US2672572A - Traveling wave tube - Google Patents

Description

March 16, 1954 J. w. TILEY 2 ,672,572

TRAVELING WAVE TUBE Filed Nov. 28, 1947 3 Sheets-Sheet l INVENTOR. JOAM/ W 11512.)

March 16, 1954 J. W. TILEY TRAVELING WAVE TUBE Filed Nov. 28, 1947 Poms-.9

5 Sheets-Sheet? JO N W 2'11; EJ/

ATTORNE Ye? March 16, 1954 Filed Nov. 28, 1947 J. W. TILEY TRAVELING WAVE TUBE 5 Sheets=$heet 3 II I .33

IXI'EX'I'OK JOHN W TILEY Ja/ZLQ BY Patented Mar. 16, 1954 UNITED STATES PATENT OFFICE.

TRAVELING WAVE TUBE John W. Tiley, Philadelphia, Pa., assignor to Philco Corporation, Philadelphia, Pa., a corporation of Pennsylvania Application November 28, 1947, Serial No. 788,724

9 Claims. 1

This invention relates in general to the electron tube art, and more particularly to a novel electromagnetic traveling wave tube having special application in high frequency and broad band electrical systems. This application is a continuation-in-part of my co-pending application Serial No. 761,798 filed July 18, 1947, now Patent No. 2,541,843.

In electrical systems such as television, pulseposition modulation and radar, it has often been extremely diificult to obtain adequate and uniform amplification over the frequency spectrum encompassed by the signal. Ordinary triodes, even when incorporated in special circuits, fail to provide a usable gain in such applications. Recent designs, such as the lighthouse tube, and the velocity modulation tube as exemplified by the klystron, can provide reasonable gain solely in narrow band width operation. An attempt to use a klystron or like tube in the microwave region with a band width of fifty megacycles or greater will result generally in a gain of less than unity.

A recent electronic development, known as the beam traveling wave tube, overcomes the limitations of the more conventional tube types, and has been successfully tested as an efficient amplifier of signals having a mean frequency of the order of thousands of megacycles with an over-all band width of the order of eight hundred megacycles. Theoretically, even much wider bands may be amplified by the device without sacrifice of gain.

For a description and illustration of this development, reference is made-to the publication Bell Laboratories Record of December, 1946, and to the article therein entitled The Beam Traveling Wave Tube by J. R. Pierce. In this reference the traveling wave tube is described as constituted of an electron gun similar to those employed in cathode ray tubes. An electron beam generated by the gun is directed in a narrow beam along the axis of a long evacuated tube and impinged upon a collector anode. Within the tube, and surrounding the beam axis, is a closely wound wire helix which is excited at the electron gun end thereof by the weak signal to be amplified and which provides at the collector end thereof the amplified output signal. The tube contains no signal grids in the conventional sense.

Broadly speaking, the applied signal travels along the wire helix as an electromagnetic wave at a speed approaching the speed of light. As is determined by the pitch of the helix, the Wave 2 travels axially of the tube at a fraction of the speed of light; and the electron gun and collector anode potentials are arranged so that the average axial velocity of the electron beam through the helix is somewhat greater than the axial wave velocity.

Interaction of the electron beam and electromagnetic fleld components extending from the helix produces the signal amplification. The greater the e-ectron current and the longer the helix, the greater is the gain. In transit through, the helix, the average electron velocity is reduced, and the energy drop represented by this decreased velocity is imparted to the signal. The tube does not require a tuned circuit in the signal path, the wire helix being in effect an all pass transmission line. Hence the tube is capable of operation over an exceedingly wide frequency range. In practice this range is limited somewhat by the impedance match of the helix to the external circuits.

In operation the signal appearing on the helix acts on the electron stream and gradually produces fluctuations in velocity and density. The density modulated electron beam delivers energy to the wave and over the helix section nearest the collector there is a substantially uniform gain per unit length of travel.

Th present invention contemplates and has as a primary object the provision of electromagnetic traveling wave tubes of novel and improved design having wide application as broad band microwave amplifiers, oscillators, frequency convertors, or the like. As in the electron tubes disclosed in my above-identified patent application, the electromagnetic traveling wave tubes of the present invention differ from prior art traveling wave tubes in that wave guides are utilized for directing wave signals around the path of an electron beam. These wave guides permit direct coupling of the electromagnetic fields therein and the tube electron beam, and preclude excessive energy radiation and undesirable coupling with external fields.

In the aforementioned patent application it was demonstrated that signal amplification could be obtained in an electromagnetic traveling wave tube by causing an electron beam to traverse the axis of first and second or input and output helical wave guide sections of predetermined pitch and number of turns. The input wave guide was energized by the weak incoming signal, whereby the electron beam was bunched or density modulated. The modulated beam was then caused to traverse a second or output helical wave guide section and to deliver energy thereto to provide the amplified output signal.

In a tube of this construction, input and output waveguides are spaced axially of the electron beam. When certain conditions of beam velocity and helix structure are properly satisfied, considerable gain is obtained and the electron tubes are capable of either amplifier or oscillator operation; the latter being accomplished by coup-ling a predetermined portion of the output signal in the correct phase to the input wave guide. In connection with traveling wave tubes utilizing independent input and output wave guide sections, it has been determined that the optimum average beam velocity through the guides is equal to the axial component of velocity of the electromagnetic traveling wave.

