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X-UV INTERFERENTIAL MIRRORS AND NEW POSSIBILITIES FOR PLASMA RADIATION
STUDIES
P. Dhez
To cite this version:
P. Dhez. X-UV INTERFERENTIAL MIRRORS AND NEW POSSIBILITIES FOR PLASMA RADIATION STUDIES. Journal de Physique Colloques, 1986, 47 (C6), pp.C6-267-C6-275.
�10.1051/jphyscol:1986634�. �jpa-00225877�
JOURNAL D E PHYSIQUE,
Colloque C6, supplement au n o 10, Tome 47, octobre 1986
X-UV INTERFERENTIAL MIRRORS AND NEW POSSIBILITIES FOR PLASMA RADIATION STUDIES
P. DHEZ
Laboratoire d e Spectroscopie Atomique et Ionique and LURE, Universite Paris XI, F-91405 Orsay Cedex, France
RESUME
La r e a l i s a t i o n d e rniroirs i n t e r f b r e n t i e l s a d a p t & au domaine X-UV e s t une p o s s i b i l i t l a p p a r u e s e u l e m e n t d e p u i s une dizaine d'annees. Dans une premiPre p a r t i e nous r a p p e l o n s l e s c a r a c t l r i s t i q u e s p r i n c i p a l e s d e t e l s miroirs. P a r comparaison avec l e s o p t i q u e s s o u s incidence r a s a n t e s e u l e s d i s p o n i b l e s & s q u J & maintenanti n o u s indiquons e n s u i t e l e s n o u v e l l e s >voies o u v e r t e s p a r l e u r u t i l i s a t i o n . Enfin n o u s d a n n o n s q u e l q u e s e x e m p l e s de diagnoctic d e plasma t i r a n t p a r t i e de c e s nouvelles p o s s i b i l i t 8 s .
ABSTRACT
F o r a b o u t t e n y e a r s new multilayered X-UV m i r r o r s have been i n p r o g r e s s and a r e now overcoming some of t h e main d i f i c u l t i e s well Rnown with t h e grazing incidence o p t i c s u s e d up t o now. The main c h a r a c t e r i s t i c s of such m i r r o r s a r e f i r s t recalled. Next some of t h e new p o s s i b i l i t i e s i n X-UV s p e c t r o s c o p y and imaging a r e given. Finally we b r i e f l y d e s c r i b e some e x a m p l e s of new s e t u p s using s u c h X-UV i n t e r f e r e n c e m i r r o r s f o r plasma diagnostic.
INTRODUCTION.
S e v e r a l p a p e r s a t t h i s f i r s t colloquium about "X-Ray L a s e r " d e m o n s t r a t e c l e a r l y t h e importance of plasma d i a g n o s t i c s t o p r o g r e s s i n t h e understanding of t h e mecanisms leading t o t h e population i n v e r s i o n and how t h e u s e of t h e X-UV r a n g e i s well s u i t e d t o s t u d y plasma t e m p e r a t u r e , d e n s i t y and c h a r a c t e r i s t i c l i n e s of highly ionized atoms. Indeed, t h e p r o g r e s s i n such d i a g n o s t i c s using s p e c t r o s c o p y o r imaging o p t i c s a r e very d e p e n d a n t t o t h e p o s s i b i l i t i e s t o build t h e correspoding X-UV o p t i c a l s y s t e m s t o achieve them.
