US2558597A - Field correction in magnetic induction accelerators - Google Patents

Description

June 26, 1951 w. F. WESTENDORP 5 5 FIELD CORRECTION IN MAGNETIC INDUCTION ACCELERATORS Filed Aug. 29, 1950 2 Sheets-Sheet 1 Fig. l.

TIME-VARVl/YG VOL 7146f & N N

Inventor: Wi Hem?) Westendor-p,

by I His Attorney.

June 26, 1951 w. F. WESTENDORP 2,558,597

FIELD CORRECTION IN MAGNETIC INDUCTION ACCELERATORS File ci Aug. 29, 1950 2 Sheets-Sheet 2 Fig. 4,

H. MA s/vg'r/c FIELD lNTE/VS/TY Fig. 5.

lnvenc'cjr'z Willem F Westendorp,

His Attorney.

Patented June 26, 1951 FIELD CORRECTION IN MAGNETIC INDUCTION ACCELERATORS Willem F. Westendorp, Schenectady, N. Y.,as-

signor to General Electric Company, a corpo ration of New York Application August 29, 19 50, Serial No. 182,025.

- 6 Claims.

The present invention relates to magnetic induction accelerator devices and, more particularly, to means for increasing the efficiency with which charged particles may be accelerated in such devices.

Apparatus for accelerating charged particles by means of magnetic induction effects is described and shown in my prior Patents 2,394,071, 2,394,072 and 2,394,073, all of which were issued February 5, 1946, and assigned to the General Electric Company, a corporation of New York. Such apparatus generally comprises a core of magnetic material including a pair of opposed rotationally symmetrical pole pieces which define a toroidal gap wherein an evacuated annular container is positioned. The core is excited by means of windings which are energized by a source of time-varying voltage to produce a time-varying magnetic flux which links an orbital path within the evacuated container and a time-varying magnetic guide field which traverses the orbital path. Charged particles, injected into the orbital path from a suitable source positioned adjacent thereto, are accelerated by the time-varying magnetic flux during a great number of revolutions to high energy levels while the time-varying magnetic guide field constrains the particles to the orbital path.

As has been denoted in'my prior patents, it is essential that the time-varying magnetic guide field existing between pole faces of the rotationally symmetrical pole pieces have a nearly I constant inverse slope with radius or radial displacement in order that the charged particles may be accelerated along a stable orbit. This condition is very often difficult to fulfill because field assymetries many times result from imperfect core and pole piece construction. Furthermore, in order that the charged particles may execute a number of revolutions within a stable orbit near the center of the evacuated container, it is necessary that the magnetic lines of force between the pole faces of the opposed rotationally symmetrical pole pieces be perpendicular to the plane of symmetry of the annular container. Very often imperfect core and pole piece construction also results in the magnetic lines of force between the pole faces being perpendicular to a plane above or below the plane of symmetry of the annular container whereby the charged particles are prematurely discharged upon the walls of the annular container. 7 n

According to one feature of the invention more fully described and explained hereinaftel', a set 2 of single-turn, electrically separate, circular coils is attached to the pole face of each of the rotationally symmetrical pole pieces and, in circuit with each of these coils, is placed a variable resistor. When the magnetic core including the pole pieces is energized, the time-varying magnetic flux which links each of the circular coils will induce a voltage therein, and since a separate circuit for each of thecoils is formed through its respective variable resistor, each of the coils will carry a current having a magnitude dependent upon the number of fiux linkages and the respective circuit constantsi The cur rent induced in eachjcoil may be varied in magnitude' by the variable resistor in circuit therewith whereby the field existing between them-- tationally symmetrical pole faces may be suitably modifiedby trial and error to provide a desired field configuration free of the defects, hereinbefore mentioned, resulting from imperfect core and pole piece construction.

