Patent Publication Number: US-7898181-B2

Title: Traveling wave tube

Description:
This application is based upon and claims the benefit of priority from Japanese patent application No. 2007-178238, filed on Jul. 6, 2007, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a traveling wave tube having a helix which causes a high frequency signal input from the outside and an electron beam to interact with each other. 
     2. Description of the Related Art 
       FIG. 1  is a schematic longitudinal sectional view of the construction of a traveling wave tube disclosed in Japanese Patent Laid-Open No. 2005-339892 (hereinafter referred to as patent document 1). 
     As shown in  FIG. 1 , traveling wave tube  1  has electron gun  10  which emits an electron beam, high frequency circuit unit  20  which amplifies a high frequency signal (microwave), collector electrodes  30  used to capture the electron beam, anode electrode  40  used to draw out electrons from electron gun  10  and to guide the electron beam emitted from electron gun  10  to high frequency circuit unit  20 . 
     Electron gun  10  has cathode electrode  11  for emitting electrons, heater  12  for supplying thermal energy for causing emission of thermal electrons from cathode electrode  11  and Wehnelt electrode  13  used to focus electrons emitted from cathode electrode  11   
     High frequency circuit unit  20  has helix  21  which causes an electron beam emitted from electron gun  10  and a high frequency signal (microwave) to interact with each other, input circuit  22  for supplying helix  21  with a high frequency signal input from the outside and output circuit  23  for outputting the high frequency signal output from helix  21  to the outside. A waveguide or a coaxial line for transmitting the high frequency signal is connected to input circuit  22  and output circuit  23 .  FIG. 1  shows an example of the construction in which waveguide  50  is connected to input circuit  22  and output circuit  23 . 
     Each of input circuit  22  and output circuit  23  has coaxial inner conductor  24  for input or output of the high frequency signal to or from helix  21 . The high frequency signal is input to helix  21  via coaxial inner conductor  24  provided in input circuit  22 . The high frequency signal output from helix  21  is radiated from coaxial inner conductor  24  provided in output circuit  23  into waveguide  50  connected to output circuit  23 . 
     Helix  21  is supported and fixed in tubular case  26  by (ordinarily three) supporting columns  25  made of a dielectric or the like. Coaxial inner conductor  24  is fixed at a predetermined position on the sealed-end side of waveguide  50  by a ceramic window provided for vacuum-sealing the interior of case  26 . 
       FIGS. 2A and 2B  show a connection between a coaxial inner conductor and a helix disclosed in Japanese Patent Laid-Open No. 5-41175 (hereinafter referred to as patent document 2).  FIG. 2A  is an enlarged longitudinal sectional view showing an essential high frequency circuit unit, and  FIG. 2B  is a sectional view of the coaxial inner conductor taken along line A-A′ in  FIG. 2A . 
     In ordinary cases, a high-melting point metal such as molybdenum or tungsten is used for helix  21  and coaxial inner conductor  24 . It is difficult to directly weld helix  21  and coaxial inner conductor  24  made of such a metal. Therefore, as shown in  FIG. 2A , helix  21  and coaxial inner conductor  24  are connected by using metal tape  27  which is made of nickel, platinum or an alloy containing nickel and platinum, and which is weldable to helix  21  and coaxial inner conductor  24 . One end of metal tape  27  is welded to an end portion of helix  21  in advance, and another end of metal tape  27  is laser-welded to an end portion of coaxial inner conductor  24 . 
     Coaxial inner conductor  24  is columnar in ordinary cases. Therefore, patent document 2 proposes a construction in which the end portion of coaxial inner conductor  24  is chamfered into a flat surface, as shown in  FIG. 2B  for example, and the flat surfaces of metal tape  27  and coaxial inner conductor  24  are brought into contact with each other to increase the area of contact between metal tape  27  and coaxial inner conductor  24 . 
       FIGS. 3A and 3B  are diagrams showing welded portions of metal tape  27  and coaxial inner conductor  24  in the related art shown in  FIGS. 2A and 2B . 
     As described above, helix  21  is supported and fixed in case  26  by supporting columns  25 . Therefore, an operation to weld metal tape  27  to coaxial inner conductor  24  is performed from the opening side of case  26  (in direction A in  FIG. 3A ). 
