US2497891A - Betatron injector structure - Google Patents

Description

Feb. 21, 1950 D; w. KERST BETATRON INJECTOR STRUCTURE Filed Sept. 19, 1945 Patented Feb. 21, 1 956 Donald W. Lflha rnpaign, 111., assignor to Board of. 1rustees. of the University of Illinois,

Urbana, Ill

Application septeniber 19, 1945'; Serial No. 617,255

8 Clai1ns. (01. 256-275) This invention; relates a betatron. injector structure,- and more particularly to improvedv shielding means for: an injector structure; for

initially accelerating charged? particles in: an in. duction accelerator apparatus; 7

This invention relatesin. general tomagnetic induction: accelerator devices which comprise an evacuated doughnut-shaped. tube, -,as of glass. or.

porcelain; withmeans (as'a h'eated filament) .forr

periodically introducing electrons. or other chargedparticles intOLthe'tube; magnetic means forcausingthe.chargedlparticles to circulate inan. orbit in the tube for a v'er'ylarge number-10f: turns,

accelerating them continuously.- during this circu- 1ation,.this means comprising magnets above and below the tube with pole faces of. outwardly tapered or divergent configu'ratiomi with means for increasing-the. magnetic field continuously duringthe circulation. of the charged particles'rand means for utilizing the kinetic. energy of the charged particles. as a targetuwhich they may:

strike to create X rays; The circulating charged particle. has a centripetal force-requirement varying with the radius. o'f'its orbit, iandthe pole-face.

tapered configuration or. divergence: islsuch as to.

provide a magneticholdingforce'also varying as a functionof the radius;theEequilibrium..orbit of the charged particle. having a radius.- where these forcesareeolualuv V c. v The: general principles. of operation of; a mag.-

netic induction accelerator are known to the art. and have heretofore; been published. in variouspatents and publications, particular reference in;

this regard beingmade to Kerstjli'atent 2,297,305 of September 29, 1342. and anarticle entitled fThe Betatronf by D. W; Kerst, which appeared in vol. 10, No. 5, of theAmerican Journalgof Rhysics (October1942) Magnetic induction accelerators of thistype have also, been the subject'of anumber of other patents. and publications including the,

following: Slepi an Patent 1,645,304.; .1. Steenbeck Patentv 2,109 303 Smith Patents..2,14=3, i59-and 39 22 Patent 2,331,788.; Kerst Patent. 2,335,014; b Other articles by D. W. Kerst arezff'lhe.Acceleration of Electrons; by Magnetic Induction? which ap-.

pe'ared'in vol. 60, No. 1.-,.of. The. Physical Review (Jul 1, i941) and "A ZO-Million.Electron volt Betatron or Induction Accelerator which appeared'in vol. 13, N o. 9, .ofThaReviewof Scientific:

Instruments (September 1942) One feature, ofthis invention. is that it rovides an improved injector structure, and more particularly an injector shielding arrange nent Penney Patent. 2,193,602; Baldwin.

whichv substantially increases the yield a (as for example where the; betatronis being:used. f0r.gen\-1. era'tiongof X rays) by preventing undesirable; e1ec.-; trostatic effects upon thecirculatin'gcharged-pan ticles. u 7

Other features and. advantages. of -this B z ten tioniwill'be apparent fromthevfollowing specificationandthedrawingsrinwhichz H Figure. 1 is atopplan. view. of. the :torus or tube of a betatron, partly'broken.awaysbetter to showthe injector structure and the. evacuating con-- nection; Figure 2 is. anuenlarged fragmentary elevational'view ofathe active end of the? injector structure; Fig-ureB: is asectional view.- of the in jectonstructui'e parts shown. in. Figure 2,.along; the; line .33: thereof 1 and Figure i is a transverse sectional view alongthe. line 4-4" of Figure 3... Inasmuch asthe general. principles of operae tionofa magnetic inductionnaccelerator have-been fullyedesc'ribed. in the patents and publications listed above, andthepresent invention is. con!- cerned only with: improvement; of...the injector. structure of such. an accelerator, the :description; Oflthe. generalconstruction andoperationof the, betatron. willbe keptbrief, and. reference may. be made to such patents and publications .to-sup plement this general description-if desired.