In accordance with the principles of the present invention a single helical wave guide is utilized for both modulating and extracting energy from an electron beam. The single wave guide employed is constructed essentially the same as the input wave guide of the systems disclosed in the above named patent application.

A particular advantage of the present construe tion to be described in detail below is that the single wave guide utilized for electron beam modulation and signal output comprises a conductive helix of uniform pitch. The incoming signal is coupled into the wave guide at the input or electron gun end of the helical wave guide and the output signal is derived from the opposite end thereof.

Successful operation as amplifier or oscillator is accomplished by the utilization of an electron beam velocity somewhat above the axial velocity of the traveling wave. In transit through the tube each group of electrons delivers energ to the traveling wave and as a consequence thereof, the output signal is at a levelconsiderably higher than that introduced at the input end of the guide.

It is therefore an object of the present invention to provide a beam-traveling wave electron tube characterized by inherent stability and high gain, and which utilizes a single helical Wave guide structure for directing the wave energy about the path of the electron beam.

A further object of thepresent invention is to provide a velocity modulation type electron tube utilizing a single helical wave guide of uniform pitch for raising theintensity level of an input electromagnetic signal.

A still further object of the present invention is to provide a traveling Wave electron tube of comparativeh simple mechanical design and rugged overall construction.

It has been observed that frequency variations of the signal being amplified tend to afiect the strength of the electric fields established by the wave guide and coupling with the electron beam. The present invention further contemplates the provision of novel helical wave guide structures which materially increase the frequency range over which wave guide electromagnetic traveling wave tubes may operate. Generally speaking, this is accomplished by capacitrxely loading the wave guide structure to mainta n the strength of electric fields more nearly constant over wide band frequency variations of the input electromagnetic energy.

It is thus another object of the present invention to provide a heli'ial wave guide structure adaptable to many types of traveling wave tubes,

4 which structure permits broad band operation thereof.

Still another object of the present invention is to provide means for capacitively loading a wave guide having an open side wall while permitting the fields extending from the wave guide to couple with an electron beam.

These and other objects of the present invention will now become apparent from the following detailed specification when taken in connection with the accompanying drawings in which Figure 1 is a general side view partially in section of a helical wave guide traveling wave electron tube.

Figure 2 is a general side view partially in section-of another embodiment of a traveling wave tube of the general type illustrated in Figure 1.

Figure 3 is a fragmentary side view partially in section of a helical wave guide tube structure illustrating a novel means of obtaining broad band wave guide operation; and

Figure 4 is a general side view partially in section of a wave guide structure similar in features and characteristics to that illustrated in Figure 3.

ith reference now to the drawings and more particularly to Figure 1, there is illustrated an electron tube incorporating the features of the present invention and comprising, generally, a centrally disposed sealed and evacuated glass or similar dielectric cylinder 2 I Sealed into the left end of the glass cylinder .21 as viewed in Figure l are the electrodes of an electron gun 2%, similar to those utilized in conventional cathode ray tube structures. As shown the electron gun 2?] comprises a heater 22 and its associated cathode 23, a centrally perforated control grid 2 and focusing and accelerating hollow cylindrical electrodes 25 and 26, respectively. When the electrodes comprising the electron gun 29 are energized from a suitable power source (not shown) an axial beam of electrons of predetermined high velocity is generated and directed along the axis 28 of cylinder 2| toward a disc shaped collector electrode 2'! sealed into the righthand end of the tube as viewed in Figure 1.

Structural means required for supporting the electron gun electrodes have been omitted for clarity.

As electron beam tubes of the type included within the structure of the tube in Figure l are sufficiently well known in the electronic art, a further description thereof and of the energizing means therefor are considered unnecessary at this point.

In accordance with the broad principles of the present invention, the axial path of travel of the electron beam through tube 2! is substantially enclosed within a single helical wave guide structure 39, preferably of uniform pitch. As is clearly illustrated in Figure 1, the wave guide structure 35? is comprised of a thin highly conductive metallic strip 3i wound edgewise about and in contacting relationship with cylinder 2i at the aforementioned uniform pitch.

The outer edge of the conductive strip 3! defines a helix lying in'electrical contacting relation with the inner cylindrical surface of an enclosin: holow metallic or otherwise conductive cylinder 33, coaxial with and extending substantially the length of tube 2!. The helical strip 3! and enclosing metallic cylinder 33 thereby define a helical passage 32 of uniform rectangular cross-section which progresses axially of tube 2 i. In ef'ect, there "ore, helical passage 32 comprises a helical wave guide enclosed at any point by two adjacent turns of strip 3|, and by the metallic surface of the enclosing cylinder 33. The innermost wall of the helical passage 32 lies in the outer surface of dielectric cylinder 2 l, and is accordingly open insofar as radiation of electromagnetic fields is concerned.

