US2531028A - Electron accelerating apparatus - Google Patents

Description

Nov. 21, 1950 N. c. CHRISTOFILOS OR N. c. PHILOS 2,531,023

summon ACCELERATING APPARATUS Filed July 25, 1946 s Sheets-Sheet 1 HIGH FREQUENCY FILTER mew F' EQuENcv FILTER HIGH FREQUENCY FILTER HIGH vovfass E Z9 .14 0 3 SOURCE 25 INVENTOR l5 ,pJG N. C/if/GfOff/OS OK 1-; W572; N. C. Phi/05 w BY wha k 20% m):

ATTORNEYS Nov. 21, 1950 N. c. CHRISTOFILOS OR N. c. PHILQS 2,531,023

ELECTRON ACCELERATING APPARATUS Filed July 25, 1946 3 Sheets-Sheet Z MAGNET sxcmmou 001 LS FIELD B CITER DIRECT CURRENT GENIRATUR FLYWH EEL INDUCTION MOTOR ELECTRONIC RELAY A INVENTOR C/HJSjZJ/IVOS 0;: N. C! H/L 05 ATTORNEYS Eatented Nov. 21, 1950 UNITED STATES PATENT OFFICE Nicolas C. Christofllos or Nicolas C. Philos,

Athens, Greece Application July 25, 1946, Serial No. 686,254 In Greece January 25, 1946 8 Claims. 1

The present invention relates to apparatus for accelerating electrons by means of a high frequency electric field while they are guided in a circular orbit within an evacuated chamber by a time varying magnetic field.

The principles and theory of this apparatus otherwise known as Synchrontron are well understood and described in prior publications, by the Russian scientist V. Veksler, the American scientist Dr. E. McMillan and others and the object of present invention is the practical application of these principles.

A major problem in the operation of apparatus of this type consists in the provision of suitable means for obtaining constancy of the orbit radius during the whole course of acceleration. As the method and the means obtaining the constancy of the orbit radius were described in application Ser. No. 752,866 (filed June 6, 1947), it is considered herein that the orbit has a constant radius without describing the means obtaining this constancy.

The main objects of the present invention are a new way for focusing the accelerated electrons electrostatically, means for producing a time varying magnetic field by means of a'pulse operated D. C. generator, the way to produce the high frequency accelerating electric field by means of a carrier whose frequency is higher than the gyration frequenc of the accelerated particles.

The invention, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawings in which:

Fig. 1 is a plan view of the apparatus with parts in section taken on line I-I of Fig. 3;

Fig. 2 is a schematic representation illustrating diagrammatically the mode of connection of the feeders, illustrated in Fig. 1, to the output coil of the high frequency source;

Fig. 3 is a section taken on line 11-11 of Fig. 1;

Fig. 4 is a schematic representation illustrating diagrammatically one mode of energization of the magnet coils by means of a D. C. generator and the manner said generator is pulse operated.

Fig. 5 is a section of the glass evacuated vessel on line II-II of Fig. 1;

Fig. 5a is a section of the glass evacuated vessel on line III-III of Fig. 5; and i Fig. 5b is a section of the glass evacuated vessel on line VV of Fig. 5a.

The apparatus consists of a glass vessel It (Figs. 1, 3 and 5) having within it an annular evacuated chamber II and a magnet l2 enclosing the glass vessel along its entire length. As will be explained later, the vessel l0 encloses a circular orbit of radius Ra in which electrons may be accelerated to a high energy value, say on the order of several hundred million volts.

The cross section of the vessel [0 is circular or elliptical. To the inner surface of the glass v ssel I0 is applied a metallic coating, such as a thin layer of silver. This metal layer is interrupted at fixed and equal intervals, so that the glass vessel is electrically divided into sectors whose number is even (in Fig. l the glass vessel 10 is divided into 4 sectors). Between the vessel I0 and the magnet 12 a tube [3, Figs. 1 and 3, is inserted, the walls of which have a small thickness and are made of copper or other non-magnetic metal of great conductivity. The tube I3 surrounding the vessel It along its entire length is continuous and has no interruptions.

