RESEARCH AND DEVELOPMENT

IN C Y C L I C A L E L E C T R O N A C C E L E R A T O R S

AT THE TOMSK POLYTECHNIC

INSTITUTE

A. A. B o r o b ' e v and V. A. M o s k a l e v

A brief description Is given of research In eycllcaI electron accelerators which has been carried out since 1946 at the Tomsk Polytechnic Institute. The basic parameters and constructional features of several betatrons ate given. Various methods of extracting the electron beam from the accelerator chamber of the betatron are considered. The chief emphasis is on a 25-Mev electron betatron. An investigation has been made of the use, In a 30-Mev synchrotron, of ctrcutar accelerating electrodes connected to an external two-conductor resonant Line.

A research program In betatron development was started In 1946 at the Tomsk Polytechnic Institute (TPI). In 1948 a betatron was built which was capable of a radiation energy of 5 Mev (the coils of the magnet were supplied with current at 500 cps); a betatron with a radiation energy of 7 Mev (the coils of the magnet were sup-plied with current at Industrial frequencies)was also constructed. In the period 1949-1955 a series of betatrons with radiation energies ranging up to 15 Mev was built. These betatrons arc now operated In scientific-research Institutes throughout the country. At the same time work was carried out on the development of a betatron with a radiation energy up to 25/VIev; features of th/s machine were the high radiation intensity and the stabilized operation of the individual units. In 1955-1056 several betatrons of this type were butlt. In the present paper there is gzven a brief description of certain of the results obtained In the research program on electron accelerators at TPI, the characteristics of one of the betatrons (radiation energy up to 25 Mev) are described. Results of the Scie_n.t!flc-E.ngineeri_ng On A c c e l e r a t o r s

Research

Program

The construction of these accelerators has been po~lble only as a resuk of a great deal of theoretical and engineering-design work by a number of people at the Institute. The original theory of electron capture In tile acceleration mode, which was verified experimentally, was proposed by M e h ~ o v [1]. The essence of this theory Is the following. As Is well known, the electron gun In the betatron Is supplied with short voltage pulses. As the injection voltage increases (leading edge of the pulse) the radius of the Instantaneous electron orbit increases and the amplitude of the electron oscillat~ons is reduced. Hence, durtng the electron Injection process the space charge dem~ty over the cross section of the beam is not uniform but Increases with time at the outer edge of the beam. This sltuauon results in an Inward displacement of the Instantaneous electron orbit, I.e., the orbR no longer colnczdes with the position of the electron gun. Calo culations indicate that the magnitude of this d~splacement may be very large, especlaUy when high currents are taken from the electron gun, the net resuh Is that electrons leave a "track" on the Inner wall of the chamber. At small gun currents the magnitude of the orbit displacement Is not great enough for the electrons to pass around the gun.

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As the injection voltage falls ofl~the instantaneous orbits are displaced outwardly and the electrons strike the gun. Mehkhov was successful in obtatmng oscillograms rbowmg the variation of current in the acceleration chamber as the electrons were introduced into the orbit. It was found that electron capture in the acceleration mode occurs mainly at the leading edge of the Injection pulse when the electron gun Is positioned outside the equilibrium orbit. By employing a voltage injection pulse with two peaks Chuehalin [2] was able to increase the radiation yield of the betatron by a factor of two. The following have been estabhshed: 1) electron capture m the acceleration mode is possible both at the leading edge and the traihng edge of the voltage ln;ecUon pulse; 2) after each electron capture process, which takes place over several revolutions, a large part of the entrained charge is collected (it may be assumed that this situation is the decisive factor which allows the remaining electrons to pass around the mjector); 3) Increasing the duration of the electron Injection process in the chamber by applying to the mjector a voltage pulse of "tdeahzed" shape, which coincides with the req~atred voltage curve, does not result m an Increase in the trapped charge; 4) an increase m the accelerated charge, lmplymg an increase m the radiation yield, can be achieved by the apphcatton to the injector of a voltage pulse with several peaks which mtersects the required voltage carve at many points, tlowever, these tntersecttons must occur only after deflrnte time intervals; these time intervals must be sufficiently long so that electrons which are captured in the acceleration mode associated with the first peaks of the voltage pulse can form a stable toroldal beam which will not interfere with the charge captured m the acceleration mode associated with the subsequent peaks m the injection voltage pulse. A method for obtatmng a unique value of the radius of the betatron equthbrium orbit for a given m a x i mum energy of the accelerated electrons was proposed by Fihppov [3]. This investigator obtained a linear equation relating the radius of the equlhbr~um orbit to the magnetic held parameters and the space between the pole paeces tn a betatron: re

