MEASUREMENT OF H- DENSITY IN A PLASMA BY PHOTODETACHMENT

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MEASUREMENT OF H- DENSITY IN A PLASMA BY PHOTODETACHMENT

M. Bacal, G. Hamilton, A. Bruneteau, H. Doucet, J. Taillet

To cite this version:

M. Bacal, G. Hamilton, A. Bruneteau, H. Doucet, J. Taillet. MEASUREMENT OF H- DENSITY

IN A PLASMA BY PHOTODETACHMENT. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-

791-C7-792. �10.1051/jphyscol:19797381�. �jpa-00219379�

JOURIVAL

DE

PHYSIQUE CoZZoque C7, supp26ment au n07, Tome

40,

JuiZZet

1979,

page

C7- 791

MEASUREMENT OF

H-

DENSITY I N A PLASMA BY PHOTODETACHMENT

M. Bacal, G.W. ~ a m i l t o n * , A.M. Bruneteau, H.J. Doucet and J.

aill let'.

+Laboratoire de Physique des Milieux l o n i s 6 s X X , Ecole Polytechnique,

91128

Pataiseau Cedex,

frame.

Office National d'Etudes e t Recherches AgrospatiaZes,

92320

ChatiZZon, France.

Improved techniques f o r measuring t h e d e n s i t y o f H- o r D- i n a plasma a r e r e q u i r e d because o f c e r t a i n l i m i t a t i o n s o f t h e techniques o f Langmuir probes and mass a n a l y s i s 1

.

I n t e r p r e t a t i o n o f probe d a t a r e q u i r e s an assumption o f t h e mass o f p o s i - t i v e i o n s and a r e r e s t r i c t e d i n u s e f u l n e s s by t h e Debye d i s t a n c e . Mass s p e c t r a measured by i o n mass a n a l y z e r s t e n d t o b e u s e f u l o n l y f o r r e l a t i v e mea- surements u n l e s s c a l i b r a t i o n techniques a r e i n c l u - ded t o determine a b s o l u t e i o n d e n s i t i e s . The r e - quirement f o r r e l i a b l e measurements of' H- and D- i s based upon t h e need f o r development o f O- i o n sources s u i t a b l e f o r n e u t r a l i n j e c t i o n i n t o c o n t r o l l e d f u s i o n devices.

Photodetachement has p r f v i o u s l y been used t o measure n e g a t i v e i o n d e n s i t i e s i n oxygen plasmas 2

.

Photodetachement c o n s i s t s o f t h e detachment ^{o f} t h e e x t r a e l e c t r o n o f a n e g a t i v e i o n by a photon (H- + photon ^---b H

+

e). T h e r e f o r e photodetachement i n a plasma produces an i n c r e a s e i n e l e c t r o n d e n s i t y , which can b e measured b y probes o r microwaves de- pending upon t h e geomerey. The photon energy s h o u l d be s e l e c t e d such t h a t i t i s l o w e r t h a n t h e t h r e s - h o l d energy o f o t h e r photon i n t e r a c t i o n s such as p h o t o i o n i z a t i o n , p h o t o e x c i t a t i o n , photoemission o r photodetachment of o t h e r species. The l i g h t f r o m a ruby l a s e r (1.8 eV photon energy) i s s u i t a b l e f o r t h i s purpose. The photodetachment c r o s s - s e c t i o n f o r H- ( 4 X 1 0 - l 7 cm 2 ) i s near i t s maximum a t t h i s photon energy3. We used a ruby l a s e r capable o f a 1 J p u l s e i n a t i m e about 30 ns.

To assure t h a t t h e photodetachement s i g n a l i s p r o p o r t i o n a l t o t h e d e n s i t y o f H- and n o t o f o t h e r n e g a t i v e i o n s such as OH-, 0-, o r 0; ( f o r which t h e c r o s s - s e c t i o n s a r e one o r two o r d e r s o f magni- tude l o w e r ) we have m a g n e t i c a l l y analyzed t h e nega-

t i v e i o n s e x t r a c t e d from t h e plasma and a l s o have v e r i f i e d ( F i g u r e 1 ) t h a t our measurements agree w i t h t h e t h e o r e t i c a l photodetachement f r a c t i o n computed from t h e c r o s s s e c t i o n f o r t h e photodetachment o f H-, b u t n o t w i t h t h a t f o r o t h e r i o n s :

photodetachment f r a c t i o n = An-/n-

=

1 - exp ( - l a s e r p u l s e energy/area x 6/hv ) _{( 1 )} The t e s t o f F i g u r e 1 s h o u l d be repeated whenever t h e e x p e r i m e n t a l c o n d i t i o n s change t o a u t h e n t i c a t e t h e mcaaurements.

