US2586494A - Apparatus for controlling electron path in an electron accelerator - Google Patents

Description

Feb. 19, 1952 R. WIDEROE 2,586,494

APPARATUS FOR CONTROLLING ELECTRON PATH IN AN ELECTRON ACCELERATOR Filed Oct. 5, 1948 4 Sheets-Sheet l Feb. 19, 1952 A R. WIDEROE 2,586,494

APPARATUS FOR CONTROLLING ELECTRON PATH IN AN ELECTRON ACCELERATOR Filed Oct. 5, 1948 4 Sheets-Sheet 2 r4;- 6 Oufimrd i l llm ard l I l J 1 I i I I 360 I Z 4,8} mm AR F9. 7. 5

R. WIDEROE Feb. 19, 1952 APPARATUS FOR CONTROLLING ELECTRON PATH IN AN ELECTRON ACCELERATOR 4 Sheets-Sheet 5 Filed Oqt. 5, 1948 Feb. 19, 1952 R. WIDEROE 2,586,494

APPARATUS FOR CONTROLLING ELECTRON PATH IN AN ELECTRON ACCELERATOR Patented Feb. 19, 1952 APPARATUS FOR CONTROLLING ELECTRON PATH IN AN ELECTRON ACCELERATOR Roll Wideroe, Zurich, Switzerland, assignor to Aktiengesellschaft Brown, Boveri & Cie, Baden, Switzerland, a joint-stock company Application October 5, 1948, Serial No. 52,842 In Switzerland October 11, 1947 In various types of apparatus for accelerating charged particles such as electrons to high velocity and hence high electron voltages, and in which the electrons travel in substantially a circular orbit, it has been found especially diflicult with apparatus so far developed to introduce the electrons into and remove them from the orbit, and to compel them to strike under satisfactory conditions an antiecathode or anode. These difficulties all arise from the fact that the additional forces, employed to modify those forces under which the stream of electrons is made to follow the orbit while accelerating them contmuously along the orbit, require for their production an extremely long time in comparison with the time required by the electrons to make one trip around the orbit, the latter being of the order of 1 10- second. Because of this the electron paths vary slowly, as if the electrons could follow the additional, deflecting forces without inertia, i. e. gradually and continuously and in general without impressed free oscilla tions. Under these conditions it has thus been found very diflicult to produce such additional forces as will cause the accelerated electrons to leave the orbit in a narrowly confined beam along a prescribed path necessary for the electrons to strike the anode or leave the tube in which acceleration takes place, or alternatively to guide the electrons along a prescribed path from an electron emissive source into the circular orbit at the beginning of the acceleration phase.

For example, ,in the betatron type which is also commonly known as a magnetic induction accelerator or ray transformer, wherein the electrons are accelerated round and round the orbit under the combined actions of an induction magnetic field and a guiding magnetic field, one method already proposed for causing the accelerated stream of electrons to be diverted from the orbit is by superimposing an auxiliary magnetic field on the main magnetic guiding field. This has the effect of altering the ratio between the induction and guiding field components with the result that the path of the electron stream expands. The electron stream after attaining the outer boundary of the orbit where the stabilizing forces become zero then travel outwardly along a spiral path. Since, however, the orbit must be axial-symmetrical due to the circular motion, the electrons leave the tube uniformly along the entire circumference contrary to the result desired. Even if the orbit is altered at the outer boundary for a certain distance by the addition of auxiliary forces acting upon the elec- 8 Claims. (Cl. 250-27) lows, this objective is attained by subjecting the stream of electrons to a first deflecting force directed perpendicular to their direction of motion and which force is applied over at most one fourth of the circumference of the orbit and also to other deflecting forces applied over the orbit in a sense which is radially opposite to that of the first force.

