ON THE THREE-BODY ELECTRON ATTACHMENT TO OXYGEN IN THE PLASMA OF A NON-SELF-MAINTAINED DISCHARGE

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ON THE THREE-BODY ELECTRON ATTACHMENT TO OXYGEN IN THE PLASMA OF A

NON-SELF-MAINTAINED DISCHARGE

A. Vasilieva, I. Grishina, V. Ktitorov, A. Kovalev, A. Rakhimov

To cite this version:

A. Vasilieva, I. Grishina, V. Ktitorov, A. Kovalev, A. Rakhimov. ON THE THREE- BODY ELECTRON ATTACHMENT TO OXYGEN IN THE PLASMA OF A NON-SELF- MAINTAINED DISCHARGE. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-103-C7-104.

�10.1051/jphyscol:1979751�. �jpa-00219457�

JOURNAL DE PHYSIQUE CoZZoque C7, suppZ6ment au n07, Tome 40, JuiZZet 1979, page C7- 103

ON THE THREE-BODY ELECTRON ATTACHMENT TO OXYGEN IN THE PLASMA OF A NON-SELF-MAINTAINED DISCHARGE

A.N. Vasilieva, I.A. Grishina, V.I. Ktitorov, A.S. Kovalev, A.T. Rakhimov.

I n s t i t u t e of Nuclear Physics, Moscow S t a t e University, 11 7234, Moscow, U.S.S.R.

The advance i n t h e physics of l a s e r s based on molecular gases e x c i t e d by non- self-sustained discharges has made it es- s e n t i a l t o understand t h e elementary pro- cesses occurring i n t h e plasma of such gases. I n t h e working mixture of t h e C02- l a s e r s , one can o f t e n f i n d t h e molecules of oxygen, which i s one of t h e t y p i c a l electronegative gases. Accordingly, one should know the constants of e l e c t r o n at- tacbment t o oxygen a t discharge parameters c h a r a c t e r i s t i c of G O 2 l a s e r s , i.e., t h e E/P r a t i o ranging from 5 t o I 5 v/cm t o r r .

The present paper d e a l s with t h e pro- cesses of e l e c t r o n attachment i n a plasma produced by an e l e c t r o n beam. The beam of IObkeV e l e c t r o n s with c u r r e n t d e n s i t y t o 4OOp/cm was produced by 2 an e l e c t r o n

gun and i n j e c t e d through an aluminum f o i l i n t o a quartz discharge chamber. The feed*

i n g system of the e l e c t r o n gun make i t possible t o vary t h e d u r a t i o n of t h e ele- ctron-beam c u r r e n t pulse fwm 0.5 t o 600 psec. While the discharge was burnin@; and

t h e plasma was decaying, t h e discharge- chamber voltage was kept constant.

ExPerimental r e s u l t s . The constants of e l e c t r b n attachment t o oxygen were d e t e r miped from t h e plasma decay r a t e . I n a l l t h e experiments, plasma was produced by f a s t e l e c t r o n beam p u l s e s of longer t h a n

3Opsec. Such a d u r a t i o n i s necessary f o r t h e r e l a x a t i o n of s t r a t a a r i s i n g a t t h e i n i t i a l i n s t ant of t h e non-self -maint ain- ed discharge burning a t E / P p I1 v/cm t o r r [I]. Fig. I shows t h e dependences of t h e i n v e r s e plasma-electron density,

/2e

t h e time t elapsed a f t e r t h e switching- off of t h e e l e c t r o n beam. The curves a r e constructed f o r gas mixtures c o n s i s t i n g of 80% of n i t r o g e n and 20% of oxygen a t a pressure of 90 t o r r . The i n i t i a l s e c t i o n of t h e time dependence of

-

4

creasing E/P i t i s only t h e i n c l i n a t i o n of t h e l l n e t h a t i s changed. A second p a r t of t h e decay curve shows t h e slow- i n g down of t h e plasma decay which is t h e more d r a s t i c t h e higher the E D r a t i o be-

comes. Moreover, with i n c r e a s i n g E D , the beginning of t h a t s e c t i o n s h i f t s toward t h e increasing higher e l e c t r o n d e n s i t i e s .

S t u d i e s of t h e time dependence of

-

f o r t h e cases of decay of plasmas with

fie

various i n i t i a l e l e c t r o n concentrations have revealed t h a t t h e i n c l i n a t i o n of the i n i t i a l p a r t of t h e curve i s independent of e l e c t r o n concentration, whereas t h e in- c l i n a t i o n of the second p a r t shows a sig- n i f i c a n t dependence on i t o

To c l a r i f y the r o l e of nitrogen and oxygen a s a t h i r d body i n t h e process of

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979751

e l e c t r o n attachment t o t h e O2 molecule, we have s t u d i e d the dependence of t h e plasma decay r a t e

d

O2 a t a given t o t a l gas pressure (Fig.2).

We have measured a l s o t h e dependence of t h e steady-state c u r r e n t of t h e non-self- maqntained discharge on t h e electron-beam c u r r e n t d e n s i t y j a t E/P=I3 v/cm t o r r . The discharge c u r r e n t has turned out t o be

r a t h e r accurately, proportional t o t h e e l e c t r o n beam current.

Discussion. The f a c t t h a t the i n i t i a l p a r t of the plasma decay curve r e p r e s e n t s , on a logarithmic s c a l e , a s t r a i g h t l i n e and t h a t the i n c l i n a t i o n of t h i s l i n e i s independent of i n i t i a l e l e c t r o n concentra- t i o n enables one t o conclude t h a t t h e i n i - t i a l p a r t of t h e curve describes pure at- tachment of e l e c t r o n s t o oxygen molecules.

The decrease of r e l a x a t i o n r a t e with in- creasing E / P (Fig.1) and t h e proportiona- l i t y of d/P t o Po .suggest t h a t t h e ob-

O2 2

served attachment i s three-body. Pig.3 pre- s e n t s . t h e dependences of the constants of the three-body e l e c t r o n attachment t o oxy- gen, KO _and KN _on E/P obtained from a n

2 2 .

examination of the s t r a i g h t l i n e s i n fig.2e One observes a reasonable agreement with the r e s u l t s of /2/ and /3/. An abrupt change i n the c h a r a c t e r of plasma decay occurring with decreasing e l e c t r o n d e n s i t y appears t o be due t o a s i g n i f i c a n t r e d i s t r i - b u t i o n of the e l e c t r i c f i e l d and charge d e n s i t y during t h e i n t e r v a l a f t e r switching off t h e source of i o n i z a t i o n . This i s a l s o 1

confirmed by t h e f a c t t h a t t h e c h a r a c t e r of plasma decay i s e n t i r e l y independent of the d u r a t i o n of t h e electron-beam p u l s e

when i t l ~ s t s from 0.5 t o 6 0 0 p s e c .

References:

I. A.N.Vasilieva e t a l . F i z i k a plasmi,

r a t u r ,

g,

3. N.A.Aleksandrov. Teplofiz.visok.tempe- r a t u r , I6, 231 (1978).