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STUDY OF X-RAY LASER SCHEMES USING NEW EXPERIMENTAL FACILITIES AT THE
RUTHERFORD APPLETON LABORATORY
M. Key, J. Boon, C. Brown, C. Chenais-Popovics, R. Corbett, A. Damerell, P.
Gottfeldt, C. Hooker, G. Kiehn, C. Lewis, et al.
To cite this version:
M. Key, J. Boon, C. Brown, C. Chenais-Popovics, R. Corbett, et al.. STUDY OF X-RAY LASER SCHEMES USING NEW EXPERIMENTAL FACILITIES AT THE RUTHERFORD APPLETON LABORATORY. Journal de Physique Colloques, 1986, 47 (C6), pp.C6-71-C6-79.
�10.1051/jphyscol:1986610�. �jpa-00225853�
STUDY OF X-RAY LASER SCHEMES USING NEW EXPERIMENTAL FACILITIES AT THE RUTHERFORD APPLETON LABORATORY
M.H. KEY, J . E . BOON, C. BROWN, C. CHENAIS-POPOVICS* * *
,
R. CORBETT', A.R. DAMERELL, P. GOTTFELDT, C . J . HOOKER G.P. KIEHN"'
,
C.L.S. LEWIS*, D.A. PEPLER, G. J. P E R T * * , C. REAGAN*, S . J . ROSE, I . N . ROSS, P.T. RUMSBY, S . SADAAT*, R. SMITH", T. TOMIE a n d 0. WILLI*'R u t h e r f o r d A p p l e t o n L a b o r a t o r y , C h i l t o n , D i d c o t , GB-Oxon OX11 OQX, G r e a t - B r i t a i n
' ~ u e e n s U n i v e r s i t y B e l f a s t , B e l f a s t , ~ o r t h e r n I r e l a n d
* * ~ r n p e r i a l C o l l e g e , P r i n c e C o n s o r t R o a d , GB-London SW7 2BZ, G r e a t - B r i t a i n
" " I ~ c o l e P o l y t e c h n i q u e , L a b o r a t o i r e P M I , F - 9 1 1 2 8 P a l a i s e a u C e d e x , France
Un n o u v e a u s y s t e m e a s t G i n s t a l l s a u R u t h e r f o r d A p p l e t o n L a b o r a t o r y s u r l e l a s e r a neodyme V u l c a n : p e r m e t t a n t d e r s a l i s e r u n e t a c h e f o c a l e a l l o n g e e e t d ' u t i l i s e r un s p e c t r o g r a p h e XUV a v e c r ~ s o l u t i o n t e m p o r e l l e . Le s y s t e m e a e t 5 d e s s i n e p o u r l a e t u d e d e d i f f e r e n t s s c h & m a s d e l a s e r X e t a
st&
u t i l i s e p o u r d e s o b s e r v a t i o n sp r G l i m i n a i r a s . On a G t u d i g p a r t i c u l i e r e m e n t l e s c h g m a d e
r e c o m b i n a i s o n d u c a r b o n e CVI e n u t i l i s a n t d e s c i b l e s c o n s t i t u g s d e f i b r e s d e l o n g u e u r p o u v a n t a t t e i n d r e 2cm. L e s m e s u r e s m e t t e n t c l a i r e m e n t e n E v i d e n c e u n e a m p l i f i c a t i o n d e l a t r a n s i t i o n H a d u c a r b o n e CVI, 6 l a l o n g u e u r d ' o n d e 18.2nm.
A b s t r a c t
?Jew e x p e r i m e n t a l f a c i l i t i e s h a v e r e c e n t l y b e e n d e v e l o p e d a t t h e R u t h e r f o r d A p p l e t o n L a b o r a t o r y f o r l i n e f o c u s i r r a d i a t i o n u s i n g t h e V u l c a n Neodymium g l a s s l a s e r i n s t a l l a t i o n a n d f o r t i m e - r e s o l v e d s p e c t r o s c o p y o f VUV e m i s s i o n .
