Abstract:
A disk-shaped cathode pellet is installed and secured by a retainer onto a heater cap that incorporates a heater. The part of this retainer that covers the periphery of the electron emission surface of the cathode pellet functions as a portion of a Wehnelt electrode. Alternatively, the retainer is formed such that the average angle of the surface with respect to the outermost shell of the electron beam matches the Pierce angle such that the part of this retainer that covers the periphery of the electron emission surface of the cathode pellet functions as a Wehnelt electrode.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an electron gun that is used in a microwave tube such as a traveling-wave tube or a klystron, and more particularly to a Pierce type electron gun that is provided with a Wehnelt electrode for focusing an electron beam.  
         [0003]     2. Description of the Related Art  
         [0004]     Traveling-wave tubes and klystrons are electron tubes for amplifying high-frequency signals by means of the interaction between a high-frequency circuit and an electron beam that is emitted from an electron gun. Such electron tubes are constructions such as shown in  FIG. 1  that include: electron gun  21  that emits electron beam  50 ; high-frequency circuit  22  for bringing about interaction between electron beam  50  that is emitted from electron gun  21  and high-frequency signals (microwaves); collector  23  for capturing electron beam  50  that is emitted from high-frequency circuit  22 , and anode electrode  24  for guiding electron beam  50  that is emitted from electron gun  21  into high-frequency circuit  22 .  
         [0005]     Electron beam  50  that is emitted from electron gun  21  is accelerated by anode electrode  24  and guided into high-frequency circuit  22 , and travels through the interior while interacting with a high-frequency signal that is applied as input from the input terminal of high-frequency circuit  22 . Electron beam  50  that is supplied as output from the interior of high-frequency circuit  22  is captured by collector  23 . At this time, the high-frequency signal that has been amplified by interaction with electron beam  50  is supplied as output from the output terminal of high-frequency circuit  22 .  
         [0006]     Electron guns  21  that are used in microwave tubes such as traveling-wave tubes and klystrons of this type are of many known types, one being the Pierce type electron gun that is provided with a Wehnelt electrode for focusing the electron beam.  
         [0007]      FIG. 2  is a sectional side view showing the configuration of a Pierce type electron gun of the prior art.  
         [0008]     As shown in  FIG. 2 , a Pierce type electron gun of the prior art is a configuration that is provided with: cathode pellet  11  for radiating thermions; heater  12  for applying thermal energy for causing cathode pellet  11  to radiate thermions; heater cap  13  for encapsulating heater  12 ; and Wehnelt electrode  14  for focusing the thermions and forming electron beam  50 .  
         [0009]     Heater cap  13  is a construction that is formed as a cylinder composed of molybdenum (Mo) with one sealed end, cathode pellet  11  being installed on the sealed surface.  
         [0010]     Cathode pellet  11  is formed from a porous tungsten substrate that is impregnated with an oxide (emitter material) of barium (Ba), calcium (Ca), or aluminum (Al). Cathode pellet  11  is formed in approximately disk form that is convex in the axial direction of electron emission and that is provided with a stepped indentation around its periphery as seen in a section taken along the axis of emission of electrons, and has a shape such that the electron emission surface is processed to a flat or concave shape that is a portion of a spherical surface, and such that the surface on the side opposite the electron emission surface is flat. Cathode pellet  11  is secured onto heater cap  13  by the pressure against the above-described indentation toward the sealed surface that is exerted by retainer  15 . A cathode pellet of this form is disclosed in, for example, JP-A-2003-346671.  
         [0011]     Retainer  15  is formed in a cylindrical shape using a refractory metal such as tantalum (Ta), molybdenum (Mo), or molybdenum—rhenium (Mo—Re) alloy, the end of retainer  15  that is not in contact with the cathode pellet being bonded to heater cap  13  by welding or brazing and soldering following placement of the cathode pellet.  
         [0012]     Wehnelt electrode  14  is formed in a donut shape that is provided with an opening in the center by machining metal such as molybdenum and is secured by welding or brazing and soldering to the rim of one of the openings in electron gun case  16 , which is formed in a cylindrical shape.  
         [0013]     Heater cap  13  to which cathode pellet  11  has been attached is supported inside electron gun case  16  by metal support members  17  that are composed of tantalum (Ta), molybdenum (Mo), molybdenum—rhenium alloy (Mo—Re), or iron—nickel—cobalt alloy (Kovar: Kv) and is secured at a position such that the electron emission surface of cathode pellet  11  and the surface of Wehnelt electrode  14  form substantially the same plane. In addition, the surface of Wehnelt electrode  14  on the side of anode electrode  24  is processed to a shape having an angle of 67.5° (referred to as the “Pierce angle”) with respect to the outermost shell of electron beam  50  (See  FIG. 2 ).  
