US2887609A - Traveling wave tube - Google Patents

Description

May 19, 1959 w. J. DoDDs TRAVELING WAVE TUBE Filed'oot. 8, 1954 .Q SGM m ,erro/MX States ,y Patent C) TRAvELnsG WAVEl Turn Wellesley J. Dodds, Allentown, NJ., assignor to Radio Corporation of America, a corporation of Delaware Application October 8, 1954, Serial No. 461,085

3 Claims. (Cl. `31E-3.6)

' This invention relates to traveling wave tubes. More particularly, it concerns a traveling wave amplifier tube adapted to reduce the space charge depression of potential within a relatively high density stream of electrons in such a tube. t

In a conventional traveling wave tube an electromagnetic signal wave is coupled to one end of an elongated metal helix of such diameter and pitch that the axial velocity of the wave along the helix is reduced to a fraction, say one-tenth, of the Velocity of light. An electron be'am of substantially uniform velocity distribution is p rojected along the thelix, either inside or outside, at a velocity approximately equal to the axial wave velocity. Under such conditions, the electron beam and the wave traveling on the helix interact to cause the amplitudeV of the wave to increase exponentially; hence, amplification of the signal wave is produced at the output end of the helix. As a matter of fact, for maximum amplification the beam velocity should be slightly greater than the undisturbed wave velocity on the helix, that is, the wave velocity without the beam. Instead of a metal helix, the wave-guiding means of the tube may be a loaded wave guide, folded waveguide, disc-loaded rod, waveguide partly filled with dielectric, or other structures capable of reducing the axial velocity of the wave to practical electron velocities.

In the design of relatively high power traveling wave tube amplifiers, use is made of an annular electron beam coupled to a delay line circuit within the tube. As is known, the electron beam within such an amplifier suffers a space charge depression `of potential. Electrons within the more inward portions of the beam are slowed down when the circuit to which the beam is coupled is disposed around the outside of the beam. This slowing down of the inner electrons causes them to fall out of step with the signal wave. Thus they may take no part in the amplification process. Often they actively detract from the net gain and power. t

It is thus desirable to use some means for avoiding the deleterious effects of space charge depression of potential upon the velocities of the electrons in the beam. In the prior art various means have been used. For example, the disc-loaded rod mentioned above has been used as the active part of the internal delay line circuit. This rod was located within an annular beam. A concentric cylinder adjacent to but outside the beam was used to raise the space potential. It is known that such an array of discs axially displaced along a central rod with an adjacent outer shield gives a poor coupling factor and makes for high voltage operation. Another tube structure used a rod as the active element in the internal circuit; it was placed centrally Within a helix. This structure set up numerous undesired field configurations. In both of these structures the undesired field configurations were due to the strong interaction of the active member with the inactive member. Accordingly, it has been desirable for sometime to devise a traveling wave tube structure wherein a streamof electrons can be maintained at a substan- Nce tially uniform velocity without the introduction of undesirable effects. l

It is an object of the present invention to devise a traveling wave tube with improved means for providing a stream of electrons with a substantially uniform velocity.

It is another object of the Apresent invention `to provide a traveling wave tube with improved means f or maintain-. ing a stream of electrons within such a tube at a substantially uniform potential and wherein substantially no de, terioration of an interaction eld from the active element` in the tube circuit is manifest. i

It is a further object of the present invention to pro-y vide a traveling wave amplifier tube with improved` means for maintaining a stream of electrons within such` a tube at a substantially uniform velocity and whereinr substantially no undesired modes of propagation within the tube are initiated.

It is yet another object of the invention to provide lal traveling wave tubey utilizing an annular electron beam and in which the velocity spread in the beam due to space charge depressionof potential is greatly reduced. n

It is still a further object of the present invention `to provide a traveling wave tube structure utilizing an annular beam of electrons with improved means for raising the gain, power output, and efficiency of the tube for a given beam configuration.

In order to accomplish these and other objects, this invention provides two adjacent delay lines within a tube envelope. For example, the delay lines may compriseg two coaxial helices. An annular, cylindrical electron. beam is confined between the two helices, the inner helix serving the purpose of maintaining the inner portion of the annular beam at substantially the same static potential as that at the outer portion of the beam. Thus, thei inner helix is the inactive one while the outer helix is the wave propagating, active helix. The two helices arev of such a pitch and diameter that a radio frequency waveI set up on the inner helix will move at a velocity substan. tially dierent from that of a radio frequency signal wave; one the outer helix. As is well known, this meansthat there Will be substantially zero coupling between the two` helices. The fields upon the outer helix will be substantially insensitive to the presence of the inner, static potential helix. The potential of the inner helix is set at a suitable value relative to that of the active helix. ,The helices may, for example, be tied to the same potential. Then the space charge depression of potential within the beam due to the presence of the electrons will be greatly, lessened. This in turn greatly reduces the differences in velocity between the electrons at the inner and outer portions of the beam and thereby improves the coupling factor and the general operation of the tube. These` effects are obtained with substantially no deterioration ofthe, radio frequency field on the active helix and without theinitiation of undesired modes of propagation in the amf plier system. l