The sectors of the metal layer of the vessel [0 constitute along with tube [3 an equal number of condensers, which have one electrode (the tube I3) common and earthed.

The metallised surface of the vessel II] as well as the tube 13 are connected by means of suitable coaxial feeders l4, [5 (Figs. 1, 2, 3) to a high frequency amplifier It.

This connection is made up in such a manner (as represented schematically in Fig. 2) that each sector should have a phase displacement of relative to the foregoing as well to the following sectors.

The feeding of each sector is efiected in a plurality of points as is shown in Fig. 1, so that no stationary waves should be produced along its length.

By this connection of the glass vessel sectors with an high frequency amplifier a high frequency electric field is produced in the gaps I"! (Fig. 1) between the sectors. The electric field produced in a gap between two sectors in succession has also a phase displacement of 180 relative to the foregoing and following gaps.

Inside the vessel l0, rings l8 (Figs. 1, 3, 5, 5a, 5b) are arranged at small intervals. These rings are of circular or elliptical shape similar to the glass vessel section and are made of metal tubes or suitably curved metallised bars of plastic material. The cross section of these rings or bars has an elliptical or circular shape. These rings are electrically insulated from the internal metal layer of the vessel Hi. All rings situated in a sector, of the metal layer of the glass vessel III, are

connected with each other by means of conductors l9 (Figs. 1, 3, 5, 5a, 5b). Thus the rings form several groups, which are electrically insulated from each other and equal in number to the sectors of the metal layer of the glass vessel. All ring groups are connected by means of conductors 20 and high frequency filters 2| (Fig. 1) to the one pole of a high voltage source 22 (in case of acceleration of electrons this pole should be the negative one) whereas the other pole is connected to the metal layer of the glass vessel.

In Fig. 1 this pole is earthed but the connection to the metal layer is effected through the feeders l4, l and the coil 23 (Fig. 2) of the H. F.

As a result of the ring charge, an electrostatic force is exerted on the particles which focuses the latter toward the axis 24 of the vessel l0 and compensates the mutual electrostatic repulsive forces.

In order to increase the horizontal electrostatic force, the magnetic poles 25 (Fig. 3) between which the initially mentioned magnetic field is generated, are formed in such a manner that a magnetic focusing force is also exerted on the accelerated particles. To this purpose, the distance between the magnetic poles 25 is variable depending on the radius so that the intensity (H) of the magnetic field varies as a function of the radius R according to the equation where K 0 and A is a constant.

Due to the above described formation of the magnetic poles a force is exerted on the particles as a result of which the vertical component of the electrostatic focusing force is slightly decreased while the horizontal component is increased.

By the focusing method explained above a high particle density (of the order of coul./cm. can be obtained, which density remains almost constant during the whole course of acceleration. For given dimensions of the evacuated glass vessel II] it is therefore possible to accelerate a great charge of particles resulting in a great efliciency of the apparatus and more economical construction thereof.

In the case of acceleration of electrons the focusing rings are negatively charged, and the coefficient K is subject to the following limitation where 130 and V0 represent the initial velocity and energy of the electrons injected into the apparatus, Ra represents the orbit radius in cm. and ey=6y /6y (;y is expressed in lrv./cm. and V0 in k. volts) where (e) is the potential produced by the charged rings and y the horizontal axis (Fig. 5). The reason for this limitation will be explained below in th description of the operation of the apparatus.

The injection of the particles into the glass vessel is effected through one or more suitable inlet tubes 26 (Figs. 5a and 5b) which are soldered at a small angle 0 on the glass vessel. The particles are injected into the vessel ill from the outside, because, the must be previously accelerated up to an energy of, at least, several hundred thousands electron volts.

Under the influence of the strong electrostatic field the particles injected at the angle 0, are prevented from striking against the walls of the glass vessel and are set in motion parallel to the orbit, simply undergoing, because of the injection at an angle 0, a vertical harmonic oscilla- 4 tion around the Z axis (Fig. 5). This oscillation is, however, rapidly damped on account of the high frequency currents inducted from the oscillating particles on the rings.

