US2951964A - Electron beam systems - Google Patents

Description

Sept. 6, 1960 c. F. QUATE 2,951,964

ELECTRON BEAM SYSTEMS Filed sept. 1s, 1955 /NVEA/TOR C. F. QUA TE ATTO NEV ELECTRGN BEAM SYSTEMS Calvin F. Quate, Berkeley Heights, NJ., assignor to Bell Telephone Laboratories, Incorporated, New York, NX., a corporation of New York Filed sept. is, 195s, ser. No. 534,090

s Claims. (cl. als- 3.6)

This invention relates to electrode systems for electrostatically focusing an electron beam, and to apparatus using such electrode systems.

The present invention relates more particularly to `an extension of the electrostatic focusing principles which are described in copending application Serial No. 514,423, tiled I une 10, 1955, by R. Kompfner and W. H. Yocom, now U.S. Patent 2,857,548, issued October 21, 1958. As is described therein, a succession of conductive elements which are arranged in a linear array positioned intermediate between a pair lof bounding plates and which Aare maintained at a positive potential with respect to such plates establishes a pair of singular equipotential surfaces, each of which winds sinuously past successive elements of the array and is so characterized that an electron beam which is introduced thereon with au appropriate velocity will flow stably therealong balanced between centrifugal and electrostatic forces. An important advantage of `an electrode system of this kindover previously known electrode systems for focusing an electron beam electrostatically is that the electron flow in this case is along an equipotential surface and so is of constant velocity in the absence of any disturbance of the beam. By way of contrast, previously known electrode systems for electrostatic focusing :an electron beam generally have involved periodic acceleration and retardation of the electron beam with consequent periodic changes in its velocity.

In the electronics art there exist many applications which make use 'of electron beams in which lit is preferable to coniine the electron ow to a relatively smooth or planar path, rather than a sinuous one as is characteristic of the electron beam systems described in .said Kompfner-Yocom patent, and yet to avoid periodic changes in the beam velocity characteristic of other forms vof electron beam systems.

Accordingly, a broad object of the present invention is to focus an electron beam electrostatically along a sub stantially smooth equipotential surface.

VTo this end, a feature of the present invention is an electrode system which establishes a singular equipotential surface which is substantially smooth and along which an electron beam which has been properly injected will iiow stably. An illustrative electrode system of this kind comprises an array yof elements in Iwhich alternate elements of the array are displaced transversely with respect to adjacent elements such that adjacent elements .are on opposite sides of a reference plane which essentially lis Vto form the singular equipotential surface =o`f Aelectron ow. This array of elements is positioned intermediate between a pair of conductive bounding members and each elec' 2,951,964 EC Patented Sept. 6, 1960 ment of the array is maintained at a positive potential with respect to the bounding members to form of the reference plane one of a pair of singular equipotential surfaces. The other singular equipotential surface will be sinuous as is characteristic of an electrode system of the kind described in the previously identified Kompfner-Yocom patent. By proper injection of an electron beam on the smooth singular equipotential surface, stable flow therealong is achieved.

The present application iinds special application 'in transverse field traveling Wave tubes of the kind described in copending application Serial No. 197,412, tiled November 24, 1950, by I. R. Pierce, now U.S. Patent 2,801,361, issued July 30, 1957, and my copending application Serial No. 465,580, lile'd October 29, 1954, now U.S. Patent 2,823,333, issued February l1, 1958. ln such transverse field tubes the interaction circuit is designed for excitation in a mode such that along a particular longitudinal smooth surface the electric field of a wave traveling along the interaction circuit is substantially purely transverse. For interaction with the electric field of the traveling wave, an electron beam is projected for flow along this surface. In general, it is true that if the electric field of a wave traveling along a slow wave circuit is purely transverse over a smooth surface, it must also have a longitudinal component at regions removed from this surface. In a transverse iield tube, it is characteristic of the mode of operation that the transverse electric fields set up by the slow traveling wave displace transversely the electrons which are iiowing longitudinally in the surface of substantially pure transverse field into the regions removed therefrom of longitudinal electric field, and such displaced electrons then interact with the longitudinal electric tield component and, as a result, give up energy to the traveling wave, whereby it is amplified.