F i g u r e 1 shows t h a t t h e photodetachment f r a c - t i o n hn-/n- i s e s s e n t i a l l y 100 % i f t h e l a s e r p u l s e energy i s more t h a n 0.1 J. Under t h i s c o n d i t i o n we can determine t h e r e l a t i v e d e n s i t y o f H- from t h e change i n t h e probe e l e c t r o n c u r r e n t d e n s i t y :

We found some l a r g e n o i s e s i g n a l s when t h e l a - s e r e n e r g y i s h i g h e r t h a n 0.2 J ; we observed an i n c r e a s e o f these n o i s e s i g n a l s when t h e l a s e r l i g h t h i t s a s u r f a c e s i t u a t e d c l o s e t o t h e probe (e.g. t h e s t a i n l e s s s t e e l w a l l o f a m u l t i p o l e ) . The n o i s e s i - g n a l s a t h i g h l a s e r power d e n s i t y (2 t o 10 W/cm ) 2 may be due t o t h e i n t e r a c t i o n o f l a s e r l i g h t w i t h s o l i d s u r f a c e s o r t o t h e onset o f m u l t i p h o t o n e f - f e c t s o r f r e e - f r e e e l e c t r o n i n t e r a c t i o n s . T h i s pro- blem can be avoided by monitoring t h e l a s e r p u l s e energy and u s i n g t h e data o n l y f o r p u l s e s w i t h i n t h e range from 0 . 1 t o 0.2 J.

We e x p l o r e d t h e photodetachment s i g n a l spa- t i a l l y by moving t h e probe i n two dimensions. We found no measurable s i g n a l when t h e probe was com- p l e t e l y o u t o f t h e l a s e r beam. The s i g n a l was appro-

x i m a t e l y c o n s t a n t when t h e probe was w v e d a l o n g t h e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797381

beam. Therefore when we compute t h e r a t i o o f probe

*

Permanent address : Lawrence Livermore Labora- c u r r e n t d e n s i t i e s i n Eq. (2) we consider t h a t only t o r y , Livermore, C a l i f o r n i a , U.S.A.

t h e s e c t i o n o f t h e probe immersed i n t h e l a s e r beam

**

Groupe de Recherche No 29 du Centre N a t i o n a l de i s e f f e c t i v e f o r photodetachment. l a Recherche S c i e n t i f i q u e .

F i g u r e 2 shows t h e experimental apparatus sche- m a t i c a l l y . The plasma was produced i n a hydrogen atmosphere o f

lo-'

T o r r by thermionic e l e c t r o n s e m i t t e d by a cathode a t a negative p o t e n t i a l o f 60 t o 120 V. D e n s i t i e s o f plasma and o f gas were con- t r o l l e d by adjustment o f t h e thermionic emission and o f t h e gas feed. L i g h t pulses from a ruby l a s e r were i n j e c t e d v i a a r e f l e c t i n g p r i s m and a 2 cm diameter g l a s s window, and were monitored by a photomulti- p l i e r through a p a r t i a l l y r e f l e c t i n g m i r r o r . The l i g h t monitor was c a l i b r a t e d versus i n j e c t e d l a s e r energy using a c a l o r i m e t e r w i t h i n t h e vacuum chamber.

Two types o f c y l i n d r i c a l tungsten probes were used t o measure t h e d e n s i t i e s o f e l e c t r o n and i o n s . For absolute measurements o f ne and o f n+ i t i s necessary t o measure t h e complete probe characte- r i s t i c and t o analyze i t e i t h e r by t h e Langmuir theory ( i f the probe r a d i u s i s l e s s than 3 Debye distances) o r by a theory f o r l a r g e r probes. On the

1. E. Nicolopoulou, M. Bacal and H.J. Doucet, Le Journal de Physique, 38, 1399 (1977).

2. J. T a i l l e t , Compt. Rend., P a r i s ,

269,

s e r i e B, 52 (1969).

3. J. Smith and D.S. Burch, Phys. Rev.,

116,

1125 (1959).

- 0 0.1 0.2

LASER ENERGY ( J 1

other hand, f o r measurements o f changes i n ne and F i g u r e 1 : Photodetachment o f s e v e r a l negative i o n species by ruby l a s e r l i g h t as a f u n c t i o n o f l a s e r n+ i t i s s u f f i c i e n t t o use large-area probes o f p u l s e energy, according t o theory o f equation (1).

which t h e r a d i u s mav be more than t h e Debve d i s - Experimental data p o i n t s are superinposed upon t h e tance, so t h e probe would be s e l f - s u p p o r t i n g and thkory o f equation' (1) f o r H-, hher; 6 / h <

=

139

c m z / ~ and t h e area o f t h e l a s e r beam is 3 cm2.

so t h e pulsed probe c u r r e n t s would be easier t o measure. The s e l f supporting probe d i d n o t r e q u i r e

t h e exposure o f a supporting i n s u l a t o r t o l a s e r

-

PULSCD RUBY

l i g h t . The best probe geometry i s such t h a t t h e probe a x i s i s p a r a l l e l t o t h e l a s e r beam, and t h e probe area w i t h i n t h e l a s e r beam i s l a r g e r than the probe area o u t s i d e t h e l a s e r beam.

The measurements by t h e technique described were r e p r o d u c i b l e and independent o f t h e probe d i a - meter, showing n- i n c r e a s i n g i n p r o p o r t i o n t o n 3 i n t h e range o f ne between 10 and 9 2 x 10'' cm-?a

This p r o j e c t was performed a t Ecole Polytech- nique as a j o i n t p r o j e c t i n v o l v i n g t h r e e laborato- r i e s , funded by t h e French Commissariat B 1'Energie Atomique and t h e U.S. Department o f Energy.

SCOPE 1 I N T E G R l T E D LASER

1

^{P U E R}

I ^I

^{SCOPE 2}

^I

F i g u r e 2 : Equipment diagram