Until now, most of t h e s e t u p s have b e e n r e s t r i c t e d t o grazing incidence and c r i s t a l o p t i c s (L). T h i s is due t o t h e X-U\/ o p t i c a l c o n s t a n t values. Over a l l t h e X-UV range; t h e s e one f o r b i d a t t h e same t i m e t h e c l a s s i c a l normal incidence m i r r a r s w i t b s i n g l e i n t e r f a c e , which need very absorbing m a t e r i a l ? and a l s o t h e r e f r a c t i v e l e n s e s , b e c a u s e r e f r a c t i v e index i s t o o c l o s e t o t h e vaccuum one. The q u i t e r e c e n t development of multilayered m i r r o r s (2) and F r e s n e l zone p l a t e (3) had considerably enhanced t h e p o s s i b i l i t i e s t o conceive X-UV o p t i c s b e t t e r a d a p t e d t o X-UV d i a g n o s t i c s . B a s e d o n d i f f r a c t i o n p r o p e r t i e s , such i n t e r f e r e n t i a l o p t i c s a r e n o t s i m i l a r t o t h e c l a s s i c a l s i n g l e i n t e r f a c e m i r r o r s and r e f r a c t i v e l e n s e s well Known in t h e v i s i b l e range. To g e t such X-UV d i f f r a c t i n g o p t i c s , s p e c i a l microfabrication t e c h n o l o g i e s have been r e c e n t l y developed. The p r e s e n t s t a t e of a r t i n t h i s new f i e l d p e r m i t s t o overcome s e v e r a l l i m i t a t i o n s which up t o now had slowed down t h e u s e of t h e X-UV range.
I n t h i s p a p e r I will t r y f i r s t t o r e c a l l t h e b a s i c p r o p e r t i e s of t h e X-UV i n t e r f e r e n t i a l mirrors. Next I will indicate new p o s s i b i l i t i e s offered by such mirrors t o design new X-UV o p t i c s . Finally I will give some e x a m p l e s of new plasma d i a g n o s t i c s using t h e s e i n t e r f e r e n c e mirrars.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986634
C6-268 JOURNAL DE PHYSIQUE
BASIC PROPERTIES OF THE INTERFERENTIAL X-UV MIRRORS
Before t o come t o interference mirrors, l e t u s f i r s t remember how the classical single interface mirror acts.
The f i r s t figure shows how the
.Pi a b s o r p t i o n c o e f f i c i e n t o f gcld i s
u l o - Au linked t o normal incidence reflectivit:,
U a, (4).
d 5-
W ---Rl..,"",vcd*' Because, i n t h e X-UV range, a l l
a, materials have almost t h e same order
2- of magnitude f o r t h e o p t i c a l
constants, they a l l exhibit the same behaviour. Fresnel formula giving the refectivity coefficient versus t h e angle af incidence p e r m i t s t o calculate t h e efficiency of X-UV
L; mirrors by using X-UV optical indices
(5).
Figure 2 is a n example of such calculations for a hafnium dioptre a t t h r e e w a v e l e n g t h s ( 6 ) . I t d e m o n s t r a t e s how r e f l e c t i v i t y
oh&& & "$a 2b0 & ' I ~ O decrease with absorption coefficient h v !eT7) when wavelength becomes shorter and how a noticeable reflectivity regime is restricted t o grazing angle range.
Some physics text books explain t h a t
- Figure 1 - only an infinitely absorbing material
i s a perfect mirrur.
In such c a s e , a l m o s t a l l p h o t o n s a r e absorbed by the f i r s t atomic layer and immediately reemittedt s o they look like reflected.
In actual materials, phatons penetrate the medium. The penetration length usually
considered in such problem is the thicUness L -
absorbing i / e of the incident intensity, about O,3.
Along the penetration path most of photons -
a r e a b s o r b e d , converted i n h e a t or 10-0
photoelectronst and those reemitted suffer o 45 9 0
also absorption on the back way before they Incidence Angle 0 (degree)
leave t h e mirror. As a rul?, good normal
incidence mirrors are obtained only from m a t e r i a l s having a p e n e t r a t i o n length - Figure 2 -
s m a l l e r t h a n , o r comparable t o , t h e wavelength of the light t o reflect.
As indicated by curves on figure 3, no material s a t i s f y this condition in the X-UV range. Sot according t o Fresnel formula only grazing incidence worKs in t h i s range for dioptre mirrurs.
Dielectric mirrors f o r visible a r e stacUs of very transparent materials, containing alternating equal thichkness of high and low index transparent materials. I t is a f i r s t example of interferential mirrors using a penetrating wave. Such a principle looks very convenient t o the X-UV range where we cannot avoid the wave penetration. In the case of transparent materialst each interface is only slightly reflecting, due t o the weaK difference between the index of both materials.
,--.
