US2771565A - Traveling wave tubes - Google Patents

Description

1956 J. H. BRYANT ET AL 2,771,565

TRAVELING WAVE TUBES Filed Aug. 19, 1952 A l A A A A A A A ATTORNEY TRAVELING WAVE TUBES John H. Bryant, Nutley, and Billy D. McNary, Pompton Lakes, N. J., assignors to International Telephone and Telegraph Corporation, a corporation of Maryland Application August 19, 1952, Serial No. 305,151

6 Claims. (Cl. SIS-3.5)

This invention relates to traveling wave electron discharge devices and more particularly to means and methods for introducing a circuit loss along the path of the R.-F. field of a traveling wave in such devices to prevent undesirable oscillations therein.

The traveling wave type of electron discharge device or tube is particularly useful in wide band microwave systems since it is capable of amplifying radio frequency energy over a very wide band of frequencies. The tube includes a form of transmission line, usually a helix, for transmission of microwave energy for interaction with an electron beam closely associated with the line. The helical characteristics of the transmission line are such that the axial velocity of microwave signals conducted along the helical path is approximately the same as or slightly slower than the velocity of the electrons of the beam, whereby the electric field of the microwave signals interacts with the electron beam for amplification of the microwave signals.

Traveling wave amplifier tubes heretofore proposed usually have an electron gun and a long, slender, glassenclosed radio frequency section wherein the interaction occurs. The radio frequency section includes an input connection for the R.-F. energy at or immediately adjacent to the output of the gun and an output connection at the other end of the section adjacent to an electron collector electrode.

In the employment of this type of tube, the useful range of amplification which can be utilized is limited by a tendency to generate self-sustaining oscillations as the .amplification is increased. This effect is usually due to mismatch between the output circuit of the device and the load circuit over all or part of the wide range of frequencies to be amplified. Due to such mismatch, energy of at least certain frequencies is reflected back toward the input end of the amplifying device. When the reflected wave is not attenuated in its travel along the 1 helix in a direction opposite to the motion of the electron stream, some energy reaching the input end of the device is reflected from the input end causing the generation of self-sustaining oscillations. Thus, the energy reflected or transmitted back to the input end must be attenuated if the tube is to remain stable.

In most traveling wave tubes heretofore proposed, attempts have been made to overcome this tendency of generating self-sustaining oscillations by employing resistive or lossy material to attenuate the reflected waves. While such proposals either distributed the resistive material along the entire length of the helix or along a major portion thereof, or in lump form disposed in spaced relation to the helical conductor but in the electromagnetic field, or as a part of the helical conductor itself, such provisions limited the gain and power output of the tube. Also, where the resistive material is spaced from the conductor, it is less effective and requires a larger amount in order to absorb the reflected energy. Where the resistive material is provided in lump form, it is either spaced from the helical conductor or used as a United States Patent ice 2 terminating resistance at some radian on one end of the conductor.

It is one of the objects of the present invention, therefore, to provide an improved means for introducing a circuit loss in the path of the R.-F. field of a traveling wave tube so as to prevent self-sustaining oscillation and yet obtain high gain and maximum output power We have found that to obtain high gain and maximum power output that the resistive material must be concentrated within the smallest axial length of the helix as possible and with proper impedance match, the conductivity of the helical conductor must be maintained high and the output section of the helix must not only be as loss-less as possible but as long as permissible. It is, therefore, another object of this invention to provide the wave propagating structure of a traveling wave tube with a body of resistive material in a concentrated body having the relationship to satisfy the above-mentioned requirements which We have found desirable for high gain and maximum power output.

One of the features of this invention is the use of a loss producing material of suitable resistivity to provide a structure which has parts that taper radially at its ends relative the helix or other propagating structure of a traveling wave tube to intercept part of the high frequency field with a minimum of reflection or radiation of the R.-F. energy and which produces an impedance transition with the R.-F. field in a minimum axial distance along the propagating structure.

A further feature of this invention is the method by which the lossy material, having uniform conductivity over large temperature variations, may be constructed and located in proximity to the propagating structure of the traveling wave tube. By spraying a colloidal graphite suspended in an air hardening binder onto a form having the required shape, a properly-shaped lossy structure is formed around the helix. After the binder has set, the form is extracted with solvents leaving the required lossy structure intact.

