SOFT X-RAY LASER CAVITIES

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N. Ceglio, D. Stearns, A. Hawryluk, T. Barbee, K. Danzmann, M. Kuhne, P.

Mueller, B. Wende, M. Stearns, A. Petford-Long, et al.

To cite this version:

N. Ceglio, D. Stearns, A. Hawryluk, T. Barbee, K. Danzmann, et al.. SOFT X-RAY LASER CAVI- TIES. Journal de Physique Colloques, 1986, 47 (C6), pp.C6-277-C6-286. �10.1051/jphyscol:1986635�.

�jpa-00225878�

JOURNAL D E PHYSIQUE,

Colloque C6, supplement au n o 10, Tome 47, octobre 1986

SOFT X-RAY LASER CAVITIES(~)

N.M. CEGLIO, D.G. STEARNS, A.M. HAWRYLUK, T.W. BARBEE,

K. DANZMANN*, M. KUHNE*, P. MUELLER*, B. WENDE*, M.B. STEARNS", A.K. PETFORD-LONG"* and C.-H. CHANG"

Lawrence Livermore National Laboratory, P.O. Box 5508, Livermore, CA 94550, U. S. A.

"~hysikalische Technische Bundesanstalt Institute Berlin, Abbestrasse 2-12, 0-1000 Berlin, F.R.G.

X I

Department of Physics, Arizona State University, Tempe, AZ 85287, U.S.A.

"'center for Solid State Sciences, Arizona State University, Tempe, AZ 85287, U.S.A.

Abstract - We report progress in the development of multilayer components for use in multiple pass soft x-ray laser cavities operating in the 1001-300A spectral range. Our work includes fabrication and characterization of multi- layer components ; simple resonant cavity design ; damage threshold assess- ment for multilayers in the x-ray laser environment ; and multipass cavity experiments for efficiency enhancement and transverse mode selection.

INTRODUCTION

Recent success i n t h e generation o f a m p l i f i e d spontaneous emission (ASE) C l , 2 , 31 i n t h e 100A-300A range has r e k i n d l e d i n t e r e s t i n o p t i c a l components f o r t h e c o n t r o l and m a n i p u l a t i o n o f s o f t X - r a d i a t i o n . Of p a r t i c u l a r i n t e r e s t are normal i n c i d e n c e m u l t i l a y e r components f o r use i n s o f t x-ray l a s e r c a v i t i e s . We r e p o r t on progress i n t h e development o f m u l t i l a y e r components f o r use i n mu1 t i p a s s s o f t x-ray l a s e r c a v i t i e s o p e r a t i n g i n t h e 100K300A s p e c t r a l range. Our program i n c l u d e s work on t h e f a b r i c a t i o n and c h a r a c t e r i z a t i o n o f mu1 t i la y e r components; simple resonant c a v i t y design; damage t h r e s h o l d assessment f o r m u l t i l a y e r s i n t h e x-ray l a s e r environment: and m u l t i p a s s c a v i t y experiments f o r e f f i c i e n c y enhancement and transverse mode s e l e c t i o n .

a) M o t i v a t i o n :

There are a t l e a s t t h r e e s t r o n g m o t i v a t i o n s f o r x-ray c a v i t y development:

enhancement o f o u t p u t e f f i c i e n c y , c o r r e c t i o n o f r e f r a c t i v e e f f e c t s , and improvement o f s p a t i a l coherence. Experiments u t i l i z i n g a 1 cm. long selenium x-ray l a s e r have produced approximately 0.5 x10-6 j o u l e o f ASE a t 206.3A and 209.6A. C21 Gain measurements, on t h e o t h e r hand, i n d i c a t e t h a t as many as 3 X

10-3 j o u l e a r e s t o r e d i n t h e p o p u l a t i o n i n v e r s i o n . A resonant c a v i t y C41 p r o v i d i n g as few as t h r e e passes through such an x-ray g a i n medium c o u l d d r i v e the o s c i l l a t i o n s i n t o s a t u r a t i o n and thereby improve t h e o u t p u t e f f i c i e n c y by t h r e e orders o f magnitude.