It is characteristic yof a transverse tield tube that if in the absence of a signal the electrons are confined to the smooth surf-ace of substantially pure transverse field Where the longitudinal iield is essentially zero, fluctuations in current densityV or velocity of the electrons will produce no significant `noise power in the circuit. Accordingly, by properly focusing and collimating an electron beam for travel along such a smooth surface in the absence of a signal, it is possible to make the noise figure of the tube very small.

' However, it has been diliicult hitherto to focus an electron beam for travel along such a smooth surface conveniently. The various expedients which previously have been suggested do not provide complete satisfaction. The use lof magnetic fields for providing the focusingforces -is not completely satisfactory, both because the extra equipment needed for creating the magnetic ux adds un- -desirably to the bulk and weight and also because the magnetic iield `affects the electron beam unfavorably from a noise standpoint. The use foi focusing the beam electrostatically of' an electrode system of the kind comprising an array `of elements in which successive elements are maintained alternately at high and low potentials is not completely satisfactory because, as previously discussed, with such a system even in the absence of a-signal the electrons in the beam are passing alternately through regions of accelerating and retarding fields so that the velocity `of the electrons is periodically undergoing Ia substantial change, an effect which is undesirable ina traveling wave tube from the standpoint of interaction eciency.

Aield for travel along the reference plane.

Accordingly, a speciic'object of the invention is -to i v focus the electron beam of a transverse field tube along the desired longitudinal surface of substantiallyl pure transverse iield in a manner that both avoids 4the need for applied magnetic fields and also does not result in large periodic variations of the velocity of the beam.

For application of the principles of the present invention to la transverse field tube, a 4focusing electrode system of the kind previously described is designed to act also as an interaction circuit of the kind characteristic of transverse eld tubes, and the-smcot'hsingular. equipoten- ,tial surface of the focusing electrode system-isV made to coincide with the surface ofrelatively pure' transverse ,field of the interaction circuit, whereby an electronbeam suitably injected on such common surfacewill flow stably .therealong for interaction with the wave traveling-along 'the interaction circuit. Y t

In an illustrative embodiment ofthe invention,a pair of flat helices are positioned alongsidemone another and vare excited in opposite phase to serve as an interaction circuit of the kind which provides a plane of substantially puretransverse field in the interspace between the :two helices. A conductive plate is inserted Yin each helix to serve as a. bounding member of the focusing system; Additionally, the iiatportions ofthe two helicesadjacent .the plane of substantially pure transverse iield serve as the succession of alternately displaced conductive elements ,of the focusing system. By maintaining the two helices at an appropriate positive potential with respect to that of -the pair of conductive plates, the-plane of substantially .traveling wave tubes in which the interaction circuit estabvlishes a longitudinal electric eld along the path of` flow of the electron beam. In an illustrative embodiment of Ythe application of the principles of the invention to this .type of traveling wave tube, a pair of comb-like arrays of nger elements are positioned on opposite sides.of a

,reference plane in a'manner such that' their projection on the reference plane forms an interdigital pattern. This structure is excited to establish aV longitudinal electric To make a singular equipotential surface of this reference plane, a pair of conductive lbounding members are positioned to venclose the two comb-like arrays and' both such members are maintained at a negative potential with respect to the finger elements of the arrays.