The s y s t e m i s d e s i g n e d f o r s t u d y o f XUV l a s e r s a n d h a s b e e n u s e d i n a p r e l i m i n a r y s u r v e y o f s e v e r a l s c h e m e s . P a r t i c u l a r e m p h a s i s h a s b e e n p l a c e d o n s t u d y of t h e CVI r e c o m b i n a t i o n . l a s e r u s i n g f i b r e t a r g e t s o f u p t o 2cm l e n g t h . M e a s u r e m e n t s s h o w c l e a r e v i d e n c e o f a m p l i f i c a t i o n o n t h e 18.2nm H a t r a n s i t i o n i n CVI.
INTRODUCTION
P r o g r e s s i n t h e s t u d y o f XUV l a s e r s h a s a c c e l e r a t e d r e c e n t l y ( 1 ) ( 2 )
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986610
JOURNAL DE PHYSIQUE
(3 ) ( 4 ) a n d work i n t h e UK s t u d y i n g t h e 2VI r e c o m b i n a t i o n scheme h a s b e n e f i t t e d from t h e c o m m i s s i o n i n g o f new f a c i l i t i e s w h i c h h a v e
e n a b l e d s i g n i f i c a n t new e x p e r i m e n t s t o b e c a r r i e d o u t
.
T h i s p a p e r o u t l i n e s o u r r e c e n t t h e o r e t i c a l o p t i m i s a t i o n o f t h e CVI s c h e m e , new f a c i l i t i e s and e x p e r i m e n t a l r e s u l t s , s h o w i n g G-
4cm -1 and GL-
4 f o rt h e 18.2nm Balmer a t r a n s i t i o n o f CVI.
THE CVI RECOMBINATION LASER SCHEME
I n r e c e n t work we h a v e i m p r o v e d o u r u n d e r s t a n d i n g o f t h e o p t i m i s a t i o n o f c o n d i t i o n s f o r XUV l a s e r a c t i o n o n t h e CVI Palmer a t r a n s i t i o n u s i n g l a s e r i r r a d i a t e d c a r b o n f i b r e t a r g e t s .
I t h a s b e e n w e l l e s t a b l i s h e d by d e t a i l e d c o m p u t a t i o n ( 5 ) t h a t t h e optimum e n e r g y c o n t e n t E o f t h e p l a s m a i s a f u n c t i o n o f i t s m a s s M and t h e d u r a t i o n o f t h e h e a t i n g p u l s e r , w i t h a s c a l i n g o v e r a l i m i t e d r a n g e o f t h e f o r m
E - M 17/11T-6/11 ( 1 1
and t h a t h i g h e r g a i n G i s o b t a i n e d f o r l o w e r mass w i t h a s c a l i n g o f t h e form G
-
M 1 7 1 1 ~ 6 ' 1 1 . The p h y s i c a l e x p l a n a t i o n o f t h i s b e h a v i o u r l i e s i n t h e f a c t t h a t s m a l l e r p l a s m a s e x p a n d more r a p i d l y p r o d u c i n g l e s s r e c o m b i n a t i o n t o t h e g r o u n d s t a t e o f t h e h y d r o g e n i c i o n . T h i s f a c t , t o g e t h e r w i t h t h e s m a l l e r r a d i u s , r e d u c e s t r a p p i n g o f L e m i s s i o n s o t h a t l o w e r t e m p e r a t u r e and more g r o u n d s t a t ea
p o p u l a t i o n c a n b e t o l e r a t e d g i v i n g h i g h e r p o p u l a t i o n i n t h e n = 3 u p p e r s t a t e of t h e l a s e r ( n ( 3 )
-
T-' l 2 e x p (E (3 , - ) / k T ) f r o m S a h a ' s e q u a t i o n ) . We h a v e p r e v i o u s l y u s e d n u m e r i c a l s i m u l a t i o n t o c o m p u t e t h e a b l a t e d m a s s and e n e r g y f o r p a r t i c u l a r t a r g e t s and l a s e ri r r a d i a t i o n c o n d i t i o n s a n d t o f i n d c o n d i t i o n s w h e r e t h e m a s s a n d e n e r g y were o p t i m i s e d ( 6 )
.