his work was performed under t h e a u s p i c e s o f t h e U . S . Department of Energy under c o n t r a c t W-7405-ENG-48

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986635

R e f r a c t i v e e f f e c t s a r e an i m p o r t a n t c o n s i d e r a t i o n f o r t h e t r a n s p o r t o f x-rays through t h e h i g h d e n s i t y l a s e r g a i n medium. The r e a l p a r t o f t h e r e f r a c t i v e index (6) i n t h e h i g h l y i o n i z e d g a i n medium v a r i e s as

where K depends o n l y on t h e x-ray wavelength and ne, t h e e l e c t r o n d e n s i t y , i s very l i k e l y a monotonical l y decreasing f u n c t i o n o f r a d i u s . Under these

c o n d i t i o n s an x-ray beam w i l l expand, i . e . diverge, as i t propagates through t h e gain medium. P r o p e r l y f i g u r e d c a v i t y m i r r o r s c o u l d perhaps c o r r e c t f o r beam divergence, keeping t h e r a d i a t i o n confined w i t h i n t h e g a i n medium. This would be p a r t i c u l a r l y u s e f u l f o r s c a l i n g t o s h o r t e r l a s e r wavelength, where h i g h e r plasma d e n s i t i e s ( l e a d i n g t o g r e a t e r r e f r a c t i v e e f f e c t s ) w i l l be r e q u i r e d t o enhance gain.

S p a t i a l coherence i s an important o u t p u t requirement f o r x-ray l a s e r

a p p l i c a t i o n s . I n c u r r e n t experiments t h e x-ray l a s e r o u t p u t i s mu1 timode and s p a t i a l l y i n c o h e r e n t . The p o s s i b i l i t y o f u s i n g a c a v i t y f o r t r a n s v e r s e mode d i s c r i m i n a t i o n l e a d i n g t o a s p a t i a l l y coherent s i n g l e mode o u t p u t would have dramatic impact on coherent x-ray o p t i c s .

b ) Chal l enges :

Laboratory x-ray l a s e r s , as c u r r e n t l y configured, p r o v i d e a number o f unique challenges f o r resonant c a v i t y development. The combination o f s h o r t l a s e r wavelength (X 5 210 W), s h o r t g a i n d u r a t i o n ( 5 200 X 10-12 sec),

r e f r a c t i v e e f f e c t s i n t h e g a i n medium C51, and m i r r o r damage from t h e i n t e n s e r a d i a t i o n f l u x make t h e achievment o f a p r a c t i c a l , e f f i c i e n t , s i n g l e mode resonant c a v i t y a formidable t a s k . Because o f t h e s h o r t wavelength. t h e Fresnel number C61,

a = c a v i t y r a d i u s L = c a v i t y l e n g t h X = wavelength

o f an x-ray l a s e r c a v i t y w i l l t y p i c a l l y be >>l. I n a d d i t i o n , t h e s h o r t g a i n d u r a t i o n a l l o w s o n l y a f e n a m p l i f i c a t i o n passes,

t = g a i n d u r a t i o n c = l i g h t speed

0 = number o f s i n g l e passes

through t h e medium C71. E f f i c i e n t c o u p l i n g o f t h e s t o r e d i n v e r s i o n energy i n t o a s i n g l e o u t p u t mode t y p i c a l l y r e q u i r e s

The prospects f o r a c h i e v i n g t h i s c o n d i t i o n w h i l e a >> X and t = L/c a r e n o t v e r y promising. For t h e near term we are forced t o consider a low e f f i c i e n c y . s i n g l e mode o s c i l l a t o r f e e d i n g a chain o f h i g h power a m p l i f i e r s , andfor t h e p o s s i b i l i t y o f an u n s t a b l e r e s o n a t o r design w i t h a l a r g e "unstable mode'' volume

C81.