The invention may be` best understood 1from the` f ollowing detailed description and drawing where like nu. metals refer to like parts, and wherein:

' Figure 1 is a longitudinal sectional view of a traveling wave tube amplifier embodying the invention. v A

Figure 2 is a transverse sectional view taken on line t 2 2 of Figure l. l

Referring now to the drawing in greater detail, there is shown in Figure l an elongated traveling wave amplifier tube 10 embodying the invention and adapted to be excited by a signal wave. Within an envelope 12, and at one end thereof, is an electron gun 14. Two de-v; lay lines in the form of concentric helices 16 and ,18X extend within the length of the envelope. The inner helixl 16is supported within the envelope 12 on anon-conduct-i ing rod 17 which is supported by the envelope at the opposite ends of the rod. The outer helix 18 is supported within the envelope by non-conducting rods 19 around the outside surface of the helix. The rods 19 are supported at their opposite ends by the envelope. The support rods for the two helices may be of ceramic. insulating support rings 21 may be used within the tube to aid in the support of the various internal tube elements. An electron collecting electrode 2t) is provided within the envelope at the end thereof remote from the electron gun.

The electron gun 14 may include a cathode 22. The cathode 22 is provided with a heater 24 adapted to be energized from a power source 26. The cathode may be provided with a ring shaped coating 27 of electron emissive material on one side of the cathode. An accelerating electrode 28 is arranged adjacent to the cathode in a known manner to produce an annular beam of electrons. The beam of electrons travels along the longitudinal tube axis in a path between the two helices 16 and 18.

As seen in Figures l and 2, the helices dene between them an annular space which extends along the length of the helices. One of the helices, for example the outer one, is the active one, i.e., the one which participates in Wave amplification. The active helix comprises a wave transmission circuit adapted to propagate a traveling wave representative of a signal wave applied to the tube. The inner helix may be the inactive one, i.e., a noninteracting helix which serves to stabilize the annular electron beam which is projected between the helices. Each of the helices is effective to conduct a traveling wave along the axis defined by the helices with a velocity less than that of light. However, the diameter and the pitch of the two helices are chosen so that the velocities of waves along the two helices are substantially different. The helices are preferably wound in the same direction so that there will be substantially no interaction between the fields on the helices even though the phase velocities of the two helices differ by only a small amount. Thus, no undesired field configurations are set up. Of course, they may be wound in opposite directions provided their wave propagating characteristics are such that substantially no coupling exists between them. While in the preferred operation, the outer helix is the active one, the tube may be operated with somewhat lesser efiiciency with the inner helix as the active one and the outer helix as the non-interacting one.

A direct current power supply 30 is connected to maintain the accelerating and the collecting electrodes at the desired potentials with respect to the cathode and to each other. The voltages at the accelerating and the collecting electrodes may be varied by means of adjustable voltage dividers connected across appropriate sections of the power supply. The inner helix is connected to the required potential with respect to the other tube elements as is the outer, active helix. The connection to the outer helix may be made through a choke 31 to isolate the radio frequency potentials on that helix from the power supply.

A solenoid 32 is positioned around the tube envelope and extends along substantially the whole length of the envelope. The solenoid 32 produces a magnetic field having components along the axis defined by the helices. As the tube envelope 12 is of a magnetically transparent material, the magnetic field is substantially unimpeded by the envelope. The active helix 18 is provided with an input 34 through which the signal wave may be passed into the tube. An output 36, provided at the opposite end of the active helix from the input, provides an exit for the amplified wave.

In operation, space charge depression of potential within the annular beam of electrons tends to effect a lower potential at the inner portions of the beam along the inner surface thereof than along the portions on the outer surface of the beam and adjacent the active conductor. A lower electron potential corresponds to a lower electron velocity. Hence, when the higher velocity portions of the beam adjacent the outer helix have a velocity substantially equal to that of the wave on that helix, so as to effect interaction and thus amplification, the lower velocity inner portions will prove less effective, if effective at all, to produce amplification. However, the invention provides an inner, static-potential helix which is at the same potential as the direct current bias of the outer helix. Thus, the portions of the electron beam adjacent the inner helix are maintained at substantially the same potential, and hence the same velocity, as the electrons adjacent the outer helix. The neutralization of space charge depression of potential improves the coupling factor and the operation of the tube. This improvement is obtained without any substantial deterioration of the field on the active helix and without the initiation of undesired modes of propagation. Electron beams of higher densities, corresponding to tubes of higher power, are thus possible.

