US2822501A - Slow-wave guide for traveling wave tubes - Google Patents

Description

Feb. 4, 1958 H. C. POULTER SLOW-WAVE GUIDE FOR TRAVELING WAVE TUBES Filed Jan. 10, 1955 v INVEN TOR. #0 0420 (I 20047:?

SLOW-WAVE GUIDE FOR TRAVELING WAVE TUBES Howard C. Poulter, Palo Alto, Calif., assignor to Research Corporation, New York, N. Y., a corporation of New York Application January 10, 1955, Serial No. 480,629

5 Claims. (Cl. 3153.5)

This invention relates to traveling wave tubes, such as are particularly adapted for amplification of wide bands of ultra-high or microwave frequencies. Particularly it relates to constructions suitable for forming the slowwave guides which form an integral and essential portion of such tubes, so as to impart to such guides characteristics which will contribute to the stability of the tubes when operated as amplifiers.

Traveling wave tubes of the type to which the present invention is applicable comprise an electron gun, adapted to generate a narrow beam of electrons. The gun is mounted at one end of an elongated tubular envelope through which the beam is axially directed by the electron gun. A collector-anode is mounted at the opposite end of the envelope, and between the two and surrounding the path of the electron beam there is positioned a slow-wave guide which, in the form to which the present invention particularly relates, comprises a long, slender helix or solenoid, having terminals at the input or electron gun end for supplying the high frequency signal which is to be amplified by the tube, and, at the output end, terminals connecting to a load.

A radio frequency wave, introduced at the input end of the slow-wave guide, travels longitudinally of the guide at a velocity, considered along the axis, which is a function of the inductance and capacitance of the guide.

conductor. To a first approximation the velocity of the wave along the conductor forming the wire is equal to the velocity of light in free space. This being the case it is at once evident that because of the helical form of the conductor forming the guide, the velocity of the wave as referred to the axis of the helix will be much less than the velocity of light.

The potential differences applied between the cathode and anode of the electron gun are so selected that the electron velocities and the velocity of the waves propagated along the guide are substantially equal throughout the band of frequencies for which the apparatus is designed. When this condition is met, there is an interaction between the fields established by the radio frequency waves and those of the electrons themselves. the beam tend to bunch in their path through the helix, and, in so doing, they build up the amplitude of the travcling wave so that there is an increase in the power delivered at the output end of the slow-wave guide over that introduced at its input terminal. The bunching of the'electrons in the beam also represents a radio frequency wave which carries power which may also be utilized under certain conditions.

The same structure which functions as atraveling wave amplifier in the manner thus briefly described, can, under certain conditions, also act as a backward wave amplifier, i. e., certain modes are possible wherein radio frequency energy will build up and be propagated along the guide in the opposite direction to that in which the electron beam is traveling. In this case the radio frequency power available at the electron gun end of the The electrons of "wherein the position 7 support is provided which it covers,

2,822,50i Patented Feb. 4, 1958 stability is extremely likely to develop, the apparatus going into oscillation on the frequency at which backward wave operation can occur.

Instability of this character may be prevented by so designing the slow wave guide that it has a narrow attenuating or stop-band at the frequency or frequencies where backward wave operation is likely to occur, and

the broad purpose-of the present invention is to provide a traveling wave tube" wherein theslow-wave guide has a stop-band or bandsof the type described. To this end, among the objects of the invention are to provide a slow-wave guidewherein stop characteristics are provided by the support for the helical conductor, the latter being of the simple and readily constructed simple solenoidal form; to provide a slow-wave guide wherein the helical conductor is supported by the envelope of the tube, asin the case of more" conventional traveling wave tubes; to provide means and methods for constructing the helix support which is applicable either to a support which is integral with the envelope or may be separately supplied; to provide a type of stop-band, slow-wave guide,

ed to amplify may be varied through a wide range at the discretion of the designer; to provide an inherently stable pense over the cost of a simple ing no stop-band characteristic.

