AN OUTPUT-CURRENT PLASMA

ELECTRON

V. A .

Gruzdev

INSTABILITY

OF

A

SOURCE UDC 539.1.01

One p r o m i s i n g method for producing e l e c t r o n b e a m s in vacuum by means of an e l e c t r i c discharge involves extraction of electrons f r o m the plasma of a Penning discharge through an a p e r t u r e in the anode of the d i s c h a r g e c h a m b e r [1]. The c h a r a c t e r i s t i c s of this type of s o u r c e can be improved considerably through the use of modified Penning tubes with one or two hollow cathodes to shape the plasma [2]. Under c e r t a i n conditions, however, the output c u r r e n t f r o m such a s o u r c e displays a c h a r a c t e r i s t i c relaxation instability with constant voltages on the e l e c t r o d e s of the discharge chamber and in the extracting gap. The experiments were c a r r i e d out with the electron gun described in [2], and the plasma electron s o u r c e shown in Fig. 1. The a v e r a g e c u r r e n t of the electron beam extracted under our experimental conditions was e s s e n t i a l l y independent of the d i s c h a r g e - c h a m b e r p r e s s u r e Pd; it depended on only the extracting voltage Ue and the e l e c t r o n density near the e m i s s i o n aperture. Despite the insensitivity of the a v e r a g e output c u r r e n t to p r e s s u r e changes in the discharge chamber over the range 5" 10-4-6 910 -3 t o r r , the nature of the c u r r e n t changes markedly, as can be seen f r o m the o s c i l l o g r a m s of the beam c u r r e n t Ib shown in Fig. 2. This figure also shows o s c i l l o g r a m s of the c u r r e n t I a in the anode circuit of the d i s c h a r g e chamber during high-voltage extraction of electrons f r o m the d i s c h a r g e plasma and the frequency s p e c t r a of the oscillations in the e l e c t r o n beam current. Pulsed beam operation, convenient for quantitative evaluations, is achieved with a pulsed d i s c h a r g e at a constant extraction voltage. Since the pulses a r e v e r y long (100 psec), the operation of the d i s c h a r g e and the gun as a whole can be assumed q u a s i s t e a d y - s t a t e . The output c u r r e n t is seen to have a relaxation nature up to a critical chamber p r e s s u r e P c r , 3,5 910 -~ t o r r in our case. At p r e s s u r e s above P c r the output c u r r e n t b e c o m e s continuous (Fig. 2b). If the c h a m b e r p r e s s u r e exceeds P c r slightly, the continuous output c u r r e n t is modulated by oscillations at a frequency near the relaxation frequency at the instant at which the relaxation is cut off (Fig. 2c). R e l a x a tion oscillations of the output c u r r e n t a r e also observed during natural e m i s s i o n of electrons f r o m the d i s c h a r g e chamber, i.e., in the case in which there is no voltage on the extracting electrode; they a r e also observed during extraction of ions. We can t h e r e f o r e conclude that during the relaxation o s c i l l a tions the periodic b r e a k s in the output c u r r e n t do not r e s u l t f r o m an instability of the plasma boundary in the e l e c t r i c field [3, 4] or f r o m a discharge in the gas in the e m i s s i o n a p e r t u r e [5]; instead, they a r e due to an instability of the plasma c h a r a c t e r i s t i c s , p a r t i c u l a r l y the density, in the r e g i o n in which the emitting boundary is formed. In an attempt to determine the nature of the instability of the plasma near the anode of the discharge, we isolated one face of the r e c t a n g u l a r anode f r o m the three other faces and s e p a r a t e l y m e a s u r e d the discharge c u r r e n t to these faces. The c u r r e n t in the circuit of each part of the anode held at the c o m m o n potential was of a relaxation nature at a constant discharge voltage; the length of the relaxation c u r r e n t pulses was proportional to the azimuthal size of the corresponding part of the anode, and the amplitudes of these pulses were equal. The phase shift of the relaxation and c u r r e n t pulses to these parts of the anode was such that when the c u r r e n t to one part reached a maximum the c u r r e n t to the other part essentially vanished. This c u r r e n t behavior implies a rotating, eccentric, azimuthal plasma inhomogeneity of the "plasma f l a r e " type in this Penning discharge with hollow cathodes 9 Using the insulated anode face and a single planar probe at the center of the face adjacent to the insulated face (Fig. 1), we found that the plasma f l a r e r o t a t e s in the e l e c t r o n cyclotron direction. The azimuthal size of the f l a r e near the anode is about 1.8 rad. T o m s k Institute of Radio E l e c t r o n i c s and Electronic Engineering. Translated f r o m Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 5, pp. 136-138, pp. 136-138, May, 1970. Original a r t i c l e submitted May 26, 1969. 9 1973 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever without permission of the publisher. A copy of this article is available from the publisher for $15.00.

