US3237046A

US3237046A - Slow wave structures including a periodically folded coaxial cable

Info

Publication number: US3237046A
Application number: US238801A
Authority: US
Inventors: Jr Hilding M Olson
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1962-11-19
Filing date: 1962-11-19
Publication date: 1966-02-22
Anticipated expiration: 1983-02-22
Also published as: GB1055580A

Description

H. M. OLSON. JR SLOW WAVE STRUCTURES INCLUDING A PERIODICALLY Feb. 22, 1966 FOLDED COAXIAL CABLE 4 Sheets-Sheet 1 Filed NOV. 19, 1962 //v VEN TOR H. M. OLSON, JR.

ATTORNEY Feb. 22, 1966 H. M. OLSON. JR

SLOW WAVE STRUCTURES INCLUDING A PERIODICALLY FOLDED GOAXIAL CABLE Filed Nov. 19, 1962 4 Sheets-Sheet 2 FIG-.4

a b c d 5 h 4 Hula INPUT new I 210 F f l -/9 u n I: J I H F ll 4, L H H L l J IL J ,'r{ f H 22 1 n J I OUTPUT Feb. 22, 1966 H. M. OLSON. JR 3,237,046

SLOW WAVE STRUCTURES INCLUDING A PERIODICALLY FOLDED COAXIAL CABLE 4 Sheets-Sheet 5 Filed Nov. 19, 1962 OUTPUT WATER PUMP FIG-8 mriu FIG. 6

0 W4 TT/2 3n/4 PHASE SH/FTPER SECT/ON Feb. 22, 1966 H. M. OLSON. JR

3,237,046 SLOW WAVE STRUCTURES INCLUDING A PERIODICALLY FOLDED COAXIAL CABLE 4 Sheets-Sheet 4.

Filed NOV. 19, 1962 FIG. 9

4 a 3 0 U 3 0 l\/ :4 a F H F a H." .j "j F I H I I 4 I w w 0 2 U y. n G I I F 5 3 [N B an "m H H EIIH n 1 1 All T II J U P T 3 T U 7 0 P 0 w 4 FIG.

M/PUT Q United States Patent 3,237,046 SLOW WAVE STRUCTURES INCLUDING A PERI- ODICALLY FOLDED COAXIAL CABLE Hilding M. Olson, Jr., Mohnton, Pa., assignor to Bell Telephone Laboratories, Incorporated, New York,

N.Y., a corporation of New York Filed Nov. 19, 1962, Ser. No. 238,801 20 Claims. (Cl. SIS-3.5)

This relates to electron discharge devices and, more particularly, to devices of the type that employ slow wave structures for propagating electromagnetic wave energy in interacting relationship with a stream of electrons.

Two broad classes of microwave frequency electron tubes that use electron stream interaction are the magnetron and the traveling wave tube. The general purpose of both of these devices is to convert direct-current energy to radio-frequency energy through the coupling of electromagnetic wave energy with a stream of electrons. The coherency of this stream is normally maintained in a traveling wave tube by a longitudinal magnetic field, and, in the magnetron, by mutually perpendicular electric and magnetic fields. Although magnetrons are usually used as oscillators, and traveling wave tubes as amplifiers, these functions can be reversed, and the distinction between the two devices can become quite hazy. It has therefore become customary to refer to all tubes that use crossedfield focused electron beams as M-type devices, and to refer to those that use longitudinally focused electron beams as O-type devices.

O-type devices are much more widely used as amplifiers than M-type devices because they normally are less noisy, more stable, and have wider bandwidths. It has long been recognized, however, that M-type amplifiers are inherently capable of more eificient operation. First, O-type devices usually require a relatively high voltage collector for collecting the electron beam; this results in dissipation rather than utilization of electron beam power. Secondly, a rather long magnetic focusing field is required because it must be parallel with the straight beam path. Finally, O-type devices must use a heated cathode while magnetrons can use cold cathodes.