' The invention will be better understood from the Vfollowingy more detailed description, taken in connection .with the accompanying drawing in which:

Fig. 1 illustrates schematically a prototype of an electron beam system in accordancewith the invention;

Figs. 2A and 2B show a longitudinal section 'and a transverse section, respectively, of a transverse eld ftraveling beam system of the kind shown in Fig.Y 1; and

face for such flow. For focusing the beam along such plane, a succession of parallel wire-like elements extending in the z direction are spaced apart in the x direction.

vAdjacent elements of the succession are displaced in the y direction so as to 1be positioned on opposite sides of the plane A whereby there are formed two sets 11, 12 of Y linearly aligned elements, each set -beiug on opposite sides but in a plane parallel to the plane A. The 'two sets-of velements are enclosed by a pair of conductive plates 13,

14 which form conductive boundaries parallel tothe t plane A.' The elements of the linear 4arrays 11,'12 are maintained at a positive potential with respect to the plates 13, 14. This is designated schematically in the drawing. Suitable choice of the potential difference results in forming of the plane A a singular equipotential surface at a potential V which is intermediate between those on the elements 11, 12 and the conductive plates 13, 14. The electrode system also forms a second singular equipotential surface B whichV winds sinuously past successive elements 11 and 12 and intersects the singular equipotential surface A periodically atl regions midway between successive elements.

The generalized relationships between the different parameters of the electrode system which will result in an equipotential pattern of the kind `desired are not amenable to a simple mathematical expression. Typically, parameters such asl the voltages to be applied to the various electrodes `are best vdetermined by electrolytic tank measurements of the kind known to workers in the art. By way of illustration, there will be set for-th hereinafter a set of parameters of an illustrative embodiment.

As is discussed in greater detail in the Kompfner- Yocom patent, it is necessary for stability that the electron beam which is to ow along the singular equipotential plane A be suitably injected thereon. In particular,

vif the potential `of the electron source from which. the electron beam originates be considered as ground or the reference level, the electron beamyshould be accelerated before injection to a velocity corresponding to the potential of the singular equipotential surface along which yitis to flow. Moreover, it is .also advantageous that the electron gun employed for injectionof the electronV beam does not appreciably disturb the focusing 'field patterns at the region of injection.

Injection arrangements suited to both .these ends are described in copending application Serial No, 514,421, iiled June 10', 1955 byk J.'S. Cook, R..Kom.pfner and W. H. Yocom, now U.S. Patent 2,939,034, issued May 31, 1960. An injectionarrangement of `the kind described in detail therein is shown included in the electron beam system illustrated in Fig. l.. Various other in- Vjectionarrangements are also described in the aforemen- 'tioned Kompfner-Yocom patent.

The beamV source comprises an electron-emissive` strip v,cathode 15 with which is associated a beam-shaping electrode 16. In these respects, the electron gun is of well known form. Additionally, the electrongun includes 'an .accelerating anode whose surface 17A is such as to Vco- ,operate with the beam-forming/electrode 16 in forming ,the electron `emission into a beam and whosesurface `17B coincideswith an extended portion of the singular` equipotential surfaceB. This anode is apertured for passage of the electron flow therepast at the region where its surface 17A, if continued smoothly, would intersect-thc singular equipotential surface A. The accelerating anode is maintained at the potential V of the singular Aequipotential surfacesA and B to result in beam injection with the correct velocity `on surface A. Y

For collecting the beam at the end ofits path of flow,

. an analogous arrangement is advantageously` employed to minimize disturbance of the focusing field pattern.Y The ,collector 18 is shielded-from the focusing field-by an electrode 19 Whose surface .19A coincides with singular equipotential surface B and whichisv apertured for Vpassage therethrough of the electron beam at the region where such surface, if continued smoothly, would intersect the y singular equipotential Vsurface A.