=

BE

*

be ,

where E is the calculated maximum energy of the accelerated electrons, be is half the radial dimension of the cross section of the accelerator chamber, B b the "betatron constant" which depends on the material of the magnetic path and does not change for betatrons with different electron energies. This equation reflects the radial focussing properties of the magnetic field of the betatron and determines the tolerances m the fabricatton and assembly of the units of the magnetic path. This equation can also be used to compute the required changes in the appropriate dimensLons for the betatron parameters In "balancing" the electron orbits. A method for direct momtormg of the posit~on of the equthbrium orbit for circular accelerators and race-track accelerators has been developed by l~shchenko [4]. A system of windings, which are placed above and below the betatron chamber, are connected in accordance with the circuit shown in Fig. 1. The e m f e c which is induced in one coil is proportional to the mean value of the induction reside the orbit while the e m f e~ in the other coil, Is proportional to the instantaneous value of the induction at the orbit. A sensitive vacuum-tube voltmeter is used as a zero Indicator when the magnet ts supplied by do while an oscilloscope is used for pulsed operauon. This device makes it possible to observe a change in the radius of the equlhbrlum orbit as small as I ram. A mlcrophasometer has been developed by Anan'ev for measuring the phase inhomogeneity of the magnetic h e l d . In order to increase the sensitivity of the mtcrophasometer, shaped pulses are used; these are generated by sensing umts. Small phase differences are % m p l , 6 e d " m this scheme. After the voltage pulse generated by one of the sensing units Is shaped, it fires a trigger in a variable grid resistance umt (a diode In parallel with a high resistance). The pulse from the other sensing unit fires a second trigger; this generates a long rectangular pulse (5-10 "s see). Thn pulse is applied to the anode of the diode. In turn there is a change In the parameters of the circuit which determines d~e duration of the pulse associated with the first trigger. 13ecause of this trigger, the duration of the rectang,lar pulse at ti~e anode

314

depends on the instant at whlcll the diode is cut off, i.e., on the difference of phase of the magnetic field at the locations of the two sensing units. In this microphasometer the phase difference factor is multiplied by 28 times. The Indicating device is a mlcroammeter with a full-scale deflection of 10 ~ amp. A full-scale deflection, due to the change In the duration of the pulse associated with the hrst trigger, corresponds to a change in phase corresponding to 10 microseconds between the magnetic fields at the two locations of the pickup units. The effect of various tolerances in the construction of the individual units of the magnet on the operation of the betatron has been conside.red theoretically and verified experimentally by Anan'ev [5]. Equations have been obtained which relate the relative variation in the radius of the equill-

I

"~C ,r, r~ ..............

1

~m===m...~ I

~?r oscilloscope ~ y o l t m e t e r ) O e,

O :D

Pig. 1. Dtagram of the device for determining the equilibrium orbit radius.

brlum orbit re and the magnetic field index with errors in the dimensions of various parts of the magnet. It has been established that the greatest effect on re is that due to inaccuracies In the fabrication of the pole pieces of the magnet; the value of r 0 may deviate by as much as 10-40% from the calculated value because of this effect. It has been proposed that the adjustmeat of the betatron be faclhtated through the use of a method of controlling the orbit radius by displacement of the core. Several original accelerator designs have also been developed in the course of this research program.

