Helix type travelling wave tube structure with supporting rods covered with boron nitride or artificial diamond

Abstract: A travelling wave tube structure is used for propagation of electron beam, and comprises a metal tube member (11) having an inner surface defining a hollow space, a helix member (12) provided in the hollow space, and a plurality of supporting rods (13a/ 13b/ 13c) provided between the inner surface and the helix member and circumferentially spaced at predetermined angle from one another, wherein each of the supporting rods is implemented by a quartz rod member (14) preferable in view of mechanical strength and covered with substance selected from the group consisting of boron nitride and artificial diamond preferable in view of dielectric constant and of thermal conductivity.

Description: FIELD OF THE INVENTION This invention relates to a helix type traveling wave tube structure and, more particularly, to supporting rods associated with a helix of the traveling wave tube structure. DESCRIPTION OF THE RELATED ART The helix type traveling wave tube structure such as a traveling wave tube or a backward traveling wave tube serves as a delay circuit structure. Since electron beam passes close thereto, part of the electron beam impinges upon the helix type traveling wave tube structure, and produces heat. The resistance loss of the high-frequency electric power is also causative of heat. If the helix type traveling wave tube structure is small in heat capacity, the helix type traveling wave tube structure reaches fairly high temperature, and the fairly high temperature increases the high-frequency resistance loss, and promotes generation of gas. This results in deterioration in output power characteristics as well as in beam transmission, and undesirable noises are increased. Moreover, these undesirable phenomena shrink the service life of the helix type traveling wave tube structure. On the other hand, the helix type traveling wave tube structure is expected to propagate higher-frequency and larger-power electron beam, and research and development efforts have been made on heat-resistive helix, supporting rods of substance with large dielectric constant and cooling technologies. Figs. 1 and 2 show a typical example of the wave traveling tube structure, and the prior art wave traveling tube structure comprises a metal tube member 1, and a helix member 2 is inserted in the metal tube member 1. The helix member 2 extends along the longitudinal direction of the metal tube member 1, and is formed of refractory metal such as tungsten or molybdenum, because the refractory metal is less deformable when electron beam impinges thereon. The helix member may be formed by a refractory metal tape. The prior art wave traveling tube structure further comprises three supporting rods 3a, 3b and 3c inserted between the metal tube member 1 and the helix member 2, and the supporting rods 3a, 3b and 3c and the helix member 2 are stationary with respect to the metal tube member 1. The supporting rods 3a, 3b and 3c are formed of dielectric substance. Beryllia ceramic has been used as the dielectric substance, because beryllia ceramic is large in heat conductivity. However, aluminum nitride or anisotropic boron nitride small in dielectric constant are developed and available for the dielectric substance. Such structure is disclosed by FR-A-2 476 908. The anisotropic boron nitride has a laminated structure, and a direction parallel to the component layers and a direction perpendicular to the component layers are respectively referred to as "a-direction" and "c-direction". Physical and mechanical properties are widely different between the a-direction and the c-direction, and the physical and mechanical properties in the a-direction are better than those in the c-direction. For this reason, the supporting rods 3a, 3b and 3c are arranged in such a manner that the a-direction is substantially perpendicular to the contact surfaces 4 with the helix member 2. Accordingly, the c-direction is substantially parallel to the contact surfaces 4. Magnetic units (not shown) are provided around the metal tube member 1 so as to confine the electron beam into the helix member 2, and the metal tube member 1 is usually formed of stainless steel. As described hereinbefore, the helix member 2 and the supporting rods 3a to 3c are stationary with respect to the metal tube member 1, and a distortion squeezing technique is applied thereto. Namely, radial force is outwardly exerted on the metal tube member 1, and, accordingly, the metal tube member 1 is increased in diameter. The helix member 2 accompanied with the supporting rods 3a to 3c are inserted into the metal tube member 1 radially expanded, and the radial force is removed from the metal tube member 1. Then, the metal tube member 1 squeezes the supporting rods 3a to 3c and the helix member 2, and the elastic force of the metal tube member 1 makes the helix member 2 and the supporting rods 3a to 3c stationary with respect to the metal tube member 1. If the supporting rods 3a to 3c are formed of beryllia ceramic or aluminum nitride, the thermal conductivity and the mechanical strength are acceptable. However, the dielectric constant is relatively high, i.e., epsilon = 6.5 to 8, and the relatively high dielectric constant is undesirable in view of efficiency of the wave traveling tube structure. If the supporting rods 3a to 3c are formed of anisotropic boron nitride, the anisotropic boron nitride is small in the mechanical strength, and the contact surfaces 4 of the supporting rods 3a to 3c are much liable to be cracked due to shearing force exerted thereon upon squeezing. The cracks deteriorates the high frequency characteristics, and the gain is lowered. The cracks tend to be developed due to heat history during long service time, and, finally, the wave traveling tube becomes inoperable. Thus, there is a trade-off between the dielectric constant and the mechanical strength. SUMMARY OF THE INVENTION It is therefore an important object of the present invention to provide a helix type wave traveling tube structure the supporting rods of which are formed of a substance excellent in the dielectric constant and the mechanical strength. To accomplish the object, the present invention proposes to form a supporting rod by using a quartz rod covered with boron nitride or artificial diamond. In accordance with the present invention, there is provided a wave traveling tube structure, comprising: a) a metal tube member having an inner surface defining a hollow space; b) a helix member provided in the hollow space; and c) a plurality of supporting rods provided between the inner surface and the helix member, and circumferentially spaced at predetermined angle from one another, each of the supporting rods being a quartz rod member covered with boron nitride or artificial