US2624020A

US2624020A - Apparatus for accelerating electrically charged particles

Info

Publication number: US2624020A
Application number: US214471A
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: 1949-12-01
Filing date: 1951-03-08
Publication date: 1952-12-30
Anticipated expiration: 1969-12-30
Also published as: NL73020C; CH280963A; DE856340C

Description

R. WIDERUE Dec. 30, 1952 APPARATUS FOR ACCELERATING ELECTRICALLY CHARGED PARTICLES 2 SHEETS-SHEET 1 Filed March 8, 1951 YWPONPW ATTORNEYS Dec. 30, 1952 R. WIDERGE Filed March 8, 1951 2 SHEETSSHEET 2 INVENTOR BY W svzl mg/fimkw ATTORNEYS Patented Dec. 30, 1952 APPARATUS FOR ACCELERATING ELEC- TRICALLY CHARGED PARTICLES Rolf Wideriie, Ennetbaden, Switzerland, assignor to Aktiengesellschaft Brown, Boveri & Cie, Baden, Switzerland, a joint-stock company Application March 8, 1951, Serial No. 214,471

In Switzerland December 1, 1949 This invention relates to accelerators for electrically charged particles such as electrons wherein the particles are forced to'travel an orbital path of substantially constant radius while at the same time being constantly accelerated along the path, and is a continuation-in-part of my application Serial No. 198,547 filed December 1, 1950, and now abandoned.

One type of accelerator, known generally as a. betatron or ray transformer," to which the present invention can be applied, is comprised of an evacuated annular tube into which electrons are introduced from an electron emissive' cathode, and a magnetic system which produces a magnetic field varying with time having a spatial distribution such that the injected electrons are accelerated along a substantially circular orbit within the tube by magnetic induction effects. The magnetic field divides into two components. duction field is responsible for acceleration of the electrons around the orbit, and the other component known as the control field produces an increasing centripetal force upon the electrons which is designed to match the increasing centrifugal forces of the electrons as the latter increases in velocity thus maintaining the electrons on a circular path of substantially constant radius. The electrons reach enormously high energy levels as a result of their acceleration and at the end of the acceleration phase can be diverted from the orbit and caused to impinge upon an anode to produce X-rays or used for other purposes.

The betatron type of accelerator usually incorporates a stabilizing device for restoring to the prescribed circular orbit those electrons which deviate from it. Such stabilization can be achieved for example by establishing a gradient in the control field such that the strength of field decreases outwardly with increasing radius of the electron path, and a full discussion of the apertaim'ng theory can be found in U. S. Patent No. 2,103,303, issued December 28,1937, to Max Steenbeck. The stabilization attributable to the decrease in field strength is effective only within a limited vicinity of the precalculated circular orbit for electron travel, known generally as the circle of equilibrium, but within the effective area provides both radial and axial stabilizing force components for any electrons which leave the precalculated orbit. In the plane of the equilibrium circle, the stabilizing force has two maxima, which lie on circles located radially inward and outward respectively from the circle of, equilibrium.

One component known as the in- 9 Claims. (Cl. 313-62) Heretofore, it has been the general practice to locate the anode and its necessary supporting structure, against which anode the stream of electrons is caused to impinge for the production of hard X-rays, outside of the efiective area of stabilization, that is, outside of the two limit circles of maximum stabilizing forces, since it was thought that too many electrons would be lost from the stream by impingement were an obstacle to be placed anywhere in the same gene eral area where electrons might be expected to travel during their acceleration phase.

Experience, however, has shown that the yield of hard X-rays for example from a betatron varies somewhat inversely as the radial distance which the electrons must travel from the circle of equilibrium to reach the anode. One possible explanation for this undesirable effect is that in the vicinity of the maximals of the stabilization forces, local disturbing fields enter into the control field space from without with the result that the inner and outer circles of maximum stabilizing force which define the efiective area of electron stabilization lie closer to the circle of equilibrium than is calculated.

The general objective of this invention is' to provide an improved arrangement for the anode within the annular tube that will assure a maximum yield of accelerated electrons available for impingement against the anode and such objective is achieved by employing a support there-- for extremely thin in the dimension perpen dicular to the plane of the circle of equilibrium and which locates the anode in the plane of the equilibrium circle at a point between the limits of one half and four fifths of the distance to the equilibrium circle from either the inner or outer limit circles of maximum stabilizing force.

The foregoing as well as other objects .and advantages inherent in the invention will be-. come more apparent from the following description and accompanying drawings illustrating two practical embodiments of the invention.

In the drawings, 1

Fig. l is a view in central vertical section of an accelerator for electrically charged particles known as a betatron, wherein electrons are accelerated by 'magnetic inductive efiects exclusively, to which the invention has been applied.

Fig. 2 is a view partly in horizontal section and partly in plan of the accelerator shown in Fig. 1;

Fig. 3 is a curve used in explaining the invention;

Figs. 4 and 5 are views in top plan and side elevation of the anode and its supporting ture drawn to-an enlarged scale; and

struc:

Figs. 6 and 7 are views similar to Figs. 4 and 5 illustrating a somewhat modified supporting structure for the anode.

