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Submitted on 1 Jan 1979
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IONIZATION OF ISOLATED DEUTERIUM PELLETS BY NEODYMIUM LASER RADIATION
H. Baumhacker, H. Brinkschulte, W. Riedmüller, M. Salvat, S. Sudo
To cite this version:
H. Baumhacker, H. Brinkschulte, W. Riedmüller, M. Salvat, S. Sudo. IONIZATION OF ISOLATED
DEUTERIUM PELLETS BY NEODYMIUM LASER RADIATION. Journal de Physique Colloques,
1979, 40 (C7), pp.C7-743-C7-744. �10.1051/jphyscol:19797359�. �jpa-00219354�
JOUENAL DE PHYSIQUE CoZZoque C7, suppZ6rnent au n07, Tome 40, J u i l l e t 2979, vage C7- 743
IONIZATION OF ISOLATED DEUTERIUM PELLETS BY NEODYMIUM LASER RADIATION
H. Baurnhacker, H. Brinkschulte, W. Riedrnuller, M. Salvat and S.Sudo.
Max-PZanck-Institut fiir Plasmaphysik, 8046 Garching, W. Germany.
The i d e a o f employing plasmas produced by t h e i r r a d i a t i o n o f s o l i d hydrogen p e l l e t s w i t h h i g h power l a s e r s f o r f i 1 l i n g o f magnetic c o n f i n e m e n t d e v i c e s appears v e r y a t t r a c t i v e . I t o f f e r s t h e p o s s i b i l i t y o f c r e a t i n g a h i g h energy plasma w i t h l o w i m p u r i t i e s and w i t h o u t any c u r r e n t whose mag- n e t i c f i e l d m i g h t i m p a i r t h e confinement o f t h e plasma i n t h e plasma machine. The p r o p e r t i e s o f t h e l a s e r produced plasma can be s e l e c t e d by p r o p e r
I t was f o u n d t h a t l e s s t h a n 10 % o f t h e p e l l e t i s i o n i z e d when t h e l a s e r energy i s a p p l i e d i n a s i n g l e p u l s e o f 25 ns d u r a t i o n . However, much b e t t e r i o n i - z a t i o n i s achieved when t h e l a s e r energy i s s p l i t i n two s u c c e s s i v e p u l s e s w i t h d i f f e r e n t e n e r g i e s . A low power p r e p u l s e i n t e r a c t s w i t h t h e p e l l e t i n such a way t h a t t h e f o l l o w i n g main p u l s e f i n d s f a v o u r a b l e c o n d i t i o n s f o r e f f i c i e n t i o n i z a t i o n o f t h e p e l l e t m a t e r i a l
.
c h o i c e of t h e p e l l e t s i z e and t h e l a s e r Parameters.
~ h ~ influence of the following parameters w h i c h are F o r s u c c e s s f u l a p p l i c a t i o n o f t h i s method i t i s
n o t independent o f each o t h e r , was i n v e s t i g a t e d : e s s e n t i a l t h a t t h e p e l l e t be c o m p l e t e l y i o n i z e d b y
t h e l a s e r r a d i a t i o n . The reasons i s t h a t any po- a ) P r e p u l s e : A t p r e p u l s e i n t e n s i t i t e s of r e n t i a i r e s i d u e m a t e r i d i o f tile p e i i e t i s c o l d atld 2
(0.2.. . 2 9 j x 10' V/cm t h e p e l l e t ?vapoiaate;
w i l l r a p i d l y c o o l t h e plasma by charge exchange i m m e d i a t e l y a f t e r h t e l a s e r s h o t and t h e p e l l e t
c o l l i s i o n s . m a t e r i a l exnands i n a p r e d o m i n a n t l y gaseous
s t a t e a t a v e l o c i t y o f a p p r o x i m a t e l y 5x10 cm/s. 5 IJe have determined e x p e r i m e n t a l l y t h e l a s e r p r o p e r - Under these c o n d i t i o n s b e s t r e s u l t s were ob- t i e s f o r maximum i o n i z a t i o n o f f r e e l y f a l l i n g deu-
t a i n e d w i t h r e s p e c t t o i o n i z a t i o n b y t h e f o l l o w - t e r i u m p e l l e t s . The envisaged a p p l i c a t i o n i s t o f i l l
ing main pulse.
t h e Garchino H I 1 b s t e l l a r a t o r , w h i c h c a l l s f o r
" e l l e t s c o n t a i n i n g 1 x 1018 d e u t e r i u m atoms and b )
-
a t a mean plasma energy o f 100 eV-
f o r a l a s e r energy i n t h e 100 J range. The p e l l e t s a r e formed by e x t r u d i n g a b a r o f s o l i d d e u t e r i u m t h r o u g h a h o l e i n t h e bottom o f a l i q u i d - h e l i u m - c o o l e d c r y o - s t a t and by c u t t i n g i t s l o w e r end w i t h two o h m i c a l - l y heated w i r e s . The c y l i n d r i c a l p e l l e t s w i t h a d i a m e t e r o f 0.3 mm and a l e n g t h o f 0.5 mm c o n t a i n ( 1...
