US2760102A - Travelling wave tubes - Google Patents

Description

` L.; M, FIELD TRAVELLING WAVE TUBES 2 Sheets-Sheet l Filed June 9, 1950 INVENTQR ESTER M. F/ELD BY af/ ATTORNEY 1 L. M. FIELD TRAVELLING WAVE TUBES Aug 21. 1956 2 Sheet's s`heet 2 Filed June 9' 1950 A INVENTOR ESM-RM HELD A'II'TORNEY United States Patent O TRAVELLING WAVE TUBES Lester M. Field, Palo Alto, Calif., assignor to The Board of Trustees of The Leland Stanford Junior University, Stanford University, Calif., a legal entity having corporate powers of California Application June 9, 1950, Serial No. 167,150 19 Claims. (Cl. S15-3.5)

This invention relates to electron discharge devices, and more particularly to improvements in travelling wave tubes, wherein amplification of electromagnetic wave energy is effected by interaction between a travelling wave and a stream of electrons moving with the Wave.

In the operation of such tubes, it is generally necessary that the electrons and the wave to be amplified travel at approximately the same velocity. This is accomplished by the use of some kind of slow wave propagating structure, such as a helix. A typical prior art device includes a relatively long small diameter helical winding which is supplied at one end with the input energy in such manner as to carry a slowly travelling wave. An electron stream is directed along the longitudinal axis of the helix, and amplilied energy appears at the other end of the helix.

To obtain a substantial amount of amplification with such tubes, it is necessary that the helix or equivalent slow wave propagating structure be made relatively long, in order that the wave and the electrons may interact throughout a substantial distance. This requirement leads directly to the problem of focussing the electron stream into a thin beam, and keeping the beam together against the forces of mutual repulsion of the electrons.

The power handling capability of the tube is limited by the beam current, which is proportional to the product of the beam density and the cross sectional area of the beam. The maximum attainable density is not high, on account of the dificulty in focussing, and the maximum usable beam cross section depends upon the size of the helix, which is largely determined by the operating frequency range. Thus, in prior art tubes wherein the Wave and the electrons move along the same axis, both the gain and the power output are limited, and as a practical design matter a compromise must be made between them. Relatively high accelerating voltages must be used, and more or less elaborate focussing means, such as external magnets, are required.

Another usually undesirable characteristic of the above described prior art tubes is that while they exhibit gain in the forward direction, i. e. in the direction of electron motion, they also propagate waves in the other direction. The amplified waves appearing at the output end of the device may feed back this way to the input end, resulting in sustained and uncontrolled oscillation. This can be, and has been, avoided by introducing losses in the helix, but such losses also reduce the desired gain in the forward direction.

It is one of the principal objects of the present inven-v tion to provide improved types of travelling wave tubes wherein the foregoing disadvantages are avoided.

More specifically, it is an object of this invention to provide travelling wave tubes in which the direction of electron flow is transverse to the general direction of wave propagation.

Another object of the invention is to provide tubes of the described type which may be designed to make the Wave-electron interaction space of any desired length,

without the introduction of any stringent beam forming or focussing requirements.

A further object of the invention is to provide tubes of the described type which are simple and rugged in construction and do not involve close mechanical tolerances.

Other objects and advantages of the invention will become apparent from the following description with reference to the accompanying drawings, wherein:

Fig. l is a longitudinal section of a presently preferred embodiment,

Figs. 2 and 3 are cross sections of the structure of Fig. 1 in the planes 2--2 and 3-3 respectively,

Fig. 4 is a plan View showing the internal structure of a modification of the device of Fig. l,

Figs. 5 and 6 are side and end elevations respectively of the structure of Fig. 4,

Fig. 7 is a transverse section of a modification of the device of Fig. 4,

Fig. 8 is a plan view of another modification of the device of Fig. 1, and

Fig. 9 is a transverse cross section of the structure of Fig. 8

The travelling wave tube shown in Figs. l, 2 and 3 includes a slow wave propagating structure in the form of a skewed conductive helix 1, of oblong or flattened cross section. The helix 1 may be made of copper wire Wound on an insulating form 5 provided with notches 7 for holding the wire in place.y The pitch of the helix is relatively small, i. e. it is the space between adjacent notches 7, but through the major portion of the helix the wire is laid at a relatively large angle to the normal pitch line so that the lower side of each turn is considerably advanced with respect to the upper side. The last few turns near each end of the structure are laid at progressively decreasing angles to provide a smooth transition from the skewed winding to a normal helix.

