H.F. INSTABILITIES IN THE HETEROGENEOUS PLASMA OF A PENNING DISCHARGE ION SOURCE WITH A HOLLOW CATHODE

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H.F. INSTABILITIES IN THE HETEROGENEOUS PLASMA OF A PENNING DISCHARGE ION

SOURCE WITH A HOLLOW CATHODE

V. Gerasimov, S. Karev, V. Obukhov, G. Shishkin

To cite this version:

V. Gerasimov, S. Karev, V. Obukhov, G. Shishkin. H.F. INSTABILITIES IN THE HETEROGE- NEOUS PLASMA OF A PENNING DISCHARGE ION SOURCE WITH A HOLLOW CATHODE.

Journal de Physique Colloques, 1979, 40 (C7), pp.C7-647-C7-648. �10.1051/jphyscol:19797314�. �jpa- 00219305�

JOURNAL DE PHYSIQUE CoZZoque C7, suppZ6ment au n07, Tome 40, J u i Z Z e t 1979, page C7- 647

HE. INSTABILITIES IN THE HETEROGENEOUS PLASMA OF A PENNING DISCHARGE ION SOUfCE WITH A HOLLOW CATHODE

V.P. Gerasimov, S.A. Karev, V.A. Obukhov and G.G. Shishkin.

Moscow, U.S.S. B.

I n gaseous d i s c h a r g e s w i t h heterogenous d i s t r i b u t i o n s of e l e c t r i c and magnetic f i e l d s t h e i n s t a b i l i t i e s of d i f f e r e n t t y p e s maygproduced / I / . The i n v e s t i g a - t i o n of t h e s e i n s t a b i l i t i e s can p o s s i b l y be used i n o p t i m i z a t i o n of some gaseous d i s c h a r g e d e v i c e s . I n t h e i n v e s t i g a t i o n p r e s e n t e d h e r e i n were o b t a i n e d measure- ments of t h e o s c i l l a t i o n s of t h e plasma p o t e n t i a l a t a number of p o i n t s i n t h e d i s c h a r g e chamber of t h e cesium i o n source o v e r range of s o u r c e o p e r a t i n g c o n d i t i o n s . Most of t h e e x p e r i m e n t a l t e s t s d i s c u s s e d h e r e were produced from 0.5 t o 1.5 MHz.

Experimental arrangment

I n t h i s i n v e s t i g a t i o n t h e 10-Sn hollow cathode cesium i o n source shown i n Fig.1 w a s used. The magnetic f i e l d was formed by t h e permanent magnet mounted on t h e back w a l l o f t h e chamber and by s p e c i a l f e r r o m a g n e t i c d e t a i l s , The magnetic f i - e l d can be c h a r a c t e r i z i e d as-divergence and c o n t r o g a t e d n e a r cathode". The mag- n e t i c i n d u c t i o n v a r i e d from 0.4 T n e a r t h e cathode t o 0.015 T n e a r t h e s c r e e n e l e c t r o d e . The source was o p e r a t e d a t t h e beam c u r r e n t s of 0.2-0.5 A. The acce- l e r a t i n g v o l t a g e was about 1.8 kV, The

d i s c h a r g e v o l t a g e v a r i e d from 1 5 t o 25 V , and t h e d i s c h a r g e c u r r e n t was c h a r a c t e - r i z e d by t h e i n t e r v a l of 2.0-3 A ( s e e Pig.3)- The plasma parameters l i k e e l e c - t r o n temperature Te, plasma d e n s i t y Ne,

lasma , x p a t e n t i a l and e l e c t r i c f i e l d

8 were measured and c a l c u l a t e d w i t h t h e h e l p of d a t a which were r e c e i v e d by u s i n g two . c i l i n d r i c a l Langmuirts p r o b e s , Two h.f. probes were a l s o used t o r e c o r d t h e plasma o s c i l l a t i o n s . The probes' s i g n a l s were measured by spectrum a n a l y z e r s and c o r r e l a t i o n r e c e i v e r s . x l l t h e r e c o r d i n g systems were matched and c a l i b r a t e d . Experimental r e s u l t s

The plasma parameteres and t h e i r d i s t r i - b y t i o n s a r e shown i n Fig.2. From Fig.2 t h o s e s k i l l e d i n a r t can s e e two a r e a s i n t h e d i s c h a r g e plasma. The f i r s t one, i . e . c e n t r a l plasma can be c h a r a c t e n z e d a s !!hot and dense", w h i l e t h e second, i.e. preanode plasma, i s more "cold and r a r e f i e d " . Fig.? shows V-A c h a r a c t e r i s - t i c s of t h e d i s c h a r g e , Fig.4 i s a t y p i - c a l s p e c t r a wich shows t h e r e l a t i v e amp- l i t u d e a s a f u n c t i o n of frequency of t h e o s c i l l a t i o n s f o r a d i s c h a r g e c u r r e n t

2.2 A , d i s c h a r g e v o l t a g e 2 0 V and beam c u r r e n t 0.42 A. T h i s s p e c t r a h a s t h e f r e q u e n c i e s of t h e dominant peaks. The f i r s t of them i s on t h e frequency of about 40 kHz, and t h e second - ^{on 0.6-}

