US2866916A - Traveling-wave tubes - Google Patents

Description

Dec. 30, 1958 R. ADLER TRAvELING-WAVE TUBES Filed April 15, 1954 HIS ATTORNEY.

Asi'

TRAVELING-WAVE TUBES Robert Adler, Northfield, lll., assigner to Zenith Radio Corporation, a corporation of Delaware Application April 15, 1954, Serial No. 423,277

12 Claims. (Cl. S15-3.6)

This invention relates to new and improved electrondischarge devices of the traveling-wave type. More particularly, the invention is directed to traveling-wave tubes suitable for use as amplifiers operable over a relatively wide range of frequencies.

In the recent past, the Federal Communications Commission has authorized the construction of television broadcasting stations operating at frequencies within the ultra-high-frequency range between 490 and 870 megacycles per second. Consequently, manufacturers of television receivers have found it necessary to provide terminal equipment' adapted to receive programs transmitted within this frequency range. One of the most dilicult problems presented in the construction of a television receiver of this type results from the fact that conventional intensity-control electron tubes (triodes, pentodes, etc.) are not well suited for use as amplifiers within the ultra-high-frequency range; more particularly, it is extremely dilicult to achieve uniform gain throughoutl the U. H'. F. range withtubes having practical dimensions and tolerances. Accordingly, it has generally been considered preferable to apply the received' signal-directly to a hetero dyning stage without amplication. In this event, however, the, picture reproduced by the receiver is often seriously disturbed by thermal noise.

One type ofelectron-discharge device which is capable of providing amplification over a relatively wide range of high frequencies is the conventional travelingwave tube. In these tubes, a radio-frequency signal is applied to a low-velocity wave-transmission line, which, in its simplest form, may comprise a helically wound conductor.. An electron stream is directed along a path closely adjacent toV the' helical line; usually, the electron beam path coincides with the axis of the helix. The velocity ofthe electrons in the'beam is made substantially equal to the eiectivez velocity of the radio-frequency signal wave traveling along the line. The electron beam isA velocity-modulated bythe electrostatic i'eld developed by the signal wave traveling along the line, and," in turn, induces current in: the line which may amplify the radiofrequency. signal. However, such traveling-wave tubes are much too largeV and .expensive for use in"V a television receiver.

A little-known Variant of the traveling-Wave tube comprises a device adaptedfor push-pull or transverse mode operation, as opposed` to the longitudinal or velocitymodulation mode of operation employed inthe conventional tubes. A transverse-mode traveling-wave tube mayf comprisev a pair of low-velocity wave-transmission lines mounted in substantially parallel spaced relationship with respect to each other. A radio-frequencyinput` signal is applied in pushLpull relationship, to 'the two wave-transmission lines, and an electron stream is projectedt along a path intermediate'thetwo transmissionlines at a velocity-substantially equal. to theeffective propagation velocity of-fthe signal-wave alongthelength of .the lines. Consequently, eachrelect-ronofthestream 1, States Patent quency signal so that exponential amplification is attained. t

In both conventionalv and transverse-mode travelingwave tubes, it is essential that the electron stream be conlined to a relatively narrow path so that the electrons are not collected by the wavetransmission lines. A magnetic field extending throughout the length `of the electron beam path is generally employed lto confine the electrons to that path and topre'v'ent dispersion of the beam. A relatively bulky and expensive electromagnetic coil surrounding the entire traveling-wave tube is usually utilized for thisV purpose. However, 'such a structure is not desirable in apparatus such as a television receiver, where space and cost considerations are of paramount importance.

It is a primary object of the invention, therefore, to provide a new and improved electron-.discharge device suitable for usel as an amplier over a relatively wide range of ultra-high frequencies.

It is a further object of the invention to provide an electron-discharge device, capable of; operating as a broad band' U. H. F. amplilier, which isrelatively small in size but which provides an acceptable degree of amplification.

It is a specific object of the invention to providev a new and` improved electron-discharge device of the transverse-mode traveling-wave type in which the electron stream is effectively confined to a predetermined" path without requiring the use ofl a magnetic collimating system.

It is another object of the inventionY to provide a new and improved transvers'emode,traveling-wave tube in which thermal noise is substantially minimized.

It is a corollary object of the invention to provide a traveling-wave tube which is relatively simple and expedient to construct and economical to manufacture.

An electron-discharge'device constructed in accordance with one aspect ofthe invention comprisesr an electron gun for projecting a sheet-like beam of electrons along a given reference path. A wave-transmission line is disposed adjacent to the reference path and'is electrostatically coupled to the electron beam; this wave-transmission line has a low wavepropagation velocity in a direction parallel kto the reference path and further has a length in that direction which is large relative to thevr effective wavelength of a signal wave. traveling along the line.

A support member isinterposed between thewave-trans-V mission line andy the reference path,and a first series of electrically interconnectedelectric signal field permeable' conductive elements tare disposed at predetermined intervalsralongthe reference pathon the surface of the support member adjacentthat path; Afsecond series of electricallyirterconnected electricv signal field permeable the invention comprises-'anlelectro"gun'for projecting' a sheet-likebeamof electrons alonga given reference path.Y A-pair ofbe'am-conlining structures, each com` prisng aplurality'ollr series of lens electrodes, arev disposed on opposite sides of the reference path and are employed to confine the electron beam to that path; these beam-confining structures are permeable to signal-frequency electric fields but are substantially impermeable to unidirectional electric fields. A wave-transmission line is disposed adjacent to the reference path but is separated therefrom by one of the beam-confining structures, so that the transmission line is electrostatically coupled for signal frequencies only to the electron beam. This wave-transmission line has a low wave-propagation velocity in a direction parallel to the reference path and further has a length in that direction which is large relative to the effective wavelength of a signal wave traveling along the transmission line.