     More specifically, metal tape  27  formed into an L-shape is joined to an end portion of helix  21  in advance so that the upright surface of metal tape  27  can be seen from the opening side of case  26 . Helix  21  to which metal tape  27  has been joined is supported and fixed in case  26  and laser welding for welding an end portion of metal tape  27  to coaxial inner conductor  24  is thereafter performed from the opening side of case  26 , as shown in  FIG. 3B . 
     In laser welding for joining metallic members by using laser beam as a heat source, a portion called a nugget is formed as a result of melting and solidification of a metal is formed in a portion irradiated with laser beam and its surrounding portion. A plurality of solid round marks shown at an end of metal tape  27  in  FIGS. 3A and 3B  represent welded portions (nuggets) of metal tape  27  and coaxial inner conductor  24  thus formed. 
     In the high frequency circuit unit in the related art, as shown in  FIGS. 3A and 3B , the welded portion is limited to the end of metal tape  27  as the boundary between metal tape  27  and coaxial inner conductor  24  which can be visually checked from the opening side of case  26 . Therefore, the high frequency circuit unit in the related art has a problem in that the area of contact between metal tape  27  and coaxial inner conductor is reduced. 
     If the area of contact between the metal tape and the coaxial inner conductor is reduced, the thermal conductivity at the portions of the metal tape and at the coaxial inner conductor that are in contact with each other becomes lower and, correspondingly, the heat dissipation capacity with respect to heat generated in helix  21  becomes lower to allow the temperature of helix  21  to rise. With this change, there is a risk of a deterioration in electrical characteristics of the traveling wave tube and a reduction in stability of the operation of the traveling wave tube. In the case of a high-output traveling wave tube having an increased amount of heat generation in helix  21  in particular, there is a possibility of a breakage in the traveling wave tube, such as melting and cutting of metal tape  27  at the worst. 
     According to the above-described patent document 2, the end portion of coaxial inner conductor  24  is chamfered into a flat surface and the flat surfaces of metal tape  27  and coaxial inner conductor  24  are brought into contact with each other to increase the area of contact between metal tape  27  and coaxial inner conductor  24 . Even in such a construction, the welded portion is limited to the end of metal tape  27  as the boundary between metal tape  27  and coaxial inner conductor  24  which can be visually checked from the opening side of case  26 . In such a case, since effective contact between metal tape  27  and coaxial inner conductor  24  is not ensured at any position other than the position of the welded position, there is a possibility of failure to sufficiently increase the contact area even when the construction disclosed in patent document 2 is used. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a traveling wave tube capable of limiting a reduction in heat dissipation capacity with respect to heat generated in a helix by limiting a reduction in thermal conductivity at a position at which a metal tape and a coaxial inner conductor are in contact with each other. 
     To achieve the above-described object, according to the present invention, there is provided a traveling wave tube including a helix which causes a high frequency signal input from the outside and an electron beam to interact with each other, a U-shaped metal tape having its bottom portion attached to an end portion of the helix, a coaxial inner conductor for input or output of the high frequency signal to or from the helix, the coaxial inner conductor being fixed in such a position that its end portion is interposed between upright surfaces of the metal tape. 
     The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings, which illustrate examples of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic longitudinal sectional view of a traveling wave tube disclosed in patent document 1; 
         FIG. 2A  is a longitudinal sectional view showing a connection between a coaxial inner conductor and a helix disclosed in patent document 2; 
         FIG. 2B  is a sectional view of the coaxial inner conductor taken along line A-A′ in  FIG. 2A ; 
         FIG. 3A  is a longitudinal sectional view showing a connection between the coaxial inner conductor and the helix in the related art; 
         FIG. 3B  is a side view of a high frequency circuit unit seen in direction A in  FIG. 3A ; 
         FIG. 4A  is a longitudinal sectional view showing a connection between a coaxial inner conductor and a helix in a first exemplary embodiment of the present invention; 
         FIG. 4B  is a side view of a high frequency circuit unit seen in direction A in  FIG. 4A ; 
         FIG. 5A  is a longitudinal sectional view showing a connection between a coaxial inner conductor and a helix in a second exemplary embodiment of the present invention; 
         FIG. 5B  is a side view of a high frequency circuit unit seen in direction A in  FIG. 5A ; 
         FIG. 6  is an enlarged sectional view of an essential portion of the high frequency circuit unit, showing another example of the shape of top portions of the metal tape shown in  FIG. 5A ; and 
         FIG. 7  is an enlarged sectional view of an essential portion of the high frequency circuit unit, showing still another example of the shape of top portions of the metal tape shown in  FIG. 5A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be described with reference to the accompanying drawings. 
