US 2940001 A
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Patented June 7, 1960 ELECTRON ACCELERATOR Richard F. Post, Walnut Creek, Califi, assignor to Applied Radiation Corporation, Walnut Creek, Calih, a corporation of California Filed Feb. 8, 1955, Ser. No. 486,865
Claims. (Cl. 315-542) The present invention relates to an improved method and means for injecting electrons into and applying accelerating energy to a traveling-wave electron accelerator.
Conventional traveling-wave electron accelerators comprising a waveguide having spaced walls defining, in effect, successive resonant cavities are normally energized by a radio-frequency vacuum tube such as a velocity-modulated electron tube coupled thereto and have associated therewith an electron gun source admitting a beam of electrons into the waveguide. The present invention provides method and means for energizing an accelerator waveguide and injecting high voltage electrons therein in a single operation. The invention contemplates the simultaneous production of radio-frequency energy and high voltage electrons with a subsequent further acceleration of said electrons by the radio-frequency energy in the form of a traveling voltage wave. One manner of carrying out this method is to couple the radio-frequency energy and electrons from a velocity-modulated electron tube into a traveling-wave linear accelerator in correct phase so that the energy excites the accelerator waveguide to further accelerate the back phase (voltage-doubled) electrons injected.
The method and means of the present invention is herein illustrated and described with respect to specific steps and structure in the interest of clarity, however, no limitations are intended or be inferred therefrom, reference made to the appended claims for a precise delineation of the scope of the invention.
The invention is illustrated in the accompanying drawing wherein:
Figure 1 is a schematic representation of a preferred embodiment of the electron accelerating means of the invention;
Figure 2 is a schematic representation of alternate collector coupler means;
Figure 3 is a section View taken at 3-3 of Figure 2; and
Figure 4 is an alternate embodiment of the collector coupler.
Considering now the invention in some detail as to the method thereof, it is contemplated that there will be produced a bunched beam of electrons. These electrons may be continuously emitted from an appropriate electron emitter such as ahot filament, for example, and the electrons are accelerated by suitable electrical fields to a relatively high potential and collimated into a beam. This beam of electrons is then velocity modulated as by time varying electrical fields to produce a bunched electron beam having periodic electron density variations. The bunched electron beam is then employed to produce electromagnetic energy as by passing same successively through one or more resonant catcher cavities whereby same are excited. There thus results a bunched electron beam and electromagnetic radiation with the latter being employed to further accelerate a small portion of the bunched beam and return energy thereto so that there is produced a resultant high voltage electron beam.
There is produced in the first portions of. the above process a voltage doubling action whereby certain relatively few electrons in the bunched beam attain substantially twice the velocity or voltage of the remaining elec trons, by virtue of passing through the catcher cavity 180 out of phase with the phase of driven oscillations in said cavity. These voltage-doubled electrons are herein chosen for further acceleration. Following production of the electromagnetic energy by the bunched beam','the
beam is collimated or otherwise operated upon to materially reduce the beam current and as the main portion of the beam tends to defocus in the process of establish ing the electromagnetic energy, collimation of beam discriminates in favor of thevoltage-doubled electrons.
