Traveling-wave tube

S. SENSIPER

TRAVELING-WAVE TUBE

2,806,972

Sheets-Sheet 1

Sept 17, 1957

Filed Dec. 8, 1954

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Zwwraf J'YA'S/YYy, &amp;

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Sept. 17, 1957

2,806,972

S. SENSIPER

TRAVELING-WAVE TUBE

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Sept. 17, 1957

S. SENSIPER

TRAVELING-WAVE TUBE

2,806,972

Filed Dec. 8, 1954

Sheets-Sheet 3

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2,806,972

Patented Sept. 17, 1957

United States Patent Office

2,806,972

TRAVELING-WAVE TUBE

Samuel Sensiper, Los Angeles, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware

Application December 8,1954, Serial No. 473,856

Claims. (Cl. 315—3.5)

This invention relates to micro-wave tubes and more particularly to a traveling-wave tube incorporating a slowwave circuit constituting a periodic waveguide device having unidirectional or selective attenuation characteristics.

In the conventional traveling-wave tube, a slow-wave structure is generally employed to propagate an electromagnetic signal wave along the path of an electron stream at a velocity to effect interaction between the electromagnetic wave and the electron stream. In that this type of tube provides gain over a broad range of frequencies, it is very difficult to provide an output circuit with an impedance that matches that of the slow-wave structure over this entire range of frequencies. This results in a portion of the energy of the electromagnetic waves appearing at the output circuit being reflected and being propagated back towards the input circuit by the slow-wave structure. When this reflected portion of the output wave arrives at the input circuit, a portion of its energy may again be reflected and propagated along with the electron stream towards the output circuit. This last reflected portion of the electromagnetic wave is progressively amplified along its return path. When this reflected portion is amplified to the extent that it has a magnitude greater than that of the initial wave appearing at the output circuit, the tube is said to be in self-oscillation. Thus, it is apparent that a traveling-wave tube may self-oscillate at any frequency where the gain is greater than the reflection and propagation losses through the tube.

In accordance with the present invention, a travelingwave tube is provided wherein the tendency to selfoscillate is substantially eliminated whereby gain and power output may be increased. Self-oscillations are eliminated by employing a periodic waveguide device having unidirectional or selective attenuation characteristics for the slow-wave structure of the tube. These unidirectional or selective attenuation characteristics are achieved by utilizing the gyroresonance properties of ferrite materials in conjunction with the magnetic field normally employed for constraining the electron stream. Properties of ferrite materials of the gyroresonance type are described in an article entitled, “Ferrites in microwave applications,” by J. H. Rowen, which appears on pages 1333-1369 of the Bell System Technical Journal for November 1953, published in New York.

In a first embodiment of the invention, a folded waveguide structure is employed to propagate an electromagnetic wave along a path through the central portion of each waveguide section. In order to provide this structure with unidirectional attenuating characteristics, slabs of ferrite material are disposed within the sections of the waveguide normal to the path of the electron stream, the successive slabs being spaced from the wall of the guide on alternate sides of the electron stream. A single longitudinal magnetic field is then developed to direct the electron stream along the path and to produce gyroresonance within the slabs of ferrite material at frequencies where the unidirectional attenuating characteristics are desired. The frequency at which gyroresonance with

IS its concomitant maximum attenuation occurs depends on the magnetic field within the ferrite material. Thus, the frequencies at which unidirectional attenuation is desired may be staggered throughout the band wherein gain is provided by controlling the magnetic field within the individual ferrite slabs. In that the same magnetomotive force energizes all the ferrite slabs, the magnetic field through any one slab may be determined by its thickness or lessened by a magnetic shunt. In addition, it may be desirable to progressively increase the coupling of the ferrite slabs to the wave being attenuated so as to distribute the heat dissipation uniformly over all of the slabs. This may be accomplished by progressively increasing the spacing between the ferrite slabs and the wall of the waveguide.

An alternative embodiment of the present invention is similar to that described above except that the ferrite slabs are spaced contiguous to the wall of the guide so as to attenuate waves propagated by the guide equally in both directions. This attenuation, however, is effected only for frequencies on both sides of the operating frequency range by suitable energization of the ferrite material by the electron stream focusing field. In this manner both forward and backward waves are attenuated at frequencies adjacent the operating range where selfoscillations are likely to occur.

Another embodiment of the present invention employs a helical waveguide coupled to an electron stream by appropriate openings at either the center or at one side thereof. A longitudinal magnetic field is used for both focusing the electron stream and for developing unidirectional attenuating characteristics in the waveguide structure by simultaneously energizing a ribbon helix constituted of ferrite material disposed parallel to and spaced from an inner wall of the guide. This structure has the advantage in that it does not have any discontinuities to detrimentally reflect portions of the electromagnetic wave being propagated along the path of the electron stream.