The features of the invention which are believed to be novel are set forth with particularity in the appended claims. The organization and manner of operation of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals refer to like elements in the several figures, and in which:

Figurel is a cross-sectional view, partially schematic, of one embodiment of an electron-discharge device constructed in accordance with the invention, and includes a schematic representation of a simplified amplifier circuit;

Figure la is an explanatory diagram illustrating certain operational features of the apparatus of Figure 1;

Figure 2 is a cross-sectional view taken along line 2--2 of Figure 1;

Figure 3 is a fragmentary cross-sectional view taken along line 3-3 in Figure 2; and

Figure 4 illustrates an alternative construction for that portion of the invention illustrated in Figure 3.

The embodiment of the invention shown in Figure 1 comprises an electron-discharge device or traveling-wave tube 20; tube 20 includes a cathode 10 having an electronemissive surface 11. A focusing electrode 12 is mounted in parallel spaced relation to surface 11 and includes a centrally located slot 13. An accelerator electrode 14 including an aperture `16 is included in device 20 and is positioned adjacent focusing electrode 12 with slot 16 opposite slot 13. A beam-limiting electrode 15 is mounted in spaced relationship to accelerator 14 on the side of the accelerator opposite electrode 12; electrode 15 includes a beam-limiting aperture 17 aligned with aperture 16. Cathode and electrodes 12, 14 and 15` comprises an electron gun 19 for projecting an electron beam along a reference path indicated by a dash line A comprising the center plane of the path;' reference path A terminates at a collector electrode 18 positioned at the opposite end of tube 20.

A first low-velocity wave-transmission line 21 is dis- I posed adjacent reference path A intermediate electrode and collector 18. Transmission line 21 comprises a helical conductive winding 22 wound upon a longitudinal support member 24; support member 24 is preferably formed from some insulating material such as glass or ceramic material suitablefor use in a vacuum tube. It should be understood that the size of conductive winding 22 has` been exaggerated in Figure l in order to facilitate the presentation of the inventive concept, and that only a relatively few turns of the winding are illustrated as compared to the number which may be employed in practice. t j

Tube `also includes a second low-'velocity wavetransmission line 25 which is disposed adjacent reference path A on the opposite side of the path from line 21. Line 25, which is essentially identical in electrical and physical characteristics with line 21,` co`mprises.a helical conductive `winding 26 mounted on an insulating `support member 27. The length Z of wave-transmission lines 21 and 25 determines theiportion of path A in which interaction `between the electron beam and a signal wave traveling along lines 21 and 25 may occur; length Z is relatively large as compared to the effective wavelength of a signal wave traveling along the transmission lines.

Preferably, the electron beam developed by gun 19 is sheet-like in form; in other words, the electron beam has one principal crosssectional dimension which is very much greater than a second principal cross-sectional dimension, these dimensions being generally determined by the configuration of slot 17. A beam of this type is normally quite advantageous in a weak-signal amplifier, due to the fact that it permits realization of increased amplification from a tube of given maximum dimensions.

As seen in the cross-sectional view of Figure 2, the thickness of the electron beam, which is determined by dimension t of slot 17, is very much smaller than its height, which corresponds to slot dimension h, so that the beam has a cross-sectional configuration corresponding to an elongated reactangle. It should be understood that although a rectangular cross-sectional configuration has been illustrated, other beam configurations may be employed. Support member 24, upon which winding 22 of line 21 is wound, is essentially I-shaped in crosssectional configuration, and the longitudinal support member 27 which carries winding 26 of line 25 is similarly shaped.

A pair of beam-confining structures 28 and 29 are included within tube 20; beam-confining structure 28 is interposed between wave-transmission line 21 and refcrence path A, Whereas structure 29 is interposed bctween line 25 and path A. Thus, each of the wave-trans-v mission lines is disposed adjacent to the reference path but is separated therefrom by one of the beam-confining structures. Beam-confining structure 28 comprises a longitudinal support member 30 which may be formed from mica or other suitable insulating material; the support member is preferably made as thin as possible consistent with adequate structural strength. As best shown in Figure 3, a first series of electrically interconnected conductive elements or lens electrodes 31 are disposed at predetermined intervals along the surface of support member 30 adjacent reference path A (Figure l). Conductive elements 31 have a relatively high resistivity so that they do not comprise a unipotential plane or shield at radio frequencies; preferably, electrodes 31 are formed as thin highly resistive coatings of carbon or other conductive material deposited on the surface of support member 30. For example, elements 31 may each comprise a coating having a resistivity of the order of ohms per square, A second series of electrically interconnected conductive elements 32 are formed on the same surface of support member 30 as conductive elements 31; lens electrodes 32 are interleaved with elements 31 and are preferably substantially identical in physical and electrical characteristics with the first series of lens electrodes.

Beam-confining structure 29 is essentially identical in construction with structure 28 and comprises a thin insu lating support member 33. A first series of conductive elements or lens electrodes 35 are disposed along the surface of support member 33 adjacent beam path A directly opposite lens electrodes 31 of structure 28. A second series of interconnected conductive elements 36 are interleaved with lens electrodes 35 and are disposed di rectly opposite conductive elements 32 of structure 28. In addition, those portions of the surfaces of support members 30 and 33 adjacent beam path A not covered by the lens electrode coatings may be provided with a leakage coating 39 of conductive material having a substantially higher resistivity than the lens electrodes in order to avoid charging the exposed insulator surface by inavoidable stray electrons; for example, the resistivity of coating 39 may be of the order of 10a ohms per square. Alternatively, a complete conductive coating of similarly high resistivity may be applied across the entire surassenzio' face of the insulating support members and the lens electrodes, or the support members themselves maybe cons tructed from `a"material having a'suitableI amountcf volume o'r'surface resistivity. i

Tube 20 may `be` provided with a suitable base for envelope 29'and ya separate 'heater larnent may be included in cathode because these structural details are familiar in the art, they are not illustrated in the drawings. Envelope 29 may be of conventional receiving-tube size. After wave-transmission lines 21 land 25, collector 18, and the electrodesv comprising electron gun 19 have been mountedwithin envelope 29, the envelope is evacuated and gettered in an'y manner known in the art.