The magnet I2 is divided in upper and lower sections (21 and 23 respectively in Fig. 3). The lower section 23 can be built up by suitable means to form a one piece unit. The upper section 21 can be built up into units weighing a few tons each, so that they may be easily lifted up. The parts of the magnet surrounding the particle injecting tubes consist of smaller pieces on which suitable holes are opened so that said tubes can pass through.

In the cross section of the magnet (Fig. 3) are also shown the glass vessel l0. the tube l3 and four coils 30, 3|, 32, 33 (Figs. 3 and 4). The magnet is excited by direct current. The acceleration takes place during the time the magnetic field is formed by the D. C. source. During this time interval the magnetic field intensity varies depending on the time accordin to the equation Where T=L/R the time constant of the magnetising coils and 92; 1 -n 1' where Ta represents the duration of acceleration and e the base of the Neperian logarithms.

The general arrangement for the operation of the apparatus by D. C. excitation of the magnetising coils and the wiring diagram thereof are schematically represented in Fig. 4. The magnet excitation coils 30, 3!, 32, 33 are energized by a D. C. generator 34. On a common shaft with the generators 34 are connected 9. fly-wheel 35, an induction motor 36 and a small D. C. generator 31. The excitation field of the generator 34 is energised by the generator 31. The excitation coil 38 of the generator 34 is connected to the generator 31 by means of two switches 39, 40. By connecting and disconnecting the switches 39 and 40 alternately, the magnetic field of the apparatus is charged and discharged periodically by means of the generator 34.

During discharge of the magnetic field, the generator 34 operates as a motor (by reversion of the current direction in the excitation coil 38 thereof), the energy of the magnetic field being transferred into the fly-wheel 35. When the magnetic field is charged, the energy required for charging it is taken from the fly-wheel 35. The motor 36 supplies the system merely with the thermal losses.

The operation of switches 39 and 40 is effected by means of the electronic relay 4|, the operation of which is controlled by the voltage drop in the resistor 42 through which passes the excitation current of the magnetic field of the apparatus. The operation is as follows:

Let us suppose that the magnetic field is charged when the switch 39 is closed. When the field intensity attains its maximum value Hmax (corresponding to an excitation current Imax and a voltage drop in the resistor 42, Emax) the switch 39 opens. After a short time durin which the energy of the excitation coil 33 is absorbed by the resistor 43, the switch 40 closes and the magnetic field of the generator 34 is reversed, the latter operating henceforth as a motor. The magnetic field of the apparatus is then discharged and when the intensity of the magnetic field on the orbit becomes zero the switch 40 opens. After a short time the switch 39 closes and the charging starts again. The maximum value of the intensity of the magnetic field at the orbit is found from the well known equation (B O-an Where Hmnx is in gauss, Ra the orbit radius is in cm., ,6 the maximum relative velocity of the particles (flamed) and vmax the maximum energy of the particles obtained in the apparatus is in electron volts.

The frequency of the high frequency accelerating field varies during acceleration according to the equation where c is the light velocity in vacuo and n the number of pairs of sectors of the metal layer of the glass vessel l, and p the velocity of the accelerated particles relative to the light velocity in vacuo.

The method by means of which the frequency f varies linearly with the velocity 5 is not described herein as it is set forth in application Ser. No. 752,866.

The energy U6 that the particles must acquire while crossing each gap is. as we know where L is the loss of energy of the particles per revolution, due to radiation. The maximum voltage Vo which must be produced in the gap ll between two sectors must be considerably higher than U6 namely U=V.Ut (6) while the frequency of the radial oscillations is c eyRa where e2=6 e/62 in kv./cm., ey=t e/6y and (e) is the potential of the electrostatic field produced by the charged focusing rings, 0 the light velocity in cm./sec., Ra the orbit radius in cm., Vo the energy of the injected particles in eKvolts.

From the initially injected particles there remain after a few revolutions n groups of flattened ellipsoid-like shape the great axis of which is an arc of the particles orbit of a radius Ra. The particles inside the groups trace elliptical orbits around the central particle (which crosses the gaps at the moment the voltage 0: U6) oscillating harmonically around it.

