US3015750A - Traveling wave electron discharge devices - Google Patents

Description

Jan. 2, 1962- J- F. SKOWRON ET AL TRAVELING WAVE ELECTRON DISCHARGE DEVICES Filed Oct. 1., 1959 2 Sheets-Sheet 1 IN VE/V TORS JOHN E SKOWRON DONALD L. Wl/VSOR ATTORNEY 1962 J. F. SKOWRON ET AL 3,015,750

TRAVELING WAVE ELECTRON DISCHARGE DEVICES Filed Oct. 1, 1959 2 Sheets-Sheet 2 CALCULATED RESONANCE CAV/TY TMO, MODE 2. FREQUENCY KM INVENTORS JOHN F SKOWRON DONALD L. W/NSOR aY j/M ATTORNEY United States Patent f corporation of Delaware Filed Oct. 1, 1959, Ser. No. 843,689 Claims. (Cl. 315-35) This invention relates to traveling wave electron discharge devices and, more particularly, to slow-wave 'interaction circuits for such devices. Although the circuit described herein may be used in other devices, such as the backward wave oscillator, immediate application may be found in the traveling wave amplifier.

The traveling wave type of tube is particularly useful in broad band microwave systems since it is capable of amplifying microwave energy over an unusually wide band of frequencies. The tube includes a form of slow wave propagating structure, such as for example a helix or a periodically loaded waveguide, for transmission of microwave energy for interaction with an electron beam closely associated with the structure. The characteristics of the propagating or interaction structure are such that the velocity of microwave energy conducted along the propagating structure is decreased until it is approximately the same as or slightly lower 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.

The basic components of a traveling wave tube are an envelope, an electron gun, a collector electrode, and a radio frequency propagating structure disposed between the electron gun and collector electrode to provide the desired interaction between the radio frequency waves and the electrons of the electron beam.

Unfortunately, one of the limitations in operating such a structure as has just been described, is that the physical size of the radio frequency propagating structure is directly proportional to the wavelength. of theradio frequency energy. At very high frequencies, the small physical size of the slow-wave structure limits the amount of electron beam power which can economically and efficiently be directed through the structure. This establishes alimit of the power which may be generated or amplified by the above-mentioned devices in very high regions of frequency. Further, as the size of the structure decreases, permissible manufacturing tolerances, which are small at best, are vastly more difficult to maintain. As is well known, the additional expense and time necessary to meet and maintain small tolerances result in high frequency devices the cost of which is grossly out of proportion to the increase in operating frequency obtained.

An important disadvantage of some prior art periodicall-y'loaded waveguide structures is that the size of the beam coupling aperture is often limited by geometricalfactors concerned with the means of achievinga usable frequency passband.

. The present invention contemplates reducing the velocity of a propagating radio frequency wave to a value less than the velocity of light by means which result in a. structure of relatively large physical size that does not limit the size of the beam. coupling aperture, that readily provides a means of adding insertion loss to the structure, that provides a large. and readily controlled frequency passband, and that makes the wave fields available for useful interaction with electrons in the open region of the structure. The invention is particularly useful as the slow wave circuit in a traveling wave amplifier or like device. In particular, the invention comprises a series of microwave cavities coupled one to Patented Jan. 2, 1962 another by metal inductive posts, and provides an interaction impedance adequate for use, for example, in a. traveling wave amplifier.

An important feature of the invention is the provision of a large structure at very high frequencies. For a given passband and frequency the structure is larger in physical size than most prior art devices and, particularly at higher frequencies, is not subject to the disadvantages and limitations of prior art devices noted hereinbefore. Further, the band width of the structure may be readily controlled through the size, number, and placement of the inductive posts, and large fractional band. widths may be obtained for a wide range of wave velocity.

Many traveling wave electron devices require the use of attenuation integral with the slow-wave structure as a means of controlling or inhibiting undesirable oscillations. Another feature of this invention is the ability to substitute lossy material for the metal inductive posts, thus supplying the required attenuation. Still further, if hollow attenuating posts are provided, cooling fluids might then be passed through them for removing heat dissipated in the attenuating material. This form of cooling is also applicable to nonattenuating regions of the structure.

