THE POSITIVE COLUMN IN THE THREE-COMPONENT MIXTURE Ne/H/H2- EXPERIMENTAL RESULTS IN COMPARISON WITH AN APPROXIMATIVE DIFFUSION THEORY

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THE POSITIVE COLUMN IN THE

THREE-COMPONENT MIXTURE Ne/H/H2- EXPERIMENTAL RESULTS IN COMPARISON WITH AN APPROXIMATIVE DIFFUSION THEORY

P. Michel, S. Pfau, A. Rutscher, R. Winkler

To cite this version:

P. Michel, S. Pfau, A. Rutscher, R. Winkler. THE POSITIVE COLUMN IN THE THREE- COMPONENT MIXTURE Ne/H/H2- EXPERIMENTAL RESULTS IN COMPARISON WITH AN APPROXIMATIVE DIFFUSION THEORY. Journal de Physique Colloques, 1979, 40 (C7), pp.C7- 157-C7-158. �10.1051/jphyscol:1979777�. �jpa-00219483�

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JOURNAL DE PHYSIQUE CoZZoque C7, suppZ6ment au n 0 7 , Tome 40, J u i Z Z e t 1979, page C7- 157

M E WSWE COLUMN IN THE THREE-COMPO~~EM MIXTURE N~/H/H~- EXPERIMENTAL RESULTS IN COMPARISON WITH AN APPROXIMATIVE DIFFUSION THEORY

P. Michel, S. Pfau, A. Rutscher and R. ~ i n k l e r * .

%E.M.A. U n i v e r s i t i i t G r e i f m a Z d , S e k t i o n P h y s i k / E Z e k t r o n i k , GreifswaZd, D.D.R.

Akademie d e r W i s s e n s c h a f t e n d e r DDR ZentraZinstitut diir E Z e k t r o n e n p h y s i k G r e i f s w a z d , DDR.

1NTRODUCTION:During the last years the ki- net ic deacript ion of electrical discharges in mult i-component gas mixtures had shown considerable progress.This was the consequence of an already high degree of per- fection in solving the BOLTZMANN equation for electrons.Especially multi-component gas mixtures are important in LASERS and plasma chemical reactors.Typica1 for such devices is a more or less marked inter action between the composition of the mixture and the properties of the discharge.

Analysing the dissociation of HL recently a first example of the influence of chemical changes(produced by the discharge itse1f)on the electrical characteristic was givedl /.It could be shown that the negative differential resistance of the positive column follows from increasing dissociation,which reached a considerable extent (about 1 00%) at high currents and low pressures.Unfortunate1y in pure hydrogen discharges the space and time be- haviour is often complicated by standing or moving striatLons,fluctuations etc.

Nore silent operation conditions are poss- ible in mixtures of hydrogen and inert gases.

In this paper a neon/hydrogen mixture is investigated by measuring the electric characteristic and the degree of dissociation.Based on a simple theoretical mod- el the experimental results are compared with calculated ones.

EXPERIMENTAL ARRANGEBBENT:The axial electric field strength of the positive column{ 1ength:l m, radius :I cm)was measured via two usual static probes.To reduce the large influence of the electrodes on the degree of dissociation( effective recombi- nation at the metallic surface)two narrow capillaries were attached in front of the electrodes.The degree of dissoeiation was determined using a YilREDE-RARTECX-probe.

In all cases the discharge tube was eon- nected with a large dead space;

TKEORETIC N0DEL:Till now no solution of the BOLTZMANN equation is known for electrons in the three-component mixture

Ne/H/H2.But the discussion of this equation shows that generally the contribution of not too large Ne constituents only

plays a rather small role in the electron kinetics oompared with the other terms

(resulting from H and H2)because of the small cross-sections and the large atomic mass of Ne.Neglecting all the Ne terms, the B O ~ T Z ~ ~ ~ A N N equation for electrons 5n the Ne/H/H2 mixture changes to that in the H/H2 mixture with the only difference that the actual(reduced)fieldstrength E/po is

replaced by a effective one ( ~ / p ~ ) ~ ~ ~ = E/p0(l-xo) ;xo :relative concentration of

Ne.In the framework of this approximation the Ne admixture in a molecular gas discharge mainly plays the role of a buffer gas.Some calculations and experiments have shown this very clearly in the Me/N2-mixture/2/.Xt could be. estimated that up to x o S O,5 and (E/po) eff

5

20 V/cmTorr the ex- plicit influence of Ne in the H/H2 mixture is very small.At higher Ne content the influence is noticeable, especially in the H/H2 ionization range of the electron

energy distribution function.