While the principles of the invention are applicable to various types of electron accelerators and other devices where the electrons follow a circular path, it will be described in connection with its application to the betatron type of accelerator to illustrate several preferred forms of construction which are considered practical for attaining the desired result. In. the drawings which show these various embodiments:

Fig. 1 is a view in central vertical section of the improved construction in which the electron deflecting forces auxiliary-to those forces which cause acceleration of the electrons are derived from electrostatic fields;

Fig. 2 is a horizontal section taken on line 2-2 of Fig. 1 showing only the tube and deflection plates, and drawn to a slightly larger scale;

Fig. 3 is a curve in development illustrating the electron path relative to the orbit when methods already known are employed to effect a.

deflection of the electrons from the orbit at the end of the acceleration phase;

. Fig. 4 is a view similar to Fig. 3 illustrating mutually opposite deflecting forces;

Fig. 7 is a view similar to Fig. 4 showing the deflection path of the electrons according to the Fig. 6 construction;

Fig. 8 shows a further modification of the arrangement illustrated. in Fig. wherein the magnetic fields producing the mutually opposed deflecting forces are positioned unsymmetrically;

Fig. 9 shows the type of electron deflection which obtains when the deflecting forces are arranged unsymmetrically as shown in Fig. 8;

Fig. 10 is a view on line Hil6 of Fig. 11 illustrating somewhat diagrammatically a modified arrangement wherein the components which produce the deflecting forces are inclined with respect to the plane of the electron orbit to produce deflecting forces which include a component of force perpendicular to the plane of the electron orbit;

Fig. 11 is a horizontal section similar to Fig. 2 illustrating use of one form of the present inventive concept for introducing a stream of electrons into the orbit at the beginning of the acceleration phase;

Fig. 12 is a view in development of the potential channel of the stabilizing forces acting upon the electrons illustrating another embodiment of the l invention for introducing electrons into the orbit at the start of the acceleration phase;

Fig. 13 is a horizontal section illustrating an arrangement wherein the deflecting forces extend substantially around the entire circumference of 1-:

the orbit, and

Fig. 14 is a development of the orbit showing the characteristic of the electron deflection which obtains when the Fig. 13 construction is utilized.

Referring now to Figs. 1 and 2, the betatron there illustrated is seen to be comprised of a magnetic field structure It made up from steel laminations of appropriate contour so as to provide a pair of mutually confronting cylindrical poles ll-l I separated by an air gap I2 and located concentrically along the axis of revolution.aa, and a pair of annular poles l3--l3 also con fronting one another and which are likewise arranged concentric to the axis 11-11, and separated by air gap 14. Yoke members l5 complete the magnetic circuit for acyclically varying magnetic flux set up in the annular and cylindrical poles. Poles lll i and l3-l3' are surrounded by an annular winding preferably split into two coil sections |6lfi' connected in series for energization from a source of alternating current of suitable frequency as for example 100 cyclessec. applied to terminals H.

An annular evacuated tube 88 rests in the air gap l4 between poles i3l3' and thereby surcathode of the injector 29 is energized periodically in timed relation with the cyclic magnetic field produced in the magnetic structure H) by energizaticn of winding 16-56 so as to produce a stream of electrons at just about the instant the magnetic field passes through the zero value. The electron stream circulating along the circular orbit is undergoes a contant acceleration under the influence of the magnetic field. The latter divides into essentially two operating components. One of these, the inducing field" component acting axially through poles H--l l and indicated by lines 1 in Fig. 1 may be considered as being responsible for the acceleration of the electron stream; the other component commonly referred to as the control field and which is indicated in Fig. 1 by lines cc acts axially through the annular poles l3-l3 to produce a centripetal effect upon the electron stream to offset the centrifugal forces of the electrons caused by their motion along the circular orbit in. Throughout the increasing acceleration of the electron stream, the centripetal force, which likewise increase, substantially matches the increasing centrifugal forces of the electrons with the result that the stream theoretically is confined to an orbit of constant radius R0. Actually, however. the electrons do deviate somewhat either radially inward or outward from the orbit and for this reason it is usually necessary to provide some means of stabilization which will return the electrons to the orbit. One practical means of providing the necessary stabilization is to taper the faces of the control poles 13-43 in the direction of the radius as shown in 1. By so doing; the distribution of the magnetic field at the control poles will then be such as to establish in conjunction with the centrifugal forces stabilizing forces which act on the electrons and form a potential channel around the tube, with the orbit k located about midway between the inner and outer boundaries of the channel. and any departure of electrons from the orbit is either radially inward or outward therefrom is counteracted by restoring stabilizing forces of the proper sense existing in the channel.