The above-mentioned and other features and objects of the invention, together with their incident advantages, will be more readily understood and appreciated from the following detailed description of the preferred embodiment thereof selected for purposes as: illustrated and shown in the accompanying drawings in which:

FIG. 1 is a longitudinal cross section of a typical traveling wave electron discharge device illustrating the principles of the invention;

FIG. 2 is a perspective view with parts broken away of a portion of the interaction circuit; and,

FIG. 3 is a graphic representation of an w-fl diagram of the measured velocity characteristics of the fundamental and a space harmonic of a structure formed in accordance with the invention.

Referring now to the drawings, the specific. embodiment illustrated in FIG. 1 comprises a traveling wave tube 10 having an electron gun pole piece member 11 and an electron collector pole piece member 12. Horseshoe magnets 14 are positioned on opposite sides of the tube 10 with the pole piece members 11 and 12 between like poles of the magnets; other means could, of course, be employed to provide the requisite magnetic field. Positioned within the pole piece member 11 is the electron gun which comprises a cathode 16 having a heater element 17 positioned therein and a focusing electrode 18. The focusing electrode 18 supports the cathode by short wire struts 20 and is in turn supported from a vitreous base 21 by leads 22 extending therethrough. Additional leads 23 extend through base 21 and support the heater element 17. The base 21 is sealed to the pole piece member 11 by an intermediate metallic ring 24.

Positioned within the pole piece member 12 is: acollector electrode 27 which may advantageously have integral therewith heat radiating fins (not shown). A part of the collector electrode forms a portion of the envelope of the tube 10 and is hermetically sealed in the pole piece member 12 by two metallic rings 29 and an intermediate glass ring 30.

Situated between the pole pieces 11 and 12 is the inter action circuit with separate input. and output coaxial line connections.

The interaction circuit comprises a hollow metallic housing 35 having apertured end plates 36 and 37, the apertures therein communicating with apertures 38 and 39, respectively, in the base pol'e piece members 11 and 12. Included within the interaction circuit are a plurality of equally spaced longitudinal conductive rods 44 forming a series of inductive posts, six rods being shown in the drawing by way of example. Although continuous rods are shown, it will be readily appreciated from the following and more complete description that separate inductive posts may be utilized, since each of the rods is conductively connected to alternate ones of the equally spaced apertured walls 42 as by brazing at the passages 41 therein through which the respective rods extend. Each set of alternate walls is provided with a plurality of aligned coupling apertures 43 through each of which an inductive post 44 extends in spaced relationship therewith to couple each cavity to an adjacent cavity, such as, for example, cavities 45 and 46. Thus, the inductive posts inter alia function as phase sensitive elements in each cavity and couple each cavity to the next succeeding cavity, with alternate walls comprising one half of the total number of walls being electrically connected to one set or group of inductive posts and insulated by air gaps from the intermediate walls, while the remaining or intermediate walls are electrically connected to another set or group of inductive posts and insulated by air gaps in like manner from aforementioned alternate walls.

. Axially aligned apertures 48 are located in each of the walls to form an electron beam path through the slow wave structure, with the inductive posts being radially disposed about apertures 48.

' Provided at one end of the housing 35 is a coaxial input line 51 comprising an outer conductor 52 connected to the housing 35 and an inner conductor 53 effective to couple an electromagnetic wave signal to be amplified to the interaction circuit. Similarly, connected to the opposite end of the housing 35 is a coaxial output line 54 comprising an outer conductor 55 connected to the housing 35 and an inner conductor 56 effective to remove the amplified signal from the interaction circuit.

In the operation of the specific embodiment of this invention, an electron stream is projected from the electron gun, suitable voltage supplies 57 and 58 being provided for the heater element 17, cathode 16 and electrode 18. The electron stream is focused to pass through the aperture 38 and through each of the apertures 48 under the restraint of the magnetic field provided by the magnets 14, and is collected by the collector electrode 27 to which may be applied a suitable bias from a voltage supply 59.