An a p p r o a c h more c o n d u c i v e t o u n d e r s t a n d i n g t h e r o l e o f e x p e r i m e n t a l p a r a m e t e r s i s t o t r e a t t h e a b l a t i o n p r o b l e m a n a l y t i c a l l y u s i n g t h e model o f s e l f r e g u l a t i n g a b s o r p t i o n b y i n v e r s e Bremmstrahlung ( 7 1 8 ) . T h i s y i e l d s arelationship o f t h e f o r m ,
I n g e n e r a l t h e r e l a t i o n s h i p s ( I ) and ( 2 ) h a v e a p o i n t o f i n t e r s e c t i o n d e f i n i n g t h e optimum p a r a m e t e r s a s i l l u s t r a t e d i n F i g 1. The
~ / l ~ c a - '
-
0.1 ( r / l p m ) 2 ' 4 ( r / 1 0 0 p S e c ) (3 o r-1
-
9 1 . 6M/lgm cm
-
5 x 10 ( r / l p m ) ( r / l O O p s e c ) ( 4 ) T h e s e r e l a t i o n s h i p s a r e u s e f u l i n c h o o s i n g e x p e r i m e n t a l p a r a m e t e r s and l e d u s t o use s h o r t e r p u l s e s and t h i c k e r f i b r e s t h a n i n o u r p r e v i o u s work f a c i l i t a t i n g t a r g e t h a n d l i n g and a l i g n m e n t , and t oo m i t t h e u s e o f a p r e p u l s e . Our c h o s e n optimum p a r a m e t e r s w i t h
-
1-
1computed g a i n c o e f f i c i e n t of G
-
8cm were E = 3 Jcm ,.r = 7OpsecX = 0.53 pm r
-
3.5pm. The r e q u i r e d i n c i d e n t e n e r g y i s t h e n 3 0 ~ c r n - I b e c a u s e o n l y 1 0 % i s a b s o r b e d i n t h e f i b r e t a r g e t " ) .1000 1000
Eowgy Gain
J/cm /cm x phc
100- - M O
1 0 -
-
1.00-1 .0.1
Kr9 10'~
ro%
UASS
Line locus oplans.
F i g 1 Numer i c a l l y computed ( p o i n t s ) and a n a l y t i c a l l y m o d e l l e d ( l i n e s ) plasma f o r m a t i o n f o r 5pm d i a m e t e r
f i b r e s i r r a d i a t e d w i t h 200psec p u l s e s
I*] K(+l H
a t X = 0 . 3 pm. 21 mm 14 mm 7mm
F i g 2
NEW FACILITIES FOR XUV LASER RESEARCH
A new t a r g e t i r r a d i a t i o n f a c i l i t y a t t h e SERC R u t h e r f o r d A p p l e t o n L a b o r a t o r y was b r o u g h t i n t o o p e r a t i o n i n September 1985 and h a s s p e c i a l f e a t u r e s d e s i g n e d f o r XUV l a s e r r e s e a r c h . F i g 2 i l l u s t r a t e s t h e l a y o u t i n which s i x l a s e r beams o f t h e Vulcan l a s e r ( 9 ) a r e b r o u g h t t o a b e r r a t i o n f r e e l i n e f o c i p r o d u c e d by o f f a x i s r e f l e c t i o n
JOURNAL
DE
PHYSIQUEfrom s p h e r i c a l m i r r o r s ( 1 0 ) . Each l i n e f o c u s i s 7mm l o n g and i t s width ( d u e t o l a s e r beam d i v e r g e n c e ) i s a p p r o x i m a t e l y 2 5 ~ m . Remote c o n t r o l of t h e p o s i t i o n o f t h e o p t i c a l e l e m e n t s e n a b l e s i r r a d i a t i o n of 7mm, 14mm and 21mm t a r g e t s by s u p e r p o s i t i o n of l i n e f o c i a s i l l u s t r a t e d i n F i g 2. T a r g e t p o s i t i o n i s monitored by two n o v e l microscope d e v i c e s u s i n g s p l i t f i e l d imaging t o p o s i t i o n f i b r e s w i t h a few m i c r o n s a c c u r a c y i n s p a c e and l e s s t h a n lmrad i n d i r 9 c t i o n
(11).
