US3181024A

US3181024A - Traveling-wave tube with oscillation prevention means

Info

Publication number: US3181024A
Application number: US196938A
Authority: US
Inventors: Sensiper Samuel
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1962-05-23
Filing date: 1962-05-23
Publication date: 1965-04-27
Anticipated expiration: 1982-04-27

Description

April 7, 1965 s. SENSIPER 3,181,024

TRAVELING-WAVE TUBE WITH OSCILLATION PREVENTION MEANS 2 Sheets-Sheet 2 Filed May 23, 1962 .Zzar;

z za 9 4 5144M; smsmsn 3,181,624 TRAVELII'lG-WAVE TUBE WITH GSQHJL Eight PREVENTlGN MEANS Samuel Sensiper, Los Angeles, Calili, assignor to Hughes Company, Culver City, Qallh, a corporation of Delaware Filed May 23, 1962, Ser. No. 196538 18 (liairns. (ill. 315-35) This invention relates generally to microwave devices, and more particularly relates to traveling-wave tubes having means for substantially eliminating oscillations in the vicinity of a predetermined frequency, such as the upper cutoff frequency of the tube passband.

In traveling-wave tubes a stream of electrons is caused to interact with a propagating electromagnetic wave in a manner which amplifies the electromagnetic energy. In order to achieve such interaction, the. electromagr etic wave is propagated along a slow-Wave structure, such as a conductive helix wound about the path of the electron stream or a folded waveguide type of structure in which a waveguide is effectively wound back and forth across the path of the electrons. The slow-wave structure provides a path .of propagation for the electromagnetic wave which is considerably longer than the axial length of the structure, and hence, the traveling-wave may be made to effectively propagate at nearly the velocity of the electron stream. The interactions between the electrons in the stream and the traveling-wave cause velocity modulations and bunching of electrons in the stream. The net result may then be a transfer of energy from the electron beam to the wave traveling along the slow-Wave structure. V

The present invention is primarily concerned with traveling-wave tubes utilizing slow-wave structures of the folded waveguide type, which structures are also known as the coupled cavity, or interconnected cell type. in this type of slow-wave structure a series of interaction cells, or cavities, are disposed adjacent to each other sequentially along the axis of the tube. The electron stream passes through each interaction cell, and electromagnetic coupling is provided between each cell and the electron stream. Each interaction cell is also coupled to an adjacent cell by means of a coupling hole at the end wall defining the cell. Generally, the coupling holes between adjacent cells are alternately disposed on opposite sides of the axis of the tube, although various other arrangements for staggering the coupling holes are possible and have been employed. When the coupling holes are so arranged, a folded waveguide type of energy propagation results, with the traveling-wave energy traversing the length of the tube by entering each interaction cell from one side, crossing the electron stream, and then leaving the cell from the other side, thus traveling a sinuous, or serpentine, extended path.

One of the problems encountered in traveling-wave tubes of the coupled cavity variety, and especially high power tubes of this type, is a tendency for the tube to oscillate at frequencies near the upper cutofi frequency of the tube passband. This problem arises from the fact that for wideband operation, the phase velocity of the slow-wave circuit wave and the velocity of the electron beam should be essentially synchronized over as large a range of frequencies as possible; hence, these velocities are also close to synchronism near the upper cutoff frequency. Since the interaction impedance is high and the circuit-to-transmission line match is poor at and inthe vicinity of the upper cutoff frequency, the loop gain for the tube, or even for a section of the tube, may be suf ficiently large for oscillations to start.

It is, therefore, an object of the present invention to provide a coupled cavity traveling-wave tube in which any tendency for the tube to oscillate in the vicinity of the upper cutoff frequency of the frequency passband of the tube is substantially eliminated.

