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SPECTROSCOPIC STUDY OF THE IONIZATION DYNAMICS IN A CO2 LASER-PRODUCED PLASMA
OF HELIUM
J. Gauthier, J. Geindre, N. Grandjouan
To cite this version:
J. Gauthier, J. Geindre, N. Grandjouan. SPECTROSCOPIC STUDY OF THE IONIZATION DY-
NAMICS IN A CO2 LASER-PRODUCED PLASMA OF HELIUM. Journal de Physique Colloques,
1979, 40 (C7), pp.C7-735-C7-736. �10.1051/jphyscol:19797355�. �jpa-00219350�
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1979,page C7-
735SPECTROSCOPIC STUDY OF THE IONIZATION DYNAMICS
IN
A C02 LASER-PRODUCED PLASMA OF HELIUMJ.C.
Gauthier, J.P. Geindre and N. Grandjouan.Groupe drEZectroniwue dans Zes
Gaz.I n s t i t u t drEZectronique Fondamentale, k b o r a t o i r e associe' au
C.N. R.S,,Universitg Paris-XI- BEt.
220, 91405Orsay Cedex, France.
T h i s paper r e p o r t s on a s p e c t r o s c o p i c s t u d y of t h e i o n i z a t i o n dynamics of C02 l a s e r induced plasmas i n medium p r e s s u r e helium gas. P r e v i o u s s t u d i e s have shown /1-3/ t h a t t h e v a r i o u s mechanisms of plasma e v o l u t i o n i n space and time a r e n o t f u l l y elucidated and may v a r y s t r o n g l y with experimental c o n d i t i o n s .
I n t h i s experiment, a W p r e i o n i z e d TEA C02 l a - s e r d e l i v e r e d approximately 1 J i n a p u l s e which c o n s i s t s of a n i n t e n s e s p i k e % I 0 0 n s wide, followed by a weaker emission a b o u t 20% o f t h e peak of some
HELlA 760 TORR
-
m- A-
1QQQ n s d u r a t i o n . The p u l s e g e p e t s t i o n r a t e $5 3
-
F i q . 1 ~ Time r e s o l v e d s p e c t r a a t l m m above f o c u s . S t a r k broadened He1 l i n e s a r e prominent above t h e p u l s e s p e r second. Plasma d i a g n o s t i c s were c a r r i e d continuum. The 4686.8
He11 l i n e a p p e a r s by 300 n s . o u t with a high degree o f p r e c i s i o n and r e l i a b i l i t y- ' L B ' i 3,
by t a k i n g advantage o f t h e good shot-to-shot r e p r o - ?
--
15 32d u c i b i l i t y and of t h e high r e p e t i t i o n r a t e of t h e
- -
-z 3
E
l a s e r . S p e c t r o s c o p i c measurements were made w i t h a
-
>
-temporal r e s o l u t i o n of 8 ns and a s p a t i a l resolutiion b e t t e r t h a n 0.1 mm i n both t h e a x i a l ( z ) and t h e r a - d i a l ( r ) d i r e c t i o n , u s i n g a computer c o n t r o l l e d data a c q u i s i t i o n system a l r e a d y d e s c r i b e d /4/.
A t y p i c a l time r e s o l v e d spectrum t a k e n a t 1 atm.