Referring now to Figs. 1-5 in particular, the electron induction accelerator, which is symmetrical about a vertical axis a-a, is comprised of a magnetic structure l made up from steel laminations of appropriate contour and includes a pair of confronting cylindrical induction poles ll|l separated by air gap' 12 located eoncen' trically along axis a-a, and a pair of confronting annular control poles l3 l 3' separated by air gap I4, the control poles being locatedradially outward from the induction poles and also concentric with axis a/a. The confronting faces of the control poles are preferably tapered in such manner that the air gap I 4'increases' as measured in the radially outward direction from axis a a, thus effecting a corresponding decrease in strength of the control field. I

Yokernembers [5 complete the magnetic circuit for'the time varied magnetic field set up in the control and induction poles. A winding pref erably split into" two annular coil sectionslfi, l6 surrounding the upper and lower control poles 13 3" respectively and wound in the same direcutn are connected in series for energizanon from a source of alternating current of suitable frequency such as for example 100 cycles/see,

such source being indicated simply by terminals H. An annular evacuated tube It, preferably of glass, is disposed symmetrically in the air gap [4 between the control poles l3l3'. The timevaried current in energizing coils lfi-IS' produces the time-varied magnetic field previously mentioned. The induction component of this field responsible for electron acceleration passes through induction poles l [-4 l across air'gap I2, and the control component responsible for maintaining the electrons on a circular path of cons'tant ra'dius' passes through poles l3l3 across air gap ['4. In Figs. 1 and 2, the circular path or circle of "equilibrium is designated k and its radius hidicated by Re, the plane containing the equilibrium circle Ic being perpendicular to axis aa.

Aspr'eviously explained, all of the electrons do not always adhere strictly to the path It but rather deviate from the same. In order to stabilize the electron path, the control field is made to decrease in the" radial direction by tap'ering the faces of the control poles i3''l3'. stabilization force P attributable to this radial gradient in" the control field has a characteristic as shown by the curve in Fig. 3 which shows the course of the force in a plane perpendicularto the equilibrium circle is. At the circle of equilibrium 7e, radius R0, force P, is zero. In a direction radially outward from circle ic, the stabilizing force Prisesluniformly in one direction reaching a'maxirnal time an outer limit circle m' of radius Re. In the direction radially inward from circle e, thestabilizing force P rises uniformly in the other direction reaching a maximal along' an in-' ner limit circle n of radius R1. The forceP drops quickly after reaching the maximal at theinnr'an'd outer limit circles. For locaticnsRd and Rathereapplies the condition R+B="max or Bibeing the strength of the control field.

The cathode 2U constituting the source of the The 4 ing limit circle n. In an experimental model which has proven very satisfactory, a very thin plate 22 whose dimension in a direction perpendicular to the plane of the circle of equilibrium is at most one-tenth of a millimeter projects laterally from housing 2| in a radially outward direction for about millimeters and at the outer end thereof the plate 22 supports the anode 23 which is spaced from the equilibrium circ'lelc by at least one half and by at most four fifths of the radial distance d to the equilibrium circle lc fromthe inner stabilizing limit circle 11. The plate 22 which may be made from platinum or a platinum-iridium alloy preferably is wider at its point of attachment to housing 2| than at the end carrying anode 23 for easier attachinent to the housing and also in order to place its mechanical inherent vibration frequency as high as possible above the frequency of the current used in energizing the coils I-6-|6. In the plane perpendicular to the plane of the equilibrium circle is, the support plate 22 therefore presents only a very small areaand constitutes, during a small, initial fraction of the acceleration period during which the electrons still deviate appreciably from the equilibrium circle, only a very small obstacle to elec-' tron travel within the total area of stabilization. Consequently the number of electrons lost from the stream because of impingement with support 22 is practically negligible; During the subsequent longer remainder of the period in which the electrons are accelerated the deviation of the electrons from the equilibrium circle becomes much smaller and none of the electrons strike the support 22 or anode 23. 7 I

Since the electrons are scattered as they enter the anode 23, it is advisable to give the anode 23 which preferably consists of platinum, a somewhat larger dimension in the direction perpendicular to the plane of the equilibrium circle is than is given the support plate 22-, as seen more clearly in the enlarged Figs. 4 and-5, so that the face of the anode ofiers to the incoming accelerated beam of electrons an area of about 0.25, or 0.5, or at most LOsquare millimeter; This area is much smaller than the area of plate 22 in the same plane, the length of the plate being about fifteen millimeters. The diniension of the anode 23 and plate 22 in the direction parallel with the plane of the equilibrium circle I k in the experimental model was about 2 millimeters, and the cathode was located approximately the same distance from the inner limit circle n as the anode 23.- I-he necessary control circuits by which the cathode 20 is energized periodically in timed relation with the variation in the magnetic field to inject a stream of electrons into the tube at the beginning of each cycle or half cycle of the alterating magnetic field have not been illustrated since; various arrangements are already well known and moreover are not considered essen an understanding of this invention which concerns only the placement of the anode and cathode elements within the tube, and the support for the anode.

