US2473123A - Electronic induction accelerator apparatus and method - Google Patents

Description

APPARATUS AND METHOD 2 Sheets-Sheet 1 Filed July 27, 1945 ill" U V {far/75f June 14, 1949.

- (5. D. ADAMS, 2,473,123 ELECTRONIC INDUCTION ACCELERATOR APPARATUS AND METHOD Filed July 27, 1945 2 Sheets-Sheet 2 @QZ/ju aa 1; 47%7 2 WZZZ/% Patented June 14, 1949 ELECTBONEC INDUCTION ACCELERATOR APPARATUS AND METHOD Gail D. Adams, Jr., Champaign, Ill., assignor'to The Board of Trustees of the University of Illinois, Urbana, Ill.

Application July 27, 1945, Serial No. 607,416

Claims. 1

This invention relates to electronic induction accelerator apparatus, and more particularly to means for improving the yield of charged particles, as electrons, by the provision of means varying the relative radial field strength as a function of time.

This invention relates in general to magnetic induction accelerator devices which comprise an evacuated doughnut-shaped tube, as of glass or porcelain with means (as a heated filament) for periodically introducing electrons or other charged particles into the tube; magnetic means for causing the charged particles to circulate in an orbit in the tube for a very large number of turns, accelerating them continuously during this circulation, this means comprising magnets above and below the tube with pole faces of outwardly tapered or divergent configuration, with means for increasing the magnetic field continuously during the circulation of the charged particles; and means for utilizing the kinetic energy of the charged particles, as a target which they may strike to create X rays. The circulating charged particle has a centripetal force requirement varying with the radius of its orbit, and the pole face tapered configuration or divergence is such as to provide a magnetic holding force also varying as a function of the radius, the equilibrium orbit of the charged particle having a radius where these forces are equal.

The general principles of operation of a magnetic induction accelerator are known to the art and have heretofore been published in various patents and publications, particular reference in this regard being made to Kerst Patent 2,297,305 of September 29, 1942, and an article entitled The betatron, by D. W. Kerst, which appeared in vol. 10, No. 5, of the American Journal of Physics (October, 1942). Magnetic induction accelerators of this type have also been the subject of a number of other patents and publications including the following: Slepian Patent 1,645,304; Steenbeck Patent 2,103,303; Smith Patents 2,143,459 and 2,289,220; Penney Patent 2,193,602; Baldwin Patent 2,331,788; Kerst Patent 2,335,014. Other articles by D. W. Kerst are: The acceleration of electrons by magnetic induction which appeared in vol. 60, No. 1, of The Physical Review form and rapidly altering the radial field strength relationship at and about the equilibrium orbit at the beginning of each period of operation of the betatron, the arrangement being such that the variation of radial field shape from a smooth curve is radually released or tapered off until it is substantially negligible at or near the end of the acceleration period.

This variation in radial field shape or relative radial field strength as a function of-time provides several advantages in the operation of an induction accelerator of this type. One such advantage is that it very substantially increases the yield of X rays where the device is being used for the generation of X rays, for example. This advantage results from the fact that a very materially great number of electrons or other charged particles injected into the tube are accelerated radially toward the equilibrium orbit fast enough to prevent loss of charged particles by collision with the injector mechanism, or otherwise, and to result in a or greater increase in the number of charged particles which are brought into the equilibrium orbit and accelerated up to the desired velocities. Another advantage of this invention is that it provides a magnetic induction accelerator with much greater radial stability of the charged particles'in their orbital circulation, much more quickly dampening down radial oscillations of the charged particles. Another advantage of this invention is that the very much greater radial acceleration toward.

the equilibrium orbit, and greater radial stability of circulation, is tapered off or relaXed-toward the end of the acceleration period, when such effects are no longer desirable, to enable desired axial or vertical stability to be secured and particle path to be changed at the end of the acceleration period to cause the charged particles to impinge on a target or otherwise produce the desired 7 effect.

Other features and advantages of this invention will be apparent from the following specification and the drawings, in which:

Figure 1 is a side elevational view of a magnetic line 22 of Figure 1; Figure 3 is an end eleva tional view of such apparatus; Figure 4 is a fragmentary vertical view, along the line l4 of Figure 2; Figure 5 is an enlarged fragmentary sectional view of the injector structure; and Figure 6 is a circuit diagram of oneparticular of my radial field shape varying arrangement.