In accordance with the objective stated above, the

traveling-wave tube of the present invention includes means for launching a stream of electrons along a predetermined path of fixed length and a coupled-cavity slow- Wave structure disposed along and about the electron stream path forpropagating an electromagnetic wave in such manner as to provide energy exchange between the electron stream and the electromagnetic wave. Electric field modifying means, preferably in the form of an electrically conductive ring concentrically disposed about the electron stream in a plane perpendicular to the electron stream path, is provided in the interaction gap of each slow-wave structure cavity. The electric field modifying means is made resonant at a preselected frequency, preferably the upper cutoff frequency of the tube passband. This reduces the electric field along the electron stream path in the interaction gap at the preselected frequency, which in turn reduces the interaction impedance section, of a traveling-wave tube constructed in accordance with the present invention; 7 q

FIG. 2 is a cross-sectional view taken along line 2-2 of FIG. 1;

FIG. 3 is a cross-sectional view, similar to FIG. 2, illustrating an alternate embodiment of the present invention; 1

FIG. 4 is a longitudinal sectional View taken along line ll of FIG. 3; g v

FIGS. 5, 6 and 7, respectively, are longitudinal views, similar to FIG. 4, illustrating further embodiments of the present invention;

FIG. 8 is a cross-sectional view, similarto FIG, 2, illustrating a still further embodiment of t e present invention; and

FIGS. 9(a)(b) are graphs of interaction impedance as a function of frequency which are used in explaining the theory of the present invention. 7

Referring with more particularity to the drawings,. in

FIG. 1 there is shown a traveling-wave tube it) in which a coupled cavityslow-wave structure 12 is utilized for. propagating m crowave energy in a serpentine path about an electron beam traveling along the longitudinal axis of the tube iii. The slow-wave structure 12 comprises a series of circular vane members, or plates, 14 which are disposed perpendicular to the longitudinal axis of the tube lit and are mounted in a cylindrical waveguide 16 which serves as an envelope for the slow-wave structure 12., The vane members 14, as well as the envelope 16, are composed of a nonmagnetic electrically conductive material, such as copper or, brass. The vanes 14 are disposed at spaced points along the envelope 16 to provide a series of interaction cells, or cavities, 20. Each cavity 2t? is defined by the Walls of the two adjacent vanes l4 and that portion of the inner surface of thewaveguide 15 located between these two vanes, Adjacent cavities 2d are interconnected by means of an olf-center coupling hole 18 provided through the intermediate vane 14 to permit the transfer of microwave energy from cell to cell along the slow-wave structure 12. As is illustrated, the coupling holes lil'may be substantially kidney-shaped 14, i.e., in bothdirections normal to the plane'of the vane V 14. The drift tubes 22 are provided with centrally and axially aligned apertures 24 to provide a passage for the flow of the electron beam. Adjacent ones. of the drift tubes 22 are separated by an interaction gap 26 in which energy exchange between the electron stream and microwave energy traversing the slow-wave structure occurs.

- At one end of the slow-Wave structure 12, there is disposed a conventional electron gun 28 which may include an electron emitting cathode 30 and filamentary heater 32. The emitted electrons are shaped and focused into a beam by a focusing electrode 34 and are accelerated to a high velocity by accelerating anode 36. A voltage source 38 is tapped at appropriate potentials, as shown,

to provide operating voltages for. the

electrodes

36, 34

and 36.

A collector electrode 40 is disposed at the opposite en of the slow-wave structure 12 to intercept the stream of electrons and dissipate their kinetic energy. A source of potential 42 is connected to the collectordt) to bias the collector 40 positive to prevent secondarily emitted electrons from the slow-wave structure 12 fromv returning to the interaction region, thereby reducing noise and other interference.

An input waveguide 44 is connected to the input'end of the slow-wave structure 12 which, although illustrated as the electron gun end in FIG. 1, may alternately be the collector end if a backward wave device is desired. The waveguide 44 -is connected to external circuitry (not shown) by a coupling flange 46, which may include a microwave window to enable a pressure differential to exist between the exterior and the evacuated interior of the travelingwave tube 10. Similarly, an output waveguide 48 is connected to the opposite end of the slow-wave structure 12. A coupling flange 50, which may be similar to the flange 46 described above, is provided at the end ofthewaveguide 48 for coupling to external circuitry (not shown). A solenoid 54, energized by a source of potential 52, is disposed concentrically about and substantially coextensive with the low wave structure 12 for providing a longitudinal focusing magnetic field which constrains the electron beam to flow along an axial path toward the collector electrode 40.

