US2934674A - Traveling-wave electron discharge device - Google Patents

Description

April 26, 1960 R. c. WERTMAN TRVELING-WAVE ELECTRON DISCHARGE DEVICE 2 Sheets-Sheet 1 Nww@ Filed Feb. 7, 1956 INVENTOR R/C//AR C WTMAN BY #UW i ,QI-n

AG ENT APYil 25, 1950 R. c. WERTMAN TRAVELING-wAvE ELEcTRoN DISCHARGE DEVICE 2 Sheets-Sheet 2 Filed Feb. 7. 1956 2,934,674 TRAVELING-WAVE ELECTRON DISCHARGE DEVICE Richard C. Wertman, Hackensack, NJ., assignor to International Telephone and Telegraph Corporation, Nutley, NJ., a corporation of Maryland Application February 7, 1956, Serial No. 564,022

s claims. (C1. sis-39.3)

This invention relates to traveling-wave electron discharge devices and more particularly to coupled helices and a method of-using them for introducing a circuit loss along the path of the radio freqency iield of a traveling wave in such devices to prevent both undesired oscillations and modes of propagation therein.

The generalstructure and theory of operation of a travcling-Wave type of electron discharge device or tube is well known, a comprehensive treatment being given in the book Traveling-Wave Tubes, by J.'R. Pierce, published by D. Van Nostrand Company, Inc., New York, 1950.

IItA is recognized that the useful range of amplification of a traveling-wave tube is limited by a tendency to generate self-sustaining oscillations as the amplification is increased. This effect is usually due to mismatch between the output circuit of the tube and the load circuit over allor part of the wide range of frequencies to be amplified. Because of such'mismatch, energy of at least certain frequencies is reflected back toward the input end of the amplifying device. When the reflected wave 'is not attenuated in its travel along the helix, or other type of propagating structure, in a direction opposite toV the motion of the electron stream, some energy reaching the input end of the device is reflected therefrom causing the generation of self-sustaining oscillations. Thus, the energy retlected or transmitted back to the input must be attenuated if the tube is to operate in ay stable manner..

Heretofore various means have been employed to overcome this tendency of generating self-sustaining oscillations. One suchk means employs. a body of' resistive material disposed on or adjacent to the propagatingstructure, either in a confined location thereabout or coextensive therewith to absorb the reflected energy. Another such means employswire-wound helices coaxial with the propagating thereto for removal of reiiected energy from the propastructure and in coupled relation gating structure for coupling to an appropriate terminav tion thereof to overcome the tendency for self-sustaining oscillations. Still another such means, described in the copending application of R. H. Geiger and' R..W. Wilmarth, Serial No. 464,282, filed October 25, 1954, provides a coupled-helixl circuit loss by coating a dielectric sleeve coaxial with the propagating structure with a conductive iilm having a predetermined conductivity.

These means for overcoming the tendency for self-sustaining oscillations have greatly improved thew usefulness of' the traveling-wave tube, at least for. certain applications. However-,in attempts to obtain high `power and high gain, stability of the attenuation at high temperatures, broadband' attenuation characteristics and reproducibility of electrical and mechanical properties of' the attenuators, it has been found that the above-mentioned types of attenuators fail to meet one or more of the above major physical and electrical properties. The lack of suitability of the above-described' methods is particularly Amarked wherey high power amplification is desired I'nfasmuch as` the most important single factor affecting etii'- an improved means vor quartz, or a nou-magnetic type cient performance at high power amplification is thev attenuator, the ability of the attenuator to rapidly dissipate the heat generated in it at high power levels is a major `factor in the tubes efficient and stable performance.

It is, therefore, an object of this invention to provide for preventing self-sustaining oscillations in a traveling-wave tube.

Another object of this invention is to provide a coupled-helix attenuator for the. propagating structure of4 a traveling-wave tube particularly suitable for use at high power levels. i

A feature of this invention is the provision of a coupled-helix circuit loss for the propagating structure of a traveling-wave tube comprising a channeled fluid-containing structure of dielectric material disposed coaxiall'y of the propagating structure, the channel within the Huid'- containing structure being in coupled relation to the electromagnetic eld ofthe traveling-wave tube and containing therein a conductive iiuid having a predetermined lossy characteristic.