As the hollow housing 35 is advantageously composed of copper or other metal with a high heat conductivity, as normally are the other components of the interaction circuit, the circuit may in conventional manner dissipate a large amount of heat which may be generated in the various elements by ordinary resistive losses from high frequency currents or by bombardment by stray high velocity electrons. The heat so generated is advantageously conducted outward by the walls to the housing and dissipated in the surrounding air. More efficient cooling, however, may be simply and economically provided for either normal or high power operation by providing a hollow passage, as shown in FIG. 2, in one or more of the rods and passing a coolant therethrough. Since the rods are symmetrically disposed within the housing in contact with the walls at relatively short intervals, a very eflicient and high degree of cooling may be obtained in this manner. It will be readily appreciated that this is an important additional attribute of the invention for high power and high frequency operation, and that the rods or inductive posts are peculiarly well adapted for this purpose. Further, in this respect the rods or inductive posts perform a new and novel dual function.

: In order to describe the action and advantages of a circuit in accordance with this invention, it may be noted that the spatial harmonic components of a propagating mode may be described by arbitrarily defining the propagation constant of the nth spatial harmonic to be p ==p +21m, where p is the propagation constant of the fundamental harmonic. The present invention behaves as a band pass filter and functions to enhance interaction with n=1 space harmonic component of the mode having the lowest value of frequency. The measured velocity characteristics of the fundamental n=0 and the space harmonic n=-l of an actual embodiment of the invention operated in the TM mode are presented in FIG. 3 in the form of a (0-13 diagram.

If the structure is operated at a frequency such that ,BL=1r phase shift per section, where L is the length of each cavity in the direction of wave propagation, the phase of the traveling wave will vary by 1r radians from cavity to cavity and the net voltage appearing across the length of each inductive post will be zero. A charge will, however, periodically accumulate at each aperture through which a post passes, and may be considered to add to the capacitance of a lumped circuit analogue of the structure. This effect, however, is more or less balanced by a decrease in inductance resulting from volume distortion of the cavity magnetic fields by the inductive posts. The net result, therefore, is that the frequency at which }3L=1r is approximately the same as the resonant frequency of each cavity resonated alone and with out the presence of the inductive posts. When the structure is operated at a frequency such that 3L=0 phase shift per section, a voltage appears across the length of the posts, and current will flow through the posts resulting in a magnetic field. Since this field is in opposition to the unperturbed cavity magnetic fields, the inductance of the system will be reduced and the frequency at which the system operates at pL=0 phase shift per section will thus be appreciably greater than the resonant frequency of the unperturbed cavity. The double resonant feature of the system, at fiL=O and 1r, results in a passband of frequencies by the resonances when the structure is matched to standard transmission lines.

Unlike a strapped magnetron type line, where the straps depress the resonant frequency at ;3L=1r from that of the unstrapped system, the inductive post structure perturbs the BL=0 resonance up in frequency, the resonant frequency at 13L=1r remaining approximately the same as that of the unperturbed cavity (see FIG. 3). This feature results in a large physical size relative to the operating wavelength, and is advantageous at high frequencies where problems with power and thermal dissipation are encountered.

The bandwidth of a structure constructed in accordance with this invention may be readily controlled through selection of the size, number, and placement of the inductive posts to provide the desired bandwidth, and large fractional bandwidths may be obtained over a wide range of wave velocities. Passbands of 70% (the frequency spread divided by the mean frequency) may be obtained. Inasmuch as known design procedures for traveling wave tubes do not generally lend themselves to accurate and dependable predictions of component size and location, and depend to a greater or lesser extent on empirical procedures and the construction of prototypes, it will be readily appreciated that provision of specific bandwidths and other desired parameters of the invention cannot be specifically defined, and that they can be determined or selected in conventional manner. If the apertures 43 in the walls 42 are too small, this will result in moving the whole frequency curve down. The optimum size of these apertures is that which reduces the capacitance between the posts 44 and the periphery of the apertures to a point at which it is negligible when compared to the capacitance of an unperturbed cavity. Further, the presence and location of the inductive posts readily lend themselves to the provision in the interaction circuit of means for introducing attenuation into the system capable of dissipating large amounts of power, for isolation, and to prevent oscillations.