A simplified amplifier circuit for tube 20 has been schematically illustratedin Figure lin'orde to facilitate the description of theoperatin of the tube. A balanced signal source 37 is connected between the ends of winding 22 of line 21 and 4winding 26 of line 25 adjacent electron gun 10. Source 37 may comprise any suitable source of radio-frequency signals, and may, for instance, constitute the termination of a balanced television antenna. A balanced lad circuit 38 is connected between the ends ofk windings 22. and 26 of transmission lines 21 and 25 adjacent collector 1S.

Cathode 10 is connected to a plane of reference potential, here illustrated as ground, and focusing electrode 12 may also be connectedto ground; if preferred, however, when the amplifier is incorporated in a television receiver or the like, electrode 12 may be connected to a suitable source of automaticy gain control potential (not s hown) to maintain a relatively constant output signal amplitude. Accelerator 14 is connected t-o a first source of positive unidirectional operating potential B1-}-, and electrode 15 is connected to a second positive voltage source B2+. A third source of po-sitive potential, B34-, is connected to conductive coatings 31 and 35 of beam-conlining structures 28 and`29 respectively; similarly, a fourth D; C. voltage source B4-l-'is connected toV conductive coatings 32 `andi36 of the beam-confining structures. Collector 18 is electrically connected to an additional source of positive operating potential B54-, It will be understood, of course, that all of the B-lvoltage sources may comprise separate taps on a single unidirectional or D. C. voltage source. Furthermore, some of the B+ potentials (e. g. B1+ andB5-}) may be of the same value.

Intraveling-wave-tubes constructed in accordance with known techniques, common experience indicates that the use of electrodes which intercept any appreciableportion of the electronbeam may produce highly undesirable partition-noise effects; accordingly, the electron guns of conventional tubes employ electrode structures in which an attempt is made to avoid interception, by the gun electrodes, of any substantial portion of the beam. One percent interception, for instance, is generally considered excessive. However, it has been found, suprisingly enough, that wthese partition-noise effects are not' significant in transverse-mode traveling-wave tubes; indeed, electron guns including electrodes which intercept fifty percent or more of the total beam current have been found to provide inherently better noise characteristics, in transverse-mode tubes, than electron guns of the type utilized in conventional tubes. Consequently, electrode 15is preferably constructed to intercept a substantial portion of the total beamcurrent, preferably greater than fifty-percent.

When vdevice 20 is placed in operation, a radio-frequency signal is applied to wave-transmission lines 21 and 25 in push-pull relationship, from source 37. Due to the helical configuration of the windings which form the wavetransmission lines, each of lines 21 1and'25 has a wave-propagation velocity in a direction parallel to reference path A which is considerably smaller than the propagation velocity o f electromagnetic radiation in free space. The actual effectivewavepropagation velocity of the lines is a matter of design choice and may be as low as one one-hundredth (0.01 of the free-space propagation velocity. Electrons emitted from surface 11 of cathode 10 are focused by passing through slot 13 of electrode 12 and are accelerated as they traverse slot 16 of accelerator 14, due to the positive operating potential applied to the accelerator from source B1-|. Part yof the electron stream then passes through aperture 17 of electrode 15, which may be held at a potential somewhat below that of accelerator 14 but positive with respect to the cathode. Preferably, the average potentials supplied to the conductive elements 31 and 35 should be different from that applied to electrode 15 from source B2-{-, so that an electrostatic focusing lens is formed between electrode 15 and lens electrodes 31a and 35a. The electrons of the beam continue along reference path A and are collected by electrode 18. The beam velocity is determined by the average of the D. C. potentials of conductive elements 31, 35 and 32, 36 with respect to cathode 10.

In order to reach a more complete understanding of the advantages and useful qualities of the invention, the arnplier illustrated in Figures l and 2 may first be consideredv as operating without the benefit of any collimating or focusing field tending to confine the electron beamv to the maximum width d 0f reference path A; that is, beam-confining structures 28 and 29 areconsidered to be omitted. The velocity of the electron beam along path A may then be adjusted so that it is approximately equal to the wave-propagation velocity of the signal wave traveling along transmission lines 21 and 25. Each electron or group of electrons instantaneously emerging into the portion of reference path A defined by transmission line length Z is subjected to a transverse electric eld established by the application of radio-frequency signals from source 37, in phase opposition, to the two transmission lines. Because the electron `and wave-propagation velocities are equal, each individual electron is continuously deected in a given direction and begins to move transversely with respect to path A as well as parallel thereto. As the electrons move away from the center plane of the reference path, they form a wave pattern which, by moving along the wave-transmission lines, induces a signal current in the lines; this induced current is out of phase with respect to the original field supplied by source 37.

Interaction between the traveling-wave` field and the electron stream leads to the emergence of three .separate waves in place of the original signal wave; only one of the three, waves grows exponentially as it travels along the transmission lines. This process has beenanalyzed in chapter 13- of the book entitled Traveling-WaveTubes by I. R. Pierce, published by D. Van Nostrand'Co., Inc. New York, 1950.

