A HOLOGRAPHIC STUDY OF LASER-PRODUCED PLASMA AT THE 10.6 µm WAVELENGTH

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A HOLOGRAPHIC STUDY OF LASER-PRODUCED PLASMA AT THE 10.6 µm WAVELENGTH

E. Barkhudarov, V. Berezovskii, T. Chelidze, G. Gelashvili, M. Taktakishvili, N. Tsintsadze

To cite this version:

E. Barkhudarov, V. Berezovskii, T. Chelidze, G. Gelashvili, M. Taktakishvili, et al.. A HOLO-

GRAPHIC STUDY OF LASER-PRODUCED PLASMA AT THE 10.6

µm WAVELENGTH. Journal

de Physique Colloques, 1979, 40 (C7), pp.C7-869-C7-870. �10.1051/jphyscol:19797420�. �jpa-00219422�

JOURNAL DE PHYSIQllE Colloque C7, suppZdment au n07, Tome 40, JuiZZlt' 1979, page C7- 869

A HOLOGRAPHIC STUDY OF LASER-PROMEED PLASMA AT THE 10.6pm WAVELENGTH

E.M. Barkhudarov, V.R. Berezovskii, T. Ya. Chelidze, G.V. Gelashvili, M.T. Taktakishvili and N.L. Tsintsadze.

I n s i t u t e o f Physics, Academg o f Sciences o f th& Georgian FSR, I n s t i t u t e o f Physics, Acadew o f S c i e q e s . o f the Georgian SSR, !Z'biZissi, U.S.S. R.

The t r a n s i t i o n from v i s i b l e t o i n f r a - r e d ( I R ) r e s u l t s i n a n e s s e n t i a l i n c r e a s e Of s e n s i t i v i t y of holographic interferome- t r y a s a method of e l e c t r o n d e n s i t y mea- surement i n a plasma. I n p a r t i c u l a r , t h e use of I R pulsed C02 l a s e r r a d i a t i o n

( A =

~ 1 0 . 6 m) allows t o study a plasma densi-

JL

t y range ( ~ ~ r * l ~ ~ ~ c r n - ~ ) , which i s inao- c e s s i b l e t o t h e i n t e r f e r o m e t r y i n v i s i b l e b u t i s of c o n s i d e r a b l e i n t e r e s t . f o r s t u - dying a s e r i e s of important plasma o b j e c t s such a s laser-produced plasma, theta-pinch, high-current diode plasma, Tokamak plasma, e t c .

I n our previous work [I] holograms produced by a pulsed C02 l a s e r have. been recorded on d i f f e r e n t thermal d e t e c t o r s . The h i g h e s t q u a l i t y holograms have been obtained on p i e x i g l a s s , which has been used f o r t h e f i r s t time a s a r e c o r d i n g ma- t e r i a l .

I n t h e p r e s e n t work t h e technique of pulsed I R holography and double-exposure holographic i n t e r f e r o m e t r y . a t 1 0 . 6 ~ m h a s been q p p l i e d t o studying laser-produced plasma, i n p a r t i c u l a r , t h e laser-produced s p a r k i n a i r has been s t u d i e d s t a r t i n g w i t h a few microseconds a f t e r breakdown.

Such l a t e s t a g e s of t h e s p a r k develapment have been chosen by two reasons: f i r s t , e l e c t r o n d e n s i t y i n t h e s p a r k plasma dec-

r e a s e s w i t h t h e time making i t a d v i s a b l e t o o p e r a t e i n i n f r a r e d a t t h e s t a g e s when t h e d e n s i t y i s a l r e a d y n o t h i g h enough;

second, while e a r l i e r phases ( t e n s o f np- noseconds t o a few microseconds) had beco- me formerly t h e s u b j e c t of i n t e n s e s t u e - i n g [2-51

,

i n p a r t i c u l a r , by means of ho- l o g r a p h i c techniques u s i n g a rub$ l a s e r

@,

5 3

-,

l a t e r s t a g e s of t h e laser-produced s p a r k i n a i r mentioned above remained out- s i d e t h e a r e a of experimental s t u d i e a .

A pulsed double-discharge TE C02 l a s e r was used f o r o b t a i n i n g I R holograms. m e l a s e r r a d i a t i o n energy was about I O J , be- i n g concentrated i n t h e most p a r t i n a pe- ak of

Z

,"100

w

width. Holograms of .the plasma were recorded u s i n g a u s u a l double- -beam scheme. P l e x i g l a s s p l a t e s of 1.5 ^ma t h i c k n e s s were used a s a r e c o r d i n g materi- a l .

A s p a r k i n a i r a t atmospheric p r e s s u r e was produced u s i n g an OGM-20 single-pulsed ruby l a s e r ( p u l s e energy O.5J, p u l s e width 2 5 n s ) synchronized w i t h the. pulsed C02 l a - s e r which served f o r 0 3 t a i n i n g holograms.

