e c u l a r r e f l e c t i o n . I n a d d i t i o n , t h e t r a e m i t t e d along t h e plasma a x i s and i n t h e ortho- plasma volume under i n v e s t i g a t i o n i s v e r y s m a l l , r e - gonal d i r e c t i o n . T h i s i s done i n t u r n i n g t h e l e n s e q u i r i n g t h e d e t e c t i o n t e c h n i q u e t o be very s e n s i t i v e . around t h e a x i s of t h e l a s e r beam, t h e s l o p e of t h e

l e n s e being k e p t c o n s t a n t . Recently, i n u s i n g a new d e v i c e f o r study-

i n g s o f t X-ray e m i t t e d by plasma, we have performed An exemple o f r e s u l t s , we have o b t a i n e d ,

"

t h e experiment p r e s e n t e d by t h e s k e t c h of f i g . 1 7 . I n i s p r e s e n t e d on f i g . 12. The s p e c t r a l l i n e a t 177.4 A a vacuuv experimental chamber t a k e p l a c e a t a r g e t - of ~ 1i o n s has been s t u d i e d p r e v i o u s l y by t h e two ~ ' h o l d e r and a lense-holder, equiped w i t h a c c u r a t e mo- plasma technique, which showed a small n e g a t i v e ab- b i l e s u p p o r t s and s t e p p i n g motors. The p r e c i s i o n of s o r p t i o n (9119'. The s e n s i t i v e n e s s t o plasma s i z e a l l displacements i s of about I ?

.

A g r a z i n g i n c i d e n - e x h i b i t e d by t h i s s p e c t r a l l i n e i n t h e p r e s e n t expe- c e t o r o i d a l m i r r o r i s l o c a t e d between plasma and spec- r i a e n t does confirm t h e l i n e of i n t e r p r e t a t i o n of t h e tromonochromator(18) s o t h a t f o c a l l i n e s a r e respec- i n t e n s i t y anomaly observed a s e a r l y a s 197l (8'. I t t i v e l y s i t u a t e d on t h e e n t r a n c e s l i t and removed t o must be p o i n t e d o u t t h a t t h i s experiment r e q u i r e s a t h e i n f i n i t e ; t h i s p r o v i d e s high l u m i n o s i t y and spa- slowly r i s i n g "long" l a s e r impulse. For i n s t a n c e , i t

0

t i a l r e s o l u t i o n a s l a r g e a s 5 f

.

does n o t appear i n t e r e s t i n g f e a t u r e a t 11 7.4 A by u s i n g a 3 n s l a s e r impulse ol' a stepped 20 n s impul- For making an elongated plasma, we have s e . R e s u l t o f f i g . 1 2 i s d u e t o G a u s s i a n 2 5 n s impulses.

used t h e astigmatism of a s l a n t e d o p t i c a l lense. This A s a p r e l i m i n a r y r e s u l t , one can n o t i c e t h e behaviour system allowes t o change e a s i l y t h e l e n g t h of t h e of an u n c l a s s i f i e d l i n e , o r perhaps a group of l i n e s ,

0

f o c u s i n g l i n e on t h e . t a r g e t . The diameter of t h e l a - a t 105.7 A which, a s f o r i t , can be a l s o observed ser-beam being of 70 mm, we g e t an impact t r a c e 2 mm w i t h a 3 n s impulse. On t h e r e c o r d i n g s of f i g . 1 2 , long f o r an a n g l e of 5O, and 4mm l o n g f o r an a n g l e s t i l l o t h e r l i n e s need f u r t h e r i n v e s t i g a t i o n s .

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