The form 5 is supported at its ends by transverse `plates 9, which are in turn supported between slightly bowed sheet metal members 11. The members 11 extend across tubular conductive sleeves 13, each disposed within and near a respective end of a tubular vacuum tight glass envelope 15.

A linear electron gun assembly 17 is provided near the lower edge of the wave propagating structure 1 and slightly in front thereof, as shown in Fig. 3. The assembly 17 includes a strip cathode member 19, focussing electrodes 21, and an accelerating electrode 23. The cathode 19 is adapted to be heated by a heater element 25. The assembly 17 is supported by conductive posts 27, 28, 29 and 30 which extend through and are sealed to the wall of the envelope 15, and act as terminals for the application of heater and accelerating potentials.

A collector electrode 31 parallel to the cathode assembly 17 is supported near the upper edge of the structure 1 by a post 33 which extends through and is sealed to the wall 15. Similar terminals 35 are brought out through the wall 15 from the sleeves 13. The ends of the helix are connected to conductors 37 and 39 respectively, which extend through seals in the Wall 15 and into external conductive sleeves to form coaxial line terminals 41 and 43. The external sleeves are supported by sleeves 45 which closely surround the envelope 15 in juxtaposition with the sleeves 13.

The operation is substantially as follows: The heater 25 is energized from an external source, not shown to` cause emission of electronls from the cathode 21. Another external source, such as a battery, is connected to the accelerating electrode 23 to maintain said electrode at a positive potential with respect to the cathode 21. The collector electrode 31 may be connected to the same point as the accelerating electrode 23 or may be supplied ,with another potential, also positive with respect to the Patented Aug. 21, 1955` cathode. The assembly 17 produces a relatively thin sheetalike stream of electro-ns flowing to the collector 31 substantially at right angles to the longitudinal axis of the propagating structure 1. Y

High frequency energy to be ampliiied is applied.y as by means of a coaxial line to the terminal 41, and it travels along the wire of the helix at approximately the velocity of light in free space. Since the input wave energy must travel. throughout. the lengthv 'of a full` turn of the helix in order to advance, the distance between two adjacent turns in the longitudinal direction of the propagating Lstructure 1, the efect is that of causing the wave to propagate relatively slowly along the helix. The phase of the Wave is substantially the same throughout the length of'any given lineal turnelement of thel hel-ix because the wave propagates along the wire at the. velocity of light. Thus in the uniformly skewed portion. of the helix, an electron travelling from the cathode assembly I7 to the collector 31 will encounter the waveV at progressively retarded space phases, as it crosses4 successive turns of the wire. Another way of stating the same thing is that the phase front ofthe wave is skewed o'r `tilted so that the wave has a component of propagation velocity perpendicular to the longitudinal axiszof the propagating structure 1 and parallel to the direction of electron. flow.

The accelerating potential is adjusted to make the electrons travel' across the structure 1` at substantially the same velocity as; the transverse component of wave velocity. Thus the electrons interact with the wave substantially as in the conventional helix, type travelling wave tube, delivering energy to the Wave as they travel across successive Vturns of 4the helix. 'The ampliiiedv wave at the upper end of each helix turn, for example at the point 47, appears at the lower end 49 of the same turn adjacent a different element of the electron stream. Accordingly' the wave as amplified by each element of the stream is further amplified by a following stream element throughout the active length of the propagating structure.

It will be apparent that the tube may be made as long as desired-:toprovide substantially any desired degree of amplification without requiring any corresponding extensionf of the lengths of the electron paths. The amplified wave appearing at the right hand end of the structure 1 is conducted through the lead 39 and the coaxial terminal 43 to suitable utilization means, not shown.

One of the problems which occurs in the design of mostV travellingrwave tubes is that of. providing a broad band impedance: match between the input and output connectors and` the slow wave propagating structure. In the` present device, the bowed conductive members 11 act as extensions of the outer conductor of the coaxial connector; 41, providing a transition from an ordinary coaxial line to ak multiple conductor open wire line. The' centrali conductor 37 merges into and becomes the iso'- lated single conductor of the helix. This' structure has` been found to provideV a satisfactory impedance matchthroughout a relatively wide band of frequencies from about 150: to 350 megacycles per second.

A tube designed substantially as 'shown and of approxima-tely the sizeshown.` n Fig. l` has been found to provide a gain ofabout 30 db at ZOO-megacycles, With a` beam voltage ofSOl volts and a beam current of 60 milliamperes. No attenuating means is required to prevent backward-amplification and feedback, although the helix itself has extremely low insertion loss.

If the tubeiis to be designed to provide relatively high power gain the cathode 19 may be arranged to emit more electrons as the output end is approached. This may be` done by making the cathode strip larger near that end, or` by using. a separate larger cathode assembly where more emission isl required.