-. A t the end, one g e t a constructive i n t e r f e r e n c e f o r t h e w a v e l e n g h t corresponding t o t h e difference o f optical path between the succesive p a r t i a l waves. By t h i s way 100%
r e f l e c t i v i t y can be obtained w i t h non absorbing media.
A n o t h e r m o d e l o f e f f i c i e n t i n t e r f e r e n t i a l m i r r o r i s the i d e a l Bragg c r y s t a l containing very t h i n l a y e r s o f a p e r f e c t l y a b s o r b i n g m a t e r i a l s e t a t a regular distance i n a perfectly transparent medium. Real c r y s t a l s are n o t l i k e that, b u t atomic planes b r i n g a periodic electronic density modulation which a l s o works c o n v e n i e n t l y . M o r e t h a n t h a t , c r y s t a l l i n e m a t e r i a l s a r e n o t a n a b s o l u t e n e c e s s i t y . I n f a c t , a s remarks very earlier, t h i n amorphous evaporated l a y e r s can i n principle leads t o a more convenient electronic Wavelength (X) density modulation, i f q u a l i t i e s o f the
- Figure 3 - i n t e r f a c e s can be controlled.
I n summary the common p o i n t t o these both kind o f interference m i r r o r s working w i t h a penetrating wave i s the need o f a periodic medium w i t h a d period matching the classical Bragg's law: 2d.sin43 =A.
L e t s t u r n now our a t t e n t i o n t o the efficiency o f such interference mirrors. Calculations show that, even w i t h s l i g h t absorbing materials, the efficiency o f such systems decrease very f a s t and by consequence are both u n e f f i c i e n t in the X-UV range. E. Spiller have been the f i r s t t o clearly p o i n t o u t t h a t w i t h absorbing materials one can s t i l l get noticeable r e f l e c t i v i t y w i t h interference m i r r o r (7). I n t h i s case, one must b u i l d evaporated periodic l a y e r s containing a f i r s t matefial w i t h the maximum absorption a t the desired wavelength a s an e f f i c i e n t s c a t t e r and a second one the more transparent a s possible used as a spacer. In a d d i t i o n one must t o adjust the r e l a t i v e thicKness o f the more absorbing material i n such a way t h a t r e f l e c t i v i t y per period is l a r g e r t h a n i t s absorption . How t o adjust t h i s r e l a t i v e thickness, called p , and how much r e f l e c t i v i t y can be obtained have been n e x t summarized o n a simple graph by A. Vinogradov (g), i n the case o f purely absorbing materials. Such an approximation i s q u i t e valuable f o r the X-UV range. F o r more precise calculations one must take i n t o account the r e f r a c t i v e p a r t o f t h e index, which leads t o a n increase o f the r e f l e c t i v i t y .
I n the case o f the W/C couple o f material, f i g u r e 4 shows the maximum o f the normal '"
incidence r e f l e c t i v i t y which can be obtained, even by optimizing the p parameter f o r each 2
w a v e l e n g t h (9). Due t o t h e q u i t e h i g h .d absorption, just before the Ck edge around 5
44A, the carbone appears clearly as a bad $
transparent material i n t h i s range. Actually, a, 0.4 such edges a r e a l s o p r e s e n t i n t h e tu r e f l e c t i v i t y curves o f any n a t u r a l o r organic 2 0.2
crystals,
t o g e t t h e f o r example most e f f i c i e n t on Ok around i n t e r f e r e n t i a l 16A. So, o,o
" " I
W/C Plultilaver
I ,
%
6 8 10 ZO (0 60 6 0 1 ~ 0 2m
m i r r o r s along a l l t h e X-UV range one needs
t o choose t h e most appropriate materials f o r Wavelength (8)
each range. Such a search have been done by
A. Rosenbluth (9) during h i s t h e s i s and t h e - Figure 4 -
computerized r e s u l t s are given in f i g u r e 5.
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- F i g u r e 5 -
" 6 8 1 0 20 40 60 100 200
W A V E L E N G T H (a].
