Broad-band travelling-wave tube

Abstract: A broad-band contains a cylindrical waveguide (56) made of a plurality of pole pieces (53) and a plurality of spacers (54) alternately arranged along its axis. A helix (51) is placed in the waveguide (56) to extend along the axis of the waveguide (56). The helix (51) is held by helix supports (52). The inner wall of the waveguide (56) has a plurality of offsets arranged at intervals along the axis of the waveguide (56). A bore-diameter-varying portion can be created inside a waveguide (56) without mounting vanes.

Description: The present invention relates to a travelling-wave tube and more particularly, to a broad-band travelling-wave tube that is effectively used for power amplification at microwave frequencies. The microwave power is in wide spread use in such a variety of application fields as communications, Electro Counter Measure (ECM), radar, industrial heating, particle accelerator, radio astronomy, and nuclear fusion. To meet the demand of society for realizing a high-level of information society for the coming century, the field of communications among the above various fields has been increasingly becoming important in application of microwave power. As a typical microwave tube for use at repeater stations of microwave communications or for satellite communications, a "traveling-wave tube" that uses an electron beam to amplify microwave power is available. The traveling-wave tube has a slow-wave structure that provides an interaction between a microwave input signal and an electron beam. A typical slow-wave structure is a helix, which is coaxially arranged in a cylindrical waveguide. The helix is held by dielectric supports not to be contacted with the waveguide. The helix provides such features as simple structure, easy fabrication, and broad-band functioning capability and therefore, it has been widely applied. An example of the conventional traveling-wave tubes equipped with the helix-type slow-wave structure is shown in Fig. 1. As shown in Fig. 1, this travelling-wave tube is partitioned into three sections, i.e., an electron beam (E-beam) source section 10, an interaction section 20, and a collector section 30. In the E-beam source section 10, an electron beam EB is generated and emitted toward the interaction section 20. In the interaction section 20, an interaction between the electron beam EB emitted from the source section 10 and a radio-frequency (RF) electro-magnetic field caused by an applied microwave input signal takes place. In the collector section 30, the electron beam EB having passed through the interaction section 20 is collected. The traveling-wave tube has a cylindrical vacuum envelope 1, an electron gun 11 fixed to one end of the envelope 1, and a collector 31 fixed to the other end thereof. In the envelope 1, a focusing electrode 12 is fixed to focus or narrow the electron beam EB emitted from the gun 11. The electron gun 11 and the focusing electrode 12 constitute the E-beam source section 10. In the interaction section 20, a conductive helix 21 forming a slow-wave structure is provided in a cylindrical waveguide 26 to be coaxial therewith. The helix 21 is made of a metal such as molybdenum (Mo) or tungsten (W). The helix 21 is held by dielectric supports 22 placed between the helix 21 and the inner wall of the waveguide 26. The supports 22, which have a shape of a straight bar, extend along the axis of the helix 21 from one end of the helix 21 to the other end thereof. Both ends of the helix 21 protrude outside from the vacuum envelope 1. A microwave input signal is supplied to the helix 21 through its one end. The input signal is amplified in the interaction section 20 due to the interaction between the traveling electron beam EB and the RF field caused by the input signal and then, it is taken out through the other end. The waveguide 26 is made of a plurality of pole pieces 23 and a plurality of spacers 24 which are alternately arranged along the axis of the cylindrical vacuum envelope 1 and which are coupled together. The pole pieces 23 are made of a magnetic material. The spacers 24 are made of a non-magnetic material. The pole pieces 23 are of a circular ring shape and have cylindrical cavities therein. The spacers 24 are also of a circular ring shape and have cylindrical cavities therein. The pole pieces 23 and the spacers 24 are connected together to couple their cavities with each other, thereby producing the cylindrical waveguide 26. A plurality of permanent magnet pieces 25, which have a circular ring shape, are placed outside the respective spacers 24. In other words, the magnet pieces 25 are arranged at regular intervals along the axis of the waveguide 26. The magnet pieces 25 and the pole pieces 23 produce a focusing magnetic field that