US3379920A - Traveling-wave tube with efficiencyenhancing focus-field jump - Google Patents

Description

April 3 8 F. L. WASHBURN, JR 3,319,920

TRAVELING-WAVE TUBE WITH EFFICIENCY-ENHANCING FOCUS-FIELD JUMP Filed Oct. 14, 1964 FIGJ.

28 so I I I MAGNETIC 32 SHUNT I4: 26 I2 22 34 26 [J 2 FIG-2.

DISTANCE l EL u EL DISTANCE EL INVENTOR. Frederick L. woshburmdr.

BYCQ g F ATTORNEY United States Patent TRAVELING-WAVE TUBE WITH EFFICIENCY- ENHANCHQG FOCUS-FIELD JUMP Frederick L. Washburn, 3n, Severna Park, Md., assiguor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Oct. 14, 1954, Ser. No. 403,811

11 Claims. (Cl. 3153.5)

ABSTRACT OF THE DHSCLOSURE The present invention relates to a traveling-wave tube in which a focus field jump section is provided in the focusing system. The provision of the focus field jump section provides means of extracting low energy electrons from the electron beam which are incapable of interacting with the RF wave on the slow wave structure to give up energy to the slow wave structure.

This invention relates to traveling-wave tube-s and more particularly to means for providing a focus-field jump in order to enhance operating efficiency.

Although traveling-wave tubes have many uses and potential uses, their relatively low efficiency, generally less than 20%, has been the concern of many workers in the field. To improve efficiency it has been proposed to decrease the pitch of the slow-wave helix at the output end in order to allow the phase velocity to be decreased to match the lower average velocity of the electrons. This proposal has proved to be only moderately successful. Depressed collector operation has also received considerable attention in the last few years. While double the apparent efiiciency has been claimed for this type of operation under circumstances of low original efiiciency, it has the disadvantage of requiring an extra power supply and requiring that the cathode be large enough to supply high current.

The present invention provides substantial improvement in efliciency without requiring an extra power supply or high cathode current.

In a traveling-wave tube operating at saturation power output, there is considerable spread in the velocity of the electrons at the output end of the tube. Many electrons are moving considerably slower than their original velocity, while some are moving faster. The largest group is moving at about the same velocity as when the electrons left the gun region, or just slightly slower. When an attempt is made to increase the output energy, and consequently the efficiency, by increasing the RF drive, the slow electrons are speeded up more than the fast electrons are slowed down, and overdrive occurs. This is a condition where less, rather than more power is obtained from the tube, even though the RF drive power has been increased.

The present invention permits an increase in power output under high RF drive operation by eliminating from the electron beam those electrons which are moving relatively slow, leaving only the faster electrons, which are capable of interacting with the RF wave in such a manner as to give up energy. The present invention utilizes the one characteristic of slow electrons which differentiates them from fast electrons, namely, their reduced speed.

Briefly stated, the present invention depends upon the provision of a focus-field jump in the focusing system, that is, the provision of a short section of focusing structure in which focusing fields, whether magnetic or electric, arc of substantially zero magnitude. Although both fast and slow electrons deviate radially at the same rate on passing into the area of no focusing field because both have the same space charge and the same angular velocity, the slow electrons, being slow, have more time in the nofield portion of the focusing structure and consequently ice deviate a greater distance. With proper adjustment of the n0-field portion of the focusing structure, it is possible to eliminate most of the slow electrons by intercepting them on the slow-wave structure, and then re-establish the focusing means before the fast electrons have had the chance to impinge upon the slow-wave structure. Not only is the efficiency increased, but the substantially fiat portion of the normal input-output curve of the tube is greatly extended, that is, near the peak output a larger range of input drive will result in only a given small percent of output power variation. The principles of the invention may be employed in substantially all of the known types of focusing systems.

It is accordingly a principal object of the invention to provide travelingwave tubes having enhanced efiiciency.

Another object of the invention is to provide unique focusing field structures for electron beams, especially as employed in traveling-wave tubes.

The foregoing and other objects, advantages and features of the invention and the manner in which the same are accomplished will become more readily apparent upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings, which illustrate preferred and exemplary embodiments of the invention (proportions being chosen for ease of explanation of theoretical principles), and wherein:

FIGURE 1 is a longitudinal sectional view of one form of traveling-wave tube in accordance with the invention;

FIG. 2 is a graphical diagram illustrating the Z or axial component of magnetic focusing field plotted against distance along the focusing structure shown in FIG. 1;

FIG. 3 is a longitudinal sectional view of another form of traveling-wave tube in accordance with the invention; and

FIG. 4 is a graphical diagram illustrating electrostatic focusing field at the beam radius, for two different angular orientations, plotted against distance along the focusing structure shown in FIG. 3.