Dimov and Noskov have developed and burr two versions of a pulsed betatron in which no iron Is used and in whzch energies up to 10 Mev have been achieved [6, '/]. The betatron consists of two coaxial coils and a central core cut from a copper plane. The coils are supplied by current pulses (the pulse repetition rate varies from 1-2 up to 20 per second). For a given energy the weight of this betatron Is approximately 10 times less than that of a betatron ha which an iron magnetic circuit Is used. Moskalev [81 has developed a betatron (15 Mcv) for therapy of cancerous tumors. A narrow beam of ?-rays Is formed by a lead collimator with an Interchangeable central insert by which the dimensions of the radiation field can be changed. Any nonuntformity In the dose distribution over the cross section of the beam is avoided by means cf a special copper filter of conical shape which is placed in the path of the beam. The properties of this beam and the dose distribution in orgamsm tissue were studied with a water dummy. The penetrating doses obtained with this betatron are considerably greater than those obtained by X-ray apparatus. The surface dose is several times smaller than the maximum dose which Is observed at 9 depth of 20 ram. Workers at this Institute, in conjunction with investigators at the Tomsk Medical Institute, have carried out experiments on animals, obtaining interesting results. In 1955, a synchrotron with electron energies up to 30 Mev [9]. using a new resonance system, was operated at TPI. In most existing electron synchrotrons the accelerating units are coaxial resonators, these, however, have a number of disadvantages (the relatively low quality factors which arise as a consequence of the limited dlmenstons of the space in which the resonator is located, inconvenient frequency adjustment, etc.). An acceleration system proposed by Vorob~ is considerably superior as regards frequency adjustment: this system consists of a two-conductor ltne loaded by the capacitance bctwcel~ 2 circular electrodes which surround the chamber along a gap in the conducting covering. Since this device occupies a very small space In the gap between the pole pieces one can select the optimum values for all geometric parameters. Thus, it Is possible

315

to obtain a quality factor and a shunt resistance which are larger than those which are achieved In synchrotrons with coaxial resonators. An analysis of the electron motion in the variable electric field produced by the circular electrodes indicates that the acceleration Is a difference effect: part of the energy obtained by the electrons in traversing the accelerating gap is lost tn the remainder of the orbit. It has been established e x perimentally that the energy lost in one tam is considerably smaller than the energy acquired in traversing the accelerating gap. The frequency adjustment is reaUzed by moving the shotting-bar on the line. The system being consldered is balanced and is especially convenrent when the radio-frequency generator usea.a push-puU ctrcutt (since a balancing arrangement is not needed). The input impedance of the device is matched to the feeder by varying the point at which the feeder is connected to the line. It should be noted that it ts easier to build a system of this kind than any kind of coaxial resonator system. One shottcommg of this system is the large amount of radiated electromagnetic energy; however, this radiation can he reduced to a comtderable degree by shielding. Kononov [lOJ has been successful m extracting an electron beam by means of a deflection condenser. A symmetric expanslon winding magnifies the magnetic flux in the central part of the pole piece, disturbing the t2 : 1 betatron condition = and the electron orbit widens. The orbit is increased gradually, reachmg an increment of 2 mm when the electrons enter the deflection condemer. The deflecting condenser consxsts of two plates, approximately 5 cm m length, which are bent to conform with the radius of curvature of the electron trajectory at the point at which the condenser Is located. The inner (interception) plate of the condemer is grounded while the outer (deflecting) plate Is fed by voltage pulses ranging from 20 to 40 kv, depending on the energy of the accelerated electrons. The voltage pulse is obtained through a pulse transformer. The useful duration of the pulse Is 300 microseconds. The pulse to the deflection condenser is apphed a little earlier than the current pulse In the expamlon winding so that the operating voltage has already been established when the electrons enter the condenser.

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,a

!Ill

N

Jk

A

n

w

i I lll

"qv

I ~lV

It

HI

~r

"f

Fig. 2. Electromagnet of the stereo-betatron. A, B) Possible locatiom for the two targets.

The , l e c t r o m are extravted from the betatron chamber through an aluminum foil 30 p thick. The extracted electron beam is focussed and at a distance of 50 m m from the wmdow is characterized by a cross section 5 -8 m m t . The beam dimensions at a distance of 50 cm from the window are 12 cm in width and 6 cm In height. These data refer to a 5-Mev b e a m . At energies of 10 Mev and above the beam focussing is better. The electron current in the beam, as determined with a Faraday cylinder and by chemical methods, is found to be 4 o10 -9 amps. The beam intensity, as measured with a thimble ionization chamber close to the output window, is 3300 r/rain.