     First Exemplary Embodiment 
       FIGS. 4A and 4B  are diagrams showing a connection between a coaxial inner conductor and a helix in a first exemplary embodiment of the present invention.  FIG. 4A  is an enlarged longitudinal sectional view of an essential portion of a high frequency circuit unit, and  FIG. 4B  is a side view of the high frequency circuit unit seen in direction A in  FIG. 4A . 
     In the high frequency circuit unit in the first exemplary embodiment, as shown in  FIGS. 4A and 4B , metal tape  27  is formed into a U-shape; a bottom portion of U-shaped metal tape  27  is attached to an end portion of helix  21 ; and coaxial inner conductor  24  inserted in case  26  from one side of the same is fixed so that its end portion is interposed between two upright surfaces of metal tape  27 . Metal tape  27  is formed by being bent at two positions so that the end portion of coaxial inner conductor  24  can be interposed between its bent portions. 
     Metal tape  27  is joined to the end portion of helix  21  while being oriented so that its U-shape is seen in direction A in  FIG. 4A , i.e., from the opening side of case  26 . 
     In the high frequency circuit unit in the first exemplary embodiment, metal tape  27  is laser-welded to two side surfaces of coaxial inner conductor  24 , as shown in  FIG. 4B , thereby joining coaxial inner conductor  24  and metal tape  27  to each other. In this welding, metal tape  27  is welded to coaxial inner conductor  24  at those portions of metal tape  27  on which the welding operation can be performed in direction A shown in  FIG. 4A , i.e., metal tape  27 &#39;s two side end portions closest to the opening of cylindrical case  26 , in the two upright surfaces of metal tape  27  between which coaxial inner conductor  24 , is interposed. 
     As a method for laser welding, a method of performing point-joining by irradiating a laser beam, for example, in pulse form, melting metallic members by thermal energy of the laser beam and solidifying the molten material is known. In the first exemplary embodiment, welding is repeated a certain number of times while moving the laser beam irradiation center along the boundaries between the side end portions of metal tape  27  and coaxial inner conductor  24 , thereby joining metal tape  27  to coaxial inner conductor  24 . A plurality of solid round marks shown at the side end portions of metal tape  27  in  FIGS. 4A and 4B  represent welded portions (nuggets) of metal tape  27  and coaxial inner conductor  24 . 
     In the high frequency circuit unit in the first exemplary embodiment, metal tape  27  and coaxial inner conductor  24  can have increased welded portions in comparison with those in the high frequency circuit unit in the related art shown in  FIGS. 2A ,  2 B,  3 A, and  3 B. As a result, the area of contact between coaxial inner conductor  24  and metal tape  27  is increased. Also, since metal tape  27  is joined to the opposite side surfaces of coaxial inner conductor  24 , stronger adhesion can be maintained between coaxial inner conductor  24  and metal tape  27  in comparison with the high frequency circuit unit in the related art shown in  FIGS. 2A ,  2 B,  3 A, and  3 B. 
     In the high frequency circuit unit in the first exemplary embodiment, therefore, the thermal conductivity at the portions of the metal tape and the coaxial inner conductor that are in contact with each other is increased in comparison with the high frequency circuit unit in the related art to limit a reduction in heat dissipation capacity with respect to heat generated in helix  21 . The increase in temperature of helix  21  is thereby limited to prevent a deterioration in electrical characteristics of traveling wave tube  1  and a reduction in stability of the operation of traveling wave tube  1 . The high frequency circuit unit in the present exemplary embodiment is particularly advantageous when applied to a high power output traveling wave tube having an increased amount of heat generation in helix  21 . 
     Second Exemplary Embodiment 
       FIGS. 5A and 5B  are diagrams showing a connection between a coaxial inner conductor and a helix in a second exemplary embodiment of the present invention.  FIG. 5A  is an enlarged longitudinal sectional view of an essential portion of a high frequency circuit unit, and  FIG. 5B  is a side view of the high frequency circuit unit seen in direction A in  FIG. 4A . 