focuses the electron beam EB travelling through the waveguide 26. Thus, the interaction section 20 contains the helix 21, supports 22, pole pieces 23, spacers 24 and magnet pieces 25. As seen from Fig. 1, the interaction section 20 is further partitioned into an input subsection 20a and an output subsection 20b. Electric power is given to the microwave input signal in the input subsection 20a, and the electric power thus given is taken out in the output subsection 20b. The electron beam EB having passed through the waveguide 26 in the interaction section 20 is collected by a collector 31 disposed in the collector section 30. With the conventional traveling-wave tube shown in Fig. 1, to make the slow-wave structure suitable for broad-band applications such as communications, the slow-wave structure must have a response as flat as possible over a wide range of microeave frequencies. Especially, in recent years, the operation band of the traveling-wave tube has been required to be ultra broad for use in such applications as communications, ECM, and radar. However, the interaction between the electron beam EB and the RF field tends to greatly vary depending upon the frequency and consequently, it is difficult for the slow-wave structure of the above conventional tube to realize a broad-band functioning capability. To solve this difficulty, an improvement of the slow-wave structure was developed, which was disclosed in the Japanese Non-Examined Utility-Model Publication No. 4-85637 in July, 1992. In this improved structure, as shown in Fig. 2, vanes 27 are fixed onto the inner wall 26a of the waveguide 26, thereby forming offsets on the wall 26a. With the use of this structure, the phase velocity of a traveling wave through the slow-wave structure can be made substantially uniform for frequency. Thus, this improved structure is effective for providing a broad-band functioning capability. The Japanese Non-Examined Utility-Model Publication No. 4-85637 also disclosed the structure as shown in Fig. 3, where projections 28 are formed on the inner wall 16a of the waveguide 26 instead of the vanes 27. The projections 28 are made by deforming the wall 26a toward inside. This variation can provide the same effect as that of Fig. 2. The effects of such vanes were, for example, clarified in the IEEE Transactions on Electron Devices, Vol. 36, No. 9, pp. 1991 to 1999, September 1989. This article disclosed the structures as shown in Fig. 4 and 5. In this article, the effects of the vanes 27 were analyzed using the structure of Fig. 5, in which the angular periodicity  t and the ratio (a v /a) were used as parameters. Here, the character a v indicates the bore radius of the vanes 27, and the character a indicates the radius of the helix 21. The article reported the following results: The phase velocity of a wave traveling through the slow-wave structure tends to slightly decrease with the decreasing value of the ratio (a v /a). This phase-velocity decreasing effect is more conspicuous at lower frequencies. When the value of the ratio (a v /a) is greater than a certain value, the phase velocity scarcely varies even with the frequency change. However, when the value of the ratio (a v /a) is reduced to the certain value or less, the phase velocity tends to decrease with lowering the frequency. The effect of the vanes 27 as stated above tends to be more conspicuous as the value of  t increases. Accordingly, in the actual design process, a compromise between the values of (a v /a) and  t is found to thereby provide a phase velocity as uniform as possible over a wide frequency range, thus making the microwave tube suitable for use in broad-band applications. Fig. 6 shows the relationship between the frequency f and the phase velocity v p disclosed in the above article, where the frequency f is normalized by the frequency f 0 and the phase velocity v p is normalized by the light speed c. The curve A shows the relationship obtained by actual measurement and the curve B shows the relationship obtained by calculation. As stated above, with the conventional travelling-wave tube, it is required to mount vanes 27 on the inner wall 26a of the waveguide 26 or to deform the wall 26a over a certain length. Concretely, for example, to realize a broad-band traveling-wave tube for use in the frequency band of 4 GHz to 12 GHz, it is necessary to be realized that the average radius of the helix 21 is approximately 1.5 mm, the bore diameter of the vanes 27 is approximately 2 mm, the angular periodicity  t is 40° to 70°, and the angular thickness of the dielectric supports 22 is approximately 0.5 mm. With these values, the angular thickness of the vanes 27 becomes approximately 1.5 mm. Thus, it is technically possible, but industrially difficult to mount three vanes 27 at angular