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I

2

J

4

J Fig. 1 Fig. 2 Fig. 1. 1, 5) Cathodes; 2) cathode cavity; 3) anode; 4) probe; 6) e m i s s i o n a p e r t u r e ; 7) e x t r a c t i n g electrode. Fig. 2. U e = 1 5 k V , Ia = 2 5 0 m A . a) Pd = 1 0 - 3 t o r r , = 130 mA; b) Pd = 5" 10 -3 t o r r , I b = 50 mA.

Ibmax

Accordingly, the o u t p u t - c u r r e n t r e l a x a t i o n s o c c u r b e c a u s e e l e c t r o n s a r e e x t r a c t e d f r o m the d i s c h a r g e a s the p l a s m a f l a r e p a s s e s the e m i s s i o n a p e r t u r e . The c u r r e n t in the anode circuit falls by a m a g n i tude equal to the output c u r r e n t at the instant of e x t r a c t i o n (Fig. 2a). The shape of the r e l a x a t i o n pulse in the output c u r r e n t r e f l e c t s the p l a s m a density distribution n e a r the f l a r e anode, and the frequency of the r e l a x a t i o n oscillations r e f l e c t s the angular rotation velocity, which d e c r e a s e s with i n c r e a s i n g d i s c h a r g e c h a m b e r p r e s s u r e and with i n c r e a s i n g a t o m i c m a s s of the working gas. The frequency i n c r e a s e s with an i n c r e a s e in the m a g n e t i c field or the d i s c h a r g e c u r r e n t . The f r e q u e n c y r a n g e of the r e l a x a t i o n oscillations in the b e a m c u r r e n t , which is cut off e x p e r i m e n t a l l y by changing the p r e s s u r e or the nature of the gas (argon, a i r , or helium), is 60-200 kHz. This r a n g e could be extended toward lower f r e q u e n c i e s through the use of h e a v i e r g a s e s and toward higher f r e q u e n c i e s by i n c r e a s i n g the magnetic field and reducing the d i s charge-chamber pressure. In conclusion we should point out that the r e l a x a t i o n instability in the output c u r r e n t of an e l e c t r o n s o u r c e based on a hollow-cathode Penning d i s c h a r g e can be used to g e n e r a t e powerful e l e c t r o m a g n e t i c o s cillations and t;o produce intense pulsed e l e c t r o n b e a m s having a s h o r t pulse length. LITERATURE

CITED

4.

Yu. E. K r e i n d e l ' , Zh. Tekh. Fiz., 36, No. 5, 903 {1966). P. P. Golik, V. A. Gruzdev, Yu. E. K r e i n d e l ' , and L. A. Levshuk, P r i b o r y i Tekh. E k s p e r i m . , No. 5, 229 {1968). K. G. Hernquist, RCA Rev., 21, 170 (1960). P. E. Belensov, A. T. Kanin:'--A. A. Plyutto, and V. N. Pyzhkov, Zh. Tekh. Fiz., 34, No. 12, 2120

5.

V. L. G r a n o v s k i i and T. A. Suetin, Zh. Tekh. Fiz., 17, No. 3, 291 (1947).

1.

2. 3.

(1964).

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