I will now be explained with reference to FIG. 9. In this figure'there are illustrated graphs of the interaction "impedance Z in a gap 26 and on the tube axis plotted as a function of the frequency of the interacting microwaves. The curve 160 intFlG. 9(a) shows the interaction impedance for a coupled-cavity traveling-wave tube of the type shown in FIG. 1 but without the inclusion of the resonantrings 60. It will be observed that the interaction impedance is amaxirnum (theoretically approaching infinity) at both the upper and lower cutoff frequencies ta and m respectively.

According to the accepted definition the interaction impedance is given by the relation Z=E /2B Wv where E is the axial electric field of the nth space harmonic in the interaction gap, {i is the propagation constant of the nth space harmonic of the traveling microwave, W is the stored energy per unit length, and 12 is the group velocity ofthe microwave. Normally at the upper cutoff frequency w the group velocityv becomes zero, and hence the interaction impedance theo retically approaches infinity, as illustrated in FIG. 9(a). However, with the inclusion of the resonant ring structure, the axial electric'field across the gap. 26 at the resonant frequency w is drawn outwardly to the ring 69, thus reducing to essentially zero the electric field in the center of the gap, i.e., at the location of the electron beam. Since the interaction impedance varies directly as the square of the electric field E and inversely as the group velocity v the interaction impedance goes to zero when the electric field goes to zero, even though the group velocity also approaches zero. Thus, as is illustrated by the curve 102- in FIG. 9(b), the resonant rings cause the interaction impedance Z to be minimum (theoretically approach zero) at the upper cutoff frequency w By reducing the interaction impedance in the vicinity of the upper cutoff frequency, while maintaining it essentially at its normal value throughout substantially the remainder of the tube passband, any tendency for the tube to oscillate at the upper cutoff frequency is substantially eliminated.

Inorder to extend the frequency range of influence of the resonant rings, a coating of lossy material, for example kanthal, may be applied to either the rings 60, the

a supporting webs 62 and 64, or both, in order to reduce the Q of the resonance and thereby broaden the range I 50 In order to minimize any tendency for the tube to osciltrically conductive ring 60 is concentrically mounted in each interaction gap 26 in a plane perpendicular to the electron beam path. The rings 60 are made resonant at a predetermined frequency, preferably the upper cutoff frequency of the tube passband, by making the circumference of each ring equal to one Wavelength corresponding to the desired resonant frequency. For typical traveling-wave tube frequencies this results in the rings 60 having a circumference slightly greater than that of the drift-tubes 22.

As is shown in FIG. 2, each ring 60 is supported at diametrically opposite points 66 and 68 by a pair of elecnected to and project inwardly from the envelope 16. The support webs 62 and 64 are substantially sector-shaped, with the, sector angle being selected experimentally for trically conductive webs 62 and 64, which webs are con- V of frequencies affected- A modified form of the resonant ring structure of th present invention is illustrated in FIGS. 3 and 4. In this embodiment the resonant ring 6f) is supported by a pair of electrically conductive rods 72 and '74 projecting inwardly from the envelope .16. Coupling probes 76and 78, disposed parallel to the longitudinal axis of the tube, are provided at the inner ends of the rods 72 and '74, respectively, adjacent the ring 6t). As illustrated in FIG. 4, the coupling probes '76 and 78 extend in opposite directions along the axis of the tube from the plane containing the ring Gil. The coupling probesensure maximum cur.- rent flow on the ring at resonance. in order to achieve 'maximum excitation of the ring at w In the embodiment shown in FIG. 5, the resonant ring as is mounted on a pair of longitudinally extending rods 82 and 84., The

rods

82 and 84 are disposed parallel to the tube axis and extend from opposite faces of the ring 60 to the .vane members 14 defining the cavity 2i) in which the ring oil is located, the

rods

82 and 84 being located on opposite sides of the axis of the tube. The rods 84 perform the dual function of supportingthe resonant ring 6% and also serving as coupling probes to ensure maximum excitation of the ring at its resonant frequency.