intervals of 120° in equally spaced locations and yet over a distance of 200 mm or longer. For making a slow-wave structure for use in the millimeter wave band, it is practically impossible because components must be worked to approximately half the dimensions as given above. A broad-band travelling-wave tube according to the preamble of claim 1 is disclosed in Patents Abstracts of Japan JP-A-63218123. This tube comprises non-magnetic cylinders and pole pieces arranged alternately along the longitudinal axis of the tube, wherein the inner wall of the non-magnetic cylinders have smaller diameters than the pole pieces and wherein grooves extending in axial direction are provided in the non-magnetic cylinders in which axial extending helix holding rods are inserted. It is an object of the invention to provide a broad-band travelling-wave tube without using vanes. Another object of the present invention is to provide a broad-band travelling-wave tube that can be fabricated even for use in the milimeter wave-band. These objects are achieved by a tube as defined in claim 1; the dependent claims are related to further developments of the invention. A broad-band traveling-wave tube according to the present invention contains a waveguide made of a plurality of pole pieces and a plurality of spacers which are alternately arranged along the axis of the waveguide. The inner wall of the waveguide has a plurality of offsets arranged at intervals along the axis of the waveguide. With the broad-band traveling-wave tube according to the present invention, the inner wall of the waveguide, which is made of the plurality of pole pieces and the plurality of spacers, has a plurality of offsets arranged at intervals along the axis of the waveguide. Therefore, an equivalent effect to that of the conventional vanes is obtained. This means that a broad-band traveling-wave tube can be obtained without using the conventional vanes. Also, the broad-band traveling-wave tube according to the invention uses no vanes and therefore, a broad-band traveling-wave tube for use in the millimeter wave band can be fabricated. In order that the invention may be readily carried into effect, it will now be described with reference to the accompanying drawings. Fig. 1 is a schematic, axial cross-sectional view of a conventional traveling-wave tube. Fig. 2 is a schematic, radial cross-sectional view of a first example of conventional slow-wave structures. Fig. 3 is a schematic, radial cross-sectional view of a second example of conventional slow-wave structures. Fig. 4 is a schematic radial cross-sectional view of a third example of conventional slow-wave structures. Fig. 5 is a schematic, radial cross-sectional view of a fourth example of conventional slow-wave structures. Fig. 6 is a graph showing the relationship of the phase velocity with the frequency for a conventional broad-band traveling-wave tube. Fig. 7 is a schematic, radial cross-sectional view of the first example of conventional slow-wave structures. Fig. 8 is a graph showing the relationship between the phase velocity and the frequency for the conventional broad-band traveling-wave tube. Fig. 9 is a schematic, axial cross-sectional view of a waveguide for a slow-wave structure according to a first embodiment of the invention. Fig. 10 is a schematic, radial cross-sectional view of the pole piece, which is used for the waveguide according to the first embodiment. Fig. 11 is a schematic, radial cross-sectional view of the spacer, which is used for the waveguide according to the first embodiment. Fig. 12 is a schematic, axial cross-sectional view of the pole piece, which is used for the waveguide according to the first embodiment. Fig. 13 is a schematic, axial cross-sectional view of the spacer, which is used for the waveguide according to the first embodiment. Fig. 14 is a schematic, axial cross-sectional view of a waveguide for a slow-wave structure according to a second embodiment of the invention. Fig. 15 is a schematic, radial cross-sectional view of the spacer placed in the input subsection, which is used for the waveguide according to the second embodiment. Preferred embodiments of the present invention will be described in detail below while referring to the drawings attached. FIRST EMBODIMENT A broad-band traveling-wave tube according to a first embodiment of the present invention has the same basic structure as that of the conventional one shown in Fig. 1. Therefore, the description relating to the same structure is omitted here for the sake of simplicity and only the different structures from the conventional one are explained below. The traveling-wave tube according to the first embodiment is equipped with a waveguide 56 as shown in Fig. 9 in the interaction section 20. The waveguide 56 is composed of