Patent Publication Number: US-6222319-B1

Title: Magnetron apparatus having a segmented anode edges and manufacturing method

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a magnetron apparatus for use in microwave ovens and the like, and a manufacturing method for the same. 
     The magnetron apparatus is a microwave oscillating tube which operates at a fundamental frequency of, for example, 2,450 MHz, and is used as a high frequency source in electric apparatuses using microwaves such as microwave heaters and microwave discharge lamps. A typical configuration of the magnetron apparatus is such that a cathode and an anode are disposed coaxially cylindrically. More specifically, the magnetron apparatus comprises a coiled cathode, an anode cylinder disposed with the cathode as the central axis, and plural anode segments radially arranged around the central axis in a space inside the anode cylinder for defining a resonant cavity. The magnetron apparatus further comprises a pair of magnetic pole pieces disposed at upper and lower open ends of the anode cylinder and magnetically associated with an annular permanent magnet, plural strap rings for electrically interconnecting the anode segments, and an antenna with one end connected to one of the anode segments for discharging microwaves. 
     In the above-mentioned magnetron apparatus, after the anode cylinder, the anode segments, the antenna, the strap rings and the magnetic pole pieces are integrally assembled as an anode assembly, the cathode is disposed in the central portion of the anode assembly. In the magnetron apparatus, as well known, the precision with which the components are assembled greatly influences the performance of the apparatus, and the arrangement of the plural anode segments for defining a desired resonant cavity inside the anode cylinder are particularly important. Therefore, it is a technical problem of the magnetron apparatus to coaxially and radially secure the plural anode segments with high precision so as to be equally spaced on the inner surface of the anode cylinder with a predetermined distance from the cathode. 
     As a conventional manufacturing method for the magnetron apparatus, a brazing and soldering method is known in which the anode segments are pressed against the inner surface of the anode cylinder by use of a temporary assembling pin and all the anode segments are secured to the inner surface at once with a brazing filler metal as disclosed in, for example, examined and published Japanese patent application TOKKO Sho 57-18823. 
     Hereinafter, the conventional magnetron apparatus and the manufacturing method will be described with reference to FIG.  16  and FIG.  17 . 
     FIG. 16 is a partially cutaway perspective view showing a configuration of a principal part of an anode assembly in a conventional magnetron apparatus before a brazing filler metal is melted. FIG. 17 is a cross sectional view showing the configuration of the principal part of the anode assembly in the conventional magnetron apparatus after the brazing filler metal is melted. 
     As shown in FIG.  16  and FIG. 17, plural anode segments  52  ( 52   a ,  52   b ,  52   c ,  52   d , as depicted in FIG. 16) are coaxially radially arranged inside an anode cylinder  51 . Specifically, for example, ten anode segments  52  are equally spaced inside the anode cylinder  51 . Each of the anode segments  52  is formed into a substantial rectangular shape having a longitudinal size of 9.5 mm and a lateral size of 13 mm, for example. In each of the anode segments  52 , one end surface on the shorter side is secured to the inner surface of the anode cylinder  51 . These anode segments  52  are pressed against the inner surface of the anode cylinder  51  by a jig pin  40 , which is a temporarily used assembling pin, shown by the dash and dotted line of the figure, and the above-mentioned one end surface is secured to the inner surface of the anode cylinder  51  by melting a wire-form brazing filler metal  56  (FIG.  16 ). 
     When a non-illustrated coiled cathode is disposed along the central axis of the anode cylinder  51 , each end surfaces of the anode segments  52  on the central side in the direction of the arrangement, i.e. an end surface each of the anode segments  52  opposed to the above-mentioned one end surface (hereinafter, the end surface on the central side will be referred to as an “inner end surface”) is situated with a predetermined distance from the cathode, so as to define a desired resonant cavity inside the anode cylinder  51 . 
     At opposite end surfaces (i.e., upper surface and lower surface) on the longer side of each of the anode segments  52 , strap ring grooves  53   a  and  53   b  are provided for brazing two pairs of strap rings  54  ( 54   a  and  54   b ) and  55  ( 55   a  and  55   b ). At the upper end surface of each of the anode segments  52  where the strap ring groove  53   a  is provided, a terminal groove  53   c  is provided for connecting one end of a non-illustrated antenna. 
     The strap rings  54   b  and  55   a  are brazed to every two anode segments  52   a ,  52   c , - - - , and the strap rings  54   a  and  55   b  are brazed to the remaining anode segments  52   b ,  52   d , - - - . A plating layer (not shown) of the brazing filler metal  56  is formed on the surface of each of the strap rings  54  and  55 , and when the brazing filler metal  56  is melted to secure the one end surfaces of the anode segments  52  to the inner surface of the anode cylinder  51 , the plating layer is also melted, so that the strap rings  54  and  55  are secured to the corresponding anode segments  52 . 
     The above-mentioned anode cylinder  51 , anode segments  52 , strap rings  54  and  55 , and antenna (not shown) are made of, for example, oxygen free copper. The jig pin  40  is made of a metal member containing silicon nitride (Si 3 N 4 ), and the surface of a cylindrical portion which comes into contact with the inner end surface of each of the anode segments  52  is formed so as to be as smooth as the mirror finished surface. The brazing filler metal  56  is made of an alloy of silver and copper, and the strap rings  54  and  55  and the antenna (not shown) are made of copper having a silver plating layer provided on the surface thereof. 
