ON THE MECHANISM OF GLOW DISCHARGE INSTABILITY FOLLOWING THE TURN-OFF A NON-SELF-SUSTAINED IONIZATION SOURCE

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ON THE MECHANISM OF GLOW DISCHARGE INSTABILITY FOLLOWING THE TURN-OFF A

NON-SELF-SUSTAINED IONIZATION SOURCE

A. Kostylev, J. Londer, A. Terentyev, K. Ulyanov, V. Fedorov

To cite this version:

A. Kostylev, J. Londer, A. Terentyev, K. Ulyanov, V. Fedorov. ON THE MECHANISM OF GLOW DISCHARGE INSTABILITY FOLLOWING THE TURN-OFF A NON-SELF-SUSTAINED IONIZATION SOURCE. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-367-C7-368.

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ON THE MECHANISM OF GLOW DISCHARGE INSTABILITY FOLLOWING THE TURN-OFF A NON-SELF-SUSTAINED IONIZATION SOURCE.

A.A.

Kostylev,

Londer,

Terentyev,

Fedorov.

A2 2-Union EZect~otechnicaZ I n s t i t u t e , Moscow

I n s t a b i l i t i e s i n a n o n - s e l f - s u s t a i n e d glow d i s c h a r g e which develop when e x t e r n a l i o n i s a t i o n p u l s e i s a p p l i e d have been d i s - cussed e a r l i e r i s many works, According t o t h e p r e s e n t dominating p o i n t of vievr t h i s i n s t a b i l i t y is a s s o c i a t e d w i t h t h e growth of t h e v a l u e of s e l f - s u s t a i n e d i o n i z a t i o n f u n c t i o n . Heating a r e r a r e f a c - t i o n of g a s p l a y an important r o l e i n t h i s case.

1 , T h i s paper r e p o r t s on t h e measure- ments of g a s d e n s i t y which were performed t o c l a r i f y t h e mechanism of i n s t a b i l i t y when t h e time Ti, exceeds t h e i o n i z e r

p u l s e d u r a t i o n Z Experiments were c a r - P

r i e d out i n a mixture o f

and N2 a t atmospheric pressure.A p u l s e d 12OkeV e l e - c t r o n beam w i t h a c u r r e n t d e n s i t y 200 m k ~ / c m ~ a t t h e anode p l a n e was i n j e c t e d

i n t o a d i s c h a r g e chamber throughan Pil-foil.

The v o l t a g e a c r o s s t h e d i s c h a r g e

was maintained c o n s t a n t and t h e v e l o c i t y of g a s flow was 3 m/s. Change i n t h e g a s den-

s i t y were recorded by a l a s e r i n t e r f e r o - meter method. Measurements were c a r r i e d o u t w i t h a He-Ne l a s e r which had a t h r e e - m i r r o r r e s o n a t o r [ ? ] . The wavelength was 0.63 mkm. O p t i c a l a x i s o f simmetry were c o i n c i d e n t .

T * ~ p i c a l o s c i l l o g r a m s of a d i s c h a r g e c u r r e n t p u l s e and of phase ov-erlapping a r e p r e s e n t e d i n Fig.1(200 mks/cm sweep). I n t h i s c a s e t h e non-ionized g a s p r o v i d e s t h e main c o n t r i b u t i o n t o t h e r e f r a c t i o n f a c t o r .

phase s h i f t 2 Z c o r r e s p o n d s t o a r e l a t i v e change i n g a s d e n s i t y AN/N~

0.01 6 f o r t h e mixture GO2-N2(1:2) and dN/No

0.018 f o r t h e mixture (1 :9), where No i s t h e i n i t i a l g a s d e n s i t y and AN =No -& ( t ) . The depen- dens of t h e r e l a t i v e d e n s i t y N ( t ) / N o on t h e d u r a t i o n of c u r r e n t p u l s e f o r 1:9 mix-

t u r e

shovrn i n Fig.?. h p o i n t i n t h e f i - gure which i d e n t i f i e s t h e development of i n s t a b i l i t y a f t e r t h e termina'tion of cur- r e n t p u l s e

marked w i t h

c r o s s .

2.