The above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanyingdrawings, in which:

Fig. 1 is a longitudinal cros-sectional view partly in block form of a traveling wave tube in accordance with the principles of this invention;

Fig. 2 is an enlarged longitudinal cross-sectional view of the helical structure shown in Fig. 1;

Fig. 3 is a cross-sectional view taken along line 3--3 of Fig. 2;

Fig. 4 is a longitudinal cross-sectional view of an alternate embodiment of the lossy material structure of this invention;

Fig. 5 is a cross-sectional view taken along line 5-5 of Fig. 4;

Fig. 6 is a longitudinal cross-sectional view of another embodiment of this invention; and V Fig. 7 is a cross-sectional view taken along line 77 of Fig. 6.

Referring to Fig. 1, there is shown an illustrative embodiment of a discharge device adapted to be used as an amplifier at ultra-high frequencies. The arrangement shown comprises an electron beam tube including an evacuated envelope having an elongated portion 1. This portion, which is of uniform diameter along its length, connects with a large electrode-containing portion 2. The envelope 1 may be constituted of a low loss insulating material, such as glass or quartz.

. The tubular envelope portion 1 is provided at one end with means, such as a known type of electron gun 3, for producing an. electron beam or stream. The electron stream emerges from gun 3 and travels along a path that is straight throughan input coupler 4 and axially down the evacuated tubular envelope 1. The electron'stream is further concentrated and guided along an axial path within the space surrounded by a helix 5 by a magnetic field produced-by cylindrical coil 6. Electrode 7 serves to collect the electrons arriving at the end of the envelope 1.

The helix 5, which serves as the path along which the wave may be propagated, is wound with several turns per wavelength along its axis, which may preferably be of a length of 30 to 40 wavelengths of the frequency to be amplified. The helix is supported by a series of nonconductive rods 8 equally spaced around the circumference which may be composed of a ceramic material and which are disposed between the helix 5 and the envelope 1 and supported by discs 8a and 8b. The helix 5 is joined to the input coupler 4 by the input impedance matching section 9 and the output coupler 10 by the output impedance matching section 11. These matching sections are simply extensions of the helix in which the spacing between turns is increased along the circumference of the helix and act as tapered transmission lines to provide a wave transmission path of uniformly changing impedance from the relatively low impedance at the end of the couplers 4 and 10 to the relatively high impedance of the central portion of the helix 5 with a minimum reflection of energy back to the signal source.

In order to utilize the device in an operable system, there is provided an incoming wave path represented by the dotted input waveguide 12 into which there is introduced the input wave signal to be amplified. An output wave path shown as the output waveguide 13 serves to transfer the amplified output Wave to the load circuit. The wave from the input waveguide 12 and coupler 4 travels along the circumference of the helix 5 at a speed approximating that of light, but at a linear velocity along the axis of the tube which is smaller in proportion to the ratio of the distance between turns to the circumference per turn. As the beam and the radio frequency wave travels along the helix, an interaction takes place whereby energy is transferred from the beam to the wave thereby greatly amplifying the wave. As the amplified wave reaches the output end of the helix 5, it is transferred to the output waveguide 13 by means of the output coupler 10. As the amplified wave reaches the impedance matching section 11 at the output end of helix 5, even with an extremely favorable termination, at a given band of frequencies, there will still exist reflected waves at frequencies outside the given band at the output end of the helix. This wave is very little affected by the electron stream and hence will propagate back along the helix 5 toward the input end with attenuation. The reflected wave will reach the input end of the helix 5 with attenuation equal to the circuit attenuation and will in turn be reflected back toward the output end of the helix. It is obvious that there will be some reflected energy which will result in self-sustaining oscillations, provided there is not enough circuit attenuation to dampen the reflected energy. It will thus be seen that the deliberate introduction of an artificial loss along the helix so as to provide a dissipation of the reflected wave will serve to greatly increase the range of useful amplification which can be achieved with a device of this type.