After the electron stream has been fully accelerated which instant occurs when the magnetic field has reached its maximum value,'there arises the problem of removing it from the orbit k in such manner as will assure its travel along a desired path to the point ofuse. The latter may, for example, be an anti-cathode located in or out of the tube or a reaction chamber located outside of the tube. One method already well known for effecting removal of the electron stream is to establish by electrical fields either electrostatic or electromagnetic in character a deflecting force in a direction perpendicular to a tangent to the orbit is over a small section of the path directly in advance of the desired place of exit. The use of both short deflecting coils or spaced electrodes and which are energized at the proper instant have. been proposed for this purpose but it has been found that these expedients are not entirely satisfactory and that the electrons leave the orbit is not at the desired place which is directly behind the deflecting means employed but rather at a point about from the deflecting means and specifically over an undesirably wide angular extent.

In Fig. 3 which has been drawn on the basis of exact calculation of the electron path B, it will be observed that the maximum deviation (ARmax) of the path B takes place 180 away from the maximum of the curve of the deflecting force F which force is for purposes of explanation assumed to be not so great as would cause the electrons to be shifted radially beyond the boundary within which the stabilizing field acting through the control poles l3i3' is able to guide the electron stream in the direction of the orbital path it notwithstanding the counter action of thedefiecting field. Path B also represents'the final path of the electrons short of actual expulsion from the orbit after the force F has had sufficient time to develop its full deflecting effect upon the electrons. As pointed out previously, development of the full effect of the deflection forces requires an extremely long time as compared with the time required by the elec v trons to make one trip around orbit k and hence transition of the electron stream from orbit to the final path B as depicted in Fig. 3 will of course be gradual and require several revolutions of the electrons of constantly increasing deflection. In the neighborhood of the point of maximum deviation the curve of the electron path B is comparatively flat with the result that the electrons tend to leave the orbit 70 over a wide angular extent.

As previously mentioned, I have found that control over the exit or entrance path of the electron stream can be materially improvedby setting up additional electrical deflecting forces upon the electrons and which have a radial oppo' site sense.

That is to say, if the first force is' trons at the desired place of exit. In Fig. 4 it will also be understood that for comparison with Fig. '3, the deflecting forces F1 F2 are likewise assumed not so great as would cause the electron stream to be deflected beyond the boundary of the orbital channel .in which the control field through poles l3-l3' is able to redirect the stream in the direction of the orbit k. Hence with such a magnitude of force the electrons would circle continuously around the orbit on path B1. Obviously to actually remove the stream radially beyond the stabilizing effect of the condirected radially outward, the additional forces will be directed radially inward, and vice versa.

In the construction illustrated in Figs. 1 and 2,

the first deflecting force is electrostatic in character and is established by a pair of arcuate' electrode plates 22-22 positioned radially in ward and outward of the orbit is immediately in advance of the desired place of exit which is the tangentially arranged tube arm I9. As measured in the direction of the path 7c, the plates 22-22 are quite short and in any event should not ex tend for more than one fourth of the complete circumference of the path. The additional detrol field it is merely necessary to establish larger deflecting forces F1 F2 in which case the final electron path would be as indicated by line Z, such path being beyond any possible stabilizing influence of the. control field and the electron stream will strike the target anode at X.