The features which I desire to protect herein are pointed out with particularity in the ap"- pended claims. The invention itself, 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 par: tially sectionalized view of an induction accelerator suitably embodying my invention; Fig. 2 is an enlarged sectionalized fragmentary view of the injector structure of Fig. 1; Fig. 3 is enlarged fragmentary view of the pole pieces of the apparatus of Fig. 1; Fig. 4 is a graphical representation useful in explaining the inven tion and Figs. 5, 6 and '7 are schematic illustrations showing circuit connections for the coils attached to the pole faces of the accelerator ap paratus. 1

Referring particularly to Fig. 1, there is shown exemplary magnetic induction accelerator a paratus comprising a magnetic core I which may be laminated to minimize the generation of eddy currents therein. Magnetic core 1 includes laminated rotationally symmetrical opposed pole pieces 2 and 3 having generally outwardly tapered pole faces 4 and 5 for the provision of a-magnetic guide field along an orbital path indicated at point x, as will be more fully described hereinafter. Coaxial with pole pieces 2 and 3 and disposed between pole faces 4 and 5 is an evacuated annular container or vessel 6 of dielectric material which, provides within its interior an annular chamber 1 wherein charged particles may be advantageously accelerated. The central portions of pole pieces 2 and 3 are terminated respectively by fiat surfaces 8 and 9 between which are disposed laminated metallic discs I and II and dielectric support spacers l2. Metallic discs I0 and II serve the purpose of reducing the reluctance of the magnetic path in the region between surfaces 8 and 9.

Magnetic core I may be excited from a suitable source of time-varying voltage I3 connected as indicated to series-connected energizing windings I 4 and I5 surrounding pole pieces 2 and 3. To minimize the current drawn from the source l3, energizing windings l4 and- I5 may be resonated by power factor correcting capacitors l6. charged particle source, such as the electron gun Within chamber 1- there isprovidedi a I! which is shown in more detail. in Fig- 2.. Gun

I! may comprise a housing member [8 within which is positioned a thermionic filament I-9 partially enclosed by a shield member 20. Conductive wires 2!: and 22,. which. may be connected toan intermittent source of electrical energy (not shown), are sealed as shown into a side arm 23 of container 6 and connected at their inner extremities respectively to filament l9 and housing member 18-. More: detailed illustration and description of suitable electron gun structure may be found by reference to my above mentioned prior patents.

As will be well understood. by those skilled inthe art, energization of windings l4 andi5 by the source" of time-varying. voltage [3 will result in av time-varying magnetic flux which traverses magnetic core I. and pole pieces 2 and 3- to; provide a time-varying magnetic flux. which links orbital. path .rand a magnetic guide field which. traverses the locus of the orbital path a:

between pole; faces 4 and 5.. Electrons emitted by gun I 1- atv a desired time instant near zero in the cycle of magnetic flux and field variations will be continuously accelerated as they execute repeated revolutions. along orbital path at. As a consequence, the injected electrons may be caused toassume energies of many millions of electron volts and then: may be automatically directed to. impinge, upon a target 24- toproduce Xrrays, the target being affixed to container 8 and a conductive coating 25 in a manner well known to those skilled in the art.

Ashas been mentioned heretofore, imperfect construction of core I including pole pieces 2 and 3 may result. in theplane of the orbital path a: being shifted above or below the plane of symmetry of the annular container 6. This effect is illustrated in Fig. 3 wherein dotted line 26 represents the plane of symmetry of annular container 6 while line 21 illustrates the plane which; would include orbital path ac when the magnetic lines of force existing between pole faces 4 and 5 have the curvature represented bydotted lines 28. The conventional designation in Fig. 3 ofpole piece 2 as a south pole and pole piece 3 as a north pole is illustrative of the respective polarities of the pole pieces for the acceleration of electrons in a counterclockwise direction. It will berealized. by those well skilled intheart that the orbital. path, which electrons undergoing acceleration follow, always lies in a plane which is perpendicular to the magnetic lines of. force between pole, faces 4 and 5 as is illustrated by lines 2'1. and 28 of Fig. 3. Consequently, if the magnetic lines. of force between polefacesA and 5 have an asymmetrical curvature, the stable. orbital path along which. the electrons'are accelerated. isshifted either above or below the plane of symmetry of annular container 6 depending upon the direction of asymmetry. It will be apparent, therefore, that the stable orbital path may be shifted suificiently above or below the plane of symmetry of annular container 6 so as to cause at least some of the electrons to impinge prematurely upon the walls of container 6 thereby reducing the efficiency of acceleration.

