US2944182A - Beam focusing system - Google Patents

Description

July 5, 1960 W. W. RIGROD BEAM FOCUSING SYSTEM Filed Nov. 13, 1958 mmm@ /f//f/ /NVENTOR W. W. R/GROD' '.4 aan, tlf

BEAM Focosnvo srsrmr William W. Rigrod, Millington, NJ., assigner to Beil Telephone Laboratories, Incorporated, New erir, N.Y., a corporation of New York Filed Nov. 13, 1958, Ser. No. 773,62

9 Claims. (Cl. SiS-3.5)

This invention relates to electron beam focusing and more particularly to such focusing in traveling Wave tubes. In a traveling Wave tube the electromagnetic wave propagates along an interaction circuit past which is projectedan electron beam in iield coupling relationship. Because of the relatively long length of the electron path and because of the space charge forces acting in an electron beam, it is generally necessary to provide focusing means to keep the electron flow cylindrical during its travel past the interaction circuit. In the past, such focusing generally has been provided by establishing a longitudinal steady magnetic field along the beam path. However, in practice, the high fluxes required in the large gap for such magnetic focusing have necessitated the use of large `permanent magnets or solenoids which have added much to the` `bulk and `Weight .of traveling Wave tubes.

Through the use of a periodically varying magnetic field it has been found that the bulk of the magnetic apparatus can be considerably reduced. This is primarily due to better utilization of the magnetic lields. In actuality, it has been found the ratios of maximum fields thereby produced to the longitudinal steady lields for the same magnetic weight may be from 5:1 to 30:1.

In their article, Focusing of an Electron Beam by Periodic Fields, Journal of Applied Physics, volume 25, pages 436 throughv 447, April, 1954, A. M. Clogston and H.Heifner discuss various techniques of utilizing the interaction between `a periodic transverse magnetic eld yand the longitudinal velocity of charged particles to provide an inward force acting on the charged particles. One structure suggested in this article is a quadrupolar arrangement of alternating north and south pole faces having hyperbolic proiiles. A plurality of such arrangements are placed successively along the length of the electron beam path, successive quadrupoles being rotated 90' degrees such that a spatially periodic field is produced both in the longitudinal and circumferential sense of the elect-ron beam. Each electron traveling along the beam then sees an alternating magnetic ux. Although the net result of the force caused by this magnetic ux is in an inward radial direction, the alternating quality ofthe flux gives rise to fluctuations in the paths of the electrons which is called ripple.

By providing periodic magnetic focusing in the circumferential sense of the beam as described above, the crosssectional area of the electron beam can theoretically be maintained at a fairly constant value. The quad-rupolar arrangement, however, tends to cause the electron beam to assume an elliptical cross-section which effectively rotates as it passes successively rotated qua-drupoles. This isv an expected manifestation of ripple caused by alternate focusing and defocusing. Because the surfaces of the beam cannot be maintained parallel with the axis thereof, and because of the erratic paths of electrons as a result, optimum interaction with the radio frequency helix is impeded. l,

2,944,182 Patented July 5, 1960 with the minimum of four pole faces in an array, mass production techniques are impracticable because of the close tolerances necessary in maintaining constant radial distances from the axis of the electron beam and proper angular correlation of the respective pole pieces. The introduction of even the slightest asymmetry may not only causeincreasedundesirable ripple but may well cause such instability in the electron beam as to result in impingement on the interaction helix by the electrons and hence a degradation of operating efficiency even to the extent of complete inoperativeness. The magnitude of linx intensity which can be introduced is limited by the magnitude of diverging force which the flux force is counter-balancing. In the Clogston and Heifner device this diverging force is due only to the electron beams space charge. The abruptness of ux change is a function of lux intensity and frequency and is therefore limited for the foregoing reasons. Thus, each of the procedures aimed at reducing beam ripple, while achieving strong lfocusing, introduces other disadvantages to the extent that the degree of ripple reduction practicably attainable is severely limited.

t is an object of the present invention to achieve strong. periodic magnetic focusing with negligible beam ripple. -t is another object of this invention to produce strong focusing with negligible beam ripple with apparatus of quite simple construction having a minimum of critical` tolerances.