Coupling means are provided for the interchange of electromagnetic wave energy with the helical wave guide passage 32. As illustrated, the outer cylindrical conductor 40 of a coaxial line 39 extends through a perforation 4| in metal cylinder 33 and enters helical passage 32. The end of inner conductor 42 of coaxial line 39 is curved as shown and electrically connected to the outer conductor 40 to form a small coupling loop 43. A similar coaxial line 44, comprising an outer conductor 45 and an inner conductor 46, extends through a perforation 41 in cylinder 33 and is terminated by coupling loop 43 within wave guide 32. It will be noted that coaxial lines 39 and 44 enter wave guide passage 32 at axially opposed ends thereof.

In order to maintain electrons traversing cylinder 2| within a sharply defined axial beam, means are provided for establishing a substan tially uniform and unvarying axial magnetic field. As illustrated, this is accomplished by means ofa multi-layer solenoid type coil 5| coaxial with tube 2| and wound upon an insulating cylindrical coil form 52 suitably secured over the metallic cylinder 33 between coaxial connectors 39 and 44. Coil 5| is preferably energized from a direct current source (not shown in the draw ings).

In accordance with well understood principles, an electron which tends to travel a path divergent from the axis 28 of tube 2 and hence angularly of the field established by coil 5|, will be urged back to axis 28' as a consequence of its interaction with the axial magnetic field.

For successful operation of the electron tube structure of Figure 1, it is desirable to preclude possible interference from stray electromagnetic fields and, further, to preclude internal wave reflections. Insofar as external electric fields are concerned, the conductive cylinder 33 and the helical strip 3| provide substantially complete shielding. A magnetic shield may be employed if found necessary.

To minimize internal signal reflections, helical wave guide structure 30 has been suitably electrically terminated and matched at the ends thereof. Thus, in the embodiment of Figure 1, impedance matching is effected at ends of the wave guide by means of two uniformly tapered, molded helical blocks 54. Blocks 54 are preferably constituted of a ceramic containing powdered graphite as the dissipating element; however, various other absorptive substances may be employed. By thus tapering the lossy material, a proper impedance match is readily obtained. This corresponds in effect with the utilization of tapered or wedge-shaped dissipative blocks as reflectionless terminations for conventional rectangular and other wave guides.

To insure reflectionless termination, each of the blocks 54 illustrated in Figure 1 is preferably made equal to at least one wave length at the lowest frequency of operation, as measured along the helix. Suitable termination may also be effected by uniformly tapering the spacing of the end turns of wave guide structure 3|) to zero, and covering the tapering sections thereof with a power absorbing, semi-conductive substance.

The operation of the traveling wave electron tube illustrated in Figure 1 will now be described when effective as a signal amplifier. The incoming weak signal to be amplified is applied to the tube through coaxial line 39 and the amplified output is extracted through coaxial line 44. The input signal travels in a helical path through passage 32 at a velocity somewhat less than that of light. Standing waves are precluded by the impedance matching dissipative blocks 54.

In accordance with the general principles of traveling wave electron tube operation, the velocity component of electromagnetic wave energy within wave guide 32 parallel to the axis 28 of tube 2|, is a fraction of the velocity of light, as determined by the pitch of the wave guide 32. As an example, if the length of the helical path through wave guide passage 32 is ten times as great as the axial distance encompassed thereby, then the velocity of wave propagation considered relative to the axis of tube 2| will be equal to one-tenth the velocity of light. It is therefore apparent that if an electron in the beam generated by the tube electron gun 23 has an average axial velocity equal to one-tenth the velocity of light, then the relative phase of the electromagnetic wave in helical passage 32 and this electron will remain unchanged.

If the electron velocities in the beam traversing the tube are of the order of one-tenth the velocity of light or less, then the following nonrelativistic equation may be utilized as a guide for determining the electron velocity:

v=5.93 10 /fi centimeters per second wherein: Y

E is equal to the voltage through which the beam electrons are accelerated.

The interaction of an electromagnetic wave traveling within helical wave guide 32 and an electron beam traversing the axis 28 of glass cylinder 2| is somewhat similar to the bunching action obtained in velocity modulation electron tubes. As will hereinbelow be demonstrated, the wave energy traveling within the wave guide 32 from the input coaxial line 39 to the output coaxial line 44 continuously density modulates the axial electron beam in accordance with the input signal variation so that the electron beam as it traverses the axis of tube 2| is progressively bunched to greater and greater bunch densities.

Simultaneously with the bunching effect of the electron beam, the beam delivers energy into wave guide 32 so that the wave energy therein progressively increases in intensity in its travel toward collector electrode 21.

The electromagnetic fields established by wave energy flowing within the helical wave guide passage 32 may best be visualized if the region between the inner surface of conductive cylinder 33 and the axis of tube 2| is considered as half a rectangular wave guide, the broad walls of which are the metallic surfaces of adjacent turns of helical strip 3|. If the coupling loop 43, applying the input signal to be amplified to the helical wave guide 32, excites this wave guide in the T1301 mode, then maximum electric field is produced between adjacent inner edges of the turns of helical strip 3|. The electric field is normal to the strip 3| and diminishes sinusoidally to zero at the inner surface of metallic cylinder 33.

Points A, B, C and D may be considered as defining the cross-section of half a rectangular wave guide operating in the aforementioned mode.