The x-ray l a s e r g a i n medium i s t y p i c a l l y a h i g h temperature, h i g h d e n s i t y . inhomogeneous plasma C l , 21. I t i s an i n t e n s e x-ray source p r o v i d i n g a h o s t i l e environment f o r t h e c a v i t y m i r r o r s , which must be c l o s e t o t h e g a i n medium because o f t h e s h o r t g a i n d u r a t i o n . I n c u r r e n t selenium x-ray l a s e r experiments

C31, c a v i t y m i r r o r s w i l l be subjected t o a f l u x o f ASE approaching 6 X l o 1 0 watt/cm2. Perhaps even more damaging w i l l be t h e continuum

r a d i a t i o n , which i s estimated t o be an o r d e r o f magnitude more intense. There i s no doubt t h a t m u l t i l a y e r c a v i t y m i r r o r s w i l l be destroyed on each l a b o r a t o r y x-ray l a s e r experiment [ g ] . However, p r e c i s e damage dynami C S and t h e p o s s i b l e s u r v i v a l o f c a v i t y m i r r o r s l o n g enough t o p r o v i d e feedback d u r i n g t h e p e r i o d of optimum g a i n a r e unresolved i ssues r e q u i r i n g experimental i n v e s t i g a t i o n .

I n a d d i t i o n t o damage problems, r e f r a c t i v e e f f e c t s due t o plasma inhomogenei t i es can l e a d t o beam s t e e r i n g a n d l o r beam blooming, making r a d i a t i o n t r a n s p o r t through t h e g a i n medium problematic C51.

a) b)

F i g u r e 1: Schematic r e p r e s e n t a t i o n s o f (a) x-ray b e a m s p l i t t e r o r p a r t i a l l y t r a n s m i s s i v e m i r r o r , and (b) a h i g h l y d i s p e r s i v e x-ray m i r r o r , comprised o f an x-ray g r a t i n g on ( o r i n ) a m u l t i l a y e r m i r r o r .

Among t h e v a r i e d issues i n v o l v e d i n x-ray l a s e r c a v i t y design i s t h e mechanism f o r c o u p l i n g t h e r a d i a t i o n o u t o f t h e c a v i t y . We a r e developing two d i s t i n c t o u t p u t c o u p l i n g components, a p a r t i a l l y t r a n s m i t t i n g x-ray m i r r o r (Fig. 1 (a)) and a h i g h l y d i s p e r s i v e x-ray m i r r o r (Fig. 1 (b)). The x-ray b e a m s p l i t t e r ( F i g . 1 (a)) r e q u i r e s a support membrane t h a t i s o p t i c a l l y f l a t , smooth, and t h i n enough f o r s i g n i f i c a n t t r a n s m i s s i o n i n t h e 100X-300X s p e c t r a l range. The

d i s p e r s i v e m i r r o r ( F i g . 1 ( b ) ) combines m u l t i l a y e r and sub-micron d i f f r a c t i o n g r a t i n g technologies t o p r o v i d e e f f i c i e n t r e f l e c t i o n a t normal i n c i d e n c e w h i l e producing a v a r i e t y of w i d e l y dispersed o u t p u t beams.

Our program i n x-ray l a s e r c a v i t y development i s addressing t h e d i f f i c u l t issues r a i s e d i n t h i s i n t r o d u c t o r y discussion. F i r s t and foremost among our e f f o r t s i s t h e development o f normal i n c i d e n c e m u l t i l a y e r components a p p r o p r i a t e f o r use as m i r r o r s and o u t p u t couplers i n a p r a c t i c a l c a v i t y . These e f f o r t s a r e reviewed i n t h e f o l l o w i n g section.

F a b r i c a t i o n and C h a r a c t e r i z a t i o n o f M u l t i l a v e r Com~onents Our e f f o r t i n m u l t i l a y e r development i s b u i l t around our c a p a b i l i t i e s i n t h e f o l l o w i n g c r i t i c a l areas:

i) t h i n f i l m d e p o s i t i o n techniques ii) submicron l i t h o g r a p h i c technology

i i i Transmission E l e c t r o n Microscope (TEM) c h a r a c t e r i z a t i o n w i t h sub-nanometer r e s o l u t i o n

i v ) h i g h p r e c i s i o n x-ray c h a r a c t e r i z a t i o n ( e s p e c i a l l y a t normal incidence)

