INFLUENCE OF MAGNETIC FIELD ON THE EFFECTS PRECEEDING THE TRANSITION OF FIELD EMISSION INTO THE VACUUM BREAKDOWN

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INFLUENCE OF MAGNETIC FIELD ON THE EFFECTS PRECEEDING THE TRANSITION OF

FIELD EMISSION INTO THE VACUUM BREAKDOWN

G. Fursey, V. Ptytsin, N. Egorov

To cite this version:

G. Fursey, V. Ptytsin, N. Egorov. INFLUENCE OF MAGNETIC FIELD ON THE EFFECTS PRECEEDING THE TRANSITION OF FIELD EMISSION INTO THE VACUUM BREAKDOWN.

Journal de Physique Colloques, 1979, 40 (C7), pp.C7-403-C7-404. �10.1051/jphyscol:19797197�. �jpa- 00219176�

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JOURNAL DE PHYSIQUE CoZZoque C7, suppZ6ment au n07, Tome 40, J u i l l e t 1979, page C7- 403

INFLUENCE OF MAGNETIC FIELD ON THE EFFECTS PRECEEDING THE TRANSITION OF FIELD EMISSION INTO THE VACUUM BREAKDOWN

G.N. Fursey, V.E. Ptytsin and N.V. Egorov.

The Leningrad EZectro technicaZ C o m n i c a t i o n I n s t i t u t e .

Introduction

It i s known t h a t when the f i e l d emitter i s overloaded by its own emis- s i o n c u r r e n t i t exploses thus stimulat- ing the vacuum breakdown /I/. It i s a l s o known / 2 , 3 / t h a t j u s t before the explo- sion t h e f i e l d emission (FE) looses i t s s t a b i l i t y and this pre-explosion s t a t e i s e a s i l y i d e n t i f i e d by i t s s p e c i f i c e f f e c t s : I . t h e bright r i n g s appear around t h e main emission image (Fig.la);

2. the emission current spontaneously increases i n time, the r e l a t i v e increase of current A ' I m a x / I o during the i n t e r v a l o f t h e voltage pulse Z i s o f the oder of magnitude e 1 .1+1.5 ( ~ i g . l b ) , depending on and emitter geometry. Up t o now these e f f e c t s have no unique treatment. I n t h e paper submitted an attempt i s made t o obtain ad- d i t i o n a l information about pre-explo- s i o n phase of the matter by placing t h e emitter i n external magnetic f i e l d .

Experiment was c a r r i e d out i n modi- f i e d Muller electron microscopes i n vacuum approx5mately equal t o 5 x I 0 - ~ ~ 1 o r r ; t h e value of external magnetic f i e l d d i - rected along the emitter axis varied i n t h e l i m i t s (0.01+0.4) T .

A t c e r t a i n conditions spontaneous ~ U r r e n t increase i n magnetic f i e l d i s slowed down and we observed s a t u r a t i o n of emission current.

I t i s established t h a t simultaneous- l y with l a r g e increase of current i n m e netfc f i e l d the "ringt* e f f e c t i s observed on t h e emission picture, the r e s u l t being presented i n Fig.2a. Absence of a x i a l symmetry ( t h a t was present when

&0), probably, shows t h a t processes of

"ringw formation and those of main emission p i c t u r e a r e d i f f e r e n t .

I n other experiments we observed e f f e c t of **accumulationw. I t was shown t h a t i f t h e consequent s i n g l e voltage pulses of constant amplitude are applied t o the F'E diode ( t h e i n t e r v a l between t h e pulses being equal t o "- 10 sec.), t h e amplitude of the pulses of emission cur- r e n t increases from pulse t o pulse (Fig.3a,b). Such current increase leads t o explosion of emitter, 1, being close t o t h e l i m i t current - 1 l i m . f o r a given emitter. I n those cases when I [ j m . > > I 0 ,

we observed a m a x i m u m of current pulse (Fig.2b, curve I ) . It i s worth noting t h a t the current pulse following t h a t with a maximum always appeared t o be Experiment a1 r e s u l t s rectangular and of e s s e n t i a l l y smaller It i s established t h a t magnetic amplitude (Fig,2b, curve 1 and 2). It f i e l d influences e s s e n t i a l l y t h e pre- should be mentioned a l s o t h a t **accumu- breakdown e f f e c t s , Spontaneous increase lation" e f f e c t probably may not be con- of current talres place t h e presence of nected with adsorption processes a t t h e magnetic f i e l d a t lower values of e l e c t - surface since the emission p i c t u r e d i d