' As has also been mentioned heretofore, im-' perfect core and pole piece construction may in addition: cause undesirable radial variations of the field strength between pole faces 4 and 5. Inmagnetic induction accelerator apparatus as above described, electrons are accelerated in a stable manner only if the following relation is observed;

H oc (1) where H is the magnetic field strength between pole faces 4. andv 5, r is the radius of the point under consideration, and n is a constant having a. value lying between zero and unity. If this relation is properly fulfilled, then electrons may be accelerated in a stable manner as. they oscillate within a limited region surrounding. orbital path as. The value of the constant 12 usually is arranged to be approximately .75 as. is indicated by'the slope of curve 29 between points a and b of Fig. 4 wherein the magnetic field strength between pole faces 4 and 5 is plotted as a function of radius. Points 11 and; b define the extremities of the region surrounding orbital path x within which electrons may be stably accelerated while points 0 and d. define the extremities of pole faces 4. and 5'. Very often, however, in the-construction of. the magnetic circuit of magnetic induction accelerator apparatus, the radial field distribution falls off in an undesired non-uniform manner as is indicated by dotted curve 30. of Fig. 4'. This means that. the above-stated stability relation isv not fulfilled within the region surrounding orbital path at and a considerable reduction in the efficiency of the accelerator apparatus consequently occurs.

According to the present invention, both of these difficulties resulting from imperfect. core and pole piece construction may be obviated by affixing with a suitable adhesive a plurality or set of single-turn, circular conductors or coils 3| along each of the pole faces 4 and 5. It has been found that, by connecting separate circuits to each of these conductors and properly adjusting the current flow therein, the magnetic field distribution. between pole faces 4 and 5 may be corrected to increase the output of the above-described accelerator apparatus as much as one hundredfold.

Referring now to Figs. 5, 6 and '7, there are shown several circuit arrangements with which the above mentioned advantages of the present invention may be suitably realized. As is shown in Fig. 5, each of the single-turn conductors 3| may be connected in series with a variable resistor 32. With this arrangement, the magnetic flux linking. the various. conductors 3|, when exciting windings. l4 and I5 are energized by the source of time-varying voltage l3, will induce. a voltage in each coil proportional to the time rate of change of enclosed flux, and the, resultant current flow may be suitably adjusted by variable resistor 32. The current flowing in conductors 3|. has a. maximum amplitude when the t me-varying-flux" passes through zero-and. therefore, has nearly maximum effect upon the field distribution when the electrons from gun 11 are injected and field symmetryis most important. The current flowing in conductors. 3|, of course, generates a magnetic field adjacent thereto which, with the proper consideration, will produce desired modification of thefield between pole faces 5 and 6 in accordance with the invention. If, as illustrated in Fig. 3, the plane of orbital path a: is too high, a desired modification of the magnetic field may be obtained by open-circuiting each of the coils 3| alon pole face 4 by means of aswitch 33 and then adjusting the current fiow through each or the coils 3| along. pole face 5 by means of-each ofv the resistors 32.. The desired lowering of the orbital :plane is indicated by an'increase in the X-ray output of the accelerator apparatus, If the magnetic field between pole pieces 4 and 5 does not fall off with radius in a uniform manner as required by Equation 1, correction may be obtained by closing the circuits of the coils 3| positioned on both pole faces 4 and 5 and adjusting the individual resistors 32 until: an increase in the output in the accelerator apparatus is also obtained. It will be apparent that, if both of the magnetic field configuration deficiencies illustrated by Figs. 3 and 4 are present; a suitable compromise as to current flow in'each of the conductors 3| may be found to produce maximum enhanced" output. In general, the.

orbital plane may be shifted up or down by employing the set of coils 3| along either pole face 4 or pole face 5. With the connection of Fig. 5 the currents in the coils along pole face 4 and in the coils along pole face 5 are all in the same direction and, therefore, both'sets of coils may be employed to produce the desired'ra'dial field configuration indicated by curve 29 of Fig. 4.