In the betatron type" of accelerator, meansmiist be provided for stream from the orbit Ic acceleration period so as against the anode 23.

accomplishing this result between the induction and control field components such as by eifecting a partialn'iagnetic diverting the electron towards the end of the to cause it toimpinge One practical way of is to change the-ratio saturation in the magnetic circuit. In the illustrated embodiment of the invention wherein the anode is disposed radially inward of the orbit is, the induction poles ll-l I would thus each include a portion Ila, Il'a of restricted area so as to effect a partial magnetic saturation in these poles as the induction field reaches a predetermined magnitude. Thereafter the centripetal forces attributable to the control field would exceed the centrifugal forces of the electrons thus driving the electrons radially inward from the orbit k.

If desired, the cathode and its housing may be located radially outward from the outer stabilizing limit circle m in which case the anode would be disposed at a corresponding position between the outer limit circle m and the equilibrium orbit it. Such an arrangement would of course require the electron stream to be diverted radially outward from the orbit is for impingement against the anodes and could be effected by producing a partial magnetic saturation effect in the control poles.

In lieu of the support plate 22, Figs. 6 and 7 show an arrangement wherein the anode support is constituted by a U-shaped stirrup 28 whose two ends are secured to the cathode housing 21. The anode 28 is secured at the outer end of the stirrup, and like plate 22, the dimension of the stirrup in the direction perpendicular to the plane of the equilibrium circle is would be very small, that is, of the order of .1 millimeter or less so as to present as small an obstacle as possible to the run of the electrons around the orbit.

In conclusion, while I have described and illustrated my invention as applied to a betatron wherein electron acceleration is produced wholly by the principles of magnetic induction, it can be applied equally as well to other types of accelerators such as the synchrotron which employs a magnetic structure similar to that used in the betatron but uses the magnetic field produced by it principally for guiding the electron stream in the circular path. The initial acceleration of the electron stream can be effected by magnetic induction but thereafter further energy is added to the stream by causing it to traverse a resonant cavity containing a gap to which a high frequency potential is applied such as described and shown in U. S. Patent No. 2,485,409, issued October 18, 1949, to H. C. Pollock et al.

Moreover while I have shown the anode as being carried by a support secured to the cathode housing, the support could be secured in some other manner within the tube without departing from the spirit and scope of the invention as defined in the appended claim.

I claim:

1. In a device for accelerating charged particles comprising a magnetic structure including a pair of axially aligned circular control poles the faces of which are arranged in confronting relation to establish a gap therebetween, an annular tube disposed in the gap between said control poles providing therein an orbital path for acceleration of the particles, means for producing a time varied magnetic field across said gap between said control poles to confine the particles to a substantially circular path coincident with a circle of equilibrium during the acceleration phase, said pole faces being divergent in a radially outward direction whereby the strength of said field decreases in a direction radially outward from the axis of said control poles to establish a stabilizing force for the particles having in the plane of said equilibrium circle two maximals lying respectively on limit circles located radially inward and outward of said equilibrium circle, an anode in said tube disposed in the plane of said equilibrium circle and spaced from the latter by at least half but not exceeding four fifths the distance to the said equilibrium circle from one of said limit circles, and a support within said tube for said anode, said support being relatively thin as measured in a direction perpendicular to said equilibrium circle.

2. A device for accelerating charged particles as defined in claim 1 wherein said anode is constituted by a body of platinum having one face disposed perpendicular to the plane of said equilibrium circle having an area not exceeding one square millimeter.

3. A device for accelerating charged particles as defined in claim 1 wherein the dimension of said anode support in a direction perpendicular to said equilibrium circle is at most one-tenth millimeter.

4. A device for accelerating charged particles 5. A device for accelerating charged particles as defined in claim 1 wherein the support for said anode consists of platinum.

6. A device for accelerating charged particles as defined in claim 1 and which further includes a cathode housing disposed within said tube at the side of the said one limit circle away from said anode, and a cathode within said housing for producing the said charged particles, said cathode being located substantially the same distance from said one limit circle as is said anode. and said anode support being attached to said cathode housing at the end opposite that at which said anode is attached.

7. A device for accelerating charged particles as defined in claim 6 wherein the support for said anode at the end attached to said cathode housing has in the plane of the equilibrium circle a larger dimension than the end supporting said anode.

8. A device for accelerating charged particles as defined in claim 7 wherein the support for said anode is constituted by a fiat plate.

9. A device for accelerating charged par- I ticles as defined in claim 1 and which further includes a cathode housing disposed within said tube at the side of the said one limit circle away from said anode, and a cathode within said housing for producing the said charged particles, said cathode being located substantially the same distance from said one limit circle as is said anode, and said anode support is constituted by a U- shaped stirrup attached by the ends thereof to said cathode housing.

ROLF WIDERE.

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

UNITED STATES PATENTS Number Name Date 2,335,014 Kerst Nov. 23, 1943 2,484,549 Blewett Oct. 11, 1949 2,550,212 Wideroe Apr. 24, 1951 2,553,312 Gurewitsch May 15, 1951 2,558,597 Westendorp June 26, 1951