Figs. 4, and 6 show the internal details ofv av tube similar to that of Fig. l but embodying a different wave propagating, structure comprising a conductivebar orl block 51 provided with a series of obliquely disposed slots S3. The cathode assembly 17 and collector electrode 31 may be the same as the correspondingly designated elements of Fig. l. An input wave guide 55 is connected to one end of the bar 51, with the interior of its lower wall 57 flush with the slotted surface. An output guide 59 is connected similarly to the other end of the bar 51. 7The slo'ts 53 are made of progressively greater depths from` the ends of the bar 51 toward the central active portion which is traversed by the electron stream, where they are of uniform, depth. The. upper walls 61 and 63 of the wave guides 55 andv 59 extend to the active portion of the wave propagating. structure, and may be made' tov'c'over this part also, forming one continuous wall from` one end of the tube to the other.

The operation of the device of Fig. 4 is essentially the same as that of Fig. 1. The slots 53 act as lumped reactive. loadingl elements, lowering the. velocity of the wave,` propagation along the bar 51. However, the propagation velocity lengthwise of any sloty is substantially the same as. that. of light in free space. As in the structure ofl Fig. 1, ai wave travelling along the member 51 willhave, its. phase front turned obliquely withy respect to the longitudinal axis ofthe member 51, and the average electron. velocity ismade substantially equal to the transversier component of wave velocity. The amplified waveV at the upper end (in Fig. 4) of each slot 5'3 appears at lthe lower end. of the same slot, and acts on a different part of the electron stream.

The structure of Fig. 4 has somewhat different opera tional. characteristics from that of Fig. l, and may be preferred. for extremely high power and high frequency applications. The nature of the retarding structure (i. e. theslotted bar) is such that the velocity of wave propagation-,will dependupon the wavelength or frequency, and the; bandwidth throughout which uniform gain. can he` obtainedwith a given adjustment of the acceleratingvoltage isy accordingly limited by this characteristic, which is calledl dispersion Asan alternative to the above described mode of operation, theY device of` Fig. 4 may be operated in a space harmonic mode, wherein the delay or retardation caused by each slot 53 is much less than the relatively large amount required to make the velocity of a given phase front match thatof the electron stream. Ordinarily, the delay per slot must be suchl that the time required for a wave. to1 advance. from one slot to the next is the same as that required. for an electron to travel from one slot to'l the next.k However, withy a repetitive structure, if in the time an electron takes to goV from one slot to` the next, the. wave goes the distance between slots plus one full' waveelength, the' electron will be synchronizedL with a travelling. field component of much lower' velocity than the waves, fundamental: mode velocity.

The space harmonic type of operation may be preferred where itis inconvenient. or impractical to construct a retardingstructurei to operate in the fundamental mode, for example in tubesintended for high power or extremely highfrequency operation. It will be apparent that the devicesopjerated in. space harmonic modes will be relatively frequency-sensitive, but this may. be advantageous under some circumstances.- It should be noted alsol that thel skewedy helix type of. structure shown in Fig. l' may be operated in space harmonic modes, if desired.

The narrow cross section of the propagating structures in the devices of Figs. 1 and 4, and the fact that the electronstream is directed along the sides thereof, makes it practicaltoiuseV a= stacked arrangement like that shown iniFig. 7. Hereapair of helices 1 and1 are'provi'ded on oppfbside` sides of the space through which an electron streamis directedrfrom a cathode 17 to a collector 31.

The helix windings are skewed in the samedirection, and' are operatdinf'parallelf, i. e. the two input ends are. connected'-te getner, and so are the two output ends; Various (Jlhr'ta'lg lrltmgements"v will be apparent; plural eleoteraction space.

ated-a tron streams may be used, for instance, and other propagating structures such as that of Fig. 4 may be combined.

y Fig. 8 shows a double-stream travelling wave tube embodiment of the present invention. In this device a skewed helical winding 1" is provided at the input end of the tube. The Winding 1" is like the initial part of the winding 1 in Fig. l, but it stops near the active portion of the tube which is traversed by the electron stream. A second winding 1'", similar to the final part of the winding 1, starts near the active portion and continues to the output end of the tube. A cathode assembly 17 is provided at` the input end, and a collector 31 is arranged at the output end to cooperate with the cathode 17'.

