US2948828A

US2948828A - Traveling wave tube interaction circuit

Info

Publication number: US2948828A
Application number: US623623A
Authority: US
Inventors: Ashkin Arthur
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1956-11-21
Filing date: 1956-11-21
Publication date: 1960-08-09
Anticipated expiration: 1977-08-09

Description

Aug. 9, 1960 A. AsHKlN TRAVELING WAVE TUBE INTERACTION CIRCUIT 4 Sheets-.Sheet 1 Filed NOV. 21, 1956 QWRDO IIIII .l E QN N @Dx /NVEN Tof? A. ASHK/N Aug. 9, 1960 A. AsHKlN TRAVELING WAVE TUBE INTERACTION CIRCUIT Filed NOV. 21, 1956 4 Sheets-Sheet 2 /Nl/E/v ro@ A. ASHK/N AHORA/5v Aug. 9, 1960 A. Asl-:KIN

TRAVELING WAVE TUBE INTERACTION CIRCUIT 4 Sheets-Sheet 3 Filed Nov. 21.11956 kbos" /NVE/vroR A. ASHK/N A ATTORNEY u8- 9, 1960 I A. AsHKiN 2,948,828

TRAVELING WAVE TUBE INTERACTION CIRCUIT '--III Filed Nov. 21, 1956 4 Sheets-Sheet 4 di Lug 2q k k w u b s zu, l a 5,' 8 le Il' N g 0: l (L 8 f S Lu x u j [j l s c Q u f N I l ,m u) m f lo N l() "vx f J ld l l V Q lu E' l f L: .J f

r Y i f n I d AML-fg 7 ATTORNEY rents. ;a traveling wave tube is quite well understood and is dis- .cussed in an article by H. Hetfner, Analysis of the Backward Wave Traveling Wave Tube, Proceedings of the j'I.R.E., volume 42 (June 1954), pages 930 through 937. `These oscillations can occur in a helix type of tube, be- -cause the helix, where uniformly wound, is actually a periodic structure and with a helix of uniform pitch, the

TRAVELI'NG WvErUE nvTERAcTIoN yCIRCUIT Arthur Ashkin, Far Hills, NJ., assi'gnor to Ben 'reiephone Laboratories, Incorporated, New York, NX., a corporation of New York ined Nov. 2'1, 1956, ser. No. 623,623

. 16 claims. (ci. sis- 3.6)

This invention relates to traveling wave tubes, which may be described as electron tubes utilizing a wave transmission circuit for propagating electromagnetic waves in eld coupling relation with an electron beam to permit interchange of energy between the transmission circuit and the electnon beam along several wavelengths of the circuit, It relates particularly to such tubes which employ in the transmission circuit a helical wave path to reduce the electromagnetic wave velocity -along the path of the electron beam so that it is substantially synchronous with the beam.

One of the diiiiculties encountered in attempting to use such tubes as high power amplifiers is the production of .unwanted backward wave oscillations at high beam cur- The production of backward Wave oscillations in `spaces between adjacent turns can be regarded as inter- ;acting gaps periodically located along the path of the .electron beam which permit spatial harmonic interaction ,between the beam and the helix circuit. Due to the fact that the impedance to the beam is lower for the sapltal ,harmonic mode of operation than for the fundamental mode, harmonic operation does not occur in helix type However, when, for

v.wave oscillation.

It is the object of this invention to provide means in V.the helix type of traveling wave tube for preventing inter- .action which will cause the objectionable operation in the spatial harmonic mode without impairing operation in :the desired fundamental mode.

This is accomplished, according to the invention, by utilizing in the wave transmission circuit a helix wound lwith a continuously varying pitch in the region of interaction with the electron beam to destroy the periodicity of the interacting gaps between adjacent turns and so dislcourage operation in the harmonic mode with consequent building up of backward wave oscillations. In order to retain eiiicient operation of the tube in the fundamental mode, the effect of the variation in the pitch of the helix upon the wave velocity therealong is counteracted by continuously varying the loading of the helix along its length to restore the desired normally constant phase velocity characteristic along the interaction region. Any

means suitable for appropriately varying the loading of` the helix may be employed. At present, it appears preferable to use either dielectric material properly distributed along and in the eld of the helix, or a second Patented Aug. 9, 1960 ic i 2 helix extending along and variably coupled to the first helix having the variable pitch.