         [0014]     In the Pierce type electron gun of the prior art that is shown in  FIG. 2 , the spacing of the cathode pellet and Wehnelt electrode, i.e., the perveance, must match a designed value with a high level of accuracy in order to focus electrons that are emitted from the cathode pellet to a desired beam diameter. It is further crucial to reduce divergence in the axial direction of electron emission between the electron emission surface of the cathode pellet and the surface of the Wehnelt electrode.  
         [0015]     A large variation in the perveance or between the cathode pellet and the Wehnelt electrode in the axial direction of electron emission results in problems such as the collision of electrons that are emitted from the cathode pellet with the anode electrode, or fluctuation of the electron beam diameter in the high-frequency circuit that causes a portion of the electron beam to hit the high-frequency circuit. These problems bring about an increase in the power consumption or a reduction in the amplification performance of the microwave tube.  
         [0016]     In addition, in the interest of reducing power consumption in an electron gun, the thermal energy produced by the heater is preferably efficiently transferred to the cathode pellet, and moreover, the heat that is conferred to the cathode pellet is preferably not diffused by way of the electron gun case or the Wehnelt electrode.  
         [0017]     In the Pierce type electron gun of the prior art that is shown in  FIG. 2 , metal support members that are secured at positions remote from the cathode pellet are used to support the heater cap inside the electron gun case such that the thermal energy that is conferred from the heater to the cathode pellet is not diffused by the electron gun case or the Wehnelt electrode. Problems were therefore encountered because high-precision jigs and tools were required to weld and secure the heater cap in order to keep the perveance and variation in the axial direction of electron emission of the cathode pellet and Wehnelt electrode within prescribed values, and further, because a wide range of variation occurred in fabrication.  
         [0018]     In addition, when the sectional form in the axial direction of electron emission of the cathode pellet is convex, electrons are emitted toward the outside from the part of the periphery of the cathode pellet that is not covered by the retainer (hereinbelow, referred to as “side emission”), and this gives rise to the previously described problems that electrons that are emitted from the cathode pellet hit the anode electrode, and the fluctuation in the diameter of the electron beam inside the high-frequency circuit causes a portion of the electron beam to hit the high-frequency circuit, thus preventing good electron emission characteristics from being obtained. As a result, the above-described high-precision jigs and tools were used to reduce the gap between the cathode pellet and Wehnelt electrode to a minimum, and the Wehnelt electrode was further arranged to precede the cathode surface (on the anode-electrode side) so as to focus electrons that have been radiated toward the outside.  
         [0019]     In microwave communication in recent years, moreover, radiowaves of even higher frequencies are preferred for achieving higher volumes and more effective use of radiowaves. The size of microwave tubes has also decreased with this move toward higher-frequency waves, and electron guns are therefore now being fabricated in smaller sizes.  
         [0020]     However, due to the convex sectional profile in the axial direction of electron emission of the cathode pellet in the Pierce type electron gun of the prior art that is shown in  FIG. 2 , the thickness of cathode pellet must be increased to a certain degree to withstand the securing force applied by the retainer. The weight of the cathode pellet increased as a consequence, and it was necessary to make the retainer thicker and stronger to secure the cathode pellet on the heater cap by brazing and soldering. This construction has therefore impeded the reduction of the size of the electron gun.  
       SUMMARY OF THE INVENTION  
       [0021]     It is therefore an object of the present invention to provide an electron gun that allows fewer individual differences that result from variations in fabrication, and moreover, that allows superior electron emission characteristics to be obtained.  
         [0022]     In the present invention that can achieve the above-described object, the periphery of a disk-shaped cathode pellet is engaged with the heater cap by means of a retainer, whereby the cathode pellet is arranged and secured on the heater cap.  
         [0023]     In addition, the cathode pellet is not only arranged and secured on the heater cap by means of the retainer, but the shape of the retainer is formed such that the average angle of the surface of the retainer with respect to the outermost shell of the electron beam matches the Pierce angle and such that the part of the retainer that covers the periphery of the electron emission surface of the cathode pellet functions as a Wehnelt electrode.  