The cathode 17' provides a thin sheet like electron stream flowing lengthwise of the tube to the collector 31', and passing in close proximity to the sides of the skewed helical windings 1" and 1. The input Winding 1" is excited, as in the tube of Fig. l, by the wave to be amplified. The electrons in the longitudinally flowing stream are bunched by the action of the field of the helix 1", the bunches being in the form of lines parallel to the turns of the helix. As the oblique linear bunches travel through the interaction space, they pass through the transverse stream from the cathode 17 in such manner that an electron in said transverse stream interchanges kinetic energy with successive portions along a given lineal bunch in the longitudinal stream.

The transverse stream itself becomes bunched, and give up energy to the longitudinal stream, intensifying the bunches therein continuously as it travels across the in- The intensified bunches then pass over the helix 1', inducing a wave therein like the input wave, but of larger amplitude. The amplified wave may betaken off and utilized as in the system ofFig. l.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. An electron discharge device, including a wave propagating structure comprising a series of conductive elements disposed in side by side relationship and defining a common planar surface of substantially uniform width and of a length greater than said width; said elements being coupled to effect wave propagation along the length of said structure from one end of said series to the other, and means including a lineal electron gun adjacent a lateral edge of said surface for producing a sheet-like stream of electrons which substantially conforms to said surface and is closely adjacent thereto for interaction with wave propagation along said conductive elements, said means including focussing and accelerating electrodes for directing said electrons across said wave propagating structure adjacent said surface transversely to the direction of wave propagation and at less than ninety degrees to the maj-or dimensions of said conductive elements.

2. An electron discharge device, including electromagnetic Wave propagating means supported along an axis, said means including a plurality `of spaced conductive element portions disposed in substantially parallel relationship transverse said axis in a comm-on plane parallel with said axis, and means including a lineal electron gun disposed laterally of said conductive element portions, said last-named means including a cathode assembly for producing and directing a sheet-like stream of electrons across said axis in a plane substantially parallel with and closely adjacent said common plane for interaction with wave propagation along said conductive element portions, said electrons travelling in a direction transverse to the direction of wave propagation along said propagating means and at less than ninety degrees to the major dimensions of said conductive element portions.

, 3. An electron discharge device as set forth in claim 2,- further including at least one further cathode assembly adjacent and in cooperative relationship with said Wave propagating means for producing a second stream ofelectrons along the axis of said wave propagating means.

4. The invention as set forth in claim 3, wherein said further cathode assembly is adjacent the part of said wave propagating means which is first traversed by a wave travelling thereon, and said first cathode assembly is adjacent a part of said propagating means which is subsequently traversed by said Wave.

5. An electron discharge device, including a wave propagating structure comprising a series of lineal conductive elements disposed in parallel side by side relationship, and defining a common planar surface, means connecting said elements to eect wave propagation along said structure from `one end of said series to the other,

means including a lineal electron gun adjacent an edge of said surface for producing a sheet-like stream `of electrons which substantially conforms to said surface, said last named means comprising means for directing said stream across said wave propagating structure closely adjacent said surface for interaction with wave propagation along said lineal conductive elements, said stream travelling in a direction transverse to the direction of said Wave propagation in said structure and at an angle of less than ninety degrees to the major dimensions of said conductive elements, means for applying wave energy to be amplified to one of said ends of said wave propagation structure, and means for leading amplified Wav energy away from the other of said ends.

6. The invention as set forth in claim 5, wherein said wave propagating structure is of oblong cross section, with one of its broad sides constituted by said conductive elements.

7. The invention as set forth in claim 6, wherein said Wave propagating structure comprises a helically Wound conductive wire, with a portion of each turn of said wire constituting one of said elements.

8. The invention as set forth in claim 7, wherein the turns of said wire in said helical winding are oriented obliquely with respect to the principal axes of said wave propagating structure.

9. An electron discharge device, including a wave propagating structure comprising an elongated bar of conductive material having a plurality of transverse slots providing a connected series of lineal conductive elements disposed in parallel side by side relationship to effect wave propagation along said structure from one end of said series to the other, and defining a common planar surface, means including a lineal electron gun adjacent an edge of said surface for producing a sheet-like stream of electrons which substantially conforms to said surface, said last named means comprising means for directing said stream across said wave propagating structure closely adjacent said surface in a direction transverse to the major dimension of said structure and at an angle ofV less than ninety degrees to the direction of said slots for interaction with wave propagation along said lineal conductive elements, means for applying wave energy to be amplified to one end of said wave propagation structure, and means for leading amplified wave energy away from the other end of said structure.

l0. The invention as claimed in claim 9, wherein said slots are oblique with respect to said major dimension of said wave propagating structure.

ll. The invention as claimed in claim l0, wherein said slots are of substantially uniform depths throughout the portion of said wave propagating structure which is crossed by said electron stream, and are of gradually decreasing depths from the ends of said portion to the ends of said structure.