-0.8 MHz. I n t h i s paper t h e f l u c t u a - t i o n s of t h e second range have been i n - v e s t i g a t e d . F i g u r e 5 shows t h e auto- and c r o s s - c o r r e l a t i o n s c u r u e s f o r t h e s e o e c i l l a t i o n s . The r a d i a l v a r i a t i o n of t h e amplitude ( a t 2=30 mm) and a x i s va- r i a t i o n of t h e frequencg ( a t 1-0) a r e g i e n on t h e Fig.2 and

.

c o r r e l a t i o n s were measured. Ik was found t h a t t h e o s c i l l a t i o n s a r e " i n phase"

b o t h o v e r t h e a x i a l plasma a r e a ( c u r v e s 1 , 3 i n Pig.5) and o v e r t h e r a d i a l o u t of t h e a r e a of t h e "hot plasma".

On t h e c o n t r a r y i n s i d e t h e a r e a of t h e

"hot plasmat* t h e wav? v e l o c i t y was mea- sur$d t o be about 1 0 cm/s ( c u r v e s 1 , 2 i n Fig.5), It seems t h a t t h i s o s c i l l a - t i o n i s an a z i m u t h a l wave. The frequency of t h i s wave i s slowly v a r y i n g on t h e l e n g t h (Fig.6). The magnitude of t h e f r e - quency t e n d s t o i n c r e a s e a s t h e f l o w r a t e of cesium and d i s c h a r g e v o l t a g e a r e i n - c r e a s e d .

D i s c u s s i o n of t h e r e s u l t s

I n t h i s c a s e t h e plasma parameters a r e c o n s i d e r e b l y heterogeneous on t h e r a d i u s a s w e l l a s a n t h e a x i s . That i s why i t i s n e c e s s a r y t o work o u t t h e t h e o r y i n three-dimensional approximation. T h i s problem could n o t be solved a n a l y t i c a l l y , Moreover i t seems t o be t o o d i f i c u l t f o r a numerical s o l u t i o n . Therefore we want t o c a r r y out a q u a l i t a t i v e a n a l y s i s . As i t f o l l o w s from t h e experiments t h e a z i - muthal wave i s s i m i l a r t o a spoke, whose frequency i s f = KVf2z According t o t h e c o r r e l a t i o n measuremenes K _x 'f/z , where r - r a d i u s of t h e "hot plasma" a r e a . I f

Vy.=V& = EZ/Bz l ~ ' - j o ? ~ ~ t h e n f=0.5-1 MHz.

T h ~ s v a l u e i s i n concordance w i t h t h e experiments. Having e x p e r i m e n t a l d a t a on t h e v e l o c i t y , t h e frequency of t h e wave

(Fig.2,6) and t h e l o c a t i o n of t h e maxi- mum amplitude $n every c r o s s - s e c t i o n of t h e chamber, t h o s e s k i l l e d i n a r t can

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797314

determine t h e wave's d i s p e r s i o n characte- r i s t i c s ( F i g , 2 ) . It seems t h a t considera- t i n g o s c i l l a t i o n i s s t i p u l a t e d by hetero- g e n e i t y of t h e d i s t r i b u t i o n of t h e e l e c t - ron d r i f t v e l o c i t y . The i n s t a b i l i t y of t h i s kind have been i n v e s t i g a t e d f o r E-H plasma a c c e l e r a t o r s with t h e closed- d r i f t of e l e c t r o n s /2/. The conclusions of t h i s a n a l y s i s a r e , a l s o s u i t a b l e f o t t h i s case. The frequency and increment of t h e wave r e s p e c t e v i l y a r e

Ap (@ i f$ ; W 8 e ) (1)

It i s s u f f i c i e n t enough t o take i n t o ac- count only t h e f i r s t member of t h e equi- t i o n (2). The c h a r a c t e r of t h e c a l c u l a - t e d and experirnerltal d i s p e r s i o n dependen- c e s a r e i n concordance. The s p a c i a l d i s t - r i b u t i o n of t h e o s c i l l a t i o n amplitude can be c a l c u l a t e d w i t h t h e h e l p of e q u i t i o n

( 2 ) and t h e expression: KtZy/vd=. I+ can be showed t h a t t h e maximum of t h e i n t e n - s i t y of t h e wave i s observed i n t h e cham- b e r a r e a where K i i s t h e g r a t e s t . The pre- sented d a t a show t h a t t h e i n v e s t i g a t e d o s c i l l a t i o n can be i d e n t i f i c a t e d as h.f.

d r i f t waves s t i p u l a t e d by t h e s l i p p i n g - -stream i n s t a b i l i t y .

References

1,Mikhailowskiy A.B. Teoriya plazmennikh neustoyichivostey. Vo1.2. Atomizdat, 1977-

2.Shishkin G.G., Gerasimov V.P. X I 1 I n t e r - nat .Cot-&. i n ionized gases. P a r t I, 1975, 310. Netherlands.