US3374388A - Traveling wave tube having tapered grooves and shims for improved thermal contact between metal envelope, support rods and slow wave helix - Google Patents

Description

3,374,388 ND SHIMS FOR OPE, SUPPORT R, E. HUBER NG WAVE TUBE HAVING TAPERED GR D THERMAL CONTACT BETWEEN META RODS March 19, 1968 TRAV IMPR OOVEJS A L ENVEL LIX WAVE H 15, 196

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RAYMOND E. HUBER j ATTORNEYS United States Patent 3,374,388 TRAVELING WAVE TUBE HAVHNG TAPERED GROOVES AND SHiMS FOR IMPROVED THERMAL CONTACT BETWEEN METAL EN- VELQPE, SUPPORT RODS AND SLOW WAVE HELIX Raymond E. Huber, Woodbine, Md., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed Nov. 13, 1964, Ser. No. 411,169 Claims. (Cl. 3153.5)

ABSTRACT 0F THE DISCLOSURE A traveling wave tube having a tubular envelope with longitudinally extending grooves of uniformly increasing depth between their ends spaced about the inner periphery, elongated tapered shims each contiguous with the radially outermost surface of a respective groove and forming an inner surface parallel to the longitudinal axis of the envelope, elongated dielectric support rods contiguous with a respective shim inner surface and forming an inner surface parallel to the longitudinal axis of the envelope, and a helical delay winding coaxially supported in the envelope by the rod inner surfaces whereby posi tioning of the shims along the length of the envelope obtains positive contact between interfaces of the winding, rods, shims and envelope and thereby affords high thermal conductivity therebetween.

The present invention relates to high frequency electron discharge devices, and more specifically to traveling wave tubes and slow-wave circuits therefor.

In a conventional traveling wave tube an electron beam is projected along the longitudinal axis of a relatively 7 long, slender, helical conductor at an appropriate velocity for interaction with high frequency signal waves traveling along the helix in the same direction, for the purpose of amplifying the signal. The essential elements of such a tube are an electron gun, a relatively long, slender, helical conductor of small diameter, a conductor, an envelope enclosing these elements, and means for coupling the input and output ends of the helical conductor to input and output transmission lines. The tube is normally mounted in a suitable magnetic field coil, or a permanent magnet, to provide a focusing magnetic field axial to the electron beam. Usually the helical conductor of a traveling wave tube is rigidly held in place within the envelope or tube between and by several ceramic rods circumferentially positioned about the outside of the helix. See US. Patent No. 3,026,445 to T. H. Stoerck for Traveling Wave Electron Discharge Tubes issued Mar. 20, 1962. e

The helix and helix-derived slow-Wave structures employed in traveling wave tubes of the prior art have good bandwidth and impedance characteristics at lower power levels of operation. However, when attempts are made to operate such structures at higher power levels thermal problems become increasingly pronounced.

Attempts have been made to solve the thermal problem by applying force on the slow-wave structure and dielectric support rods of traveling wave tubes to assure good physical contact between the metal slow-wave structure, the support rods, and the metal tube envelope: good physical contact is requiredfor high thermal conductivity at the aforesaid interfaces.

One process now employed to obtain high thermal conductivity involves the deformation of the tube envelope to a point slightly below the elastic limit thereof. The slow-wave structure and support rods are then inserted into the now out-of-round envelope. The support rods are placed at the points of maximum envelope width. When the deforming force is removed the envelope attempts to return to its original cylindrical shape clamping the slow-wave structure and support rods in place. The force impressed upon the slow-wave structure and support rods varies with the differences between the outside diameter of the slow-wave structure and support rods and the smaller inside diameter of the envelope.

Another process uses two envelope halves, the inside diameter of which is smaller than the outside diameter of the slow-wave structure and support rods. The slowwave structure and support rods are placed on one envelope half and the other half is placed on this assembly. This unit is then fixtured to close the gap between the body halves and brazed to seal the halves together. The force exerted on the slow-wave structure and support rods varies with the difference in diameters as in the process discussed above.