Because of the above advantages, considerable effort has recently been expended in developing M-type amplifiers such as that shown, for example, in the US. patent of Gewartowski, 2,926,285, granted February 23, 1960. This device is of the type known in the art as a coaxial magnetron and comprises a circular array of anode resonators which are coupled by a series of anode slots to an outer waveguide. Electromagnetic wave energy from the waveguide fiows through the slots to the resonators and interacts with an electron stream from a cylindrical cathode. The electromagnetic wave travels in the same direction as the electron stream which qualifies the device as a forward-wave amplifier.

One characteristic of virtually all forward-wave M-type amplifiers is their bulk. For example, the radial anode vanes that define the anode resonators of the Gewartowski amplifier must be approximately one-quarter wavelength long at the operating frequency. When one additionally considers that waveguide diameters usually increase with lower frequencies, it can be appreciated that at lower frequencies the radial dimensions of the device will be quite large.

This disadvantage can be avoided by substituting interdigital structures, ladder structures, or the like, for the anode resonators for synchronizing the electromagnetic wave with the electron stream, as is required for interaction. Unfortunately, these comparatively trim structures are basically backward-wave devices; that is,

aza eas Patented Feb. 22, 1966 strongest interaction is achieved when the wave is propagated in a direction opposite that of the electron flow. Backward-wave devices are inherently of limited bandwidth because they couple at one fairly specific electron stream velocity. Although this obviously limits their usefulness as amplifiers, it should be pointed out that they make good oscillators because the output frequency can be tuned by the simple expedient of adjusting electron stream velocity.

It is an object of one embodiment of my invention to attain forward-wave interaction in an M-type device with a compact slow wave structure.

This objective is attained in an illustrative embodiment comprising a cylindrical cathode for forming and projecting a stream of electrons that is forced to flow in a roughly circular path around the cathode through the combined focusing action of crossed electric and magnetic fields. The electric field is produced between the cathode and a surrounding slow wave structure while the magnetic field is parallel with the axis of the cathode and transverse to both the electric field and the electron stream. The principal purpose of the slow wave structure is to transmit electromagnetic wave energy in interacting relationship with the electron stream to produce amplification.

According to one feature of this invention, the slow wave structure comprises a coaxial cable that is periodically folded upon itself to form a series of straight sections interconnected by bends in the coaxial cable. Each of the section extended parallel with the cathode axis and transverse to the direction of electron flow. With this arrangement, electromagnetic wave energy is transmitted in the same direction as the electrons at a much slower net velocity than the velocity of light because of the tortuous path of the cable. As is known, interaction requires the net velocity of the wave to be approximately equal to that of the electron stream.

It is another feature of this invention that a conductive vane be attached to the inner conductor of the cable at the mid-point of each straight section. The vane extends through a slot in the outer conductor to a point in proximity to the electron stream. Its purpose is to create a modulating field across the electron stream and it basically differs from the vanes used in the Gewartowski device because it does not define a resonator and because it is short with respect to one-quarter of a wavelength. I have found that this type of structure inherently results in strong interaction in the fundamental forward-wave mode. Hence, forward-wave amplification can be achieved in an M-type device having a compact slow wave structure with small radial dimensions.

One limitation on the power capabilities of a device such as this is the heat generated in the vanes by a high power electron beam. Accordingly, it is a feature of another embodiment of the invention that the inner conductor of each straight section be directly connected at both ends to the outer conductor so that the outer conductor and associated elements can act as a heat sink for the vanes.

It is a feature of still another embodiment of this invention that the inner conductors of each section be hollow and be adapted to transmit water or some other appropriate coolant. With this type of cooling there is practically no limit to the quantity of electron beam power that can be used.

For some types of operation the short-circuit connection between the inner and outer conductors of the last two embodiments may result in comparatively weak coupling between adjacent coaxial cable sections. Accordingly it is a feature of still another embodiment of this invention that the inner and outer conductors be contube envelope.

ductively connected through a quarter-wavelength stub at each bend of the coaxial cable. The stub comprises a quarter-wavelength cylinder with an open end attached to the outer conductor, the other end being closed. The closed end is connected by a thermal conductor to the inner conductor so that heat can be transmitted to the outer conductor. This structure affords strong coupling between adjacent cable sections.

