TEMPERATURE OF HEAVY-CURRENT CYLINDRICAL HOLLOW CATHODE

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TEMPERATURE OF HEAVY-CURRENT CYLINDRICAL HOLLOW CATHODE

N. Rykalin, A. Nikolaev, A. Borzhov

To cite this version:

N. Rykalin, A. Nikolaev, A. Borzhov. TEMPERATURE OF HEAVY-CURRENT CYLINDRI- CAL HOLLOW CATHODE. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-229-C7-230.

�10.1051/jphyscol:19797113�. �jpa-00219084�

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JOURNAL DE PHYSIQUE Colloque C7, suppl6ment au n07, Tome 40, J u i l l e t 1979, PQae C7- 229

TEMPERATURE OF HEAVY-CURRENT CYLINDRICAL HOLLOW CATHODE

N.N. Rykalin, A.V. Nikolaev and A.P. Borzhov.

Baikov I n s t i t u t e o f Metallurgy Moscow U. S. S.R.

I n t r o d u c t i o n . The first p a p e r s on d i s - charge w i t h hollow cathode were published about twenty y e a r s ago /I,2/. The discharge of t h i s type we used a s a heavy-current source of ions. L a t e r i n v e s t i g a t i o n s on h o l l o v cathode a p p l i c a t i o n were developed i n l a b o r a t o r i e s of d i f f e r e n t c o u n t r i e s . P h y s i c a l i n v e s t i g a t i o n s under t h e l e a d e r - s h i p of Prof ,Delcroix /3/ and p r a c t i c a l . use of t h e discnarge w i t h hollow cathode by Ulvac Co, i n metallurgy should be espe- c i a l l y mentioned /4/.

A t Baikov I n s t i t u t e o f P e t a l l u r g y experiments t o s t u d y energy-physical and tech- n o l o g i c a l parameters of heavy-current d i s - charge w i t h hollow cathode a r e under way /5-7/. This p a p e r p r e s e n t s r e s u l t s of tem- p e r a t u r e c o n d i t i o n a t u d i e s f o r c y l i n d r i c a l tungsten cathode. The o b t a i n e d d a t a charac t e r i z e e l e c t r i c t r a n s f e r on t h e cathode, i t s energy balance and e r o s i o n mecnanism,

Experimental assembly and i n v e s t i g a t i o n c o n d i t i o n s . The assembly t o study cathode temperature c o n d i t i o n c o n s i s t a of a cathode u n i t w i t h a hollow t u n g s t e n e l e c t r o d e , water cooled copper anode w i t h a c r a t e r f o r l i q u i d metal, s e a l e d chamber, d,c.power source, gas-evacuation system and mea- surement equipment.

Brightness temperature of t h e cathode providing t h e wave l e n g t h A =0,65 mkm was measured i n experiments. Power d i s s i p a t e d from cathode by h e a t c o n d u c t i v i t y was determined by t h e temperature of water cool- i n g cathode. The a r c c u r r e n t was i n t h e range from 300 up t o 3300 A, consumption of plasma, forming gas - from 0,05 up t o

0,26 cm /see, t h e cathode diameter 3 w a s equal t o 0,4-2 cm, t h e w a l l t h i c k n e s s -

0,2-0,5 cm and t h e cathode l e n g t h - 6-12 cm. Experiments were mainly conducted i n argon by p r e s s u r e i n t h e chamber of 0,4 t o r r e In some t e s t s helium, n i t r o g e n and hydrogen were used as a plasma forming g a s

Cathode temverature, By l a r g e c u r r e n t s two m a U m s of temperature a r e observed

on t h e cathode s u r f a c e : t h e first maximum

Tim - i n t h e cathode a c t i v e zone, she s e - cond maximum T2, - n e a r cathode f i x i n g i n a c o o l i n g hold; ( F ~ ~ . I )

T'X E& TZp Pig,I. Change

3000

t

4500 ^r\ of temveratu-

f? \ n e n t s of ener- r gy balance f o r

a c y l i n d r i c cathode alonz

I i t s a x i s (I= ^w

=20ooA; G,,=

2 6 b 9

-

cm; ^cm^{3 ' -}/ s e c ; , , 1 cm; d=Z,6 h=0,4 cm).

T G ~ t y p i c a l temperature of t h e a c t i v e zone f o r t u n g s t e n cathode i s e q u a l t o 2900- 3 3 0 0 ~ ~ . Value and p o s i t i o n of t n e tempera- t u r e m a x i m u m e s s e n t i a l l y depend on t h e a r c c u r r e n t and g a s consumption through cathode. 13y c u r r e n t i n c r e a s i n g from 800 up t o 3000 A t h e maximum temperature r i s e s accor- ding t h e empiric e q u a t i o n

%.n = 1750 1 ~ 9 ~ 7 5

and i t s p o s c i i o n i s s h i f t e d t o t h e working end of t h e cathode. Providing I=800 A t h e maximum i s a t a d i s t a n c e of 4 cm from t h e

o u t l e t end, and by 1=3000A - ^{2 cm.}

By varying g a s consumption t h e maximum temperature changes n e g l i g i b l y . Gas consumption e s s e n t i a l l y e f f e c t s t h e maximum p o s i t i o n , Thus, by argon consumption t h r o - ugh cathode of 26 and 0,5 cm /sec i t s maxi- 3 mum temperature d i d n o t change p r a c t i c a l l y and w a s equal t o 3 3 0 0 ~ ~ . By g a s consumption of 26 cm /sec t h e maximum temperature 3 was a t t n e e l e c t r o d e t i p ^andby consumption of 0,5 cm3/sec - a t t h e d i s t a n c e of I8 mm from i t . S h i f t of t h e a c t i v e zone pro- v i d i n g c u r r e n t and g a s consumption change i s obviously explained by varying pressu- r e i n t h e e l e c t r o d e c a v i t y .