An a n a l i t i c a l assessment of e a s r a r e f a c t i o n

made on t h e b a s i s of a t n o - l e v e l model which t a k e s i n t o account t h e t r a n s f e r of o s c i l l a t o r y energy away from t h e d i s c h a r g e zone. The c o n t i n u i t y e q u a t i o n s f o r t h e e x i t e d N2 and C02 mole- c u l e s have t h e form :

JNN. + A , ~ ~ Q = - X(N;N~ - N;NQ- b*

I

J T ZN

C

N"

&YC +N:J'"F= ⁺ K(N,"N~-N,"N~)- -f ⁽²⁾

d t Ec ^c

Here NN ,Nc a r e t h e d e n s i t i e s of N 2 and

mol~eaules i n t h e ground s t a t e ; w,,w,

denote t h e povrer d e n s i t y used f o r t h e e x i - t a t i o n o f o s c i l l a t o r y l e v e l s i n W2 and Go2; EN, EC a r e t h e e n e r g i e s of o s c i l l a - t o r y l e v e l s i n

and

/C - c o n s t a n t r e l a t e d t o t h e o s c i l l a t o r y quanta exchange r a t e betrveen

and Go2;&, & - ~ e l z x n - t i o n time of o s c i l l a t o r y l e v e l s i n N2 and Cog. Equation

and ( 2 ) should be sup- plemented by g a s dynamical e q u a t i o n s which may be expressed f o r i s o b a r i c ex- pansion of g a s a s f o l l o w s

Here N i s t h e t o t a l d e n s i t y of molecules i n a g a s mixture. The s o l u t i o n of system ( 1 ) - ( 4 ) may be derived. w i t h an assumption tha-t due t o t h e h i g h r a t e o f energy cx- change between t h e

and C02 o s c i l l a t o r y quantn, t h e e s t s b l i s h m e n t of e q u i l i b r u i m betneen them i s p r a c t i c a l l y a momentary p r o c e s s , i . e . t h a t NN/fic = N ' / N ~ = 6 .

The s e t of e q u a t i o n s

may now

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797180

now be reduced to a single non-linear se- cond-order differential equation for gas densif y

Solution of (5) is as follows

A = 2 f B

l p - ~ ( i - ~ J 2 ' 4 If 1, expression (6) may be simpli- fied as follows :

n = r + ~ ( i + p )

I

B

B ~ . e x p -f i - ~ ( i + p )

e x p ( ~ ~ ) ( ) . 6 In another limiting case when B ( / - ~ ) > > 1

0'

we obtain I'rom (6) :

In both cases the solution does not de- pend on Zc

. It should be noted that af- ter termination of the pumping pulse, the gas concentration decreases for some time due to oscillatory energy relaxation. The solution of equations (1 )'-(4) at k'/

yields

n2(Z;.) ( Z q

"(') = Nz/- n'IQ)p-exp(G-&J

4

(7)

dotm

voltage of this discharge gap shows that at

the latter is three times as high as Vd.

A simular situation is true for the (1:2) mixture.

Thus the rarefaction of the gas is not a sufficient factor for the development of a static breakdown (direct Townsends ionisation), The development of instabi- lity may be associated with the growth of stepped self-sustained ionisation in the decaying plasma i.n an external field. In this case the increase of E/IT due to gas rarefaction plays a sip;nif ican-t role.

[I] - D.E.Ashby, D.F. Jephcott, Appl.Phys.

Lett.,3,13,1963.

= -@/ _{d Z Z= TP} ^{Pig. I}

To allow comparisnn with experimental da- ta of &(t) was computed from formulas C6a), (7) for mixture C02-N2(l 89) at Vd=3 kV and V d ' 5 kV (curves 3 and

res- pectively in Fig.2). Note, that with the increase of Vd and at earlier moments of observation, the deviation from the con- ditional P=const which is the basic ap- proximation of the theoretical model be- comes greater. Also, the discrepancy bet- ween calculation and experiment increases.

3. Discussion of results. From Fig.2 it may be seen that at Vd=6 kV and 3lOmks

I 1 1 a 1 I .

duration current pulse, instability deve- 42 96

t,w

lops at 375 aks (&> 5

. By this time the gas density decreases approximately

by a factor of 2.5. Cornparision between Fig. 2

the applied voltage and the static break-