Although the invention was conceived for use in an M-type forward-wave amplifier as described above, it gives advantages that are useful and desirable in other microwave tubes. Most such tubes, for example, presently require a coupling device between the slow-wave structureand the input and output circuits; here, the slow-wave structure can be an integral part of a coaxial cable input or output circuit and no coupling devices are required.

Another advantage is the inherent broad band characteristics of coaxial cables. Other slow-wave structures, particularly the coupled resonator type, are not capable of transmitting energy over such a large frequency band. Secondly, the particular vane configuration results in a high interaction impedance with respect to the electron stream, which, as is known, gives stable operation at high gain.

Still another inherent advantage of coaxial cable structures is that virtually all of their transmitted wave energy is contained within the cylindrical outer conductor and is therefore shielded from surrounding elements .such as the This eliminates problems such as stray shunt capacitance which inevitably affects the propagation characteristics of conventional structures.

Of course it is necessary to modify the coaxial cable slow-wave structure before it can be used in backwardwave devices or O-type devices. It is a feature of an embodiment of this invention that the outer conductors of adjacent straight sections of a coaxial cable slow-wave structure be conductively connected and support therebetween a vane that extends toward the electron stream. The stream then alternately sees a vane at the outer conductor potential and a vane at the inner conductor potential. As will be shown later, this arrangement constitutes a backward-wave structure which is often desirable for use as an oscillator because of the characteristic of voltage tunability. When used as a backward-wave oscillator, the downstream end of the cable is terminated. Electromagnetic wave energy is excited in the cable which then propagates in a direction opposite that of beam fiow to be extracted at the upstream end of the cable. The output frequency depends on the velocity of the electron stream, which, in turn, depends on the electric field between the cathode and slow-wave structure. By adjusting this field one can conveniently adjust the output frequency.

In modifying the device for O-type operation, it is necessary to provide for the formation of predominately longitudinal radio-frequency fields which are fairly uniform throughout the beam. Accordingly, it is a feature of still another embodiment of this invention that an aperture be contained in each of the vanes of a coaxial cable slow-wave structure for permitting passage of a longitudinally focused electron stream. In an O-type device the electron stream follows a straight line between the cathode and a collector and no transverse fields are produced between the slow-wave structure and the cathode. With the structure described above, radio-frequency fields are produced between adjacent vanes which have a proper configuration for longitudinal space-charge wave electron stream modulation and interaction.

These and other features of the invention will be more fully appreciated from a consideration of the following detailed description, taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a cross-sectional view of one embodiment of the invention;

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

FIG. 3 is a development of the slow-wave structure of the device of FIG. 1;

FIG. 4 is a schematic illustration of an interdigital type slow-wave structure;

FIG. 5 is a schematic illustration of a forward-wave structure formed from a dual conductor transmission line;

FIG. 6 is a graph of frequency versus phase shift per section for the slow-wave structure of the device of FIG. 1 for different values of capacitance;

FIG. 7 is a development of another slow-wave structure illustrating another embodiment of this invention;

FIG. 8 is a development of a slow-wa ve structure illustrative of still another embodiment of the invention;

FIG. 9 is a development of a slow-wave structure illustrative of still another embodiment of the invention;

FIG. 10 is a partially schematic cross-sectional view of another embodiment of the invention; and

FIG. 11 is a partially schematic cross-sectional view of still another embodiment of the invention.