Taking into account Re only as a buffer gas the diffusion theory of the positive column in the Ne/H/H2 mixture can be form- ulated using the electron energy distribution functions of H/H2, mentioned in /I /.

Then the balance equations of the charge carriers,of the H atoms and of the discharge current give the basis for calcu- lating the column characteristic,the degree of dissociation etc.With some further simplifications these equations read:

(~/r,)~(~,/b,) (b+/$) = xl 5 ,

+

x2e2 X1X2ZD

-

⁽x ~ z ~ N ~ + x , ZRl No+x2zR2N0 )

+

-&x1Z/4r N = 0 i =

x

eoro N0xIbeE O2O

(A=2,405;D ,b :diffusion coefficient and mobility ofee18ctrons;W:mobility of ions;

N, :total concentrat ion of neutral part ic- lee-x * o * x x 4 ' 2 :relative concentrations of

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979777

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Be, H, and Hz p a r t i c l e s ; z,,

,

z a : i o n i z a t i o n r a t e of H and H2;zB:rate of HI d i s s o z i a - t i o n by e l e c t r o n c o l l i s i o n s ; zn:recombina- t i o n r a t e by t h r e e body c o l l i s i o n s o f H atoms; :wall recombinat i o n c o e f f i c i e n t of H atoms;E:thermal velocity;r,:column r a d i u s ;x :degree of i o n i z a t i o n ) .

The s i m p f i f i c a t i o n s mentioned a r e :Use of only one value f o r t h e m o b i l i t y of t h e d i f f e r e n t i o n s H+, H~', H~~ (BLANCS law shows t h a t t h i s i s c o r r e c t w i t h i n a f a i l u r e of about 20%; b+= 1 , 3 m 2 / V s a t 1 T o r r ) ; n e g l e c t

o f t h e i o n i z a t i o n of Ne and n e g l e c t of t h e PEmIMG i o n i z a t i o n of H and H2 v i a meta-

s t a b l e Be atoms.

RESULTS :Pig, I shows t h a t w i t h i n c r e a s i n g Ne c o n t e n t x o t h e degree of d i s s o c i a t i o n xD=xfi1+x2) i s i n c r e a s e d too.This i s t h e

. -

consequence o f t h e i n c r e a s e of t h e e f f e c t - i v e f i e l d s t r e n g t h.Althlough t h e hydrogen component d e c r e a s e s by t h i s change of t h e mixture r a t i o t h e a b s o l u t e value of t h e H atom c o n c e n t r a t i o n remains n e a r l y c o n s t a n t i n t h e beginning(Pig,2).Pig.3 shows t h e degree of i o n i z a t i o n and d i s s o c i a t i o n in dependence on t h e d i s c h a r g e c u r r e n t . The measured v a l u e s xD a g r e e q u i t e w e l l w i t h t h e t h e o r y . l i n a l l y Fig.4 compares t h e measured and c a l c u l a t e d e l e c t r i c charac- t e r i s t i c . T h e two dashed l i n e s correspond t o t h e l i m i t i n g c a s e s of c o n s t a n t chemical c o n s t i t u t hon:Ne/H2(upper curve 1, Ne/H(lower c u r v e ) . ~ n e x p l i c i t c o n s i d e r a t i o n of Ne i n t h e BOLTZMANN e q u a t i o n of t h e mixture would i n c r e a s e t h e s e l i m i t s which roughly correspond t o t h e e x p e r i m e n t a l values.But t h e t r a n s i t from t h e upper t o t h e lower l e v e l d i f f e r s considerably.T~~ainly t h i s must be a t t r i b u t e d t o t h e n e g l e c t of t h e g a s h e a t i n g a t h i g h e r c u r r e n t s . 8 ~ a bound- a r y c o n d i t i o n we used No=const(dogether w i t h nl+n2=const).But a c t u a l l y No w i l l de- c r e a s e w i t h i n c r e a s i n g c u r r e n t , g i v i n g h i g h e r e x p e r i m e n t a l reduced e l e c t r i c a l f i e l d s t r e n g t h s t h a n shown i n Tig.4.

REFERENCES:

/~/I?IICHEL,P. e t . a l . 1 3 t h ICPIG,Berlin 1977, P - 245 1 4 t h ICPIG, Grenoblel979

013

-

\ po= 6,4 Torr

5

I - x o

-

0 , I

0 012 OIL 0,6 018 l I O

Fig. 1

3 p, = 6,4Torr

= 5.10-' Fig. 2

1 - 0'

c