The v e r t i c a l l i n e s i n d i c a t e where new couple of m a t e r i a l s must be u s e d t o keep t h e maximum normal incidence peak r e f l e c t i v i t y . The optimum p a r a m e t e r p and t h e n e c e s s a r y number of p e r i o d s N t o g e t such r e f l e c t i v i t y a r e given.
Conslderlng a glven wavelength and changlng s l i g h t l y t h r p p a r a m e t e r o f f e r t w o l n t e r e s t l n g p o s s ~ b l l l t i e s .
T h e f l r s t o n e 1s t o optiml:e t h e m u l t l l a y e r s f o r maxlmum p e a k o r i n t e g r a t e d reflectivity, o r f o r t h e b e s t r e s o l u t i o n fiO). The second one
C / W Multalayer 1s t o mlnlmize t h e overlapping o r d e r .
T h e f i g u r e 6 1s a n e x a m p l e of
Perlod d=35& '
calculation f o r t h e c a s e o f W/C multllayer a t % A ( i f ) . With a s m a l l d e c r e a s e of reflectivity ~t IS p o s s i b l e t o minimlse a t t h e s a m e time t h e second and t h l r d order. Around p=0.5 w e g o t t h e equivalent o f a c e n t r o s y m e t r l c c e l l well known In d l f f r a c t l n g s y s t e m a s a b l e t o cancel a l l t h e e v e n o r d e r s . An e x p e r i m e n t a l o 2 .4 6 8 check of t h e s e p o s s l b l l l t i e s have been 8=dw/d done (11) and t h e damping of any
- F i g u r e 6 - choosen o r d e r h a s b e e n observed.
I t i s n o t h e r e t h e r i g h t place t o give d e t a i l s a b o u t c a l c u l a t i o n s , e v a p o r a t i o n and t e s t i n g methods developed t o obtained good X-UV m u l t i l a y e r s mirrors. L e t u s j u s t s a y t h a t p r e s e n t l y , r e s u l t s o b t a i n e d in d i f f e r e n t c o u n t r i e s a r e very encouraging, a r e c e n t meeting s p e c i a l l y devoted t o X-Ray Multilayered O p t i c s i i 2 ) can be considered a s a good summary of t h e p r e s e n t s t a t e .
Two e x a m p l e s of r e f l e c t i v i t y curve a r e just given h e r e t o i l l u s t r a t e t h e r e s u l t s .
F i g u r e 7 i l l u s t r a t e s some p e c u l a r i t i e s of such m i r r o r s working i n f a c t with f e w p e r i o d s compared t o a c r y s t a l . The r e f l e c t i v i t y curve v e r s u s t h e incidence angle h a s b e e n g o t with t h e 1.54A Cuk line and a 12 p e r i o d s W/C m u l t i l a y e r s 2d=120A (13). One can s e e t h e t o t a l r e f l e c t i v i t y r a n g e f o r t h e most grazing incidence r a n g e and n e x t t h e Bragg p e a k s corresponding t o t h e N = 112 and 3 t h o r d e r of t h e c l a s s i c a l Bragg formula 2d.sin0 = N A .
Secondary maxima b e t w e e n t w o s u c c e s i v e Bragg p e a k s a r e s i m i l a r t o t h o s e o b s e r v e d i n t h e m u l t i s l i t e x p e r i m e n t s i n v i s i b l e with a s m a l l number of d i f f r a c t i n g objects. So, adding 2 t o t h e number of o b s e r v e d secondary maxima g i v e s t h e number of working p e r i o d s of t h e mirror.
When a f i x e d angle of incidence is u s e d and t h e band p a s s of t h e multilayer i s r e g i s t r a t e d v e r s u s wavelength, such secondary maxima can a l s o been o b s e r v e d on each s i d e of t h e r e f l e c t i v i t y peak.
INCIDENCE ANGLE BP 8 B R A G G ANGLE (Deg.)