Inasmuch as the general principles of operation of a magnetic induction accelerator have been fully described in the patents and publications listed above, the description of the general construction and operation of the betatron will be kept brief, and reference may be made to such other publicationsto supplement this general de or any other suitable material; and it is prefereu.

ably coated on the inside with a high resistance concluctivemoating,as withn very thin film of palladium. Th e .tube is disposed betwen two gen erally .circularQpole pieces hereidentified as l2 and l3, (these .pole pieces being of outwardly tapered or diverging I configuration, as a may; be I best gseenin Figure ,4, and substantia ly -.coaxial withhthe tube, l0, such that the field strength di-;- minishes radially toward the outside of the annular chamber, and so that the fiux l-ines curve in a verticalplane .to provide the desired vertical stabilitylanddamping of. vertical oscillations of the charged particles. when moving in theircircularpathh As hasbeen more fully described. in the aboveementioned patents, and particularly Kerst ,Patent 2,297,3(15, the ,outwardly tapered configuration ofthe pole piece faces is such thatthe magnetic fieldstrength varies-inversely with v the radial displacement 1' according to the proe portionality.

where. n is .a constant between zero and. unity.

Thismconstant has heretofore been preferably. kept, by proper design, between .5 and .8, generally in the neighborhood of .75; but by the use .of my radial field shape varyingmeans;

poleiace designmaybe usedwith a constant down 7 near .zeroand even into the negative range, this range. being included when -n is hereinafter termed. as beingless than unity. I In the space in the center of the torus or annular tube is means also comprising a factor in the field shape and strength-relationship, this comprising a pair .of

disks and i5 ofmagnetically permeable material, and anintermediate spacenof non-magnetic, material,.as stone or insulating material.

Surrounding the pole pieces 12 and 13 are themain fieldproducing coils l1 and i8, Surrounding all of theparts-just described is a frame structure, of iron or other magnetically permeable ma-a terial completing the magnetic circuit, this com. o

prising top, bottom. and. side portions and being hereidentifiedin general as l9. Themagnet coils I! and i8 are energized by an alternating. current of considerable a rnplitude, these coils preferably being connected'in parallel with condensers and. energ-iaedirom an alternating current-genera tor, whichmay,have a frequency of 180:-cycles,.. per second and is preferably above the;conven-;,,

tional 60 cycles per second, the magnetcoil-and condenser providing a resonant circuit at this, frequencyin which highcurrents circulate them generatonmierely providing the actuating our:

rentijeplacinglosses in the system; The;elect ron-,; injectingmechanism is intended, as byqexc ta tion of a thyratron grid by voltage from a pickup or peaking strip in the field of the main magnet coils, to inject the electrons into the tube at the beginning of the rising quarter cycle of current on the positive side of the cycle. By the electron gun or injector means shown in Figure 5 and more fully "describedshereafter; the electrons are injected-into the tube substantially tangent to the circular path provided, with substantial initial energy of more than several hundredselectron.volts. Coaction between the movingelectronsyand the vertical lines of force of the magnetic field cause them to be accelerated radiallyebycthe\magnetic force, inwardly if the injector'means-iswutside of the equilibrium orbit. Atthe-same time, the increasing magnetic fieldaccelera-testhe: electrons in their orbital path. The period of acceleration of speed of electronscontinues during the entire rising portion of the positive cycle of the current, as the magnetic; fieldis increasing during this time; and. since; the, electrons; may .@.have described several thousand revolutions iin .:this. time, strong: mage netiofields will. have. worked-their .energy.;up to 1 fiui: density;throughoutthe. area :withinatherorbit is double: the flux idensity at .the: :orbit :.and verti cal or axial .stabilityiis secured by the; curvature.