As shown in Fig. 6, each of the coils 3| may have connected in circuit therewith a fixed or rent is shown in Fig. 7 wherein each of the coils 3| are illustrated as separately connected to a source of direct current (not shown) in series upon the requirements of the situation.

It will be realized by those skilled in the art that to secure desired correction of field inhomogeneities and asymmetries according .to the present invention, the currents in the abovementioned electrically separate coils may all b different from each other.

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

7 What I claim as new and desire to secure by Letters Patent of the United States is:

1. Apparatus for accelerating charged particlescomprisin a closed vessel defining an annular chamber within which charged particles variable resistor 34 and an autotransformer 35,

the input of which may be connected to the source of time-varying voltage I3. arrangement, the voltage derived from autotransformers 35 and the time-varying voltage source l3 may be employed to oppose'the voltage induced in each of the coils 3| and to produce a 'net voltage of opposite phase and of a desired magnitude across resistors 34 and coils 3| whereby the current in each of the coil l3| is in an opposite direction from that which would result from voltages induced therein by the timevarying flux. Conversely, net voltages of the same phase as the voltage induced in coils 3| may be obtained if the outputs of'autotransformers 35 are respectively arranged to be slightly less than the voltages induced in coils 3|. The arrangement of Fig. 6 permits the employment of the sets of coils 3| along both pole faces 4 and 5 to shift the plane of the orbit up or down; because, inorder to shift the orbital plane up or down, either only one set of coils along the pole faces must be employed or the currents in the two sets must be in opposite directions. With the circuitry of Fig. 5 no current reversal in coils 3| is possible, while with the circuitry in Fig. 6 current reversal may be accomplished and hence both sets of coils 3| may be employed to shift the orbital plane.

It is also contemplated according to the present invention that direct current may be utilized to achieve the desired magnetic field modification. A circuit suitable for utilizing direct cur- With this may move in a generally circular orbital path; a magnetic structurev outside said vessel and including opposed, rotationally symmetrical pole pieces which are coaxial with said vessel; means for producing a time-varying magnetic field between said pole pieces, the pole pieces having outwardly. tapered pole faces adjacent said vessel; 'means for projecting charged particles within said vessel; and means for causing the field between said outwardly tapered pole faces to conform to the proportionality where H is the field intensity between said pole faces, r is the radial displacement and n is an exponent having a value lying between zero and one, and coincidentally therewith for causing the magnetic lines of force between the pole faces of said pole pieces to be perpendicular to the plane of symmetry of said vessel including a set of electrically separate current-carryin singleturn circular coils afiixed to each of said pole faces.

2. Apparatus for accelerating charged particles comprising a closed vessel defining an annular chamber within which charged particles may move in agenerally circular orbital path; a magnetic structure outside said vessel and including opposed, rotationally symmetrical pole pieces which are coaxial with said vessel; means for producing a time-varyin magnetic field between said pole pieces, the pole pieces having outwardly tapered pole faces adjacent said vessel; means for projecting charged particles within said vessel; and means for causing the field between said outwardly tapered pole faces to conform to the proportionality H oc where H is the field intensity between said pole faces, 1' is the radial displacement and n is an assets? exponent having avalue lying between zeroand one, and coincidentally therewith for causing the magnetic lines of force between the" pole faces of said pole pieces to be perpendicular to the plane of symmetry of said vessel including a set of single-turn circular current-carrying coils aifixed to each of said pole faces, each of said coils being electrically separate and having connected thereto a separate circuit for determining the current flow in each coil.