-

r i c f i e l d strength. An e s s e n t i a l l y new not vary during t h e experiment.

r e s u l t i s t h e f a c t t h a t A m a y reach va- Further measurements were carried l u e s ^C= 5+7 during t h e pulse i n t e r v a l 2 out i n a constant e l e c t r i c f i e l d , I n (Fig.2b, curve 1 and Fig.3a, curve 3 ) . these experiments voltage applied t o t h e

F.E. diode was chosen i n such a way t h a t

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797197

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i n i t i a l c u r r e n t density - ^dohad a value

2 3

l y i n g i n the l i m i t s 10 + I 0 ~ / c m ~ . Re- s u l t s of our experiment a r e presented i n Fig.4.

We established t h a t t h e r a t e of cur- r e n t i n c r e a s e i n magnetic f i e l d becomes l a r g e r with i n c r e a s e of i n i t i a l c u r r e n t density. The keeping of e m i t t e r i n a mag- n e t i c f i e l d without c u r r e n t does not influence the character of c u r r e n t increase w i t h subsequent switching t h e e l e c t - r i c f i e l d . It should be mentioned t h a t decrease of c u r r e n t a f t e r switching off t h e magnetic f i e l d i s going on essen- t i a l l y slower than the c u r r e n t increase.

Rate of c u r r e n t decrease i n c r e a s e s w i t h i n i t i a l d e n s i t y f o at Bzconst. Slope of c u r r e n t increase and decrease i n c r e a s e s with magnetic f i e l d .

A s t a t e induced by magnetic f i e l d i s desfroyed by heating. !.Enimal tempe- r a t u r e wheb t h e induced s t a t e i s dest- royed i s equal t o =1 1 0 0 0 ~ ~ .

Main Conclusions

1. It i s established t h a t magnetic f i e l d i n e r t i a l l y i n c r e a s e s emitting ca- p a c i t y of a f i e l d emitting cathode.

2. It i s shown t h a t magnetic f i e l d e s s e n t i a l l y influences s t a b i l i t y of a f i e l d e m i t t e r i n a pre-breakdown phase and i n a process of i n i t i a t i n g vacuum breakdown.

B=O. 6 - p u l s e of c u r r e n t :

2 = 4 0 0 p s e c , B = 0 .

Fig.2 a- deformation o f emission p i c t u r e i n t h e magnetic f i e l d a t t h e pre-breakdown phase, B=0,4T; 8 - p u l s e s of c u r r e n t , B=0,4T, =I , 0 msec.

. -

F i g . 3 Effect o f accumulation i n a pre- breakdown phase.

a- r = 5 0 0 p s e c , I3=0,4T; 4- r = 5 0 q u s e c ,

References

1. G.M,Fursey, 9.N.Voronteov-Veliaminov.

Sov.Phys., JTP, 2, 10, 1880 (1967) 2. W.P.Dyke, J.K.Trolan. Phys. Rev.,

3, ⁴(1953).

3 , G.N.l?ursey, V.E.Ptytsin, N.V.Egorov.

Sov.Phys., Abstracts of the I11 Con- f erence on High Current; &mission Electronics. Tomsk, 1978.

I ^1.m ²⁰⁰ t,sec.

F i g .4 1

.

j o = 3 . f 0 2 ~ l c ~ ; 2.4. j0=5101~,!.3 3. a,= ^/f13%

I n t h e r e g i o n of c u r r e n t i n c r e a s e &d s a t u r a t i n ( f o r c u r v e s 1 , 2 , 3 ) B i s e q u a l t o 0,04T andho 0,4T f o r c u r v e 4.In t h e r e g i o n o f c u r r e n t d e c r e a s e E=O _{f o r}J t 4 .