In addition to collimating the beam, the electromagnetic energy waves are directed along the beam path for returning energy to the beam. In order; for the beam to be accelerated by the electromagnetic waves 'the phase of the latter is varied with respect to the voltage-doubled electron beam pulses, or vice versa, and in this respect certain alternative steps are possible. In order to obtain the proper phase relation, either the phase of the electromagnetic Waves may be advanced with respect to the beam pulses or it may be retarded with respect thereto. Advancing the phase of the electromagnetic waves may be accomplished by passing both the energy waves and electron beam through a transmission line wherein the wave velocity is substantially equal to or greater than the speed of light and the electron beam velocity is much less so that in a predetermined distance a desired phase advance is achieved. Retarding of the energy wave phase may be accomplished by directing both energy waves and the beam through a slow wave structure wherein the wave velocity is reduced to less than that of'the beam and wherein a predetermined distance of travel shifts the relative phase to the desired relationship. Although'both of the above alternatives are feasible, a minimum length of travel of beam and wave results either from the slow wave propagation or the propagation of the wave at a velocity greater than that of light for in both cases a maximumized velocity difference between wave and beam is attained. Certain advantages will be seen to attach to the minimization of distance required to accomplish this step of the method. 7
Acceleration of the voltage doubled portion of the bunched beam is accomplished by establishing traveling voltage waves with the electromagnetic waves originally produced by the beam and passing the beam therethrough in proper phase to receive acceleration therefrom. As to the proper phase relation same is produced in the above-noted manner so that the voltage waves established do accelerate the voltage-doubled electrons of the beam. There is produced by the above process a high voltage electron beam of periodically varying electron density. The original electron beam produced in the process will be seen to have actually supplied energy to a small portion of itself to drive same to a high voltage and this is accomplished by producing electromagnetic energy from the beam and then feeding same back into a selected portion thereof. Various structural arrangements may be employed to carry out the described method and referring to Figure 1 of the drawing wherein one preferred means is illustrated, there is shown a velocity-modulated electron tube 11 axially aligned with the waveguide 12 of a traveling-wave linear accelerator 13 and communicating therewith through a cylinder 14 disposed between same. The electron tube 11 may consist of a conventional electron-beam radio-frequency tube such as klystron and including an electron emitter 16 having a filament and heating means therefor to emit electrons within the tube. Electrons are accelerated from the emitter 16 by an anode 17 which may be formed as a transverse tube Y the collector-coupler.
time d thence through a buncher cavity 18 aligned with v emitter and adapted for excitation by an externally time-varying voltage of a radio frequency, for example. The electron beam is velocity modulated by above, the proper choice of length of the collector-coupler 22 will place the voltage-doubled electrons in proper phase relation to the cavity excitation so that only bunches of voltage-doubled electrons are accelerated thereby. The
this yoltage in the buncher cavity 13 in-a conventional manners6 that the electron beam in traversing a relatively field tree. space beyond the cavity 18 becomes bunched h pe iodic variations in electron density: Following ncher cavity 18 is asecond cavity 2 lconnected .toth first' jbyi an elongated passage 19 through which the eleetron beam travels. V'Fhe secondjcavity '21 is designed to resonate at the frequency of the bunched beam so that there is thus produced in a conventional manner a resonant energy exchange within the cavity 21 whereby electromagneticenergy is produced therein.
The above-described' elements and operation are conventional in tubessuch as a 'klystron, however, the electron beam is conventionally intercepted by a collector beyond the output cavity 21 with the radio-frequency energy developed in the tubebeing fed into other apparatus asby coupling loops or the like for use therein. The. presentiinvention removes both the electron'beam and if:idjo-freq-uency energy from the tube and forthis purpo'sej provides'a collector-coupler 22 in, axial align mentwith the tube 11 communicating with the output cavity21thereof... This collector-coupler 22 includes the cylinder 14 and an inner member 23 shown as a cylinder or solid of revolution having an axial passage therethrough similar to a reversed venturi with a diameter decreasing with separation from the cavity 21 and having an enlarged opening at the exit-thereof, as shown. The member 23 may be mounted as by a pair of quarterwave stubs 2'4 wit hinthecylinder 14 to minimize insulation problems as shown in Figure. l. A small opening therethrough will .beseent to intercept and collect a large portion of the electron beam from the electron tube 11. 'Within the 11 a certain small portion of the electrons inthe beam experience an additional. acceleration to substantial- -ly twice the voltage of the restof the beam electrons by virtueof their phase'being such as to be accelerated in V the outputcavity 21 rather than decelerated. These electrons are called volt-agedoubled electrons; It is these ;vo ltage doubled electrons which are preferentially chosen inftheprsent invention for additional acceleration, since I itl canibeshown that an increase in injection voltage increasesithe efiiciency of, and simplifies the design er,-
] trave ing Wave accelerators. V 1 Within the col1ector-coupler22 of Figure l the radioirequency energy from the tube 11 travels at the speed of;;-light,between the cylinder 14"and member 23'while.
even .the voltage-doubled electrons travelingrthrough the coupling theenergy waves advance in phase exactly the proper amount in relation to thebunches of voltagevdoubled electrons asrnoted below. Within the accelerator by the collectorcouplerfn. There is thus established '1 in the accelerator 12 a travelingwavereceiving its energy,
from the tube 11 and giving up energy to electrons passing @t-herethrough ,in proper phase relation thereto. As noted collector-coupler 22 thus acts as'a collector for the electron beam of the tube 11 whileyet admitting passage of a portion of said beam therethrough and also couples the tube energy into thewaveguide ,1 3-f or exciting the cavities 27 thereof. V V r The portion of the electron beam accelerated in the waveguide 12 maybe utilized exteriorly of'the apparatus by the provision of'a window 28'at the closed end of the waveguide on the axisdhereof. The window 28 is pervious to the accelerated electron beam and yet seals the system to maintain a vacuum therein which is advantageous in the operation thereof.
