MECHANISMS OF ELECTRON DISAPPEARANCE IN A DECAYING PLASMA ARC

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MECHANISMS OF ELECTRON DISAPPEARANCE IN A DECAYING PLASMA ARC

A. Gleizes, H. Kafrouni, S. Vacquie

To cite this version:

A. Gleizes, H. Kafrouni, S. Vacquie. MECHANISMS OF ELECTRON DISAPPEARANCE IN A DECAYING PLASMA ARC. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-235-C7-236.

�10.1051/jphyscol:19797115�. �jpa-00219086�

JOURNAL DE PHYSIQUE CoZZoque C7, suppZe'ment au n07, Tome 40, JuiZZet 1979, page C7- 235

A. Gleizes, H. Kafrouni and S. Vacquie.

C e n t r e de Physique Atomique, Laboratoire Associe' au C.N.R.S. n o 277, U n i v e r s i t e ' Paul . S a b a t i e r , 31 077 Tou Louse France.

It is generally accepted that during argon plasma decay a t a t m o s p h e r i c p r e s s u r e , the d i s a p p e a r a n c e of c h a r g e d p a r t i c l e s i s brought about by e l e c t r o n - ion recombination. F o r a t e m p e r a t u r e of about 1 eV, recombination i s p a r t i a l l y counterbalanced by ionization. When the p l a s m a i s confined the e l e c - t r o n 10s s e s through a m b i p o l a r diffusion a r e not ne- gligible. T h e a i m of this communication i s to show the r e l a t i v e influence of the v a r i o u s p r o c e s s e s by c o m p a r i n g the e x p e r i m e n t a l r e s u l t s with t h e o r e t i - c a l e s t i m a t i o n s .

EXPERIMENTAL RESULTS. T h e e x p e r i m e n t a l s e t

-

up i s d e s c r i b e d in r e f e r e n c e / l / . T h e a r c i s a wall stabilized one and the decay i s g o v e r n e d by a f a s t t h y r i s t o r .

In steady s t a t e conditions the e l e c t r o n n u m b e r den- s i t y , n , w a s d e t e r m i n e d with r e s p e c t to the r a - d i u s , by m e a s u r i n g the absolute intensity of the continuum radiation a t 423 nm. T h e e l e c t r o n t e m - p e r a t u r e , T

,

was deduced f r o m r e l a t i v e intensity

e

m e a s u r e m e n t s of the l i n e s .

In the extinction phase, the e l e c t r o n number den- s i t y and the v a r i a t i o n of the a t o m density n

a' w e r e m e a s u r e d by l a s e r i n t e r f e r o m e t r y , a t two wavelengths, along the a x i s of the d i s c h a r g e . A l s o n ( r , t ) w a s d e t e r m i n e d by m e a s u r i n g the continu- u m radiation (fig. 1 ) . T h e values a t r = O a r e i n good a g r e e m e n t with the r e s u l t s deduced f r o m 'interfe- r o m e t r y . Finally, the v a r i a t i o n s of light intensity of s o m e A r I l i n e s w e r e m e a s u r e d along the a x i s of the d i s c h a r g e (r=O). C e r t a i n r e s u l t s a r e given i n a n o t h e r communication/2/ p r e s e n t e d a t this ICPIG : f o r c u r r e n t i n t e n s i t i e s l e s s than 25 A , the intensity of t h e s e l i n e s r o s e s h a r p l y a f t e r cutting off the c u r r e n t ( t i m e of i n c r e a s e - 5 ps) then fell

to z e r o a f t e r 250 to 300 p s . It i s shown/2/that this phenomenon i s due to the rapid r e l a x a t i o n of the e l e c t r o n t e m p e r a t u r e to the g a s t e m p e r a t u r e .

0 1 2 r (mm)

F i g . 1 . Radial profile of n a t different t i m e s

e .

a f t e r cut-off (I=52 _A A).

.

^{continuum 423 n m}

i n t e r f e r o m e t r y .

THEORETICAL ANALYSIS. In a decaying a r g o n p l a s m a , with c y l i n d r i c a l s y m e t r y and no a x i a l g r a - dients, the v a r i a t i o n of n- i s d e s c r i b e d by :

ane 1

a

^t:

-

a t t

-

^{- (rn v} )=-on3+sn n r a r e e e e 1

w h e r e v i s the a v e r a g e velocity of the e l e c t r o n s , a the recombination coefficient, S the ionization coefficient and n l the density of a t o m s i n the ground s t a t e .