In the particular. embodiment illustrated. here-,v Q with the, tube providing, the. annularchamber. in;

which thecharged particles, as electrons, circulate, is here identifie'dias A'; .an'd isprovided at; one point in: its periphery, withi an injector: ar rangementhere. identifiedcin general as,B,-. to be more. fully described hereinafter. It is provided at anotherpoint in itsiperiphery with a connector arrangement here identified asC, this latter arrangement serving to evacuate; the tube and. to. provide a groundcormectionfon the.conductive. inner surface thereof. The tube is preferably. made of'porcelainlforeease in formation, as;- is more fully described and claimed inthe co pending application; of. Almy and Hursh, Serial No. 5536,6 60, filed. May. 29, 194:5, -.now abandoned..

although it may bemade. ofUglass-or any other suitable. material and it 1 is, preferably coated on the; inside withal high resistance. butlconductive 1 coating, asa very thin. .fi1rn. of. palladium this. inner conductive coatingse rving as a shieldto prevent strayelectrostatic. impulses-from; having any eifect on the. electrons circulating in the tube or torus. Accelerationof the chargedpare,

ticles-in the-tube is effected by mounting .thetube between two generally, ,circularpo1cpieces of out.- wardly tapered .oridiverging. configuration to pr vide lines ofmagnetic force crossing the tube vextlcally (when the tube has-its axis-vertical, as is generally the case in practice), this magnet structure not being shown here as reference may be had to Kerst Patent 2,297,305 for a showing and description of a representative field creating arrangement. The pole pieces have magnet coils associated with them and these magnet coils are energized by an alternating current of considerable amplitude, the coils preferably being connected in parallel with condensers and energized from an alternating current generator at the resonant frequency of the tuned circuit thus provided (as at a frequency of 180 cycles per second), very large circulating currents thus flowing in the magnet coils with the generator merely providing an actuating current replacing losses in the system.

The injector structure with which this application is particularly concerned comprises in general a filament within the tube and spaced from the inner surface of a circular wall thereof, electron focusing and initial accelerating means in the form of a cup which may be given a brief high negative excitation with respect to the filament, and shielding means to prevent the voltages and currents being supplied to the filament and tube from undesirably affecting the path of the circulating electrons. The pulse of negative voltage supplied to the cup must be correlated with the frequency of the current flowing in the magnet coils, as by completing this circuit by excitation of a thyratron grid by voltage from a pick-up or peaking strip in the field of the main magnet coils, so as to inject the electrons into the tube at the beginning of the rising quarter cycle of current on the positive side of the cycle. injector structure shown in detail in Figures 2, 3 and 4, and more fully described hereafter, the electrons are injected into the tube substantially tangent to the circular path provided, with substantial initial energy of more than several electron volts. Coaction between the moving electrons and the vertical lines of force of the magnetic field cause them to be accelerated radially by the magnetic force, this acceleration being so proportioned as to cause the particles to move toward the equilibrium orbit, inwardly if the injector means is outside of the equilibrium orbit, and then to be so accelerated radially as to maintain them in that orbit. At the same time, the increasing magnetic field accelerates the electrons in that orbital path. The period of acceleration of speed of electrons continues during the entire rising portion of the positive cycle of the current, as the magnetic field is increasing during this time; and since the electrons may have described several thousand revolutions in this time, strong magnetic fields will have worked their energy up to millions of electron volts. At the end of this time means is used to swing the electrons out of their equilibrium orbit, as by a change in the distribution of magnetic field, into collision with a target, for example. During the time the electrons are describing revolutions in and about the equilibrium orbit, maintenance of the electrons at and near the equilibrium orbit is effected, in the case of radial stability, by proportioning of the field relationships such that the average fiux density throughout the area within the orbit is double the flux density at the orbit; and vertical or axial stability is secured by the curvature of the lines of force in a vertical plane.

