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
DYNAMICS 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>
HAL Id: jpa-00219350
https://hal.archives-ouvertes.fr/jpa-00219350
Submitted on 1 Jan 1979
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JOURNAL DE PHYSIQUE
CoZZoque C7, suppZ6ment au n07, Tome SO, JuiZZet 1979, page C7- 735
SPECTROSCOPIC STUDY OF THE IONIZATION DYNAMICS IN A C02 LASER-PRODUCED PLASMA OF HELIUM
J.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, 91405 Orsay Cedex, France.
This paper r e p o r t s on a spectroscopic study of
HELlA 760 TORR
-
t h e i o n i z a t i o n dynamics of C02 l a s e r induced plasmas
m
-
A
-
i n medium p r e s s u r e helium gas. Previous 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
evolution i n space and time a r e n o t f u l l y elucidated
and may vary s t r o n g l y with experimental conditions.
I n t h i s experiment, a W preionized 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 about 20% o f t h e peak of some
1QQQ n s duration. The p u l s e g e p e t s t i o n r a t e
$5
3
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
F i q . 1 ~Time r e s o l v e d s p e c t r a a t lmm 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
continuum. The 4686
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
by t a k i n g advantage o f t h e good shot-to-shot
repro-
d 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
- ' L B'i 3,
--
?
--
15 32
3
z
-
E
l a s e r . Spectroscopic measurements were made with 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 than 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 , using 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 described /4/.
A t y p i c a l time resolved spectrum taken a t 1 atm.
about Imm above t h e focus i s shown on Fig.1.
Time
~lDiE1440
and space-resolved measurements taken along t h e l a 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 -
I,
cxF-it- IP
d
SE76.BZ
1132
-
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
region 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 9
3
3
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 obtained over t h e range
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797355
8.
He11 l i n e a p p e a r s by 300 n s .
C7- 736
0.7-2.5
atm. were q u a l i t a t i v e l y t h e same. A break-
3/ E.V.GEORGE,
Fluids
down plasma c o n s i s t i n g mainly of ~ e i+o n s (only t h e
G-BEFEKI and B.YAIAKOBI
14,2708
(1971)
-
Phys.
.
4/ N. GRANDJOUAN, J-C.GAUTHIER and J-P-GEINDRE
continuum and He1 s p e c t r a a r e observed) appears near
C.R.
Ac. S c i .
284, 437
-
(1977).
t h e f o c a l plane (z=O) during t h e r i s i n g edge o f t h e
pulse. 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 while t h e t r a i l i n g edge 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 decrease 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 be d i r e c t l y proport 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 contained i n
- 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 emiss i o n (5876 A) ; c ) HeII emission (4686 i).
Fig.2
-
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 be 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 occur s i n c e t h e el 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 too 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. Radial 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 hypothesis. 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 with
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 consider a b l e r e d u c t i o n o f t h e hot 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 studies of plasma ,parameters mder &mi.
l a r conditions a r e presented in the following paper.
1/ Yu.P. RAIZER
-
Sov. Phys. Uspekhi
2/ N. AHMAD, B.C.GALE
A310, 231 (1969).
and M.H.KEY
8,
650 (1966).
- PrOc. ROy.Soc.
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 emission (5876 A) ; b) HeII ,-I.,
e m i s s i o n (4686 A ) .
i