Referring now to FIGS. 1 and 2 there is shown an M-type forward-wave amplifier 11 comprising a cylindrical cathode 12 for forming and projecting a stream of electrons on an approximately circular path around the cathode. Surrounding the cathode is a slow-wave structure 14 biased at a positive direct-current potential with respect to the cathode. A magnetic field across the electron stream is maintained by a pair of pole pieces 15 and 16. An envelope 17 of soft iron or other suitable material forms a magnetic circuit between the pole pieces and maintains the electron stream in a substantial vacuum. The electric field between the cathode and slow-wave structure cooperates with the magnetic field to constrain the electron stream to follow a predetermined curvilinear path around the cathode. As is known, the electron velocity is a function of the direct-current electric field intensity.

Slow-wave structure 14 comprises a coaxial cable having a solid inner conductor 19 surrounded by ahollow cylindrical outer conductor 20. FIG. 3 is a development ofthe coaxial cable slow-wave structure and is included to show how it is periodically folded upon itself to form a series of straight sections connected by bended sections. At the middle of each straight section, a conductive vane 21 is connected to the inner conductor 19 and extends through a rectangular slot in outer conductor 20 toward the electron stream. This inner conductor is conveniently supported and separated from the outer conductor by a plurality of dielectric spacers 22.

The main purpose of slow-wave structure 14 is to transmit electromagnetic wave energy in approximate synchronism with the electron stream as is required for electron stream interaction. To this end, wave energy is transmitted at approximately the speed of light from the input end to the output end as indicated by the arrows in FIGS. 1 and 3. To the electron stream, however, the wave energy appears to travel from vane to vane at a much lower net velocity. Proper phase synchronism between the wave and the beam is maintained by making the electron stream velocity approximately equal to the net velocity of the wave energy. The electron stream flows in the same direction as the direction of the net velocity of the: wave energy, which in FIG. 2 is clockwise. Under these: conditions the radio-frequency fringing fields of the vanes; 21 will interact with the electron stream; this action results in an amplification of the wave energy propagated between inner and

outer conductors

19 and 20 of the coaxial cable. It should be noted parenthetically that the beam velocity is reduced during interaction and that adjustments in the electrical distance between adjacent vanes can be made to compensate for this reduction.

In the device of FIG. 1, strongest interaction isattained when the wave is transmitted in the same direction as the electron flow. This is because the electron stream of a traveling wave amplifier interacts most strongly with the fundamental of the Fourier space c0m-.

ponents of the wave and in the device of FIG. 1 the phase velocity of the fundamental component is in the same direction as the group velocity of the wave. Conversely, the phase velocity of the fundamental space component of most similar M-type slow-wave structures, such as the interdigital structure, has a direction opposite that of the wave group velocity. That is an important distinction because with a conventonal interdigital structure, for- Ward-wave operation is possible only by interaction of the stream with a relatively weak space harmonic of the Fourier space components, While backwand-wave interaction with the funamental component is inherently possible only over a relatively narrow frequency band. The following comparative analyses of interdigital and coaxial cable structures will further illustrate their differences.

It is known that the qualification of a slow-wave structure as a forward-wave type or a backward-wave type can be determined by the slope of its dispersion curve, i.e., a plot of frequency versus phase shift per section as shown by the graph of FIG. 6. If the curve slopes positively between and 1r radians per section, the fundamental space component is forward, and if it slopes negatively it has a backward direction. Referring now to FIG. 4, consider the voltages produced at points a through d by a wave propagating along a conventional interdigital structure. The wave meanders back and forth along the line as shown by the arrows. If the phase delay along one section of line (between successive designated points) is denoted by 6, then eb ab 6 d0= bc (2) where V is the complex voltage between points c and b, V is the complex voltage between points a and 12, et cetera.

Equation 1 can be written as bo ab Equation 3 in turn, can be rewritten as be ab At zero frequency 6 equals zero but the phase shift per section is 1r radians because of the reversal of the conductor taken as the voltage reference conductor. As the frequency rises, 6 increases and the phase shift per section drops, until at some finite value of frequency the phase shift per section becomes zero. When these values are graphed, they form a dispersion curve that slopes negatively, which demonstrates that the interdigital structure is fundamentally a backward-wave structure.

