ANODE REGION OF HIGH CURRENT ARC DISCHARGE

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ANODE REGION OF HIGH CURRENT ARC DISCHARGE

G. Dyuzhev, N. Mitrofanov, S. Shkolnik, V. Yur’Ev

To cite this version:

G. Dyuzhev, N. Mitrofanov, S. Shkolnik, V. Yur’Ev. ANODE REGION OF HIGH CUR- RENT ARC DISCHARGE. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-463-C7-464.

�10.1051/jphyscol:19797225�. �jpa-00219207�

JOURNAL DE PHYSIQUE CoZloque C7, suppZdment au n 0 7 , Tome 40, J u i Z Z e t 1979, page C7- 463

ANODE REGION OF HIGH CURRENT ARC DISCHARGE

G.A. Dyuzhev,N.K. Mitrofanov, S.M. Shkolnik and V.G. Yur'ev.

A. F. I o f f e PhysicaZ-TechnicaZ I n s t i t u t e , Academy o f S c i e n c e s o f t h e U. S . S . R., Leningrad, K-21, U.S. S. R.

Wide a p p l i c a t i o n s of high current a r c discharge i n some plasma devices make t h e i n v e s t i g a t i o n s of p h y s i c a l phenomena i n near-electrode regions t o be very import-

a n t .

The experimental i n v e s t i g a t i o n s of t h e anode near-electrode l a y e r i n high

current a r c ( I " 102- 1 0 ~ 8 ) burning f r e e l y i n a l k a l i n e metal vapours a t p r e s s u r e s p10a2- I 0 Torr and i n noble gases a t p r e s s u r e s 1-760 Torr have been c a r r i e d out. One of methodical p e c u l i a r i t i e s of a p r e s e n t work i s use of a low temperature

e r o s i o n l e s s multichannel cathode /I/ which allows t o c a r r y high current d e n s i t i e s

3 2

( 8 -10 A/cm ) with no contamination of near-anode plasma with cathode erosion ma-

t e r i a l , which i s inherent i n spot operat ion mode. The experiments were c a r r i e d out i n p u l s e operation, a voltage s t a b i l i z a t i o n t a k i n g place. This circumstance permitted t o study anode sheath breakdown conditions and anode spot formation mechanism. A p u l s e supply of discharge ( r e g u l a r p u l s e s with duration W I m s and r e p e t i t i o n r a t e from I H5 up t o 12.5 Hz and s i n g l e p u l s e s ) permitted t o vary a thermal regime of

anode and t o c a r r y out a probe d i a g n o s t i c s of dense plasma (plasma concentration n -10'3- 10'7cm-3, e l e c t r o n temperature

Te is up t o 3 eV). I?he plasma parameter r a d i a l d i s t r i b u t i o n s ( a, T, and plasma p o t e n t i a l % ⁾ were measured a t various distances from anode by using t h e probe and spectroscopical techniques. Evaluation of probe measurement d a t a was performed by means of a d i f f u s i o n theory, assuming a

charge p a r t i c a l production i n t h e near- probe sheath. / 2 /

It was showed t h a t a t low and i n t e r - mediate p r e s s u r e s a current t r a n s f e r i n

anode region has been provided by a f i e l d current component 4s 6 E whew G i s a highly ionized plasma conductivity, E -

an e l e c t r i c f i e l d s t r e n g t h . Under t h e s e conditions t h e spreaded a t whole anode surface discharge existSwith a negative anode drop (AD) i n t h e space charge, sheath. The negative AD r e t a r d s an e l e c t - ron flow from near-anode plasma region t o anode so t h a t AD 9, i s determined by re- l a t io n

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r e n t d e n s i t y t o anode, dl ⁼ t%n(a)G(a) - random e l e c t r o n current density)(Fig.I)

.

It i s showed t h a t t h e negative AD s t a b i - l i z e s an anode sheath i n r e s p e c t - t o a spot format ion, and anode c o l l e c t i o n o f a high current d e n s i t y can n o t r e s u l t i n change of AD s i g n and i n anode spot r i s e .

The change of AD _sign t a k e s place on- l y a f t e r near-anode plasma t r a n s i t i o n t o a high i o n i z a t i o n s t a t e i f a cathode pro- v i d e s a s u f f i c i e n t l y high anode current d e n s i t y

&--it .

O u r s t u d i e s showed t h a t discharge with a p o s i t i v e AD i s unstable i n r e s p e c t t o anode spot formation because an anode evaporation and following i o n i z a t i o n of evaporised atoms r e s u l t s i n increasing

?%(a) and removed t h e current l i m i t a t i o n mentioned above. I n discharge current 0s- cillograms a sharp current p p w t h r e l a t e d

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797225

t o anode sheath breakdown i s observed i n time i n t e r v a l Z a f t e r t h e discharge w a s i g n i t e d . This time i n t e r v a l depends on a power supplied by e l e c t r o n flow t o anode

and on thermal q u a l i t i e s of anode mate- rial.

In Fig. 3 t h e current-volt age charac- t e r i s t i c s , plasma concentration t'& and

f l o a t i n g p o t e n t i a l values v+. of t h e probe s i t u a t e d near anode are presented f o r

Z = 0,5 msec a f t e r t h e discharge i g n i

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t i o n i n cesium plasma at p r e s s u r e s p = I, 2 and 4 t o r r . One can s e e t h a t anode spot breakdown r e s u l t s i n a sharp concentra- t i o n growth and p o t e n t i a l d i s t r i b u t i o n reconstruction. In t h e case of t h e spot operation t h e plasma parsmeters a r e inde- pendent on p but a r e determined by anode

material vapor pressure according t o t h e spectroscopical d a t a . A s u r f ace tempera- t u r e evaluation of copper anode gives

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The f e a t u r e of high c u r r e n t high pres- s u r e a r c (p>IOO t o r r ) i s a presence of a s u f f i c i e n t l y . extensive thermal 1 ayer i n anode region which r e s u l t s i n e l e c t r o n cooling near t h e anode and i n diminution

of plasma c o n d u c t i v i t y l 4 l . It r e s u l t s i n changing of a c u r r e n t t r a n s f e r mechanism i n anode region. The c u r r e n t t r a n s f e r t o anode i s produced by e l e c t r o n d i f f u s i o n .

I n Fig.4 t h e d i f f u s i o n c u r r e n t compo- nent value is represented along t h e dis-

charge gap f o r one of t h e modes studied.

One can see t h a t t h e c o n t r i b u t i o n of t h e d i f f u s i o n c u r r e n t component i n c r e a s e s near t h e anode s o t h a t a negative AD occurs.

AD value is determined by t h e r e l a t i o n mentioned above. The change of a current t r a n s f e r nechanism from a f i e l d one ( i n

a p o s i t i v e a r c column ) t o d i f f u s i o n one ( i n anode thermal l a y e r ) l e a d s t o a r c c o n t r a c t i o n n e a r t h e anode which t a k e s place a t a negative 0 without anode spot formation. A t high discharge currents t h e c o n t r a c t i o n develmpment can r e s u l t i n change of AD s i g n and in t h e spof forma- t i o n .

/I/. G.A.Dyuzhev, E.A.Startsev, S.M.S+kol n i k , V.G.Yur ev. J . Techn. Phys., 48, 2113, 1978.

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/2/. F. G.Bakst , G. A.Dyuzhev, N .K .Mitrof a- nov, s .Bd.Shkolnik, V. ~ . ~ u r ' i e v . J

.

.

ev. J. Thchn. Phys., 48,1195,1978.

/4/. L.Peretz, V.A.Nemchinsky.

J. Tech.. Phys.,

a,