linear arrays'lland l12 of. 33.5 mils. The plates 13,14 are maintained at 130 Volts above the reference'level which is the potential ofthe cathode 15 of the electron The accelerating anode 17A, 17B and the collector shielding electrode 19 are each maintained at 100G volts which corresponds to the potential of the singular equipotential surfaces A and B. The wire elements 11, 12 .are maintained at 1130 volts, 1000 volts above the potential of the plates 13,14. The collector 18 is also maintained at 1130 volts. An electrode system of this kind is designed to focus an electron beam having a density of 100 milliamperes/centimeters2. Figs. 2A and 2B show a transverse eld traveling wave tube which includes an electrode beam system of the kind Yshown in Fig. 1. The various tube elements are enclosed within a vacuum envelope 20, typically of glass. In the interest of simplicity, details such as support rods, spacer elementsand lead-in connections have been omitted in the drawing. An identical pair of helices 21 and 22, which are wound in attened form as seen in Fig.. 2B, are positioned alongside one another with their flat surfaces parallel and spaced apart to define therebetween an interaction space through which is to be projected an electron beam. The two helices are wound in the same Vsense and are positioned so that their fiat portions 21A, 22A adjacent the interaction -space form a succession of elements which resembles and is the electrical equivalent of the succession formed by the two linear arrays 11,- 12 ofthe electrode system shown in Fig. 1. In respective helices 21 and 22, there are inserted conductive plates 23 and 24 to serve as the electrical equivalents of plates 13 and 14 of the electrode system shown in Fig. 1. Accordingly, by suitably relating the parameters of the electrode system formed by helices 21 and 22 and the conductive inserts 23 and 24, there is established a pair of singular equipotential surfaces, corresponding to surfaces A and B of the electrode system shown in Fig. l. Of these, one will be relatively smooth and extend longitudinally in the interspace between the two helices.

Beyond one end of the pair of helices, an electron gun of the kind discussed with reference to Fig. 1 and including an electron-emissive cathode 25, beam forming electrode 26 and suitably apertured accelerating anode 27 is arranged to inject a flat elect-ron beam for stable flow yalong the singular equipotential surface set up in the interspace. At the opposite end of the pair of helices, a collector 28 and suitably apertured shielding electrode 29 are used Ito collect the spent beam in a manner analogous to that described for the arrangement shown in Fig. 1.

In operation, by a coupling connection to the input end of the pair of helices, an input wave from a signal source is applied to the interaction circuit formed by the pair of helices for travel therealong for interaction with the electron beam flowing in the interaction space. In order that the singular equipotential surface of electron ow will correspond to a surface along which the electric field of the traveling wave is almost purely transverse, the two helices are excited such that the radio frequency potentials of adjacent flattened portions of the two helices remain substantially 180 degrees apart in phase. This end is readily achieved, for example, by connecting the .input ends of the two helices to different conductors of a parallel-wire transmission line 31 as shown. The distance around a complete turn of each helix is adjusted relative to the separation between turns, or pitch, such that the phase velocity of the electric iield traveling along the intersection surface in the interspace is substantially equal to the velocity of electron ow therealong, in the manner characteristic of traveling wave tubey operation.

' Y As a consequence, there is interaction between the electric iiield of the signal wave and the electron ow, whereby the signal wave is amplified. At the collector end of the interaction circuit, the amplied wave is abstracted for `transfer to a load by a parallel-wire coupling connection Vto self-oscillation by the insertion of loss along the interaction circuit in the manner known to workers in the art.

It is, of course, feasible to incorporate an electron beam system of the kind shown in Fig. l in various other forms of traveling wave tubes, both of the transverse iield type and the longitudinal field type. By way of another example, in Figs. 3A and 3B, there is shown such an electron beam system incorporated in a longitudinal field tube.

The various tube elements are housed in a vacuum envelope40. The interaction circuit is of the interdigital type in which an interdigital array of conductive elements form a slow wave circuit. To this end, the circuit com# Y prises a pair of support plates 41, 42 spaced apart parallel to one another. tends a separate set of coplanar conductive iinger elements 43, 44 to form a comb-like structure. The two sets are interleaved in an interdigital pattern, but eachis poistioned in one of two separate planes on opposite sides of the plane which is to act as the interaction surface along which the electron beam is made to iiow. Such an arrangement serves as a slow wave circuit and a wave suitably applied to one end will travel -therealong and establish longitudinal electric elds in the interspace between the two sets of elements.