316

Beam extraction by electromagnctlc methods has been achieved by Sokolov [10] in two ways. In the firstsystem~ a magnetic shunt is used, tlus shunt Is parallel to a tangent of the electron trajectory for electrons which move In a widening spiral at tlle instant of extraction. The effmlency of the magnetic shunt is small and the method makes It posnble to extract only about 2% of the total number of accelerated electrons, approximately 3 -10 "st i m p s . This low efficiency is due to three factors. 1) the presence of the shunt In the betatron gap space produces radial oscillations of the electrons even before they reach tile shunt channel {because of the weakening of the guiding magnetic field near the shunt)~ thus some of the accelerated electrons are lost, 2) at extraction the height of the electron beam Is considerably greater than the vertical dimensions of the shunt channel and thus the channel intercepts a small fraction of the accelerated elections; lncreasmg the height of the shunt channel does not result in any practical increase In the number of captured electrons since there ts a sharp drop in the efficiency with which the shunt affects the magnetic field. 3) the presence of a steel shunt In the operating volume of the betatron has a negative effect on electron capture In the acceleration mode at inJection as a result of which the radiation yield of the betatron Is reduced. The second method of electron extraction from the betatron chamber by an electromagnetic method uses a sectored winding which u placed inside the acceleration chamber above and below the equilibrium orbit, this winding is supplied by current pulses and has not been described In the literature. In this system a wInding is used such that the current pulse weakens the magnetic field by 10froover an azimuthal angle of 9 0 ; The divergence angle produced In the electron beam ts approximately 70 L The electrons in the beam are dtmd.buted nonuniformly. Depending on the fringing magnetic field of the betatron it is possible to obtain one, two, or more beams, which are directed within the limits of the angle indicated. In accordance with certain theoretical considerations developed by us the extraction of the electron beam from the betatron chamber may be considered as a hmiting case of the displacement of the accelerated electrons in the equihbrium orbit. Considering both symmetrical and asymmetrical displacements of the orbit (the first effect arises by virtue of the expansion winding while the second results from the segment winding which is located at a definite azimuthal region) It is found that up to 75~ of the accelerated electrons can be extracted. Recently two I5-IViev betatrons with external electron beams have been constructed at the Scientific-Research Institutes in Moscow. A small-size betatron yielding energies up to 30 Mev for use In medicine and for defectoscopy of large metal structures under factory conditions has been developed by Anan'cv [5]o The construction of the pole pieces of this betatron makes tt possible to operate the accelerator at inductions in the central core which are greater than the saturation induction of a given magnetic material. This pole-piece design allows a reduction in the weight of the magnet by a factor of 2-2.5. To reduce the losses in the steel a pulse supply Is used and the central core Is de-magnetized by a current pulse. The duration of the de-magnetizing current pulse is 5-10 times greater than the duration of the main current pulse. In order to reduce the size of the apparatus the de-magnetizing winding is located Inside the pole piece Itself. Thus, the %adtus of the equilibrium orbit and the dimensions of the working region are not affected. The Improved utilization of the magnetic properties of the material and thc reduchon In the size of the winding because of the pulsed supply result in a total reduction of 30-40% In the weight of the magnet. The magnet is symmetric; It is in the form of a closed cylinder. The dimensions of the apparatus are: height of the magnet 42 c m , diameter 59.5 cm, required power 6.5 kw, total weight, approximately 1000 kg. A machine provldmg this same energy, developed in Switzerland, weighs seven tons. The destgn of a stereo-betatron for defectoscopy and medical purposes, in which two accelerating chambers are used In conjunction with a nonbranchmg magnetic flux, has been proposed and carried out by Moskalev [11]. In essence this betatron Is a combination of two betatrons In one system with which it is possible to obtain four radiation beams which lntcrsect at one point {Fig. 2). When the betatron is used for medical purposes this arrangement makes it possible to carry out a quadruple radiation of a deep-lying tun)or. Consequently, the therapeutic dos (on the tumor) can be incrcased by a factor of four for a given surface dose.