of thelines of force in..a vertical.;;plane;o

Referring. now more particularly) to Figure -5,

the tube J [-0 will. be :seen; to :have taneck portion a :to which. is :sealedatherbody: :of the itube; I i This; -tube.;is providedwith concentric generally i cylindrical; :wall portions: I. l a,'.;l lb and. .I I0, candy. H

with 'acvacuum nipple. I 1111.:- aThewallat-l b and-.1 ic serve as supporting means for .the :electron :gun

structure 1 comprising; as its ".principle; parts :the. 1 I hotdcathodecor filaments-l le;v shielding. means therefor; comprising inner-sand 10111381. ashieldszl If and 11g; and the supportinggandtconnecting rod. members. The filament 1 la is caused to. emit elece tronsat .the; desiredziinstantxas .byractuation of re renaasnmentionemabove to. complete: a circuitefiecting; applicationzof appropriate nega-;

tive {pot nt a as110,000'yoltsatoutheiinneizshielde:; ins 1 6311 511lfanduthe-filamentfle,:sothaLt-rthesa parts are highly; ;;negative: with; respect-....to: the:

r nd d; i r acoatinaof -th.e.i-..tube. anrlmouter 7 Shield 1 l t d fielfentialxeausingz electronssto be m tted:thrcu h thershield slitdlsieestnaaming'1;v to the; left;; as yiewedri-n Figure :5y'with1'an initiai: .1

velocityin the, ordenof sever.alzthousimd. electron.

sa :eleQtr0ns -areactediupomby the mag.-..

netiofield in sucha mannenas :not, onlygto accel-r erate theiryelocity, but-also .to tendrto accelerate 1 them ;inwardly=:toward-.;the reenter: of .-.the': tube towardthe: equilibrium orbit IBbWhiChciS. pref-. erablyijust to :thefinside .of theielectronigunstruc-i ture in the; form :shown .-:in the j-drawingsi The electrons. are then caused'. to .travelaround in or near the; equilibriumz-nrbitnforias many z-revolutions;'as may be -.desired,-'. and are 'finallyflcaused: U to follow an instantaneous orbit of slightly greater radiusthanthe.equilibrium orbit and to impinge upon. atarget 11;] h thisgimpingement-effecting the desired,=,;,generati0n .of :X mayswas is 'more fully described and. claimed in the copending Kerst and Serber application Serial No.'534,060, filed May 4, 1944, now U. S. Patent 2,447,255.

It will be apparent that, in order to avoid loss of electrons and poor yields as a result of electrons impinging upon the electron gun structure when they have only revolved a couple of times or so and have no substantial energy, the electrons initially injected or emitted by the gun must be accelerated inwardly, in the first few revolutions, suificiently to miss the target. The electrons initially oscillate rather considerably about their instantaneous orbit, being to the in.- ner side of the instantaneous orbit at the end of their first trip around the tube so that the time of at least two revolutions is provided within which to effect a sufiicient inward acceleration or average change of instantaneous orbit such that they will miss the target and back of the shield on the second revolution. If this can be accomplished, and the field strength relationships are proper radially, the electrons will then continue traveling in instantaneous orbits getting closer and closer to the equilibrium orbit. In the particular induction accelerator which I am describing here the equilibrium orbit has a radius of about 7 inches and the size of the electron gun parts are such that the electrons must be accelerated inwardly about two millimeters in the first two revolutions, i. e., the electrons must be accelerated radially inwardly through a distance of the order of two millimeters in about 31 of a microsecond.

I have found that the relative radial field strength relationships, or radial field shapes, heretofore used in magnetic induction accelerators are not satisfactory for this purpose and not capable of producing high yields, even though these relationships may be satisfactory in theory when the electron is already in the equilibrium orbit. I have found that much higher yields may be obtained by the use of radial field shapes Which are not proportional, particularly at and near the instant of injection, to

with n a constant. I still find it convenient to have the conditions at the equilibrium orbit such that the flux density at that point is only half the average flux density within the area of the orbit; but I find it results in much improved yield and radial stability to have the field strength immediately within and without the equilibrium orbit considerably different from that which would be indicated by the above proportionality, at least at and about the equilibrium orbit and for at least the early portion of the period of rising magnetic strength. Generally speaking, I have found that it is desirable to have the field strength outside the desired equilibrium orbit, and at least as far as thepoint where the electrons are injected, substantially stronger than would be indicated by the above proportionality; and to have the field strength immediately within the equilibrium orbit substantially less. Moreover, I find it is highly important to have this radial field shape distortion, if it may be so termed, a function of time and created very rapidly near the inception of the operative period of the induction accelerator.