While the potential on the inactive conductor has been described as being chosen to be the same as the bias on the active conductor, the potential on the inactive conductor may be made higher than the bias on the other conductor to effect a more uniform overall neutralization of space charge for electron beams having an appreciable width. Thus, in such beams the space charge potential starting from the outer, active helix tends to be slightly depressed at first but is then slightly increasedv so that the greater portion of the beam is at substantially the same potential as that on the outer helix.

In Figure 2 there is seen a cross-sectional view of the tube shown in Figure l. An annular electron beam 38, that is, a hollow beam of annular cross-section, is seen positioned between the inner helix 16 and the outer helix 18. A traveling wave initiated at the input of the outer helix f3 is propagated along that helix at substantially the same axial velocity as that of the electro-n beam. As substantially all of the electrons in the beam are traveling at substantially the same axial velocity as the axial velocity of the wave, interaction is effected between substantially all of the electrons and the wave.

Some of the kinetic energy of the electrons is converted into wave energy causing the wave to increase in amplitude as it travels along the helix toward the output thereof. The rest of the energy of the electrons in the beam is dissipated at the collecting electrode after the travel of the electrons along the helices.

It will be appreciated that other embodiments of the invention will be apparent to those skilled in the art. For example, in place of the active helix described above, a delay line having a signal wave structure in the form of a Ibaflied waveguide with a coaxial static-potential helix may be used. Two parallel helices side by side may be used instead of the concentric ones shown in Figures 1 and 2, an electron -beam being projected in the space defined between them. While the helices may be of circular cross-section, a greater efficiency may be achieved by the use of helicesk of elliptical or rectangular crosssection. Here, one conductor would serve as the active one and the other as the inactive, static-potential one. As a further alternative, two parallel, flat helices may be used with a flat electron beam projected in the space defined between them; the whole wave transmission assembly would then take the shape of a pair of zig-zag conductors in substantially parallel planes sandwiching between them an electron beam.

In any of the embodiments disclosed the delay line and the stabilizing helix may be connected together inside the tube, through a suitable choke, for operation at the same direct current potential.

It will be apparent from the foregoing description of the invention that a novel means is disclosed for providing a traveling wave tube amplifier having a uniform velocity electron beam and in which an increased gain,

power output, and efciency is produced without the initiation of undesired modes of propagation.

What is claimed is:

1. An elongated traveling wave amplifying tube adapted to be excited by a signal wave, and comprising a delay line adapted to propagate a rst traveling wave representative of said signal Wave in a path along the longitudinal axis of the tube with a velocity less than that of light, means for projecting a beam of electrons in a path adjacent to said path of said traveling wave and at a velocity substantially equal to said velocity, whereby interaction is effected between said beam and said traveling wave for producing amplification of said wave, means comprising a helical conductor adapted to propagate thereon a second traveling wave with a velocity which is substantially different from that of said rst traveling wave and said beam and being disposed on the side of said beam remote from said line for stabilizing the beam velocity by suppressing space charge depression of potential between said delay line and said helical conductor, resulting in improved amplication of said signal wave, and input and output signal coupling means coupled to the ends of said delay line only.

2. The traveling wave amplifying tube as defined in claim 1 and in which the delay line and the helical conductor are connected together for operation at substan tially the same direct current potentials.

3. A traveling wave tube structure comprising an envelope, an active wave-propagating helix within said envelope, an inactive wave-propagating helix concentric with said active helix and disposed within said active helix, said helices defining an annular space therebetween,

means for producing an annular electron beam with a path of travel within said annular space and adjacent to said helices, said electron beam having electrons therein having a velocity which is substantially less than that of light, said active helix having a pitch and a diameter effective to propagate a Wave thereon with a velocity component along said electron beam path which is substantially equal to said velocity of said electrons, said inactive helix having a pitch and a diameter effective to propagate a wave thereon with a velocity component along said electron beam path which is substantially different from said velocity of said electrons, whereby interaction between said Wave on said inactive helix and said wave on said active helix is substantially eliminated, means for biasing said helices with predetermined direct current potentials relative to each other such that differences in velocity between said electrons are substantially suppressed, and input and output signal coupling means coupled rto the ends of said active helix only.

References Cited in the le of this patent UNITED STATES PATENTS 2,610,308 Touraton et al. Sept. 9, 1952 2,643,353 Dewey June 23, 1953 2,652,513 Hollenberg Sept. 15, 1953 2,687,777 Warnecke et al. Aug. 31, 1954 2,707,759 Pierce May 3, 1955 2,725,499 Field Nov. 29, 1955 2,742,588 Hollenberg Apr. 17, 1956 2,801,361 Pierce July 30, 1957 2,824,257 Branch Feb. 18, 1958

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