The invention and its application in specific embodiments will be better understood from the following description and the accompanying drawings of which:

Fig. 1 is aschematic drawing illustrating an embodiment of the invention in a traveling wave tube utilizing in the wave transmission circuit a helix of uniform diameter 'and varying pitch variably loaded along its length by means of dielectric material to maintain uniform phase velocity along the circuit;

Fig. 2 is a schematic drawing illustrating an embodiment of the invention ina traveling wave tube utilizing in the wave transmission circuit a rst (inner) `helix of uniform diameter and varying pitch which is variably loaded valong its length by means of a second (outer) helix which is coaxial with the first, has a varying pitch and is of varying diameter for variable coupling to the iirst helix to maintain uniform phase velocity along the circuit in the principal region of interaction with the electron beam;

Fig. 2A is a diagram indicating relative phase velocities at points along the lengths'of the helices of Fig. 2 in the principal interaction region when the helices are separated from each other (uncoupled) and also under the two possible modes of operation when coupled as in Fig. 2;

Fig. 2B is a diagram indicating relative phase velocities along the lengths of the helices of Fig. 2 in'both the coupling and the principal interaction regions when the helices are separated from each other (uncoupled);

Fig. 3 illustrates schematically an embodiment of the invention in a traveling wave tube utilizing a tubular electron beam and in the wave transmission circuit a first (inner) helix of uniform diameter and varying pitch variably loaded along its length by means of. a second (outer) helix coaxial with'the first and of uniform diameter and varying pitch for variable coupling to the iirst helix to maintain uniform phase velocity along the cir- Acuit in the principal region of interaction With the elec# Fig. 3 to maintain uniformphase velocity, However,

whereas in Fig. 3 the pitches of the inner and outer helices increase and decrease respectively with the length along the entire length of the principal interaction region, `and the uncoupled phase velocities are similar in the center,

in Fig. 4 the pitches of the inner and outer helices in-f crease and decrease, respectively, with the length from` one end to the center of the principal interaction region, L and then decrease and increase, respectively, with the.v length from the center of the principal interaction region, to the yother end, and the uncoupled phase velocities are,

similar at the two ends;l

Figs. 4A and 4B are diagrams similar to FigsrSA andi, 3B but indicating relative phase velocities at points along;

the lengths of the helices of Fig. 4; and Y Figs. 5 and 5A illustrate Vin perspective and longit dinal section, respectively, an embodiment of the inven tion in a magnetron type of traveling wave tube. Herel is shown a single iiattened variablepitch helix variably loaded by means of a tapered dielectric block to maintain constant phase velocity along the length ofthe helix,fas

is shown in Fig. l2

To simplify thedrawings, the usual vacuum tight'enve' lope required, potential sources, the commonly employed electron beam focusing means extending along the beam path (except in Fig. and energy absorbing means associated with the circuit in the interaction region to prevent oscillations due to reected energy are not shown.

The various-figures, illustrative of -the several embodiments of the invention described, are intended-merely to clarify the descriptions land consequently-'do not Vindicate-sizes ortpropo'rtions. 'For instance, the tubes would ordinarily be much more elongated than shown,rtherle'ngth indicatedas interaction region would be much longer in proportion tothe lengths'of the couplers-than is shown, and't'he diagramsshowing r'elationsbet'weenphase velocity and length along the -helices indicate only-thecha'racter ofthe variations,`not actual dimensions or slopes. Furthermore, the portion of the length of each Itube marked rinteraction lregion-'fis theprin'cipal interaction region where thewavephase velocity is kept constant and vvirtually all `of the amplilication takes place. The relatively `small interaction in the coupler regions is neglected.

