US2982879A - Travelling wave tube - Google Patents

Description

May 2, 1961 J. P. F. NALOT TRAVELLING WAVE TUBE 2 Sheets-Sheet 1 Filed April 4, 1957 INVENTOR JEAN PAUL FERNAND NALOT ATTORNEY May 2, 1961 J.\ P. F. NALOT TRAVELLING WAVE TUBE 2 Sheets-Sheet 2 Filed April 4, 1957 INVENTOR JEAN PAUL FERNAND NALOT ATTORNEY TRAVELLING WAVE TUBE Jean Paul Femand Nalot, Paris, France, assignor to Compagnie Generale de Telegraphic sans Fil, Paris, France Filed Apr. 4, 1957, Ser. No. 650,630 Claims priority, application France Apr. 25, 1956 l 11 Claims. ci. sis-3.5

The present invention relates to travelling wave tubes, and more particularly relates to delay lines which may be utilized in these types of tubes. 7

It is well known that heating of the delay line during the operation of a travelling wave tube poses grave problems in the use of such travelling wave tubes, and that the heat thus produced in the delay lines actually limits the power such tubes may produce. This heating efiect results principally, as is well known, from the absorption of electrons by the delay line.

This heating problem is particularly grave in the case ofbackward wave oscillators, such as described in the copending application, Serial No. 281,347, filed on April 9, 1952, by B. Epsztein, now Patent No. 2,932,760, in which the elements of the delay line adjacent the cath- 'ode,=that is, the elements of the delay line which are exposed to the action of the electron beam having maximum intensity, are subjected to a maximum ultra-high frequency field, and, consequently, to the maximum electronic bombardment. This is so since in such backward wave oscillators the energy producing the ultra-high frequency field travels in a backward direction, i.e., opposite the movement of the beam and is constantly amplified in this backward direction by interaction with the beam whereby the ultra-high frequency field reaches maximum proportions near the end thereof adjacent the cathode where ordinarily. the output coupling is also located.

, The present invention has for its object an arrangement whichlreduces the resultant heating in such tubes.

Accordingly, it is an object of the present invention to provide a tube with. an electron beam which propagates parallel to the delay line, the delay line. including a portion, on the inside of which oneof the. dimensions of the section of the delay elements decreases from one element to the next elementin the direction of'the beam.

Theory. and experience have proved that the-organization of the electron beam with a View tointeraction with the ultra-high frequency field ofthe delay line may be; effected in a more favorable manner if, in the initial part-of thebeam, i.e., in the immediate neighborhood of the cathode,;the energy utilized for that .purposeis nottoo considerable or not too large.

Under these conditions, the utilization of the .delay line ac'co'rdingto the presentinventionpermits an improve- 1A further object of the present invention is the provi I sionf of 'anf arrangement in which' the amounti'offheat produced, which is ordina" y ,lirriitative'of theamount of po "er" output possiblefby' the tube, is effctivelyidei creased byt the, arrangement of the delay line in i m ate ,Pr t u rlitw such a v United States Patent 2 i.e., a greater output with a tube of given physical dimen sion.

A still further object of the present invention resides in the construction of thedelay line which facilitates grouping of the electron beam.

Still another object of the present invention is to provide a travelling wave tube which has a particularly favorable arrangement of the various parts as regards production of heat due to absorption of electrons by the delay line.

These and other objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawing which shows, for purposes of illustration only, two..preferred embodiments in accordance with thepresent invention and wherein:

Figure 1 is a cross sectional view of a travelling wave tube having crossed electric and magnetic fields taken along line 11 of Figure 2; v

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

Figure 3; illustrates anembodiment of the delay line with cylindrical bars which may be used in a tube according to the present invention.

The travelling wave tube in accordance with the present invention comprises the usual elements of such tubes such as, inter alia, a cathode, a heater for the cathode, a collector, focusing means, a delay line and 'an electrode having a conductive surface essentially parallel to the delay line; potentials are applied to the respective elements to provide an electric field which extends from the electrode to the delay line and a magnetic field'is also provided which has lines of flux essentially perpendicular to the electric field and which is obtained in any suitable manner as, for example, by means of pole pieces arranged coaxially with the axis of the tube. The present'invention resides in the particular configuration of the delay line, in which at least one dimension of each element forming the delay line varies in a predetermined manner. I

More particularly, the oscillator according to the present invention is provided with a delay line in which one of the transverse dimensions of the elements thereof decreases progressively fromone element to the other, beginning with the end of the line adjacent the cathode and extending over a predetermined portion thereof, the

distance between the sole and thedelay line remaining essentially constant throughout.

