US2550212A

US2550212A - Magnetic induction accelerator

Info

Publication number: US2550212A
Application number: US716816A
Authority: US
Inventors: Wideroe Rolf
Original assignee: BBC Brown Boveri France SA
Current assignee: BBC Brown Boveri AG Germany; BBC Brown Boveri France SA
Priority date: 1945-02-17
Filing date: 1946-12-17
Publication date: 1951-04-24
Anticipated expiration: 1968-04-24

Description

April 1951 R. WIDER CSE MAGNETIC INDUCTION ACCELERATOR Filed Dec. 17, 1946 Patented Apr. 24, 1951 r PATENT O F FICE.

MAGNETIC "INDUCTION ACCELERATOR? Rolf lwideriie Oslo; Norway,.assignor to Aktiengesellshaft'Biown, Boveri & Cie., Badcn, 'Swit' zerland, a joint-stock company Application December 17, 1946',

Serial'No. 716,816

In Germany February 17,1945.

Section 1, Public Law 690, August 8, 1946 Patent expires February 17, 1965 Thissinvention relates to devices for accelerat- 3 Claims. (Cl. 313-432) ing electrically charged particles such as elec-" trons to a high velocity and hence high potential by means of magnetic induction-effects. These devices-which are sometimes referred to as ray transformers are comprised generally of an evacuated annular tube into which electrons are introduced from an electron emissive cathode or electron gunand a magnetic system which produces a magnetic field varying with time hav-' ing a space distributionsuch that the injected electrons are accelerated by the field along a circular orbit. The magnetic field divides into components, onecomponent being known as the; in-

ducing field which produces the electron accelera tion and the other component being known as the control or guiding field which produces a centripetal effect upon the electrons to offset the centrifugal forces J resulting from the circular motion. The electrons have been found to reach can 'thenbe diverted from the circular orbit for producing desired results such as for example to bombard'a target to produce X rays.

Arrangements already known for diverting th enormously high velocities at which time they circling electrons'from their circular orbitinelude "electrostatic or auxiliary'magnetic fieldproducing means which'are switched in towards the end of the acceleration period and function to drive the electrons into a path of constantly increasing radius, i. e. into a spiral outwardly.

In another known device for carrying the electrons from their circular path, two deflecting electrodes similar to the deflecting plates of a Braun tube are arranged at opposite sides of the circularpath andv a voltage is applied to these electrodes at the proper time. The suddenly applied electrostatic field between the electrodes causes the electrons 'to pull out of their circular path and enter asecond constant electrostatic field in whichthey are finally deflected to the target or into a reaction chamber. Inorder that'disturbing infiuences should not cause the circling electronsi-n the vacuum tube to deviate from" the equilibrium" circle and pass to the walls of the tube, it'isnecessary to intro duce stabilizing forces which force the electrons from all. sides towards the centre of the tube.

Such stabilizing-forces can be produced-in a known manner as disclosed in United States Patent No. 2,l03,303--St'eenbeck, so that the magnetic field strength of the control field of the electrons decreases outwardly, namely somewhat less'than proportionately with 1/r.(for instance 0 F;

trol over the electron stream so'that the latterv maybe directed accurately into a reaction cham- 1' being the distance as measured radially out-e" ward from; the central axis a-a of. Fig. 1.. In

this 'Jwayit isi possibleto produce-a stabilizingci force which. increaseswhn movingfloutwar'dlyf frQmtheequiIib'riumi circle. On account-of the" width of the control'nfield being limited, due to' constructional reasons, the stabilizing force produced in .this. manner 'lWill reach a maximum value a certain distance awaymfromthe equilibrium circle and then rapidly decrease toze'ro asthe distanceincreasesstillfurther." Because of thisgthe electron "stream when pulled outwardly from the: equilibrium circle for target-'- bombardment or for lead-off to a reaction cham-' ber enlarges so rapidly immediately'upon passing theimaximum of the stabilizing field that it results inrvirtu'ally an explosion of the'eleotron stream and therefore renders it mostdifiicult to rthereafter control the'path of the stream, with the result rthatithe stream misses the target ore reaction chamber completely.

As an example of the difficulties encountered:

in enlarging the'electron streamafter the latter; has=attained its-ultimate velocity, let it be as--;-

sum'ed thattheinduction accelerator-is designed-- for 15 million" volt operation with an equilibriumcirclediame'terof 14 cm.-and the maximumvalue of the auxiliary stabilizingfield lying outwardly on'a circlehaving a' diameter of about 17.2 cm;-

When the electrons havereached their final-" velocity and'the equilibrium circle is then gradually-expanded, such as-by saturation ofthe condistance increases at a most rapid'rat'e so' that after 'elev'en revolutions it has: increased by .5

mm.."wh ile; after twelve revolutions it has in creased by 3' mm, and after-.twelvef'and a half revolutions bylO mm. With about'12.8'revolu;

tions the distance will haveincreasedto an ex tremely great value.