Next consider a slow-wave structure made by folding one wire conductor above a ground plane as shown in FIG. 5. Assume that the transmission line is excited such that a TEM Wave between the wire and the ground plane meanders back and forth along the line. Using the same form of notation as used above, and denoting the ground plane with the subscript g gives ab ug+ gb but V131: ers Hence Vnb= a Similarly bc bg go bc bz[ bc aEe U- l bo abe At zero frequency 6 equals zero and the phase shift per section is zero because there is no reversal of the voltage reference conductor. As frequency increases 5 increases until it equals 1r at some finite positive frequency. Hence, the dispersion curve is positive as shown in FIG. 6 and the structure is fundamentally forward-Wave. Next, imagine that the ground plane is made to surround the wire conductor as in a coaxial line. In order to maintain the forward-wave nature of the structure it becomes necessary to open the outer conductor at points a, b, c, et cetera, and to avoid intervention of the outer conductor between these points. This is, of course, conveniently accomplished by the vane structure of FIGS. 1-3. It can be appreciated, however, that other dual-conductor transmission lines could also be used in which the potential of one of the conductors were projected toward the beam only at successive and periodic discrete points.

A certain amount of inter-vane capacitance is inevitable in a structure of this type, and FIG. 6 shows different dispersion curves for different values of capacitance. In practice, the vanes should ordinarily be designed to minimize this capacitance because it tends to reduce the interaction impedance and bandwidth of the device. Also, it sometimes complicates design of the device by necessitating certain compensations such as a shorter coaxial cable length between successive vanes.

As mentioned above, one of the primary virtues of the embodiment of FIG. 1 is its convenient size, and particularly its relatively small radial dimensions at the lower microwave frequencies. The following specifications demonstrate this advantage. They are, however, presented only as an illustrative example and are not intended to limit the invention.

1215 to 1400 me. per sec. 25 kilovolts.

Frequency band Anode voltage With an operating frequency of less than 1.5 kilomegacycles and a diameter of less than 5 inches, the exemplary device gives forward-wave M-type amplification at a relatively low frequency without being unduly bulky.

It has been observed that the beam power capabilities of the device of FIG. 1 may be limited by the heat generated in the vanes. Extremely high temperatures can deform the comparatively thin vanes. Referring to the embodiment of FIG. 7, heat is drained from the vanes by connecting the inner conductor of the coaxial cable to the outer conductor at each end of each straight section. When this is done, the outer conductor and the magnetic circuit are thermally connected to the vanes and act as a heat sink. Adjacent straight sections are coupled by circular coupling apertures 24 rather than being directly connected as in FIG. 3. The other elements of FIG. 7 correspond to those of FIG. 3 and are numbered accordingly. Tight coupling between the sections can be attained by making them physically close and by using a large aperture to give high magnetic field penetration.

Even greater cooling is possible by making the inner conductor portions hollow and transmitting water or some other appropriate coolant through them as shown by the embodiment of FIG. 8. A water pump 26 circulates water through a pipe 27 to which a plurality of branch conduits 28 are connected in parallel. The branch conduits. also constitute inner conductors for coaxial cable slow-wave structure 14.

While the type of coupling between the cable sections of FIGS. 7 and 8 results in a short, simple structure it tends to reduce the attainable bandwidth. This potential disadvantage is obviated by the embodiment of FIG. 9 wherein successive straight sections are directly connected, rather than being magnetically coupled. The inner conductor is thermally connected to the outer conductor through a quarter wavelength stub 30. The stub 30 is a conductive cylinder having an open end connected to the outer conductor of the cable and a closed end connected by a thermal conductor 31 to the inner conductor 19. The short-circuit connection between conductor 31 and stub 39 essentially constitutes a zero impedance, but the wave propagating in the cable sees a high impedance at the stub connection because of the quarter wavelength phase difference between cable and the short-circuit connection. Hence, heat is drained from the vanes with a minimum amount of interference with the propagating wave.