-Referringnowfto'Fig 1, internal elements of a beamhelix type of traveling wave tube, according to one ernbodiment of the invention,are illustrated schematically in the formfof a longitudinal section. VFrom the electrongun comprising cathode 1, heater 2, focusing and accelerating electrodes 3 and 4, the electron Vbeam 5 is projected along the axis of and in coupling relation with the helix wave transmission circuit 6 to the electron collector 7. The rinput'andoutput connections to the helix are indicated at 8` and 9, respectively. In accordance with the invention, the helix 6 is wound with continuously varying pitch to destroy the periodicity of the interacti'nggapsbetweenv turns, rand thereby discourage the building up of the undesirable backward wave oscillations. Further in accordance with the invention, a block of dielectric material 10 tapered gradually but not necessarily uniform with length is disposed along and coupled to the helix toprovi'de loading of the helix in varying amount along its length to compensate for the effect upon phase velocity of the varying pitch of the winding. This loading is so varied that the phase velocity of wave propagation `along'the Vthe length of the helix is substantially uniform in spite-,of the varying winding pitch. Thus, by varying thelhelix winding pitch, the unwanted backward wave oscillations are avoided and, 'by compensating for such variations to maintain constant wave phase velocity, the'normal'functionin'g of the traveling wave tube is retained. a

It may be noted that while the desired variation in loading by nthe dielectric 4material 10 is shown as 'being accomplishedjby tapering thedblock of dielectric material, itmay be'had also by other suitable meansysuch as by positioning of the block or by varying the composition of the material. Also, the direction of the tapering -of the block of dielectric 10- and of 'the variation in pitchuof the helix 6 may be the reverse of what is shown in the figure.

Fig. 2*'is an illustration showing in the same general manner -as Fig. l another embodimentof the invention ina beam-helix type of traveling wave tube. Where appropriate, designations the same as in Fig. l are employed. In this embodiment the varying loading of the inner helix 21, having gradually decreasing pitch along the Iength'of the tube indicated as interaction region, is had by means of an outer helix 22., preferably wound oppositely to the winding of helix 21 to enhance the mutual coupling. -Terminations not'shown may be attached to the otherwise free ends of the inner helix 21. Helix 22 also has agradually decreasing pitch along the interaction region, but has additionally a gradually changingu-diameter to provide varying coupling between the twohelices in that region. This coupling is adjusted to obtain a constant phase velocityalong the interaction region.

The gradually decreasing pitch of the inner helix 21 corresponds toV a gradually decreasing phase velocity in the uncoupled state, that is, separated from the inuence of helix 22. The portion of the outer helix 22 in the interaction region `also having gradually decreasing pitch likewise has a decreasing phase velocity in the unconpled state or when separated from the influence of helix 2l. The portions of

helices

21 and 22 in the i.teraction region are designed to have the same phase velocity characteristic when uncoupled or separated from each other. That is, at each :point along the length in the interaction region l, the two helices have the same phase velocity when uncoupled,a's is indicated bythe solid line graph a of Fig. 2A.

Such a system of two coupled helices 'has two modes of wave propagation, one faster than that of the individual helices and one slower. The upper and lower dotted line graphsb and c of Fig. 2Ashow phase characteristics typical of the faster and slower modes of propagation, respectively, of vthe system consisting of the two helices 21 and `22'coupled together "as shown in Fig. 2.

Fig. 2A thus shows three phase velocity,characteristics typical ofthe helices Vof Fig. .2, phase velocity being plotted against'length l along the yhelices in .the` region indicated interaction regionin Fig.\2 where both helices have varying pitch.

The coupling between the two. helicesmay be adjusted so that the phase velocity of either `theifasteror the slower modeof propagation-risfconstant along the length I. in this particular illustrative embodiment, the coupling between the two helices is adjusted so that the phase velocityof the faster'mode of the coupledsystem is constant along thelength of'theinteraction region, as is'shown lbythegraph 'b ofFig. 2A. .In this manner, the desired objective'of maintaining?constantzphase velocity alongthe interaction regionwhile at the same time employing a -helical'wave circuit with non-uniform pitch to avoid unwanted backward wave oscillation is attained. To obtain this-result in operation it is, ofcourse, `necessary that the coupled helix system operate in thedesired mode. In this instance, itis.therefore'necessaryto.provide means lto launch the fast mode at the input of the helices and to extract the power in the fast` mode at the output end. This may be doneby means of `what have been termed step couplers, which are short lengths of appropriately coupled helices, attached to cacherd of the portions of the two kcoupled helices located in the interactionregion indicated-.in Figs. 2 and 2B. lt may be noted VinI Fig. 2 that the short lengths of helcescom prising the couplers are `actually extensionsofand partsv of the coupled

helices

21 and 22. Such couplers 'are described in US. Patent 2,885,593,"issued May 5, 1959, of J. S. Cook.