This unique combination o f f a b r i c a t i o n and c h a r a c t e r i z a t i o n c a p a b i l i t i e s enables us t o c o n t r o l f a b r i c a t i o n parameters and c o r r e l a t e them w i t h s t r u c t u r a l d e t a i l and x-ray performance i n o r d e r t o r a p i d l y evolve toward h i g h e r performance x-ray o p t i c a l devices. A t present we a r e focussing our f a b r i c a t i o n and

c h a r a c t e r i z a t i o n e f f o r t s on t h r e e types o f normal incidence m u l t i l a y e r components: x-ray m i r r o r s , x-ray b e a m s p l i t t e r s , and h i g h l y d i s p e r s i v e x-ray mi r r o r s .

a) Normal I n c i d e n c e X-ray M i r r o r s (X = 208 A)

Normal incidence mu1 t i la y e r c a v i t y m i r r o r s f o r use on t h e neon-l i ke selenium l a s e r C l , 21 have been f a b r i c a t e d and characterized. Performance c h a r a c t e r i s t i c s f o r one such m i r r o r a r e presented i n F i g u r e 2 . M i r r o r r e f l e c t i v i t y i s p l o t t e d versus wavelength. The x-rays a r e i n c i d e n t on t h e m i r r o r a t 8 = 0.5" from surface normal. Peak r e f l e c t i v i t y i s approximately 26% a t X = 210 A. The bandpass (FWHM) i s approximately 25 R. A TEM micrograph o f a cross s e c t i o n o f t h a t same m i r r o r i s presented i n F i g u r e 3. An i n depth a n a l y s i s o f t h e TEM data Cl01 as w e l l as d e t a i l s o f sample p r e p a r a t i o n [ l 1 1 w i l l be provided elsewhere.

However, a number o f i n t e r e s t i n g c h a r a c t e r i s t i c s o f t h e mu1 t i la y e r s t r u c t u r e are immediately e v i d e n t :

i) The m u l t i l a y e r p e r i o d measured by e l e c t r o n d i f f r a c t i o n ( d = 107 A) a p p a r e n t l y d i f f e r s from t h a t measured by Cu K, x-ray d i f f r a c t i o n (d = 115 A).

ii) Much o f t h e Molybdenum l a y e r i s composed o f a p r e f e r e n t i a l l y - o r i e n t e d p o l y c r y s t a l l i n e s t r u c t u r e .

iii) The l a y e r s appear q u i t e f l a t and u n i f o r m i n thickness, b u t t h e 5 n t e r f a c e s between t h e Molybdenum and Si l i c o n a r e q u i t e complex, exhi b i t i n g considerable i n t e r d i f f u s i o n . I n p a r t i c u l a r , t h e Molybdenum on S i l i c o n i n t e r f a c e i s more d i f f u s e than t h e S i l i c o n on Molybdenum i n t e r f a c e .

A Mo/Si rnultilaver mirror I

Wavelength A

F i g u r e 2: R e f l e c t i v i t y o f a m u J t i l a y e r x-ray m i r r o r a t v i r t u a l l y normal incidence.

F i g u r e 4: Photograph of f i v e x-ray b e a m s p i i t t e r "windows". Each b e a m s p l i t t e r i s + a MoISi m u l t i l a y e r supported on a t h i n Si3N4 membrane.

Efforts are underway to model the complex multilayer structure observed in the TEM data. Conventional bi-layer models of mu1 ti layer mirrors (and beamsplitters) are too crude and simplistic to provide a quantitative accounting of the x-ray optical performance of real mirrors .

Silicon i

-

( r l58A)

K Silicon su bstrate Figure 3: TEM micrograph of a cross section of the x-ray mirror characterized in Figure 2.

b) Normal Incidence X-ray Beamsplitters (X = 130 R and 208 R)

The first soft x-ray beamsplitters have been successfully fabricated and characterized. They will be used as output coupling mirrors in x-ray laser cavities at X = 130 R (neon-like molybdenum laser [31) and X = 208 R (neon-like selenium laser C l , 21). The beamsplitter is a mu1 tilayer mirror supported on a very thin (= 300 R) silicon nitride (Si3N4) membrane. The

fabrication technique for the beamspl i tter is easily summarized. A p01 i shed, 250 pm thick silicon wafer is coated with a thin Si3N4 layer by chemical vapor deposition. The Mo/Si multilayer mirror is subsequently deposited on top of the Si3N4 by magnetron sputtering techniques. The silicon support wafer is then chemically etched, taking care to protect the multi layer from attack by the chemical etchant. A typical result of this process is presented in Figure ^4.