Gperation of `theamplilierV of Figures l-and V2'under the conditions just described might be highly successful if the electrons projected from emissive surface 11 of cathode 10 all followed paths exactly parallel to reference path A and if space-charge effects could be ignored. In practice, however, this is not possible. On the average, each of the electrons emerging from electronfgun 19 has some initial transverse velocity which causes the beam todispersel and,` in addition, space-charge effects tend further to spread the beam. Consequently, if no provisions are made for focusing or collimating the electron beam as it traverses reference path lengthZ,it is extremely difiicult to secure any appreciable Ygain from traveling-wave tube 20, since the electronrbeamrapidlydispersesand is collected by the beam` confining structures. Conventionally, magnetic collimating fields have been employed to restrict the transverse excursions of the beam electrons; the structuresemployed todevelop the magnetic collimating field, however, are 'consideredexcessively bulky and expensive forl domestic television receivers and similarV applications.

Traveling-,wave-tube 2f?, onthe other hand, includes electrostatic focusing system, comprising structures 28 and 29, for confining the electron beam within maximum width d of reference path A; width d is limited by and must be smaller than the spacing between the two beamconfining structures. Lens electrode series 31 of beamconfining structure 28 is maintained at the same potential as conductive element series 35 of structure 29. Lens electrode series 32 and 36, on the other hand, are maintained at a different D. C. potential, so that there is a substantial potential difference between the first set of' lens electrodes, elements 31 and 35a, and the next set of lens electrodes 32a and 36a. Thus, throughout interaction space length Z, each set of opposed lens electrodes is maintained at a different D. C. potential from the adjacent sets of conductive elements. This condition is illustrated in Figure 1a, which comprises a schematic representation of the lens electrodes; the relative sizes and spacings of the elements illustrated therein have been distorted somewhat in order to assist in explaining the figure and to provide more space. resulting from the signal wave traveling along lines 21 and 25 (Figure l) is relatively small in comparison with the difference in D. C. potential between the individual windings of each line.

As shown in Figure la, an electron entering the portion of reference path A bounded by the wave-transmission lines may have a velocity component in the transverse direction indicated by arrows y as well as a principal velocity component parallel to reference path A. lThe transverse velocity component may result from thermal or space-charge effects or other factors. The electron may enter the interaction space bounded by lens electrodes 31, 32, 35 and 36 along a hypothetical path A', and, at the outset, is subjected to an electrostatic field primarily determined by the average or steady-state potential applied to the first set of conductive elements 31a and 35a from source B34- (Figure l). As the electron continues along path A', it reaches the space between elements 31a, 35a and the next set of lens electrodes, designated 32a, 36a. Because elements 31a and 35a are connected to source 133+ and are maintained at a considerably different potential from lens electrodes 32a, 36a, the electron encounters a convergent electron lens action which tends to deflect the electron toward center plane A of the reference path. T'he electron continues along path A', and, uoon reaching the space bounded by conductive coatings 32a, 36a and 31b, 35h, enters another convergent electrostatic lens. Consequently, the electron is again deflected toward center plane `A. Thus, as the electron proceeds along path A it is subjected to a periodic electrostatic lens field effectively constituting a series of convergent electrostatic lenses. The periodic lens field tends to deflect the electron toward reference path center plane A with a force which is proportional to the displacement of the electron from that plane; accordingly, the lens field is generally equivalent to a transverse elastic field and confines the electrons `of the'beam to maximum path width d. y

The focal length `ofeach of the electrostatic lenses formed between adjacent sets of lens electrodes, such as the lens between elements 31a, 35a and 32a, 36a, is proportional to the transverse spacing between those sets of conductive elements and is a function of the ratio between the D. C. potentials of the sets of lens electrodes with respect to cathode 10. Thefocal lengths of the electrostatic lenses, in conjunction with the spacing s between adiacent lenses, determines the distance along path A which is traversed by an electron having a given initial transverse velocity before that electron crosses the center plane ofthe reference path. Consequently, as shown by trajectoryA, each electron (other than those having no initial transverse velocity) followsa substantially sinusoidal'path which is symmetrical with respect to referencepath center plane A. A second hypothetical electron The signal-frequency potential path A" illustrates the trajectory of an electron having a different initial transverse velocity and a different starting position from the electron following path A'. As indicated by trajectory A, the magnitude and direction of the initial transverse velocity and the original displacev ment of the electron with respect to center plane A do not affect the wavelength Le of the sinusoidal paths followed by the individual electrons.

For a given structure, wavelength Le is determined by the strength of the lens field produced by the D. C. po tential difference between adjacent sets of lens electrodes, and by the average of their individual D. C. potentials which establishes the average velocity of the electron stream. As each electron follows its individual trajectory, it carries out a transverse harmonic motion at a frequency we equal to its average velocity divided by wavelength Le. This transverse motion is analogous to the motion of a mechanical resonator such as a vibrating reed; a periodic force having a frequency equal to the natural frequency of such a resonator produces a periodic motion of linearly increasing amplitude. Consequently, the electron stream may be said to exhibit a transverse resonance at the frequency we.

The frequency of the signal applied from source 33 may be designated wo and the propagation velocity of the undisturbed signal wave traveling along the lines may be taken as v0. The average velocity of the electrons may be designated ve.

By proper choice of the D. C. potentials applied to the wave-transmission lines, the velocity of the electron beam may be adjusted so that Under these conditions, the individual electrons of the beam are subjected to a periodically changing signal field having a frequency we; because the electrons tend to resonate at that frequency, they begin to move at increasing amplitudes in the y direction. It should be recalled that the electrostatic lens field is relatively strong in comparison to the signal wave field, so that the transverse excursions of the electrons induced by the signal field do not cause the electrons to be collected by the wave-transmission lines.

