INVESTIGATIONS OF THE CONTRACTION MECHANISM IN A HELIUM MEDIUM PRESSURE DISCHARGE

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INVESTIGATIONS OF THE CONTRACTION MECHANISM IN A HELIUM MEDIUM PRESSURE

DISCHARGE

Yu. Golubowsky, R. Sonnenburg

To cite this version:

Yu. Golubowsky, R. Sonnenburg. INVESTIGATIONS OF THE CONTRACTION MECHANISM IN A HELIUM MEDIUM PRESSURE DISCHARGE. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-155-C7-156. �10.1051/jphyscol:1979776�. �jpa-00219482�

JOURNAL' DE PHYSIQUE CoZZoque C7, suppZ6ment au n07, Tome 40, JuiZZet 1979, page C7- 155

Yu.B. Golubowsky and R. ~ o n n e n b u r ~ ?

I n s t i t u t e of Physics, Leningrad State University Department of Optics and Spectroscopy, Leningrad U. S. S . R.

f ~ . n r s t - ~ o r i t z - ~ m d t - ~ n i v e r s & t , Sektion Physik/EZektronik GreifswaZd, DDR.

Introduction I n a s e r i e s of p u b l i c a t i o n s

%he r a d i a l inhomogeneous heating of gas c h a r a c t e r i s t i c of medium pressure dis- charges i s considered t o be the reason f o r t h e contracted s t a t e of t h e p o s i t i v e column i n rare-gas discharges. Experi- raental and t h e o r e t i c a l i n v e s t i g a t i o n s i n argon and neon discharges a t pressures of s e v e r a l hundred t o r r / l - 4 / demonstra- t e d t h a t t h e abrupt c o n t r a c t i o n a s well a s t h e appearances linked t o i t can be explained without regarding t h e r a d i a l gas temperature gradient. The involve- ment of t h e energy balance of t h e atoms

i n t h e i n i t i a l equation system, however, r e s u l t s i n a b e t t e r q u a n t i t a t i v e corre- l a t i o n of theory and experiment /4/. The present study i n v e s t i g a t e s t h e contrac- t i o n mechanism of t h e column of a hellum discharge a t medium pressures.

Experimental r e s u l t s Fig.1 shows the mo- notonous change of the p o t e n t i a l gradient E(curves 1,2,3,4,5 correspond t o values poR=l 00,200,300,500,700 t o r r cm, R = l cm) i n dependence on t h e discharge conditions a t a constant wall temperature 6 ^% 320'~.

The e f f e c t i v e current c r o s s s e c t i o n

p

( a r a t e f o r t h e degree of c o n t r a c t i o n )

density, i s p s O , 12(in t h e case of Bessel d i s t r i b u t i o n u, ;& 0,241. This weak current

balance equation of t h e charge c a r r i e r s , the energy balanae equation of t h e atoms and e l e c t r o n s and t h e equation f o r pres- sure and discharge current/4/. The ioni- z a t i o n model of paper /4/ w a s used with I and P a s e f f e c t i v e r a t e s of i o n i s a t i o n and recombination considering t h e presence of atomic and molecular ions i n t h e dis- charge. The experimental r e s u l t s can be q u a l i t a t i v l y explained with t h e h e l p of t h i s equation system taking i n t o account t h e p e c u l i a r i t y of t h e elementary pro- cesses i n a helium discharge. ~ ' i n c e t h e electron-atom c o l l i s i o n frequency in t h e e l a s t i c range f o r helium does not depend on t h e e l e c t r o n v e l o c i t y and thus t h e i o n i s a t i o n frequency is not a nonlinear function of t h e e l e c t r o n concentration, i t s r a d i a l decrease i s determined by a term o f t h e k i n d I