Referring now more particularly to the drawing,

wherein like reference numerals are used throughout the various views to designate like parts, and more particularly to Figures 1 and 2 thereof, reference numeral 1 designates a metallic 'e'nvelope of the travelling wave tube. The travelling" wave tube itself may be of the type as described in the aforementioned copending application by B. Epsztein which relates to a backward wave oscillator. L v

The cylindrical metallic envelope 1 is closed by ,two cover members 2 and 3. p A cathode 10 heated by a'filament 11, and an electronic-optical focusing means of suitable construction-are disposed within the'tube. The electronic-optical focusing means may, for example, in-

clude a so-called, Wehnelt electrode 12 and, an intermediate electrode 13which faces the cathode 10. ,. API'9 tures 4 and 5 are prpvidedinthe cylinder ,1 which permit, respectively, passage therethrough of an output conductor] adapted to transfer the energy produced in the tube and of a-plurality of conductors orrl'ead-in wires '-intended to applydiiferent potential to the various electrodes of the tubes from any suitable conventional power supply" i(not shown) 'I wo of these lead-in wires are of the tube illustrated I earn Figure 1 by referencefnum eralsQ"and .9,; 1 v

A delay line of the interdigital type 14, with a constant pitch, and which is composed of two combs 19- and 20, is fixed to the interior of the cylindrical envelope 1 as it is shown in Figure 2. The internal conducting surface of the envelope 1 plays the role of ceiling or base for the delay line. The element 15 of the delay line which is located nearest the cathode 10 is coupled to the output conductor 7. All of the elements of the delay line which are included between the elements 16 and 17 inclusively are covered with any suitable attenuating means, although it is understood thatattenuation of these elements of the delay line may also be obtainedin any other suitable manner, as for example, by suitable attenuating'means either internal or external.

An electrodeZl, commonly referred 'to as sole, of which the characteristic form is shown in Figure 2-, is socured to and supported by three insulating sleeves 22, 23 and 24 which are made of ceramic material and which in turn are secured in any suitable manner, for example,

soldered or brazed, to the cover member 2 of the tube. A collector 25 is fixed to the cylindrical envelope 1 at the end of the portion of the delay line provided with the attenuating means.

According to the manner "of realization of the present invention as illustrated in the drawing, the height of the fingers of the delay line, that is, the dimension of the fingers in the direction perpendicular to the ceiling or base of the line, decreases regularly over a substantial portion of the line from one finger to the next beginning with the finger 15 located at the end i of the delay line adjacent the cathode up to the finger 28 which is equidistant of the two ends of the delay line. The fingers of the delay line included between the finger "28rand the finger 1'7 have been illustrated as being provided with heights which again increase regularly.

in effect, the fingers of the delay line have been cut off in such a manner as to include the volume delimited by two cylinders having parallel axes 26 and 27. The surfaces of the sole and of the delay line facing each other being essentially parallel, it follows that the active part of the sole 21 must, therefore, be constituted by the 'extern-a1 surface of a cylinder having an axis 26, whereas the internal surface of thercrown constituting the enyelope 1 of the tube is formed by a cylinder with an axis 27. H

inthe embodiment illustrated in Figures 1 and '2, elemerits of the delay line are generally equi-distant from each other and the distance of the delay line from the ceiling formed by the'internal surface of the cylindrical envelope 1 is the same from one end of the line to the other. -However, this-.isnot absolutelyinecessary, and the internal surface of the envelope 1 may also be a V pylinder having an axis26. In that case, the spacing or distance of the delay line to theceiling or base thereof formed by the internal conductive surface of the envelope 1 would be greater at the place ofthe finger28 than at the place of fingers 1 5 and i1.

. A suitable permanent magnet or electromagnet' of I which the pole pieces are schematically indicated by reference numeral 2h and 34} in Figure}, provides a magnetic field having flux lines essentially: parallel to the axes 2'6 and 27. A radial electric field in the interaction space between the delay line 14 and the sole 21 is obtained by the difference in potential between. these two electrodes as applied thereto frorna source of power by means. of

- later has been fully described inflthe' aforementioned copendin'g'application. 1 7

As described therein," itis based onfthe interaction, in

wave induced in the delay line and propagated along the latter.