With regard to the variation in phase of the electromagnetic waves and the; voltage-doubled electrons of the radians) with respect to vthe phaseoi the voltage-doubled electrons. With the. above described coaxial collectorcoupler .of Figure l having 3a length L, the rfollowing V wherein:
relationshipis attained ,for' 180 phase shi-ftin same:
2TB 21rL V an; Ber 7 k =freespace wavelength at operating frequency Be=velocity of electron beam relative to free space velocity of light (0) ng=phase velocity of wave relative to free space velocity of light (0) Solving this relationship for L gives:
'4 My 2 rig-5 The phase velocity of the wave is substantially. equal to the free space velocity of light so that 'fig=1 and in a typical case of 300 thousandelec't'ron volts, fle=0. 8 so that substituting r in the above relationship, 7
and in a typical velocity modulated electron tube, as a .klystron,;wherein the, wave. length is, 10.57 centimeters,
L =21 centimeters.
Although the above determined :lengthof. the cellectorcoupler isnot excessive it, may be reduced by alternative structure suchas shown in Figures 2 and 3.
. Theialternative embodiment ofFig'ures '2 and '3 illustrates acollec'tor-coupler 31 including, a cylinder 14 conlar in structure to the like numbered member of Figure l faxial passagein member-23 have a much lower velocity soJthatthe, energy waves advance in phase with respect -to-the bunched. electrons of the beam in passing through By choosing the proper. lengthof nected between the; output cavity-2:1 of .a-tube' 11, and the waveguide 12 of the accelerator 13. Coaxially within this cylinder 14 is mounted'a cylindricalrnember 23 simibut shorter, as noted below. In this embodiment a fundamental coaxial, mode at eachend of the coupler is transformed in the central part of the coupler to a special waveguide mode. Thus ateachendthe coupler 31 and surrounding cylinder 14 are symmetrical so-that the electric'fiield ofthe'transmitted electromagnetic waves is directed radially outward from the coupler axis and uniform about the member 23; This coaxial mode of transmission becauseof its 'axialxsymmetryispartic'ularly desirable. for coupling out of 'thetcavity 21 and into the accelerator 13 The central portionof the coupler 31 is' supported by: a wall orupstanding member 32,.as shown in Figure 2 and 3, paralleltothe lcoupler'axis'and below 'same between the cylinder 14 and member- 23; This thereby to become circumferential wherein the wave ve locity exceeds that of light. Small vertical posts 33 are disposed upon the cylinder 14 below the member 23 at each end of the wall 32 thereof for impedance matching between the coaxial and waveguide modes and these posts are conventionally termed matching posts. While the aforesaid matching posts serve to illustrate an embodiment of the invention, various other means for accomplishing the same results will suggest themselves to those skilled in the art.
In the embodiment of Figures 2 and 3 wherein the wave velocity in the center section coupler exceeds the speed of light the length L of the center section having the wall 32 attached thereto may be determined from the relation:
section is determined by the boundary conditions for the transmission mode which may be stated as:
sin (K p)= or K =g wherein: p=mean circumferential dimension of the coupler and K +K =K where Solving this for Bg gives and in a typical case wherein the diameter of the member 23 is 2.5 centimeters and the cylinder diameter is 3.5 centimeters, p=7.0 centimeters, so that with A =10.5 centimeters and 52:0.8, as above, g=1.60.
Thus, substituting in the above equation for L gives L=0.8)\ or 8.4 centimeters. This dimension of 8.4 will be seen to be materially less than the length of 21 centimeters calculated above for a typical structure of the embodiment of Figure 1 and even with the addition to this of the end sections of the coupler the resultant length is substantially less than that of the coupler of Figure 1.