Although the devices of FIGS. 1-9 are particularly suitable for M-type forward-wave operation, they also offer advantages that can be used in backward-wave and O-type devices. FIG. shows a linear M-type backward-wave oscillator 34 which usesa coaxial cable slowwave structure 35 that is periodically folded upon itself in the same manner as in the foregoing embodiments. FIG. 10 also illustrates how an M-type device can assume a predominately linear, rather than circular, geometry. A magnetic field B extending into the paper cooperates with a direct-current electric field between the slowwave structure and a sole electrode 36 to focus an electron stream flowing between them. The electron stream is formed and projected by a cathode 37 and is collected by a collector 38. A vacuumis maintained within the device by an evelope 40.

The coaxial cable comprises an outer conductor 41 surrounding an inner conductor 42. Vanes 43 are connected to the inner conductor at the mid-point of each straight section as in FIG. 1, and, in addition, vanes 44 are connected to the outer conductor 41 between each straight section. As a result, the potential of the outer conductor is interposed between the vanes of the inner conductor, making the slow-wave structure 35 basically backward-wave. This characteristic can be appreciated by comparing the structure 35 with the interdigital structure of FIG. 4 and its accompanying analysis.

The general principles of operation of the device of FIG. 10 are the same as those of known backward-wave oscillators. The electron stream excites currents in the vanes which excite a wave in the cable. The component of wave energy traveling in the direction opposite that of the electron stream and at some particular frequency couples much more strongly to the stream than other components and becomes amplified. This inherently selective process generates a fairly uniform output frequency which is removed at the upstream end of the slow-wave structure as shown. It is to be noted that if vanes 44 connected to the output conductor 41 were removed, the device would be fundamentally forward-wave as in FIG. 1.

FIG. 11 shows how my invention can be modified for use in an O-type device. An amplifier 45 comprises a cathode 46 for forming and projecting an electron stream toward a collector 47. A magnet 48 focuses the beam by producing a magnetic field B that is substantially parallel with the electron stream. Wave energy to be 8 amplified is propagated by a coaxial cable slow-wave structure 50 that is periodically folded upon itself in the same manner as in the foregoing embodiments. Vanes 51 are attached only to the inner conductor at the middle of each straight section which makes the cable a forward wave structure.

Convention-a1 space-charge wave interaction in an 0- type device differs from magnetron interaction in that only those electric fields that are parallel with the electron stream interact with it. Accordingly, apertures 52 are provided in each of the vanes so that the electron stream can travel through the vanesinstead of beside them. The radio-frequency fields between the vanes are then primarily parallel with the. stream as is needed for optimum space-charge wave interaction.

It is. clear from a consideration of all of the above embodiments that numerous other devices can employ the concepts of the present invention. For example, the O-type-device of FIG. 11 can be made into a backwardwave device by adding. vanes connected to the outer coaxial cable conductor as in FIG. 10. The device of FIG. 10 can take a circular geometry as in FIG. 1, if desired. The slow-wave structure. of the device of FIG. 11 could be helically wrapped around the electron stream as this would not affect the longitudinal modulating fields producedin the beam. It should also be pointed out that my invention is drawn to dual-conductor slowwave structures of which coaxial cables are, strictly speaking, only a species. It is not necessary, for example, that the inner and outer conductors be coaxial, or indeed, even parallel. It is only necessary that the electric field surrounding one of the conductors be projected toward the beam at appropriate discrete points and be shielded from the beam along the rest of its length so that the net velocity of the wave defined between adjacent projections corresponds to the velocity of the beam. Of course the coaxial cable is the most convenient dual-conductor transmission line to employ for this purposebecause it is so simple and so widely used. Numerous other modifications can be made by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. An electron discharge device comprising:

means for forming and projecting a stream of electrons;

means for transmitting electromagnetic wave energy in interacting relationship with the stream;

said transmitting means comprising a coaxial cable having inner and outer conductors and which is folded upon itself at periodic spatial intervals to provide a series of straight portions intermediate between bends;

a vane connected to the inner conductor at each straight portion of the cable and extending through a slot in the outer conductor to a point in proximity to said stream;

and a quarter wavelength stub connected to both the inner and outer conductors at each bend of the cable.