It can be appreciated that the two sets of elements 43, 44 correspond to the sets 11, 12 of the arrangement shown in Fig. 1. Bounding conductive plates 45, 46 are positioned parallel to and on opposite sides of the planes formed by elements 43, 44 to correspond to plates 13, 14 of the arrangement shown in Fig. l. By suitably relating the parameters of the electrode system formed by elements 43, 44 and plates 45, 46 a singular equipotential surface may be established in the interspace between elements 43, 44 to serve as the path of flow for a beam which is to interact with the electric field est-ablished thereby a wave traveling along the slow wave interdigital circuit formed. i

The lengths of the elements 43, 44 'are chosen so that the phase velocity of the traveling wave is suited for interaction between the traveling wave and the electron flow.

In the manner previously described, an electron gun including a strip cathode 47, a beam formingy electrode 483 and asu'itably apertured accelerating anode 49 serves as the source of the electron How and the collector'50 and the suitably apertured shielding electrode 51 serve to collect the spent beam.

An input wave is applied to the input end of the interaction circuit by a coaxial connector 52 comprising an inner member which forms an extension of the end finger element 43A of the array and an outer member which surrounds such extension and is connected to side plate 42. A similar coaxial connector 53 is used to abstract amplified wave energy from the output end of the circuit.

It should now be obvious that the principles of the in- Vention are not limited to traveling wave tubes. Electron beam systems of the kind shown in Fig. 1 may be incorporated in switching tubes, counting tubes, and lany apparatus in which it is desired to focus electrostatically an electron beam for flow along a relatively smooth surface.

Accordingly, it is to be understood that the specific embodiments described are merely illustrative of the general principles involved. Various modifications may be made by one skilled in the art even in the basic electrode arrangement without departing from the spirit and scope of the invention. In particular, it is unnecessary to have uniformity in either the size and shape of the elements of the succession, the spacing between successive elements, or the spacing between the elements and the bounding members so long as proper compensation is made for any deviation from such uniformity, `as by the application of diierent potentials to successive ele- From each of these support plates exl 7 rnentsor to the two bounding members: Moreover, it 1s feasible also to eliminate the bounding members completely by proper compensation.Y The general principles of compensation for-non-uniformities arejset forth inthe aforementioned Kompfner-Yocom patent. v

What is claimedis: Y p vj j VVVl. In high frequency apparatus which employs the Yinteraction between an electron beam and a traveling wave to amplify the Wave, an electron source providing `an electron beam; means for focusing the electron beam for ow yalong a relatively smooth `path comprising a succession of conductive elements spaced apart in the desired direction of flow, alternate elements of the succession forming different arrays of elements and the different arrays of elements lying in different planes which .are on opposite sides of and substantially parallel to the desired path of flow and adjacent thereto, a pair of conductive members on opposite sidesrof the desired path f 110W extending substantially the entire length thereof and enclosing therebetween the succession of elements, and means for maintaining the succession of elements at a positive potential with respect to said pair of conductive members; means intermediate the electronsource and the focusing means for injecting the electron beam from the source on the desired path of ow with the correct Velocity; and means including said succession of elements for propagating a traveling wave to have an electric field along the desired path of flow with a phase velocity substantially equal to that of the electron flow.

2. In high frequency apparatus, an electrode arrangement for providing a relatively smooth singular equipotential surface comprising a succession of conductive elements, alternate elements of the succession forming different sets of elements, and the different sets lying'in different planes which are on opposite sides of land substantially parallel to the desired singular equipotential surface and adjacent thereto, a pair of conductive members on opposite sides of said surface and extending substantially the entire length thereof `and enclosing therebetween the succession of elements, means for maintainling the succession of elements at a positive potential with respect to said pair of members for establishing the desired singular equipotential surface, means includingV an electron source and electron accelerating means for injecting an electron beam on said established singular equipotential surface with the correct velocity for stable ow therealong, and means for propagating an electromagnetic wave having an electric field in the re- .gion of said singular equipotential surface for interaction with the electron flow.