Five etched "windows" of Si3N4 supporting a MoISi mu1 ti layer structure are shown. A typical beamsplitter area is = 5 mm2. The largest soft x-ray beamsplitter that we have fabricated to date has an area of

-

Performance c h a r a c t e r i s t i c s f o r normal incidence b e a m s p l i t t e r s are presented i n Figures 5 and 6. Again, t h e x-rays a r e i n c i d e n t a t €I = 0.5" from s u r f a c e normal. The b e a m s p l i t t e r represented i n F i g u r e 5 i s designed f o r use a t X = 130 A. I t has a peak transmission approaching 50%, and a peak r e f l e c t i v i t y o f 8%. The bandpass (FWHM) i s approximately 20 A . A second b e a m s p l i t t e r , designed f o r use a t X = 208 A i s represented i n F i g u r e 6. I t has a peak r e f l e c t i v i t y o f approximately 17% and a bandpass (FWHM) o f approximately 28 A. I t has a transmission a t X = 208 A o f o n l y 5% due t o a b s o r p t i o n i n t h e Si3N4 membrane and t h e MoISi m u l t i l a y e r .

TEM analyses i n d i c a t e t h a t performance l i m i t a t i o n s i n t h e x-ray b e a m s p l i t t e r s may be due t o i m p e r f e c t i o n s i n t h e chemical vapor deposited Si3N4 support l a y e r . This i s i l l u s t r a t e d i n F i g u r e 7. A TEM micrograph of a cross s e c t i o n o f t h e 208 A beamspli t t e r ( f r o m F i g u r e 6) i s presented C121. Roughness i n t h e Si3N4 l a y e r i s f a i t h f u l l y r e p l i c a t e d i n t h e MoISi l a y e r s above it. These l a r g e s c a l e f l u c t u a t i o n s i n t h e l a y e r e d s t r u c t u r e d i s r u p t resonant processes

Incident I I

Mo/Si multilayer (d = 7 1 i , N = 11) on 3004 Si3N4 membrane Incident

- - - -

- - - +

Reflected I I Transmitted

F i g u r e 5: Performance c h a r a c t e r i s t i c s o f an x-ray beamspl i t t e r a t v i r t u a l 1 ~ normal i n c i d e n c e (resonant wavelength z 130 A ) .

Beamsplitter is Mo/Si m u l t i l a y e r (d--115K, N = 7 ) supported o n a 300.4 Si3N4 membrane

25

Transmission 2 0 = 1 °

20 15 10 5

0 1 6 0 180 200 220 240 2 6 0 280 3 0 0

F i g u r e 6: Performance c h a r a c t e r i s t i C S o f an x-ray beamspl i t t e r a t v i r t u a l l y normal i n c i d e n c e (resonant wavelengtn 2 208 A ) .

M o l y b d e n u m d 2: 108A

/= 55 + 18) ^{N = 6}

Silicon

( 2 52 f 18)

/-- Si3N4 s u p p o r t

/ ^{( 2} 4 4 0 ~ )

. - ^c S i l l c o n s u b s t r a t e

,*

F i g u r e 7: TEM micrograph o f a cross s e c t i o n o f t h e x-ray bearnsplifter c h a r a c t e r i z e d i n F i g u r e 6.

a f f e c t i n g beamspl i t t e r r e f l e c t i v i t y and transmission C101. A number o f b e a m s p l i t t e r s t r u c t u r e s have been analyzed, and a s t r o n g c o r r e l a t i o n between Si3N4 i m p e r f e c t i o n s and reduced x-ray performance has been demonstrated.

c) H i g h l y D i s p e r s i v e X-ray M i r r o r s ( X 1 2 0 8 R )

The f i r s t h i g h l y d i s p e r s i v e m u l t i l a y e r x-ray m i r r o r s f o r use a t normal incidence have been f a b r i c a t e d and characterized. They will be used as novel o u t p u t c o u p l i n g m i r r o r s i n x-ray l a s e r c a v i t i e s a t X .I208 A (neon-like selenium

l a s e r C l , 21). F i g u r e 8 i l l u s t r a t e s t h e simple c a v i t y concept. The z e r o t h order r e f l e c t i o n from t h e d i s p e r s i v e m i r r o r provides c a v i t y feedback, and t h e

d i f f r a c t e d o r d e r s p r o v i d e a v a r i e t y of o u t p u t beams.