Aspoints C and D are .metallically-connected, there .is no potential agradient therebetween. However, an electric field is established between points A and Bbyrt-heexcitingwave, which field cyclically variesin intensity as afunction of time from axpredetermined maximum in one direction through zerotoa corresponding maximumin the opposite direction. The electric field between points .A :and. B, of course, extends through the dielectric cylinder 2i and into the region of'electronnflowtherethrough. ;In view of the factthat the :input .electromagnetic wave constantly :progrosses from left ,to:right;as-viewed in Figure 1, agivenipotential gradient may be considered as traveling around theinneriedges of the helical strip i 3l 'ata velocity approaching the velocity .of light. 2Thus,\if a voltage: maximum in aparticular-zdirection appears between points :Aand ;B atuone instant, it will appear at correspondingly later intervals of time between-points E and It,

B and G, .F and'l-Liand so on, until down the guide.

For purposes of illustration, let it be assumed t'hatthe dimensions of .wave guide 32 are such that, electromagnetic wave energy, introduced by coupling .loop 43, travels around one complete turn or the helical passage 32in a time period corresponding to one-half.cycle of the input wave. Underlsuchconditions there is a sinusoidal voltage distribution-between opposed inner edges of the'helical strip'3l. .If, ,under these conditions, andiana particular instant of time, the voltage between pointsArandB-is a maximum in one direction, then the potential difference between points E-andl w'illbezero and between points Y B and G a maximum in the opposite direction.

Within the glass cylinder 2!, the electricfiel'ds extending :between opposed points on adjacent turns of the helical strip: 3! may be resolved into components, one of which is parallel to the axis 28 of electron flow. "The sense of these parallel field-components is dependent upon the-instantaneous polarity of the originating points. As wave energy passes from left to right through the helical 'wave guide passage '32, these field com- "ponentsmay be considered as progressing axially from left' to right, while simultaneously rotating "about the tube axis. As previously mentioned, the axial velocity oi'propogation or the electromagnetic field components will be determined 'primarily'by the pitch of the helical-guide '32.

If'the electron gun potentials were to be adjuste'd to provide-an axialelectron beam having a velocitysubstantially equal to the axial velocity of propagation of theinput'electromagnetic wave (asdescribedin theaforementioned patent application and herein discussed for illustrative purposes only), a bunching action will take place ass. result 'ofthe interaction of the traveling electronbeamand the axially moving electric field-components. l'n a given electron beam, individual electron velocities extend over aspectrum from a velocity considerably below to one considerably'above the average velocity. If at the timeelectrons enterthat portion-of the axis ponent in its .travel between points ;:A :an'tisB, :;E and F, B and G, and the like. 'O-nzthe other hand, electrons which enterztub 2 I theireg-ion of helical guide structure 30 with a tvelocity greater than the axial velocity of'the traveling wave, will be retardedsomewhatand will T6011- tinue to decrease in velocity intransitth-rough the tube.

Electrons which enter the region of the helical wave guide 30 when point Ais positive'withrespect topoint B, will be retarded. Of these retarded electrons, the ones having the highest velocities may escape the retarding held and enter into a region of anaccelerating field; of-the type previously described. The lowest velocity electrons may be suificiently retarded lid-cause them to be overtaken by the following cycle-of an accelerating fieldcomponent.

Thus, as the electron beam traverses the section of tube 2! spanned by the helical wave guide structure 30, it is acted upon by the traveling wave field components so that electron 'willbecome progressively bunched in the region ofi max imum positive potential points of the electric field. Th degree of bunching, ordensity mod- .ulation, depends of courseupon the intensityci the input electromagnetic wave and the' effective length of the helical wave guide passage 32. .The density modulated beam which is .formed, -as hereinabove described, .finally impinges upon collector plate 2?.

Since the electron beam is pr.ogressively..rednced in velocity as ittravels. throughthe region of helical wav guide 32, it is preferable that.,,the collector electrode 2'? flbeconnecteduto a source of potential (not shown) which is thatltequirfidlio produce such electron velocity. as determinedby the above equation.

In order for the electronbeam travelingwave tube illustrated in Figurelto amplify electromagnetic wave ener y, applied overcoaxialco hector til, it is essentialthat therebea nettransfer of energy from the electron beam totthei'elem tromagnetic wave traveling Withinthe wave guide passage'32. From the foregoing discussion1it'is apparent that to accomplishthis ener y exchange it is necessary that the electronbeam actually enter the region spannedbythewaveguidestructure 30 With an average electron velocity somewhat greater than theaX-ial velocityoitheelectric field components which extend from ,the open wall oi guideztflinto theregionofthe electron beam.

As in the aboveexarnpla-electrons enteringsa region of positive potential gradient will-berati- .celerated in the direction-pi the collector-electrode 2'! while thoseventeringga region sir-negative potential gradientwill be retarded. j .How- -ever,.-since in operation the electrons en.ter;the region of wave guide struotureeilxwith an-average velocity greater than the axialyelocity of the axial field components electronswill remain in a region of retarding field for a longer-period than in a region of accelerating field. At-agiven instant, therefore, a-greater number of electrons are being retarded than accelerated, whereby the amount of energy given up to the travelingwa-ve by the electrons at a give-n'instant will be greater than the amount of energy absorbed by the electron beam from the traveling wave. 'Thus as the electromagnetic wave travels. its helical path from the input coaxialline39 to the output coaxial line 44, the intensity of the wave energy therein progressively,buildslup, resultingin a net amplification for the system, the amplified signal being extracted at coaxial lin 54.