a plurality of pole pieces 53 and a plurality of spacers 54, which are alternately arranged along the axis of the cylindrical vacuum envelope 1 and which are coupled together by brazing. The pole pieces 53 are made of a magnetic material. The spacers 54 are made of a non-magnetic material. Each of the pole pieces 53 has a circular ring shape and an approximately cylindrical cavity therein. Each of the spacers 54 also has a circular ring shape and an approximately cylindrical cavity therein. The cavities of the pole pieces 53 and those of the spacers 54 are connected together to thereby produce the cylindrical waveguide 56. A plurality of permanent magnet pieces 55, which have a circular ring shape, are placed outside the respective spacers 54. In other words, the magnet pieces 55 are arranged at regular intervals along the axis of the waveguide 56. The magnet pieces 55 and the pole pieces 53 produce a focusing magnetic field that focuses the electron beam EB travelling through the waveguide 56. Similar to the conventional tube as shown in Fig. 1, a conductive helix 51 forming a slow-wave structure is provided in the waveguide 56 to be coaxial therewith. The helix 51 is made of a metal such as molybdenum (Mo) or tungsten (W). The helix 51 is held by three dielectric supports 52 placed between the helix 51 and the inner wall of the waveguide 56. The supports 52, which have a shape of a straight bar, extend along the axis of the helix 51 from one end of the helix 51 to the other end thereof. Both ends of the helix 51 protrude outside from the vacuum envelope 1. A microwave input signal is supplied to the helix 51 through its one end. The input signal is amplified in the interaction section 20 due to the interaction between the traveling electron beam EB and the RF field caused by the input signal and then, it is taken out through the other end. As shown in Figs. 10 and 12, each of the pole pieces 53 has two circumferential grooves or depressions 53c at each end. The inner surface 53a of the piece 53 has a relatively smaller diameter D p1 . The inner surface 53b of the piece 53, which is the bottom of the groove 53c, has a relatively larger diameter D p2 . Similarly, as shown in Figs. 11 and 13, each of the spacers 54 has two circumferential grooves or depressions 54c formed at regular intervals along the axis of the spacer 54. The inner surface 54a of the spacer 54 has a relatively smaller diameter D s1 , where D s1 = D p1 . The inner surface 54b of the spacer 54, which is the bottom of the groove 54c, has a relatively larger diameter D s2 , where D s2 = D p2 . With the traveling-wave tube according to the first embodiment, since the grooves 53c and 54c are periodically arranged along the axis of the waveguide 56 to thereby produce a plurality of offsets on the inner wall of the waveguide 56, an equivalent effect to that of the conventional vanes is obtained. As a result, a broad-band traveling-wave tube can be obtained without using the conventional vanes. Also, the broad-band traveling-wave tube according to the first embodiment uses no vanes and therefore, a broad-band traveling-wave tube for use in the millimeter wave band can be fabricated. Next, the reason why the equivalent effect to that of the conventional vanes is obtained in the tube according to the first embodiment will be clarified. The equivalent effect is due to hollow spaces generated by the offsets. With the conventional vane-loaded tube, the waveguide is divided into three sets of four different regions R 1 , R 2 , R 3 and R 4 , which are arranged at angular intervals of 120°, as shown in Fig. 7. In the region R 1 , the vane exists and therefore, the inner diameter of the bore is relatively small. In the region R 2 , no vane and no support exist and therefore, the inner diameter of the bore is relatively large. In the region R 3 , the support exists. In the region R 4 , no vane and no support exist and therefore, the inner diameter of the bore is relatively large, which is identical with the region R 2 . For given vane dimensions, computer simulating the phase velocity of a wave traveling through the interaction section 20 for frequency provides the relationship of Fig. 8. From Fig. 8, it is seen that, depending upon the selected value of (a v /a), the phase velocity can be made substantially uniform with the changing frequency, and thus the broad-band functioning capability can be provided. This tendency also depends upon the selected value of the angular periodicity  t of the vane. In this technique, the slow-wave circuit including the vanes is treated as a model in which the helix is developed along a linear line and the helix supports are arranged in parallel to the linear line with respect to the electro-magnetic field that travels along the helix, while rotating. On