US2830220A - Traveling-wave tube - Google Patents

Description

APriI 8; 1958 s. E. WEBBER 2,830,220

TRAVELING-WAVE TUBE Filed June 29, 1950 Inventor: Stanieg E. Webber',

HiS Attorney.

United States Patent TRAVELING-WAVE TUBE Stanley E. Webber, Ballston Lake, N. Y., assignor to General Electric Company, a corporation of New York Application June 29, 1950, Serial No. 171,179

Claims. (Cl. 315- 3.5)

This invention relates to improvements in electron discharge devices of the type generally called traveling-wave tubes, and its principal object is to increase the gain and eificiency of such tubes. Other objects and advantages will appear as the description proceeds. I

The features of this invention which are believed to be novel and patentable are pointed out in the claims which form a part of this specification. For a better understanding of the invention, reference is made in the following description to the accompanying drawing, in which the single figure is a schematic longitudinal section of a traveling-wave tube embodying principles of this inventron Referring now to the drawing, the traveling-wave tube amplifies an electromagnetic wave transmitted between input terminal 1 and output terminal 2. Connecting these terminals is a structure which transmits electromagnetic waves at relatively low velocity. This structure may be an electrically conductive wire helix 3, as shown in the drawing. Since an electromagnetic Wave travels along the wire of helix 3 at substantially the speed of light, the velocity of the wave along the axial direction of the helix is' less than the speed of light by a factor substantially proportional to the ratio between the pitch of helix 3 and its diameter. It is known in the art that other structures which transmit electromagnetic waves at relatively low'velocity may be used in place of the helix, and it will be understood that such other structures may be used in the practice of my invention. Examples of structures which have been proposed for such use are a rod which is electrically loaded with a plurality of rather closely spaced transverse metal discs, or a cylindrical wave guide loaded with a plurality of apertured discs. Connection of terminals 1 and 2 to external circuits may be made by direct connections to the wire, or

by inductive or capacitive coupling; for example, by

hand end of envelope 4 there is an electron gun comprising a cathode 5, a cathode heater 6, and a focusing electrode 7. Electrode 7 may be tubular in shape, so that a beam of electrons, indicated by. dotted lines 8, is directed axially through helix 3 to a collector plate 9 positioned within the right-hand end of envelope 4. Means (not shown) may be provided to produce a longitudinal magnetic field through the envelope to keep the electron beam in focus. It is known in the art that the electron gun structure may be modified, if desired, to insure a more uniform current density in the beam, or to provide'a hollow electron beam so that maximum current density will ice.

occur very close to the wire of helix 3, where the field" of the transmitted electromagnetic wave is strongest. A battery 10, or other suitable voltage source, is connected to maintain electrode 7 and collector plate 9 at a large positive potential with respect to cathode 5, and a battery 11, or other current source, is provided to supply operating current to heater 6. v

The ratio of pitch to diameter of helix 3 and the elec tron accelerating potential provided by battery 10 are so related that electron beam 8 travels in the same direction and at near the same velocity as electromagnetic waves transmitted by helix 3 between input terminal 1 and output terminal 2. The adjustment is such that the velocity of the electron beam is slightly greater than that of at least" one component of the electromagnetic wave. Under these conditions, it is known that interaction between the beam and the wave occurs in such a way that the electromagnetic wave is amplified at it travels along the helix. One explanation of this interaction, by J. R. Pierce, may be found in The Bell System Technical Journal, volume 29, No. 1 (Jan. 1950), pp. 6-19.

To prevent oscillations, an attenuator 12 is placed within the field of electromagnetic waves transmitted. by helix 3. Since this field extends through envelope 4, attenuator 12 may conveniently be a band of lossy, electrically conductive material placed around the outside, or the inside, or as a part of envelope 4. The attenuator has a value such that the attenuation of the wave in the backward direction along helix 3 exceeds the gain in the forward direction, and thereby prevents oscillations due to successive reflections at the input and output terminals. As explained in my copending application, S. N. 171,178 filed June 29, 1950, and assigned to the same assignee as the present application, attenuator 12 preferably is positioned at input terminal 1, or spaced therefrom a short distance, preferably in the order of one wavelength. Y 7

According to a theory for explaining the operation of traveling-wave tubes, electron beam 8 is velocity modulated by interaction with the electromagnetic wave transmitted by helix 3 in that portion of the helix between input terminal 1 and attenuator 12. The portion of the helix adjacent attenuatorll acts as a drift space in which electron bunching takes place; The bunched electrons interact with the electromagnetic wave transmitted by the helix in that portion between attenuator 12 and output terminal 2, and thereby amplify the electromagnetic wave. The present invention provides means for increasing the field strength of the electromagnetic wave near input terminal 1, thereby increasing the amount by which electron beam 8 is velocity modulated and increasing the gain and eificiency of the tube. Althoughexperimental results tend to confirm this theory of operation, it should be understood that the theor is given in this specification for clarity only, and that applicants' invention is not limited to any particular theory for the operation of traveling-wave tubes.