An additional feature of this invention resides inthe circulation of the lossy uid through a heat-exchange system so that, in addition to providing desired attenuation, the fluid serves to remove heat generated in the attenuator in a convenient and expeditious manner.

The above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, in which:

Fig. l is a diagrammatic illustration. of a travelingwave tube incorporatingone form of the coupled-helix attenuator of this invention and illustrating the. iluid circulation system;

Fig. 2 is an elevational view,.partly in section, of anotherembodiment of this invention;

Fig. 3 is a diagrammatic illustration of an annular beam traveling-Wave tube incorporating one form of the coupled-helix attenuator of this invention;v and f Fig. 4 is a sectional view, partly yin elevation, of still another embodiment of this invention.

Referring, to Fig. 1, there is, shown an illustrative embodirnent of a traveling-wave tube adapted to be` used as an amplilier at ultra-high frequencies. The arrangement shown comprises an electron beam tube including an evacuated envelope 1.. The envelope 1 may be con.- stituted of a low-loss insulating material, such as glass of metal. The. shape of envelope 1 may have a uniform diameter as illustrated in Fig. l or may include an elongated portion coextensive with the interaction section 2 and in supporting relation,- ship with the propagating structure 3 included therein. The envelope 1 is provided at one end with means, such as a known type of electron gun 4 for producing an electron beam or stream. The electron stream. emerges from the electron gun 4 and travels along a rectilinear path through an input coupler 5 an-d axially down the evacnated envelope 1. The electron stream is further concentrated and guided along this substantially axial path by a longitudinal magnetic field produced by magnet 6 which may be either an electromagnet or a permanent magnet. Electrode 7 serves to collect the electrons` arriving at the end of envelope 1.

The propagating structure 3, which serves as a path along which the radio frequency wave may bek propagated, is illustrated as including a helix 8. Helix 8 is wound with several -turns per wavelength along its axis, which may preferably be of a. plurality of wavelengths of the frequency being amplified. The helix is illustrated as being supported by a seriesV of non-conductive rods 9 equally spaced` about theA circumference thereofY and` sup'- supported by means of a ceramic tubing or by so shaping envelope 1 to provide an'elongated portion in contact with the helix 8 for support thereof.

The helix 8 is joined at the input coupler 5 by an input matching section 11 and the output coupler 12 by the output impedance matching section 13. These matching sections are simply extensions of the helix in which the spacing between turns is increased along the circumference of the helix. Thereby, transmission lines to provide a wave transmission path of uniformly changing impedance from the relatively low impedance at the end of the couplers and 12 to the relatively high impedance of the central portion of helix 8, with a minimum reflection of energy back to the signal source.

In order to utilize the device in an operable system, there is provided an incoming wave path represented by the dotted input waveguide 14 into which there is introduced the input wave signal to be amplified. An output wave path, serves to transfer the amplified output wave to a load circuit. As the electron beam and the radio frequency Wave travel axially of the helix at substantially the same linear velocity, an interaction takes place whereby energy is transferred from the beam to the wave, thereby greatly amplifying the wave. As the amplified wave reaches the output end of the helix 8, it is transferred to the output waveguide 15 by means of output coupler 12. As the amplified wave reaches the impedance matching section 13, even with an extremely favorable termination, at a given band of frequencies, there will still exist reflected waves at frequencies both inside and outside the given band at the output end of the helix. This reflected wave is very little affected by the electron stream and hence will propagate back along the helix 8 toward the input end with attenuation. The reflected wave will reach the input end of the helix 8 with attenuation equal to the circuit attenuation and will in turn be reected back toward the output end of the helix. It is obvious that where there is not enough circuit attenuation to dampen the reflected energy, this energy will result in self-sustaining oscillations. It will thus be seen that the deliberate introduction of an artificial loss Within the electromagnetic field of the helix so as to provide a dissipation or removal of the refiected wave will serve to greatly increase the range of useful amplilication which can be achieved with a device of this type. It is of course an essential feature of traveling wave tube construction that this attenuation must be provided. Furthermore, where high power must be dissipated, in tubes handling powers ranging from one kilowatt to megawatts, the removal of the heat generated in the dissipation of this power becomes very difiicult and troublesome.