a b o u t Imm above t h e f o c u s i s shown on Fig.1. Time
~ l D i E 1 4 4 0
and space-resolved measurements t a k e n along t h e l a - I, cxF-it- IP d
SE76.BZ 1 1 3 2
s e r a x i s a r e shown on Figs.2a-c. They g i v e r e s p e c t i -
-
v e l y t h e i n t e n s i t y of t h e continuum i n a s p e c t r a l r e g i o n f r e e from atomic and i o n i c l i n e s , t h e i n t e n -
3 3 9
s i t y of t h e 3 D-2 P (5876 A) He1 t r a n s i t i o n , a n d t h e i n t e n s i t y o f t h e n=4 t o n=3 (4686 A) He11 t r a n s i t i o n both c o r r e c t e d from t h e i r underlying continuum. The time s c a l e o r i g i n corresponds t o t h e beginning of t h e l a s e r p u l s e . R e s u l t s o b t a i n e d over t h e range
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797355
C7-
7360.7-2.5 atm. were q u a l i t a t i v e l y t h e same. A break- down plasma c o n s i s t i n g mainly of ~ e + i o n s ( o n l y t h e continuum and He1 s p e c t r a a r e observed) a p p e a r s near t h e f o c a l p l a n e (z=O) d u r i n g t h e r i s i n g edge o f t h e p u l s e . An expansion f r o n t w i t h a n i n i t i a l v e l o c i t y of 5x10 6 cm/s i s observed t o move i n a d i r e c t i o n op- p o s i t e t o t h a t o f t h e i n c i d e n t l a s e r beam. By 300 ns t h e plasma v e l o c i t y d e c r e a s e s w h i l e t h e t r a i l i n g ed- g e of t h e luminous f r o n t moves toward t h e focus.ThiS p e r i o d i s dominated by a s h a r p r i s e of HeII emission ( F i g . 2 ~ ) and a s t r o n g d e c r e a s e of both continuum and He1 emissions (Fig.2a-b). These r e s u l t s confirmed t h e time-resolved spectrum o f Fig.1. The e l e c t r o n temperature i n c r e a s e c h a r a c t e r i z e d by t h e a p p a r i t i o n of HeII r a d i a t i o n was found t o b e d i r e c t l y propor- t i o n a l t o t h e f r a c t i o n of l a s e r energy c o n t a i n e d i n t h e hil. I n a d d i t i o n , t h e i n i t i a l speed o f t h e plasma f r o n t and t h e time of HeII r a d i a t i o n appearance were a l s o found t o b e p r o p o r t i o n a l t o t h e g a s p r e s s u r e . This r u l e s o u t a r a d i a t i o n d r i v e n d e t o n a t i o n mecha- nism /1/ of expansion under t h e p r e s e n t c o n d i t i o n s . Also, p h o t o i o n i z a t i o n of t h e unheated g a s ahead of t h e plasma f r o n t i s u n l i k e l y t o o c c u r s i n c e t h e e- l e c t r o n temperature deduced from t h e i o n i z a t i o n s t a - t e i n t h e i n i t i a l plasma i s t o o low (4 eV). I n s t e a d , plasma e v o l u t i o n i s governed by l a s e r d r i v e n i o n i z a - t i o n dynamics. R a d i a l o b s e r v a t i o n s shown i n Figs.3a -b f u r t h e r s u b s t a n t i a t e t h i s h y p o t h e s i s . The h b t plasma c o r e (800 ns on Fig.3b) b u i l d s up from t h e bulk o f t h e c o l d plasma through absorbed IR r a d i a - t i o n from t h e l a s e r t a i l . Indeed, experiments w i t h a C02 p u l s e f r e e from tail e m i s s i o n d i s p l a y a conside-
r a b l e r e d u c t i o n o f t h e h o t c o r e s i z e and temperature The b l a s t wave mechanism s t a t e d i n Ref.3 4 s t h u s t o b e rejected. Further s t u d i e s of plasma ,parameters m d e r &mi.
l a r c o n d i t i o n s a r e presented in the following paper.
1/ Yu.P. RAIZER
-
Sov. Phys. Uspekhi8,
650 (1966).2/ N. AHMAD, B.C.GALE and M.H.KEY - PrOc. ROy.Soc.
A310, 231 (1969).
3/ E.V.GEORGE, G-BEFEKI and B.YAIAKOBI
-
Phys.F l u i d s
14,
2708 (1971).
4/ N. GRANDJOUAN, J-C.GAUTHIER and J-P-GEINDRE
-
C.R. Ac. S c i .
284,
437 (1977).F i g . 2
-
Time r e s o l v e d i n t e n s i t i e s a l o n g z . L a s e r--
from t e f t t o o r i g h t a ) continuum (4750
i)
; b ) He1 emis- s i o n (5876 A) ; c ) HeII e m i s s i o n (4686i).
Fig.3
-
Time r e s o l v e d * n t e n s i t i e s aloong r a t 1 mm above f o c u s . a ) oHeI e m i s s i o n (5876 A ) ; b) HeII ,-I., ie m i s s i o n (4686 A ) .