Rear cavity m i r r o r

Gain m e d i u m

Highly dispersive m i r r o r f o r o u t p u t coupling Figure 8: Novel x-ray l a s e r c a v i t y concept u t i l i z i n g a h i g h l y d i s p e r s i v e mu1 t i la y e r m i r r o r f o r o u t p u t coup1 ing.

I n t h e p a r t i c u l a r embodiment r e p r e s e n t e d i n F i g u r e 9 , t h e d i s p e r s i v e m i r r o r i s comprised o f a 2000 A p e r i o d . 500 A t h i c k , g o l d g r a t i n g a t o p a MoISi

mu1 ti la y e r m i r r o r d e s i g n e d f o r a peak, normal i n c i d e n c e r e f l e c t i v i t y a t X 2 208 A . The x-rays r e p r e s e n t e d i n F i g u r e 9 a r e i n c i d e n t a t 8 = 0.5"

f r o m s u r f a c e normal. The z e r o t h o r d e r r e f l e c t i v i t y ( a t 28 = 1") i s

a p p r o x i m a t e l y 2%. The h i g h e r d i f f r a c t e d o r d e r s a r e each s e p a r a t e d b y

-

A l t h o u g h o u r most s i g n i f i c a n t accomplishments t o d a t e have been i n t h e f a b r i c a t i o n and c h a r a c t e r i z a t i o n o f mu1 t i la y e r c a v i t y components, s i g n i f i c a n t work i s on-going i n a r e a s of p r a c t i c a l " c a v i t y e n g i n e e r i n g . " These a c t i v i t i e s a r e d i s c u s s e d i n t h e n e x t s e c t i o n . *

mirror

Angle (degrees)

F i g u r e 9 : A h i g h l y d i s p e r s i v e mu1 t i la y e r X - r a y . m i r r o r : SEM photo; schematic concept; performance c h a r a c t e r i s t i c s a t X = 208 A f o r v i r t u a l normal

i n c i d e n c e .

F u t u r e D i r e c t i o n s

Work i s on-going i n t h e areas o f p r a c t i c a l c a v i t y design, x - r a y damage

assessment, and m u l t i p a s s c a v i t y experiments. S i n c e some c r i t i c a l experiments have y e t t o be completed, t h e s e t o p i c s w i l l be d i s c u s s e d o n l y i n c o n c e p t u a l terms. E x p e r i m e n t a l d e t a i l s and r e s u l t s w i l l be r e p o r t e d elsewhere [131.

a) C a v i t y Design:

As i n d i c a t e d e a r l i e r , r e f r a c t i v e e f f e c t s i n t h e x - r a y l a s e r medium c a n n o t be n e g l e c t e d . Beam s t e e r i n g and blooming e f f e c t s have a l r e a d y been observed a t 206.3 A and 209.6 A i n g a i n media 1 3 cm. l o n g 1 3 , 141.

The chal l enges f o r p r a c t i c a l c a v i t y design f a 1 l i n t o two general c a t e g o r i e s : compensation f o r r e f r a c t i v e e f f e c t s and p r o d u c t i o n o f a s p a t i a l l y coherent s i n g l e mode o u t p u t . The problem o f e s t a b l i s h i n g a s p a t i a l l y s t a b l e i n t e n s i t y

d i s t r i b u t i o n ( n o t n e c e s s a r i l y a s i n g l e resonant mode) o s c i l l a t i n g between two m i r r o r s separated by an inhomogenous g a i n medium r e q u i r e s t h e design o f a p a i r o f phase conjugate m i r r o r s matched t o t h e i n c i d e n t phase f r o n t s . M i r r o r design w i l l be s t r o n g l y dependent on t h e inhomogeneous s t r u c t u r e o f t h e medium C51.