To increase the electromagnetic field strength along that portion of helix 3 near input terminal 1, an abrupt impedance discontinuity is placed at a distance from input terminal 1, preferably in the order of one-half wavelength of the electromagnetic wave as transmitted by the helix. This discontinuity may be a notch 13 in the helix wire, as shown in the drawing, or any other structural anomoly which presents an abrupt impedance discontinuity at this point to the wave transmitted by the helix. For example, a metal ring about the inside (or outside) of envelope. 4 or a probe coupled externally to a resonant circuit might be employed. The impedance discontinuity ofnotchJlS reflects a portion of the electromagnetic wave back toward input terminal 1, where; due to normal imperfections in the impedance match betweentheex ternal circuit and the helix, reflections again occur. Since these multiple reflections occur at points which are spaced apart by one-half wavelength, they produce a resonance effect which substantially increasesthe electromagnetic field strength within thisportion of the helix. A consequent increase in the gain of the tube has been experimentally verified. The attenuator may immediately follow the impedance discontinuity.

It is not essential that the impedance discontinuity be exactly one-half wavelength from the input terminal. In fact, if the traveling-wave tube-is used to amplify modulated electromagnetic wave, it'is inevitable that for some frequency components of the wave the distance willnot be exactly one-half wavelength. If the Q of the resonant circuit between the input terminal and the impedance discontinuity is relatively low, as is generally the case, the bandwidth which the tube can amplify may be quite large, although smaller than the extremely Wide bandwidths which some other travelingwave tubes. are capable of amplifying. Therefore, the present invention is most useful in applications where extremely wide bandwidths are not necessary. Also, it

will be appreciated that increased gain can be obtained in a similar manner if the impedance discontinuity is located at a distance from the input terminal of about one-quarter wavelength, or three-quarters wavelength, or some other integral number of quarter wavelengths. Preferably, the

distance between the input terminal and the impedance output terminal 2 to input terminal 1, in the conventional manner; I

Having described the principle of my invention and the best mode in which I have contemplated applying that principle, I wish it to be understood that the apparatus described is illustrative only, and that other means can be employed without departing from the true scopeof the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electron discharge device comprising a transmission structure having input and output terminals "between which electromagnetic waves are transmitted at low velocity, means for providing an electron beam in proximity to said structure traveling in the same direction and at a velocity slightly greater than-the velocity of an electromagnetic wave so transmitted, said electron beam beingcontinuously coupled with said transmission struc ture between said input and output terminals whereby interaction between said beam and said wave occurs to amplify the wave, and said transmission structure having a single abrupt impedance discontinuity at an intermediate point along its length.

2. An electron discharge device comprising a transmission structure having input and output terminals be tween which electromagnetic waves are transmitted at low.

velocity, means providing an electron beam in proximity to said structure traveling in the same direction andat near the same velocity as an electromagnetic wave so transmitted, whereby interaction between said beam and said wave occurs to amplify the wave, said transmission structure having a single abrupt impedance discontinuity spaced from the input terminal of said structure a distance not exceeding one wavelength of the electromagnetic wave as transmitted by said structure.

p 3. An electron discharge device comprising a transmission structure having input and output terminals belit) 2,830,220 g i l r tween which electromagnetic waves are transmitted at low velocity, means providing an electronbeam in proximity to said structure traveling in the same direction and at near the same velocity as an electromagnetic wave so transmitted, whereby interaction between said beam and said Wave occurs to amplify the Wave, said transmission structure having a structural anomoly which presents a single abrupt impedance discontinuity to waves so transmitted located at a distance along said structure from said input terminal not exceeding one-fifth the total length of said. structure.