Although the introduction of the resonant ring structure advantageously serves to reduce the interaction impedance at the upper cutofi frequency, there may also be a slight narrowing of the passband of the fundamental mode due to a lowering of the upper cutoff frequency. One Way to maintain the upper cutoff frequency ca of the tube at the desired value is to make the resonant frequency a of the ring 6%? occur at a value slightly higher than ca Alternately, additional capacitance may be added to the low-wt-ve circuit by the structure of FIG. 6. For this purpose annular ridges 85 are provided on the vanes 14, which ridges project into the cavities at points radially outwardly of the

ring support rods

82 and 84.

Another way of introducing the additional capacitance necessary to maintain the desired upper cutofi frequency of the tube passband is illustrated in FIG. 7. In this embodiment drift tubes 22 are constructed with first tubular steps $7 of increased outer circumference and second tubular steps 39 of the same inner circumference but a smaller outer circumference than the steps 8'7 projecting beyond the steps S7 into the gaps 26. The

rods

82 and 84 which support the resonant ring 69 are connected to the inner steps 87 of the drift tubes 22.

A further embodiment of the present invention is illustrated in FIG. 8, The resonant ring as is supported in the gap 26 by an annular disk 90 of dielectric material which extends from the inner circumference of the envelope to to the outer circumference of ring 6%.

It should be understood, of course, that the foregoing disclosure relates only to preferred embodiments of the invention, and that numerous modifications or alterations may be made therein withoutdeparting from the spirit and scope of the invention as set forth in the appended claims.

I claim:

1. A traveling-wave tube comprising: means for providing a stream of electrons along a predetermined path, slow-wave structure means disposed along and about said path for propagating electromagnetic Wave energy within a predetermined frequency range in such manner as to provide energy exchange between said electron stream and said electromagnetic wave energy in energy exchange regions along said path, and resonant electrically conductive means disposed in proximity to said stream and adjacent at least one of said energy exchange regions for modifying the electric field in said one region of electromagnetic wave energy within a small portion of said predetermined'frequency range Without substantially affecting the electric field of electromagnetic wave energy within the remainder of said predetermined frequency range.

2. A traveling-wave tube comprising: means for providin a stream of electrons along a predetermined path, slow-wave structure means disposed along and about said path for propagating electromagnetic wave energy within a predetermined frequency range in such manner as to 7 provide energy exchange between said electron stream and said electromagnetic Wave energy in energy exchange regions along said path, and resonant electrically conductive means disposed in proximity of said stream and adjacent at least one of said energy exchange rgeions for reducing the interaction impedance of said electron stream with electromagnetic wave energy within a small portion of said predetermined frequency range without substantially affecting the interaction impedance of said electron stream with electromagnetic wave energy Within the remainder of said predetermined frequency range.

3. A traveling-wave tube comprising: means for providing a stream of electrons along a predetermined path, slow-wave structure means defining a plurality of intercoupled cavities disposed sequentially along and about said path for propagating electromagnetic wave energy within a predetermined frequency range in such manner as to provide energy exchange between 'said electron stream and saidelectromagnetic wave energy, and elecbrically conductive means disposed in at least one of said cavities in proximity to said stream and resonant at a preselected frequency'within substantially said predetermined frequency range for reducing the electric field along said path in said one cavity of electromagnetic wave energy within a small portion of said predetermined frequency range encompassing said preselected frequency without substantially aifecting the electric field of electromagnetic wave energy Within the remainder of said predetermined frequency range.

4. A traveling-wave tube comprising: means for providing a stream of electrons along a predetermined path, slow-wave structure means defining a plurality of intercoupled cavities disposed sequentially along and about said path for propagat ng electromagnetic wave energy within a predetermined frequency range in such manner as to provide energy exchange between said electron stream and said electromagnetic wave energy, and an element of electrically conductive material mounted in l at least one of said cavities adjacent said predetermined path, said element being resonant at a preselected frequency within substantially predetermined frequency range.