'Cook refers to each-suchcoupler as a transducer section, and teaches that a desired mode of propagation may be excited in a pair of coupled transmission lines by associating therewith a properly designed transducer' section of lines as an-extensionof the pair of couple' lines, and applying an 'input wave to the end of one of the two lines forming the transducer section. The two lines forming the transducer section must be chosen to have different average characteristic phase propagation constants. The faster of the two normal modes of the coupled trans mission lines may then be launched therein b v exciting, the component line of the input transducer section having'the faster average phase velocity. and conversely, the slower normallmode may be` launched by exciting the component line of the input transducer section having theslower average phase velocity. Similarlypfrom reciprocity considerations, the faster of the two lines forming an output transducerlsection will be excited selectively when the faster normalirnode is in the main coupled line, and the slowerline when the slower modeis in the main coupled line. Reference can rbe made to this above-cited patent fora detailed description of the principles of operation and design of these couplers or transducer sections.

the proper mode of wave propagation, that is, the mode for which the phase velocity has been made constant (such as indicated by the graph b of Fig. 2A). It may be seen from Fig. 2B, which shows typical variations in phase velocity along the helices (when uncoupled) in the interaction and coupler regions, that the coupler sections of helix 22 (the outer helix) have higher phase Velocity characteristics than the coupler sections of helix 21 (the inner helix), and therefore, to operate the system in the faster mode (for which the phase velocity has been made constant), input and output connections are made to the coupler sections of the outer helix. If it were desired to operate the system in the slower mode of wave propagation, appropriately dilferent coupler sections or connections thereto would be employed as taught by Cook in the aferomentionad patent.

` Fig. 3 illustrates in the same general manner as Fig; 2 another embodiment of the invention in a traveling wave tube utilizing two coupled helices, preferably wound oppositely, to efect'variable loading to maintain constant wave phase velocity along the interaction region. Where appropriate, designations are the same as for Figs. 1 and 2. However, in this ligure the cathode and other elements of the electron gun are arranged to pro-` ject a tubular beam of electrons through the annular space between the two coupled helices 31 and 32 to the collector 7, although `an axial beam as shown in Fig. 2 could be used. In this embodiment is a constant diameter inner helix 31 with pitch and phase velocity gradually increasing along its length in the uncoupled state. The outer helix 32, however, has a pitch and phase velocity decreasing with length along the interaction region and, unlike the outer helix of Fig. 2, it has a constant diameter. Since the separation of the phase velocities of these helices (when uncoupled) varies with length, the effective coupling varies with length too when they are coupled together. This coupling is weakest at the ends of the interaction region and strongest in the middle. The variations of the phase velocity of each of the helices can be adjusted to achieve a situation in which the coupled system has a mode of wave propagation with constant phase velocity whereby, according to the invention, efficient traveling wave tube operation may be had together with the advantage of non-uniform helix pitch in preventing backward wave oscillation.

Fig. 3A illustrates typical phase velocity variations along the length l (the interaction region) of the inner and outer helices 31 and 32. The solid line graphs a1 and a2 show variations of phase velocity along the two helices when uncoupled, and the dotted line graphs b and c show phase velocity variations along the two helices forthe faster and slower modes ofwave propagation when the helices are coupled together, as in Fig. 3. The phase velocity may be and is shown constant for both modes of propagation.

ln order to operate the Fig. 3 embodiment in the desired mode, step-couplers or transducer sections are shown at each end of the interaction region, as has been explained in connection with the description of Fig. 2.

Typical variations in phase velocity along the helices in the coupler and the interaction regions when the helices are uncoupled are shown in Fig. 3B. The outer helix portions of the couplers have the higher phase velocities,

and to these the input and output connections are made in Fig. 3 to operate .the system in the faster velocity mode of wave propagation. As mentioned in the description of Fig. 2, terminations not shown m-ay be connected to the otherwise 'free ends of the inner helix 31.