Referring now more particularly to the injector structure, the tube A will be seen to have an outer circular wall Ill with an opening I I therethrough, the inner surface of this circular wall being By the r otherwise coated with a conductive material, as has heretofore been mentioned, this material being normally grounded in use through a connection taken oil through the arm C. Around the opening II is a neck portion [2 of the material of which the tube is made, as porcelain, to which is sealed the body l3 of the injector structure tube, which may be of glass. Extending up inside the neck I2 of the porcelain tube are concentric generally cylindrical glass members of the injector structure tube, the ends of these carrying supporting and connecting means for the filament, cup and shield which at least partly project through the opening ll into the interior of the main tube.

Referring now more particularly to Figures 2, 3 and 4, connecting and supporting members 14 and I5 carry at their outer ends the filament l6, which may be of helical form and which, when heated to incandescence, provides the electron emitting means. Surrounding the filament I6 on three sides is a cup or electrode member ll adapted to be rendered considerably negative with respect to the filament to give electrons a desired initial direction and acceleration in excess of at least several electron volts, the electrons being started off generally tangent to the circle formed by the main tube and thereafter accelerated and controlled by the effect thereon of the magnet lines of force. The negative pulse applied to the cup is normally or only very brief duration, as for example a few microseconds, but of relatively high potential difierence with respect to the filament, which might be at ground potential, sometimes as high as 50,000 or 60,000 volts. This voltage is applied to the cup through a connecting and supporting member here identified as I8, and the voltages in the connecting members l4, l5 and i8, and particularly the latter, have heretofore created a loss of electrons by electrostatic deflection of some of the electrons from the desired orbit.

While shielding means for the filament and cup have heretofore been used, I have discovered that this was not suificient, but that shielding means must be employed in such manner as to prevent electrostatic fields from the voltages and currents in the connecting members from undesirably infiuencing the electrons in their circulating path. Moreover, the shield arrangement previously used had a cross piece on the same side of the filament as the connecting members, with frequent arcing over between the connecting members and the grounded shield when an attempt was made to use high voltages on the cup, limiting the cup voltages to much lower figures than those now deemed desirable.

I have overcome these and other difficulties by providing a shield member of sheet metal formed in a U-shape with the plane of the sheet metal parallel to the lines of force and the leg por- I tions extending generally radially of the circular path of the electrons, with the filament cup and connecting members lying within the shield. Moreover, I have found it undesirable to have any shield cross piece on the side of the filament and cup from which the connecting members extend; and have found that the leg portions of the shield must extend at least half way from the filament to the arc of the inner surface of the adjacent generally circular wall, and preferably past this arc and into the injector opening in the wall.

Referring now more particularly to the specific structure shown in the accompanying drawings, 'the shield is identified in general i and is shown as comprising a generally U-shaped piece of sheet metal with the plane of the {sheet metal parallel to the lines of force passing through the tube, the Lb-shaped rcrm comprisin radially extending leg portions Ha and-I917 (although it will be understood that these could diverge or converge somewhat, if desired, or both be parallel but not exactly radial, all these situations being contemplated in the term generally radial) and a cross piece I90. This latter cross piece l9c is the only shield portion extending between the leg portions and lies on the other side of the filament and cup members 1 6 and ll fr'omthe connecting and supporting members. The shield is in turn sup ported by grounded connecting"andsupporting members here identified 85 ahd 2]. The cross piece is here shown as carr ing on one surface thereof the target piece 22 on which the electrons are adapted to ir'np'inge to create xi ays. The ends of the leg portions 19a and 19b of the U-shaped shield member are preferably fiah'gedfout slightly for rigidity; and these ends extend into the injector opening H well beyond the arc of the inner surface of the circular wall It, as may be best seen in Figure 1, so that the shielding provided by this means extends down ast the line of effectiveness of the shielding provided by the conductive coating on the inner surface (if this circ'lll'ar wall [0, these two shielding means preferably being at the same "poten tion, as ground potential. one leg portion of the shield, as the One here identified as [9a. includes a slot here identified as ltd, this slot registering with the filament l6 and providing an exit opening for the electrons.