- F i g u r e 7 - - F i g u r e 3 -
The f i g u r e 8 s h o w s t h e r e f l e c t i v i t y o b s e r v e d on t h e copper e m i s s i o n l i n e s iLa,pf a t i3.2A with a n organic c r y s t a l , l e a d s t e a r a t e in t h i s c a s e , and a 2 1 p e r i o d s W/C multilayer prepared w i t h a l m o s t t h e s a m e 2d. Such a d i r e c t comparison d e m o n s t r a t e s t h a t optimized m u l t i l a y e r s can be much more e f f i c i e n t t h a n organic c r y s t a l s (14) f o r i n t e g r a t e d and peak r e f l e c t i v i t y . Especially optimized multilayer f o r a given wavelength can be more e f f i c i e n t t h a n t h i s one.
The r e s o l u t i o n of t h e m u l t i l a y e r s is l o w e r d u e t o t h e s m a l l number o f l a y e r s .
NEW POSSIBILITIES GIVEN BY X-UV MULTILAYERS
S e v e r a l new w a y s t o d e s i g n multilayered o p t i c s have been t e s t e d . In a d d i t i o n of t h e p o s s i b i l i t i e s t a aptirnize such m i r r o r s f o r any wavelength a t any choosen incidence, including t h e normal one, we can o b t a i n e d more e f f i c i e n t and more s t a b l e l a y e r s under high flux t h a n with organic c r y s t a l s . S t a b i l i t y of m u l t i l a y e r s under high pulsed X-ray b u r s t begin t o r e c e i v e a t t e n t i o n f o r plasma d i a g n o s t i c s (15).
A s s u g g e s t e d b y s e v e r a l people, it is p o s s i b l e t o p r e p a r e m u l t i l a y e r s with a g r a d i e n t of t h e period along one direction. Keeping f i x e d t h e i n c i d e n t angle and moving such multilayer along t h e graded d i r e c t i o n i n f r o n t of a n X-UV beam will change t h e r e f l e c t e d wavelentgh. So w i t h o u t any change in t h e incidence angle, t h e e q u i v a l e n t of a double monochromator can be o b t a i n e d hy using a s i n g l e t r a n s l a t i o n and t w o p a r a l l e l m u l t i l a y e r s (16). More r e c e n t l y i t h a s b e e n proposed t o u s e such g r a d e d m u l t i l a y e r on p o i n t t o p o i n t f o c u s s i n g o p t i c s t o r o i d a l mirrors. Changing t h e period d locally a s d e s i r a t e d l e a d s t o enhance t h e r e f l e c t i v i t y o r t o r e s t r i c t t h e band p a s s of t h e focused beam 117).
F i g u r e 9 s h o w s a p r o p o s a l t o o b t a i n q u a s i n o r m a l X-UV e f f i c i e n t g r a t i n g (101.
R e f l e c t i v i t y w o u l d b e o b t a i n e d by t h e multilayer and d i s p e r s i o n by t h e g r a t i n g . Obviously f o r a choosen wavelength it IS n e c e s s a r y t o match t h e blaze a n g l e and t h e groove d i s t a n c e of t h e g r a t i n g with t h e multilayer period. I n f a d n o t only t h e e c h e l e t t e g r a t i n g b u t any Rlnd of groove s h a p e c a n b e u s e d , a s d e m o n s t r a t e d by r e c e n t electromagnetic t h e o r e t i c a l calculations on such l a y e r e d g r a t ~ n g s (18).
A n o t h e r way opened by m u l t i l a y e r s is t h e p o s s i b i l i t y t o build X-UV p o l a r i z e r s and
p o l a r i m e t e r s . LiRe in o t h e r s e l e c t r o m a g n e t i c - F i g u r e 9 -
r a n g e , t h e u s e of t h e B r e s w t e r angle permit
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t o r e f l e c t only one o f t h e component o f l i g t h and s o t o polarize a beam by reflection. That happens f o r the incidence angle such t h a t the refracted and the r e f l e c t e d d i r e c t i o n o f the wave are a t 9 0 ' . Because r e f r a c t i v e index i s almost the u n i t y over a l l the X-UV range, the 45' incidence provide always a very good polarization r a t e a s appears on t h e f i g u r e 2.
Unfortunatly, such a p o s s i b i l i t y i s forbidden w i t h classical single i n t e r f a c e m i r r o r s because f o r such large angles the r e f l e c t i v i t y efficiency i s t o o low. On t h e contrary, a multilayer optimized f o r such incidence can have good r e f l e c t i v i t y and so t o be an e f f e c t i v e polarizers.