5. A traveling-Wave tube according to claim 4 wherein said element is coated with a lossy material.

6. A traveling-wave tube comprising: means for pro viding a stream of electrons along a predetermined path, slow-wave structure means defining a plurality of intercoupled cavities disposed sequentially along and about said path for propagating electromagnetic wave energy within a predetermined frequency range in such manner as to provide energy exchange between said electron stream and said electromagnetic Wave energy, and an electrically conductive ring mounted in at least one of said cavities and being concentrically disposed about said electron stream in a plane perpendicular to said predetermined path, said ring being resonant at a preselected frequency within substantially said predetermined frequency range.

7. A traveling-wave tube comprising: means for providing a stream of electrons along a predetermined path, slow-wave structure means defining a plurality of intercoupled cavities disposed sequentially along and about said path for propagating electromagnetic wave energy within a predetermined frequency range in such manner as to provide energy exchange between said electron stream and said electromagnetic wave energy, and an element of electrically conductive material mounted in selected ones of said cavities adjacent said predetermined path, each said element being resonant at a frequency in the vicinity of the upper cutoff frequency of said predetermined frequency range.

8. A traveling-wave'tube comprising: means for providing a stream of electrons along a predetermined path, slow-wave structure means defining a plurality of intercoupled cavities disposed sequentially along and about said path for propagating electromagnetic wave energy within a predetermined frequency range in such manner as to provide energy exchange between said electron stream and said electromagnetic wave energy, a pair of electrically conductive support elements aflixed to said slow-wave structure means and projecting therefrom into at least one of said cavities on opposite sides of said electron stream, and an electrically conductive ring mounted on the ends of said support elements remote from said slow-Wave structure means, said ring being concentrically disposed about said electron stream in a plane perpendicular to said predetermined path and having a circumference equal to a preselected wavelength corresponding to a frequency Within substantially said predetermined frequency range.

9. A traveling-wave tube according to claim 8 wherein said support elements are coated with a lossy material.

16. A'traveling-wave tube comprising: an envelope, means disposed adjacent one end ofsaid envelope for envelope and cooperating therewith to define a plurality of intercoupled cavities disposed sequentially along and about said path for propagating electromagnetic waveenergy within a predetermined frequency range in such.

manner as to provide energy exchange between'said electron stream. and said electromagnetic wave energy, a pair of electrically conductive support elements afiixed to said envelope and projecting inwardly therefrom on opposite sides of said electron stream in at least one of said cavities, and an electrically conductive ring mounted on the inner ends of said support elements, said ring being concentrically disposed about said electron stream in a plane perpendicular to said predetermined path and having a circumference equal to 'a preselected wave-length corresponding to a frequency within substantially said predetermined frequency range.

11. A traveling-wave tube according to claim wherein said support elements are substantially sector-shaped webs of a predetermined sector angle.

12. A traveling-wave tube according to claim 10 wherein said support elements are parallel rods disposed per- 7 pendicular to said predetermined path.

13. A traveling-wave tube according to claim 10 wherein conductive probes are disposed parallel to said electron stream path at said inner ends of said support elements.

14. A traveling-wave tube comprising: an envelope, means disposed adjacent oneend of said envelope for launching a stream of electrons along a predetermined path within said envelope, means disposed within said envelope and cooperating therewith to define a plurality of intercoupled cavities disposed sequentially along and about said path for propagating electromagnetic wave energy within a predetermined frequency range in such manner as'to provide energy exchange between said electron stream and said electromagnetic wave energy, a di electric plate defining a central circular aperture afilxed to said envelope and projecting inwardly therefrom in at least one of said cavities, and an electrically conductive ring mounted in said aperture and being concentrically disposed about said electron stream in a plane perpendicular to said predetermined path, said ring having a circumference equal to a preselected wave-length corresponding to a frequency within substantially said predetermined frequency range.