In accordance with the principles of this invention, the artificial loss or circuit attenuation is introduced along the portion of the helix by a coupled-helix attenuator 16 Aappropriately disposed longitudinally along the helix 8 and in coupled relation with respect to the electromagnetic field existing about the helix. As illustrated in Fig. 1, the coupled-helix attenuator 16 is shown as a fiuidcontaining structure of dielectric material 17 disposed coaxially of the propagating helix 8 and in the electric field of the propagating structure. This fluid-containing structure is shown as a helix wound about the propagating helix 8. A conductive fluid having a predetermined lossy characteristic is contained in the hollowed channel of the attenuator 16. For dissipation of large amounts of power, this fluid may be circulated through the attenuator in any convenient manner; for example, a circulating pump 19 may be used to circulate the lossy fluid through the attenuator. A heat exchanger Ztl, cooled by air or water or in any other convenient manthese sections act as tapered Cit shown as the output waveguide 15,

ner, is used to remove heat from the circulating fluid. As is known in this art, in order to achieve maximum coupling, the pitch of the larger-diameter helix must be wound somewhat looser than the smaller-diameter helix, in a ratio approximately determined by the respective diameters of the helices. In addition, for proper coupling, the pitches must be disposed in opposite directions, i.e., counterwound to one another. Thus, the helical attenuator 16, as illustrated in Fig. 1, is wound about the propagating helix 8 and in a looser pitch than the winding of the propagating helix. The dielectric material 17 used for the fluid-containing structure should preferably have a low loss factor and a low dielectric constant. Where the attenuator is located Within the vacuum envelope, as is the case for Fig. 1, it is preferable to form the attenuator 16 of a material such as quartz, glass or ceramic. These materials may be readily degassed and a proper vacuum obtained, which is not always the case with lowloss plastic materials. The desired properties for the conductive fluid may be readily obtained by choice of a fiuid which has the desired lossy characteristic and suitable heat-transfer properties. The selection of the fiuid is considered a matter of choice and may be experi-mentally determined by those skilled in this art. Among gases that may be used are the chlorinated fluorocarbons, commercially known as the Freons,ammonia and carbon dioxide. In general, for the fluid to be circulated, it is considered preferable to use various liquids such as alcohol, silicone oils and metallic colloidal suspensions. If a liquid such as a silicone oil, for example, is not sufficiently lossy, the degree of loss may be adjusted by the addition of colloidal graphite thereto in desired portions. The desired degree of heat transfer may, of course, also be readily controlled. As the liquid absorbs the microwave power, it will heat up; but it may readily be cooled by passing it through the heat exchanger at a desired rate and recirculating it.

In Fig. 2 is illustrated an alternative embodiment of the attenuator of this invention. In the embodiment illustrated, the propagating coil 8 is contained within the envelope 1, whichis made of a dielectric material such as glass, quartz or ceramic. In this embodiment, the electromagnetic field extends outside of the envelope, and the attenuator 16 is in the form of a helical coil wound about the envelope and in coupled relation to the electromagnetic field. In general, the same considerations of coupling apply to this embodiment as were mentioned with respect to Fig. 1. Because the attenuator is located on the outside of the vacuum envelope, there is no need for degassing it. Therefore, low-loss polymeric dielectric materials, such as polystyrene, polyethylene and polytetrauoroethylene, may be used for the attenuator 16 nasmuch as these materials are more readily shaped than corresponding glass or ceramic helices. It is, of course, apparent that maximum coupling is obtained when the two helices are as close to one another as is feasible and differbut slightly in their respective diameters.

In Fig. 3 is illustrated another embodiment of a traveling-wave tube utilizing the attenuator of this invention. In this embodiment, the propagating structure 8 and electron gun 4 are arranged so as to propagate an annular electron beam. The electromagnetic field associated with the propagating structure is essentially contained within the propagating structure. Therefore, the coupled helix 16 is of smaller diameter than the propagating helix 8 and is contained Within the propagating helix. Because of its smaller diameter, the coupled helix is wound more tightly than the propagating helix. However, for a maximum transfer of energy, it is preferable that the diameter of the coupled helix approach that of the propagating helix.