2. An electron discharge device comprising:

means for forming and projecting a stream of electrons;

means for transmitting electromagnetic wave energy in interacting relationship with the electron stream comprising a coaxial cable having inner and outer conductors;

the coaxial cable being periodically folded upon itself and comprising a series of straight sections;

all of the sections being substantially perpendicular to the direction of flow of the electron stream;

and a vane attached to the inner conductor at the middle of each straight section and extending through a slot in the outer conductor toward the electron stream.

3. The electron discharge device of claim 2 wherein:

the inner conductor is connected to the outer conductor at each end of each straight section, whereby heat is transmitted from each vane to the outer conductor via the inner conductor.

4. The electron discharge device of claim 3 wherein:

the inner conductor of each straight section comprises a hollow pipe;

and means for circulating a coolant through the hollow pipes, thereby cooling the vanes.

5. The electron discharge device of claim 2 wherein:

the outer conductors of adjacent straight sections are conductively connected and support a vane which extends from the conductive connection toward the electron stream;

and wherein the coaxial cable is adapted to transmit electromagnetic wave energy in a direction opposite to the direction of flow of the electron stream.

6. A magnetron-type device comprising:

a cylindrical cathode for emitting a stream of electrons;

a coaxial cable surrounding said cathode;

crossed-field focusing apparatus for focusing the electron stream and for constraining it to follow a curvilinear path around the cathode;

said apparatus comprising means for producing a magnetic field between the cable and the cathode in the same direction as the cathode axis, and means for maintaining the coaxial cable at a higher positive direct-current voltage than the cathode;

the coaxial cable being periodically folded upon itself and comprising a plurality of straight sections that are substantially parallel with the cathode axis;

each of the straight sections having a vane connected to the inner conductor of the coaxial cable which extends through the outer conductor to a point in proximity to the electron stream.

7. The device of claim 6 wherein:

the coaxial cable is adapted to transmit electromagnetic Wave energy in the same direction as the direction of flow of the electron stream; and further compriss;

means for transmitting heat from the vanes comprising a metallic connection between the inner and outer conductors at each bend of the coaxial cable.

8. The device of claim 7 further comprising:

cylindrical extensions connected to the outer conductor at each bend of the coaxial cable, each extension being approximately one-fourth wavelength long at the mean frequency of the electromagnetic wave energy;

the metallic connections connecting the inner conductor with the extreme end of the cylindrical extensions.

9. The device of claim 6 wherein:

the coaxial cable is adapted to transmit electromagnetic wave energy in a direction opposite that of the flow of the electron stream;

the outer conductors of adjacent straight sections being cond uctively connected an supporting therebetween a vane which extends to a point in proximity to the electron stream.

10. An electron discharge device comprising:

means for forming and projecting a stream of electrons along .a path;

means for producing a magnetic field along the path that is substantially parallel with the path;

a collector for collecting the stream;

said transmitting means comprising a dual-conductor transmission line having first and second conductors and which is folded upon itself at periodic spatial intervals;

and a vane connected to the first conductor at each straight portion of the transmission line and extending through a slot in the second conductor toward the path.

10 11. The electron discharge device of claim 10 wherein! all of the vanes have an aperture; and the path threads through all of the apertures. 12. The electron discharge device of claim 10 further comprising:

a quarter wavelength stub at each bend of the transmission line which is connected to both the first and second conductors.

13. An electron discharge device comprising:

means for forming and projecting a stream of electrons;

means for transmitting electromagnetic wave energy in interacting relationship with the electron stream comprising a serpentine-shaped dual-conductor transmission line having first and second conductors;

and a conductive vane attached at periodic spatial intervals to the first conductor and extending through an opening in the second conductor to a point in proximity to the electron stream.