It is therefore an object of the present invention to provide an improved traveling-wave tube wherein the tendency to self-oscillate is minimized.

Another object of the invention is to provide a travelingwave tube incorporating a slow-wave structure having unidirectional attenuating characteristics.

Still another object of the invention is to provide an improved apparatus employing ferrite materials in conjunction with the electron stream focusing field to effect unidirectional attenuation in the wave guiding structures of traveling-wave tubes.

A further object of the present invention is to incorporate a plurality of ferrite slabs in a traveling-wave tube to effect unidirectional attenuation wherein substantially equal quantities of heat are dissipated in each ferrite slab.

A still further object of the present invention is to provide a traveling-wave tube having selective bidirectional attenuating characteristics adjacent its operating frequency range.

The novel features which are believed to be charactertic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which several embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention.

Fig. 1 is a diagrammatic sectional view of an embodiment of the traveling-wave tube of the present invention together with associated circuitry;

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Figs. 2 and 3 are sectional views of the tube of Fig. 1 taken on lines 2—2 and 3—3 of Fig. 1, respectively;

Figs. 4, 5 and 6 are explanatory views of the magnetic field configuration in the tube of Fig. 1;

Fig. 7 is a graph illustrating the permeability characteristic of a typical ferrite for positive and negative circularly polarized waves;

Fig. 8 is a graph illustrating the relative attenuation of forward and backward waves in the wave propagating structure of the tube of Fig. 1;

Figs. 9 and 10 are respectively a sectional and an elevational view illustrating an embodiment of a magnetic shunt for the ferrite members of the tube of Fig. 1;

Fig. 11 is a sectional view illustrating a modification of the embodiment of Fig. 1;

Fig. 12 is a graph illustrating the relative bidirectional attenuation characteristic for the wave propagating structure of the tube of Fig. 11; and

Figs. 13 and 14 are diagrammatic sectional views of an alternate embodiment of the device of the present invention.

Referring now to the drawings, there is illustrated in Fig. 1 an embodiment of the present invention comprising an evacuated envelope 10, an electron gun 12 disposed within the envelope 10 at the left extremity thereof as viewed in the figure for producing an electron stream, a folded waveguide structure 14 for propagating an electromagnetic signal wave to be amplified along a predetermined path for the electron stream, and a solenoid 16 disposed concentrically about the path for producing a magnetic field along the longitudinal axis of the tube.

More particularly, electron gun 12 comprises a cathode 29 with its associated heater 22 for providing an electron emitting surface, a focusing electrode 24, and an accelerating anode 26. Heater 22 is energized by a connection across a battery 28, one terminal of which is connected to cathode 20. In operation, the cathode 20 is maintained at a potential of the order of 2000 volts negative with respect to ground. This is accomplished by means of a connection from the cathode 20 to the negative terminal of a battery 30, the positive terminal of which is connected to ground. The focusing electrode 24 provides a conductive surface of revolution about the path of the electron stream at approximately 67.5° thereto. In accordance with this configuration, electrode 24 is maintained at the same potential as that of cathode 20 by means of a connection therebetween. Accelerating electrode 26 is disposed concentrically about the path of the electron stream to the right of and adjacent the focusing electrode 24, as shown. Electrode 26 is maintained at a potential of the order of 200 volts positive with respect to ground by means of a suitable connection to battery 32.

Folded waveguide structure 14 for propagating an electromagnetic wave along the path of the electron stream comprises a length of rectangular waveguide 40 that is periodically folded back and forth across the path of the electron stream. Suitable apertures are disposed in the waveguide walls to enable the stream electrons to proceed along the path. The electric field within the waveguide is caused to concentrate about the path of the electron stream by means of ferrules 42 which connect adjacent apertures of the folded waveguide structure and protrude slightly inwards into the guide. Annular rings 43 and 44 are disposed about the periphery of the first and last apertures in the waveguide along the path of the electron stream, respectively, to provide similar protrusions into the guide from the end apertures. Dielectric seals 46, 47 are disposed across the input and output ends of the waveguide, respectively, as shown so as to enable the envelope 10 to be evacuated. The waveguide 40 is maintained at ground potential by suitable connections thereto.

A collector electrode 50 for intercepting and collecting the electron stream is disposed at the extremity of its path farthest from the electron gun 12. Electrode 50 is maintained at a potential of the order of from 100 to 200 volts positive with respect to ground so as to minimize secondary electron emission from its surface exposed to bombardment by the electron stream. The above is accomplished by a connection to the positive terminal of a battery 51, the negative terminal of which is connected to ground.