Referring to the drawing, and initially to FIGURE 1 thereof, reference numeral 10 designates a traveling-wave tube comprising a conventional envelope 12 formed of suitable materials, such as glass and/or metal, the envelope containing at one end an electron gun 14 and at the other end a collector or target electrode 16. The elec trodes are shown diagrammatically because their details are well known and are not per se the present invention. The electron gun 14 may include, in accordance with conventional practices, a cathode 18 and an accelerating electrode 20 for projecting electrons along the length of the tube toward the collector 16. Interposed between the electron gun and the collector is a conventional slow-wave structure or RF circuit 22. In the form shown this structure is of the single helix type, although many other types of slow-wave structure may be employed. Suitable input and output terminations, such as the conventional wave guides 24 and 26, are provided for coupling radio frequency energy to and from the slow-wave structure. The helical conductor 22 is supported within the envelope by conventional means, such as dielectric rods (not shown) and is positioned for proper interaction of the radio frequency energy with the electron beam, which may be a solid cylindrical beam projected through the circular tunnel of the helix 22. In the embodiment illustrated the accelerating electrode 2-0, the helix 22, and the collector 16 are connected to ground (or chassis) potential, while the cathode 18 is connected to a source of potential negative with respect to ground.

It is conventional practice in traveling-wave tubes or similar discharge devices to provide some means for focusing the beam of electrons in order to prevent undesired radial divergence of the electrons from the beam. Such a focusing means may take the form of a magnetic focusing coil. In the embodiment of FIGURE 1 a magnetic focusing coil 28 is shown diagrammatically. The coil 28 surrounds the envelope 12 throughout the length of the slow-wave structure, and somewhat beyond. The coil 28 may be enclosed by a magnetic shield 30, which may be formed of a material such as soft iron (the term iron being used generically herein to include other appropriate magnetic materials). The shield 39 may extend inwardly substantially to the envelope 12 and in the form shown extends substantially to the enlarged end portions of the envelope 12 where these portions join the narrower main portion of the envelope 12. The focusing coil 28 has an axial passage 32 through which the envelope 12 passes with some clearance, as shown, so as to receive the input and output terminations and another element to be described hereinafter.

In the operation of the magnetic focusing coil conventionally employed with traveling-wave tubes, a magnetic field having a substantially uniform axial or Z component is produced throughout the length of the slow-wave structure. In accordance with the present invention, however, a focus-field jump is provided. This jump may be provided by means of a magnetic barrier or shunt, such as a sleeve 34 of soft iron. The sleeve 34 fits within the passage 32 of the focusing coil and surrounds, but may be slightly spaced from, the envelope 12. In the preferred embodiment illustrated the sleeve 34 is located a distance from the electron gun which is approximately three-quarters of the distance between the gun and the collector 16. The sleeve 34 has an axial length of the order of one-tenth of this distance, or on the order of one or two wavelengths of the RF energy on the slow-wave structure.

FIGURE 2 illustrates a plot of the Z component B of the magnetic focusing field versus the distance along the focusing coil. It will be observed that the field is substantially uniform along the major part of this distance, but there is a jump j along the minor part of this distance through the sleeve 34, where the field drops substantially to zero.

In the operation of the device of FIG. 1, the electron beam in the region of the focus-field jump is permitted to de-focus, that is, the electrons of the beam are permitted to diverge radially. While both the slower and faster electrons of the beam diverge at the same rate, the slower electrons remain in the no-field region for a longer period of time than the faster electrons and hence diverge a greater distance. The length of the focus-field jump along the axis of the slow-wave structure is made great enough so that the slow electrons diverge sufiiciently to be intercepted by the slow-wave structure 22, which is an equipotential surface. The length is, however, made insufficient to permit interception of the faster electrons, which are focused after passing the focus-field jump and are ultimately collected by the collector 16. By virtue of the invention, the substantial numbers of slow electrons which exist adjacent the output end of the tube under high RF drive conditions are extracted from the beam, and their relatively low energy (almost entirely kinetic energy) is given up to the heiix in the focus-field jump region. The result of this extraction of the slower electrons is a significant increase in the efiiciency of the tube.

FIG. 3 illustrates another embodiment of the invention. Here a traveling-wave tube 36 has an envelope 38, which may be similar to the envelope 12 of FIG. 1, and which contains an electron gun 40 at one end and a collector electrode 42 at the other end. The electron gun includes a cathode 44 and an accelerating electrode 46. In the form shown the electrodes are constructed and energized, in accordance with conventional techniques, for the projection of a hollow cylindrical electron beam along the length of the tube. The electrons interact with RF energy on slow-wave structure 48, which may be supported in the envelope in a conventional manner, as by the dielectric rods mentioned previously. The slow-wave structure is a bifilar helix type comprising a pair of similarly wound helical conductors 50 and 52. In accorda ce wi h t e invention the bifilar Winding has a discontinuity or gap 54, so that the slow-wave structure actually comprises a pair of separated bifilar coils. The gap 54 is surrounded by a single helical conductor 56 which bridges the gap and extends along a substantial part of the bifilar windings at opposite sides of the gap. The purpose of winding 56 is to transfer RF energy over the gap. Conventional input and output terminations, such as the wave guides 58 and 60, may be employed to couple RF energy to and from the slow-wave structure.

