INVESTIGATIONS OF INSTABILITIES IN A NON-SELF-SUSTAINED REPETITIVELY PULSED DISCHARGE

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INVESTIGATIONS OF INSTABILITIES IN A NON-SELF-SUSTAINED REPETITIVELY PULSED

DISCHARGE

A. Kostylev, V. Perevodchikov, A. Terentyev, K. Ulyanov, V. Fedorov

To cite this version:

A. Kostylev, V. Perevodchikov, A. Terentyev, K. Ulyanov, V. Fedorov. INVESTIGATIONS OF INSTABILITIES IN A NON-SELF-SUSTAINED REPETITIVELY PULSED DISCHARGE. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-369-C7-370. �10.1051/jphyscol:19797181�. �jpa-00219159�

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JOURNAL DE PHYSIQUE CoZZoque C7, suppZ6ment au n07, Tome 40, J u i Z Z e t 1979, page C7- 369

INVESTIGATIONS OF LNSTABILITIES IN A NON-SELF-SUSTAINED REPETITIVELY PULSED DISCHARGE

A.A. Kostylev, V.I. Perevodchikov, A.P. Terentyev, K.N. Ulyanov and V.A. Fedorov.

AZZ-Union EZectrotechnicaZ I n s t i t u t e , Moscow U.S.S.R.

Non-self-sustained d i s c h a r g e s a r e known t o be u n s t a b l e . I n s t a b i l i t i e s which l i m i t t h e maximum energy i n p u t i n p u l s e d modes of o p e r a t i o n may develop i n d i f f e r e n t ways.

A t a s u f f i c i e n t l y h i g h d i s c h a r g e v o l t a g e an i n s t a b i l i t y which occurs d u r i n g a cur- r e n t p u l s e can be observed. The mechanism of t h i s i n s t a b i l i t y h a s a l r e a d y been s t u - d i e d i n d e t a i l . A t lower d i s c h a r g e (vol- t a g e s i n s t a b i l i t y may develop when t h e ex- t e r n a l s o u r c e of i o n i z a t i o n h a s been t u r - ned o f f . A s a r e s u l t t h e d i s c h a r g e v o l t a - ge should be decreased f u a t h e r . T h i s l e a d s t o a reduced energy i n p u t p e r p u l s e i n a C02 r e p e t i t i v e l y p u l s e d l a s e r when t h e frequency i s h i g h o r t h e g a s v e l o c i t y i s s m a l l [I, 27.

T h i s paper r e p o r t s on t h e r e s u l t s of experiments on i n s t a b i l i t i e s o c c u r r i n g a f - t e r t u r n i n g o f f t h e i o n i z a t i o n source of a non-self-sustained d i s c h a r g e when a s i n g l e o r two subsequent c u r r e n t p u l s e s of t h e e l e c t r o n beam c u r r e n t were a p p l i e d .

The experiments were c a r r i e d out i n pure N2 o r i n mixture of g a s e s C02:N2=1:9 at atmospheric p r e s s u r e . Pulsed 120 keV e l e c t r o n beam having a c u r r e n t d e n s i t y 200 mk~/mn* a t t h e anode p l a n e was i n t r o d u c e d through an A l - f o i l i n t o a d i s c h a r g e cham- ber. T y p i c a l o s c i l l o g r a m s of c u r r e n t p u l s e forms w i t h o u t i n s t a b i l i t y a r e shorn i n Fig. l a , F i g . l b r e l a t e d t o an i n s t a b i l i t y developed a f t e r ? t u r n i n g o f f t h e i o n i z a t o r and F i g . 1 ~ - t o a c a s e when an i n s t a b i l i t y o c c u r when t h e i o n i z a t o r i s a c t i v e . The in- s t a b i l i t y development time was s t u d i e d when s i n g l e p u l s e s Zp and 2

rp

o r sequence a o f two p u l s e s w i t h a d u r a t i o n each and a p e r i o d T were a p p l i e d . P

Dependence of i n s t a b i l i t y development

time

r,

i n a d i s c h a r g e plasma f o r a mix- t u r e CO ₂:N _2--1 : 9 upon E / P i s g i v e n i n Fig. 2a. It shows t h e time of i n s t a b i l i t y development r e l a t i v e t o t h e f r o n t of t h e c u r r e n t p u l s e a f t e r t h e a p p l i c a t i o n of a s i n g l e current pulse. P o i n t s r e l a t e d t o

'Gp=0.3ms, a r e marked( x ) and t h o s e t o - 0 . 1 5 ~ 1 ~ - ( U ). Data o b t a i n e d i n a sequ- P -

ence of two p u l s e s a t

Z,,,

=0.15ms and T=

0.75ms a r e i d e n t i f i e d by ( A ) . The same f i g u r e a l s o shows t h e dependence

r,

^(E/P)

f o r a c a s e when t h e i n s t a b i l i t y develops b e f o r e t h e end o f t h e c u r r e n t p u l s e i . e . when

'd, < rp.