l2. An electron discharge device including means for propagating electromagnetic energy along an axis in the form of Waves with their phase fronts inclined with re- 7 spect. to 'saldi axis, said means having anl oblong cross section, and means for producing and' directing 'a' stream of elec rons or oblong 'cross section transversely acrossv said for encountering said waves and interact-ion therewith at progressively diierent' space phases from one S'i'de of said propagating meansto the other, said elect'rons having a vvelocity component perpendicular to said phase fronts, the narrow dimension of the cross section of said electron stream being orientedl substantially parallel with 'the narrow dimension of the cross section of said electromagnetic energy propagating means and the wide dimension. -o'f the cross section 'of said electron stream being 'oriented at' an angle withr respect to the wide dimensien of the croSssect-ion of said wave propagating means.

The; invention as set forth in claim 1'2", further including meansv for producing and'l directing a second stream ofj electrons along said axis, said second stream being ofI substantial extent transversely of its direction of motion, saidpropagating means comprising rst and second sections with saidv rst section comprising means for modulating said` second stream to form` transverse linear bunches therein, said bunches being obliquely inclined withr'espect t-o said axis and with respect to the direction of motion of the electrons in said lirs't mentioned stream.

14. An electron discharge device, comprising an evacuajted@ tubular envelope, a conductive helix of oblong cross section supported in said envelope, the longitudinal axis of! said helix extending along the axis of said tubular envelope, the respective ends -of said helix comprising in put and output terminals for coupling to input and output electromagnetic transmission line structures, respectively, an elect-'ronl gun supportedy within said envelope adjacent one of the narrow sides of said helix for producing and directing an electron beam of oblong cross section transverse the axis of said helix and across one of the wide sides of said helix with the beam electrons travelling at an angle with respect to the adjacent helicalV turn portions of said'- one wide side in energy exchanging relationship with said turn portions from one to the yother edges of said one' wide side ofsaid helix, the narrow dimension of the cross section of said electron beam being orientedsubstantially parallel with the narrow dimensionl of the cross section of said helix, and collector electrode means supported within said envelope adjacent the other of the narrow sides of said helix for receiving said electron beam from said electron gun.

115. An electron discharge device as set forth in claim 174, wherein the turns yof said helix adjacent said electron beam are` skewed with respect to the axis of said helix.

16. Anelectron discharge device, comprising a slow wave electroniagneticv wave energy propagating structure having a longitud inaily axis", saidv structure ccrnpri'singl means ferk guiding microwave energy from one region to another` region spaced? from said: oneV region along said longitudinal axis, said propagating Astructure including 'a plurality of conductive' elements disposed" transverse said axis witlr portions of saidi elementsv lying substantially in a common planeparal-lel with said axis, and means in-l cluding a strip*y cathode member disposed= laterally of said propagating structure for producing and directingl a stream of electrons of substantially rectangular cross section in a transverse direction relative to saidcon'ductive element portions and the off said propagating structure for modulation by electromagnetic wave 'energy along said conductivegelement's.

17. An electron discharge4 device, comprising a travel'- ling electromagnetic wave helix of oblongcross section, said helix having a longitudinal axis andr said helix inw cluding a plurality of adjacent lineal turn elements lying ina common plane, andtmeans including a lineal electron gun disposed laterally of said helix for producing and directing an electron stream of substantially rectangular cross section inE a transverse direction relative to said longitudinalalxis of said helix andy the lengths of said lineal turn elements.

183. electron discharge device as set forth in' claim 17 wherein said lineal turn elements arel substantially' parallel with each other and in an oblique relationship relative to-said axis.

19; A11l electron discharge device as set forth in claim 18', wherein said'c lineal turn elements of said helixv are inI a plane-parallel withthe direction ofVv electrons from said' electron gun, and said lineal' electron gun includes a stripf cathode having a substantially planar faceof rectangular` crossys'ecticmvin a plane at right' angles to 'said plane of said lineall turnV elements.

References'. Cited in the nie of thispatent UNITED STATES' PATENTS 2,122,538 Potter July 5 1938 2,300,052 Lindenblad Oct. 27, 1-942 2,439,401 Smith Apr. 13, 1948 2,487,656 Kilgore Nov. 8, 1949 2,511,407 Kleenetal. a June 13, 1950 2,516,944 Barnett Aug. l, 1.950 2,531,972 Doehler et al' Nov. 28, 1950- 2,559,581- Bailey,..- July 10, -1` 2,687,494 Adler Aug; 2,4, 1954- N-fr

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