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SPACE AND TIME RESOLVED MEASUREMENTS OF ELECTRON AND ATOMIC EXCITED STATES DENSITIES IN A CO2 LASER-PRODUCED PLASMA
OF HELIUM
J. Gauthier, J. Geindre, N. Grandjouan
To cite this version:
J. Gauthier, J. Geindre, N. Grandjouan. SPACE AND TIME RESOLVED MEASUREMENTS OF ELECTRON AND ATOMIC EXCITED STATES DENSITIES IN A CO2 LASER-PRODUCED PLASMA OF HELIUM. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-737-C7-738.
�10.1051/jphyscol:19797356�. �jpa-00219351�
JOURNAL DE PHYSIQUE CoZZoque C7, supplgment a u n07, Tome 40, JuiZZet 1979,- page
€7- 737
SPACE AND TIME RESOLVED MEASUREMENTS OF ELECTRON AND ATOMIC EXCITED STATES DENSITIES
IN
A C02 LASER-PROWCED PLASMA OF WIUMJ.C.
Gauthier, J.P. Geindre and N. Grandjouan.Groupe d1EZectronique dans Zes gaz. I n s t i t u t dtEZectronique FondamentaZe, k b o r a t o i r e a s s o c i i a u C.
N.
R.S., U n i v e r s i t i Paris-XI- B Z t . 220, 91405 Orsay Cede?, f i a n c e .Plasma spectroscopy i s a n e f f i c i e n t means t o
w2191563
.-r S
s t u d y l a s e r induced plasmas. I n a companion paper /I/ 11 V G B B 41 2.65
we have shown t h e r o l e played by t h e i o n i z a t i o n dyna- mics on t h e plasma s t r u c t u r e through measurements of r e l a t i v e atomic and i o n i c l i n e i n t e n s i t i e s . Here, we may e x t r a c t information a b o u t e l e c t r o n d e n s i t i e s and temperatures from an a n a l y s i s of l i n e widths and s h i f t s . Also, a measure of atomic e x c i t e d s t a t e s po- p u l a t i o n s h a s been o b t a i n e d from t h e a b s o r p t i o n o f a v i s i b l e , a u x i l i a r y l a s e r .
The experimental c o n d i t i o n s were s i m i l a r t o t h a t d e s c r i b e d i n Ref.1. The a p p a r a t u s , a l r e a d y d e s c r i b e d i n Ref.2, h a s been s l i g h t l y modified t o i n s e r t a n i - t r o g e n pumped dye l a s e r f o r a b s o r p t i o n measurements.
The C 0 2 and t h e dye l a s e r s were s y n c h r o n i z e d t o b e t - t e r t h a n 10 ns. The probe beam diameter was a b o u t O.2m. R e j e c t i o n of plasma l i g h t was c a r e f u l l y s t d i e d t o allow a b s o r p t i o n measurements down t o t h e p e r c e n t l e v e l . R a d i a l d i s t r i b u t i o n s of t h e emission c o e f f i - c i e n t were o b t a i n e d by performing a n Abel i n v e r s i o n of t h e observed emission and a b s o r p t i o n p r a f i l e s u s i n g s t a n d a r d techniques o f i n v e r s i o n with s e l f ab- s o r p t i o n / 3 / . ~ y p i c a l r e s u l t s a r e shown on Fig.l.Peak a b s o r p t i o n c o e f f i c i e n t s o f 50% a r e seen t o s t r o n g l y modify t h e emission p r o f i l e , p a r t i c u l a r l y i n t h e c o l d o u t e r plasma d e p i c t e d i n Ref.1. T h i s p o i n t s t o t h e u s e l e s s n e s s of s p e c t r o s c o p i c d i a g n o s t i c s i n l a - s e r - c r e a t e d plasmas i f s e l f a b s o r p t i o n i s n o t pro- p e r l y t a k e n i n t o account.