Under the conditions described immediately above, the current induced in wave-transmission lines 21 and 25 by the pattern of transversely vibrating electrons moving along path A is in phase with the signal Wave applied from source 37, so that gain is achieved. The amplitude of the signal wave, at any point along the lines, may be eX- pressed as exist along wave-transmission lines 21 and 25; this has been determined to be true, The fact that the initially applied signal is effectively divided into these two waves accounts for the factor 1/2 inthe above equation. For a tube which is long enough to afford substantial gain, the attenuated wave is of negligible effect as compared to the amplified wave, so that a simplified expression for the gain of such a long tube is (3) nel/2meZ This compares quite favorably with the conventional veexpression for the growth constant indicates division of the applied signal into three waves, so that the equation for a long tube includes a factor of only 1/3 instead of 1/2.

It should be noted that the phase conditionswithn tube 20 whichresult in the direct addition of the induced current tothe original signal current in wave-transmission lines 21 and 25 prevail accurately only so long as the amplitude of the driving or signal field does not change along the length of the wave-transmission lines. Actually, the driving field increasescontinuously, so that a phase error occurs. This phase error,y however, is relatively small and may be readily eliminated by minor adjustments in either the transverseresonance frequency orthe electron beam velocity; such adjustments may conveniently be made by varying the applied potentials from one or more of the D. C. sources B34- or 134+.

In order to achieve effective operation of tube 20, several conditions should be met. ,To avoid difficulties presented by lens aberrations, the spacing s between adjacent lenses (Figure la) should be greater than three times the maximum permissible width d of reference path A (the proportions illustrated in yFigures l and la-are at variance with this condition to avoidV overcrowding). The effective wavelength Le of the lens field (Figure la) should be equal to or greater than three times lens spacing s. Furthermore, transmission-line length Z must be large in relation to the effective wavelength of the signal wave as it travels along the wave-transmission lines; this latter condition must be met in order to achieve appreciable gain. Support members 30 and 33 and the conductive coatings comprising lens electrodes 3l, 32, 35 and 36 must, of course, be thin enough and have sufficiently high resistance so as to have only a small effect upon the signal frequency electric fields, in order that the wavetransmission lines may be effectively coupled to the electron beam. The resistance of conductive coatings 3l, 32, 35 and 36 is made low enough so that the D. C. potential of the lens electrodes remains substantially uniform through each lens electrode surface and relatively close to the desired value despite the unavoidable voltage drop produced by stray electrons fro-m the beam impinging upon the lens electrodes. Because the lens coatings are positioned within the signal-frequency field, reduced gain results if the resistance of electrodes 31, 32, 35 and 36 is made too low; a certain amount of such loss may be desirable, however, in order to increase the operating stability of the device, in accordance with techniques well known in the traveling wave tube art. i

The electrostatic lenses established along path A determine the transverse resonant frequency of the electron beam and, at the same time, confine the beam within width d so that it does not impinge upon the wave-transmission lines. Consequently, it is possible to make lines 21 and 25 suiciently long to achieve useful gain from tube .20. The electrostatic. lens field centers the beam about center plane A, asV contrasted to the mere collimating action of a-conventional magnetic field, so that width d -may be held to avminimum. This facilitates close coupling between the electron beam and the wave-transmissionlineszand permits the realizationof greater amplification ina tube of given overall size.

i It has beenrdeterrnined that the exponential gain or growth constant a of a transverse-mode traveling-wave tube such as device 20 is vadversely affected by stray capacities valong the'wave-transmission lines. In order to minimize such stray capacities, support members 24 and 27 preferably have a transverse cross-sectional area which is.. as` small as possible; .in addition,- the material from which the supports are made should have a low dielectric constant.` The-Ishaped `cross-sectional configuration for the ksupport members illustrated in Figure -2 is advantageous in this respect -in thatl it presents a relatively small cross-sectional areawhich is widely separated from the individual turns... Support- members 24 and 27 may be Iformed from ceramic, glass, or other dielectric materials adapted for use in a vacuum. Helix windings 22 and-'26',

of course,` may be formed from copper, molybdenum,-or other conductive material suitable for'use in a vacuum; In the embodiment of the'invention illustrated in Figures 1 and-2, the electrostatic lens field utilized to confine the electron beam Within Width d is established solely by the D. C. potential between the individual sets of lens electrodes of beam-confining structures 28 and 29, which shield the electron beam from any zero frequency or unipotential electric field developed by the transmission lines, although they are effective in coupling to the beam the signal-frequency field 'created by the signal wave traveling along transmission lines 21 and 25. It is possible to construct devices in which portions of the wave-transmission lines themselves function as lens electrodes and'in which the wave-transmission lines may be coupled to the electron beam by means of separately identifiable lens electrodes which form a part of the lines. Structures of this general type are described in the copending applications of Robert Adler, Serial Nos. 394,797 and 394,798, both filed November 27, 1953. Moreover, periodic electrostatic lens structures may also be advantageously employed intraveling-wave tubes including only a single wave-transmission line and specifically adapted for longitudinal-mode operation, as described and claimed in the copending application of Robert Adler, Serial No. 401,149, filed December 30, 1953; all of these applications are assigned to the same assignee as the present invention.

Figure 4 illustrates another type of beam-confining structure which may be employed in place of structures 28 and 29 of Figures 1 3. As shown in Figure 4,`the beam-confining structure 43 may comprise a relatively thin insulating support member 49 substantially similar to the previously-described members l3A0-and 33. Support member 49 may be provided with a very highly resistive leakage coating (not shown) to preclude the collection of undesirable charges on the' support member surface.