Fig. 4 illustrates an embodiment of the invention which, like Fig. 3, employs two helices, each with constant diameter. Here, however, `alon-g the lengthjoff the interaction region, the pitch and phase velocity of the inner helix increase and then decrease to the initial value, while the pitch and phase velocity of the outer helix along that region decreasey and then increase to the initial value. As with Fig. A3, when -coupled together, these sections of helix can have modes of propagation with constant phase velocity. n

Where appropriate, the designations on Fig. 4 are the same as for the previous gures, and it may be noted that, an axial electron 'beam is shown as in Fig. 2, although a tubular beam as in Fig. 3 could be used. The inner yand outer coupled helices are designated 41 and 42, respectively. As with Fig. 3, since the separation of the phase velocities of these two helices (when uncoupled) varies with length, the effective coupling varies with length'too when they a-re coupled together. Here, however, the coupling is strongest Iat the ends of the interaction region, and weakest in the middle. Again (as with Fig. 3), the variations of the phase velocity of each of the helices can be adjusted so that the coupled system h-as a mode of wave propagation with constant velocity whereby the objective of the invention may be achieved.

Fig. 4A illustrates typical phase velocity variations along the length l (the interaction region) of the, inner Iand outer helices 41 land 42. The solid line graphs a1 and a2 show variations of phase velocity along the two helices uncoupled, and the dotted line gaphs b and c show phase velocity variations along the two helices for the faster and ,slowerV modes of wave propagation when the helices are coupled together, as in Fig. 4.

To operate the Fig. 4 embodiment in the desired mode, step-couplers,l or transducer sections, are shown at each end of the interaction region, as has lbeen explained in connection with the descriptions off Figs. 2 and 3. Typical variations in phase velocity along the helices in the coupler and interaction regions when the helices are uncoupled are shown in Fig. 4B. 'The outer helix portionsl of the couplers 'have the higher phase velocities, and these are used for the input and output connections in Fig. 4 to operate the system in the Vfaster Velocity mode of Wave propagation. As previously mentioned, terminations not Ishown may be connected to the otherwise free ends `of the inner helix 41. Figs. `5 land 5A illustrate an embodiment of the inveny tion in a linear magnetron type ofutravelinggwave tube employing a 'helix type of wave propagating circuit. A

flattened helix 51 is shown as an example. In accord? lance with lthe invention, the pitch of the helix is varied continuously along its' length, land further by way of example, a tapered block of dielectric material10 is located along and in proximity to the helix to variably load'the into Va magnetron, crossed electric `and magnetic field, type In Figs. 5 and 5A, the fiat electron beam 50 s projected from the cathode 1 of the electron gun to the collector 7 along the helix 51 between the 'helix and the oppositely polarized plate 52. The magnetic field producin'g means,

pole pieces

53 and 54, produce a magnetic field in the space between plate 52 and the helix which is perpendicular to the electric field there and to lthe direction of travel of the electron beam 50. The region of interaction between the electron beam 50 and the helix 51 is along the space between plate 52 and the helix. The focusing strips 55 are maintained at different direct voltages with respect to the helix 51, and are adjusted to' counteract the Varialble magnetic force in the fringe field.

region by maintaining the proper ratio of electric to magY netic field in the fringe magnetic field. This means for focusing an electron beam prior to its entrance into the magnetic `feldrofthe.interaction space in a linear magnetron is described in my US. Patent 2,843,793, issued July 15, 1958. While it is -part of`-,thevil1ustrative.magnetron structure, itis not relatedto the :invention .of the instant application, which briefly stated, `relates tor-avarying-pitch constant phase velocity `helical type .of interactioncircuit for a traveling Wave tube.

It is to be understood .that the several arrangements which have beentdescrihed .are merely illustrativeofthe invention, and that other `modifications may be devised by one skilled in fthe art Withoutxdeparting `from theA spirit and scope of the application. In particular, the I.principles described may be extended to 'traveling wave tubesr which employ other .forms of slow wave circuits which like the helix do .not involve spatial harmonic operation and so do not need to beperiodic in structure, but which ordinarily are made ,periodic in structure to facilitate achieving a Vconstant, phase velocity.