In accordance with the principles of this invention, the artificial loss is introduced along the portion of the helix by a structure 14 of a suitable dissipative material, such as graphite. Referring to Figs. 2 and 3, wherein one type of structure found particularly suitable is shown, it is seen that the lossy material structure 14 is thickest at its center where it is closest to the helix 5, and it has parts that are tapered ralially outward toward each end. The center of the lossy structure is in close proximity to and preferably actually touching the helix 5. When the lossy structure 14 is in direct contact with the helix, the attenuation introduced into the circuit will be greatest. The loss-producing structure 14 is supported by the dielectric supporting members 8 of the R.F. propagating structure and is situated between and adjacent to the supports. The location of the lossy material around the propagating structure of the traveling wave tube permits the interception of part of the high frequency field associated with the electromagnetic energy being propagated on the helix and therefore attenuating the electromagnetic wave while the tapering of the/ends provides a gradual transition with substantially no reflection of the R.-F. energy. The disposition of the loss is generally such that most of the electromagnetic energy in the propagating structure is removed, and the signal is transmitted through the loss section by the modulation signal energy in the electron stream. At or near the output end of the loss section, the electromagnetic wave is re-excited by the modulated electron beam. The location of the lossy material is a predetermined distance from the input end such that the equation CN=-.25, where C is the coupling parameter relating the degree of interaction between the electron beam and electromagnetic wave propagated along the propagating structure of thhe traveling wave tube and N is the number of wavelengths from the input end. Pro vision of an input portion of helix having a length corresponding to CN=-.25 insures that the electron stream becomes sufliciently excited according to the R.-F. signal from input 12 that an electromagnetic wave is re-excited beyond the loss section in the output portion of the helix. This rc-excited wave in turn interacts with the modulated beam in a continuous manner as the wave and the beam progress down the tube at practically the same velocity in such a manner that the electromagnetic wave gains in amplitude. Providing a length of input portion of helix greater than that corresponding to CN=-.25 would re sult in little or no added gain if the attenuation of the electromagnetic wave by the lossy material is high enough to remove most of the electromagnetic energy.

One method of constructing the lossy material structure, that we have found satisfactory, comprises the steps of first preparing the helix assembly to insure its cleanliness and freedom from oxides, acids, and salts. The dielectric supporting rods 8 are equally spaced around the helix 5 so that the magnitude of loss desired may be easily reproduced. The helix 5 is then painted with a thin film of organic plastic, such as nitrocellulose containing the necessary solvents and plasticizers, to give a thin, tough cylindrical coating over the helical structure when the plastic is dry. Care must be exercised that the cylindrlcal film does not contain holes between turns of the helix wire and that a uniform coating is obtained which makes contacts with supporting rods 8 but which does not extend up the sides of the rods 8. The helical structure, or a desired part thereof, is then dipped in or otherwise subjected to molten parafiin or other similar material, and since the helical structure is cooler than the paraffin at its melting point, the paraffin adheres in a uniform coating over the helix. The coat of wax is built up by repeated applications until a paraffin wax form equal substantially to the outside diameter of the supporting rods is obtained. The wax, for example, may also be applied with a brush or a smooth glass rod. The wax is then scraped or molded into a bell-shaped taper down to the organic film. The nitrocellulose film can also be tapered or removed so that turns of the helix are exposed to contact the lossy material by using a smooth glass bead which has been dipped in acetone. Rubbing the acetone over the dry organic film will dissolve the surface layers, and thus the thin lacquer film can be removed. Colloidal graphite in a binder is then sprayed or otherwise applied onto the prepared form around the helical structure. After the binder has set, the wax is extracted by using carbon tetrachloride or other suitable solvent, andnitrocellulose film being extracted with acetone or other suitable solvent. Although it is not essential, the structure can be air fired at 375 C. after the wax form is removed and may then be vacuum fired at 800 C. to remove gases from the lossy material and other parts of the tube. When it is necessary to place the loss structure in contact with the helix to produce the required amount of attenuation in the shortest possible axial length of helix, the degree of electrical contact must remain constant through large variations in the temperature of both the helix and the energy absorbing loss structure. Since the thermal expansion coclficients of the helix material and of suitable loss material cannot be exactly matched and since a temperature gradient of unpredictable magnitude will exist between the helix and the loss structure, it is desirable to bind particles of loss material of microscopic size in definite contact with the surface of the helix wire. A slightly flexible binder is required to hold the particles of loss material in contact with the helix wire and in contact with each other. As shown by the above steps, the requirements on the binder for the colloidal graphite are extremely severe since it must mix with graphite, be adaptable to spraying, must wet wax, collodian, and glass sufficiently well to start a uniform film. The binder must set by drying in air adequately at room temperature within a reasonable time to withstand the action of hot carbon tetrachloride and acetone and must then harden when baked. Shrinkage must be low enough that the lossy material is not pulled away from the helix or develop cracks and discontinuities in the surface thereof. We have found that the silicates with their high melting and decomposition temperatures are the best choice as a binder, particularly sodium silicate which we have found to have the most desirable properties for fabrication.