Since many changes could be made in the above-described construction of this invention, for example, various figures, shapes and sizes of elements may be employed, and many apparently widely difierent embodiments of the invention could be made without departing from the scope thereof, it is intended that all material contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in alimited sense.

What is claimed is:

1. An electron discharge device comprising: a tubular metallic base member and a plurality of transverse metallic walls defining a series of microwave cavities therein; said walls each having a centrally located aperture and a plurality of coupling apertures and passages disposed away from said central aperture in a predetermined manner, said coupling apertures and passages in each said wall being orientated such that a passage is interposed between and aligned with an adjacent pair of coupling apertures to form a plurality of sets of alternate and aligned coupling apertures and passages; a hollow conductive post passing in predetermined spaced relationship through each coupling aperture and in registry and electrical contact with the periphery of said passages adjacent the coupling aperture through which it passes, whereby a coolant may be passed through and retained within said passages and said posts; means for introducing an electromagnetic wave to said device; means for projecting a beam of electrons through said aligned apertures; and means for removing said wave from said device.

2. An interaction circuit for an electron discharge device comprising: a space-defining tubular base member having conductive portions; a plurality of transverse metallic walls positioned within said base member in electrical contact with said conductive portions defining a series of substantially identical microwave cavities, said walls each having a centrally located aperture and comprising two groups alternately arranged, the walls of the first of said groups having a plurality of aligned coupling apertures and passages and the walls of the second of said groups having a plurality of coupling apertures and passages in alignment with said first-mentioned passages and coupling apertures respectively; and a hollow conductive post extending in spaced relationship through each said coupling aperture and in register and electrical contact at its ends with each passage adjacent the coupling aperture through which it extends whereby a coolant may be passed through and retained within said passages and said posts.

3. An electron discharge device comprising: a tubular base member and a plurality of transverse walls defining a series of microwave cavities therein, said Walls each having a centrally located aperture, said walls comprising a first group having coupling apertures and a second group having coupling apertures, a respective one of said second group being interposed between each adjacent pair of said first group with said centrally disposed apertures in substantial alignment; conductive members extending between adjacent walls of each of said groups through respective ones of said coupling apertures in the other of said groups in electrically insulated relationship with respect to said coupling apertures to couple together the two of said cavities adjacent each said coupling aperture for the passage of microwave energy therebetween; means for introducing an electromagnetic Wave into said device; and means for projecting a beam of electrons through said aligned central apertures. I

4. An electron discharge device comprising: a tubular metallic base member and a plurality of transverse metallic walls defining a series of microwave cavities therein, said walls each having a centrally located aperture and a plurality of coupling apentures disposed away from said central aperture, said walls comprising a first group having first aligned coupling apertures and a second group having second aligned coupling apertures, a respective one of said second group being interposed between each adjacent pair of said first group with said centrally disposed apertures in substantial alignment; conductive members extending between adjacent walls of each of said groups through respective ones of said coupling apertures in the other of said groupsin electrically insulated relationship with respect to said coupling apertures to couple together the two of said cavities adjacent each said coupling aperture for the passage of microwave energy there between; means for introducing an electromagnetic wave into said device; means for projecting a beam of electrons through said aligned central apertures; and means for removing said wave from said device.

5. An electrondischarge device comprising: a tubular metallic base member and a plurality of transverse metallic walls defining a series of microwave cavities therein, said walls each having a centrally located aperture and a plurality of coupling apertures disposed away from said central aperture in a predetermined manner, said walls comprising a first group having first aligned coupling apertures and a second group having second aligned coupling apertures, a respective one of said second group being interposed between each adjacent pair of said first group with said centrally disposed apertures in substantial alignment; conductive members extending between adjacent walls of each of said groups through respective ones of said coupling apertures in the other of said groups in electrically insulated relationship with respect to said coupling apertures to couple together the two of said cavities adjacent each said coupling aperture for the passage of microwave energy therebetween; means for introducing an electromagnetic wave into said device; means for projecting a beam of electrons through said aligned central apertures; and means for removing said wave from said device.