In the particular embodiment of my invention illustrated here I secure this time-dependent radial field shape by a coil comprising two turns of wire here identified as 20a and 20b(see Figures with the condenser.

be smaller or larger with a retention of beneficial effects, I prefer to choose a radius such that the lines of magnetic force passing through the center of the wires (when they are open circuited) will pass through the equilibrium orbit, this being a slightly smaller radius than that of the orbit due to the curvature of the magnetic lines of force in a vertical plane. By using this coil 20 as the secondary of a transformer having the coils I! and it as a primary, in effect, a circuit connected to the coil 20- will cause disturbance in the relative radial field strength distribution or radial field shape which is a function of conditions in such circuit, shifting lines of force which would have passed inside the coil to the area outside the turns Ziia and 2%; i. e., distorting what might be termed the static magnetic radial field shape in such manner that the field strength immediately within the equilibrium orbit is reduced and that without it increased. This change in field shape from the smooth radial field shape curve heretofore provided by design in accordance with the above proportionality (see Figure 2 of r the October 1942 article entitled The betatron mentioned heretofore, for example) causes the electron to accelerate in toward the equilibrium orbit :much more quickly in its initial couple of turns, accelerators embodying this invention increasing the yield or more. Once the electron gets into the equilibrium orbit or swings within it on its radial oscillations, the electron encounters a sharply lower field strength tending to return it more quickly to the equilibrium orbit, so that any variation in radial field shape not only results in getting the electron away from the injector gun structure in the required length of time to avoid unduly large loss of electrons but also results in much greater radial stability of the electron in its subsequent acceleration period during its revolutions in its circular path.

Referring now more particularly to Figure 6, the two turns 20a and 2th of the field shape changing coil 2&3 are shown as connected in series with the output leads of the coils connected to a circuit including a thyratron 22, which may be of tube type No. B G-154, a resistor 23, a condenser 2 i, and another resistor 25 in parallel Excitation of the thyratron at the proper interval is effected through lead wires tide and 2% leading to a peaking strip or any other appropriate source of energizing Voltage synchronized with the rise and fall of the main magnetic field in the accelerator. The lead 262) is connected directly to the cathode of the tub 22 and the lead Ziia is connected to the grid of such tube through a resistor 21 and bias means here indicated as a bias battery 28. The coil 29 is divided into two turns, with one located on each side of the tube, in order to secure balanced effects on the field. It is preferably kept to only two turns, and must be kept to a low number of turns, in order to get the -maximurn rate of current change in the coil upon change in the magnetic field about it, to give th desired speed of distortion or change of the radial field'shape. The lowest possible number of turns gives the maximum rate of current change in the coil, and thus the maximum change of magnetic field about it, since the voltage developed in the coil only goes down in direct ratio to the number of turns while the inductance drops as the square of the reduction of turns. J Whilea series connection of thetwo emanate 7 coilshalveszfld-and 20b' isshown it is understood: that a parallel connection could-ibe used- 1f "a different i circuit arrangement were connected to '1 the coil, andparticularly' if an appreciable part; of the resistance r in the circuit :were: "in the connection between thetwo coil parts.

In theparticularcircuit illustrated thei:resisto1"'*' 23 is'given a value in accordancewith thezpealr currents-to be permitted-to circulat'ecin thisicira cuiti and this zvalue should be *of theiorder of l ohmszin the: particular 'Iaccelerator. being: de scribed. The fcapacity of thelcondenserill is then: 1' madesuchv that :the: time :constanttcr *decayvcon stantnof thearesistor-condenser ccombinationaiii andillliwill beof the: order. of 100-microseconds;.: which is approximately the period of time through c: whichvthe; :device canmhave desirable? :efiectszin a theiiacceleratorfbeing: described; ;.The;".value of resistor c25 is chosen such' that 'theitimeiconstant of the combination 2lijand 251s large compared-= with?v that unentioneds in the! previous sentence but smal-l.tccmpared with theperiod' of onescycle at thezfrequency at whichthe. accelerator isbeing 1; operatechas /180 second." The: time "constant? of thislatter.combinationmustbemuchilarger, than" v thatzof the first in order: that current delivered thlOl-lghfilthe=I'ESlStOlFr23 twill charge-the cone: denserflduringxthei rising portionnof thewmagt-s netic cycle; and. having thestime constant of then; second combination considerabl smallen than the 3 period: of Chef cycle assures discharging of 1 thecondensera2li before the beginning of thernext-tn operative period. The condenser-24 andithe re-s sistor =25 .cannbe eliminated altogether and the:- desired control-'of current in the coil 20 effected by the' resistor '23; this-requires a much-heavier: dutytthyratron. While other types of circuit'arrangements'and connectionsrto thecoil 20 may-y be made these preferably include: some :meansw for havingthe coil open circuitedat the instant of beginning of the-rising. portion of the 'positive 5* current fiow in the magnet windings, for closing the "-circuitiias by actuation of the thyratron) at thee-time of electron injection, and forlimitinga I the. current how in the coil so that-the relative efiect of this radial'field-shapechanging arrangement' decreases as thepower in the main magnetarrangement rises toward maximum; In the par-- ticularecircu-it here being described, forexample; Y therpower in the thyratron circuit s limited to about-SOOwwatts, whereas the power circulating, in themagnet coils is of the order of 2,000 km.- The powercirculating in the thyratron circuit; and-its efiecton the radial field-shape-at and' about the-:equilibrimnvorhit, is very substantial in theearly portion of the operative quarter cycle, but-substantially negligible near theend-thereof. Grid excitation for the thyratroncan be fromthe same; source used to actuate the injector circuit; also. containing a thyratron control'arrangement; or by a separate peaking. strip or voltage source between the poles or" the accelerator magnet,so thatttherelative time of firing of the field c0n-" trolthyratron 22 and the injiector-circuit thyratron canbe controlled.- i