3. In an electron beam system, means for focusing the beam comprising a succession of conductive elements, alternate elements ofthe succession forming Vseparate sets of elements and the separate sets lying in separate planes extending on opposite sides of and parallel to la reference plane adjacent thereto, conductive mem- .bers on opposite' sides of the reference plane for enclosing the two sets of elements, means for maintaining the conductive elements i; at the same potential positive With respect to the bounding conductive members for mak- -ing the reference plane substantially a singular equipotential surface, and means for injecting an electron beam for stable flow along said singular equipotential surface.

4. In an electron beam system, an electrode system for establishing a singular equipotential surface which is relatively smooth comprising a succession of conductive elements equally spaced in the ydirection desired for electron ow, alternate elements of the succession forming two linear arrays of elements and different linear .rarrays lying in different planes extending adjacent to, Aon opposite sides of and parallel to a reference plane -which substantially ydefines the relatively smooth singular equipotential surface, conductive members extending on lopposite sides of .and parallel to the reference planefor 8 enclosing the two linear Aarrays of elements, and -means for maintaining the conductive elements at the Samepotential positive with respect to the conductive members, and means for injecting an electron'beam with'the correct velocity for stable flow alongsaid-relatively'smooth singular equipotential surface. l t

5. In an electron beam system, lan electrode arrangement for establishing a pair of singular equipotentialsurfaces of which one is relatively smooth and the other sinuous, comprising a succession of conductive elements, alternate elements 0f the succession being on opposite sides of and adjacent to a reference plane which corresponds to the desired smooth equipotential surface, a pair of conductive elements on opposite sides of said reference plane enclosing the succession of elements, and means for maintaining the conductive elements at a positive potential with respect to the conductive members; and means for injecting an electron beam on said relatively smooth equipotential surface at a regionwhere it intersectsthe sinuous singular equipotential surface comprising an electron source for providing the electron beam and an accelerating anode having a surface which substantially coincides with the singular equipotential surface and which is aperturcd for passage of the electron beam therethrough at a region of intersection with the relatively smooth sinuous equipotential surface.

6. In high frequency apparatus which 'employ the interaction between an electron beam and a traveling wave, means providing an electron beam, meansrfor focusing electrostatically the electron beam for ow along a singular equipotential surface which is relatively smooth comprising a succession of conductive elements, alternate elements of the succession forming two sets of elements, different sets lying in different planes extending adjacent to and on opposite sides of and parallel to a reference plane which substantially defines the relatively smooth singular equipotential surface, conductive members spaced on sides of the reference plane for enclosing the two different sets of elements, and means for maintaining the conductive elements at the same potential positive with respect to said conductive members, and means including the succession of conductive elements for establishing an electric field for travel along said reference plane for interaction with the electron flow. Y

7. In high frequency apparatus, a pair of longitudinally extending conductive helices spaced apart alongside one another, the axes of said helices being spaced apart along the length of said helices the turns of each helix including a flat portion adjacent the interspace between the two helices, adjacent flat portions of the two helices being displaced longitudinally with respect to one another, means for exciting the two helices in opposite phase to establish in the interspace therebetween a longitudinal plane along which the electric eld of the excitation is substantially purely transverse, and means for making said plane substantially a singular equipotential surface, said means comprising said pair of helices, a conductive member within each of said helices and extending-axially therealong, and means for maintaining said helices at a positive potential with respect to said conductive members; and means for providing an electron beam for stable flow along said4 singular equipotential surface.

8. In high frequency apparatus, a Wave propagating structure comprising a pair of spaced conductive supports, a linear array of conductive elements extending from each support towards the other support, each of the two References Cited in the tile of this patent UNITED STATES PATENTS 2,296,355 Levin Sept. 22, 1942 2,575,383 Field Nov. 20, 1951 2,622,158 Ludi Dec. 16, 1952 2,725,499 Field Nov. 29, 1955 10 Dohler et al Jan. 10, 1956 Walker May 15,' 1956 Charles Dee. 18, 1956 Pierce July 30, 1957 Adler Oct. 8, 1957 Kompfner et al Oct. 21, 1958 Guenard et al. June 6, 1959 FOREIGN PATENTS Great Britain May 25, 1955 France June 27, 1951 France July 18, 1951

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