in operation of the injector structure, ,the filament I6 is continuously heated to incandescence by relatively low voltage high current power delivered through the connecting members- H and I5. When the injection operation is brought into action by triggering of a thyratron tube by means the main magnet held, as a peaking strip, the cup I1 is made highly negative with respect to the filament, as .for example as much as 60,000 volts negative, for a brief period of a few microseconds. This voltage differential causes electrons to stream out of the opening or slit l9a in the shield, causing them to stream clockwise in the tube as shown in Figure 1.,preferably with an initial velocity of the order of several thousand electron volts. These electrons are then acted upon by the magnetic held in such manner as not only to accelerate their velocity but also to tend to accelerate them inwardly toward the center of the tube, toward an equilibrium orbit which is preferably just to the inside of the electron gun or injector structure here shown as passing through the outer wall of the tube. The electrons are then caused to travel around in, or near the equilibrium orbit for as many revolutions as may be desired, and for as long as the magnetic field of force is risin'g in strength,and are finally caused to follow an instantaneous orbit of slightly greater radius than the equilibrium orbit (as by changes in the magnetic field distribution) and to impinge on the target 22, this impingement effecting the desired generation of X-rays, as 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 No. 2,447,255 issued Aug. 17, 1948.

my omitting any cross piete in the shield stfiiew tu're on the same side of the filament and cup as the connecting members, it is possible-to 'use very much higher initial accelerating voltages on the "cup, "and thus "to start the electrons on with a nitialelectron voltage hundreds of times that heretofore possible, with a resultant higher eventual electron energy .in the same period of magnetic v acceleration. Moreover, by having a s eld arrangement of the kind shown where the leg portions "of the -U-shaped shield extend at least halfway to the arc of the circular wall in which the injector structure is mounted, "and preferably past that arc, it is possible to insert the filament further into the tube, particularly desirable in a porcelain tube, and yet completely to 'avoid any undesirable electrostatic efiects from the connecting members "for the filament and cup, and "particularly the latter. The particular injector structure here shown and "described has provided increased 'X-ray yields amounting "to as much as 300% in some tubes, with other conditions being unchanged.

While I have shown and described certain embodiments of my invention, it is to be under stood "that it is capable of many modifications. Changes, therefore, in the construction and .arrangement'may bemade without departing from the spirit and scope of the invention as disclosed in the appended claims.

I'claim:

1. 'Inan accelerator for charged particles 'having a closed vessel with at least one generally circular wall defining a chamber within which such particles may move in a generally circular path and said accelerator having means providing magnetic lines of force across said path for accelerating the particles, the inner surfaceof said wall comprising conductive material, an. injector structure including: means for emitting. charged particles, this emitting means being: within the vesseland spaced from said wall; an: electrode member for effecting acceleration of said particles away from said emitting means;. connecting members extending from said emit ting ;means and electrode member generally radially of .said circular path; and a shield member of sheet metal former in a U-shape with them-plane of the sheet metal parallel to the lines of force and the leg portions extending generally radially of said circular path, said emitting means and connecting members lying within said shield member, one leg portion having an opening therethrough adjacent the emitting means, the crosspieceof the U-shape comprising the-only transversely extending part of the shield member and lying on the other side of the emitting means from the connecting members, the leg portions of g the shield member extending at least half way from the emitting means to the arc-of the inner surface of said generally circular wall.