One further embodiment of the coupler is herein illustrated in Figure 4 wherein a member 23 substantially identical to that of Figure 1 except for size is mounted as by quarter wave length stubs 24 in a cylinder 14 between the tube 11 and accelerator 13. In this instance a periodic structure is formed within the coupler 34 formed by the above mentioned elements so as to produce a periodically loaded coaxial section to delay the phase of the wave with respect to the electrons. Such a structure is well known for its properties of slow wave propagation and in the illustrated embodiment there are provided upon the coupler a plurality of radial flanges or discs 36 spaced evenly along the coupler upon the exterior surface thereof. As the wave is propagated between the cylinder 14 and central member 23, the discs 36 form such a periodic structure as to materially reduce the wave velocity. Within practical limits the wave velocity may be made as small as desired and, for example, a value of fig=0.5, wherein fig here again equals the phase velocity of the wave relative to the velocity or" light, may be achieved with a ratio of disc to member 23 diameter of about 1.5 to 1 and a ratio of cylinder to member 23 diameter of about 1.7 to 1. The relationship noted in relation to the case of Figures 1 and 2 is here again ap- 21rL 21rL A BeBg) B s By and 2 5 -1 9 Substituting 52:05 and Bg=0.8 in the above relation gives L=0.67)\ so that with A =l0.5 centimeters, L=7 centimeters. In this case, as in that of Figure 2, L is taken as the length of the center periodic section of the coupler 34 so that an addition is to be made thereto of the unloaded end sections to determine the overall length.
There has been described above an improved method and means for electron accelerationwherein but a single source is employed to produce a pulsed electron beam and energy for accelerating same. With regard to the alternative embodiments of the coupler illustrated and described, same are presented as illustrative only and particularly the calculations herein included are to be taken only as examples and in no way limiting upon the scope of the invention which is precisely delineated in the following claims.
What is claimed is:
1. An electron accelerator comprising a velocity modulated electron tube, a waveguide disposed in axial alignment with said electron tube and having transverse partitions defining connected resonant cavities, and coupling means intermediate and interconnecting said tube and waveguide with coaxial means therein directing radiofrequency energy from said tube into said waveguide and having a small opening therein limiting electrons transmitted from said tube into said waveguide, said coupling means also including means for varying the relative phase of said radio-frequency energy and beam to provide substantially degrees phase difference therebetween.
2. Improved electron accelerating means comprising a traveling-wave linear accelerator including a waveguide, a velocity modulated electron tube generating a bunched beam of electrons for producing radio-frequency energy, said electron tube and accelerator wave-guide disposed in axial alignment, and coupling means connecting said tube and waveguide for admitting to said waveguide a portion of the bunched electron beam of said tube and coupling said radio-frequency energy to said waveguide with an advance in the relative phase of said radio-frequency energy whereby said waveguide is energized to accelerate said bunched electrons therein.
3. An improved ejector for an electron accelerator comprising a velocity modulated electron tube including means producing a bunched electron beam and defining a resonant cavity traversed by said beam wherein electromagnetic waves are excited thereby, and coupling means communicating with said tube at said cavity in axial alignment with the electron beam thereof, said coupling means defining coaxial openings therethrough for passing said electromagnetic waves and a portion of said electron beam in predetermined shifted phase relationship and adapted for connection to an electron accelerator to inject therein electromagnetic waves and a bunched electron beam.
4. Improved coupling means for coupling a velocitymodulated electron tube to a traveling-wave linear electron accelerator accelerating Waveguide comprising an elongated tube adapted for axially aligned communicable connection between the output cavity of'said velocitymodulated tube and the input of said waveguide, and a coupling cylinder disposed coaxially within said elongated tube and having a beam collimating aperture therethrough for transmitting a collimated bunched electron beam through the aperture and electromagnetic waves through the annulus between said cylinder and elongated tube from the output cavity of the tube to the input of the Waveguide.