6. An electron discharge device comprising: a tubular metallic base member and a plurality of transverse metallic walls defining a series of microwave cavities therein, said walls each having a centrally located aperture and a plurality of coupling apertures disposed away from said central aperture in a predetermined manner, said walls comprising a first group having first aligned coupling apertures and a second group having second aligned coupling apertures, a respective one of said second group being interposed between each adjacent pair of said first group with said centrally disposed apertures in substantial alignment; conductive members extending between adjacent walls of each of said groups through respective ones of said coupling apertures in the other of said groups in electrically insulated relationship with respect to said coupling apertures to couple together the two of said cavities adjacent each said coupling aperture for the passage of microwave energy therebetween with zero phase shift at one frequency and degree phase shift at another frequency; means for introducing an electromagnetic wave into said device; means for projecting a beam of electrons through said aligned central apertures; and means for removing said wave from said device.

7. An interaction circuit for an electron discharge device comprising: a tubular metallic enclosing member; a plurality of metallic walls positioned within said enclosing member defining a series of microwave cavities, said walls having aligned centrally located apertures, each of said walls having a coupling aperture; and a plurality of conductive members each extending through the coupling aperture of a respective one of said walls in electrically insulated relationship therewith into conductive contact with both the walls next adjacent such respective one of said walls to couple together the two of said cavities having such respective ones of said walls in common for the passage of microwave energy therebetween.

8. An interaction circuit for an electron discharge device comprising: a space-defining tubular base member having conductive portions; a plurality of transverse metallic walls positioned within said base member in electrical contact with said conductive portions defining a series of microwave cavities, said walls each having a centrally located aperture and comprising two groups alternately arranged, the walls of the first of said groups having a plurality of aligned coupling apertures and the walls of the second of said groups having a plurality of aligned coupling apertures displaced from said first-mentioned coupling apertures; and conductive post members extending between adjacent walls of each of said groups through respective ones of said coupling apertures in the other of said groups in electrically insulated relationship with respect to said coupling apertures to couple together the two of said cavities adjacent each said coupling aperture for the passage of microwave energy therebetween.

9. An interaction circuit for an electron discharge device comprising: a space-defining tubular base member having conductive portions; a plurality of transverse metallic walls positioned within said base member in electrical contact with said conductive portions defining a series of microwave cavities, said walls each having a centrally located aperture and comprising two groups alternately arranged, the walls of the first of said groups having a plurality of aligned coupling apertures and the walls of the second of said groups having a plurality of aligned coupling apertures displaced from said first-mentioned coupling apertures; and conductive post members extending between adjacent walls of each of said groups through respective ones of said coupling apertures in the other of said groups in electrically insulated relationship with respect to said coupling apertures to couple together the two of said cavities adjacent each said coupling aperture for the passage of microwave energy therebetween with zero phase shift at one frequency and degree phase shift at another frequency.

' 10. In an electron discharge device, the combination comprising: a tubular base member and a plurality of transverse walls defining a series of microwave cavities therein, said walls each having a centrally disposed aperture and at least one coupling aperture radially displaced therefrom, said centrally disposed apertures all lying in substantial registry along the central axis of said tubular member; a conductive coupling member connected to and extending between each pair of alternate ones of said walls, each of said conductive coupling members passing through a coupling aperture in the Wall intermediate each such alternate pair in insulated relationship therewith to couple together the two of said cavities adjacent each such intermediate wall for the passage of microwave energy there'between; means for introducing an electromagnetic wave into said device; and rrieans for projecting a beam of electrons through said aligned cent-rally disposed apertures.

References Cited in the file of this patent UNITED STATES PATENTS 2,637,001 Pierce Apr. 28, 1953 2,820,923 Wilbur Jan. 21, 1958 2,827,589 Hines Mar. 18, 1958 2,844,797 Dench July 22, 1958 2,850,671 Dench Sept. 2, 1958

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