The helical conductor which forms the slow-wave guide in a conventional traveling wave tube is formed of fine wire or strip, wound, usually, with many turns to the inch. The .over-all length of the helix may be somewhere in the neighborhood of a foot. A solenoid of this type is obviously extremely flexible and requires support throughout its entire length to maintain it in alinement with the path of the electron beam, so that the latter can pass axially through itE Normally such bythe tube envelope itself, the latter having a boreor lumen which is substantially equal to the outer diameterof the helix. A tube within the envelope, the sole'purpose of which is to support the helix, can, of course, be provided, but this would ordinarily be merely an expensive and useless complication, although the possibility of such use cannot be ignored.

In accordance with the present invention the helical conductor constituting the slow-wave guide is held in position by a tubular insulating support' which maybe and preferably is part of the envelope of the tube itself.

This support, however, differs from that customarily used spaces being substantially equal" to one-half.wavelength,- along the conductor itself, of the desired stop-band fre quency. In general, the rifling will,'itself, be helical and 1 uniform throughout the length of the conductor helix and the locatioup f lhs vS p. and is .de-s

of the stop-band with relation to; the broad, traveling wave band which the device is intend contact between the conisinterrupted for uniform length along the conductor at the desired stop-band treq e y is es t an t e l n thrqf one turaoinhe sour ductor helix, the pitch of the groove will be in the pp direction i cppcs te s n esiwmJha fztl conductor helix; if the half wavelength is greater than the lengthof one turn of the conductor the pitchpf both helices will be in the same direction,fwhile it the half wavelength is equal to the length of one conductor turn the pitch of the helical groove becomes infinite, the helix, degenerating into astraight line. Obviouslythe pitch of the riding and that of the conductor must be difierent, either in magnitude or in sense, since if rifling and condoctor had equal pitches in the same sense there would be no interruption in the support of conductor and no non-uniformities in conductor capacity introduced. at regular intervals to produce a stop-band.

In the formation of a rifled support tube in accordance with the invention a mandrel is constructed whose. diameter is the external diameter of the conductor helix.

On this mandrel there is formed one or more projecting helical threads which may be similar in cross-section to an ordinary screw thread, although a squareor trapezoidal form is preferred as being mechanically stronger and less likely to breakage. Preferably this thread is formed by fitting a helix wound of metal, tape, or wire. of the desired crossesection, onto the mandrel. The mandrel is then inserted in a vitreous tube, of large enough internal diameter to slip over the mandrel and its thread, and the tube is then heated to a plastic condition and shrunk down over the mandrel, preferably by sealing the end of the tube and exhausting it. The mandrel, being of metal, has a higher coefiicient of ther-,- mal expansion than the glass or quartz of thetube and shrinks away from the latter so that it can be unscrewed after the glass has cooled. This construction will leave aridge on the rifled tube; if desired this ridge can .be ground off, on a centerless grinder or on a suitable mandrel, or it may be,le ft to project if desired. Because, in tubes of this character, the electron beamis .ordinarily confinedto its axial path by meansof a longitudinal magnetic field developed by a solenoid surrounding the envelope, the grinding procedure isusuallypres ferred.

-In the detailed description of a preferred embodiment of the invention which follows, reference willbe made to the accompanying drawings, wherein:

Fig. 1 is a schematic diagram of a traveling wave tube embodying the instant invention;

Fig. 2 is an axial cross-sectional view of .a portion of the envelope of the tube, of Fig. 1, illustrating the riding within the envelope;

Fig. 3;is a view similar to Fig. 2 showing the position of the conducting helixwithin the rifledenvelopej Fig. 4 shows a. portion of the mandrel upon which the envelope shown in- Figs. 2 and 3 is formed;

Fig. 5 is a longitudinal cross-sectional view illustrating the shrinking operation.

Since, with the single exception of the wave guide.

and its support, the traveling wave tube of the present invention may be nearly identical with substantially any other form of traveling wave device, one of the simplest of such forms has been chosen to illustrate symbolically the inventive idea. Such a tube is shown in Fig. 1, wherein the envelope, generally designated by the reference character 1 comprises a somewhat enlarged or bulbous base portion 3 and elongated tubular portion or barrels. Within the base portion is mounted acathode 7, enclosing a heater element 9, and having an electron emissive surface 11. Electrons frorn this surface are'attractedtoa tubular anode 13, the combination "of cathode and anode forming an electron gun for directing abeam of electrons axially through: the elongated barrel of -the apparatus. Inoperatioii the barrel bet een will normally be enclosed in a solenoidal coil (not shown) for maintaining the beam in a compact conformation in its progress through the tube.