The disadvantage of the above-mentioned processes is the required high degree of accuracy in fabrication necessary to obtain the desired diameter difference and the desired force thereby impressed upon the slow-wave structure. Also, the prior art methods exhibit a lack of control of the concentricity of the slow-wave structure with respect to the inside diameter of the envelope. Furthermore, the prior art methods exhibit little control over the amount of force applied to the slow-wave structure and support rods other than that force developed due to the initial difference in diameters discussed above. It is readily observed that in the first process there is a risk of exceeding the elastic limit of the metallic tube envelope while deforming it to accept the slow-wave structure and support rods. In the second process there is a requirement for elaborate fixturing or machining to maintain alignment while brazing the body halves together, and during this process there is the possibility of decreasing the amount of attenuation on the dielectric support rods due to furnace brazing the body halves together in a hydrogen atmosphere.

The present invention solves the problem of applying suflicient force to the slow wave structure and dielectric support rods of traveling wave tubes and assures good physical contact between the metal slow-wave structure, the support rods, and the metal tube envelope, thereby obtaining high thermal conductivity at the aforesaid interfaces. This is obtained by employing an envelope, the inside diameter of which tapers from one end to the other or alternatively an envelope with grooves which'taper from one end to the other and metal shims of the same taper angle but of greater length. The shims are placed between the support rods of the slow-wave structure and the tube envelope with the tapers opposing each other'to present parallel surfaces to the support rods.

It is an object of the present invention to provide a loading mechanism for a traveling wave tube slow-wave structure for providing high thermal conductivity.

Another object of the present invention is to provide a loading mechanism for a traveling Wave tube slow-wave structure which enables the control of the concentricity of the slow-wave structure with respect to the inside diameter of an enclosing envelope.

It is a further object of the present invention to provide a loading mechanism for a traveling wave tube slow-wave structure which permits easy control over the amount of force applied to the slow-wave structure and support rods.

A still further object of the present invention is to provide a loading mechanism for a traveling wave tube slowwave structure which provides facile control over the amount of force applied to the slow-wave structure and support rods.

. .Stillanotherobject of the present invention is to pro vide a loading mechanism for a traveling wave tube slowwave structure for adjusting the attenuation characteristiqs of the traveling wave tube as well as for adjusting the phase of the velocity characteristics of the slow-wave structure.

A still further object of thepresent invention is to provide .a loading mechanism fora traveling wave tube slow-wave structure, ,which provides control over the amount of force exerted on the slow-wave structure and support rods, control over the concentricity of the slowwave structure with respect to the, envelope inside diameter, and which, by virtue of control of the amount offorce exertedonthe slow-wave structure, controls the attenuation and phase velocity characteristics of the slowwave structure. a I

.These. and other objects and features of the invention will become apparent to those skilled in the art as the .disclosureis made in the following description of an embodimentof the invention as illustrated in the ac- .cornpanying drawings which show, so far as is necessary to an understanding of the said invention, a traveling wave tube of the kind referred to and embodying the said invention.

.FIG. 1 is a schematic elevation, broken away at the ends, of. the embodiment in question; and

FIG. 2 is a sectional view taken along the lines 2--2 of FIG. 1..

As illustrated in the drawings, the traveling wave tube .10, partly rrepresented, has within a cylindrical metallic envelope 11, shown broken away, a helical delay winding 12 ,hav ing one end thereof attached to an RF input coupling means, not shown, and having the other end thereof coupledto an RF output coupling means, also not shown. The winding 12 coaxial'ly surrounds an electron discharge path, not represented and emanating from an electron gun known and used in the art. The path extends axially and interiorally of the winding 12 in the customary manner. The. winding 12 is supported by four insulating rods 15 of dielectric materialspaced at 90 intervals about the winding periphery and contacting it along the periphery thereof. I a

In accordance with this invention the envelope 11 is provided ,with four grooves 29 formed within the interior surface ofthe envelope 11 and spaced at 90 intervals about. the inside circumference thereof. Grooves are tapered from the insertion end 110 of envelope 11 to the other end 11b of the envelope 11. In other words, the distance 1.,the distance between the longitudinal central .axis of envelope 11 to the outermost wall 21 of groove 20, decreases with increased distance from insertion end 11a;

.- ."Dielectric support rods 15 are of constant thickness throughout the longitudinal length thereof and are par- .tiallylseatedwithin grooves 20.. To obtain good physical contact necessary for ,high thermal conductivity between the metalslow-wave structure 12, the support rods 15 and .the metal tube envelope 11, metal shims are inserted within groove 20 between each support rod 15 and the tapered wall 21. Each metal shim 30 is tapered from one end tothe other at the, same taper angle as groove 20 but is of-greatertlength. Shim 30 includes a horizontal rodabutment surface 31 and a tapered surface 33, and is insertedbetween the support rods of the slow-wave structure 12 and the tube envelope 11 with the tapered surfaces33 opposing the tapered surface 21 of groove 20 to thereby presenttparallel surfaces to the support rods.