317

@

, of

i

:~ '

t'fSO

Fig. 3. Magnet of the 25-Mev betatron. ~) Magnet yoke; 2) pole pieces; 3) inserts; 4) coils; 5) textolite gaskets.

318

It Is possible to arrange the intersections are located at different sides of the apparatus. 9 stereo-picture of two units simultaneously. possible in the determination of the depth of a

of the beams in pairs in such a way that the intersection points In defeetoseopy of indhstrlal products it is possxble to obtain The large base line for each pair of bantus makes high accuracy de~ect under the surface of the ptece.

A betatron has been used by Gorbunov [12J for laboratory research m defectoscopy of thick-walled p~eces. Using aspectally developed dostmeter apparatus it ~s possible to regulate the dose requtred for ~rrad~atzng the piece. Exposure tables and curves have been drawn up for various film-blackening densxt~er for dtfferent thicknesses of irradxated pieces. In 9 to those which have been described, a number of other accelerators have been developed at TPl; these include htgh ra&atzon-yield accelerators, a small-s~zed betatron for defectoscopy of factory products under xndustnal con&tions and a pulsed betatron wtth very htgh radzauon intensity. The betatrons developed at TPI are relanvely inexpensive, rehable, stable m operation, and easy to use. They are betng used successfully m natxve industry and medxcme where, as ts well known, the betatron yxelds valuable sctentff~c and techmcal results. Features

of the 25-Mev

Betatron

The magnet of the betatron (F~g, 3) has an E -shape yoke fabricated from electrotechmcal sheet steel 0.35 mm thick (E.-42). The sheets are insulated w~th a plastic lacquer. The shaped pole p~eces of the accelerator are assembled from stamped steel sheets of the same kind. The sheets are positioned radially. The total pole piece consists of 224 sectors w~th 19 sheets m each sector (F~g. 4). Two dLsks (inserts) of radially positioned steel sheets are placed in the central part bf the gap space.

Fig. 4. Pole piece of the betatron magnet.

The magnet Is supplied by an Industrial-frequency, alternating-current Une, thereby ehmtnating the need for comphcated and expensive high-frequency generators. The supply system is simple and rehable m operation (Fig. 5). The hne voltage (220-380 volts) is fed to the primary wmdlng of the magnet. The magnetic flux energizes 9 second (de-magnetizing) winding, in the circuit of which a condenser bank C with a capacity of '/0 ~f is connected (power of about 1000 kva) for compensating the reactive power requlred by the apparatus. With a voltage Uz = 6 kv at the terminals of the secondary winding of the magnet a current of 130 amperes Is drawn. Under these conditions the Induction in the steel in the central inserts is 13,100 gauss while the Induction in the pole pieces and the yoke ts 9,000 gauss. The shaped region of the air gap of the betatron occupies 11.5 cm in the radial direction. The height of the air gap at radius r0, the equilibrium orbit (r0 = 21 cm) is "/era. The weight of the steel In the magnetic c~rcuit is 3200 kg. The radial magnetic field index in the air gap is 0.75. Curves of the azimuthal inhomogenelty In the magnetic field are shown In Fig. 6.

319

The azimuthal inhomogeneity in the magnetic field at the equilibrium electron orbit has a sharply defined first harmonic which is as high as 8-I0 gauss If no correction measures are taken, The azimuthal inhomogeneity is reduced by means of special compensatzng windings which are located on the vertical yoke of the magnet circuit.

~ ~au~S

Recently a method has been developed in whzch equal "flux loadzng" of the yoke and supports in the magnetic crrcuzt zs achieved; thzs is done by appropriate d~vision of the yoke into secuons. In this case

/ #

|

i

,: i

.,a 9

90"

~0" ~, degrees

3

0"

Fig. 5. Diagram of the power supply for magnet of the betatron.

ZTO"

~SO"

Fig. 6. Curve showing the azimuthal inhomogeneztzes in the magnetic field of one of the betatrons, a) Before correction: b) after correctzon,

the azimuthal inhomogeneity in the magnetic field is redaced by a factor of 5-'/wzthout requmng any further correction. This method was developed by Flhppov, a worker at this Institute, and has not been pubhshed earhcr,,

9

Pig. 7, Glass accelerating chamber of the betatron with the injector and the deflector which extracts electrons from the chamber. The aluminum extraction window can be seen in the foreground.