Biasing potential for the anode may be applied through a sealed-in connection 15, and this connection may be used to introduce the radio-frequency signals to be amplifiedh Such signals can also be introduced through a coupling cavit'y from a Wave guide or by other well known coupling means, but since the particular method of introduction ofthe energy into the tube hassno relationto the present invention, such refinements are not shown.

The vanode 13 connects to the slow-wave guide 17, illustrated by a symbolically shown helix which termimates in a tubular output electrode 19 which may be coupled, in any of the ways mentioned for the input anode 13, with an output or load circuit, but is shown asso connected through a sealedrin lead'21. Spaced slightly along the tubular portion of the envelope from the outputelectrode 19 is a collector-anode 23, which connects to an energizing circuit through a .sealedain lead 25. Positive biasing potential for the anode is appliedthrough the lead 25, and, as in the case of the conventional traveling wave tube, this lead also may couple to a load circuit.

The tube as thus described differs from a conventional traveling wave tube only in the construction of the' ;barrel 5, the inner surface of which isprovided with :isacross-sectional view of a short length of the envelope ,of an actual tube design, drawn on a considerably enlarged scale. In this particular design the internal diameter is 0.225 inch. The helical groove 27, constituting the rifiing, is 0.050 inch wide .and 0.030 inch deep, the riding having a left-hand pitch of one and onehalf turns per inch.

The slow-wave guide 17 is introduced within the barrel thus formed and is supported thereby as is shown in Fig 3. Inthis case the conducting helix is formed of thin metal tape, about 50 mils wide and a few mils thick, wound with a right-hand pitch of 14 turns per inch. Such a helix may be introduced into the barrel of the .tubehtly wound on a mandrel of such size that itclears the barrelvery slightly, and then allowed toexpand through its ownresiliency intocontact with the supporting barrel, afterwhich the mandrel can be withdrawn.

.Sinceihe rifting groove helix has a left-hand pitch offone and a half turns per inch and .the conductor helix has. aright-hand pitch of fourteen turns per inch,

therewilLbe fifteenand one-half cross-overs per inch between the twohelices measured along the axis of the ry early nine; t enths of a turn. of the conductor lihe'ye l ity of wave propagation along the helix is inversely proportionaLto x LC, where L is the inductanceper-unit length and C. is;the capacitance per unit length. This velocity. .willbe materially less than the fpr opagation of a wave along a straight confreespace, approaching the speed oflight, 1s; increase doby the coupling between adjacent turns-pf;the]helixv and C isincre'ased because of the close juxtaposition of theconductor and the glass IQL ghQU -IQQJQQELportion or thelength of the condugtorffiheremgyal ofjthis contact where the crossovers take placedoes not afiect the inductance, but it does atfectthe capacitance. Accordingly slight perturon-uniformities in the constants of steedsg ii ih fiiil ltl tqiasaline, occur every nine-tenth' 'bf ik t eloaeits t ;..that is, at intervals of0.647i h ltis well known that any departure from uniformity w ter in the characteristic impedance of a line causes a certain amount of reflection of energy back toward the source from the point at which irregularity occurs. Where these reflections occur at uniform intervals of one-half wavelength they become cumulative, the reflected waves reinforce, and the repeating structure becomes a band-elimination filter. The result is an attenuation of waves at or closely adjacent to the half-wavelength frequency. At frequencies materially different from the center of the stop band the reflections are not cumulative and the attenuation falls off very rapidly. With the structure here disclosed the number of perturbations is verylarge, the wave guide corresponds to a filter of from 100 to 200 sections. Even a small attenuation per section is therefore very effective, despite the relatively small attenuation per section.

If the pitch of the groove helix is in the same direction as that of the conductor the number of perturbations per axial inch becomes equal to the difference of the lineal pitches instead of their sum. There is then more than a single turn of the conductor between perturbations. It follows, of course, that if the pitch of the rifling is iden tical with the pitch of the conductor no perturbations are introduced, whereas as if one perturbation per turn is desired the helix of the rifling has a pitch approaching or equal to infinity and the helix degenerates to a single straight groove. Where the pitches of the two helices are in the same direction the number of perturbations for a slow-waveguide of a given length is, of course, reduced, and the effectiveness of the construction as a filter drops off materially before equality of pitch is reached. In practice, however, a stop band which would necessitate any such arrangement will not normally be required.