4. A traveling-wave tube comprising input and output terminals, a long, electrically conductive helix between said terminals for transmitting electromagnetic waves therebetween at low velocity, an evacuated envelope enclosing said helix, means for providing an electron beam axially through said helix, said last-named means comprising an electron gun within one end of said envelope and a collector plate within the other end of said envelope, an attenuator within the field of electromagnetic waves ransmitted by said helix, said helix having a notch on a turn thereof which provides an abrupt impedance discontinuityto electromagnetic waves transmitted thereby, said notch being at a distance from said input terminal not exceeding one wavelength of the electromagnetic wave so transmitted. t

5. An electron discharge device comprising a transmission structure having input and output terminals be tween which electromagnetic waves are transmitted at low velocity, means providing an electron beam in proximity to said structure traveling in the same direction and at near the same velocity as an electromagnetic wave so transmitted, whereby interaction between said beam and said wave occurs to amplify the wave, said structure having astructural anomoly which presents an abrupt impedance discontinuity to the electromagnetic wave so transmitted, to cause multiple reflections between said structural anomoly and the input terminal of said transmission structure and therebyincrease the electromagnetic field strength throughout the length of said transmission structure between said input terminal-and said anomoly, said anomoly being located from said input terminal a distance in the order of an integral number of quarter wavelengths less than one wavelength of the electromagnetic waves as transmitted by said structure.

6. An electron discharge device comprising a transmission structure having input and output terminals be-' tween which electromagnetic waves are transmitted at low velocity, means providing an electron beam in proximity to said structure traveling in the same direction and at near the same velocity as an electromagnetic wave so transmitted, whereby interaction between said beam and said wave occurs to amplify the wave, said structure having a structural anomoly which presents an abrupt impedance discontinuity to the electromagnetic wave so transmitted, to cause multiple reflections between said structural anomoly and the input terminal of said transmission structure and thereby increase the electromagnetic field strength throughout the length of said transmission structure between said input terminal and said anomoly, said anomoly being located from said input terminal a distance in the order of one-half wavelength of the electromagnetic waves as transmitted by said structure.

7. An electron discharge device comprising .a trans-i and said wave occurs to amplify the wave, said structure having a structural anomoly which presents an abrupt impedance discontinuity to the electromagnetic wave so transmitted, to cause multiple reflections between said structural anomoly and the input terminal of said transmission structure and thereby increase the electromagnetic field strength throughout the length of said transmission structure between said input terminal and said anomoly, said anomoly being located from said input terminal a distance in the order of one-quarter wavelength of the electromagnetic waves as transmitted by said structure.

8, An electron discharge device comprising a transmission structure having input and output terminals between which electromagnetic waves are transmitted at low velocity, means providing an electron beam in proximity to said structure traveling in the same direction and at near the same velocity as an electromagnetic wave so transmitted, whereby interaction between said beam and said wave occurs to amplify the Wave, said structure having a structural anomoly which presents an abrupt impedance discontinuity to the electromagnetic Wave so transmitted, to cause multiple reflections between said structural anomoly and the input terminal of said transmission structure and thereby increase the electromagnetic field strength throughout the length of said transmission structure between said input terminal and said anomoly, said anomoly being located from said input terminal a distance in the order of three-quarter wavelengths of the electromagnetic waves as transmitted by said structure.

9. An electron discharge device comprising a transmission structure having input and output terminals between which electromagnetic waves are transmitted at a low velocity, means for providing an electron beam in 6 t proximity to said structure traveling in the same dire'c'- tion and at a velocity slightly greater than the velocity of an electromagnetic wave so transmitted, and an attenuator in proximity to said structure and spaced from said input terminal one wavelength or less and extending over a length less than half of said transmission structure, said transmission structure having an abrupt impedance discontinuity at a position intermediate said input terminal and the end of said attenuator remote from said input terminal.

10. An electron discharge device comprising a transmission structure having input and output terminals between which electromagnetic waves are transmitted at low velocity, means for providing an electron beam in proximity to said structure traveling in the same direction and at a velocity slightly greater than the velocity of an electromagnetic wave so transmitted, and an attenuator in proximity to said structure and spaced from said input terminal one wavelength or less and extending over a length less than half of said transmission structure, said transmission structure having an abrupt impedance discontinuity at a position intermediate said input terminal and the end of said attenuator adjacent said input terminal.

References Cited in the file of this patent UNITED STATES PATENTS 2,367,295 Llewellyn Jan. 16, 1945 2,541,843 Tiley Feb. 13, 1951 2,575,383 Field Nov. 20, 1951 2,669,674 Diemer Feb. 16, 1954

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