Alignment o f t h e l i n e f o c u s on a f i b r e t a r g e t i s d e t e r m i n e d by
o b s e r v a t i o n o f t h e ' F o n c a u l t ' shadow i n t h e beam t r a n s m i t t e d p a s t t h e t e r g e t , u s i n g CW g r e e n l i g h t from an Ar i o n l a s e r t o s i m u l a t e t h e 0.53 Pm main l a s e r p u l s e .
A new time r e s o l v i n g XUV s p e c t r o m e t e r h a s been developed ( I 2 ) based bn a 5m r a d i u s of c u r v a t u r e g r a z i n g i n c i d e n c e d i f f r a c t i o n g r a t i n g of v a r i a b l e l i n e s p a c i n g used a t 3 a n g l e o f i n c i d e n c e t o p r o j e c t a spectrum w i t h a f l a t f i e l d . An XUV s t r e a k camera w i t h a t r a n s m i s s i o n p h o t o c a t h o d e r e c o r d s t h e s p e c t r u m w i t h 150psec t i m e r e s o l u t i o n over a s p e c t r a l r a n g e o f l O O A w i t h 0.5A r e s o l u t i o n , l i m i t e d by t h e d i a m e t e r of t h e plasma column i n t h e s l i t l e s s mode of o p e r a t i o n i l l u s t r a t e d i n F i g 3 .
A d d i t i o n a l d i a g n o s t i c s i n c l u d e an a r r a y of plasma c a l o r i m e t e r s , measuring a n g u l a r l y r e s o l v e d plasma e n e r g y and X-ray p i n h o l e cameras r e c o r d i n g s o f t X-ray e m i s s i o n images (hv > 0.5 kev) of t h e plasma, w i t h 20pm s p a t i a l r e s o l u t i o n .
EXPERIMENTAL RESULTS
An X-ray image from a 12mm l e n g t h of i r r a d i a t e d f i b r e i s shown i n F i g 4 . The o v e r a l l l e n g t h L o f t h e i r r a d i a t e d r e g i o n was d e t e r m i n e 3 from such images and t h i s , t o g e t h e r w i t h t h e absorbed e n e r g y ET, o b t a i n e d by a n g u l a r i n t e g r a t i o n of t h e plasma c a l o r i m e t e r s i g n a l s , was used t o d e t e r m i n e a b s o r b e d e n e r g y p e r u n i t l e n g t h E = E
d L .
P l o t t i n g t h e r e l a t i o n s h i p between a v e r a g e b r i g h t n e s s of t h e X-ray image and E gave a p r o p o r t i o n a l i t y from which t h e v a r i a t i o n o f absorbed e n e r g y a l o n g t h e l e n g t h c o u l d be e s t i m a t e d from images s u c h a s F i g 4. The r e s u l t s i n d i c a t e d t y p i c a l l y
+
50% RMS, v a r i a t i o n i n E on a s p a t i a l s c a l e of t h e o r d e r of 100um.l a s e r is shown i n F i g 6. The Palmer s p e c t r u m o f CVI i s m o s t
p r o m i n e n t w i t h some w e a k e r l i n e i n c l u d i n g o x y g e n i m p u r i t y a l s o s e e n . The r a t i o s o f t h e l i n e i n t e n s i t i e s a r e d i s t o r t e d b y t h e i n s t r u m e n t a l r e s p o n s e w h i c h a c c e n t u a t e s s h o r t w a v e l e n g t h s r e l a t i v e t o l o n g
w a v e l e n g t h s and a l s o b y t h e o v e r l a p o f 4 t h o r d e r CVI L w i t h CVI H
a 8 '
CVI H a n d CVI Ha a r e f r e e o f s i g n i f i c a n t c o n t r i b u t i o n s f r o m o t h e r
Y
VARIABLE LINE S?ACING GRATING
STREAK CAMERA
SOil;lCE
FLAT FIELD
F i g 3
F i g 5 S p e c t r u m f o r 1.5mm l e n g t h a t 2 ~ c m - I a b s o r b e d e n e r g y .