14. The electron discharge device of claim 13 further comprising:

means for producing crossed electric and magnetic focusing fields along the electron stream path; and wherein:

the electron stream forming means comprises a substantially cylindrical cathode;

the transmission line surrounds the cathode;

and the electromagnetic wave energy is transmitted by the transmission line in the direction of beam flow.

15. The electron discharge device of claim 13 further comprising:

means for producing a magnetic focusing field along the electron stream path that is substantially parallel with the path;

an aperture in each of the vanes;

and wherein the electron stream follows substantially a linear path through each of the apertures.

16. In combination:

a source of electromagnetic wave energy;

a load;

means for transmitting wave energy comprising a coaxial cable connected at one end to the source and at the other end to the load;

means for amplifying the wave energy comprising apparatus for forming and projecting a stream of electrons;

the coaxial cable describing a tortuous path along an interaction region proximate to the electron stream;

and a conductive vane attached at perodic spatial intervals to the inner conductor of the cable and extending through an opening in the outer conductor toward the electron strea 17. In combination:

a source of electromagnetic wave energy;

a load;

the coaxial cable being periodically folded upon itself along an interaction region proximate to the electron stream and comprising a series of straight sections .all of which are substantially perpendicular to the direction of flow of the electron stream;

and a vane attached to the inner conductor of the coaxial cable at the middle of each straight section and extending through an opening in the outer conductor toward the electron stream.

18. The combination of claim 17 further comprising:

a conductive cylinder attached to the outer conductor at each bend of the coaxial cable;

the cylinders each having an open end attached to the outer conductor and a closed end connected through an opening in the outer conductor to the iner conductor.

19. In combination:

a source of electromagnetic wave energy;

a load;

means for amplifying the wave energy comprising apparatus for forming and projecting a stream of electrons; I

the coaxial cable being periodically folded upon itself along an interaction region proximate to the electron stream thereby to form a series of straight sections connected by a series of bended portions;

a vane attached to the inner conductor at the middle of each straight section and extending through an opening in the outer conductor toward the electron stream;

and an auxiliary conductor connected at one end to the outer conductor of the coaxial cable at each of the bended portions;

each of the auxiliary conductors being approximately one-quarter wavelength long at the frequency of the electromagnetic wave energy and being connected at its other end to the inner conductor of the cable.

20. An electron discharge device comprising:

means for forming and projecting a stream of electrons;

said transmitting means comprising a pair of substantially parallel conductors which are spatially periodically bended to describe a tortuous path along an interaction region in proximity to the stream;

and a vane connected to one of the conductors between each of the bends, the vane extending toward the electron stream;

the distance along the transmitting means between successive vanes being substantially one-half wavelength at the frequency of the electromagnetic wave energy;

and the protrusion of the vanes toward the electron stream being substantially less than one-fourth wavelength at the aforementioned frequency.

References Cited by the Examiner UNITED STATES PATENTS 2,988,669 6/1961 Collier et a1 3153.5 X

25 GEORGE N. WESTBY, Primary Examiner.

Claims

2. AN ELECTRON DISCHARGE DEVICE COMPRISING: MEANS FOR FORMING AND PROJECTING A STREAM OF ELECTRONS; MEANS FOR TRANSMITTING ELECTROMAGNETIC WAVE ENERGY IN INTERACTING ELECTROMAGNETIC WAVE ENERGY COMPRISING A COAXIAL CABLE HAVING INNER AND OUTER CONDUCTORS; THE COAXIAL CABLE BEING PERIODICALLY FOLDED UPON ITSELF AND COMPRISING A SERIES OF STRAIGHT SECTIONS; ALL OF THE SECTIONS BEING SUBSTANTIALLY PERPENDICULAR TO THE DIRECTION OF FLOW OF THE ELECTRON STREAM; AND A VANE ATTACHED TO THE INNER CONDUCTOR AT THE MIDDLE OF EACH STRAIGHT SECTION AND EXTENDING THROUGH A SLOT IN THE OUTER CONDUCTOR TOWARD THE ELECTRON STREAM.