Clearly the actual gain obtained by a tub of the type illustrated in Figure 1 is dependent upon numerous factors including the length of the helical wave guide 32 between input and output connectors, the actual velocity differential between the average electron velocity and the axial velocity of the traveling wave, The precise design factors involved in the construction of a tube of this type will not be treated in greater detail in the present application.

The inter-action of th electric field component extending into the cylinder 2! in the region of the electron beam may also be explained by considering the axially moving electrons as encountering electric fields whose magnitudes are continuously increasing as the electrons traverse the cylinder 2! from the electron gun 29 to the collector anode 21.

When considered in this manner, each retarding field will effectively be greater in intensity than the previous accelerating field with the result that electron bunching will occur in the regions of retarding field. As a phenomenon of this type results in the transfer of energy from the beam to the wave, energy will, be delivered by the electrons to the electric field to increase further the amplitude of the traveling wave energy. Since the electron velocity in the beam when considered on an average is greater than that of the axial velocity of the traveling electric fields, the electron bunches will not continuously be comprised of the same individual electrons; rather electrons will drift between electron bunches.

The electron tube illustrated in Figure 1 has a particular tendency to become unstable or act as an oscillator unless considerable care is given to the matter of preventing reflections at both ends of the structure. When employed as an ampliher, as in the above description of Fig. 1, it is particularly important to terminate properly and match the wave uide 32 to the external circuits.

On the other hand. if it is desired to operate the electron tube illustrated in Figure l as an oscillator for the generation of high frequency electromagnetic wave energy, a feedback circuit delivering energy from the coaxial line A l to the input terminal of coaxial line 39 of proper phase may be provided. If the gain of the system is comparatively high, then merely mismatching the terminations at in ut and output connectors will result in self-oscillation at a natural frequency which may be determined by placing suitable frequency sensitive means in the energy input and output coupling systems.

Referring now' to Figure 2, there is illustrated a microwave amplifier or oscillator having the general electrical features illustrated and described in connection with Figure 1, but differing.

somewhat in construction. The electron tube of Figure 2 comprises a glass, or similar dielectric cylinder 6!, sealed and evacuated to permit the passage of an electron beam therethrough. Sealed into the left-hand end of cylinder 6|, as viewed in Figure 2, is an electron gun comprising an indirectly heated cathode 52, a perforated disc-type grid 63, and focusing and accelerating cylindrical electrodes 64 and 65, respectively. As

described in connection with Figure 1, when theseelectrodes are energized from a suitable power source (not shown), an electron beam is generated having an average electron velocity determined as a function of the total accelerating 16 potential in accordance with the equation set forth above.

The generated electron beam is directed along axis 56 toward a collector electrode 87, suitably sealed into the right-hand end of the cylinder 6|. For amplifier and oscillator operation, the potential of collector electrode 6'! is determined in a manner similar to that described hereinabove for collector electrode 21 shown in Figure 1.

In accordance with the principles of the present invention, a helical wave guide H is coaxially disposed with respect to the glass cylinder 6| and positioned between the ends thereof. Helical wave guide H is preferably formed of a coil of unitary U-shaped metallic channel comprising side walls 73 and M rigidly spaced by integral outer metallic strip '55. As illustrated the inner open face of this U-shaped channel is formed to lie in the outer surface of cylinder 6|.

In order to prevent signal reflections and stand-- ing waves within the wave guide H, helical dissipative blocks 18 and 18' are secured within the ends of this wave guide. As described in connection with the impedance matching blocks 54 of Figure 1, blocks 18 and 18' are preferably uniformly tapered and constituted of a material capable of eiiectively dissipating microwave energy. The length of a block 78 when measured helically about the guide is greater than a wave length at the lowest frequency of tube operation.

Electromagnetic wave energy is introduced into the wave guide H through a coaxial line 16, the

outer wall of which enters the wave guide section through a perforation H in the outer metallic strip l5. A loop 8| from the coaxial line inner conductor provides ample signal coupling. in a corresponding manner a coaxial cable 82 is utilized to extract signal energy from the wave guide H. Thus the outer conductor of coaxial cable 82 extends into the wave guide H through a perforation 83 therein, and a loop 84 formed of the inner conductor serves as the coupling means.

The outer helical surface of the wave guide H is substantially enclosed within a Bakelite or similar insulating cylinder 85, upon which a uniform, solenoid-type coil 86 is wound. When coil 86 is energized from a direct current source (not shown), a magnetic field is established having a large component parallel to the axis 65 of electron travel. This magnetic field minimizes de-. focusing of the electron beam in its comparatively long axial transit through the cylinder 6! between the electron gun and the collector electrodev Complete structural means for rigidly positioning the elements of the electron tube of Figure 2- in the relation shown have not been illustrated.

, The operation of the electron tube illustratedin Figure 2 is essentially similar to that alreadyanode 6?. The actual axial velocity of the travel-;-

ing wave is a fraction of the velocity through the wave guide H due to the helical path traversed therethrough.