Beam-conning structure 48 further includes a first series of lens electrodes 56 each includingl aplurality of thin, highly conductive strips --51 extending across the surface of support member 43 transversely with respectv to the direction of beam travel, generally indicated yby arrow K. Conductive strips 51 may, for example, comprise thin lines of silver printed or otherwise deposited upon support member 49. Each'of strips 51' is electrically connected to a conductive member 52 which has a high impedance at signal frequencies; member 52Hm'ay comprise a'radio-frequency choke or a resistor or any combination of resistance and inductance having a relatively high impedance at signal frequencies.

Structure 48 further includes a Vsecond series of lens electrodes 53 which are substantially similar to electrodes 50; lens electrodes `53l each comprise a plurality of thin conductive strips 54Vwhich are electrically connected 'to a second conductive member 55 having a relatively high impedance at signal frequencies. Impedance members 52 and 55 are electrically connected to D. C.` operating potential sources B34- and BHL, sol that lens electrode strips 5l are maintained at a given constant D. C. potential and electrode strips 54 are maintained at a-different constant positive potential.

It will be readily apparent that the structure illustrated in Figure 4 may be substituted for beam-confining structure 28 and that a similar structure may be substituted for structure 29 in tube-2 4) (Figure l)- Without effecting any substantial change in the operation of the tube. Because members 52 and-55 have a relatively high impedanceat signal frequencies, they cannot'constitute'a short circuit for the signal-frequency electric fields and thus do not adversely-affect operation of the 'device'.

Moreover, because vstrips 51 and 54 do not extend 'for any substantial distance in the direction of beam travel and wave propogation (arrow K) they cannot'constitute equipotent'ial planes extending in this direction andl therefore do not interfere with the normal operation of the tube. Although the structure of Figure 4 requires a greater number of individual elements for each of the lens electrodes, the necessity of using highly resistive materials for the lens electrode coatings is obviated and it is therefore somewhat easier to obtain uniform operating characteristics for the lens electrodes.

Traveling-wave tubes constructed in accordance with the invention may be made to provide relatively constant amplification characteristics throughout a broad band of frequencies; more specifically, these amplifiers may be constructed to provide substantially constant amplification throughout the U. H. F. television range of frequencies, The tubes do not require the bulky and heavy external magnetic structures utilized in prior art devices to form magnetic collimating fields for the electron beams; consequently, tubes constructed in accordance with the invention may be made relatively small and are well suited for mounting in a domestic television receiver or similar device. Moreover, because the electrostatic lens system of the invention effectively centers the electron stream about a center plane rather than merely collimating the electrons to constrain them to paths parallel to such a plane, it is inherently more effective than the prior art magnetic systems in restricting dispersion of the electron stream attributable to thermal and/or space charge effects. The operating voltages required are relatively low and do not unduly burden the power supply of a television receiver; all of the electrode structures are relatively simple in form and may be readily constructed by known methods, so that the tubes are not unduly expensive if manufactured on a mass production basis.

While particular embodiments of the present invention have been shown and described, it is apparent that changes and modifications may be made without departing from the invention in its broader aspects. The aim of the appended claims, therefore, is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

`1.\An electron-discharge device of the traveling-wave type comprising: an electron gun for projecting a sheetlike beam of electrons along a given reference path; a wave-transmission line disposed adjacent to said reference path and elcctrostatically coupled to said electron beam substantially uniformly throughout the length of said line, said wave-transmission line having a uniform low wave-propagation velocity in a direction parallel to said path and further having a length in said direction which is large relative to the effective wavelength of a signal wave traveling along said line; a support member interposed between said wave-transmission line and said reference path; a r'st series of electrically interconnected electric signal field permeable conductive elements disposed at predetermined intervals along said reference path on the surface of said support member adjacent said reference path; and a second series of electrically interconnected electric signal field permeable conductive elements disposed on said surface of said support member and interleaved with said first series.

2. An electron-discharge device of the traveling-wave type comprising: an electron gun for projecting a sheetlike beam of electrons along a given reference path; a pair of wave-transmission lines disposed adjacent to opposite sides of said reference path and electrostatically coupled to said electron beam substantially uniform throughout the length of said lines, said wave-transmission lines each having a uniform low wave-propagation velocity in a direction parallel to said path and further having a length in said direction which is large relative to the effective wavelentgh of a signal wave traveling along said line;` al `first support member interposed `between one of said wave-transmission lines and said reference path; a first series of electrically interconnected electric signal field permeable conductive elements disposed at predetermined intervals along said reference path on the surface of` said support member adjacent said reference path; a second series of electrically interconnected electric signal field permeable conductive elements disposed on said surface of said support member and interleaved with said first series; a second support member interposed between the other of said wave-transmission lines and said reference path; a third series of electrically interconnected electric signal field permeable conductive elements disposed opposite said first series on the surface of said second support member adjacent said reference path; and a fourth series of electrically interconnected electric signal field permeable conductive elements disposed opposite said second series on said surface of said second support member.

3. An electron-discharge device of the traveling-wave type comprising: an electron gun for projecting a sheetlike beam of electrons along a given reference path; a pair of wave-transmission lines disposed adjacent to opposite sides of said reference path and electrostatically coupled to said electron beam, said wave-transmission lines each having a l-ow wave-propagation velocity in a direction parallel to said path and further having a length in said direction which is large relative to the effective wavelength of a signal wave traveling along said line; a first insulating support member interposed between one of said wave-transmission lines and said reference path; a first series of electrically interconnected high-resistivity coatings periodically disposed along said reference path on the surface of said support member adjacent said reference path; a second series of electrically interconnected high-resistivity coatings disposed on said surface of said support member and interleaved with said first series; a second insulating support member interposed between the other of said wave-transmission lines and said reference path; a third series of electrically interconnected highresistivity coatings disposed opposite said first series on the surface of said second support member adjacent said reference path; and a fourth series of electrically interconnected high-resistivity coatings disposed opposite said second series on said surface of said second support member.