In the embodiment of FIG. 3 electrostatic focusing of the electron beam is employed, and the focusing means utilizes conductors 50 and 52 of the slow-wave structure. Typical relationships of energizing potential are shown in FIG. 3, the accelerating electrode 46, collector 42, and helix 56 being connected to ground (or chassis) potential, the cathode 44 being negative with respect to accelerator potential, and conductors 50 and 52 being connected, respectively, to sources of poten ial which are positive and negative with respect to accelerator potential.

FIG. 4 illustrates a plot of electrostatic focusing field, showing the electric field E at the beam radius for two different angular orientations, in this case the upper and lower portions of the beam 180 apart. Curve E represents the focusing field at the upper portion of the beam, while curve E represents the focusing field at the lower portion of the beam. These fields correspond to the potentials indicated in FIG. 3 by the plus and minus signs. The focus-field jump is clearly indicated at J in FIG. 4, corresponding to the gap 54 in FIG. 3. The position and length of the gap may, for example, correspond to the dimensions given with respect to FIG. 1. The electrostatic focusing field along the major portion of the length of the slow-wave structure is interrupted at the minor portion corresponding to the gap 54 to permit the de-focusing of the beam as described in connection with FIG. 1. Under high RF drive conditions substantial numbers of slow electrons diverge sufliciently to be intercepted by the helix 56, and the energy of these electrons, which is almost entirely kinetic energy, is given up to the helix 56. Radio frequency energy readily bridges the gap by virtue of the coupling effect of the helix 56. Elimination of the slower electrons results in improved efficiency, as described previously.

While preferred embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that changes can be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims. Accordingly, the foregoing embodiments are to be considered illustrative, rather than restrictive of the invention, and those modifications which come within the meaning and range of equivalency of the claims are to be included therein.

What is claimed is:

1. In a traveling-wave tube or the like having an electron gun at one end of the tube for emitting electrons toward a collector electrode at the opposite end of the I tube, and having a slow-wave structure along said tube for the interaction of radio frequency energy with said electrons, means extending along the major portion of the length of said tube for focusing said electrons into a beam, said focusing means comprising means for producing a focusing field along said major portion of the length of said tube, but not along a minor portion of the length of said tube intermediate the ends of said tube, said minor portion of sufficient length to provide a focusfield jump to permit electrons having relatively low velocity incapable of interaction with said slow-Wave structure in such a manner as to give up energy compared to other electrons in the beam to move radially sufficiently to be collected by said slow-wave structure rather than by said collector electrode.

2. In the traveling-wave tube or the like of claim 1, said focusing means comprising means for producing an electrostatic focusing field having a gap at said minor portion.

3. In the traveling-wave tube or the like of claim 1, said focusing means comprising a focusing coil having a passage through which said electron beam is projected, there being a body of magnetic material in said passage at said minor portion to provide said focus-field jump.

4. In the traveling-wave tube or the like of claim 2, said body comprising an iron sleeve.

5. In the traveling-wave tube or the like of claim 3, said coil having an iron shield surrounding it.

6. In the traveling-wave tube or the like of claim 3, said sleeve being located at a distance from said electron gun approximately three-quarters of the distance from said electron gun to said collector electrode.

7. In the traveling-wave tube or the like of claim 5, the length of said sleeve along said tube being about onetenth of the total distance between said electron gun and said collector electrode.

8. In the traveling-wave tube or the like of claim 1, said slow-wave structure and said focusing means comprising a bifilar helix having a discontinuity at said minor portion.

9. In the traveling-Wave tube or the like of claim 9, said tube having an additional helix wound about said bifilar helix and extending across said discontinuity in order to couple radio frequency energy between the portions of said bifilar helix at opposite sides of the discontinuity.

10. In the traveling-Wave tube or the like of claim 9, said bifilar helix having means for connecting the respective conductors thereof to sources of electric potential which are respectively positive and negative with respect to the potential of an accelerating anode of said electron gun.

11. In the traveling-wave tube or the like of claim 11, said electron gun comprising means for projecting a hollow electron beam through said slow-wave structure.

References Cited UNITED STATES PATENTS 2,797,353 6/1957 Molnar et a1. 3153.5 2,964,671 12/ 1960 Chang 3 l53.6

FOREIGN PATENTS 841,767 6/ 1952 Germany. 772,091 4/ 1957 Great Britain.

OTHER REFERENCES Multiple-Severed-Helix Microwave Amplifier Tube, Beam, RCA Technical Note No. 344, November 1959, 3l53.6.

HERMAN KARL SAALBACH, Primary Examiner. ELI LIEBERMAN, Examiner.

P. L. GENSLER, Assistant Examiner.

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