P o s s i b i l i t i e s o f X-UV polarizers and polarimeters have been demonstrated recently (6, 19).
Multilayered o p t i c s being able t o work a t the normal incidence, i s i n principle possible t o r e t u r n t o the w e l l known o p t i c a l system using normal incidence mirror f o r telescope or microscope. Such p o s s i b i l i t y have been f i r s t t e s t e d a t 44.4A w i t h a spherically bended s i l i c o n wafer previously coated w i t h a m u l t i l a y e r s (20). Schwarzchild microscope w i t h t w o spherical confocal m i r r o r s have been n e x t designed f o r synchrotron source (21). Another example o f imaging multilayered optics i s the telescope b u i l t a t the I n s t i t u t d'optique d'Orsay f o r the french astrophysicists (22). I t i s a Ritchey-Chretien type system where aspherization o f the f i r s t mirror have been achieved by a graded single l a y e r evaporated on the spherical blank before t h e m u l t i l a y e r coating (23).
X-ray Fabry-Perot, first demonstrated by T. Barbee, i s an other major advance which must b e underligned (24,25).
EXAMPLES OF THE USE OF MULTILAYERS FOR PLASMA DIAGNOSTICS
Figure 10 shows the principle o f t h e system designed by t h e L ~ v e r m o r e group t o receive simultaneously on the same streak camera photocathod several small band pass o f the spectrum e m i t t e d by a l a s e r plasma (26). I t i s certainly the f i r s t system t a k n g advantage of t h e m u l t i l a y e r possibilities, i n t h i s case t o choose f r e e l y the 2d spacing. By t h i s way they can work a t the Bragg condition f o r d i f f e r e n t choosen wavelengths, b u t w i t h the same Q Bragg angle.
L- irradiated Drgt L- irradiated Drgt
- F i g u r e 10 -
Imaging o f plasma w i t h grazing incidence KirKpatrick's m i r r o r s o r pinhole camera optics have always been l i m i t e d by the d i f f i c u l t y t o select a small band pass o f the spectrum passing through these optics. M e t a l l i c f i l t e r s are very u n e f f i c i e n t i n t h e X-UV and m u l t i l a y e r s appear a s a good s o l u t i o n f o r such f i l t e r i n g . Such a band pass selection f o r X-UV pinhole camera had been used a t the PM1 Laboratory f o r l a s e r plasma imaging. A metallic f i l t e r is j l s t added t o p r o t e c t t h e m u l t i l a y e r s from the plasma b u r s t b u t a l s o t o block the v i s i b l e and U V which would be r e f l e c t e d by the multilayers. With such a system, images o f aluminum i o n s emitted a t i 5 4 A have permitted t o compare the l a t e r a l expansion o f t h e plasma f o r t h e W and 2w frequency o f the l a s e r (27).
Combining a f l a t and a focussing m u l t i l a y e r s with matched p e r i o d s , t h e s a m e group of t h e PM1 l a b o r a t o r y a l s o achieved t h e f i r s t d e m o n s t r a t i o n of t h e p o s s i b i l i t y t o experimentally e v a l u a t e i n X-UV t h e r e f r a c t i v e index of l a s e r produced plasma (28). The scheme of t h e s e t up is given on f i g u r e 11.
Laser
AI F i l t e r Beam 1
Knife Spherical
--I---
M u l t i l a y e r e Mirror
- F i g u r e 11 -
Beam 2
One p a r t of t h e l a s e r beam produces t h e X-IJV probed plasma. The 45" f l a t multllayered mirror w a s optlmlzed t o r e f l e c t only t h e 154A f o r whlch t h e y wlsh t o t e s t t h e r e f r a c t i v e index. No s t r o n g line of t h l s wavelength 1s e m l t t e d by t h e probed plasma. A p a r a l l e l beam of t h e d e s i r a t e d wavelentgh, a s t r o n g 154A l i n e e m i t t e d b y a Cu t a r g e t , 1s s e l e c t e d and r e f l e c t e d In a p a r a l l e l beam ny u s i n g a n o t h e r spherical multilayered mirror. Only t h i s p a r a l l e l beam a f t e r refraction through t h e plasma pruduces a s p o t on t h e photographic plate, and t h e measured deflection h a s been u s e d t o e v a l u a t e t h e refractive ~ n d e x .