5. Improved coupling means as claimed in claim 4 further defined by said coupling cylinder having smoothly curving convex inner walls tapering radially inward away 7 from the inletend thereoffor collecting the majority of electrons of an entering'electron beam and passing onl a small percentage thereof. a
6. In an improved injector for an electron accelerator including a periodically loaded accelerating waveguide,
, the combination oomprisingaklystron' having an openended outputcavity, a cylinder connected in axial alignment with said klystron in unimpeded communication with the output'cavity thereof and adapted for communicable-connection to theaccelerating Waveguide of an elec- ;,tron linear accelerator, and an inner coupling cylinder disposed in said cylinder, said inner coupling cylinder having" a passage therethrough' tapered radially inward for collimating an electron beam from said klystron' to pass'only a small portion thereof and having a length sufficient to advance the relative phase of electromagnetic Wave energy radiated from the klystron through the annulus between said inner coupling cylinder and outer .cylinder a predetermined amount over that of the electron beam passing through said passage.
7. An electron accelerator comprising a velocity modulated electron tubeproducing a bunched electron beam and resonant electromagnetic energy, a traveling wave linear accelerator having a waveguide, and means coupling saidtube and waveguide in alignment and providing direction communication for coupling the electromagther defined by said coupling means having a central aperture of 'small diameter collimating said electron'beam to intercept all but the central core thereof and having an elongated waveguide structure wherein the relative phase of the energy and'electron beam varies in propor-' tion to the length thereof-for establishing such a phase relationship that 'said'voltage waves accelerate predetermined bunches of electrons in said beam. 7
' further defined by said couplerhavinga length sufficient 9. An electron accelerator comprising a velocity-moda lated electron tube producing resonant electromagnetic energy with a bunched electron-beam, a traveling wave electron accelerator structure'axially aligned with the electron beam of said tube at the end thereof, and a coupler communicating between saidtubeand said .ac-
. celerator structure with; said-coupler including 'a coaxial member for coupling electromagnetic energy from said tube into said accelerator structure anddefining an axial aperture for collimating the electron beam from said tube to substantially original. cross section prior to entry ,of sameinto said accelerator structure: V g 1 '10. An electron accelerator comprising tube 'means es-, 'tablishing resonant electromagnetic energy with .a bunched electron beam, coupling means communicating "with said tube means in line withthe ,electronbeam of same for. passage of said beam. therethrough and including a periodically loaded waveguide structure coaxial section wherein saidelectromagnetic energyis'transmitted at a low propagation velocity so that the phase of said electron beam advances over that of the vtransmitted energy, and a plurality of communicating resonant cavities of successively increasing volume'connectedto said coupling means wherein said electromagnetic energy estab- V lishes traveling voltage waves for accelerating portions of said electron beampassing therethrough.
' to retard the phase of the electromagneticenergy one hundred and eighty degrees relative to the phase of said bunched electron beam. J 1 1 12. In'an injector for an ele'ctr onaccelerator' including a plurality of communicating resonant cavities, the'co'nrbination comprising means producing a bu'nched electron beam, means defining a cavity 'resonant zit-the frequency of said beam bunches and disposed for beam traverse therethrough whereby resonant electromagnetic energy waves are established insaid cavity, and couplingmeans admitting a collimated portion of. said electron beam intoisaid cavities for passage therethrough, said coupling means including a coaxial section coupling said electromagnetic energy into-said cavities .for exciting same and including axially spaced radial disc loading said section for reducing Wave propagation velocity therein whereby V bunches of said beam. 1
13. An electron accelerator comprising a velocity modulated electron tube including means establishing an electron beam, means bunching said beam'to produce spaced high density main beam bunches and intermediate secondary beam bunches of lesser density, and means defining a cavity traversed by said beam and resonant at the passage frequency of said main beam bunches for excitation thereby to produce electromagnetic energy waves accelerating secondary beam bunches; a periodic waveguide structure defining consecutive cavities resonant at the frequency of said energy Waves; and a coupler-collector communicating between said electron lube .and waveguide and including an; elongated coaxial section coupling energy waves into said waveguide for energizing same and beam collimating means collecting divergent low energy electrons of saidmain electron bunches and passing high energy s'econdaryelectron bunches into said waveguide for acceleration therein. 7
14. An electron accelerator as claimed in claim 13 a further defined by said collector-coupler including means varying the relative phase of output energy waves'to substantially correspond with the phase of said secondary electron bunches.
15. An electron accelerator as 13 further defined by said collector-coupler having a-predetermined field free length with means controlling propag'ation velocity of said "energy waves'therethrough for setting the relative phase of'exciting energy waves-and f electron bunches.
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