These and other objects of this invention are attained in one illustrative embodiment of the invention wherein two annular metal members of opposite magnetic polarity are placed at opposite ends of the interaction` circuit of a traveling wave tube. Elongated extensions of high permeability protrude from each of these members along the length of the interaction region and are interleaved with one another to form a circular array ofelongated magnetized elements of transversely spatially alternating magnetic polarity surrounding the traveling wave tube. Variable magnetic strength is imparted to the annular members and their elongated extensions by connecting the two annular members by electrical solenoid means and by energizing the solenoid -with a variable direct current. The electron beam of the traveling wave tube is given a transverse velocity component beforeentering the focusing region defined by the magnetized members such that each electron thereinV travels a substantially helical path to a collector electrode.

During its travel to the collector, each electron passes? rst a north magnetic pole and then a south magnetic pole and hence sees magnetic field. In addition to providing strong focusing, the periodicity of the magnetic tield in the circnmferential sense ofthe electron beam maintains a substantially circular cross-sectional. area on the electron beam. Rapidly alternating focusing, accomplished in the instant invention by increasing the rotational component or twist `of the electron beam,` whereas such increased rapidity in `the Clogston and Helner device woulddemand an increased number of successive arrays or quadrupoles.` in either device, -a more uniformly circular cross-section on the electron. beam, and hence, reduced ripplecan be: producedby a spatially alternating periodic and thus reduced ripple, can be providing a greater number of pole pieces in each circular array. In the instant invention, however, there is only one array of pole pieces, and an increase in the number of pole pieces in the array not only improves the configuration of the electron beam but also increases theV rapidity of alternation of the focusing. In the afore-mentioned device there must necessarily be a large number of arrays, each of which must contain additional pole pieces merely to improve the beam cross-section. In applicants device not only is there a comparatively small number of elements which must 4be correctly oriented for proper functioning, but any small increase in that number will both improve the beam cross-section and reduce ripple by the resulting increase in periodicity. The rotating beam of the instant invention also plays a dual role in that, besides permitting periodic focusing, the centrifugal force thereby exhibited permits a stronger focusing force which not only restricts random movement of the electrons, but also increases the abruptness of fiux change, thereby further reducing ripple.

From the foregoing description of this embodiment of the invention, it is apparent that, since there rare a plurality of members of high permeability extending longitudinally throughout substantially the entire length of the focusing region, each of the members being of substantially constant polarity throughout its length, there will be little or no longitudinal magnetic field in the region ofthe electron beam. Further, the magnetic intensity of the transverse magnetic field is substantially constant, both as to magnitude and direction, along any line in the interaction region parallel with the axis of the electron beam.

In another embodiment of the invention, the focusing apparatus comprises a circular array of elongated pole pieces connected along their lengths by sectors of permanently magnitized material. to form a cylinder which surrounds the tubes interaction region. The permanent magnets are so arranged as to impart a north magnetic polarity on alternate pole pieces and a south magnetic polarity on the other alternate pole pieces. This embodiment Ais then used in conjunction with a traveling wave tube electron beam having a transverse velocity component as previously described.

It is a feature of this invention that there is established, along the path of ow of an electron beam, a beam focusing field which is longitudinally uniform for substantially the entire length of the beam path and which is spatially alternating transversely to the path of flow.

It is another feature of this invention that the electron beam have imparted thereto a transverse velocity component prior to its entry into the focusing region such thatY electrons in the beam travel substantially helical paths and encounter periodically alternating magnetic focusing fields.