- A
0 m $-
F i g 6 D e n s i t o m e t r y
a t 6 0 0 p s e c a f t e r t h e l a s e r p u l s e i n P i g 5.
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-. 193 125 159 175 230
T WAVELENGTH (PI
JOURNAL DE PHYSIQUE
F i g 7 A x i a l s p e c t r u m f o r l l m m i r r a d i t e d l e n g t h a t 2 J c n -
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200l i n e s and t h e i r a p p r o x i m a t e l y e q u a l o b s e r v e d b r i g h t n e s s i n F i g 5 i m p l i e s a t r u e b r i g h t n e s s r a t i o o f H :H o f a b o u t 2:l ( e s t i m a t e d f r o m
a Y
f i l t e r t r a n s m i s s i o n c h a r a c t e r i s t i c s b u t n o t y e t d i r e c t l y c a l i b r a t e d o r c o r r e c t e d f o r t h e w a v e l e n g t h s e n s i t i v i t y o f t h e p h o t o c a t h o d e ) .
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The s p e c t r u m from a n l l m m l e n g t h o f p l a s m a shown i n F i g 7.
D e n s i t o m e t r y a t a t i m e d e l a y o f 6 0 0 p s e c i n F i g 8 shows a marked i n c r e a s e i n t h e r e l a t i v e i n t e n s i t y o f H which h a s ( w i t h d i f f e r e n t
a
f i l t e r i n g ) a n a p p a r e n t r a t i o H /H o f 3 and a t r u e r a t i o e s t i m a t e d a t a Y
Ha/Hy > 9.
100 125 150 175 200
WAVELENGTH IAI
The t e m p o r a l v a r i a t i o n o f t h e i n t e n s i t y i s a l s o d i f f e r e n t f r o m t h e o t h e r l i n e s e x h i b i t i n g a p e a k a f t e r 7 0 0 p s e c w i t h a h a l f w i d t h o f a b o u t 6 0 0 p s e c .
The v a r i a t i o n o f i n t e n s i t y w i t h l e n g t h f o r H i s a p p r o x i m a t e l y
Y
l i n e a r , F i g 9 b u t Ha i n c r e a s e i n a n o b v i o u s l y e x p o n e n t i a l f a s h i o n , F i g 1 0 . F i t t i n g t o t h e f u n c t i o n e x p (GL)-1, a p p r o p r i a t e t o a m p l i f i e d s p o n t a n e o u s e m i s s i o n , shows G
-
4cm -1 and GL-
4 f o r Ha.F i g 9 I n t e n s i t y o f H Y
13
.