The frequency In of this oscillation, otherwise called by Dr. McMillan phase oscillation" is (Pug where Io is the gyration frequency of the particles, n the number of pairs of sectors of the metal layer of the vessel I0, V the particles energy,

It results from Equation 8 that phase stability and accordingly the possibility of particle acceleration exists as long as It results from this inequality that phase stability is possible in two cases.

(where dU/dt 0) p This case is the known principle of phase stability of the apparatus Synchrontron proposed by the Russian scientist V. Veksler, the American Dr. E. McMillan and the present inventor (Greek Patent No. 10,579 filed January 25, 1946) independently of each other, the first two scientists using the known focusing system of Betatron apparatus (where K 0), whereas the present inventor proposed the focusing system described above.

In this latter case to have 0 during the whole course of acceleration it is necessary (as it results from Equation 10) that the following inequality must be fulfilled:

2M1 K+ fioVo fi0 (108') This inequality is identical with the inequality 1a.

2. a 0, uo 0 This case proposed for the first time is possible only with the focusing arrangement proposed herein. In this case the following inequality must be fulfilled.

cmRa a 7 must be fulfilled. Considering that po -1 this inequality can be written:

eyRa' where V and rg oRa in mv. and 410:6116/81 where yo is the potential of the electrostatic field, produced by the charged focusing rings, at the time of the im'ection of the electrons or where Ra is in meters and V0 in megavolts. At the end of acceleration the following inequality must be fulfilled:

volts/cm. (13) where Ra being in meters and vmax in mv. From the inequalities 12 and 13 it is obvious that eymsxy0- As both inequalities must be fulfilled, the voltage of the focusing rings should be variable during acceleration. This is obtained by connecting the focusing rings to the negative pole of the high voltage source 22 (Fig. 1) through a resistor 29 (Fig. 1) this connection being effected by means of a switch (not shown in Fig. 1) closing at the moment the switch 38 (Fig. 4) closes. The resistance R0 of the resistor 29 is Ta R=M- where Ta is the duration of acceleration, Co the capacity of all ring groups (including the capacity of the interconnecting conductors I3, 20) and M, a numerical constant. The value of the constant M is 1 to depending on the value of the voltage of source 22 and the value of 62 max so that 62 varies approximately during acceleration according to the equation 2 20I o After the electrons have been accelerated to the desired degree they are deflected from their orbit so that they may be intercepted by an X-ray producing target or may be collected into a beam for ejection outside the glass vessel 10. These operations could be obtained by means of suitable devices, already described by other inventors, provided for Betatron" apparatus and which could be, applied in the described apparatus.

While the invention has been described by reference to a particular embodiment, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the invention. The appended claims therefore are designed to cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:

1. An electron accelerating apparatus comprising means for producing a high frequency accelerating field, means for producing a time varying magnetic field for guidin the accelerated particles in a circular orbit within an annular evacuated glass vessel, said vessel having its inner surface metallised, said metallised surface being divided into an even number of, electrically insu- 8 lated sectors, said sectors being connected to a high frequency amplifier in such a manner, so that in the gaps between said sectors a H. F. electric field having a phase displacement of to the preceding and following gaps is produced.

2. An electron accelerating apparatus comprising means for producing a high frequency accelerating field means for producing a time varyin magnetic field for guiding the accelerated particles in a circular orbit within an annular evacuated glass vessel, said vessel having its inner surface metallised, said metallised surface being divided in even number of, electrically insulated sectors, said sectors being connected to a high frequency amplifier in such a manner so that in the gaps between said sectors a H. F. electric field having a phase displacement of 180 to the preceding and following gaps is produced, said sectors being connected to the high frequency amplifier at a plurality of points so that no stationary waves may be produced along the length of said sectors.

3. An electron accelerating apparatus comprising means for producing a high frequency accelerating field, means for producing a timevarying magnetic field for guiding the accelerated particles in a circular orbit within an annular evacuated glass vessel, said vessel having its inner surface metallised, means for producing an electric potential gradient sloping from points inside said vessel which are radially displaced from said circular orbit inside said vessel, the polarity of said potential gradient being such as to accelerate the accelerated particles toward said circular orbit.