For a tube of this type, provided with an interdigital delay line, calculations made by the applicant have indicated that the coupling resistance is given by the following equation:

In this expression:

K is a constant which depends on the units.

I is the constant of propagation of the wave in interaction.

I1 is the distance between two consecutive fingers.

p is the pitch of the line.

d is the distance between the sole and the delay line.

I: is the height of the fingers D is the distance of the delay line to the ceiling formed by the inner surface of the crown.

x is the distance between the sole and the beam.

-F or the normal values of these quantities, the coupling resistance is approximately inversely proportional to the height of the fingers.

On the other hand, experience has indicated that the output of the tube increases considerably if the height of the fingers is decreased. Therefore, it is of interest to reduce the height thereof, but in that case, the capacity for dissipation of the heat also decreases with such reduction.

According to the present invention, by increasing the height of the first few fingers of the delay line, on the one hand, the coupling resistance is decreased and therewith the amplitude or magnitude of the high frequency field at the entrance of the interaction spacewhile, on the other, thecapacity for dissipation of these fingers is increased. The decrease of the high frequency field has as result that the electrons of the beam which are absorbed thereat by the delay line are distributed over. a larger surface, and that therewith the maximum dissipationvis further, increased. In addition thereto, the interaction is not established with its maximum intensity directly at the beginning of the interaction space but very progressively, which improves the mechanics of the electronic grouping. Furthermore, it follows therefrom that the maximum dissipation isvery considerably increased without decreasing the output in an appreciable manner; for the conditions of operation are thereby regularized, The inventor has experimentally noted that the power provided by the tube of Figures'l and 2 is superior by at least fifty per- 1 cent to that of a tube having analogous characteristics butprovided with a classical delay line, i.e., with a delay line ofthe prior art as-discussed hereinabove.

The delay line of the tube according to Figures 1 and 2 is particularly easy -to realizeinpractice. Eacho'f the combs is obtained by millin'g, a hollow metallic cylinder of which, to start with, the external and internal surfaces are cylinders with parallel axes non-coaxial to each other so that these'cylinders are eccentric with respect to each other.

- Figure 3 shows a different embodiment of a delay the interaction spiicebetween the sole-21 and the delay which may 'bensed in a tube according to the present in vention. This embodiment relates to any type of a line with bars, such as'a ladder-type delay line, a comb-type delayline, or any other suitable type of delay line. delayhne is illustrated .in EigureB of the drawing in cross sectionin a plane perpendicularto the bars thereofQl t In. Figure 3; the sole, for example, of, a linear tube which utilizes thelineis designated by reference nunieral '80, the ceiling or base of the line by reference numeral 81, and the cross section of some of the successive bars by reference numerals 82 through 87. Depending on the particular case, these bars of the delay line may be supported at one of the ends thereof or at both ends thereof by means of lateral walls (not shown) which are rigidly secured to ceiling or base 81. According to the present invention, the diameter of these bars decreases regularly over a substantial portion of the line from the bar 82 to the bar 87. In an oscillator which utilizes such a delay line, the output conductor would be fixed to bar 82.

The rules which must beobserved for purposes of realization of a tube according to the present invention which utilizes a delay line having cylindrical bars are as follows: I

(1) The ratio of the capacities of each bar with respect to the ceiling or base formed by a conductive surface (and with respect to the following bar is essentially constant; and

(2) The surface of the sole is parallel to the surface which is tangent to the bars on the side thereof nearest the sole.

It is understood, of course, that the present invention is not limited to the embodiments illustrated and described herein which have been illustrated only for exemplary purposes. In particular, the delay line of the type represented in Figures 1 and 2 may be used in a linear tube, and particularly in a linear oscillator, and the type of the delay line of Figure 3 may be used with a cylindrical tube, and more particularly in a cylindrical oscillator. Moreover, the present invention is also applicable to travelling wave tube amplifiers with suitable input and output means whereby the attenuating means may be omitted. In addition thereto, the ceiling or base of the delay line may be formed by a separate conductive element other than the envelope, while any suitable attenuating means may be used with the respective elements of the delay line.

Thus, it is clear that the present invention is susceptible of many changes and modifications within the spirit of the present invention and I intend to cover all such changes and modifications as encompassed by the appended claims.