By the use of a radial' fieldshape control arrangement of thiskind 'the'field shape curve can be modified,.-particularly'at and about the equi librium orbit,- so that it may be expressed as a proportionality where where .n' is note-aconstant but a function-bothe the elec'tronsprojectedrintoi theitube are initially subjected'to a high :relative radial field strength variation from what is being termed'the static field condition; and: iwherein this differential 'is substantiallykreduceda Thi'seffect should pref erably" be 'heldifwithinslimits such'that the field strength of an increment immediately outside the equilibriumprbit:never exceeds the'field streng h of an increment-immediatelyiwithin the 'equilibrium orbitgt-butrwhereinthe difierential from the: smooth curve which .would: normally be prm videdi: byitheataperedpr divergingpole pieces is considerable', difier'entialrbeing here spoken of as a difference fromiitherradialtmagnetic field shape which-would be produced if. my invention were not employed: a

While: I .have 'shownwancl described certain embcdimentscf rhyxinvention; it is to be understood thatit is I-capablei zof' many *modificationsm Changes; :therefore, in the construction and arrangementzmay. be made without departing from the-spiritiandrscope of theinventionxas disclosed in therappendedclaims;

I claim:

1. A magnetic" induction accelerator for charged!particles, comprising; a closed vesseldefining'aanwannular. chamber within which 'such panticlesxmaytmove'in :a circularpath; a mag-'-' netic structure outside said vessel and comprisingropposed;"generally circularv pole pieces-which are -coaxial? with s'aidcannular chamber; said pole pieces 'being :of outwardly taperedconfiguration and having additional field-influencingapparatus associated .thereWith Such' that the r magnetic field astrength: ivaries inversely with: the: radial displacementzrr, atl'andabout said'path; accordto theproportionality' where...n. is a-r-apidly. changing variable; at least at the'Itimev-v. charged particles. are.=injected;.

means for..producing..a time-varying. magnetic field between said .poleflpieces; andtmeansfor v. projecting charged particles within the annular chamber with a substantial initial velocity.

2. A magnetic induction accelerator for chargedparticles, comprising :'-'a closed vessel de- 1 fining anvannular-ichamberwithin which such gration and having. 2 additional field infiuencing apparatuswassociated.therewith 'suchrthat the magnetic field strength varie :inverselywith the radial :displacement r, at and about saidpath, according? to the prop ortionality:

1 Her wherein is a; rapidly'changing .variabie less than unitygrat leastrat the time charged particles are injected meansafor producingaa timeevarying magnetic field between said :pcle piecesgzthe arrangement beingrrsuch that n approaches a conconstant as the .magneticifield between said pole 1' plecesrincreasesyand -means' for: projecting.

'cha'rged particleswwithin :the annular chamber with a. substantial initial velocity."