Snm F i g 1 0 I n t e n s i t y o f H a18.2nm
DISCUSSION
The e x p o n e n t i a l v a r i a t i o n o f Ha i n t e n s i t y g i v e s v e r y s t r o n g e v i d e n c e o f l a s e r a m p l i f i c a t i o n , w h i c h is c o r r o b o r a t e d by t h e l i n e a r v a r i a t i o n o f H i n t e n s i t y . U n p u b l i s h e d t h e o r e t i c a l m o d e l l i n g , i n w h i c h l e v e l
Y
p o p u l a t i o n s a r e computed i n e a c h z o n e o f a L a g r a n g i a n h y d r o - c o d e t r e a t m e n t o f t h e p l a s m a f o r m a t i o n , i l l u s t r a t e d i n F i g 11 p r e d i c t s a n a n n u l a r g a i n r e g i o n o f a b o u t 15OVm r a d i u s w i t h a g a i n c o e f f i c i e n t s i m i l a r t o t h e o b s e r v e d v a l u e a t a d e l a y t i m e o f 6 0 0 p s e c . The
c o m p u t a t i o n a l s o s u g g e s t s up t o 2 times h i g h e r g a i n o c c u r r i n g e a r l i e r i n t h e e x p a n s i o n b u t t h i s i s p o s s i b l y o b s c u r e d by t h e l a c k o f
s p a c e / t i r n e m a t c h i n g o f t h e g a i n r e g i o n s f r o m p a r t s o f t h e f i b r e i r r a d i a t e d a t
+
50% RMS v a r i a t i o n of a b s o r b e d e n e r g y .F i g 11 Hydro-code s i m u l a t i o n s s h o w i n g d e n s i t y , t e m p e r a t u r e and g a i n p r o f i l e s . Time = 6 0 0 p s a f t e r t h e l a s e r p u l s e , E = 4J/cm
JOURNAL
DE
PHYSIQUEThe g a i n shown i n F i g 1 0 i s s i m i l a r t o t h a t r e p o r t e d f a r S e 24+ (1) b u t t h e e n e r g y ( 2 0 J c m - ' i n c i d e n t a n d 2J crn-'absorbed i n t h e p l a s m a ) c a n b e c o n t r a s t e d w i t h more t h a n 3 O O J cm-'in 4 5 0 p s e c n e e d e d t o i r r a d i a t e t h e Se l a s e r t a r g e t s .
R e c o m b i n a t i o n l a s e r s o f t h i s t y p e a r e a t t r a c t i v e b e c a u s e o f t h e r e l a t i v e l y low e n e r g y n e e d e d f o r t h e p l a s m a f o r m a t i o n and a l s o
b e c a u s e r e f r a c t i o n i s n o t s u c h a s e v e r e p r o b l e m . G a i n is p r o d u c e d i n t h e CVI scheme w i t h ne
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1 0 l ~ c m - ~ and i n t h e ~e~~~ scheme w i t hn e
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5 x w i t h a p r o p o r t i o n a l d i f f e r e n c e i n r e f r a c t i v e i n d e x . D e s p i t e t h e s t r o n g g r a d i e n t o f p l a s m a d e n s i t y i n t h e p l a s m a p r o d u c e d from t h e f i b r e ( F i g ) i t i s e s t i m a t e d t h a t t h e XUV l a s e r beam c o u l d p r o p a g a t e w i t h i n t h e g a i n r e g i o n f o r s e v e r a l c e n t i m e t r e s , g i v i n g good p r o s p e c t s f o r a c h i e v i n g s a t u r a t e d A S E ~ p e r a t i o n ' ~ ) .I s o e l e c t r o n i c s c a l i n g t o s h o r t e r w a v e l e n g t h ( A
- z - ~ )
i s a l s o a n a t t r a c t i v e p o s s i b i l i t y i n t h e r e c o m b i n a t i o n scheme b u t l e s s s o f o r t h e 3p-3 s n e o n - l i k e i o n s c h e m e , f o r w h i c h t h e 3p-3 s e n e r g y g a p i n c r e a s e s w e a k l y w i t h Z r e l a t i v e t o t h e 2p-3p e x c i t a t i o n e n e r g y .CONCLUSIONS
We h a v e shown, f o r t h e f i r s t t i m e , a n e x p o n e n t i a l v a r i a t i o n o f i n t e n s i t y w i t h l e n g t h f o r t h e ZVI r e c o m b i n a t i o n l a s e r scheme u s i n g new f a c i l i t i e s c a p a b l e o f a w i d e r a n g e o f XUV l a s e r s t u d i e s .
Our t h e o r e t i c a l u n d e r s t a n d i n g o f t h e C V I l a s e r h a s b e e n improved and is i n q u i t e good a g r e e m e n t w i t h t h e o b s e r v a t i o n s . E x t e n s i o n o f t h e work t o w a r d s s a t u r a t e d A S E l a s e r a c t i o n i n C V I and s h o r t e r w a v e l e n g t h i n i s o e l e c t r o n i c i o n s a p p e a r s f e a s i b l e and w i l l b e i n v e s t i g a t e d i n f u t u r e work.
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