These e x p e r i m e n t a l p o i n t s a r e marked w i t h 0. From t h e f i b w e i t may be s e e n t h a t t h e l a g o f i n s t a b i l i t y development i n a sequence of tvro p u l s e s coin- c i d e s w i t h t h e l a g observed i n a s i n g l e p u l s e of double d u r a t i o n i f t h i s time i s g r e a t e r t h a n 0.9ms a t t h i s v a l u e t h e i n - s t a b i l i t y i s formed d u r a t i o n t h e second p u l s e of t h e sequence. T i t h f u r t h e r i n c r e a - s e of E/P t h e i n s t a b i l i t y development l a g r e d u c e s s l i g h t l y . \\%en

r,

=0.75ms a t

E/P=

3.5 k ~ / c m - a t m ,

za

s t a r t s a g a i n t o i n c r e a s e r a p i d l y . When t h e v a l u e

ra

d i m i n i s h e s t o 0.15ms, t h e i n s t a b i l i t y o c c u r s d u r i n g t h e f i r s t p u l s e .

S i m i l a r dependences were o b t a i n e d i n N2 ( f i g . 2 b ) w i t h s i n g l e 0.1 and 0.2ms pul-

s e s ( i n d i c a t e d ( 0 ) and ( x ) , r e s p e c t i v e l y ) and w i t h a sequence of two p u l s e s w i t h

~ p = O , l m s and T=0.9ms( a ) , o r w i t h Zp=0.5 and

rp

^=l.O'ms⁽ ^and ^{+ )}^{a n d}-.vith two p u l s e s

rp

=0.5ms and T=l.5ms ( A ) .

I n our o p i n i o n t h e n a t u r e of i n s t a b i - l i t y a t

ra>Zpis

t h e same a s t h a t observed a t

r,< rp.

When t h e e l e c t r o n beam i s i n t e r - r u p t e d t h e plasma s t a r t s t o d e i o n i z e and t h e c u r r e n t decreases. However, simultane-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797181

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o u ~ l y w i t h t h i s p r o c e s s t n e s e l f - s u s t a i n - Phenom. i n I o n i z e d g a s e s , B e r l i n , 1977.

ed i o n i z a t i o n c o n t i n u e s t o i n c r e a s e due t o t h e growth of t h e mean energy of e l e c - t r o n s i n t h e a p p l i e d f i e l d ( ~ 1 1 ~ i n c r e a s e s a s gas c o n c e n t r a t i o n IT d e c r e a s e s ) . Another r e a s o n i s t h e i n c r e a s e of t h e number of f a s t e l e c t r o n s w i t h i o n i z i n g c a p a b i l i t y . I n t h i s c a s e

u4/3t

(Be-the e l e c t r o n con- c e n t r a t i o n ) may become p o s i t i v e w i t h subsequent c u r r e n t and i n s t a b i l i t y development. The n o n - s e l f - s u s t a i n e d d i s c h a r g e t r m s f o r m s i n t o a s e l f - s u s t a i n e d mode.The second p u l s e merely a c c e l e r a t e s t h i s pro- cess. The time of e s t a b l i s h m e n t of a con- s t a n t g a s d e n s i t y a f t e r t u r n i n g o f f t h e source of i o n i z a t i o n i s r e l a t i v e l y s h o r t . I f i s of t h e o r d e r of o s c i l l a t i n g r e l a x a - t i o n time f o r t h e mixture of g a s e s and corresponds t o t h e r o t a t i o n a l r e l a x a t i o n time f o r N2 where t h e V-T r e l a x a t i o n time i s on t h e c o n t r a r y v e r y long, Thus t h e i n s t a b i l i t y w i l l n o t develop a f t e r t h e f i r s t p u l s e but r a t h e r a c e r t a i n s t a b l e d e n s i t y may be e s t a b l i s h e d . A f t e r t h e se- cond p u l s e t h e d e n s i t y s t a r t s a g a i n t o r e - duce,dnl,/d$ changes i t s s i g n and l e a d s t o i n s t a b i l i t y . Thus, t h e r a r e f a c t i o n of t h e g a s i n f l u e n c e s e s s e n t i a l l y t h e time of i n - s t a b i l i t y development.

Discharge v o l t a g e i n l a s e r s when an e x t e r n a l i o n i z a t o r i s used f o r c u r r e n t mo- d u l a t i o n should be chosen such t h a t E/P does n o t exceed t h e minimum v a l u e a t which t h e i n s t a b i l i t y develops, o r

r,

should be g r e a t e r t h e n t h e time of g a s t r a n s i t through t h e d i s c h a r g e gap. I n b o t h c a s e s t h i s v o l t a g e i s a p p r e c i a b l y l o v e r t h a n t h e li- m i t i n g v o l t age a t vrhich

ra

⁼

rp,

t h u s pum- ping would n o t be performed i n t h e o p t i m a l c o n d i t i o n s . It t h e r e f o r e a p p e a r s t h a t t h e most promising means of punping would be p r o v i d e s anode v o l t a g e modulation i n such a way t h a t t h e p r o b a b i l i t y of i n s t a b i l i t y development be e l i m i n a t e d .

References

1. G. S. Dzakowic, S. A.Y?ut zke, 3. Appl. Phys. ,

44,11,5061, 1973.

2. A.A.Kostylev,J.I,Londer,A.P.Terentiev, K.N.Uljanov, X I I I - t h I n t e r n . Confer. on

Fig. I

Fig. 2a

F i g , 2b