D i r e c t imaging of plasma by l a r g e angle collecting o p t i c s is a n o t h e r p o s s i b i l i t y open by normal incidence s p h e r i c a l o p t i c s , with t h e a d d i t i o n a l a d v a n t a g e of magification. A Schwarzschild microscope have been r e c e n t l y b u i l t a t t h e I n s t i t u t d'Optique d t O r s a y f o r t h e PM1 Lab t o o b t a i n X-UV image a t t h e r e a r s u r f a c e of a f o i l i n view t o s t u d y t h e h e a t t r a n s f e r t i n s i d e t h e t a r g e t during t h ~ . plasma formation. R e s u l t s o b t a i n e d a t 304A with a 6p s p a t i a l r e s o l u t i o n during t h e f i r s t t r i a l s look very promising ( 2 9 ) .
X ray l a s e r r e s e a r c h , which began i n t h e s e v e n t i e s (30) is a n o t h e r f i e l d where m u l t i l a y e r s can be hepful, e v e n if a complete r e s o n a t o r is n o t y e t working. A s proposed i n 1982 by o u r group (31) and d e m o n s t r a t e d l a s t y e a r a t Princeton 132), a s i n g l e normal incidence multilayer mirror set i n normal incidence can r e f l e c t enough i n t e n s i t y through t h e plasma column t o praduce a n o t i c e a b l e i n t e n s i t y change o n t h e t e s t e d line.
0 1 l ' ! " ' l ! 1
1 0 5 1 1 0
W A V E L E N G T H C A )
- F i g u r e 12 - - 1 0
Z 3 m C 4
>
t 5
V) Z Y l-
?
The f i g u r e 12 s h g w s t h e predicted e f f e c t w i t h a n a c h i e v a b l e m i r r o r r e f l e c t i v i t y and o b s e r v e d g a i n plasma amplification i n o u r experiment 130).
Because n o change i n t h e spectrum can a p p e a r o u t s i d e t h e r e f l e c t e d band p a s s of t h e mirror, a comparison b e t w e e n s h o t s , w i t h a n d w i t h o u t m i r r o r , is s t r a i g t h f o r w a r d . I n addition, t h e r e l a t i v e l i n e i n t e n s i t i e s o v e n t h e unchanged r a n g e give good information a b o u t t h e l a s e r s h o o t and h e l p s f o r s t a t i c t i c a l e r r o r s e v a l u a t i o n s .
-
"with mirror R = 0 . 1 5 ,
-
-
-
+without mirror
A1 Plasma Length 2Gm. ,
C6-274 JOURNAL DE PHYSIQUE
Obviously the realisation of the second multilayer worUng i n t h e semi transparent mode a s an exit window of an X-UV l a s e r is a very exciting problem. Another talks in t h i s meeting will review t h e questions and the s t a t e of a r t on t h i s field (32,33).
CONCLUSIONS
Limitations brought by the obligation t o build only X-UV grazing incidence optics a r e now removed a f t e r a long sixty years hlstory. The quite recent X-UV mult~layer mirrors bring completely new possibilities for X-UV optical systems. Some examples of uses, summarized here a r e just a f i r s t blooming of applications. As when a new tool arrives, we are just presently solving the f i r s t class of problems which a r e the most stringent in our work and which look the easiest. No doubt t h a t t h i s new allowance t o conceive X-UV optics, with large collecting apertu* and worKing a t the same time a s a n efficient band p a s s f l l t e r over all the ranget will lead t o a large diversity of s y s t e m s helping t h e plasma diagnostics. We have not yet a l l the equivalent optical systems of t h e visible range, but t h i s s t e p ahead with interference mirrors have reduced the gap which s e p a r a t e s t h e X-UV optics of t h e numerous possibilities available in other ranges.
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