The construction shown in Fig. 5 differs from that shown in Figs. 1 and 2 only in that the two sets of deflecting forces are established by the use fleeting force, of opposite sense, is likewise estab-i lished by a pair of arcuate electrode plates:

23-23 positioned radially inward and outward of the orbit is and which are located 180 away from the plates 22-22 as measured along the orbit. The two sets of plates 22-22 and 23-23.

are connected in parallel to a source of potential at terminals 24 through a switch 2| and the latter (assuming control over the exit path of the electrons) is arranged to be closed for a brief instant at the end of the acceleration phase by any one of a number of well known control circuits one of which can be found described in my co-pending application, Serial No. 751,680, filed June 2, 1947, now U. S. Patent No. 2,533,859 issued December 12, 1950.

The polarity of the field produced between plates 22-22 will as shown in Fig. 4 be such as to efiect a radially inward force upon the electron stream since the exit path is located radially out and inner plate 23 together and similarly con-" meeting together the inner plate 22 and the outer plate 23.

The beneficial effect of the additional field established by the set of electrode plates 23-23 is quite apparent from an inspection of Fig. 4. In this view, the deflecting force established by the first set of electrode plates 22-22 relative to the orbital path It, here drawn in development," is shown by the curve F1, and the additional deflecting force of opposite sense established by the second set of electrode plates 23-23 relative to the path k but 180 removedv from plates 22-22 is shown by the curve F2.

The respective center of coils 25-25 which enclose opposite sides of the control poles l3-I3. However, the eifect of the oppositely directed magnetic fields produced bythese coils on the electron stream is the same as that produced by the electrostatic fields set up between'the sets of electrodes 22-22 and the deflected electron path will be substantially the same as path B1 of Fig. 4.

If desired, the two sets of oppositely directed deflecting forces may be located adjacent one another as" shown in Fig. 6 instead of at opposite sides of the orbital path is. Here the first deflecting force is established electromagnetically by a coil 26, enclosing the control poles l3-l3 and located in advance of the exit arm I9 of the tube, and the oppositely directed deflecting force is established by two coils 21-21 also enclosing the control poles l 3-13 and which lie adjacent to and on either side of coil 26. As shown in the drawing,'-coils 21-21 are so interconnected with respect to their common source of power supply that the fields produced by'coils 21-21- are both opposite in direction to that produced by coil 26.

Figure '7 illustrates graphically the effect of the deflecting fields produced by coils 26, and-21-2'l on the electron path. The force produced by coil 26 is designated F3 and the forces produced by coils 27-27 are designated F4. Here again; it is seen that the maximum deflection (AR-zmax.) of the electron path B2 from the orbit is coincides with the center of the deflecting force F3 pro duced by coil 26 which is the most favorable condition for leading off the electron stream at the desired point of exit.

Figures 8'and 9 illustrate still another possible arrangement of the two sets of deflecting forces. In this form of the invention, the first deflectingforce is established by a coil 28 enclosing the control poles 13-13 near the exit arm IQ of the evacuated tube. The oppositely directed force is against divided into two components as in the construction shown in Figs. 6 and '7. However, the coils 251-23 surrounding the control poles l3-l3 which produce the oppositely directed force insteadof being-locatedat each side of .coil 28 .are seen to be arranged nnsymetrically with respect to the latter. That is to say, one of the coils such as coil-29 is located next to coil 28 but the other coil 29' is locatediapproximately 180 away from the othertwo. Their effect upon the path of the electron stream isillustrated inFig. 9 where it will be observed that the maximum deviation (AR3max.) of the electron path Bawhile still located substantially at-the position of the deflecting coil 28, its field force being shown by curve F5, is quite unsymmetrical to the latter and to the field forces Feproduced by. the other two deflecting coils 29 and 29.

