FORMATION OF HIGH-CURRENT AND DENSITY ELECTRON BEAM ON GALLIUM CATHODE WITH THE LIMITED EMISSION SURFACE

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FORMATION OF HIGH-CURRENT AND DENSITY ELECTRON BEAM ON GALLIUM CATHODE WITH

THE LIMITED EMISSION SURFACE

G. Fursey, V. Zhukov, L. Shirochin, Alexander Aleksandrov, S. Galuso

To cite this version:

G. Fursey, V. Zhukov, L. Shirochin, Alexander Aleksandrov, S. Galuso. FORMATION OF HIGH- CURRENT AND DENSITY ELECTRON BEAM ON GALLIUM CATHODE WITH THE LIM- ITED EMISSION SURFACE. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-415-C7-416.

�10.1051/jphyscol:19797203�. �jpa-00219183�

JOURNAL DE PHYSIQUE CoZZoque C7, suppZ&ment au n07, Tome 40, JuiZZet 1979, page C7- 415

FORMATION CIF HIGH-CURRENT AND DENSITY ELECTRON BEAM ON GALLIUM CATHODE WITH THE UMITED EMISSION !XJRFACE

G.N. Fursey, V.M. Zhukov, L.A. Shirochin, A.F. Aleksandrov and S.Y. Galuso.

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

A s was shown i n papers /1,2/, the explosive emission (EE) of s o l i d and li- quid gallium cathodes is characterised by high s t a b i l i t y of E.E current. The authours of paper /3/ had shown t h a t the use of the point gallium cathodes with the limited emission s w f a c e permits t o decrease t h e space charge i n a diode and t o r e v e a l the.emission properties of EE.

Use of such cathodes i n diodes of micro- second duratiqn with magnetic i n s u l a t i o n makes i t possible t o obtain sharply fo- cused electron beams with a t o t a l cur- r e n t about 300A, energy of 150 Kev and t h e current den i t y measured on the t a r - !?

g e t of about 10 A /cm2 /4/.

It is known /5,6,7/, t h a t i n magne- t i c a l l y insulated diodes the potencial drop and t h e influence of the magnetic f i e l d of electron beam on the transport- ing f i e l d decrease when the r a t i o p=r.k

tends t o unity, where Zi and "Lr

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t e r n a l and external r a d i i of t h e beam.

These are t h e beams with a weak diamag- netism, i.e, with a small r a t i o of trans- v e r s a l and the t o t a l energies of elect- rons. I n t h e conventional constructions

of diodes t h e emitting cathode surface i s unlimited and t h e majority of e l e c t - rons i s emitted i n t h e d i r e c t i o n which is normal t o magne.tic force l i n e s . It leads t o g r e a t e r diamagnetism i n t h e beam,

We made t h e butt-ended cathode of s p e c i a l form ( t h e emitting surface fac- ing t h e guidance channel). I n this case t h e t r a n s v e r s a l motion of electrons is possible because of nonuniiormity of e l e c t r i c f i e l d near t h e cathode edge

1 2, =(I&) *10-2cm 7 and t h e influence of high space charge of t h e beam e l e c t - rons. The first f a c t o r may be diminished by deeping the cathode edge.

width "a" of cathode r i n g abo%?li% mm.

t h e edge sinking is of t h e same order of value. The second f a c t o r i s decreased by l i m i t i n g t h e emitting surface of cathode /3/.

A s was noticed i n /5/, processes i n diode influence g r e a t l y formation and current t r a n s p o r h g o? t h e beam. Ne- v e r t h e l e s s v e r s a t i l i t y of processes on t h e cathodes with unlimited and conti- nuously changing emitting surf ace makes it d i f f i c u l t t o completely solve the problem of beam formation i n a whole beam

-

We supposed t h a t i n case when t h e butt-ended cathode with limited emitting surface is -used the exact theory de- veloped i n /6,7/. may be used t o describe the diode p a r t of beam formation too.

Estimates made i n /6,7/ show t h a t it is s u f f i c i e n t t o c r e a t e the magnetic f i e l d

Be =8KHs t o provide t h e guidance o f beam with the external radius & =O.5 cm, energy[ 12 beam current Ig =IOU. It i s a l s o s own i n /7/, t h a t increase of mag- n e t i c f i e l d upto B* Be produce compress- ing of annular beam which r e s u l t s in re- duction of external radius. It may be supposed t h a t t h e beam which i s formed i n such a manner has the r a t i o pr: 1 and i t s diameter i s determined by the cathode emitting surface Dc. It makes possible t o accord the diode impedance with the d r i f t space of accelerator by varing %he cathode geometry. Analysis of experimen- t a l d a t a may be carried out basing.on t h e r e s u l t s /8/, which lead t o t h e following formula, supposing the small diamagnetism of t h e beam:

where ze

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The experimental t e s t of v a l i d i t y of supposition concerning small diamagnetism of beams formed on cathodes with limited emitting surface is t h e subject of pre- sent paper. Fig.1 demonstrates the scheme of high voltage diode, measuring devices and gallium cathode. Total diode current 10, d r i f t tube and anode electrodes cur- r e n t Id , evacuated beam current I b and accelerating voltage were fixed i n expe- riment. The vacuum was o f 10-5 Torr.

Teflon was used as a cathode insulation.

We used t h e cathode with Dc=10;15;22 mm i n diameter with t h e r i n g cathode thick- ness a d . 2 mm. The cathode was immersed i n homogeneous magnetic f i e l d B = l O kHs.

Distance between t h e emitting edge of cathode and t h e wall of d r i f t tube was

d u - c ⁼² cm. The d r i f t space was equal t;o 100 mm and i t s diameter was 26 mm.

Pig.2,3 give t h e oscillograms of current and voltage and current t o vol- tage c h a r a c t e r i s t i c s of t h e evacuated beam f o r d i f f e r e n t cathodes. Fig.3 gives t h e values of current calculated (dashed l i n e s ) by formula (I) neglecting t h e po-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797203

Fig.3 c u r r e n t t o voltage characteris*':.

t i c s , 1

-

-

-

Fig.1 The scheme of high voltage diode and gallium cathode.

$0 loo 450 zoo t , n s 7.

"I&---

Pi6.2. The oscillograms of c u r r e n t s and voltage.

t e n c i a l drop i n t h e beam. It corresponds t o t h e model of "Thin" annular beam a = 0. For comparison, on t h e same Piwe we give t h e d a t a f o r t h e s t e e l cylindrz- c a l cathode with unlimited emitting sur- f ace. The s i g n i f i c a n t d i f f e r e n c e between t h e evacuated beam and t h e aalculated beam c u r r e n t f o r t h e cathode with D c = 22 mm may be explained by t h e f a c t t h a t e l e c t r o n Larmour r a d i u s i n t h e t r a n s p o r t i n g f i e l d i s of t h e same order of value a s t h e distance between emitt- ing edge of cathode and d r i f t channel wall. This r e s u l t s i n t h e l o s s of beam i n a d r i f t space.

Thus, the use of butt-ended catho- des with limited emitting surface per- m i t s produce thin annular beams and t o c a l c u l a t e c u r r e n t i n diode with magnetic i n s u l a t i o n over t h e whole space of for- mation and beam t r a n s p o r t i n g by approxi- mate formulae (I).

References

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JTP, 1976, 46, 2.

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.

V,S.Voronin, A.N.Lebedev. Sov. Phys., JTP, 1973, 43, 12, 2591.

A.V.Agafonov, V.S.Voronin e t a l . JTP, 1974, 44, 9 , 1909.

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