Dissipative blocks 78 and 18' ensure the the axial direction of electron flow. Between any two opposed points, such as M and N of the open side of the helical wave guide ll, an electricficld is established with a component within the ab-- sence of traveling waves in a direction other than 1'1" tube Erl parallel to the axis 85 of the'electronfiovw This electric field component may be visualized as spinning about the axis of the tube ill at somewhat lessthan the velocity of light as the wave travels'irom left to right as viewed in Figure 2.

Theaverage velocity of electrons entering'the space within tube fil encompassed by the helical wave guide ll is greater than the axial velocity of the traveling wave. As described above the electron beam is density modulated as it traverses the region of the helical wave guide toward the collector anode til. In View ofthe velocity relationships specified for the axial component of the traveling wave and the electron beam, the bunching operationdescribed is accompanied by a general retardation of the beam electrons with the result that energy is delivered from the electron beam to the traveling wave.

As the traveling wave thus approachegthe col lector anode 6-1, the energy level thereof is greatly increased whereby the energy'delivered'to coaxial cable 82 is of considerably higher level than that introduced at coaxial cable it.

As a result of the separation of individual turns of helical wave guide ii, there are regions as for example between points N and P within the cylin-- der 6! ofcomparatively uniformly weal: electric field; In other words, the electric field is at all times concentrated between opposed broad walls I3 and IA-0f the waveguide. In effect, therefore, regions within the cylinder 6!, such as that between points N and P, act as drift spac s through which the velocity-of electrons remains substantially unaffected.

The electron tube of Figure 2'may be connected for oscillator operation by connecting a portion of the signal output obtained in coaxial cable 82 through proper phasing means to the input coaxial cable 16L circuit of suitable form (not shown) is preferably connected into the feedback means.

As in the case of the electron tube illustrated in Figure l, the beam traveling wave tube of Figure 2 is substantially unaffected by signal frequency changes between wide limits or by the -2 bandwidth thereof. It is preferable that the diameter of cylinder 6! be chosen such that when the volume enclosed therebyis considered as a cylindrical wave guide, it has a cut-oil frequency above the range of operation of the tube, whereby direct wave propagation and wave reflections at the signal frequency tending to distort the normal electric fields are precluded.

Inter-action with the electron beam of wave energy in the wave guide inter-turn space 33 is highly undesirable. By filling the inter-turn spaces 88 by an absorptive material (not shown) the eflect'of inter-turn energy may be substantially eliminated. Although an inner-turn space dissipative material has not been illustrated in 1 Figure 2 of the'present application, there is illustration and discussion thereof in aforemen-- tioned co-pending patent application.

The electron tubes illustrated in Figures 1 and- 2 of the present application are capable of operation' as microwave amplifiers or oscillators overextremely wide bands. However, it has been ob servedthat frequency variations alter the strength of the electric fields established between opposed points such as M and'N of Figure 2' of the helical wave guide.

The range of operation'of the electron tubes of Figures 1 and 2 may be extended considerablyin frequency by incorporating wave guide constructions' as illustrated in Figures 3 and 4.

For stability of operation, a filter acra-wa Referring now to Figure 3', there is illustrated. a portion of an electron tube generally constructed in accordance with theprinciples illustratedin Figure l, but modified to include a frequencybroadening means.

Thus in Figure 3 there is shown a dielectric cylinder 21 surrounded by an edge-wound helical strip 3i of conductive material which forms a guide 30 extending" helical channel or wave through th electron tube. The edge-wound strip 31 is wholly enclosed within a metallic cylinder 33 which in turn is covered by a Bakelite coilform 52 for the solenoidal coil 55. The coinponents of Figure 3, namely members 2!, Si, 33,

5| and 52, function as the correspondingly desig-- nated elements of the electron tube of Figural.

The sole modification incorporated in the structure of Figure 3 is the addition of a flat helical metallic strip lfil.

strip 3! which defines the sides of helical Wave guide 333. Further, the helical gap 562 defined by eling wave within the wave guid 38 in turn estab- 35 lishes an electric field directed across the helical gap $62, which field is in cilect similar to the field described asformed between points 5 and'G of the electron tube of Figure l with the exception of. increased intensity.

The'field extending acrcssithe gap I02 also'extends through dielectric cylinder 21 and spins helically as it progresses down the guide between the inputand output couplersinot shown in Figure 3).

Electrically the capacitiveloading introduced by the helical strip Sill tends to maintain the strength of the electric field existing across the gap substantially constant for wide frequency variations of the input wave energy. As a consequence of this construction, the traveling Wave electron tube of Figure 3 is operable over. a wider frequency band. than the corresponding tube shown in .Figure l. The application of a helical stripsuch as it?! thus provides a certain design freedom when constructing the wave guide structure" 39.

Referring now toFigure l, thereis illustrated in fragmentary forma'modification of the electron tube shown in Figure 2 corresponding essentially to the. above described modification of the elec-- tron tube of Figure .l as illustrated in Figure 3.