Since the conductor helix is, in effect, supported by a helical strip of insulation in close proximity to it (defining helical to include helices of infinite pitch) it would obviously be possible to form such a helical strip of insulating material separately from the envelope of the tube and insert it into the tube barrel, the result being the equivalent of forming a groove in the barrel itself. One of the advantages of the present invention, however, is its simplicity and the possibility of rifling the barrel itself easily and economically.

The means of accomplishing this are illustrated in Figs. 4 and 5. A mandrel is first constructed of cylindrical form having a diameter substantially equal to the desired external diameter of the conductor helix. A helical thread is formed thereon, the dimensions of the thread, as regards cross-section and pitch, being equivalent to the dimensions desired of the groove to be formed Within the barrel. The cross-sectional shape of the thread is not particularly important, since the effect of the contacting insulator upon the capacitance of the conductor falls off with great rapidity as the distance between the conductor and its insulating support increases, and hence the difference between a thread of triangular, square, trapezoidal or semi-circular cross-section is very small indeed. A triangular, V-shaped thread has the disadvantage that its sharp corner may unduly weaken the envelope, owing the concentration of stress at the apex of the V. Such a thread could, of course, be formed integrally with the mandrel by conventional screw-cutting technique. It is much simpler and more economical, however, to wind a tape or strand of metal of the desired cross-section on the mandrel to form the thread, and since the rectangular shape is as effective as any other and tape of rectangular cross-section is both easiest to handle and commercially available it will ordinarily be chosen for the purpose.

The Winding can be done on an ordinary screw-cutting lathe, geared to advance the carriage at the proper rate to give the winding the desired pitch. The tape is wound under heavy tension to make the helix cling tightly to the mandrel. The tape may first be wound on a smaller mandrel. Then it can be slipped over the proper size 6 mandrel and will cling tightly to this mandrel. The resultant structure is illustrated in Fig. 4.

Preferably the coefficient of thermal expansion of the mandrel should exceed that of the tape, so that the tension on the latter will be increased when the structure is heated and the tape will cling more closely to the mandrel. This is especially important if the pitch of the winding is infinite, and the tape is stretched parallel to the axis of the mandrel. While this latter arrangement is entirely feasible it is preferable for purely mechanical reasons so to design the structure that a finite pitch will give the proper stop-band.

There are many combinations of metals or alloys which meet the requirements as to differential coefficients of expansion. For example, a nickel-iron alloy of low coeflicient may be used for the tape and a mild steel for the mandrel, if the barrel is to be made of glass. If fused quartz is to be used for the tube barrel higher melting point materials will be preferred, such as a molybdenum mandrel and a tungsten tape.

The vitreous tubing 36 which is to form the rifled barrel is of sufficient size to slip over both mandrel and tape. One end is sealed vacuum tight, the mandrel is inserted, and the other end is connected to an exhaust line 37 leading to a vacuum pump 39, as shown schematically in Fig. 5, and the structure is evacuated. The portion of the tubing 36 which is to form the tube barrel 5 is then heated to the flow-point, as by the gas torches 41, whereupon the tubing collapses under atmospheric pressure, shrinking to conform tightly to the threaded mandrel. The shrinking can be done progressively, fro-m the sealed end toward the exhaust end, the flames being moved ahead gradually as the tubing shrinks.

The whole is preferably allowed to cool before the vacuum is broken, especially if quartz tubing is used, to prevent oxidation of the mandrel and tape. After cooling the sealed end of the now rifled tubing is cut off and the mandrel is simply screwed out of the barrel.

Ordinarily the next step in the formation of the envelope would be to anneal the rifled tube, although with quartz or some types of glass this may be an unnecessary refinement. The barrel may be used in the form in which the shrinking has left it. Under these circumstances it will have a projecting helix 43 corresponding generally to the helical rifling within it. If desired, however, this external helix may be removed by grinding, as was done in the case of the tube illustrated in the Figs. 2 and 3. With a helix of finite pitch this will be most readily accomplished in a centerless grinder; where the helix is degenerated into a single straight groove more uniform results can be obtained by grinding between centers; it can be done by mounting the tube on a mandrel similar to or identical with that on which the shrinking was done.