3. -A I "magnetic- :inductionnr'r accelerators.v forw charged-particles:comprising: :a closed vessel 'de-V finingranu-iannular chamber within which such of rand-of time -=this'iunctlon:belng'. such' thatwsparticlesonayimovein :a circularipathpa magw netic structure outside said vessel and comprising opposed, generally circular pole pieces which are coaxial with said annular chamber, said pole pieces bein of outwardly tapered configuration and having additional field-influencing apparatus associated therewith, said apparatus being energized at least at the beginning of the operative period and influencing the field such that the magnetic field strength varies inversely with the radial displacement r, at and about said path, according to the proportionality Halwhere n is a rapidly changing variable less than unity and has an instantaneous value which is a function both of time and of r at least at the time charged particles are injected; means for producing a time-varying magnetic field between said pole pieces; and means for projecting charged particles within the annular chamber with a substantial initial velocity.

4. In a magnetic induction accelerator for charged particles, comprising a closed vessel defining an annular chamber within which such particles may move in a circular path, a magnetic structure outside said vessel and comprising opposed, generally circular pole pieces which are coaxial with said annular chamber, said pole pieces being of outwardly tapered configuration to produce a predetermined relative radial field strength relationship, means for producing a time-varying magnetic field between said pole pieces, means for projecting charged particles within the annular chamber with a substantial initial velocity, means for varying the relative radial field strength at and about said path as a function of time, including: a coil with a very small number of turns between said pole faces, this coil being coaxial with said circular path and of substantially the same radius means for completing a circuit includin said coil in predetermined relation to the changes of the timevarying field to energize said coil during the time particles are projected within said chamber; and means for limiting the current flow in said coil.

5. In a magnetic induction accelerator for charged particles, comprising a closed vessel defining an annular chamber within which such particles may move in a circular path, a magnetic structure outside said vessel and comprising opposed, generally circular pole pieces which are coaxial with said annular chamber, said pole pieces being of outwardly tapered configuration to produce a predetermined relative radial field strength relationship, means for producing a time-varying magnetic field between said pole pieces, means for projecting charged particles within the annular chamber with a substantial initial velocity, means for varying the relative radial field strength at and about said path as a function of time, including: a coil with a very small number of turns between said pole faces, this coil being coaxial with said circular path and of substantially the same radius; a circuit connected to said coil, this circuit including a gas filled, grid controlled triggering tube and current limiting means; and apparatus rendering said tube conductive at the beginning of a period of rising magnetic field strength.

6. In a magnetic induction accelerator for charged particles, comprising a closed vessel defining an annular chamber within which such particles may move in a circular path, a magnetic structure outside said vessel and comprising opposed, generally circular pole pieces which are coaxial with said annular chamber, said pole pieces bein of outwardly tapered configuration to produce a predetermined relative radial field strength relationship, means for producing a time-varying magnetic field between said pole pieces, means for projecting charged particles Within the annular chamber with a substantial initial velocity, means for varying the relative radial field strength at and about said path as a function of time, including: a coil with a very small number of turns between said pole faces, this coil being coaxial with said circular path and of substantially the same radius; a circuit connected to said coil, this circuit including a gas filled, grid controlled tube connected directly to said coil and current limiting means comprising a resistor and a condenser; and apparatus rendering said tube conductive at the beginning of a period of rising magnetic field strength.

7. In a magnetic induction accelerator for charged particles, comprising a closed vessel defining an annular chamber within which such particles may move in a circular path, a magnetic structure outside said vessel and comprising opposed, generally circular pole pieces which are coaxial with said annular chamber, said pole pieces being of outwardly tapered configuration to produce a predetermined relative radial field strength relationship, means for producing a time-varying magnetic field between said pole pieces, means for projecting charged particles within the annular chamber with a substantial initial velocity, means for varying the relative radial field strength at and about said path as a function of time, including: a coil with a Very small number of turns between said pole faces, this coil being coaxial with said circular path and of substantially the same radius; a circuit connected to said coil, this circuit including a gas filled, grid controlled triggering tube connected directly to said coil and current limiting means; and apparatus rendering said tube conductive at the beginning of a period of rising magnetic field strength, the coil having a substantial effeet on the relative radial field strength at the beginning of the period of rising magnetic field strength and negligible effect thereon near the end of said period.

8. A method of bringing a group of charged particles to a desired velocity in a desired path, comprising initially accelerating said charged particles by a first force, radially accelerating said charged particles to cause them to tend to travel in a circular path and accelerating their velocity in such path by another force, and rapidly varying the relative radial accelerating force at least at the beginning of the operative period, as a function of time.