4. An electron-discharge device of the traveling-wave type comprising: an electron gun for projecting a sheetlike beam of electrons along a given reference path; a pair of wave-transmission lines disposed adjacent to opposite sides of said reference path and electrostatically coupled to said electron beam, said wave-transmission lines each having a low wave-propagation velocity in a direction parallel to said path and further having a length in said direction which is large relative to the effective wavelength of a signal wave traveling along said line; a first insulating support member interposed between one of said wave-transmission lines and said reference path; a first series of electrically interconnected conductive elements disposed at predetermined intervals along said reference path on the surface of said support member adjacent said reference path; a second series of electrically interconnected conductive elements disposed on said surface of said support member and interleaved with said first series; a first conductive leakage coating, having a substantially higher resistivity than said conductive elements, covering all of said surface of said second support member exposed to said beam; a second insulating support member interposed between the other of said wave-transmission lines and said reference path; a third series of electrically interconnected conductive elements e disposed opposite said first series on the surface of said second support member adjacent said reference path; a fourth series of electrically interconnected conductive elements `disposed opposite said second series on said surface of said second support member; and a second conductive leakage coating, having a substantially higher resistivity than said conductive elements, covering all of said surface of said secondA support member exposed to said beam. l n

5. An electron-discharge dcviceof'the traieling-wave type comprising: an electron `gun for projectingpasheetlike beam of electrons alonga given referen e Vpath; a pair of wave-transmission lines' disposed` adjacent j to oppositesides of said reference path'and elecitrostatic'ally coupled to said electron beam, said y wave-transmi'ssion lines each having a low wave-propagationvelocity in a direction parallel to said path and further Ahaving a length in said direction which'is'; large `relative tol the effective wavelength of a signal wave traveling alongsaid line; a first support member interposed between one of said wave-transmission lines and said referencefpath; a ijrst series of electrically interconnected conductive elements disposed at predetermined intervals along said reference path on the `surface of said support member adjacent said reference path; a V'second yseries of electrically interconnected conductive elements disposed on said surface of said support member and interleaved with), said first series; a second support'niembery interposed between the other of said Wave-transmissionflines and'said reference path; a third series of electricallyinterconnected conductive elements disposed opposite saidfi'rst" series at corresponding predetermined intervals along said reference path on the surface of said second support member adjacent said reference path; a fourth series of electrically interconnected conductive elements disposed'opposite said second series on said surface of said second support member; means for maintaining said rst and third series of conductive elements at a first predetermined average potential; and means for `maintaining `said second and fourth series of conductive elements at a second predetermined average potential different from said first potential to establish a series of electron lenses along said path and substantially confine said beam `'to said path throughout said' lengths of said wave-transmission'-lines. 6. An electron-discharge device of the traveling-wave type comprising: an electron gun for projecting a sheetlike beam of electronsalo'ng a given reference path; a pair of wave-transmission.lines disposed adjacent to opposite sides of ,said referencel path and electrostatically coupled to said electronbearn, saidVwave-transmission lines each having a low wave-propagation velocity in a direction parallel to said path and further havingV a length in said direction which is large relative to the effective wavelength of a signal wave traveling along saidwlineg a first support member interposed'between one of said wave-transmission lines and said reference path; a rst series ofV electrically interconnected conductive'elernents, each comprising al plurality of conductive strips extending in a direction transverseV to thedirection of travel of said electron beam, disposed at predetermined intervals along said reference path on the surface of said support member adjacent said reference path; a second series of electrically interconnected conductive elements, each comprising a plurality of conductive strips extending in a direction transverse to the direction of travel of said electron beam, disposed on said surface of said support member and interleaved with said first series; a second support member interposed between the other of said wave-transmission lines and said reference path; a third series of electrically interconnected conductive elements disposed opposite said first series at corresponding predetermined intervals along said reference path on the surface of said second support member adjacent said reference path; and a fourth series of electrically interconnected conductive elements disposed opposite said second series on said surface of said second support member. 7. An electron-discharge device of the traveling-wave type comprising: an electron gun for projecting a sheetlike beam of electrons along a given reference path; a pair of wave-transmission lines disposed adjacent to opposite sides of said reference path and electrostatically coupled to said electron beam, said wave-transmission lines each having a low wave-propagation velocity in a direction parallel to said path and further having a length inA saiddirection which is large relative tothe effective wavelengtho'ffa signal wave traveling along said line; a first support member interposed between one of said wave-transmission lines and said `reference path; a first series of conductive elements, each comprising a plurality of conductive strips extending in a direction transverse to the directionA of travel of said electron beamdisposed at predetermined intervals along said reference path .on the surface of said support membertadjacent said reference path; a second series of conductive elements, each comprising a plurality of conductive strips extending in a direction transverse to Vthe directionof travelof said electron beam, vdisposed on said surface of said support mem ber and interleaved with said first series; arsecond support member interposed between the other of said wavetransmission lines and said reference path; a third series of conductive elements disposed opposite said first series the surface `of said second support member adjacent said reference path; a fourth series of conductive elements Ldisposed op'posite said secondtseries, on said surface of said second support member; and connecting means, having a relatively high impedance atr signal frequencies, for electrically Vinterconnectingvvith respect to lower frequencies each of said conductive elements of each of said four series with the remaining elements of said series.