What is claimed is:

l. In a traveling wave tube utilizing a wave transmission circuit for `propagating electromagnetic Waves in field coupling relation with an electron stream, the combination including a transmission circuit comprising a helix portion having predetermined variations in pitch along its length in .the region of said iield coupling relation, and means coupled tosaid helix portion for counteractingy the effect of the variations in `pitch upon the uniformity of phase velocity of a Wave propagated therealong.

2. A device .according `to claim 1 in which the variations in pitch .of the said helix portion are substantially uniform with length therealong.

3. A device according to claim 1 in which the pitch of the helix portion increases in a single direction therealong.

4. A device Aaccording to claim 1 in which the pitch of the helix portion increases in one direction therealong in one `part of the length and in the other direction therealong in another part of the length.

5. A device according to claim 1 in which the means for counteracting the effect of variations in pitch of the helix portion upon the uniformity of phase velocity comprises a member of dielectric material coupled to the helix portion and :electrically loading it variably `along its length.

6. A device according to .claim 5 in which the variable loading of the dielectric .material along the fhelix portion compensates vfor the variations in Vwave phase velocity therealong which .are ydue to the variable pitch, and maintain substantially Aconstant -wave phase velocity along the length of thehelix portion.

7. A device :according to claim l in which the means for counteracting `the effect -of variations in pitch of the said helix portion upon -the uniformity of phase velocity comprises a second helix Vportion coupled tothe said helix portion fhaving predetermined variations in pitch along its length.

8. A device according to claim 7 in which variations inthe coupling between the two helix portions along their lengths compensate for the variations in wave phase velocity along the lengthof the first said helix portion which are due to variable pitch, and maintains constant wave phase velocity along the length of the first saidfhelix portion.

9. A device according to-clairn 7 in which said second helix portion has `predetermined variations in pitch along its length.

10. A device according to claim 7 in which said second helix portion has predetermined variations in diameter along its length.

l1. A device according to claim 7 in which said second helix portion has predetermined variations in .pitch and diameter along its length.

12. In a traveling Wave tube utilizing a Wave transmission circuit for propagating electromagnetic -waves in eld coupling relation with an electron stream, the combination including a transmission circuit comprising a helix portion having predetermined variations in pitch along `its length Vin the region of said ield coupling relation, and means coupled to said helix portion for compensating for the variations in ywave phase velocity therealong which result from the variations in pitch, said means comprising amember of dielectric material dimensioned and coupled in such a way to the helix portion that the wave `phase velocity remains substantially constant, `therealong.

13. In a traveling wave tube utilizing a wave transmission circuit .for propagating electromagnetic waves in field coupling relation with an electron stream, the combination including a transmission circuit comprising a irst helix having a first portion of uniform pitch, a second intermediate portion of varying pitch, and a third portion of uniform pitch, and means coupled to said second intermediate helix portion for compensating .for the variations in wave phase velocity therealong which result from the variable pitch, said means including a second helix coupled to said rst helix with a rst portion of uniform pitch, a second intermediate portion of varying pitch, .and a third portion of uniform pitch, each `of said portions of said second helix being in coupling relation with the respective portions of said first helix.

14. A device according to claim 13 in which the variations in `pitch of the intermediate portions of said rst and second `helices are substantially uniform with length therealong.

15. A device according to claim 13 in which the pitch of the intermediate portions of said lirst and second helices increases in one direction therealong in one part of the length and in the opposite direction thcrealong in another part of the length.

16. A device according to claim 13 in which the variations in pitch along the intermediate portion of said first helix is substantially uniform with length therealong and the intermediate portion of said second helix has predetermined variations in pitch and diameter therealong.

References Cited in the le of this patent UNITED STATES PATENTS 2,489,082 De Forest Nov. 22, 1949 2,541,843 Tiley Feb. 13, 1951 2,630,544 Tiley Mar. 3, 1953 2,641,731 Lines June 9, 1953 2,727,179 Lally et al. Dec. 13, 1955 2,730,648 Lerbs Jan. 10, 1956 2,773,213 Dodds Dec. 4, 1956 2,784,339 Lindenblad Mar. 5, 1957 2,807,744 Lerbs Sept. 24, 1957 2,825,841 Convert Mar. 4, 1958 2,851,630 Birdsall Sept. 9, 1958 2,888,596 Rudenberg May 26, 1959