In accordance with the present invention, ferrite slabs 60, 61 are disposed on alternate sides of the electron stream for- at least one period of the folded waveguide structure 14. Sectional views indicating representative locations of ferrite slabs 60, 61 are illustrated in Figs. 2 and 3. Referring to these figures, ferrite slabs 60, 61 are spaced distances a and b, respectively, from the walls of the waveguide section 40. The dimensions a and b of the slabs 60, 61, respectively, may both equal approximately 25% of the width of the waveguide for maximum unidirectional attenuation. Alternatively, if it is desired that the power dissipated in each of the ferrite slabs 60, 61 be substantially equal, the distances a and b may be made to progressively increase for each half-period of the waveguide section 40 but retained within the range of 10% to 40% of the waveguide cross section. On the other hand, the attenuation of an electromagnetic wave propagated by folded waveguide section 14 may be made bidirectional by making each of the distances a and b equal to zero. In this latter case, the selective attenuation characteristic of the ferrite material is made to occur outside of and adjacent to the operating frequency range as will be hereinafter explained.

Ferrite slabs 60, 61 have a thickness of the order of 0.025 inch and preferably extend completely across the narrow dimension of the waveguide 40 coextensive with the portion thereof normal to the path of the electron stream. The exposed edges of the ferrite slabs 60, 61 are slightly tapered, as shown in Fig. 1, so as to minimize their tendency to reflect portions of electromagnetic waves propagated through the guide. Ferrite slabs 60, 61 may be composed of, for example, ferrite material known commercially as “Ferramic A,” “Ferramic G,” “Ferramic R-l,” “Ferroxcube 104,” or “Ferroxcube 106.”

The solenoid 16, for producing the longitudinal magnetic field, is connected across an adjustable potential source 54. The voltage output of source 54 is adjusted so that solenoid 16 develops a magnetomotive force along the length of the tube depending upon the frequency of operation and the type of ferrites used which may be of the order of 1500 oersteds. In accordance with the present invention, this magnetic field is employed to both focus and direct the electron stream along its path and to produce transverse magnetic fields through the ferrite slabs 60, 61.

To illustrate more clearly the magnetic field portion of an electromagnetic wave propagated along the path of the electron stream, reference is made to Fig. 4, which shows a developed portion of the folded waveguide section 40 of Fig. 1. In this figure, the instantaneous magnetic field for an electromagnetic field being propagated along the developed waveguide section 40 is shown by dash lines 70. In the operation of the tube of the present invention, magnetic fields of this configuration are propagated along the folded waveguide section 40 at a velocity designated as the phase velocity of the wave.

In order to consider the characteristics of the magnetic fields represented by dash lines 70 in the region occupied by ferrite slab prime e. g. 61', the field will be viewed from a single point within the slab such as, for example, at point A. It is seen that the portion of the magnetic field that will intercept point A for the particular instant shown may be represented by vectors a, b, c and d as illustrated in Fig. 4. As the wave is propagated along the waveguide section 40, point A will be successively cut by these points of the magnetic field. Thus, as viewed from point A, it is seen that, as shown in Fig. 5, the mag2,806,972 netic field appears as a vector that rotates in a clockwise direction at an angular velocity «j. When the moment of this rotating vector coincides with the direct-current magnetic field through the ferrite slab 61', the component of the wave described is said to be positive circularly polarized. Alternatively, if the moment of the rotating vector is in an opposite direction, the particular component described would be negative circularly polarized. Similarly, point B within slab 60 sees successive magnetic vectors e, f, g, and h as indicated in Fig. 4. As before, these vectors appear at point B as a single vector rotating in the counter-clockwise direction at an angular velocity w₂ as illustrated in Fig. 6.

In accordance with the present invention, a direct-current magnetic field is produced in opposite directions through ferrite slabs 60 and 61 with respect to the direction in which the electromagnetic wave is propagated through the developed waveguide structure. This directcurrent magnetic field is directed in a manner such that the magnetic field components of the wave being propagated in the forward direction will be negative circularly polarized. In that the magnetic field intercepting ferrite slabs 60, 61 rotates in opposite directions, it is apparent that in order to have components of the propagated wave on both sides of the waveguide section 40 polarized in the same direction it is necessary that the direct-current magnetic field through ferrite slabs 60 be in a direction opposite to that through slabs 61. In accordance with the present invention, this is accomplished by folding back-to-back the successive portions of the waveguide containing ferrite slabs 60 and 61 which are disposed on opposite sides thereof.