3. Apparatus-for accelerating" charged particles: comprising a closed vessel defining an annular chamber within which charged particles may move in a generally circular orbital'path; a. magnetic structure outside said vessel and including opposed; rotationally symmetrical pole pieces which are coaxial with said vessel, said pole. pieces having outwardly tapered pole faces adjacent said vessel; means for projecting charged particles within said vessel; means for producing a time-varyingmagnetic field between said pole pieces; and means for causing the magnetic lines of force between the-pole faces of said. pole pieces to be perpendicular to the plane of symmetry of said vessel to correct-for imperfect magnetic structure including a set of electrically separate circular coils affixed to one of said pole faces, each of said coils having in circuit therewith a resistor for limiting current flow through each of said coils resulting from the voltage induced therein by said time-varying, magnetic field.

4-. Apparatus for" accelerating charged particles comprisin a closed vessel defining an annular chamber: within which charged particles may move in a generally circular. orbital path; a-magnetic structure outside. said vessel and including opposed, rotationally symmetrical. pole pieces, which are coaxial with said vessel; means for producing a time-varying magnetic field between said pole pieces, the pole" pieces having outwardly tapered pole faces adjacent said vessel; means for projecting charged particles. within said vessel; and means for causing the field between said outwardly: tapered pole faces to conform to the proportionality whereI-I is the field'intensity between said pole faces, 1" is theradial displacement and n has a value lying between Zero and one, and coinci-- dentally therewith for causing the magnetic lines of force between the polefaces of: said pole pieces to be perpendicular to the plane of symmetry of said vessel including a set of electrically separate circular coils affixed" to each of i said pole. faces, each ofsaid coilshavingconnectedin circuit therewith only a switch and a variable resistor.

5. Apparatus for" accelerating charged parti-- cles comprising a closedvessel defining an annular chamber within which charged particles may move in a generally circular path; a magnetic structure outside said vessel andincluding opposed rotationally symmetrical pole pieceswhich are Goa-mar with said vessel; energizing windings upon said magnetic: structure connected to a source of time-varyingvoltage for producing a time-varyin magnetic. field between'said pole pieces, the pole piece having outwardly tapered polefaces adjacent said vessel; means for projecting charged particles within said vessel; and meansforcausin'g the field between said outwardly tapered pole faces to conform to -the proportionality Him- Tn where HT is the "field-:- strength between said pole faces,,r is: the radial displacement andn ha ;a value lying between zero and:one;.and1coincidentally therewith for causing. the magnetic lines of:- forcebetween the p'olet'faceso'f said pole pieces to be perpendicular to the: plane of symmetry of said vesselfincluding. a' set'of electrically'sepa rate single-turn circular coils distributed alongthev pole faceof each: of. said-pole pieces and a plurality of autotransformers, eachof which has a secondary winding connected to one of said coils through a current-limiting resistor and a primary windin connected to said source of time-varying voltage.

6. Apparatus-foraccelerating charged particles comprising aclosed vessel defining an annular chamber within which charged particles may move" in'a" generallycircular'orbital path; a magnetic structure outside said' vessel and including opposed, rotationally symmetrical' pole pieces which are coaxialwith said vessel; means for producin a time-varying magnetic field between said pole' pieces, the pole pieces'having outwardly tapered pole faces adjacent saidvessel; means for projecting'charged particles within saidivessel; and: meansfor causing the fieldi between said: outwardly tapered pole faces; to conform to" the proportionality Hoc whereH is the fieldrbetween said polefaces 'r isthe radial displacement and n has avalue lying between zero and one, and coincidentally therewith :for causing the magnetic linesof force between the pole faces or said pole pieces-to be perpendicular to theplane of symmetry of 'said vessel; including: a set' of electrically separate single-turn circular"- coilsdistributed along the: pole-faces-of each pole: piece,.a' source-of direct current, and aseparate circuit for each-of said coils comprising a-switch,.a-reactor and a resis tor connected; in-- seriesewith: eachof said coils: to said sourceof direct current.

- WILLEM F. WESTENDORP.

REFERENGES. CITED The-following references are of record in the file ofthis' patent:

' UNITED STATES PATENTS Number Name Date 2,491,345 Westendorp'n Dec. 13, 1949'

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