G a s type e f f e c t s catho.de temperature con-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797113

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d i t i o n n e g l i g i b l y . Thus, i f plasma-forming gars i s argon t h e temperature of o u t e r s u r - f a c e w a s e q u a l t o 3 2 5 0 ° ~ , i n c a s e of nitro gen and helium - 3270 and 3 3 0 0 ' ~ correepon d i n g l y .

The second maximum n e a r cathade f i x i n g i n t h e h o l d e r i s obviously e x p l a i n e d by two f a c t a r s : I ) cathode i s no,t being cooled by e l e c t r o n emission and 2 ) e l e c t r o d e is h e a t e d by f u l l discharge c u r r e n t i n t h i s r e g i o n ( o u t s i d e t h e a c t i v e zone). The temperature may be determined w i t h enough accuracy from Lenz-Jaule and Stefan-Boltz- rnann e q u a t i o n s 4

TF~-- ^{( 2 )}

Here e - i n t e g r a l c o e f f i c i e n t of r a d i a - t i o n , G - Stefan-boltzmann c o n s t a n t , p -

s p e c i f i c e l e c t r i c resistance, d - ^cathode

diameter and h - ^cathodewall t h i c k n e s s , As i t f o l l o w s from E q , ( I ) and ( 2 ) t h e temperature o u t s i d e t h e s p o t r i s e a more ra p i d l y by t h e c u r r e n t than i n t h e spot.This i s t h e r e a s o n f o r l i m i t i n g c u r r e n t o f t h e c y l i n d r i c hollow cathode caused by e l e c t - rode m e l t i n g n e a r i t s f i x i n g i n t h e h o l d e r C r i t i c a l c u r r e n t d e n s i t y i n t h e t u n g s t e n cathode body, determined experimentally by i t s m e l t i n g , i s e q u a l t o 4,6-5,6 kA/cm ²

Fim.2, Chsnge of t h e f i r s t (I) _{and se-} cond 92) temperature maximum depending on t h e c u r r e n t d e n s i t y i n t h e e l e c t r o d e .

.

^,

.

^-by b r i g h t n e s s temperature (d=1,6- ,

2 cm; h =0,4-0,5 cm), x,+, o , ⁸ - by e l e c t r o d e m e l t i n g (d=0,4-0,6 cm; h=0,2-0,3 cm) Cathode energy balance. Study of cathode temperature c o n d i t i o n allowed t o d e t e r - mine components of i t s energy balance. Po-

c u r s due t o r a d i a t i o n Pr; e l e c t r o n s emission Pe, h e a t c o n d u c t i v i t y P and evaporati- on of cathode ma_teria3..Pv.

Power d i s s i p a t e d from t h e cathode by r a - d i a t i o n essentkaLly depends on t h e a r c cur- r e n t and forms 45-75s of f u l l power emer- gea on t h e cathode. Power d i s s i p a t e d by he- a t c o n d u c t i v i t y i s equal t o 8-14%. Power con~uming on e l e c t r o n s themnoemission may be e q u a l t o 20-40%. The e l e c t r o n c u r r e n t p a r t i s 0,6-0,8. It r j s e s w i t h i n c r e a s i n g t h e d i s c h a r g e c u r r e n t .

Rate of e l e c t r o d e e r o s i o n i s s a t i s f a c t o - r i l y d e s c r i b e d by henginour equation and re- q u i r e s about I% of t h e energy.

I o n s moving t o t h e cathode a r e t h e main energy source i n t h e a c t i v e zone. I f t h e d i s c n a r g e c u r r e n t i s e s s e n t i a l l y lower ( i n some t i m e s ) than t h e l i m i t i n g one, t h e power t r a n s f e r r e d by i o n s i s e q u a l t o 85-90%

o f t h e f u l l energy emerged i n t h e a c t i v e zone.

Cathode v o l t a g e drop c a l c u l a t e d on t h e b a s i s of energy balance i n t h e a c t i v e zone i s equal t o 16-20V,

Conclusion, Analysis of t h e temperature c o n d i t i o n of a t u n g s t e n hollow-cathode h a s shown t h a t e l e c t r i c c u r r e n t of such a cathode i s provided by tnermo-emission me- chanism. The l i m i t i n g c u r r e n t of a cylind- r i c cathode h a s been d e f i n e d experimental- l y . Its value i s caused by e l e c t r o d e melt- i n g due t o J o u l e o v e r h e a t i n g of an e l e c t r o -

de o u t s i d e t h e cathode spot.

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Phys*49(4), 1978, 2380.

4. Tokei H., Ishigami Y. Trans, of t h e In- tern.Vac.Met.Conf.Anaheim, Calif.1970.

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mater., 1974, i!o 2.

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wes i n t r o d u c e d i n t o t h e cathode i n t h e ac- t i v e eone c o n s i s t s of energy streams cau- s e d by i o n s energy moving t o t h e cathade Pi and Joule h e a t i n g Pa. Energy o u t l e t oc-