-

^{-e)(lo (-d4/%)}

with V l - f i r s t e x c i t a t i o n p o t e n t i a l , Te

-

e l e c t r o n temperature. There a r e no d a t a in the l i t e r a t u r e on t h e c o e f f i c i e n t of the d i s s o c i a t i v e recombination i n helium a t medium pressures. Due t o t h e p o s i t i o n c o n t r a c t i o n i s caused by t h e r a d i a l in- of t h e i n t e r s e c t i o n of t h e p o t e n t i a l homogeneous heating of gas. The charac- curves of the moleoular i o n and t h e quasi- t e r of discharge fundamentally changes in molecule i n t h e range of t h e t h i r d vibra- t h e case of f r e e temperature exchange of

t h e discharge tube with t h e surrounding a i r . The examples of E a n d p i n fig.2 show an abrupt change of a l l plasma para- meters i f a c r i t i c a l current value i s reached(poR=lOO t o r r cm). A weak r a d i a l contract i o n of the continuum r a d i a t i o n and a s t r o n g c o n t r a c t i o n of t h e l i n e ra- diatiori is c h a r a c t e r i s t i c of t h i s a p p e a r a c e .

Discussion The s t a r t i n g point f o r the c a l c u l a t i o n of t h e plasma parameters i s

t i o n a l l e v e l we can assume t h a t the re- combinat i o n of molecular ^' i o n s i n higher v i b r a t i o n a l s t a t e s occurs with slow elec- trons. On the assumption t h a t the d i s t r i - bution of these i o n s over t h e vibratiohal, l e v e l s a t medium pressures i s mainly de- termind by c o l l i s i o n s with atoms the num- b e r of t h e recombination events p e r u n i t time and u n i t volume must contain a term

r -

I L I ( I ~ D ( - ~ ~ ~ / & ~ ) withV+-potent i a l of t h e v i b r a t i o n a l l e v e l in the range of the i n t e r s e c t i o n of the p o t e n t i a l curves, an equation system c o n s i s t i n g of the Gatom temperature. I n t h i s model the

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979776

t r a n s i t i o n t o t h e column i n t h e recombi- n a t i o n range is possible which i s not accompanied by a c u r r e n t c o n t r a c t i o n i f t h e r a d i a l decrease of t h e i o n i z a t i o n r a t e proceeds more planar than t h a t of t h e recombination r a t e .

This i s the case i f i n t h e r a t i o I / F

the parameter i s

In t h e opposite case c o n t r a c t i o n w i l l occur. ~ i g . 3 a ) b ) show t h e resu1t.s of the c a l c u l a t i o n s f o r the case of constant w a l l temperature 0 ~ 5 3 2 0 ~ ~ . For these conditions we can assume t h a t in-equat ion ( 2 ) i s v a l i d f o r a l l current values and the. c a l c u l a t e d monotonous current depen- dence of the parameters correaponds t o t h e experimental observed. A t f r e e tem- perature exchange with the surrounding a i r t h e wall temperature of t h e discharge tube i n c r e a s e s with increasing c u r r e n t and t h e gas temperature

eo

in t h e c e n t r e of t h e discharge a s well. A s a r e s u l t of t h e c a l c u l a t i o n s the e l e c t r o n temperature can be regarded a s constant. A t a d e f i n i t e c u r r e n t value in-equat ion ( 2) changes the symbol and t h u s c o n t r a c t i o n follows. The r e s u l t s of t h e corresponding c a l c u l a t i o n f o r increased wall temperatures f o r poR = 100 t o r r cm a r e shown i n figa3c)d).The z- l i k e characteristics(fig.3c)d)) obtained by the s o l u t i o n of the i n i t i a l equation system correspond t o t h e abrupt t r a n s i t ion from t h e d i f f u s e t o t h e contracted s t a t e . The c o n t r a c t i o n appearance i n a helium diecharge i s t h u s due t o t h e inhomogeneous heating of gas.

References

e t .al., Zur.Tech.Phys.

G(197612327 e t .al., Zur.Tech.Phys.

u ( 1 9 7 7 ) 1 8 5 2 e t a al., Zur..Tech.Phy~.

a ( 1 9 7 7 ) 1 4 7 8

, Sonnenburg, R.

2ur.Tech.Phys. 1979

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