As is generally known, the permeability characteristics for positive and negative circularly polarized waves of ferrite material are different. More particularly, the permeability for the negative circularly polarized wave is substantially constant, whereas the permeability for the positive circularly polarized wave goes through resonance for changes in frequency or magnetic field. During this resonance the power dissipated in the material increases substantially, thereby making the permeability for a positive circularly polarized wave a complex quantity. A typical permeability characteristic for a ferrite material is shown in Fig. 7 wherein represents the permeability for the negative circularly polarized wave and μ₊=/—/μ represents the permeability for a positive circularly polarized wave. As illustrated in this figure, lines 74, 76 and 78 represent the variation in μ_, μ’ and μ, respectively, versus frequency for circularly polarized waves. The magnitude of μ, i. e. the imaginary part of μ₊, determines the extent to which the positive circularly polarized wave will be attenuated. As shown in the figure, maximum attenuation of the positive circularly polarized wave occurs at the resonant frequency. The point at which this resonance occurs is both a function of the frequency of the circularly polarized wave as seen by the ferrite material and the strength of the direct-current magnetic field through the ferrite.

Thus, in the operation of the device of the present invention, the current through solenoid 16 which determines the direct-current magnetic field is adjusted by means of adjustable potential source 54 to produce a magnetic field within the ferrite material corresponding to resonance at the operating frequency.

In operation of the device of the present invention, the function of the ferrite slabs 60, 61 is to attenuate electromagnetic wave energy being propagated by the waveguide section 40 in the direction from the collector 50 to electron gun 12, i. e. in the “backward” direction, without appreciably attenuating waves being propagated in the forward direction. The relative attenuation characteristics for a forward and backward wave for the device of the present invention are shown in Fig. 8 as lines 80 and 82, respectively.

As is evident from Fig. 8, when a single magnetic field strength is used through the ferrite slabs 60, 61, the attenuation of backward waves occurs for only a comparatively narrow range of frequencies. This attenuation range may be broadened by causing resonance within the ferrite slabs 60, 61 to occur at different frequencies within the band wherein it is desired to attenuate the backward waves. This may be accomplished by employing different types of ferrite material or, alternatively, by developing different magnetic fields within each of die ferrite slabs 60, 61. The magnetic fields within the ferrite slabs may be varied, for example, by employing slabs of different thicknesses or magnetic shunts 84, 86 shown in Figs. 9 and 10. In the latter case, the magnetic shunts 84, 86 are composed of a material which presents a low reluctance to the magnetic field produced by solenoid 16 so as to shunt a portion of the field around the ferrite slabs 60' and 61'.

In an alternative embodiment of the present invention, ferrite slabs 90 and 92 as shown in Fig. 11 are disposed in contact with the walls within the portions of the folded waveguide section 40 normal to the path of the electron stream. When disposed within a waveguide in this manner, both forward and backward waves are attenuated by substantially equal amounts. This attenuation, however, is a maximum at frequencies corresponding to resonance for the ferrite slabs 90 and 92. In accordance with the present invention, each of the ferrite slabs 90 and 92 are of at least two different thicknesses. Thus, when energized by the magnetic field produced by solenoid 16, different magnetic field intensities are developed within each of the slabs 90 and 92 which produce resonance at frequencies /i and /2 which are adjacent the operating frequency range of the tube. In this manner, attenuation of both forward and backward waves· is effectuated in the regions adjacent the operating frequency range of the tube as indicated, for example, by lines 94, 96 of Fig. 12 thereby minimizing any tendency of the tube to break into self-oscillation due to a poor impedance match of folded waveguide structure 14 with the input and output circuits in these ranges. It is evident that this selective attenuation may be produced by magnetic shunts or by the use of different ferrites in the manner previously described.

In another embodiment of the present invention illustrated in Fig. 13, a helical wave propagating structure 100 is employed in lieu of the folded waveguide structure 14 of the device shown and described in connection with Fig. 1, the remaining elements being substantially the same. Helical wave propagating structure 100 comprises a length of rectangular waveguide 102 which is edge-wound about the path of the electron stream at a uniform pitch. The wall of the waveguide within the evacuated envelope and adjacent the path of the electron stream is removed so that an electromagnetic wave propagated within each turn of the waveguide will combine to form a wave capable of interacting with the electron stream.

In accordance with the present invention, a ribbon helix 104 of ferrite material is disposed within the waveguide 102 coextensive with the edge-wound portion thereof and spaced from the outer wall 106. In general, the distance between the ribbon helix 104 and the outer wall 106 of the waveguide may be from 10% to 30% of the width of the guide to produce optimum unidirectional attenuation. In operation, the magnetic field produced by solenoid 16 provides focusing for the electron stream and in addition, constitutes the field which energizes the ferrite helix 104 in a direction transverse to the waveguide 102. As before, voltage source 54 is adjusted to produce a magnetic field necessary to effect resonance in the ferrite material at the desired frequency. In the event that it is desired to effect resonance over a broad range of frequencies, the thickness of the ribbon helix 104 could be tapered to cause the magnetic field within the ferrite material to progressively increase or decrease.

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