320

"the accelerator chamber, fabricated from porcelain or glass (Fig. 7). has a port for connection to the vacuum pump and for installation of the electron injector. The chamber is ellipsoidal in cross section with a x e of 11 and 5.6 cm respectively.

6

t

k~

Le

/-m

i Fig. 9. Diagram of the pulse supply for the betatron injeCtor~

Fig. 8. Diagram of the three-electrode InJector of the betatron. 1) Cathode. 9) focmsmg eleCtrode, 3) anode, 4) target a = l m m , and b = 1 . 1 ram.

In Fig. 8 is shown a diagram of the three-electzode electron inJeCtor used in the betatzon. In Fig. 9 is shown s diagram of the pulsed injector supply. The shaping line 1 is charged through the rectifier 2 to a voltage of lo5 kv. At zero time.a pulse from a pem~alloy pickup unit located in the magnetic field of the betatron triggers thyratron 3 and the condensers are discharged through the primary winding of the pulse transformer. A voltage pulse, multiplied up to :30-35 kv, is applied to the cathode of the injector. The working hfe of the injector is determined by the useful life of the cathode.

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f

P.U.

R

-1".

_-~%

!lYI1 Pig. 10. Diagram of the supply for the winding which displaces accelerated electrom from the equihbrium orbit. Vt-VG-213, Vt-VG-129, V$-TGI-200, V4-TGI-400/16,

At the end of the acceleration cycle a current pulse is applied to a special winding located in the central core or in the pole pieces of the accelerator above and below the acceleration chamber, thereby disturbing the betatron relation, as a result, the electrons move in an ever-increasing spiral and are collected on the target (a tungsten slab fastened to the back end of the injector anode). The accelerated electrons are decelerated in the target and emit very hard radiation. A diagram of the supply which furnishes current pulses to the displacement (or expansion) winding is shown in Fig. 10. Condenser C x is charged through the gaseous rectifier 1 (Vs) to s voltage of 200-250 volts while condemer C t f= charged through rectifier 2 (V1) to a voltage of 4 kv. A trigger pulse from the permalloy pickup unit is fed through a delay circuit to the grid of thyratron V=. Condenser C1 discharges through thyratron V= and resistance Rt as a result

321

of which a triggering pulse is applied to the grid of the following thyratron V4. Condenser Ct discharges through V 4 and the winding, expanding the orbitof the accelerated electrons. All the electronlc gear Is built in units and is mounted in a small control panel.

The radiation yield of the betatron is 60 r/rain at a distance of one meter from the target. The 25-Mev betatron has been installed at the Tomsk Medical Institute and is being used for clinical and research puzposes. LITERATURE C I T E D [1] V, S. Melikhov, Dissertation TPI (Tomsk Polytechnic Institute) (1954).,

~J I. P. Chuchalin, Dissertation TPI (Tomsk Polytechnic Institute) (1955). [3] M.F. Filippov, Dissertation TPI (Tomsk Polytechnic Institute) 87, 67 (1957). [4] I.G. Leshchenko, Dissertation TPI (Tomsk Polytechnic [I~titutr

(1955).

[5] L. M. Anan'ev, Dissertation TPI (Tomsk Polytechnic Institute) (1956). [6] G. I. Dimov, Dissertation TPI (Tomsk Polytechnic Institute) (1954). [7] D. A. Noskov. Dissertation TPI (Tomsk Polytechnic Institute) (1054)o [8] V. A. Moskalev, Dissertation TPI (Tomsk Polytechnic Institute) (1953). [9] B. A. Solntsev, Dissertation TPI (Tomsk Polytechnic Institute) (1955).

[10] Borob'ev et al., lzvestla TPI 82, 149 (I956). [11] V. A. Moskalev, I. Tech. Pays. 26, 2060 (1956). [/2] V . l . Gorbunov, lzvestta TPl 87,411 (I957).

Received September 20, 1957.

Enghsh Translation.

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