         [0024]     Because the part of the retainer that covers the periphery of the electron emission surface functions as a Wehnelt electrode in the above-described construction, the perveance and the divergence in the axial direction of electron emission between the electron emission surface of the cathode pellet and the retainer surface that functions as a Wehnelt electrode are uniform, and individual differences in the positional relation of the Wehnelt electrode and cathode pellet surface are reduced.  
         [0025]     Thus, despite the occurrence of variations in fabrication in the gap between the retainer and the Wehnelt electrode that is arranged at the periphery of the retainer, influence upon the electrical field of the cathode pellet surface is reduced. In addition, side emission does not occur because the periphery of the cathode pellet is covered by the retainer. As a result, electron guns can be obtained that have excellent electron emission characteristics and in which individual differences are reduced.  
         [0026]     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  
       [0027]      FIG. 1  is a side sectional view showing an example of the configuration of a traveling-wave tube;  
         [0028]      FIG. 2  is a side sectional view showing the configuration of an electron gun of the prior art;  
         [0029]      FIG. 3  is a side sectional view showing an example of the configuration of an electron gun of the present invention;  
         [0030]      FIG. 4  is a side sectional view showing the configuration of a modification of the electron gun that is shown in  FIG. 3 , and  
         [0031]      FIG. 5  is a side sectional view showing the configuration of another modification of the electron gun of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]      FIG. 3  is a sectional view showing an example of the configuration of an electron gun of the present invention.  
         [0033]     As shown in  FIG. 3 , the electron gun of the present invention is a configuration in which cathode pellet  31  is formed in a disk shape and the periphery of cathode pellet  31  is engaged with and pressed against the sealed surface of heater cap  33  by means of retainer  35 , whereby cathode pellet  31  is secured onto heater cap  33 .  
         [0034]     As with the prior art, cathode pellet  31  is secured at a position such that its electron emission surface and the surface of Wehnelt electrode  34  form approximately the same plane. Here, retainer  35  is a construction that protrudes by only its thickness with respect to the electron emission surface of cathode pellet  31 . The construction is otherwise identical to an electron gun of the prior art and explanation of this construction is therefore here omitted.  
         [0035]     In the electron gun of the present invention, the part of retainer  35  that covers the periphery of the electron emission surface of cathode pellet  31  is not only used as a securing member for securing cathode pellet  31  but also functions as Wehnelt electrode  34  for focusing electrons.  
         [0036]     As previously described, in an electron gun of the prior art, the convex shape of the sectional form of the cathode pellet in the axial direction of electron emission results in an increase in the electric field strength at the periphery (edge portion). In addition, an electric field is not formed parallel to the surface of the cathode pellet, and electrons are therefore radiated toward the outside. As a result, electrons that were radiated toward the outside were focused by reducing the gap between the cathode pellet and Wehnelt electrode to the minimum, and further, by arranging the Wehnelt electrode to precede the cathode surface (on the anode electrode side).  
         [0037]     In the electron gun of the present invention, the field strength at the edge portion of retainer  35  increases because the periphery (edge portion) of the electron emission surface of cathode pellet  31  is covered by retainer  35 , but the laminar flow characteristic of the electron beam does not deteriorate because electrons are not radiated from retainer  35 .  
         [0038]     In the electron gun of the present invention, moreover, the positional relation of retainer  35  that functions as Wehnelt electrode  34  and the surface of cathode pellet  31  is fixed, and the perveance and divergence in the axial direction of electron emission between the electron emission surface of cathode pellet  31  and the surface of the surface of Wehnelt electrode  34  are also fixed.  
         [0039]     The field strength of the periphery of cathode pellet  31  is determined substantially by the positional relation with retainer  35  and therefore is virtually unchanged. In addition, side emission therefore does not occur because the periphery of cathode pellet  31  is covered by retainer  35 . The reduction of individual differences in the positional relation of retainer  35  and the surface of cathode pellet  31  results in a reduction of the influence exerted upon the electric field of the surface of cathode pellet  31  despite fabrication variations in the gap between retainer  35  and Wehnelt electrode  34  that is arranged at the periphery of retainer  35 . Accordingly, electron guns can be obtained in which individual differences are limited and that are provided with excellent electron emission characteristics.  
         [0040]     In the electron gun of the present invention, moreover, cathode pellet  31  is formed in a disk shape, and the thickness of cathode pellet  31  in the axial direction of electron emission can therefore be reduced from that of the prior art. The heat capacity of cathode pellet  31  is thus reduced and the heat conductivity from heater  32  to cathode pellet  31  is improved. The device will thus function with less heater power, whereby the power consumption of the microwave tube can be reduced. In addition, the thermal response speed can be accelerated, and the start-up time from the introduction of the power supply to the operation of the electron gun can therefore be shortened.  