Nevertheless, f o r a known, s t a b l e plasma inhomogenei t y , m i r r o r s could i n p r i n c i p l e be designed which would match t h e shape o f an i n c i d e n t wavefront and merely reverse i t s phase. Then by o p t i c a l r e v e r s i b i l i t y , a s t a b l e p a t t e r n c o u l d be e s t a b l i s h e d between t h e two a p p r o p r i a t e l y shaped m i r r o r s .

The achievement o f a s p a t i a l l y coherent, s i n g l e mode x-ray l a s e r o u t p u t would have s i g n i f i c a n t impact i n x-ray l a s e r a p p l i c a t i o n s . The mode a n a l y s i s f o r a r a d i a l l y inhomogeneous g a i n medium i s mathematically e q u i v a l e n t t o t h a t f o r g r a d i e n t index o p t i c a l f i bers except t h a t t h e r e f r a c t i v e index increases ( i n s t e a d o f decreases) w i t h r a d i u s , and t h e boundary c o n d i t i o n s a r e somewhat modified.

A f t e r i d e n t i f y i n g t h e s p a t i a l d i s t r i b u t i o n o f t h e lowest o r d e r mode, m i r r o r s and i n t r a - c a v i t y s p a t i a l f i l t e r s need be designed t o d i s c r i m i n a t e a g a i n s t h i g h e r order modes i n j u s t a few a m p l i f i c a t i o n passes.

b) X-ray Damage

The assessment o f x-ray damage i s c r i t i c a l t o t h e e v a l u a t i o n o f t h e v i a b i l i t y o f mu1 ti la y e r m i r r o r s f o r x-ray l a s e r c a v i t i e s .

The s h o r t g a i n d u r a t i o n o f c u r r e n t LLNL x-ray l a s e r schemes C l , 2, 31 r e q u i r e s t h a t t h e m i r r o r s be placed c l o s e t o t h e g a i n medium, From p a s t experiments C91 we know t h a t c a v i t y m i r r o r s are destroyed on each x-ray l a s e r shot. However, the p r e c i s e t i m i n g and mechanism o f m i r r o r f a i l u r e a r e unknown. These issues form f i e focus o f n e a r term experiments on t h e Nova f a c i l i t y . We w i l l perform t i m e r e s o l v e d spectroscopi c measurements o f ASE r e f l e c t e d o f f mu1 ti la y e r m i r r o r s d u r i n g x-ray l a s e r experiments. We expect t o determine damage t h r e s h o l d s and p r e c i s e symptoms o f m i r r o r f a i l u r e . Such symptoms c o u l d range from a sudden and c a t a s t r o p h i c f a i l u r e , t o a gradual s h i f t and broadening o f t h e resonance as t h e m u l t i l a y e r heats and expands i n an ordered fashion.

c) Multi-Pass C a v i t y Experiments

Our u l t i m a t e goal, scheduled on t h e Nova f a c i l i t y f o r F a l l '86, i s t h e

implementation o f a m u l t i p a s s x-ray l a s e r c a v i t y . T h i s w i l l be attempted on t h e neon-like selenium l a s e r a t 206.3 X and 209.6 X u s i n g the normal i n c i d e n c e m i r r o r o f F i g u r e 2, and an improved v e r s i o n o f t h e x-ray b e a m s p l i t t e r o f F i g u r e 6 f o r o u t p u t coupling. (For a simple q u a n t i t a t i v e a n a l y s i s see C41). These i n t e g r a l experiments w i l l c o n f r o n t a1 l t h e p o t e n t i a l d i f f i c u l t i e s reviewed i n t h i s paper. Speculation about r e s u l t s a t t h i s t i m e could n o t hope t o match t h e eloquence o f experimental r e s u l t s .

References

1) D. L. Matthews, e t . a l . , Phys. Rev. L e t t . 54, ! l 0 (1985);

M. D. Rosen, e t . a l . , Phys. Rev. L e t t . 54, 106 (1985)

2) N. M. Ceglio. P r o c e e d i n ~ s 7 t h I n t e r n a t i o n a l Workshop on Laser I n t e r a c t i o n and Re1 ated P1 asma Phenomena, (Monterey, Cai i f o r n i a) October, 1985; a1 so LLNL Report #UCRL-94257.