15. A traveling-wave tube comprising: means for providing a stream of electrons along a predetermined path, an electrically conductive envelope disposed about and axially, aligned with said path, a plurality of electrically conductive plates mounted perpendicular to said path at spaced points along said envelope to define therewith a plurality of cavities, said plates defining aligned apertures in their central regions to provide a passage for said electron stream and further defining coupling apertures in regions radially outwardly of said central regions for inremote from said pair of plates, said ring being concenviding a stream of electrons along a predetermined path,

an electrically conductive envelope disposed about and axially aligned with said path, a plurality of electrically conductive plates mounted perpendicular to said path at spaced points along said envelope to define therewith a plurality of cavities, said plates defining aligned apertures in their central regions to provide a passage for said electron stream and further defining coupling apertures in regions radially outwardly of said central regions for interconnecting adjacent cavities whereby a propagation path is provided for electromagnetic Wave energy within a predetermined frequency range in a manner to provide energy exchange between said electron stream and said electromagnetic wave energy, each of said plates defining a .tubular portion adjacent its central aperture which projects beyond the remainder of said plate on at least one side thereof in a direction along said electron stream path whereby an interaction region is provided in the cavity into which said tubular portion projects of an axial length less than the axial length of said cavity, and a ring of electrically conductive material concentrically mounted about said electron stream in said interaction region and in a plane perpendicular to said electron stream path, the circumference of said ring being equal to a preselected wavelength corresponding to a frequency within substantially said predetermined frequency range and being slightly greater than the circumference of said tubular portion.

17. A travelin wave tube comprising: means for providing a stream of electrons along a predetermined path, an electrically conductive envelope disposed about and axially aligned with said path, a plurality of electrically conductive plates mounted perpendicular to said path at spaced points along said envelope to define therewith a plurality of cavities, said plates defining aligned apertures in their central regions to provide a passage for said electron stream and further defining coupling apertures in regions radially outwardly of said central regions for interconnecting adjacent cavities whereby a propagation path is provided for electromagnetic wave energy within a predetermined frequency range in a manner to provide energy exchange between said electron stream and said electromagnetic wave energy, each of said plates defining a tubular portion adjacent its central aperture which projects beyond the remainder of said plate on at least one side thereof in a direction along said electron stream path whereby an interaction region is provided in the cavity into which said tubular portion projects of an axial length less than the axial length of said cavity, at least a pair of electrically conductive support elements respectively afiixed to a pair of adjacent plates at points radially outwardly of said tubular portions thereof, said support elements projecting from said pair of plates on opposite sides of said electron stream into the cavity bounded by said pair of plates, each plate of said pair of plates further defining an annular ridge portion concentrically disposed about said tubular portion thereof and projecting from said plate in the direction in which said support element afiixed to said plate projects, and an electrically conductive ring mounted on the ends of said support'ele ments remote from said pair of plates, said ring being concentrically disposed about said electron stream in a plane perpendicular to said electron stream path and hav ing a circumference equal toa preselected wavelength corresponding to a frequency within substantially said predeterminedfrequency range.

18. A traveling-Wave tube comprising: means for pro- I conductive plates mounted perpendicular to said path at spaced points along said envelope to define therewith a plurality of cavities, said plates defining aligned apertures in their central regions to provide a passage for said electron stream and further defining coupling apertures in regions radially outwardly of said central regions for interconnecting adjacent cavities whereby a propagation path is provided for electromagnetic wave energy within a predetermined frequency range in a manner to provide energy exchange between said electron stream and said electromagnetic Wave energy, each of said plates defining a first tubular portion adjacent its central aperture Which projects beyond the remainder of said plate on at least one side thereof in a direction along said electron stream path and a second tubular portion of the same inner circumference and smaller outer circumference than said first tubular portion projecting beyond said first tubular portion in said direction whereby an interaction region is provided in the cavity into which said first and second tubular portions project of an axial length less than the axial length of said cavity, at least a pair of electrically conductive support elements respectively afiixed to the first tubular portions of the plates defining said cavity, said support elements projecting from said first tubular portions on opposite sides of said electron stream into said cavity, and an electrically conductive ring mounted on the ends of said support elements remote from said first tubular portions, said ring being concentrically disposed about said electron stream in a plane perpendicular to said electron stream path and having a circumference equal to a preselected Wavelength corresponding to a frequency within substantially said predetermined frequency range.

