AN

IRON-FREE

LINEAR

INDUCTION

ACCELERATOR

A. I. Pavlovskii, A. I. Gerasimov, D. I. Zenkov, V. S. Bosamykin, A. P. Klement'ev, and V. A. Tananakin

UDC 621.384.6.03

L i n e a r induction a c c e l e r a t o r s have in recent y e a r s been u s e d to obtain pulsed electron c u r r e n t s of hundreds of a m p e r e s at energies of s e v e r a l m e g a e l e c t r o n v o l t s [1-2]. Units of this type consist of a s e r i e s of t r a n s f o r m e r s having o n e - t u r n coils, which a r e connected in s e r i e s . The a c c e l e r a t i o n of the p a r t i c l e s is accomplished by an e l e c t r i c field which is induced as a consequence of the synchronous variation of the fluxes in the f e r r o m a g n e t i c c o r e s of the t r a n s f o r m e r s . Elimination of the c o r e s allows a substantial i n c r e a s e in the t r a n s f o r m e d power and simultaneous simplification of the a c c e l e r a t o r construction, which results in a d e c r e a s e in its dimensions and cost. The authors consider s e v e r a l s t r u c t u r a l s c h e m e s for linear induction of a c c e l e r a t o r s with a i r cooling (see, for example [3, 4]), which a r e of interest p r i m a r i l y for obtaining single pulses or pulse t r a i n s of c u r r e n t s up to tens of thousands of a m p e r e s consisting of charged p a r t i c l e s having an energy of tens of m e g a e l e c t r o n v o l t s . In 1967 an experimental prototype of such an a c c e l e r a t o r was constructed. F i g u r e 1 shows a d i a g r a m of a v e r s i o n of the a c c e l e r a t i n g s y s t e m which was implemented. Its special feature lies in the combination of all the elements of the p r i m a r y impulsing circuit - the ring capacitor C, the controlled s p a r k gap SG and the buses (disks) AB and DE which connect them - into a toroidal magnetic core ABDE which confines a closed alternating magnetic flux that is created by the c u r r e n t during the discharge of the s t o r a g e capacitor C. The induced e l e c t r i c field is concentrated in the a c c e l e r a t i n g gap AH by the grounded s e c o n d a r y impulsing circuit AFNH which includes the magnetic core. The thickness of the disks AB and DE is l a r g e r than the depth of the skin l a y e r at the frequency of the discharge current, and this allows one of the disks to be used as a portion of the s e c o n d a r y impulsing c i r cult. The minimum magnitude of the s t r a y magnetic flux is r e a c h e d by using a capacitor and a c o m m u t a t o r having a ring g e o m e t r y in the i r o n - f r e e magnetic core. The a c R c e l e r a t o r s t o r a g e capacitance C= 0.053 pF is a s s e m b l e d N f r o m ten low-inductance sicond capacitors designed for an operating voltage of 50 kV.* The cylindrical electrodes of the s p a r k gap SG together with four p a r a l l e l c u r r e n t SG~ discharge channels, which a r e situated s y m m e t r i c a l l y around the c i r c u m f e r e n c e and simultaneously short the i n t e r e l e c t r o d e gap, f o r m the internal c u r r e n t cylinder. The a c c u r a c y with which the s p a r k gap is connected is at least ~2 nsec. The coupling coefficients between the p r i m a r y and s e c o n d a r y impulsing circuits is ~95%. The a m plitude value of the d i s c h a r g e current in the p r i m a r y i m pulsing input circuit is 105 A, while the frequency of the oscillations is 6.2 MHz. The i r o n - f r e e magnetic core, together with the s e c o n d a r y impulsing circuit which inc o r p o r a t e s it, f o r m s an a c c e l e r a t i n g cell having a height Fig. 1. D i a g r a m of the a c c e l e r a t i n g s y s t e m ; of 50 m m and a d i a m e t e r of 1050 mm. The 2 IVIeVa c c e l e r C and C1) c a p a c i t o r s ; SG and SGI) controlator is based on 48 identical cells combined into t h r e e led s p a r k gaps; R and R1) r e s i s t o r s ; L)

IL

solenoid; I) the c u r r e n t in the p r i m a r y i m pulsing circuit.

* Sicond capacitors of the K15-10 type w e r e developed under the supervision of M. I. Neiman and G. P. Blokhina.

.s

T r a n s l a t e d f r o m Atomnaya Energlya, Vol. 28, No. 5, pp. 432-434, May, 1970. Original letter submitted August 4, 1969.

9 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West I7th Street, New York, N. Y. lO01I. 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.

549

Fig. 2. Overall view of the a c c e l e r a t o r . blocks. The overall view of the a c c e l e r a t o r is shown in Fig. 2. Electrons having an energy of s e v e r a l hundred kiloelec. tronvolts a r e injected into it from a plane diode having a straight-channel cathode. The s o u r c e of the injection voltage consists of a low-inductance o s c i l l a t o r having a coaxial construction, which is b a s e d on the A r k a d ' e v - Marx circuit. Magnetic limiting is used to guide k i l o a m p e r e b e a m s from the injector to the a c c e l e r a t o r target. A longitudinal magnetic field is c r e a t e d by means of a solenoid L. which is placed in the a c c e l e r a t i o n region and simultaneously s e r v e s to improve the uniformity of the a c c e l e r a t e d e l e c t r i c field. The a c c e l e r ating s y s t e m is loaded by the solenoid through the capacitor C 1 which is charged f r o m an external s o u r c e . For the case in which s m a l l e r c u r r e n t s a r e accelerated, the focusing of the electron beam is achieved by a s y s t e m of magnetic lenses, and the a c c e l e r a t i o n region is shielded from s t r a y pulsed magnetic fields. Under these conditions the beam at the a c c e l e r a t o r output has a low angular divergence. The storage capacitors C a r e charged by pulses in 5 ,asec.

Fig. 3. O s c i l l o g r a m s : 1 ) a c c e l e r a t i n g voltage; 2) injection voltage; 3) electron current at the a c c e l e r a t o r output (100 MHz m a r k e r s ) ; 4) b r e m s s t r a h l u n g pulse (10 MHz m a r k e r s ) .

F i g u r e 3 shows o s c i l l o g r a m s of the s e c o n d a r y voltage (across one of the blocks), the injection voltage, the current pulse .~1300 A, and the b r e m s s t r a h l u n g pulse, which w e r e obtained during one cycle of a c c e l e r a t i o n of the electrons to an energy ~2 MeV. (The maximum value of the a c c e l e r a t e d current is approximately 2000 A and is limited by the i n j e c t ing device.)

During the p r o c e s s o[ operating the a c c e l e r a t o r a study was made of its basic c h a r a c t e r i s t i c s . Experiments w e r e c a r r i e d out on the a c c e l e r a t i o n of the electrons of a p l a s m a which had been created in advance in the a c celeration region; a f a s t - e l e c t r o n c u r r e n t of ~4.103 A was recorded. Injection of an e l e c t r o n b e a m f r o m an i r o n - f r e e linear induction a c c e l e r a t o r into a magnetic field of the betatron type was achieved with subsequent a c c e l e r a t i o n up to s e v e r a l tens of MeV. LITERATURE 1.

2. 3. 4. 550

N. A. V. V.

CITED

Christofilos et al., Rev. Sci. Instrum., 35, 886 (1964). I. Anatskii et al., At. ]~nerg., 2__!1, 439 (1966). F. Bosamykin et al., Byulleten' Izobretenii, No. 23 (1967). F. Bosamykin, Byulleten' Izobretenii, No. 9 (1969).