Once formed, the barrel can be fused to the bulbous base end 3, either directly or through a length of unrifled tubing. The collector anode may similarly be inserted directly into the rifled portion of the barrel or additional length of tubing may be added. In the latter case the seal through which the lead 21 is brought out may be conveniently formed at the junction between the rifled and unrifled tubings. Such a junction is indicated schematically in Fig. 1, where the sealed-on, unrifled portion of the barrel is shown at 5'.

Such a splicing-on procedure offers one method of introducing two stop bands in the same traveling wave tube, in case there may be two critical frequencies at which oscillation might occur. Two pieces of tubing, rifledwith grooves of different pitch, may be spliced together. In this case neither stop band will give the same degree of attenuation that would be offered by a single band, but it may be that sufficient attenuation may be thus introduced to prevent the instability of operation. Whether or not this will prove to be the case depends upon the operating parameters of the tube. Another method of introducing two stop bands is to provide a mandrel which is separable into two parts; the thread formed on one p b n ift ea rom h we Q theiqmci wa piece of glass tubingis shrunk onto the composite mandrel in the same fashionas'before, and atter the glass and mandrels have cooled the two halves of the mandrel are screwed out of the tubing'fro m opposite ends,

As will be apparentto those skilled in the art, the present invention provides not only ameans of extreme theoretical simplicity for solving a problem which has heretofore required complex and expensive equipment, but also a ready and relatively inexpensive method oi constructing the apparatus to apply the theory, It is, accordingly, ap' plicable to apparatuswhichdilfers greatly in details from that shown. The detailed descriptions herein are therefore not intended to be limiting, all intended limitations being specifically expressed in the following claims.

What is claimed is:

l. A traveling'wave tube comprising arr electron gun for generating a beam of electrons, an anodepositioned to receive the electrons of said beam, and'aslow-wave guide interposed between said electron gun and said'- anode and surrounding the path of saidbeam and comprising a helically wound conductor, and a tube of insulatin'gimaterial surrounding and supporting said conductor, the wall of said tube having a single internal generally longitudinal groove formed therein to interrupt its contact? withthe conductor for regularly spaced and substantially uniform intervals along the length thereof.

2. A traveling wave tube comprising an electron gun for generating a beam of electrons, an anode positioned to receive the electrons of said beam, and a -slow-wave guide interposed between saidelectron gun andibetween said electron gun and said anode and, surrounding the path of saidbeam and comprising a helically wound conductor, and a tube of insulating material surrounding and supporting saidconductor, the wall of said tube having a singlefinternal "groove extending longitudinally there through'to interrupt thejcontact between said'wall and said conduetor'ff or substantially uniform and equally spaced distances along the length of said'conductor, the major portion 6f nductor being in contact with said ll QT l'fl'f tr 1'; t V

3. A traveling wave tube as defined in claim 2 wherein said roore aescns s' ahelix of substantially uniform pitch differing from inspires of'the conductor helix.

4. A traveling wave tube comprising an evacuated envelope ericlo'sir'igaafeicesn g'u'n' fo'r'developing a beam of electrons, ac'dllector'anode positionedto receive said beam, 21 s1o'w wa'v guideiiite'rposed between said'cathode and anode comprising a helix of conducting material, and a support for said helix comprising a helix of insulating material substantially surrounding the" conducting helix and of differentpitch' whereby contact between said conducting helix and said supportis interrupted for substantially uniform'equally spaced intervals therealong.

5. A traveling wave tube asdefined in claim 4 wherein said support' 'is integral with said evacuated envelope.

References ,Citedinthe file of this patent umlaasm'rm ATENI 1,999,525, :Mor s chol; Apr. 30, 1935 2,2 9,739: Meyer. July 30, 1940 5975019; ',Ev r t M y 9, 9. .i6 ,0 Mallafisrhek Sep 1 9. 2 707 59 Blame May 3, 1955 2,730,649 Dewey Jan. 10, 1956 tFQllt ilqNv ilfi Ts 664,663, ,GreatBritain Jan. 9, 1952

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