The general objectof this invention is to provide means for counteracting-the unduly rapid expansion of the electron equilibrium circle after it passes the maximum of the auxiliary stabilizing field force to thereby maintain sufficient conber or against a target anode. 7

As will be explained in more detail hereinafter the I desired result; is attained through: the use lof 3 one or more electrostatic deflecting devices in the tube.

The invention is illustrated in the accompanying drawings in which Fig. 1 is a vertical section through one form of induction accelerator to Which the invention is applied, Fig. 2 i a horizontal sectionon lines 2--2 Of Fig. 1 with only the tube element being shown, and Fig. 3 is a horizontal section of the annular electron chamber illustrating a "modified form of the invention.

Referring now to the drawings, the electron induction accelerator shown in Fig. 1 which is symmetrical about axis aa is comprised of'a magnetic field structure made up from steel laminations of appropriate contour and includes a pair of cylindrical pole pieces I ll I separated by air gap I2 located concentrically along axis (1-11 and a pair of annular poles |3-l3' facing each other and separated by air gap I4. Yoke members l5 complete the magneticcircuit for the time varying flux set up in the annular and cylindrical pole pieces. Poles ll-| l and |3 l3 are surrounded by an annular winding preferably split into two coil sections l'6|6' which are wound in the same relative direction and connected in series for energization from an alternating current source I! of suitable frequency which may, for example, be 100 cycles/sec.

An annular evacuated glass tube [8 rests in the air gap l4 between the annular poles l3l3' and thereby surrounds a part of the axial poles II- II'. Charged particles are introduced periodically into the tube from a coiled filamentary electron emissive cathode which is energized periorically in known manner in timed relation with the current alternations in the alternating current source I1 so that a stream of electrons are produced each time that the current passes through the zero point in its swing positive. The cathode 20 may be located within the tube l8 proper or, as illustrated, in an arm portion 22 arranged tangentially of the tube with electron guide means, not shown, to get the electrons into the orbit in which they are to be accelerated. As the magnetic flux builds up in a positive direction in poles. I l-I l, the injected electrons are caused to be constantly accelerated from their injection velocity. As the electrons accelerate under the influence of the changing flux in poles Ill I, their centrifugal tendencies are offset or counterbalanced by a centripetal force that is produced by the component of the time varying fiux that threads axially across the tube t8 at the air gap I 4 between the annular pole pieces I3-l3'. Thus the electrons are forced to follow a circular orbit while being accelerated round and round the tube as the energizing current in coils l6--l6' is increasing positive in its cycle.

Theoretically, the centripetal force produced by the magnetic field existing at the annular or guiding poles I3-'l 3' will balance the centrifugal tendencies of the electrons when the following relationship is established V =21rT 2H1 where is the flux included within the electron orbit, 1" is the radius of the orbit, and Hr is the field strength at the orbit.

The field relationships in accordance with the above equation are usually attained by making the reluctance of the magnetic path appropriately greater at the particular electron orbit than its average reluctance within the orbit.

After the electrons'have been accelerated to their final velocity, which occurs as the magnetic indicated by the line 32.

flux reaches its maximum they are drawn off their orbit in known manner such as disclosed in the aforesaid Steenbeck patent by eifecting a saturation of a section I 3a of the guide poles l3-I3 having a reduced cross-sectional area which produce flux Hr and caused to impinge upon a target 23 that may be located in another tangentially arranged arm 24 leading from tube is diametrically opposite arm 22.

The stabilizing field which when a disturbance occurs, prevents the electrons from leaving the equilibrium circle, has its maximum value on a circular orbit 21. To prevent the electron explosion effect which takes place as the electrons pass the stabilizing field force maximum orbit 21 while being drawn outwardly from their equilibrium circle 25, I provide an electrostatic deflect ing device comprising a pair of spaced electrodes 2829 which are supplied with direct current from suitable source illustrated by battery 35] or alternatively by a voltage impact at the proper instant. While both electrodes may be constituted by plates, I prefer to construct the inner electrode 28 as a thin wire placed perpendicular to the tangential and radial movement of the electrons and the outer electrode 29 as a plate.