I claim:

1. A backward wave travelling wave tube comprising a constant pitch delay line constituted by a chain of parallel conductive bars, a sole electrode having a conductive surface opposite said delay line and bounding therewith an electron and wave interaction space, an electron beam source near one end of said interaction space, and means for establishing in said interaction space crossed electric and magnetic fields for propagating said beam therethrough, the dimension of the cross section of said bars in the direction perpendicular to said sole surface gradually decreasing from one bar to the next over a first portion of the delay line away from said source,

, and all of said elements being spaced apart from said sole electrode by a constant interval.

2. A backward wave travelling wave tube according to claim 1, wherein said dimension increases away from said source beyond the end of said first portion of said delay line. A

3. A backward wave travelling wave tube comprising a constant pitch interdigital delay line made up of intercalated metallic fingers, a sole electrode having a conductive surf-ace opposite said delay line and bounding therewith an electron and wave interaction space, an.

said s'ole electrode by a constant intervaln apart from '4. A backward wave travelling wave tube according to claim 3, wherein said dimension increases awayfromsaid source beyondthe end of said first portion of said delay line.' J H v p 5. A backward wave; travelling wave tube comprising a constant pitch delay line constituted by a chain. of parallel cylindrical bars, a sole electrode having a conductive surface opposite said delay line'and bounding therewith an electron and wave interaction space, an electron beam source near one end of said interaction spacegandmeans for establishing in said interaction space crossed electric and magnetic fields for propagating said beam therethrough, the diameters of said bars gradually decreasing from one bar to the next over a first portion of the delay line away from said source, and all of said bars being spaced apart from said sole electrode by a constant interval.

6. A backward wave travelling wave tube according to claim 5, wherein said diameter increases away from said source beyond the end of said first portion of said delay line.

7. A backward wave travelling wave tube according to claim 2, wherein said delay line is circularly curved and comprised between two cylindrical surfaces having noncoincident parallel axes.

8. A backward wave travelling wave tube comprising a constant pitch delay line constituted by a chain of generally equi-distant parallel conductive bars, a sole electrode having a conductive surface opposite said delay line and bounding therewith an electron and wave interaction-space, an electron beam source near one end of 9. In a microwave oscillator of the type utilizing interaction between an electron beam and an electromagnetic wave propagating along a retardation line in the direction opposite to the direction of flow of said electron beam, said oscillator including an electron source and having an output coupling means disposed adjacent said source, the improvement comprising a retardation line having a periodic structure positioned adjacent the path traversed by said electron beam, said retardation line having its coupling resistance gradually increased in value over a substantial portion of said line in the direction of flow of said electron beam from a relatively low value in the vicinity of said source to a relatively high value at a region along said line substantially removed from said source. V

10.-In a microwave oscillator of the type utilizing interaction between an electron beam and an electromagnetic wave propagating along a retardation line in the direction opposite to the flow of said electron beam, said oscillator including an electron source and having an output coupling means disposed adjacent said source, the

improvement comprising a retardation line having peri-.

5 'pling resistance of said line to gradually increase in value said source.

crossed electric and magnetic fields for propagating said beam therethrough, the dimension of a cross section of said fingers in the direction perpendicular to said sole I surface gradually decreasing from one finger to the neat over a first portion of the delay line away from said over a substantial portion of said line in the direction of flow of said electron beam from a relatively low value in the vicinity of said source to a relatively high value at a region along said line substantially removed from 11. A microwave oscillator comprising an electron source and a collector electrode defining therebetween a path of electron flow,,'a' delay'line'having periodically spaced digits positioned along said path in which there is travel, the end of said line remote from said electron source being terminated by digits of increased size, and attenuation means inserted in said line at said remote end for absorbing wave energy.

References Cited in the fiie of this patent UNITED STATES PATENTS 2,643,353 Dewey June 23, 1953 2,681,427 Brown et a1. June 15, 1954 Warnecke et a1 Aug. 3 1, 1954 Pierce May 10, 1955 Peter Oct. 4, 1955 Lerbs Jan. 10, 1956 Field Aug. 21, 1956 Miller et a1. Oct. 22, 1957 Munushian Sept. 2, 1958 Birdsall Sept. 9. 1958 FOREIGN PATENTS France May 30, 1950 Germany June 13, 1952 France -a-'--- Mar. 10, 1954

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