4. An electron accelerating apparatus comprising means for producing a high frequency accelerating field, means for producing a timevarying magnetic field for guiding the accelerated particles in a circular orbit within an annular evacuated glass vessel, said vessel having its inner surface metallised, means for producing an electric potential gradient sloping from points inside said vessel which are radially displaced from said circu'ar orbit inside said vessel, the polarity of said potential gradient being such as to accelerate the accelerated particles toward said circular orbit, said potential gradient being effected by means of suitable conductors placed inside said glass vessel, said conductors being charged at an electrical potential being of the same polarity relatively to the inner metallised surface of said glass vessel, as the polarity of the accelerated particles.

5. An electron accelerating apparatus comprising means for producing a high frequency accelerating field means for producing a timevarying magnetic field for guiding the accelerated particles in a circular orbit within an annular evacuated glass vessel, said vessel having its inner surface metallised, means for producing an electric potential gradient sloping from points inside said vessel which are radially displaced from said circular orbit inside said vessel, the polarity of said potential gradient being such as to accelerate the accelerated particles toward said circular orbit, said potential gradient being effected by means of suitable conductors placed inside said glass vessel, said conductors being charged at an electrical potential being at the same polarity -relatively to the inner metallised surface of said glass vessel, as the polarity of the accelerated particles, said conductors being rings, surrounding said circular orbit, arranged at small intervals along said orbit inside said glass vessel.

the plan of said rings being normal to said orbit, said rings being connected electrically to each other along a sector of the inner metallised surface of said glass vessel forming ring groups whose number is equal to the number of sectors of said metallised surface, each group being connected through suitable high frequency filters t the one pole of a high voltage source the other pole being connected to the inner metallised surface of said glass vessel, so that the polarity of the rings being the same as the accelerated particles.

6. An electron accelerating apparatus comprising means for producing a high frequency accelerating field, means for producing a timevarying magnetic field for guiding the accelerated particles in a circular orbit within an annular evacuated glass vessel, said vessel having its inner surface metallised, means for producing an electric potential gradient sloping from points inside said vessel which are radially displaced from said circular orbit inside said vessel, the polarity of said potential gradient being such as to accelerate the accelerated particles toward said circular orbit, said potential gradient being effected by means of suitable conductors placed inside said glass vessel, said conductors being charged at an electrical potential being of the same polarity relatively to the inner metallised surface of said glass vessel, as the polarity of the accelerated particles and in combination with said focusing potential gradient, means for producing a magnetic field substantially perpendicular to the orbit plan and increasing in intensity the said orbit radius increases.

7. An electron accelerating apparatus comprising means for producing a, high frequency accelerating field, means for producing a timevarying magnetic field for guiding the accelerated particles in a circular orbit within an annular evacuated glass vessel, said time-varying magnetic field being substantially perpendicular to the orbit plan, said magnetic field produced by means of a magnet surrounding said glass vessel along its whole length, the excitation coils of said magnet being energized by means of a direct current generator, the excitation field of said generator being alternately reversed by means of suitable switches as th magnetic field intensity at said orbit reaches predetermined maximum and minimum values so that said generator operates alternately as a motor discharging the magnetic field energy and transferring said energy to a fiy-wheel and as a generator taking up energy from said fly-wheel and charging the magnetic field, the acceleration of the particles being efiected during the charge of said magnetic field.

8. An electron accelerating apparatus comprising means for producing a high frequency accelerating field, means for producing a timevarying magnetic field for guiding the accelerated particles in a circular orbit within an annular evacuated glass vessel, means for injecting the accelerated particles inside said vessel, said injection being effected through suitable tubes soldered under a small angle on said glass vessel.

NICOLAS C. CHRISTOFILOS.

OX NICOLAS C. PHILOS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITEDSTATES PATENTS Number Name Date 2,084,476 Brown June 22, 1937 2,193,602 Penney Mar. 12, 1940 2,245,670 Hollman June 17, 1941 2,331,788 Baldwin Oct. 12, 1943 2,349,011 Smith May 16, 1944 2,394,070 Kerst Feb. 5, 1946 2,394,072 Westendorp Feb. 5, 1946 2,394,073 Westendorp Feb. 5, 1946

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