References Cited by the Examiner UNITED STATES PATENTS 2,885,641 5/57 Birdsall et al 3153.5 X 2,939,993 6/60 Zublin et al. 3338l X 2,967,968 1/61 Nalos 3l55.39 X

GEORGE N. WESTBY, Primary Examiner.

ARTHUR GAUSS, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No a 3 181, 024 April 27, 1965 Samuel Sensiper It is hereby certified that error appears in the above numbered patent requiring correction and th at the said Letters Patent should read as correotedbelom Column 3, line 48, for "low wave" read column 5, line 60, for "of" read to after "substantially" insert said slow-wave column 6, line 22,

Signed and sealed this 12th day of October 1965.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Allosting Officer Commissioner of Patents

Claims

18. A TRAVELING-WAVE TUBE COMPRISING: MEANS FOR PROVIDING A STREAM OF ELECTRONS ALONG A PREDETERMINED PATH, AN ELECTRICALLY CONDUCTIVE ENVELOPE DISPOSED ABOUT AND AXIALLY ALIGNED WITH SAID PATH, A PLURALITY OF ELECTRICALLY CONDUCTIVE PLATES MOUNTED PERPENDICULAR TO SAID PATH AT SPACED POINTS ALONG SAID ENVELOPE TO DEFINE THEREWITH A PLURALITY OF CAVITIES, SAID PLATES DEFINING ALIGNED APERTURES IN THEIR CENTRAL REGIONS TO PROVIDE A PASSAGE FOR SAID ELECTRON STREAM AND FURTHER DEFINING COUPLING APERTURES IN REGIONS RADIALLY OUTWARDLY OF SAID CENTRAL REGIONS FOR INTERCONNECTING ADJACENT CAVITIES WHEREBY A PROPAGATION PATH IS PROVIDED FOR ELECTROMAGNETIC WAVE ENERGY WITHIN A PREDETERMINED FREQUENCY RANGE IN A MANNER TO PROVIDE ENERGY EXCHANGE BETWEEN SAID ELECTRON STREAM AND SAID ELECTROMAGNETIC WAVE ENERGY, EACH OF SAID PLATES DEFINING A FIRST TUBULAR PORTION ADJACENT ITS CENTRAL APERTURE WHICH PROJECTS BEYOND THE REMAINDER OF SAID PLATE ON AT LEAST ONE SIDE THEREOF IN A DIRECTION ALONG SAID ELECTRON STREAM PATH AND A SECOND TUBULAR PORTION OF THE SAME INNER CIRCUMFERENCE AND SMALLER OUTER CIRCUMFERENCE THAN SAID FIRST TUBULAR PORTION PROJECTING BEYOND SAID FIRST TUBULAR PORTION IN SAID DIRECTION WHEREBY AN INTERACTION REGION IS PROVIDED IN THE CAVITY INTO WHICH SAID FIRST AND SECOND TUBULAR PORTIONS PROJECT OF AN AXIAL LENGTH LESS THAN THE AXIAL LENGTH OF SAID CAVITY, AT LEAST A PAIR OF ELECTRICALLY CONDUCTIVE SUPPORT ELEMENTS RESPECTIVELY AFFIXED TO THE FIRST TUBULAR PORTIONS OF THE PLATES DEFINING SAID CAVITY, SAID SUPPORT ELEMENTS PROJECTING FROM SAID FIRST TUBULAR PORTIONS ON OPPOSITE SIDES OF SAID ELECTRON STREAM INTO SAID CAVITY, AND AN ELECTRICALLY CONDUCTIVE RING MOUNTED ON THE ENDS OF SAID SUPPORT ELEMENTS REMOTE FROM SAID FIRST TUBULAR PORTIONS, SAID RINGS BEING CONCENTRICALLY DISPOSED ABOUT SAID ELECTRON STREAM IN A PLANE PERPENDICULAR TO SAID ELECTRON STREAM PATH AND HAVING A CIRCUMFERENCE EQUAL TO A PRESELECTED WAVELENGTH CORRESPONDING TO A FREQUENCY WITHIN SUBSTANTIALLY SAID PREDETERMINED FREQUENCY RANGE.