the basis of this concept or technique, a model in which bore-diameter-varying pole pieces and spacers are arranged along the helical line of the helix can be treated as the same model. In other words, whether bore-diameter-varying pole pieces and spacers are arranged in the radial direction of the waveguide or they are arranged in the axial direction thereof, both the structures are equivalent to each other. The present invention utilizes this principle and consequently, the same effects as those by the conventional tube can be obtained. The optimum dimensions of the offsets are as follows: With the conventional tubes using vanes, selecting the proper bore diameter of the vanes can provide a uniform phase velocity with respect to the frequency, as shown in Fig. 8. On the other hand, with the present invention, reducing the bore diameter by means of the convex offsets is equivalent to decreasing the bore diameter by means of the vanes in the conventional one. If the vane has the angular periodicity  t of 60°, a width equal to the pitch of the helix multiplied by (60°/360°) must be provided for the convex offset in the present invention. The smaller bore diameter must be specified to be approximately 1.5 to 2 times the average radius of the helix 51 so that the phase velocity is substantially uniform with respect to the frequency, while the larger bore diameter must be 2 to 3 times as large as the average radius of the helix 51. The reasons why such smaller and larger bore diameters are specified are that, if the larger bore diameter exceeds the above upper limit, the required bore diameter of the electron-beam focusing magnet 55 increases, resulting in the required magnetic field being not achieved, and thus focusing the electron beam being difficult, and that, if the smaller bore diameter is under the lower limit, the impedance of the interaction section 20 decreases, resulting in the lowered efficiency of the microwave tube. The pole pieces 53 are preferably made of iron, and are approximately half as thick as the spacers 54, which is made of a non-magnetic material. In addition, the thickness of the pole pieces 53 and that of the spacers 54 is preferably minimized so long as the proper magnetic field for focusing can be obtained. SECOND EMBODIMENT A broad-band traveling-wave tube according to a second embodiment of the present invention is shown in Figs. 14 and 15. The difference of the second embodiment from the first embodiment is that only the waveguide 56 for the output subsection 20b of the interaction section 20 contains the offsets while a waveguide 66 for the input subsection 20a does not contain offsets. The waveguide 66 has conventional vanes (not shown). The other structure is the same as that of the first embodiment. Accordingly, the second embodiment is equivalent to a tube which is obtained by applying the present invention to a vane-loaded traveling-wave tube. With the second embodiment, the same advantage as that of the first embodiment can be obtained. While the preferred forms of the present invention have been described, it is to be understood that modifications will be apparent to those skilled in the art without departing from the scope of the invention as defined by the following claims.

Claims: A broad-band travelling-wave tube comprising: a waveguide (56, 66) made of a plurality of pole pieces (53, 63) and a plurality of spacers (54, 64), which are alternately arranged along the axis of said waveguide; a helix (51, 61) placed in said waveguide along the axis of said waveguide; and a helix support (52, 62) for supporting said helix, said helix support being arranged between said inner wall of said waveguide and said helix; characterized in that the inner wall of said waveguide has a plurality of circumferential offsets (53c, 54c) arranged at intervals along the axis of said waveguide; and said circumferential offsets are produced by outwardly depressed areas or inwardly protruded areas formed on said inner surfaces of said pole pieces (53) and on those of said spacers (54). The tube as claimed in claim 1, wherein said waveguide is partitioned into a first portion (20a) for an input subsection and a second portion (20b) for an output subsection; and wherein only said second portion (20b) has said offsets. The tube as claimed in claim 1, wherein the inner surfaces of said pole pieces (53, 63) are substantially equal in diameter to the inner surfaces of said spacers (54) and wherein said outwardly depressed areas or inwardly protruded areas of said pole pieces are substantially equal in diameter to those of said spacers.

Inventors: TONEGAWA SATOSHI, TONEGAWA, SATOSHI, Tonegawa, Satoshi, c/o NEC Corp.
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1996
Main classification: H01J23
IPC codes: H01J23