A complete understanding of this invention and of these and other features thereof may be gained from consideration of the following detailed description and the accompanying drawing, in which:

Fig. '1 shows, in longitudinal cross-section, a conventional traveling wave tube of a preferred embodiment of the invention in which a rotating annular elec-tron beam is focused by spatially alternating electromagnets;

Fig. 2 shows a transverse cross-section taken along lines 2--2 of Fig. 1; and

Fig. 3 shows, in transverse cross-section, another embodiment of the invention in which an electron lbeam is focused byy spatially alternating permanent magnets.

Referring now more particularly to the drawing, an

-evacuated glass envelope 11 houses the various tube elements. At one end of the envelope is an electron gun 12, generally designated las the Pierce-type electron gun, which serves as the electron source for providing a rotating hollow beam of substantially uniform density over its cross-section. This gun 12 is merely exemplary of many types of electron guns which could be used. Indeed, although an annular beam has generally been shown to give better results, an electron gun generating a solid beam could be used, the only restriction being that said beam must contain a rotational velocity component. The gun illustrated comprises an annular cathode 12A, an annular beamdorming electrode 12B and an annular accelerating anode 12C. Consistent with the well-known principles of the Pierce-type gun, the surfaces of electrode 12B and anode 12C are of such configuration that, in the presence of complete space charge, the electric field component in a direction parallel to the emissive sur' face of cathode 12A is zero. The electrode 12B and the anode 12C each include a central or inner portion and an outer portion defining annular passages therethrough for the electron beam. By the inclusion of such portions, the usual form of Pierce gun may be advantageously modified vfor providing a hollow cylindrical beam of substantially uniform current density. In operation, the cathode 12A is maintained at reference potential, the electrode 12B at a potential slightly lnegative thereto, and the anode '12C at Va potential positive thereto. In the interest of simplicity, Ylead-in connections and electrode support means have not been shown.

At the end of the tube opposite the electron gun llt is a collector '13 which receives the spent electron beam. Intermediate the gun and collector and coaxial with the generated cylindrical electron Vbeam is an 'interaction helix 14. A radio frequency signal from a source, not shown, is introduced in the helix in any suitaple manner well known to workers in the art. For purposes of illus-' tration the tube shown is designed for operation as afer-'1 ward wave amplifier. To .this end, the input Wave 'to be amplified is applied to the electron source end of the' helix and the ou-tput wave is abstracted at the collector end. Input and `output wave coupling means 18 andA 19, respectively, are shown schematically for purposes of simplicity. Suitable arrangements for coupling wave guide or transmission line connections to such a traveling wave tube are well known in the art.

A magnetic fiux producing means, such as solenoid 15,v is positioned adjacent the source of the electron beam current. Such means serves a dual purpose. First, it provides a longitudinal magnetic field in which the electron gun is immersed to confine the beam until it can be focused by the periodic magnetic field. Secondly, it provides a transverse component of magnetic flux on` the beam immediately before said beam enters the periodic magnetic field such that `a rotational velocity component is imparted thereon. To shape themagnetic. flux to the desired configuration, two annular disks 16 and 17, of highly permeable material are positioned` transverse to the path of flow, extending as pole pieces from a solenoid 1S. In the arrangement shown, the diskl 16 is external and the disk 17 internal the tube envelope.; By properly positioning these pole pieces, the magnetic` flux linking them is longitudinally oriented when it passes through electrodes 12A, 12B and 12C and is transverselyl orientedr where the electron beam passes the disk 17 and enters into the region of alternating field. Y