9. A method of bringing a group of charged particles to a desired velocity in a desired path, comprising initially accelerating said charged particles by a first force, and both radially accelerating said charged particles to cause them to tend to travel in a circular path and accelerating their velocity in such path by a varying magnetic force, this being a different force than said first force, and rapidly varying the radial accelerating component of said magnetic force, at least at the beginning of the operative period, as a function of time, relative to the accelerating component at right angles thereto.

10. A method of bringing a group of charged particles to a desired velocity in a desired path,

comprising initially accelerating-saidcharged particles by an electrostatic force; and" both radially accelerating said charged particles to cause them to tend ton-ave! m a circular path and accelerating their velocity in such path by a varying magnetic force, and r'apidly varying the relative radial field strength, at least at the beginning of the operative period} as a function of time.

11. A method of bringing agroup of charged particles to a desired velocity-ina" desired path, comprising initially acceleiatin'g' said charged particles by an electrostatic force', and both radially accelerating'said charged particles to cause them to tend-to" travelin a circular path and accelerating their'velocity in such path by a varying magnetic force, and-varying the relative radial field strength as a'function of time, this variation being so arranged as to provide greater relative radial field strength variations at and about the desired path nea'r'the'b'eginning of the accelerating period'than'later iii-said period.

12. A magnetic induction accelerator for charged particles, comprising: a'closed vessel defining an annular chamber within which such particles may move in acircular path; a magnetic structure outside said vessel and comprising opposed, generally circular pole pieces which are coaxial with said annular chamber; means for producing a timevarying' magnetic field between said pole pieces; means for projecting charged particles withintheannular chamber with a substantial initial velocity; and conducting apparatus in said magneticfield affecting the radial position of said charged-particles at least at the beginning of the operative period, said conducting apparatusbeing' operable to vary the relative radial field'strength, at least at the beginning of the operative period, as a function of time.

13. A magnetic induction accelerator for charged particles, comprising: a closed vessel defining an annular chamber within which such particles may move in'a circular path; a magnetic structure outside said vessel and comprising opposed, generally circular pole pieces which are coaxial with said annular chamber, said pole pieces being of outwardly tapered configuration to produce a predetermined relative radial field strength relationship; means for producing a time-varying magnetic field between said pole pieces; means intermittently operative for projecting charged particles within the annular chamber with a substantial initial velocity; and conducting apparatus'in saidmagnetic field aifecting the radial positionof said charged particles at least at the beginning of the operative period, said conducting apparatus being operable to vary the relativeradial field strength at and about said path as a function of time, and initially to subject the projected particles to a I2 rapidly created high relativeradial field'strength difierential' and subsequently to reduce such differential.

14. A magnetic induction accelerator for charged particles, comprising: a closed vessel defining an annular chamber within which-such particles may move in a circular path; a magnetic structure outside said vessel andcomprising opposed, generally circular pole pieces which arecoaxial with said annular chamber, said pole pieces being of outwardly tapered configuration to produce a predetermined relative radial field strength relationship; means for producing a time-varying magnetic field between said pole pieces; means intermittently operative for projecting charged particles within the annular chamber with a substantial initial velocity; and apparatus affecting the radial position of said charged particles at least at the beginning of the operative'period, this apparatus comprising a. coil with a very small number of turns between said pole faces and coaxial with said circular path,. and a circuit for energizing said coil to varythe relative radial field strength at and about said path,. at least at the beginning of the operative period, as a function of time.

15. A magnetic induction accelerator for charged particles, comprising: a closed vessel defining an annular chamberwithin which such particles may move in a circular path; a magnetic structure outside said vessel and comprising opposed, generally circular pole pieces which are coaxial with said annular chamber, said pole pieces being of outwardly tapered configuration toproduce a predetermined relative radial-field strength relationship; means for producing a time-varying magnetic field between said pole pieces; means intermittently operative for projecting charged particles within. the annular chamber with asubstantial initialvelocity; and apparatus afiecting the radial position of said charged particles at least at the beginning. of the operative period, this apparatus comprising a coil witha very small number of turns between said pole faces and-coaxial with said circular path and of substantially the same radius as said path, and a circuit for energizing said coil to vary the relative radial fieldstrength at and about said path, at least at the beginning. of the operative period, as a function of time.

GAIL D. ADAMS, J a.

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

UNITED STATES PATENTS Number Name Date 2,297,305 Kerst Nov. 13, 1940 2,335,014 K'erst Nov. 23, 1943 2,394,072 Westendorp Feb. 5, 1946

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