The ruby l a s e r r a d i a t i o n was foceised i n a i r by a l e n s of 2.5 cm f o c a l length. Botb single-exposure and double-exposure I R holograms of t h e laser-produced s p a r k have been obtained f o r a s e r i e s of times from 2 t o 2 5 p a f t e r b r e a M ~ w n .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797420

S p a r k images r e c o n s t r u c t e d from s i n g - le-expobure holograms a l l o w one t o t r a c e t o t h e dynamics of i t s expansion a t l a t e r s t a g e s . H o l o g r a p h i c i n t e r f e r o g r a m s of t h e s p a r k (Fig.?) o b t a i n e d by r e c o n s t r u c t i o n

Fig.1 Holographic i n t e r f e r o g r a m s of t h e l a s e r - p r o d u c e d s p a r k .

Delay time: a

-

^{6.2; b}

-

9 . 6 ~ 1 s . f r o m double-exposure holograms show i n t e r - f e r e n c e f r i n g e s h i f t s t o d e c r e a s e w i t h t h e t i m e g i v i n g e v i d e n c e of d e c r e a s i n g of mean e l e c t r o n d e n s i t y i n t h e s p a r k plasma a s a r e s u l t of e x p a n s i o n and r e c o m b i n a t i - on p r o c e s s e s ; t h i s f a c t i s a l s o r e s p o n s i b - l e f o r observed g r a d u a l r e d u c t i o n of i n - t e r f e r o g r a m a r e a s where measurable f r i n g e s h i f t s a r e p r e s e n t c o r r e s p o n d i n g t o t h e r e d u c t i o n of plasma a r e a w i t h a n e l e c t r o n d e n s i t y exceeding t h e s e n s i t i v i t y l i m i t of h o l o g r a p h i c i n t e r f e r o m e t r y a t 1 0 . 6 ~ m . By means of a q u a n t i t a t i v e t r e a t m e n t of measured f r i n g e s h i f t s u s i n g Abel i n v e r - s i o n t h e s p a t i a l and t e m p o r a l d i s t r i b u t i - on of e l e c t r o n d e n s i t y i n d e c a y i n g s p a r k plasma was determined.

Apart from t h e ruby l a s e r , a p u l s e d d o u b l e - d i s c h a r g e TE C02 l a s e r w i t h t h e maximum r a d i a t i o n energy of ~ 1 0 0 5 was a l s o used f o r plasma p r o d u c t i o n . The

p u l s e w i d t h v a r i e d from 0.1 t o 1.5 s de-

J

pending on t h e g a s m i x t u r e composition.

The r a d i a t i o n was f o c u s e d i n a i r o r on t h e s u r f a c e of a s o l i d t a r g e t by a concave cop- p e r m i r r o r o r a NaCl l e n s o f

15

cm f o c a l

l e n g t h . Experiments were c a r r i e d o u t i n a s t a i n l e s s s t e e l chamber a t a i r p r e s s u r e s v a r y i n g from a t m o s p h e r i c t o a few T o r r .

The r e s u l t s o b t a i n e d show t h a t a t ne- a r - a t m o s p h e r i c p r e s s u r e t h e C 0 2 l a s e r - produced s p a r k h a s s u c h a c o m p l i c a t e d s t r u c t u r e t h a t no q u a n t i t a t i v e t r e a t m e n t of h o l o g r a p h i c f n t e r f e r o g r a m s i s p o s s i b l e . However, when a i r p r e s s u r e i s d e c r e a s e d

t h e s t r u c t u r e of t h e C02 l a s e r - p r o d u c e d plasma i s g r a d u a l l y s i m p l i f i e d and i n t h e p r e s s u r e r a n g e 20 t o a few T o r r we suc- ceeded i n p e r f o r m i n g a q u a n t i t a t i v e t r e a t - ment of o b t a i n e d - i n t e r f e r o g r a m s .

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C h e l i d z e , V.V.Chichinadze. P i s f m a v Zhurn. iP ekhn. Biz.,

2,

1079, 1976.

2.T.P.Evtushenk0, G.U.Malish~ev,G.V.Ostrov- skaya, V.V,Semenov, T.Ya.Gbeli&ze. Zhurn.

Tekhn.Fiz.,

&,

1115, 1966.

3.A.J.Alcock, S.A.Ramsden. Appl.Phys.Letts.

8, 187, 1966.

-

4.A.N.Zaide1, G,V.Ostrovskaya, Yu.I.Ost- r o v s k i i , 'I,Ya,Chelidze. Zhurn.Tekh,Fiz.,

2,

2208, 1966.

5.I,I.Komissarova, G.V.Ostrovskaya, L.L.

S h a p i r o . Zhurn.Tekh.Fiz., 40, 1072, 1970.