2. .Inan'accelerator for charged particles having a closed vessel with at least one generally circular wall defining a chamber within which such particles may move in a generally circular path and said accelerator having means providing magnetic lines of force across said path for accelerating the particles, said wall having an opening therethrough and the inner surface of the wall comprising conductive material, an injector structure including: means for emitting charged particles, this emitting means being within the vessel and spaced from said wall; an electrode member for effecting acceleration of said particles away from. said emitting :means; connecting members extending from said emitting means and electrode member generally radii ally of said circular path; and a shield'member of sheet metal formed in a U-shape with the plane of the sheet metal parallel to thelines of force and the leg portions extendi generally radially of said circular path, said emitting means andv connecting members lying within said shield member, one leg portion having an opening therethrough adjacent the emitting means, the cross- ;piece of the U-shape comprising the onl ytrans ing a closed vessel with at least one generally.

circular wall defining a chamber within which such particles may move in a generally circular path and said accelerator having means providing magnetic lines of force across said path for accelerating the particles, the inner surface of said wall comprising conductive material, an injector structure including: means for emitting charged particles, this means being within the vessel and spaced from said wall; and a shield member. of sheet metal formed in a U-shape with the plane of the sheet metal parallel to said lines of force and the leg portions extending generally radially of said circular path at least half way from the emitting means to the arc of the inner surface of the said generally circular wall.

4. In an accelerator for charged particles having a closed vessel with at least one generally circular wall defining a chamber within which such particles may move in a generally circular path and said accelerator having means providing magnetic lines of force across said path for accelerating the particles, said wall having an opening therethrough and the inner surface of the wall comprising conductive material, an in-'- jector structure including: means for emitting charged particles, this means being within thevessel and spaced from said wall; and a shield member of sheet metal formed in a U-shape with the plane of the sheet metal parallel to said lines of force and the leg portions extending generally radially of said circular path and into said opening in said wall past the arc of the inner'surface thereof and the shield member and the conductive material being maintained at approximately the same electrical potential. 5. In an accelerator for charged particles having a closed vessel defining a'chamber within which such particles may move in a closed path and said accelerator having means providing magnetic lines of force across said path for ac celerating the particles, an injector structure including: apparatus within said vessel for emitting charged particles; and a shield member Ups ture including: apparatus within said vessel for;

emitting charged particles; and a grounded; shield member of sheet metal formed in -a U; shape with the plane of the sheet metal parallel tosaid lines of force and the'leg portions extending.- generally radially of said circular path,

the interior of said U-shaped shield; member be ing entirely open. l, p '7. In an accelerator for charged particles have: ing. a closed vessel defining a chamber within. which such particles may move in a closed path. and 'sa'i'daccelerator having means providing, magnetic lines of force across said path 'for 'accelerating the particles, an injector structure in cluding: apparatus within said vessel for emit'-. ting charged particles; connecting members'ex v tending from said means; and a shield member, of sheet'metal formed in a U-shape with the plane of the sheet metal parallel to the lines 'of force, and the leg portions extending generally parallel to the connecting members, said emit-" ting' means and connecting members lying within said shield member.

. 8. In an accelerator for charged particles hav-j ing a closed vessel defining a chamberwithin; which such particles may move in a generally cirg cular path and said accelerator having means'' providing magnetic lines of force across said; path for accelerating'the particles, an injector structure including: apparatus within said ves-., sel for emitting charged particles; an electrode member for effecting acceleration of said parti cles away from said emitting means: connecting members extending from said emitting means and electrode-member generally radially of said" circular path; and a shield member "of sheet metal formed in a U-shape comprising leg por-" tions and across-piece,'with the plane 'of the sheet metal parallel to'the lines of force and the leg portions extending generally radially of said circular path, said emitting means and connecting members lying within said shield mem-" ber, one leg portion having an opening therethrough adjacent the emitting means, the crosspiece of the U-shape comprising the" only trans versely extending part of the shield member and M: lying on the other side of the emitting mean's The following references are of record in the file of this patent:

,UNITED STATES PATENTS Name Date I Number Kerst Sept. 29, 1942- Certificate of Correction Patent N 0. 2,497,891 February 21, 1950 DONALD W. KERST It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 5, lines 32 and 33, for potention read potential; column 6, line 48, for former read formed;

and that the said Letters Patent should be read with these corrections therein that the some may conform to the record of the case in the Patent OflEice.

Signed and sealed this 4th day of July, A. D. 1950.

LSEAL] THOMAS F. MURPHY,

Assistant Oommz'saz'oner of Patents.

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