3) D. L. Matthews, e t . al., paper presented a t t h i s conference.

4) The h y p o t h e t i c a l c a v i t y envisioned f o r t h i s experiment has t h e f o l l o w i n g parameters: r e f l e c t i v i t y o f back m i r r o r = 0.25; r e f l e c t i v i t y of o u t p u t m i r r o r = 0.15: t r a n s m i s s i o n o f o u t p u t m i r r o r = 0.05. A d i s c u s s i o n o f t h e a p p r o p r i a t e c a v i t y g a i n c a l c u l a t i o n i s contained i n : N. M. Ceglio, e t

a l . . Aool i c a t i o n s o f D

O o t i c s , SPIE 563, 360 (1985).

5) C e r t a i n l y an i m p o r t a n t c o n s i d e r a t i o n i s t h e p o s s i b i l i t y o f time v a r y i n g r e f r a c t i v e e f f e c t s i n t h e g a i n medium. There i s a t p r e s e n t no experimental v e r i f i c a t i o n o f t h e importance o f such phenomena, so we have n o t e x p l i c i t l y i n c l u d e d them i n o u r d e l i b e r a t i o n s a t t h i s time.

6) As a f i g u r e o f m e r i t , N i s approximately t h e r a t i o o f t h e c a v i t y volume t o t h e volume o f t h e lowest o r d e r t r a n s v e r s e mode o f t h e c a v i t y . N >> 1 imp1 i es t h a t t h e c a v i t y wi l l operate mu1 ti-mode, and t h e coup1 i ng e f f i c i e n c y i n t o t h e lowest o r d e r mode w i l l be small.

7) Many passes through an a m p l i f i c a t i o n medium are t y p i c a l l y r e q u i r e d f o r d i s c r i m i n a t i o n a g a i n s t h i g h e r l o s s modes. Thi S i s p a r t i c u l a r l y t r u e f o r h i g h g a i n media.

8) W. F. Krupke and W. R. Sooy, IEEE Journal o f Quantum E l e c t r o n i c s OE-5, ⁵⁷⁵

(December, 1969).

9) I n double pass x-ray l a s e r c a v i t y experiments a t LLNL t h e c a v i t y m i r r o r was destroyed on each l a s e r shot. For more d e t a i l s see Ref. C41.

10) Data and a n a l y s i s w i l l be presented by D. G. Stearns, e t . a l . a t t h e SPIE conference on M u l t i l a y e r S t r u c t u r e s and X-ray Laser Research i n San Diego (August. 1986).

11) A. K. Petford-Long, e t . a l . . submitted t o Journal o f A p p l i e d Physics.

12) An unetched p o r t i o n o f t h e b e a m s p l i t t e r sample was used f o r t h e TEM

a n a l y s i s , thereby e x p l a i n i n g t h e presence o f t h e s i l i c o n wafer s u b s t r a t e i n F i g u r e 6.

13) Data w i l l be presented by N. M. Ceglio, e t . a l . a t t h e SPIE conference on M u l t i l a y e r S t r u c t u r e s and X-ray Laser Research i n San Diego (August, 1986).

14) B r i a n MacGowan and James Trebes p r i v a t e communication.

Acknowledaements

We wish t o acknowledge t h e valued c o n t r i b u t i o n s o f o u r colleagues i n t h e LLNL l a b o r a t o r y x-ray l a s e r e f f o r t headed by Dennis Matthews. I n a d d i t i o n , we wish t o recognize t h e q u a l i t y e f f o r t s o f Gary Howe and Ed Hee f o r sample p r e p a r a t i o n f o r t h e x-ray c h a r a c t e r i z a t i o n experiments; David Gaines f o r c a l c u l a t i o n a l work on x-ray l a s e r c a v i t i e s ; and E. M. Campbell f o r h i s continued support and

a p p r e c i a t i o n o f t h e v a l u e o f t h i s work.