In operation, electrons emitted from the cathode 12A` are formed into 'an annular beam by the action of the beam forming electrode 12B and accelerated toward the` accelerating anode 12C. The electrons are confined tol longitudinal paths through this gun region. As they ap-` proach the pole piece 17, where the magnetic flux ac`` quires a transverse direction, they are given a rotational.`

component which sets them spinning with an angularv This magnetic field is advantageously formed to be` spatially alternating in the circumferential sense of the electron beam and of constant uniform strength in the longitudinal sense. The focusing means which produces abituati the magnetic iield in the interaction region of the traveling wave tube comprises solenoid 22 which includesannular pole piece members of high permeability, 20 and 21. Current is supplied through voltage source 23 such that a magnetic polarity is imparted to member 20 and an opposite magnetic polarity is imparted on member 21. In the illustrative embodiment of Figs l and 2, member 20 is of a north magnetic polarity, while member 21 is of a south magnetic polarity. Annular member 20 surrounds the envelope l1 at the electron source end, while annular member 21 surrounds the envelope at the electron collector end. Extending from annular members 20 and 21 and parallel with the electron beam axis 'are elongated pole pieces of high permeability, 24 and 25. lPole pieces 24 extend from annular member 20, while pole pieces 25 extend from annular member 21. 'Since the magnetic reluctance along pole pieces 24 and 25 is very small, pole piece 24 is of a substantially uniform north polarity, whilepole 25 is of a substantially uniform south polarity. Pole pieces 24 and 25 interleave one another in alternate relationship such as to form` a circular array surrounding the envelope 11, as is best seen in Fig. 2. The magnitude of polarization on pole pieces 24 and 25 can be varied by varying the output. of direct current voltage supply 23. As can be appreciated from a study of Fig. 1, the magnetic flux intensity along any line in the interaction region parallel with the axis of the electron beam is substantially constant and unvarying, both as to magnitude and direction. Further, the longitudinal component of magnetic ux intensity is substantially zero. As is apparent from Fig. 2, the magnetic flux intensity varies with spatial periodicity'in the circumferential sense.

Individual electrons projected from electron gun 12 follow a substantially helical path to collector anode i3 due to the rotational velocity component introduced into the electron beam by the magnetic field in the aperture of member i7. During its traverse, each electron passes, in `alternate succession, first a north magnetic pole piece 24, than a south magnetic poie piece 25. Hence, each electron sees a spatially alternating magnetic field.

-The effect of an yalternating magnetic field on any electron is a force thereon which is proportional to the root mean square of the alternating flux intensity. In the instant case, it can be shown that the net inward focusing force on an electron of the beam is proportional to the root mean square of the magnetic flux intensity component in the direction tangential to the path of said electron.

Because of the spiral motion of individual electrons during their traverse, they encounter a relatively large number of polarity reversals. By increasing either or both the rotational velocity components of the beam and the number of pole pieces 24 and 2S in the array, the number of such reversals can be increased such that the ripples caused thereby are so close together and of such small magnitude as to be" negligible. The increased number of pole pieces also tends to promote a uniform circular cross-section on the electron beam. Further, centrifugal force caused by the beam rotation tends t make the beam diverge. l'fhis permits a stronger converging focusing force which can be applied through voltage supply 23 which reduces random movement of the individual particles. The forces acting on the cylindrical beam are hence quite large and the surfaces of the cylindrical beam become sharply dened. Further, because of increased magnetic intensity, the iiux change becomes more abrupt, thereby further reducing ripple.

Fig 3 illustrates another embodiment of the invention` in which alternately polarized pole pieces 24 and 25 are joined by sectors of permanently magnitized material 26 to form `a cylinder surrounding the traveling wave tube from its electron source end to its electron collector end. Each pole piece 24 is in contact, throughd out its length, with the north magnetic pole. of eachadjacent permanently magnetized sector 26, while eachpole piece 25 is in contact, throughout Vits length, with the south magnetic pole of each adjacent sector of permanently magnetized material. Pole pieces 25 and 26 .are arranged in alternate relationship to form a circular-array around the envelope 14 such as to present a field configuration which is uniform in the longitudinal sense and periodic in the transverse sense.

lFor the sake of simplicity, only the enevolpe andhelix of a conventional traveling wave tube have been shown in the embodiment of Fig. 3. The tube` apparatus is intended to be the same as that shown in the embodiment of Figs. 1 and 2.