This arrangement has the advantage that the lines of electrical force from plate 29 become drawn together at the surface of the wire 28 and hence any electron entering between them and close to wire 28 approximately on a path following the line 3! will be subjected to a much higher field strength than would be an electron traveling along a further outward path such as Thus the electrons which have already travelled a greater path in the radial direction than the electrons located still further inward are deflected somewhat less and the collection of the rays into a bundle is improved thereby over a deflecting device consisting of two like electrode plates.

The use of a thin wire as the inner electrode is also preferred because it can absorb from the impinging electrons an energy that is small in comparison with the wire surface and therefore does not heat up to an unallowable extent. The ratios become better as the wire electrode 28 is made thinner. A mechanically compact material, melting at a high temperature is preferred for the wire. The explosive effect of the electrons as they pass outwardly of the auxiliary field force maximum circle 21 is avoided by placing the electrodes 28-29 at such a distance from each other and so selecting the thickness for the inner electrode 28 that the electrons, led spirally outwardly from the equilibrium circle 26 such as by saturating the control or guiding field poles l3l3 to the auxiliary stabilizing field force maximum circle 21, thereafter describe in one revolution a greater path in a radial direction than the thickness of the inner deflecting electrode 28 but which nevertheless is smaller than.

the distance between the two electrodes 2829. With the electrodes so situated, substantially all the electrons in the outwardly spiraling stream pass between them and are deflected into a bundle which is then passed to the target anode 23.

In a modified construction of the invention shown in Fig. 3, a second electron deflecting device comprising a pair of plates 3334 energized from a direct current source 35 is placed in the tube It at a angle from the electrodes 28.29 to provide a further deflection of the electrons after the latter leave the electrodes 28-29. Upon an inspection of the drawing, it will be seen that when the two sets of electrostatic deflecting electrodes are located at 90 away from each other, the position of the electrodes 28-49 can be adjusted slightly in relation to the auxiliary stabilizing field force maximum 2'! by shifting the tube I8 in the direction of the double arrow 35 without changing the position of the electrodes 33-3 1 relative to the auxiliary field force maximum. The two sets of electrodes can thus be made fast within the tube 33 and any fine adjustment which may become necessary is made by shifting tube l8 laterally in one direction or the other along the axis of the double arrow 36.

If desired, an electron lens which may be of the magnetic type or an electrostatic type as illustrated in the drawings by the plates 3l-38 charged from a suitable source can be installed between the target 23 and deflecting electrodes 33--34, the lens being energized for a brief instant at the moment that the electron stream is pulled out of its acceleration orbit 26 to lead the electrons out of the tube.

I claim:

1. A magnetic induction accelerator including an evacuated tube within which charged particles such as electrons may follow a closed orbital path,

- means for providing charged particles within said tube, means adjacent said chamber for cyclically producing a time varying magnetic field having a space distribution such as to produce a stabilizing field on said electrons to confine said electrons within said tube to substantially a predetermined circular orbit while continuously ac celerating them along said orbit, themaximum value of said stabilizing force being located on a circle lying outwardly of said electron orbit, means for expanding said electrons spirally outwardly from their orbital path, and an electrostatic electron deflecting device within said tube for guiding said electrons after the latter, pass the maximum of said stabilizing force in their spirally outward path, said deflecting device comprising a pair ofspaced electrodes within said tube and which are spaced radially from each other, the innermost of said electrodes lying rae dially outward of said circle which defines the maximum of said stabilizing force and occupying a position such that th path taken bythe spirally expanding electron passes through the space between said electrodes.

2. A magnetic induction accelerator as defined in claim 1 characterized by the fact that the inner electrode of said electrostatic deflecting device is comprised of a thin wire placed perpen' dicular to the tangential and radial movements of said electrons.

3. A magnetic induction accelerator as defined in claim 1 characterized by the fact that th inner electrode of said electrostatic deflecting device is comprised of a thin wire placed perpendicular to the tangential and radial movements of said electrons, and further including a second electrostatic deflecting device within said chamber following the first deflecting device, said second deflecting device being comprised of a pair of spaced plates through which said electrons are drawn after leaving the first deflecting device and which plates are disposed at an angle of from the first deflecting device as measured on the orbital path of said electrons. I

' ROLF WIDEROE.

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

UNITED STATES PATENTS Number Name Date 1,948,384 Lawrence Feb. 20, 1934 2,103,303 Steenbeck Dec. 28, 1937 2,153,190 Hollmann Apr. 4, 1939 2,193,602 Penney Mar. 12, 1940 2,229,572 Jonas Jan. 21, 1941