In Figure 4 the dielectric cylinder 6 l, the helical wave guide l l, the enclosing Bakelite coil form 85, and the solenoid-condo correspond to the similarly designated components of the electron tube of Figure 2. Other structural features of Figure 2- have been omitted as they are unessential to the description of th present modification.

To increase the frequency band of operation possible for a beam electron tube of this type, a helically wound flat metallic strip l l i has been positionedupon the outer surface of cylinder 81, and

has apitch corresponding to that of helical wave Theaxia'lwidth of the strip i l l is such that; at this pitch, there-is provided a uniform The strip Hit as illustrated in Figure 3 is of a pitch equal to that of the helical helical air gap I I2 between adjacent turns thereof, similar to air gap I02 described in connection with Figure 3.

The air gap I I2 which is of helical form is arranged as illustrated in Figure 4 as to lie centralling of the open inner wall of the U-shaped wave guide section II. It may also be seen that the helical strip Ill completely spans the interturn space 88 between adjacent turns of the wave guide ll.

In operation, traveling wave energy appearing within wave guide H establishes a particularly intense electric field across the gap H2 which functions in the manner already described in connection with the travelin fields of Figures 1 and 2, with the advantage that the construction illustrated is operable overa much greater frequency range.

From the foregoing description of the present traveling wave electron tubes, it is apparent that these offer design possibilities unattainable with other known means. Although described above as functioning as signal amplifiers, these tubes may be employed as microwave modulators and detectors, and in other circuits where conventional tube types fail to give the desired performance.

Numerous modifications oi the apparatus herein disclosed are possible without departing from the spirit of the present invention. As described, a helical path is employed for signal propagation to reduce the axial velocity of wave propagation to the average electron velocity. A large pitch, necessitating fewer turns for the input and output wave guides, may be used if the wave guides are filled with a solid, low loss dielectric substance. This is possible since the velocity of wave propagation in a solid dielectric is substantially less than the velocity in free space.

In order to increase further the energy output of the tubes illustrated, the helical wave guides may be operated with a uniform increasing potential gradient. Thus, the guides may be formed of a resistive material rather than of highly conductive substances as hereinabove described, and

a direct voltage maintained between the ends thereof, the positive end being furthest along in the direction of electron travel. This positive gradient will accelerate and add to the kinetic energy of the electrons, and permit the extraction I of increased power from the bunched beam.

It will now also be obvious to those skilled in the art that the inventive principle herein disclosed may, without departure from the invention, take the form in which both the wave guide and electron beam system are enclosed in a common evacuated envelope as illustrated in Figure 3.

of the parent ap lication of this application, Serial No. 761,798 filed July 18, 1947.

In view of the many possible structural and electrical design modifications possible, it is preferred that the present invention be defined solely by the appended claims.

I claim:

1. An electron tube comprising an evacuated dielectric cylinder, means for generating an electron beam therein, a helical wave guide formed of channel shaped conductor having substantially rectangular cross-section, a flat helical conductive strip formed in engagement with the outer surface of said dielectric cylinder, the open end of said channel engaging said strip, said strip being formed with adjacent turns spaced from each other so as to effect a helical gap of pitch equal to the pitch of said helical wave guide, said gap being disposed centrally of said open end of said wave guide and permitting energy radiation from said wave guide into the region of said elec-. tron beam, and means for introducing energy to and extracting energy from said wave guide.

2. An electron tube comprising an evacuated dielectric cylinder, means for generating an electron beam therein, a helical wave guide formed of channel shaped conductor having substantially rectangular cross-section, a fiat helical conductive strip formed in engagement with the outer surface of said dielectric cylinder, the open end of said channel engaging said strip, said strip being formed with adjacent turns spaced from each other so as to efiect a helical gap of pitch equal to the pitch of said helical wave guide, said gap being disposed centrally of said open end of said wave guide and permitting energy radiation from said wave guide into the region of said electron beam, and means for introducing energy to and extracting energy from said wave guide, said wave guide being formed to permit inter-action of electromagnetic wave energy flowing therein with said electron beam.

3. An electron tube comprising an evacuated dielectric cylinder, means for generating an electron beam therein, means for directin said beam along a predetermined path, a continuous helical wave guide formed of channel shaped conductor having substantially rectangular cross-section, a

flat helical conductive strip formed in engagement with the outer surface of said dielectric cylinder, the open end of said channel engaging said strip, said strip being formed with adjacent turns spaced from each other so as to effect a helical gap of pitch equal to the pitch of said helical wave guide, said gap being disposed centrally of said open end of said wave guide and permitting energy radiation from said wave guide into the region of said electron beam, and wave energy coupling means associated with each end of said wave guide.

4. An electron tube comprising an evacuated dielectric cylinder, means for generating an electron beam therein, means for directing said electron beam along a predetermined path, and a continuous helical wave guide formed of channel shaped conductor having substantially rectangular cross-section, a flat helical conductive strip formed in engagement with the outer surface of said dielectric cylinder, the open end of said channel engaging said strip, said strip being formed with adjacent turns spaced from each other so as to effect a helical gap of pitch equal to the pitch of said helical wave guide, saidgap being disposed centrally of said open end of said- Wave guide and permitting energy radiation from said Wave guide into the region of said electron beam, said wave guide being formed to permit inter-action of electromagnetic wave energy flowing therein with said electron beam, and said wave guide having energy input and output couplings displaced longitudinally of said path.