In Fig. 4 is shown an additional embodiment of atten-l ausgew dielectric cylinderifZZ;` Before assemblypazserieszfof heli-l cal. channels r 23. forming.; a=1 continuous'f passageway is machined abouti the: facing#4 outer' surfacev of. the. inner cylinderv 2.1. ori the facing' inner surface of' the' outer oylinderZZ. When the'two cylinders are fitted together in-a-uid`tight manner, a lossy, conductive'tluid may be circulatedY through thisk uidecontaining structure. The fluid enters at'fopeningl 24 andemerges at exit'25 and may be recirculated by means of a pump through a heatexchangev system, as describedk forv Fig. l. Where this cylindrical type of' attenuator is located outside of the vacuum envelope, as shown in Fig'. 4, it-is preferable to shape it of a suitable low-loss plastic material. The dielectric cylinder may be also contained within the vacuum envelope', as occurs, for example, in traveling-wave tubes of the type illustrated infFig. l. It is' then preferable to use glass-or ceramic for the material forming. the uidcontaining structure. In fabricatingk these cylinders, the same considerations with respect to pitch to obtain maximum coupling would apply as discussedl with respect to Fig. l.

The embodiments hereinabove described, in conjunction with proper'construction-techniques, willv provide a long life and rugged type of attenuatorfor use in travelingwave tubes. Thesetraveling-wave 'tubes'have their most useful range from- 100 to 4,000 megacycles, although, Where tolerances may bemaintained within very close limits, this range maybe extended to higher frequencies. The'useof these tubes for high-power applications'is of considerable importance, and the-novel type of attenuator provided therein makes for tubesV thatmay be usefully employed to dissipate powers ranging from one vkilowatt up to severalmegawatts.Y By` using the' type of. construction described, it is seen that the coupled-helix attenuator need not be located in intimate contact with the main helix of the traveling-wave tube. This arrangement reduces the possibility of mismatch on the propagating helix due to the attenuation. Also, as is illustrated in the various embodiments, the coupled-helix attenuator may be located either inside or outside the vacuum envelope. Because a flexible and controllable loss may be provided, inasmuch as the lossy liquid used and its composition is a matter of choice, levels of attenuation within close liimts may be readily achieved. Furthermore, inasmuch as the heat exchanger may be used for rapid or gradual dissipation of heat from the attenuator and this may additionally be controlled by the manner and rate in which the liquid is circulated through the heat exchanger, a simple, flexible, readily ocntrollable manner of removing heat from the attenuator and, at the same time, providing a desired degree of loss has been achieved.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set in the accompanying claims.

I claim:

l. lin a traveling wave electron discharge device including an elongated propagating structure having a given diameter for the propagation of radio frequency energy therealong, radio frequency energy input means coupled to one end of said propagating structure and radio frequency energy output means coupled to the other end of said propagating structure, land means for propagating a beam of electrons parallel to the axis of said propagating structure for interaction with the electromagnetic field of said energy; an attenuator disposed intermediate said input means and said output means and in spaced relation to said output means to provide said propagating structure with a substantially lossless portion adjacent said output means comprising a uid- Y quency forth in the objects thereof and l containingstructure of i dielectric material? disposedl in i al coaxial relationship .to'said propagatingfstructureandrhav ing a diameter d'fferent from=saidz givenfdiametenofsaid propagatings'tructure, said fluid-.containing structure in# cluding a channelztherein disposed in Said electromagnetic field, and a. conductive, non-'ionized uid having a` prei. determined lossy characteristic; contained in said channel in a* predetermined energy couplingirelationship with the radio frequency energy` on said propagating structure'to reduceA said radio frequency energy traveling along said propagating structure fromsaid input meansto said outputl means to a predetermined useful leveland to reduce said radio frequency energy reflectedy from said output meansfor travel along: said propagatingV structurefrom said' output means to said input;l means to aV negligible level.

2; A` deviceaccordingto claim'1,.wherein said uidcontaining structure has aV helical; forml 3. A device according to claim 1 wherein said fluidcontaining structure consists of two closeefttingA cylinders nested in'l an Huid-tight:relation'y to one` another and said channel is providedin said cylinders by a continuous helical passageway onta facing surfaceofat least one of said cylinders.