diamond. Quartz is as large in flexural strength as 7 kg/mm², and the dielectric constant is of the order of 3.9. However, the thermal conductivity of quartz is about 1 watt/m·k, and is too small to use as the substance of a supporting rod in comparison with that of beryllium oxide of 250 watt/m·k. On the other hand, boron nitride and artificial diamond are as large in thermal conductivity as about 60 watt/m·k, and the dielectric constants ranges between 3 to 6. Therefore, composite material thereof is Preferable for a supporting rod rather than the prior art substance. BRIEF DESCRIPTION OF THE DRAWINGS The features and advantages of the helix type wave traveling tube structure according to the present invention will be more clearly understood from the following description taken in conjunction with the accompanying drawings in which: Fig. 1 is a partially cut-away perspective view showing the structure of the prior art wave traveling tube structure; Fig. 2 is a cross sectional view showing the arrangement of the prior art wave traveling tube structure; Fig. 3 is a partially cut-away perspective view showing the structure of a wave traveling tube structure according to the present invention; Fig. 4 is a cross sectional view showing the arrangement of the wave traveling tube structure shown in Fig. 3; Fig. 5 is a partially cut-away perspective view showing the structure of another wave traveling tube structure according to the present invention; and Fig. 6 is a cross sectional view showing the arrangement of the wave traveling structure shown in Fig. 5. DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment Referring to Figs. 3 and 4 of the drawings, a wave traveling tube structure embodying the present invention comprises a metal tube member 11 of stainless steel, a helix member 12 of tungsten is inserted in the inner hollow space of the metal tube member 11, and supporting rods 13a, 13b and 13c. The helix member 12 extends along the longitudinal direction of the metal tube member 11, and is formed from a tungsten tape having width of about 1.5 millimeters and thickness of about 1 millimeter. The helix member 12 is about 2 millimeters in inside diameter. Each of the supporting rods 13a to 13c has a rectangular cross section of 1 millimeter by 2 millimeters, and is about 100 millimeters in length. The supporting rods 13a to 13c are spaced apart from one another at about 120 degrees, and each of the supporting rods 13a to 13c is formed of a quartz rod 14 covered with a boron nitride film 15. The boron nitride film 15 is deposited to thickness of about 50 microns by using a plasma-assisted chemical vapor deposition process. The helix member 12 and the supporting rods 13a to 13c are fixed to the metal tube member 11 through the distortion squeezing technique. Namely, radial force is outwardly exerted on the metal tube member 11, and, accordingly, the metal tube member 11 is increased in diameter. The helix member 12 accompanied with the supporting rods 13a to 13c are inserted into the hollow space of the metal tube member 11 radially expanded, and the radial force is removed from the metal tube member 11. Then, the metal tube member 11 squeezes the supporting rods 13a to 13c and the helix member 12, and the elastic force of the metal tube member 11 makes the helix member 12 and the supporting rods 13a to 13c stationary with respect to the metal tube member 11. Since the quartz is large enough in mechanical strength to withstand the elastic force, no crack take place in contact surfaces of the supporting rods 13a to 13c with the helix member 12, and high reliability is achieved. Moreover, the boron nitride films 15 are low in dielectric constant and high in thermal conductivity, and the wave traveling tube structure implementing the first embodiment achieves high efficiency and large high-frequency output characteristics. Second Embodiment Turning to Figs. 5 and 6 of the drawings, another wave traveling tube structure embodying the present invention is illustrated. The wave traveling tube structure shown in Figs. 5 and 6 are similar in structure to the first embodiment except for supporting rods 23a, 23b and 23c, and the other components are labeled with the same references designating the corresponding components of the first embodiment without any detailed description for the sake of simplicity. Each of the supporting rods 23a, 23b and 23c is about 100 millimeters in length, and has a generally rectangular cross section of 1 millimeter by 2 millimeters. The supporting rods 23a to 23c are implemented by respective quartz rods 24 covered with artificial diamond films 25, respectively, and the thickness of each artificial diamond film 25 ranges from about 5 microns to about 100 microns. The artificial diamond is deposited by using a plasma-assisted chemical vapor deposition technique, and the helix member 12 and the supporting rods 23a to 23c are fixed to the metal tube member 11 through the distortion squeezing technique. Since the artificial diamond is large enough in mechanical strength to withstand the elastic force, no crack take place in contact surfaces of the supporting rods 23a to 23c with the helix member 12, and high reliability is achieved. Moreover, the artificial diamond films 25 are low in dielectric constant and high in thermal conductivity, and, accordingly, the wave traveling tube structure implementing the second embodiment also achieves high efficiency and large high-frequency output characteristics. Although particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that various changes and modifications may be made. For example, the helix member may be formed of another refractory, and a refractory metal wire may be available for the helix member. Various deposition techniques are available for the boron nitride films and the artificial diamond films. Moreover, the metal tube member is not limited to stainless steel.

Claims: A wave traveling tube structure, comprising: a) a metal tube member (11) having an inner surface defining a hollow space; b) a helix member (12) provided in said hollow space; and c) a plurality of supporting rods (13a/ 13b/ 13c; 23a/ 23b/ 23c) provided between said inner surface and said helix member, and circumferentially spaced at a predetermined angle from one another, characterized in that each of said supporting rods is a quartz rod member (14; 24) covered with boron nitride (15) or artificial diamond (25). A wave traveling tube structure as set forth in claim 1, in which said substance has thickness ranging from about 5 microns to about 100 microns. A wave traveling tube structure as set forth in claim 1, said substance is deposited to the entire surface of said quartz rod.

Inventors: NISHIDA KAZUHISA, NISHIDA, KAZUHISA
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1992
Main classification: H01J23
IPC codes: H01J23