5. An electron tube comprising an evacuated dielectric cylinder, means for generating an electron beam therein, means for directing said electron beam over a substantially linear path, a wave guide of substantially rectangular cross-secticn helically wound around said beam path, a flat helical conductive strip formed in engagement with the outer surface of said dielectric cylinder, the open end of said guide engaging said strip, said strip being formed with adjacent turns spaced from each other so as to effect a helical gap of pitch equal to the pitch of said helical; wave. guide, said;- gap being; disposed cone trally of saidopen. endofsaid; wave: guide and permitting; energy radiation iromlsaid wave guide into-the regional saidelectron-beam, said wave guide being; adaptedto permit density modulationof said electron beam. in accordance with electric wave energy flowing therein; and to ex!- tractenergy from. a: density modulated,- electron beam;

6. A beamtraveling wave electrontubeoomprising. in combination: an. evacuated dielectric cylinder, an electron. gun disposed at an endof said.- cylinder forgenerating an electron beam of predetermined average electron. velocity, a collector electrode positionedwithin the opposed end 01 said dielectric-cylinder, means iordirecting said generated electron" beam axially through'said dielectric cylinder and impinging said .beamupon said: collector electrode, a helical. Wave guide formed of obannelshaoed conductor, having substantially rectangular cross-section, a fiat :helical conductive strip formed in-engagementwith the outer surface of soididielectric cylinder, theopen end of said channel engaging said strip; said strip; being formed with adjacent turns spaced from each other so as: toefiect. a helical gap of pitch equal to the pitch of-said helical Wave guide, saidgap being disposed centrallyof said open .end of said a wave guide and permitting energy. radiationfrom said waveguide into the region or said electron beam, and means for introducing and extracting energy fromisaid wave guide;

7. An; electron. tube comprising an evacuated dielectric cylinder, means'ior generating an; elec tronbeanrtherein, a helical waveguide-of substantially rectangularrcross-section. having one of its conductive-bounding: surface in engagement with. the, outer: surface of said: dielectric oy-lindcn, said: one bounding: surface being providedwith: a can runningglengthwise thereof and, following the helicalkconvolutions otsaid surface; said gap being disposed centrally of: said one bouncingsurface and. permitting energy radiation from said: waveguide into the. region: of, said electronbeam; and means for, introducing energy to and extracting energyfrom'said waveguide, the: axial dimension. of said gap, being: less than the axial dimension of thecross section ofrsaid' wave guide.,

8; A beam". traveling'wavc': electron.-. tube: comprising: in combination an evacuated: dielectric cylinder; an e ectron gun disposed: atan end of said: cylinder for. generating an; electron beam of predetermined average: electronz velocity; a col-'- leotorv electrode positioned within the: opposed and of said dielectriccylinder; meansiondirect ing'said. generated electronibeam: axially through said. dielectric cylinder. and impinging saidbeam uponsaid; collector electrode, a helical Waveguide of, substantially rectangularcross sectionhaving one-- at its. conductive bounding surfaces: in: en gagement with the outer surface of said choice-.- tric cylinder, said one bounding surface bein provided with a gap running lengthwise thereoi and. following the helical convolutions of said surface, said gap being disposed. centrally of said one bounding surface and permitting energy radi ation from saidwaveguide into the region of said. electron beam, and means for introducing energy toiand extracting energy from said. Waveguide, the axialdimensionof said'gap being less than the axial dimension of the cross-section ofsaid waveguide.

Q. A beam traveling Wave electron tubeicom prising in. combination evacuated dielectric cylinder, an electron gun,v disposed at aniendof said cylinder for generating an electron beamof predetermined average electron velocity, acollector electrode positioned Within. the opposed end of said dielectriccylinder, means for direct ing said generated electron beam axially through said-dielectric cylinder and impinging saidbeam upon said collector electrode, a helical Waveguide of substantially rectangularcross-section having one of its conductive bounding surfaces in one gagement with the outer surface of said diBlec trio cylinder, said one bounding surface; being provided with. a gap running lengthwise thereof? and following the helical convolutions of said surface, said gap being disposed centrally-ofsaid one bounding surface and permitting energy radiation from .said waveguide-into the region of'said electron beam, and, means for introducingenergy; to and extracting energy'from said waveguide; said Waveguide being adapted to permit density modulation oi S8.-id':10l31011 beam in accordance; with electricvvave energy flowing therein,.and-to. extract energy from adensity modulatedelectron beam, the axialdimension ofsaid gapbeing less than the axial dimensionof thecross-section of;- said wave guide.

JOHN W. TILEY.

References Cited inthefile of this patent UNITED STATES PATENTS Number Name Date 2,300,052 Lindenblad Oct. 27, 1942' 2,367,295 Llewellyn Jan..16, 1945 2,368,031 Llewellyn Jan. 23, 194:5 2,413,608 Di Toro Dec. 31, 1946 2,439,401 Smith Apr. 13, 1948. 2,578,434 Lindenblad Dec. 11, 1951" OTHER REFERENCES.

Article: by: Pierce, Bell Lab. Record, December-

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