4. In a traveling wave electron` discharge device including an elongated propagating structure having a given diameter for the propagation ofy radio frequency energy therealong, radio frequency energy input means coupled to one end of said propagating structure and radio frequency energy output means coupled to the other end of said structure, and means for propagating a beam of electronsl parallel to thev axis of said propagating structure fori interactionv with the electromagnetic. eld of said energy; an attenuator disposed; intermediate, saidv input means and said output means and in spaced relation to said output means to provide said propagating structure with a substantially lossless portion adjacent said output means comprising a fluid-containing structure of dielectric material disposed'in a coaxial relationsihp to said propagating structure and having a diameter different from said given diameter of said propagating structure, said fluid-containing structure including a channel therein disposed in said electromagnetic eld, and a conductive, non-ionized uid having a predetermined lossy characteristic contained in said channel in a predetermined energyv coupling relationship with the radio frequency energy on said propagating structure to reduce said radio frequency energy traveling along said propagating structure from said input means to said output means to a predetermined useful level and to reduce said radio freenergy reflected from said output means for travel along said propagating structure from said output means to said input means to a negligible level and means for circulating said iiuid in said channel and for removing heat from said uid.

.5. A device according to claim 4, wherein said uid is selected from the gases consisting of chlorinated fluorocarbons, carbon dioxide and ammonia.

6. A device according to claim 4, wherein said uid is selected from those conductive liquids having a predetermined lossy characteristic consisting of as major constituents silicone oils, metallic colloidal suspensions and alcohol.

7. In a traveling'wave electron discharge device including` a helical propagating structure having a given diameter and a given pitch direction for the propagation of radio frequency energy therealong, radio frequency energy input means coupled to one end of said propagating structure and radio frequency energy output means coupled to the other end of said propagating structure, Y

and means for projecting a beam of electrons parallel to the axis of said propagating structure for interaction with the electromagnetic field of said energy; an attenuator disposed intermediate said input means and said output means and in spaced relation to said output means lossless portion propagating structure with a substantially adjacent said output means comprising a fluid-containing structure of dielectric material disposed in a coaxial relationshipto said propagating structure and having a diameter different from said given diameter of said propagating structure, said fluid-containing structure including a channel therein in the form of a continuous helical passageway having a pitch direction opposite to said given pitch direction of said helical propogating structure, and a conductive, non-ionized fluid having a predetermined lossy characteristic contained in said channel in energy coupling relationship with the radio frequency energy on said helical propagating structure to reduce said radio frequency energy traveling along said propagating structure from said input means to said output means to a predetermined useful level and to reduce said radio frequency energy reected from said output means for travel along said propagating structure from said output means to said input means to a negligible level.

8. In a traveling wave electron discharge device including a helical propagating structure having a given diameter and a given pitch direction for the propagation or radio frequency energy therealong, radio frequency energy input means coupled to one end of said propagating structure and radio frequency energy output means coupled to the other end of said propagating structure, and means for projecting a beam of electrons parallel to the axis of said propagating structure for interaction with the electromagnetic field of said energy; an attenuator disposed intermediate said input means and said output means and in spaced relation to said output means to provide said propagating structure with a substantially lossless portion adjacent said output means comprising to provide said a duid-containing structure of dielectric' material disposed in a 'coaxial relationship to said propagating structure and `having a diameter different from said given diameter of said propagating structure, said Huid-containing structure including a channel therein in the form of a continuous helical passageway having a pitch direction opposite to said given` pitch direction of said helical propagating structure, a conductive, non-ionized fluid having a predetermined lossy characteristic contained in said channel in energy coupling relationship with the radio frequency energy on said helical propagating structure to reduce said radio frequency energy traveling along said propagating structure from said input means to said output means to a predetermined useful level and to reduce said radio frequency energy reected from said output means for travel along said propagating structure from said output means to said input means to a negligible level and means for circulating said fluid in said channel and for removing heat from said fluid.

References Cited in the le of this patent UNITED STATES PATENTS 2,463,428 Rieke Mar. 1, 1949 2,669,696 Ward Feb. 16, 1954 2,721,953 Rothstein Oct. 25, 1955 2,752,572 Bird June 26, 1956 2,758,242 Samuel Aug. 7, 1956 2,788,464 Geiger Apr. 9, 1957 2,800,605 Marchese `uly 23, 1957 2,811,673 Kompfner Oct. 29, 1957 FOREIGN PATENTS 699,893 Great Britain Nov. 18, 1953

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