2) x 1018 d e u t e r i u m atoms. A f t e r a f r e e f a l l o v e r 20 cm t h e y a r e i o n i z e d i n t h e f o c u s o f a Q-swi t c h e d neodymi um 1 a s e r w i t h a maximum energy of 100 J. The i o n i z a t i o n o f t h e p e l l e t and t h e e a r l y expansion phase w i t h o u t magnetic f i e l d was i n v e s t i - g a t e d w i t h h o l o g r a n h i c i n t e r f e r o m e t r y ( a t r u b y l a s e r wavelength) and w i t h i o n c o l l e c t o r probes.A b a l a n c e o f t h e l a s e r l i g h t was made i n o r d e r t o check t h e e f f i c i e n c y o f t h e method w i t h r e s n e c t t o t r a n s f o r m a t i o n o f l a s e r energy i n t o plasma energy.
P u l s e i n t e r v a l and l a s e r beam d i a m e t e r : A t a
9 2
c o n s t a n t n r e n u l s e i n t e n s i t y o f 4 x 10 W/cm t h e t i m e s e p a r a t i o n between t h e p r e p u l s e and m a i n p u l s e and t h e d i a m e t e r of t h e l a s e r beam a t t h e s i t e o f t h e p e l l e t were v a r i e d . I t was found t h a t f o r a l a s e r beam d i a m e t e r o f 3
m
t h e p l a s - ma p r o d u c t i o n reaches a b r o a d maximum f o r t i m ei n t e r v a l s between 300 ns and 500 ns. F o r d i a - m e t e r s o f 1.5 mm and 5 mm, r e s p e c t i v e l y , much
l e s s o f t h e p e l l e t i s i o n i z e d .
Energy o f main l a s e r p u l s e : A t a p r e p u l s e i n - t e n s i t y o f 4 x 10 ~ / c m ~ , 9 a l a s e r beam d i a m e t e r of 3 mm and a p u l s e s e p a r a t i o n o f 500 n s t h e p e l l e t becomes 80 %
2
20 % i o n i z e d a t t h e maxi- mum main p u l s e energy o f 100 J. However, b y comparing t h e v a l u e s f o r t h e t o t a l number o f e l e c t r o n s w i t h t h o s e o f t h e i o n s-
measured a t a l a t e r t i m e w i t h Faraday cups-
i t can b e con- c l u d e d t h a t a s u b s t a n t i a l p o r t i o n o f t h e plasmaArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797359
recombines q u i t e r a p i d l y . Numerical c a l c u l a - t i o n s show t h a t t h e i o n i z a t i o n o f t h e nlasma drops w i t h i n 200 ns t o 40.. .70 % o f i t s i n i t i a l value and then decays v e r y s l o w l y .
The s i g n a l s o f the i o n c o l l e c t o r probes f a c i n g the plasma o r i g i n from d i f f e r e n t d i r e c t i o n s revealed t h a t t h e plasma expansion i s n o t symmetrical when t h e p e l l e t i s i r r a d i a t e d from o n l y one d i r e c t i o n . I n t h i s case, most o f the plasma expands i n the d i r e c t i o n onoosi t e t o t h a t o f the l a s e r beam. A much more u n i f o r m plasma c l o u d i s formed when the
l a s e r r a d i a t i o n imninges on the p e l l e t from two o?posi t e d i r e c t i o n s . However, the t o t a l i o n number does n o t grow w i t h the number o f l a s e r beams. I t grows o n l y w i t h the l a s e r energy no m a t t e r from how many d i r e c t i o n s t h e ~ € 1 l e t i s i r r a d i a t e d .
I t was demonstrated t h a t an i s o l a t e d deuterium o e l l e t c o n t a i n i n 9 ( 1 . . . 2 ) x 1018 deuterium atoms can be comnletely i o n i z e d by a Q-switched neo- dymium l a s e r w i t h an energy s l i g h t l y above 100 J.
The process i s v e r y e f f i c i e n t w i t h r e s n e c t t o t r a n s f e r o f l a s e r energy i n t o plasma enerqy:
Up t o 80 % o f t h e i n c i d e n t l a s e r energy i s absorbed by the el let and roughly 1 / 2 o f t h i s value i s measured as i o n k i n e t i c energy. The averaged t r a n s l a t i o n a l energy o f t h e ions i s t y n i c a l l y 100.. .ZOO eV.