, As may be. readily seen from the figures, longitudinal movementofthe shim will cause a radial displacement of the support rods 15 and slow-wave structures 12. By

.manipulating opposing shims, concentricity of the slowwave structure 12 with respect to. the inside diameter of .the tube envelope 11 may be attained. By manipulating the shims collectively, after concentricity has been attained, the desired force may be imposed on the slowwave structure. By knowing the amount of force required for good physical contact of the slow-wave structure members and the mechanical advantage derived from the taper angle, the required displacement of the shims to attain the desired force can be calculated. Should good physical contact necessary for high thermal conductivity be less important than attentuation characteristics, as in low power traveling wave tubes, the attenuation characteristics may be adjusted slightly by varying the force imposed on the slow-wave structure 12. The phase velocity characteristics of the slow-wave structure may be adjusted in a similar manner.

It will be further observed that the inward pressure forces applied by the shims 30 are uniform over the entire length of the slow-Wave structure 12 so that a very good and stable positioning and clamping action is obtained.

It will be understood that various changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.

, What is claimed is:

1. A traveling wave electronic discharge tube of the type having a delay winding which is coaxial with the electron beam path of the tube, said tube including a plurality of rods which support said delay winding and are in contact with the outsidethereof along a plurality of lines extending parallel to the axis and equally spaced aboutthe periphery thereof comprising:

a plurality of grooves equally spaced about the inner circumference of the tube and extending longitudinally along the length thereof, each of said grooves having a surface inclined from longitudinal alignment and extending from one of the groove ends to h the other;

and elongatedshim means inserted in said grooves, each said shim means being tapered along the length thereof and having an inclined surface defining the taper of said shim means in abutting engagcrnent'with said inclined surface of a respective said groove whereby parallel surfaces are presented to the support rods.

2. The invention as defined in claim 1 wherein said tapered shim means is of a length greater than the longitudinal extent of said grooves. I I

3. The invention as defined in claim 1 wherein said shim means is constructed of thermal conducting material.

4. The invention as defined in claim 1 wherein said shim means is constructed of metal.

, 5 The invention as defined in claim 1 wherein four grooves are provided at intervals about the interior circumference ofthe tube,

a 6. The invention as defined in claim 1 wherein each said inclined surface defines a respective groove depth progressively decreasing from one of its respective ends to the other. i

7. The invention as defined in claim 6 wherein said inclined surfaces of said grooves have inclinations relative to the tube axisof thesame magnitude,

'8. The invention as defined in claim 1 wherein said inclined groove surfaces diverge relative to the tube axis in the same longitudinal direction.

9. The invention as defined in claim 8 wherein:

said grooves have rectangular cross-sections; and

said inclined surfaces of said grooves have inclinations relative to the tube axis of the same magnitude.

10. In a high frequency electron discharge device of the type including an electron gun disposed at one end of a predetermined path for generating and directing an electron beam along such predetermined path, a collector structure disposed at the other end of said predetermined path for collecting said beam, a slow-wave circuit disposed along said predetermined path between said electron gun and said collector structure, said slow-wave circuit being adapted and arranged such that energy exchange 5 between the electron beam and radio-frequency energy propagating on said slow-wave circuit is achieved, said slow-wave circuit including a helix disposed around said predetermined path, and further including a plurality of rods supporting said helix and in contact with the outside thereof and equally spaced about the periphery thereof, the improvement comprising:

a thermally conductive tubular envelope surrounding the predetermined path and including an interior circumferential surface;

a plurality of equally spaced grooves formed in said interior circumferential surface and extending parallel to the longitudinal axis of said envelope, each of said grooves having an inclined surface defining a groove depth progressively decreasing from one of 1 its ends to the other; and

longitudinally extending shim means each having an inclined surface imparting a longitudinal taper thereto and each inserted in a respective one of said grooves between said groove and said support rods, each said shim means being inserted within said respective grooves with said inclined surfaces opposing each other and presenting parallel surfaces to said support rods.

References Cited UNITED STATES PATENTS 9/1966 Washburn 315-35 HERMAN KARL SAALBACH, Primary Examiner. PAUL GENSLER, Examiner.

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