A modified arrangement is shown in Figs. 10 and 11, it being noted that here the deflecting forces are used for deflecting an electronstream emitted from source 33 into. the orbit k for acceleration rather than for guided expulsion of the electron stream after acceleration as in the previously described embodiments. The deflecting forces are derived electrostatically as in Fig. 2 from two sets of electrode plates 3 4-34 and 35-45 located diametrically opposite one another in the tube |8 at the inner and outer sides of orbit is. Since the two deflectingvforces are opposite in direction, i. e. one radially inward and the other radially outward, theinner plate 34 and outer plate 35 are connected together to the same side of potential source 36. Similarly, outer plate 34' and inner plate 35 are connected together to the same but other side of potential source 36. A switch 37 is included in the connections and is adapted to be closed at the instant at which the electron stream is injected intothe tube from the electron emissive source 33.

Located between electron source 33 and the pair of'electrodes 34-34 are a pair of spaced and charged condenser plates 3|-3| through which the electrons pass. As seen in Figs. 10' and'll, electron source 33 and plates 3|-3| are located outside of and upwardly from, the orbit of acceleration k. The two sets of deflecting plates 34-34 and 35-35 areinclinedwith respect to the plane of orbit k and the effect of this is to establish by each set a deflecting force having a vector quantity AR in the plane of the electron orbit is and another vector quantity AV in the plane perpendicular to the plane of the electron orbit.

After the electron stream is, emitted from source 33 it follows the path designated B5. The electrostatic field produced between the negatively charged condenser plate 3| and the positively charged condenser plate 3| starts the :stream radially inward and the deflecting forces set up by the two sets of plates 3434' and 3535' will serve to guide the stream radially inward and downward into the plane of'the acceleration orbit is. Thereafter the path of the stream will be along orbit it due to the effect of normal magnetic stabilizing forces previously mentioned.

If desired, the arrangement shown in Fig. 11 can be used in reverse to discharge an electron stream from the orbit is at the end of the acceleration phase. In such case the switch 31 would be closed at the instant expulsion of the stream isdesired, and the source 33 would be replaced by an anti-cathode, assuming the accelerated electron stream to be used for the production of Roentgen rays. To prevent the accelerated stream from striking-the inner condenser plate 3| and being absorbed by the latter, plate 3| is made slightly shorter than the outer plate 3 I as shown clearlyvin Fig. 10.

In Fig. 12, where the plane of the paper corresponds to the plane of the electron orbit there is shown still another arrangement employing oppositely directed deflecting forces on the electron stream for introducing the latter into the potential channel. Here the dimensions of the potential channel are shown in development. Its inner and outer boundaries in the radial direction are indicated by lines 38-38, and the acceleration orbit It lies about midway between the boundaries. An electron source 39 is located just outside of the potential channel and is adapted to shoot a stream of electrons in a direction parallel to, or at a flat angle to, the boundary line 38%. The path 40 of the steam is then shifted parallel in the direction of the acceleration orbit k by the field force acting in radial direction and produced by a coil 4| located with its winding parallel to the plane of the electron orbit, the force of the same being indicated by the vector 42. The electron stream after passing coil 4| is then immediately subjected to a force (vector 43) of opposite direction and which is produced by coil 44 located adjacent coil 4| and connected for instance in series with the latter to source terminals 45. The direction of the current flow through the two coils 4|, 44 is shown by arrows. The combined action of the two forces 42, 43 on the electron stream is the same as that already described in connection with the preceding embodiments of the invention (especially Figs. 6 and 7), and the electron stream will accordingly be deflected into the potential channel and subsequently reaches the orbit k.

Still another arrangement for settin up the two deflectin forces of opposite direction is illustrated in Figs. 13, 14. Here the first deflecting force is produced by a pair of electrode plates 46-46 extending for a short distance along the circumference of the acceleration path is at the place of electron exit, and the additional deflecting force of opposite sense is produced by a coil 4'! enclosing the entire guiding field and producing radial directed forces along the entire electron orbit k. The electron injector is shown at 20 and the anti-cathode at 50. The coil 41 and the deflecting plates 46-46 are connected to the same voltage source 24 and energized by closing the switch 2|. The effect upon the electron stream is illustrated in Fig. 14. Here as before, the path is is shown in development, and the deflecting forces due to the coil and the electrode plates are designated respectively by curves F8 and F1. The

course of the electron stream is shown by curve B4 and it will be observed that the latter is shifted radially relative to the orbit is when compared with the curve shown in Fig. 3, so that the point of maximum deflection (A R4 max.) from the orbit It occurs at the location of electrode plates 46-46, i. e. at force F7.