8. A n electron-dischargedevice ofthe traveling-wave type comprising.: an electron gun for projecting a sheetlike beam of electrons along a given reference path; a pair of wave-transmission lines disposed adjacent to opposite sides of s aidI reference path andA electrostatically coupled to said electron beam, saidwave-transmission lines each having a low wave-propagation velocity in a direction parallel to said path and further having a length in said direction which is largev relative tothe effective wavelength of arv signal wave traveling, alongtsaid line; a first' support Vmemberinterposed betweenL one lof Ysaid wave-transmission lines and saidtreference path; a rst series of conductive elements, each comprising a plurality of conductive strips extending in adirection transverse to the direction of travel, of ,saidtelectron beam,k disposed at predetermined intervals along said reference pathon the surface" of said support member adjacentsaid reference path; a second series ,of conductiveelements,\each comprising a plurality of` conductive strips extending in a direction transverse tothe directiontof travel of said electron beam, disposed on/saidsurface ofgsaid support member and interleaved with said firstseries; a second support member interposed between .the wother of said wave-,transmission lines andrsaid` reference path; a third series of conductive elements disposedopposite said first series the surface of ,said second support member adjacent said reference path; a fourth series of conductive elements disposed opposite said second series onv said surface of said second support member; and four inductive connector members, each having a relatively high impedance at signal frequencies, for electrically interconnecting each of said conductive elements of each of said four series with the remaining elements of said series.

9. An electron-discharge device of the traveling-wave type comprising: an electron gun for projecting a sheetlike beam of electrons along a predetermined reference path; a wave-transmission line, disposed adjacent said reference path in spaced relation thereto and electrostatically coupled to said beam, having a uniform low wavepropagation velocity in a direction parallel to said path and further having a length in said direction which is large relative to the effective wave length of a radiofrequency signal wave traveling along said line; means for applying a radio-frequency signal to said wave-transmission line to establish said traveling signal wave and thereby to provide a traveling electric signal field which propagates along said reference path at said low wavepropagation velocity; and means, including a beam con fining structure substantially permeable throughout its length to said electric signal field but substantially impermeable to unidirectional electric fields and disposed interjacent said Wave-transmission line and said reference path, for confining said beam to said path while permitting substantially uniform electrostatic coupling between said line and said beam for signal frequency electric fields but shielding said line therefrom for unidirectional electric fields.

10. An electron-discharge device of the traveling-wave type'cornprising: an electron gun for projecting a sheetlike beam of electrons along a predetermined reference path; a pair of helical wave-transmission lines, individually disposed adjacent opposite sides of said refer,- ence path in spaced relation thereto and electrostatically coupled to said beam, having a `uniform low wave-propagation velocity in a direction parallel to said path and further having a length in said direction which is large relative to the effective wave length of a radiofrequency signal wave traveling along said line; means for applying a radio-frequency signal to said pair of wave-transmission lines in push-pull relationship to establish said traveling signallwave and thereby to provide a traveling transverse electric signal field which propagates along said reference path at said low wave-propagation velocity; and means, including a pair of beamconfining structures individually disposed inten'acent respective ones of said pair of wavetransmission lines and said reference path, for confining said beam to said path while permitting substantially uniform electrostaticY coupling between said lines and said beam for signal frequency electric fields but shielding said lines therefrom for unidirectional electric fields.

11. An electron-discharge device of the traveling-wave type comprising: an electron gun for projecting a sheet-4 like beam of electrons along a predetermined reference path; a pair of helical wave-transmission lines, indi-4 vidually disposed adjacent opposite sides of said refer,- ence path in spaced relation thereto and electrostatically coupled to said beam, having a uniform low wave-propagation velocity in a direction parallel to said path and further having a length in said direction which is large relative to the effective wave length of a radio-frequency signal wave traveling along said line; means for applying a radio-frequency signal to said pair of wave-transmission lines in push-pull relationship to establish said traveling signal wave and thereby to provide a traveling transverse electric signal field which propagates along said reference path at said low wave-propagation velocity; and means including a pairof beam confining structures, each comprising two interleaved series of periodically spaced lens electrodes, individually disposed interjacent respectivel ones of said pair of wave-transmission lines and said reference path for confining said beam to said path while permitting substantially uniform electrostatic coupling between said lines and said beam for signal frequency electric fields but shielding said lines therefrom for unidirectional electric fields.

12. An electron-discharge device of the traveling-wave type comprising: an electron gun for projecting a sheetlike beam of electrons along a predetermined reference path; a pair of helical wave-transmission lines, individually disposed adjacent opposite sides of said reference path in spaced relation thereto and electrostatically coupled to said beam, having a uniform low wave-propagation velocity in a direction parallel to said path and further having a length in said direction which is large relative to the effective wave length of a radio-frequency signal wave traveling along said lines; means for applying a radio-frequency signal to said pair of wave-transmission lines in push-pull relationship to establish said traveling signal wave and thereby to provide a traveling transverse electric signal field which propagates along said reference path at said low wave-propagation velocity; and means including a pair of beam confining structures, each comprising two interleaved series of periodically spaced lens electrodes, individually disposed interjacent respective ones of said pair of wave-transmission lines and said reference path with each series of lens electrodes of each structure aligned with the corresponding series of the other of said structures and further including means for maintaining each of said series of lens electrodes of one of said structures at an average unidirectional potential equal to the average unidirec- 1 tional potential of the corresponding series of lens electrodes of the other of said structures but of an average unidirectional potential substantially different from that of the remaining series of said electrodes to establish a space-periodic electrostatic lens for confining said electron beam to said path while permitting substantially uniform electrostatic coupling between said lines and said beam for signal frequency electric fields but shielding said lines therefrom for unidirectional electric fields.

References Cited in the file of this patent UNITED STATES PATENTS 2,183,398 Hehlgans Dec. 12, 1939 2,410,863 Broadway et al. Nov. 12, 1946 2,489,082 De Forest Nov. 22, 1949 2,509,374 Sunstein May 30, 1950 2,653,270 Kompfner Sept. 22, 1953 2,683,256 Kumpfer July 6, 1954

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