With the method described above, it is possible to taper the lossy material structure as near to the helix and in any profile required to give the proper impedance match. Such a structure has stable electrical properties with respect to temperature and is able to withstand the usual mechanical vibration tests.

Referring to Figs. 4 and 5, an alternate embodiment of a lossy material structure in accordance with the principles of this invention is shown wherein a graphite block 15 is constructed to fit into and around the space between the dielectric supporting members 8 of the propagating structure 5. Since the field is confined to a region within an extremely small part of a wavelength from the helical transmission line in order to produce the magnitude of attenuation necessary to prevent self-oscillation, we have found that the graphite block 15 must be situated in extremely close proximity to the helix 5. The close tolerances required and the necessary taper are obtained by machining the graphite blocks to fit the specifications of the helix and supporting rods. Graphite appears to have the most desirable electrical and vacuum properties than any material used for RAF. loss by absorption. The electrical properties of graphite are sensitive to metallic impurities, and for reproducible results we have found it desirable to use spectroscopically pure graphite produced artificially by electrolytic methods.

Referring to Figs. 6 and 7, another embodiment of a lossy material structure in accordance with the principles of this invention is shown for use with a traveling wave electron discharge device of the hollow electron beam type wherein said lossy material structurae 16 is axially disposed within the propagating structure 17. The propagating structure 17 is supported on rods 18. The lossy material structure 16, disposed between support rods 18, is tapered on both ends away from the propagating structure 17 and at its center is in close juxtaposed relation with or touching the helix 17. The lossy material structure 16 may be formed and then slid over support rods 18 inside the helix 17, during the construction of the propagating structure.

While we have described above the principles of our invention in connection with specific apparatus, it is to be clearly understood that this description is made only by Way of example and not as a limitation to the scope of our invention as set forth in the objects thereof and in the accompanying claims.

We claim:

1. In a traveling wave electron discharge device having a conductor in the form of a helix for transmission of radio frequency energy and means to project a beam of electrons parallel to the axis of said helix having interaction with the electromagnetic field of the radio frequency energy transmitted by said conductor; an attenuator disposed along certain of the turns of said helix, said attenuator comprising a concentrated body of resistive material having its central portion only in direct contact with at least one of said certain turns for maximum attenuation and spaced relative to other of said certain turns for impedance transition, said helical conductor being supported by a plurality of dielectric rods disposed parallel to the axis of said helix and said portion of said body being disposed transversely between at least two of said rods.

2. In a traveling wave electron discharge device according to claim 1, wherein said dielectric rods are disposed along the outside of said helix and said body of resistive material surrounds said helix and rod assembly and said portion extends radially inwardly between two of said rods.

3. In a traveling wave electron discharge device according to claim 1, wherein said rods are disposed on the inside of said helix and said body is disposed within said helix with said portion extending radially outwardly between two of said rods.

4. A method of forming an attenuator in operative relation with respect to the helical conductor of a traveling wave electron discharge device comprising the steps of assembling in association with a part of said helix a body of material conformed to the configuration of said helix, one part of said material being electrically resistive and non-soluble in certain dissolvents while other of said material is soluble and removing said soluble part by subjecting the assembly to a dissolvent, said soluble part being first applied to form a body about said helix, selectively removing a portion of said body adjacent certain of the turns of said helix, applying onto said body a coating of said non-soluble resistive material whereby when said soluble part is removed the resistive material is disposed in desired relation to the turns of said helix, said resistive material comprises graphite and a binder of silicate.

5. A method of forming an attenuator according to claim 4, wherein the soluble part includes a wax material and said certain dissolvent comprises carbon tetrachloride.

6. A method of forming an attenuator according to claim 4, wherein said soluble part includes a first layer of organic plastic of nitro-cellulose and a second layer of parafiin, and said certain dissolvent includes carbon tetrachloride for removing the wax and acetone for removing the organic plastic.

References Cited in the file of this patent UNITED STATES PATENTS 1,769,599 Niedich July 1, 1930 2,359,302 Curtis Oct. 3, 1944 2,471,732 Feenberg May 31, 1949 2,588,831 Hansell Mar. 11, 1952 2,602,148 Pierce July 1, 1952 2,611,101 Wallauschek Sept. 15, 1952 2,626,371 Barnett et al. Jan. 20, 1953 2,636,948 Pierce Apr. 28, 1953 2,646,549 Ragon et al. July 21, 1953 2,679,019 Lindenblad May 18, 1954

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