As can be readily appreciated from the foregoing vdescription, the focusing apparatus of either of the embodiments is so constructed that it canI A'be conveniently fitted over the envelope of ya conventional traveling wave tube. The embodiment of Figs. l and 2 is advantageous when a variable flux is desired. The embodiment of Fig. 3 presents the advantage of being even more compact than the embodiment of Figs. l and 2.

It is to be understood that the above arrangements are merely illustrative of the application of the principles of the present invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is: l

l. In an electron discharge device, an electron gun for forming and projecting a cylindrical electron beam having transverse and longitudinal velocity components, a collector -for collecting said electron beam, and means for producing a focusing field about said electron beam which is of substantiaily uniform strength in the longitudinal sense of said electron beam and which is spatially alternating in the circumferential sense of said elect-ron beam, said means surrounding said electron beam and extending substantially the entire distance from said electron gun to said collector.

2. In combination, means for forming and projecting a cylindrical electron beam having a rotational velocity component, means for collecting said electron beam, and means for focusing said electron beam comprising means for producing magnetic flux throughout said electron beam, the intensity of said magnetic flux being substantially uniform along any line parallel with the axis of said cylindrical beam and being spatially alternating with a substantially uniform periodicity in the circumferential sense of said electron beam.

3. An electron discharge device comprising electron gun means for forming and projecting a cylindrical beam of electrons, collector means for collecting said beam of electrons, means included in said electron gun means for forcing said electrons to follow substantially helical paths to said collector means, and means for focussing said beam comprising a circular array of magnetized bars extending substantially the entire length of said cylindrical beam and coaxial therewith, alternate ones of said bars being of like magnetic polarity and adjacent ones of said bars being of opposite magnetic polarity.

4. The combination of elements of claim 3 wherein alternate ones of said bars comprise extensions of one pole piece of an electromagnet, while the remaining ones of said bars comprise extensions of a second pole piece of said electromagnet.

5. The combination of elements of claim 3 wherein alternate ones of said bars are in contact, throughout their length, with permanently magnetized pole faces of like polarity, while adjacent ones of said bars are in contact, throughout their length, with permanently magnetized pole faces of opposite polarity.

6. In an electron discharge device, means for forming and projecting a cylindrical beam of electrons, a collector for collecting said beam, means for imparting to said electrons a transverse velocity component, and means for focusing said beam comprising a circular array of elongated members surroundingsaid beam and extending substantially the entire distance between said beam forming means and said collector and means for establishing acircumferen-tially spatially alternating focusing eld comprising means for maintaining alternate ones of said members at the same polarity and adjacent ones of said members at opposite 'polarities 7. An electron discharge device comprising a cylindrical envelope, an electron source at one end of said envelope for projecting a hollow electron beam, means for imparting'to the electrons in the beam a transverse velocitycomponent, electron collector means at the end of said envelope opposite said electron source end for collecting said electron beam, a helix surrounding said electron beam for propagating an electromagnetic wave in interacting relationship with said beam, and beam focusing means comprising a circular array of elongated magnetized elements surrounding said envelope and extending from the electron source end of said envelope to the electron collector end of said envelope, alternate ones of said elongated elements being of a uniformly north magnetic polarity and the remaining ones of said elongated elements being of a uniformly south magnetic polarity.

9. The combination of elements of claim 7 whereinI said alternate elongated elements are in contact, throughout their length, with permanently magnetized pole faces of a uniformly north magnetic polarity, while said remaining elongated elements are in contact, throughout their length,l with permanently magnetized pole faces of a uniformly south magnetic polarity.

References Cited in the file of this patent UNITED STATES PATENTS 2,844,754 Cioli July 22, 1958 2,855,537 Mendel Oct. 7, 1958 2,860,278 Cook et al. Nov. 1l, 1958 Y FOREIGN PATENTS L 772,091 Great Britain Apr. 10, 1957- Y

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