Space-resolved l i n e p r o f i l e s were recorded f o r
0
Fig.1
-
R a d i a l e m i s s i v i t y of He1 ( 5 8 7 6 A ) a f t e r Abel i n v e r s i o n . a) d i r e c t emission ; b) t r u e emission when measured a b s o r p t i o n i s t a k e n i n t o account.rameter l e a s t s q u a r e s method t o Griem's S t a r k broa- dening t h e o r y /4/. T h i s is done on Fig.:! a t z = I t
3 3 0
r = 0.5mm f o r t h e 3 P
-
2 S He I emission a t 3889 A.D e t a i l e d agreement i s remarkable, e s p e c i a l l y f o r t h e s t r o n g asymmetry of t h i s l i n e a t e l e v a t e d e l e c t r o n d e n s i t i e s . From t h e f u l l width of s e v e r a l t e n s of p r o f i l e s such t h o s e g i v e n on Fig.2, we ob- t a i n a "map" o f t h e time e v o l u t i o n of t h e e l e c t r o n some He1 l i n e s and f i t t e d by a non-linear multipa- d e n s i t y a s shown On Fig.3. R e s u l t s f o r t h e 6678 A,
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797356
0 0
5876 A and 3889 A l i n e s p r e d i c t s l i g h t l y d i f f e r e n t 5/ J-C.GAUTHIER, J-P. GEINDRE ( t o b e p u b l i s h e d ) . d e n s i t i e s , w i t h d i f f e r e n c e s n o t exceeding 30% i n t h e
low d e n s i t y o u t e r plasma. On a x i s , t h e r e s u l t s a r e w i t h i n a f a c t o r o f two a t e a r l y t i m e s ; t h e r e a s o n f o r t h e d i s c r e p a n c y i s n o t y e t f u l l y understood. S i - m i l a r remarks a p p l y t o t h e l i n e s h i f t f o r which we found t h e goad s i g n and o r d e r o f magnitude. The e l e c t r o n temperature was i n f e r r e d from r e l a t i v e l i n e t o continuum i n t e n s i t i e s . T y p i c a l r e s u l t s g i v e Te=
4 t 1 eV i n t h e c e n t r a l r e g i o n and Te = 2.5
+
0.5eV----
FIT (MIIU)a t r = 1.3 mm by 500 n s , t h a t i s b e f o r e plasma c o r e m m n t i , h e a t i n g by t h e l a s e r t a i l ( s e e Figs.3a-b i n 1/).Du-
r i n g l a s e r t a i l - h e a t i n g , t h e e l e c t r o n temperature r e a c h e s 5.8 f 0.5 eV on a x i s , a s deduced from Abel i n v e r t e d He11 t o He1 i n t e n s i t y r a t i o . From a b s o l u t e a b s o r p t i o n and emission measurements, we a l s o o b t a i -
3 3 3
ned t h e a b s o l u t e p o p u l a t i o n s o f 2 P, 3 P, 3 D and
1 3
3 D l e v e l s . R e s u l t s f o r t h e 2 P Level p o p u l a t i o n i n u n i t s of Cmc3 a r e given on Fig.4. S i m i l a r s t r u c t u r e was observed f o r t h e o t h e r e x c i t e d s t a t e s . f i e i n t e r n a l c o n s i s t e n c y o f t h e experiment was checked by v e r i f y i n g w i t h i n 10% t h a t t h e e q u a l energy 3 3 P
0
F i g . 2
-
Time-resolved l i n e p r o f i l e o f He1 (3889 A) e m i s s i o n a t z = 1, r = 0.5 mm.m
and 3 D l e v e l s were i n Boltzmann e q u i l i b r i u m . 1 These r e s u l t s c l e a r l y show t h a t l a r g e g r a d i e n t s
F i g . 3
-
Space a n d t i m e r e s o l v e d e l e c t r o n d e n s i t y of e x c i t e d s t a t e s d e n s i t i e s ( r e l a t e d t o e l e c t r o n tern- 'Orresponding to the raw data Of Fig-2.p e r a t u r e g r a d i e n t s ) m a y be expected i n l a s e r c r e a t e d plasmas. Comparison of experiment w i t h t h e p r e d i c - t i o n s o f a one-dimensional l a g r a n g i a n code /5/ i n c l u - d i n g hydrodynamics, i o n i z a t i o n dynamics and r a d i a t i - ve t r a n s f e r g i v e s encouraging r e s u l t s s i n c e most o f t h e s p a t i a l s t r u c t u r e and temporal e v o l u t i o n of plas- ma p a r a m e t e r s and e x c i t e d s t a t e s p o p u l a t i o n s a r e w e l l reproduced.
1/ J-C.GAUTHIER, J-P.GEINDRE, N.GRANAJOUAN : t h i s conference.
2/ Reference 4 of t h e preceeding paper.
3/ P.ELDER, T.JERRICK and J.W.BIRKELAND
-
Applied5
O p t i c s
5,
589 (1965). F i g . 4-
2 P l e v e l p o p u l a t i o n d e n s i t y a s a f u n c t i o n o f s p a c e and t i m e .4/ H.R. GRIEM " S p e c t r a l l i n e broadening by plasmas"
(Academic, New York, 1975).