         [0041]     In the electron gun of the present invention, moreover, when the thickness of retainer  35  is made greater than 0.2 mm, or when the thickness of retainer  35  is greater than approximately 10% of the diameter of the cathode pellet, the electric field strength becomes non-uniform from the central portion to the peripheral portion of the surface of cathode pellet  31 , whereby the concern arises that electrons of the peripheral portion of cathode pellet  31  will be radiated toward the central portion and the laminar flow characteristic of the electron beam cannot be maintained. The thickness of retainer  35  therefore preferably meets one of the conditions of being less than 0.2 mm or being less than 10% of the diameter of cathode pellet  31 . The thickness of retainer  35  need only be sufficient to provide the strength to secure cathode pellet  31 , and a thinner and lighter cathode pellet  31  facilitates the reduced thickness of retainer  35 .  
         [0042]     In the electron gun of the present invention, moreover, if the Pierce angle (67.5°) is realized as the average angle, with respect to the electron beam, of the part of retainer  35  that functions as Wehnelt electrode  34  and the surface of Wehnelt electrode  34 , no particular limitation need be placed on the surface area of cathode pellet  31  that is covered by retainer  35 . However, too much coverage of the surface of cathode pellet  31  by retainer  35  interferes with the effective use of cathode pellet  31 . On the other hand, insufficient coverage of the surface of cathode pellet  31  by retainer  35  diminishes the function of retainer  35  as Wehnelt electrode  34 . Accordingly, the inner diameter of retainer  35  that covers the periphery of the surface of cathode pellet  31  is preferably approximately 90% of the diameter of cathode pellet  31 .  
         [0043]     As previously described, retainer  35  is formed from a thin refractory metal plate that is composed of, for example, tantalum (Ta), molybdenum (Mo), or molybdenum—rhenium alloy (Mo—Re). On the other hand, tungsten is used as the main material of cathode pellet  31 , as previously described. The difference between the thermal expansion coefficient of retainer  35  and the thermal expansion coefficient of cathode pellet  31  is not great, and the difference in thermal expansion coefficients causes virtually no decrease in the fixed strength of cathode pellet  31  due to retainer  35 . However, to prevent even a slight decrease of strength, the end of retainer  35  that contacts cathode pellet  31  should be processed to a turned-back shape as shown in  FIG. 4A  or an arc shape as shown in  FIG. 4B .  
         [0044]     In addition, the electron emission surface of cathode pellet  31  need not be flat as shown in  FIG. 3 , but may be processed to a concave shape that forms a portion of a sphere as shown in  FIG. 4C . In such a case, the end of retainer  35  that contacts cathode pellet  31  should be a turned-back shape as shown in  FIG. 4A , an arc shape as shown in  FIG. 4B , or a shape that is bent to an angle more acute than 90° with respect to the concave surface.  
         [0045]     As previously described, retainer  35  functions as Wehnelt electrode  34  in the present invention, but this fact shows that any configuration is possible as long as the average angle of the part of retainer  35  that functions as Wehnelt electrode  34  and the surface of Wehnelt electrode  34  have the Pierce angle with respect to the electron beam. In other words, the Wehnelt electrode function of retainer  35  may be realized by forming retainer  35  on the electron emission surface side of cathode pellet  31  as a funnel shape or as a shape that includes a funnel shape as shown in FIGS.  5 A-C. In such a case, Wehnelt electrode  34  is unnecessary.  
         [0046]     According to the electron gun of the present invention, the part of retainer  35  that covers the periphery of the electron emission surface functions as Wehnelt electrode  34 , thereby fixing the perveance and the divergence in the axial direction of electron emission of the electron emission surface of cathode pellet  31  and the surface of the retainer that functions as Wehnelt electrode  34 , and reducing individual differences in the positional relation between Wehnelt electrode  34  that is formed by retainer  35  and the surface of cathode pellet  31 . As a result, influence upon the electric field of the surface of cathode pellet  31  is reduced despite the occurrence of variations in fabrication of the spacing between Wehnelt electrode  34  that is arranged on the periphery of retainer  35  and retainer  35 . Further, side emission does not occur because the periphery of cathode pellet  31  is covered by retainer  35 . As a result, electron guns can be obtained that have fewer individual differences and that are provided with excellent electron emission characteristics.  
         [0047]     While preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.