     The high frequency circuit unit in the second exemplary embodiment is constructed in the same manner as the high frequency circuit unit in the first exemplary embodiment. That is, metal tape  27  is formed into a U-shape in advance; a bottom portion of U-shaped metal tape  27  is attached to an end portion of helix  21 ; and coaxial inner conductor  24  inserted in case  26  from one side of the same is fixed so that its end portion is interposed between two upright surfaces of metal tape  27 . 
     Metal tape  27  is joined to the end portion of helix  21  while being oriented so that its U-shape is seen in direction A in  FIG. 5A , i.e., from the opening side of case  26 . Metal tape  27  is formed by being bent at two positions so that the end portion of coaxial inner conductor  24  can be interposed between its bent portions. 
     In the high frequency circuit unit in the second exemplary embodiment, top portions of metal tape  27  formed into the U-shape are tapered, as shown in  FIG. 5A , and the tapered portions are laser-welded to coaxial inner conductor  24 , thereby joining metal tape  27  to coaxial inner conductor  24 . 
     As a method for laser welding, a method of performing point-joining by irradiating a laser beam, for example, in pulse form, melting metallic members by thermal energy of the laser beam and solidifying the molten material can be considered, as in the first exemplary embodiment. 
     In the second exemplary embodiment, welding is repeated a certain number of times while moving the laser beam irradiation center along the boundaries between the tapered portions of metal tape  27  and coaxial inner conductor  24 , thereby joining metal tape  27  to coaxial inner conductor  24 . A plurality of solid round marks shown at the side end portions of metal tape  27  in  FIGS. 5A and 5B  represent welded portions (nuggets) of metal tape  27  and coaxial inner conductor  24 . 
     The tapered top portions are formed so that a front end portion as seen in direction A has the lowest height and a portion remoter from the front end portion has a higher height, to allow welding from direction A shown in  FIG. 5A , that is, the height of the upright surfaces forming the U-shape is minimized at the position closest to the opening of case  26  and is maximized at the position remotest from the opening. 
     In the high frequency circuit unit in the second exemplary embodiment, the tapered portions of metal tape  27  are welded to coaxial inner conductor  24  to enable the welded portions, after the welding operation, to be visually checked in an oblique direction, thus facilitating determination as to whether or not the portions to be welded are welded with reliability in comparison with the first embodiment in which the welded portions after the welding operation, are visually checked in a flat plane. 
     Thus, in the high frequency circuit unit in the second exemplary embodiment, an improvement in reliability of the operation to weld metal tape  27  to coaxial inner conductor  24  is achieved as well as the same advantages as those of the first exemplary embodiment. Further, thermal conductivity at the portions of metal tape  27  and coaxial inner conductor  24  that are in contact with each other is increased in comparison with the first exemplary embodiment to limit a reduction in heat dissipation capacity with respect to heat generated in helix  21 . The increase in temperature of helix  21  is thereby limited to prevent a deterioration in electrical characteristics of traveling wave tube  1  and a reduction in stability of the operation of traveling wave tube  1 . The high frequency circuit unit in the present exemplary embodiment is particularly advantageous when applied to a high power-output traveling wave tube having an increased amount of heat generation in helix  21 . 
     While an example, in which the top portions of metal tape  27  that are formed into a U-shape are tapered, is illustrated in  FIGS. 5A and 5B , the shape of the top portions of metal tape  27  is not limited to the U-shape. The top portions of metal tape  27  may have any other shape provided that the height of the upright surfaces forming the U-shape is minimized at the position closest to the opening of case  26  and provided that the height of the upright surfaces forming the U-shape is maximized at the position remotest from the opening. For example, the same advantages as those of the present embodiment can also be obtained in a case where the top portions of metal tape  27  have an inverse circular-arc shape such as shown in  FIG. 6 . Also, the same advantages as those of the first exemplary embodiment can be obtained in a case where the top portions of metal tape  27  are stepped as shown in  FIG. 7 , although no improvement in reliability of the welding operation can be expected. 
     While the invention has been particularly shown and described with reference to exemplary embodiments thereof, the invention is not limited to these embodiments. It will be understood by those ordinarily skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the claims.