The arrangement shown in Fig. 13 serves the purpose of modifying the normal ratio between the control and induction field components produced by coils l6|6', and thus produce the already described second force acting in an opposite sense and simultaneously with the first named deflecting force upon the electrons. This alteration of the ratio between the control and induction field components can also be established in some other well known manner, such as by effecting a saturation of the induction poles or the control poles |3-|3 at the proper instant, or by causing a phase displacement between the control and induction field components. The means required to produce the saturation of the induction poles or the control poles are well known and are described in detail for instance in U. S. Patent No. 2,103,303. Similarly the phase displacement referred to above can be achieved by well known means such as described in the U. S. Patent No. 2,297,305.

In conclusion, I wish it to be understood that while the various constructional arrangements have been illustrated as applied to an electron accelerator of the betatron type, the principles of the invention are equally applicable to other types of electron devices where the electrons describe circular paths such as for example in the synchrotron and beta ray spectographs.

I claim:

1. In an electron device such as an electron accelerator and the like wherein the electrons travel along a circular orbit, means for efiecting a deflection of the electrons at a preselected point along said orbit comprising, means establishing a first deflecting field immediately in advance or said point of deflection, said field extending along said orbit for at most one-fourth the circumference thereof and which produces a generally radial deflecting force upon the electrons, and means simultaneously establishing a second deflecting field elsewhere along said orbit and, which also produces a generally radial deflecting force upon the electrons but which force is of opposite sense to the force produced by said first field.

2. An electron device as defined in claim 1 wherein the respective means producing said first and second fieldsare s0 oriented with respect to said orbit as to yield a component of deflecting force perpendicular to the plane through said orbit.

3. An electron device as defined in claim 1 wherein the means establishing said second field extends substantially over the circumference of said orbit.

4. An electron device as defined in claim 1 wherein at least one of said field producing means is located in such position that the deflecting force produced thereby is efiective in the radial direction over only a portion of the width of the potential channel containing said orbit.

5. An electron device as defined in claim 1 wherein the respective means for producing said first and second deflecting forces are comprised of coils surrounding at least parts of said orbit.

6. An electron device as defined in claim 1 wherein the respective means for producing said first and second fields are comprised of pairs of electrodes located radially inward and outward of said orbit.

7. An electron device as defined in claim 1 wherein the said respective means establishing said first and second fields are disposed adjacent one another.

8. In a device for accelerating electrons comprising an annular tube within which a stream of electrons following a circular orbit may be accelerated to high velocity, a magnetic field structure associated with said tube and which includes a central induction pole surrounded by said tube and a pair of annular control poles in juxtaposed relation of said orbit, and a winding surrounding said induction and control poles adapted to be energized with alternating current to establishing magnetic fields varyin with time in said induction and control poles and under the influence of which said stream of electrons is accelerated; means for efiecting a deflection of the electron stream at a preselected point along said orbit, said deflecting means comprising means establishing an electrical field immediately in advance of said point of deflection, said field extending along said orbit for at most one-fourth the circumference thereof and which produces a first and generally radially acting defleeting force upon the electron stream, and means for changing the magnitude of the control field in said control poles relative to the magnitude of the magnetic field in said induction poles thereby establishing a second deflecting force on said electrons acting in a radial opposite sense to and simultaneously with said first deflecting force.

ROLF WIDEROE.

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

UNITED STATES PATENTS Number Name Date 2,103,303 Steenbeck Dec. 28, 1937 2,193,602 Penney Mar, 12, 1940 2,297,305 Kerst Sept. 29, 1942 2,394,070 Kerst Feb. 5, 1946

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