T h e m o s t r i g o u r o u s calculation of a and S w a s m a - de using a collisional r a d i a t i v e model/3/. F o r a r - gon the d e e p e s t s t u d y was m a d e by ~ a t s o n i s / 4 / . In o r d e r to apply h i s r e s u l t s to equation ( I ) , ne, T e , T h and the diffusion flux m u s t be locally known.

F o r high values of n (n

)

1

o1

6 c m - 3 ) the p l a s m a e e

a p p r o a c h e s L T E and the v a l u e s of the different pa- r a m e t e r s m u s t be known with g r e a t a c c u r a c y : f o r T e and n l , the e x p e r i m e n t a l r e s u l t s a r e not a c c u - r a t e enough. F o r t h i s r e a s o n we developed a m o d e l which allowed u s to calculate T , T h , and the a - t o m d e n s i t i e s f r o m e x p e r i m e n t a l values of n ( r ) and the a x i a l e l e c t r i c field E. T h i s model i s based

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797115

on the resolution of a coupled s y s t e m of balance e - quations for the population of each r e a l e n e r g y l e - v e l of A r I and the e l e c t r o n energy balance, i n the steady s t a t e p l a s m a . F o r l e v e l j :

w h e r e C.. and F.. a r e r a t e coefficients f o r collisio-

'J J 1

nal excitation and de-excitation ; S. the collisional J

ionization r a t e coefficient ; R . , Q . r a t e coefficients

J J

f o r radiative and collisional recombinations ; A j'

A ..,

e s c a p e f a c t o r s of radiation. T h e e l e c t r o n e n e r -

1J

gy balance (equation 3 ) i s d e s c r i b e d i n detail in/2/.

F o r the excited l e v e l s , diffusion i s negligible and we can w r i t e :

L L ( r n v

= - ' 2 -

r a r

e e

r

" 1 ~ 1 )

A m o d e l with such a make-up allows the calcula

-

tion of T , T h , n l , n2.

. .

^, and the deduction of the ionization and recombination coefficients. T h e r a - t e s of these reactions a r e the population and depo- pulation r a t e s of the ground s t a t e by collision and radiation.

A s a l r e a d y stated, when E--- 0, the e l e c t r o n t e m - p e r a t u r e d r o p s to T before n and n have had the

h a

time to change. T h e coefficients a and S t h e r e f o r e m u s t be calculated f o r a n e l e c t r o n t e m p e r a t u r e e - qual to T

h '

COMPARISON O F THE RESULTS. The following table gives values of a and S with r e s p e c t to some(

of the values of the (n

,

T ) couple corresponding to r e a l c a s e s .

I n e ( 8 . 8 10t16 1 4 . 0 10t1611.5 1 0 ' l ~ ~ : ~ > i ~

T h e values of Katsonis, under the conditions of r e a b s o r p t i o n found i n the a r c , a r e lower than o u r s ; this i s due to the choice of the excitation c r o s s section of the f i r s t levels of A r I.

In fig. 2 a l l the experimental and calculated r e s u l t s a r e p r e s e n t e d . T h e 3 c u r v e s of the calculated r e - sults c o r r e s p o n d to 3 different p a r a m e t e r s : a i s the recombination coefficient ; y 5 a

-

Sn

/n

²

1 e the apparent recombination coefficient and (an /at)/

n: the e l e c t r o n disappearance coefficient. A g:od a g r e e m e n t i s s e e n between the calculated and m e - a s u r e d values of (a ne/a t)/ne which would s e e m to 3 justify our calculation of a and S (Katsonis' values a r e not i n such good agreement). A s the electron density i n c r e a s e s , s o do the ionization phenomena;

this can be s e e n by comparing a and y

.

^Finally,

the comparison between l a n d (ane/a t)/n3 shows the influence of diffusion which m a y r e p r e s e n t 30

%

of the e l e c t r o n disappearance m e c h a n i s m s .

Fig. 2. Experimental and calculated values of recombination and disappearance coefficients

of the e l e c t r o n s .

/1/ H. Kafrouni and a l i i , to be published in JQSRT (1 979).

/2/ H. Kafrouni and a l i i , ICPIG Grenoble (1979).

/3/ D. R. B a t e s and alii, P r o c . Roy. Soc. London, 267, (1962), 297.

-

/4/ K. Katsonis, Thkse d1ktat, n o 1637, O r s a y (1976).

(1) our values, (2) Katsonis /4/