     In such a conventional manufacturing method for the magnetron apparatus, first, the plural anode segments  52  and the strap rings  54  and  55  are placed in the respective positions inside the anode cylinder  51  by use of a non-illustrated temporary assembling jig. Then, the jig pin  40  is moved along the central axis of the anode cylinder  51  and press-fit from below into the central portion in the direction of the arrangement of the anode segments  52  (the central portion of the anode cylinder  51 ) as shown by the arrow Y of FIG.  16 . So that the jig pin  40  contacts with the inner end surfaces of the anode segments  52 . Thereby, the anode assembly is maintained in a preassembled condition where the one end surface each of the anode segments  52  are pressed against the inner surface of the anode cylinder  51  by the jig pin  40 . Hereafter, only the temporary assembling jig is detached, and the brazing filler metal  56  is placed on the end surfaces on the longer side of the anode segments  52  so as to be in contact with the inner surface of the anode cylinder  51  as shown in FIG.  16 . After one of the magnetic pole pieces (not shown) is attached to an upper open end of the anode cylinder  51 , one end of the antenna (not shown) is attached to one of the anode segments  52 . Then, the anode assembly in the preassembled condition is heated to a predetermined temperature (for example, 800 to 900° C.) in a non-illustrated furnace. Thereby, the brazing filler metal  56  is melted and flows into a clearance between the inner surface of the anode cylinder  51  and the one end surface each of the anode segments  52  caused by expansion. At this time, the plating layers on the strap rings  54  and  55  and the antenna (not shown) are also melted. Hereafter, by taking the anode assembly out of the furnace while maintaining the preassembled condition, and cooling it, the inner surface of the anode cylinder  51  and the one end surface each of the anode segments  52 , the strap ring grooves  53   a  and  53   b  and the corresponding strap rings  54  and  55 , and the one of the anode segment  52  and the antenna (not shown) are secured. 
     Consequently, after the jig pin  40  is downwardly pulled out, the other of the magnetic pole pieces (not shown) is attached to a lower open end of the anode cylinder  51 , and thereby the assembly of the anode assembly is finished. 
     In the conventional magnetron apparatus and the manufacturing method as described above, when the jig pin  40  is press-fit or taken out by moving it in the direction of the central axis, the jig pin  40  comes into contact with and rubs against the inner end surface of each of the anode segments  52  over the entire surface in the direction of the central axis. That is, in the conventional magnetron apparatus and the manufacturing method, the contact surface of the jig pin  40  and each the anode segments  52  equal the length of the inner end surface in the direction of the central axis, and the length of the contact surface (shown at A in FIG. 16) is long. For this reason, in the conventional magnetron apparatus and the manufacturing method, during the while the jig pin  40  is being press-fit or being taken out, contact pressure exerted on the anode segments  52  through the contact surfaces increases, so that the anode segments  52  are apt to be deformed. When such deformation is caused on the anode segments  52 , the molten brazing filler metal  56  does not deposit onto the entire surface of the one end surface each of the anode segments  52  but the anode segments  52  come off due to insufficient brazing. Further, the deformation of the anode segments  52  changes the configuration of the strap ring grooves  53   a  and  53   b , so that deformation of the strap rings  54  and  55  are caused and the strap rings  54  and  55  come off because the strap rings  54  and  55  are not secured to the strap ring grooves  53   a  and  53   b.    
     When the components such as the plural anode segments  52  are mass-produced, it is difficult to form these components so as to have uniform outer dimensions and it is impossible to completely prevent the outer dimensions from varying. For this reason, in the conventional magnetron apparatus and the manufacturing method, there are occasions when the anode and the cathode are short-circuited because of the variation in outer dimension. Specifically, in the case that the outer dimensions of the anode segments  52  are greater than predetermined outer dimensions and the outer dimensions of the inner surface of the anode cylinder  51  are smaller than predetermined outer dimensions, when the jig pin  40  is press-fit from below, the inner end surface each of the anode segments  52  is extended in the movement direction of the jig pin  40  by stress caused by the press fitting of the jig pin  40 , so that copper foil burrs  57  as illustrated in FIG. 18 are caused at the upper end of the inner end surface. As a result, when the cathode is placed along the central axis of the anode assembly (anode cylinder  51 ), it often happens that the burrs  57  come into contact with the cathode and the contact causes a short circuit. Further, in the case that the anode cylinder  51  or the anode segments  52  are formed to have outer dimensions which are different from predetermined outer dimensions as mentioned above, greater power is necessary when the jig pin  40  is press-fit or taken out, thus resulting in dents and scratches on the jig pin  40  that require the jig pin  40  to be replaced. 
     Further, in each of the anode segments  52 , as has been explained in the above, the strap ring groove  53   a  and the terminal groove  53   c  are provided at one of the end surface on the longer side, and the strap ring groove  53   b  is provided at the other end surface. For this reason, in the conventional magnetron apparatus and the manufacturing method, when the jig pin  40  is press-fit so as to be in contact with the inner end surface each of the anode segments  52 , the pressing force which the anode segments  52  receive from the jig pin  40  and the anode cylinder  51  is not uniform in the direction of the central axis. Specifically, when each anode segment  52  is divided into three areas, for example, an upper area Va, a central area Vb and a lower area Vc in the direction of the central axis as shown in FIG. 17, the central area Vb does not include the grooves  53   a ,  53   b  and  53   c . Therefore, the pressure exerted on the central area Vb is greater than that exerted on the upper and lower areas Va and Vc. When the anode assembly in the preassembled condition is heated, since the anode segments  52  expand and the molten brazing filler metal  56  flows into the clearance between the anode cylinder  51  and the anode segments  52 , the pressing force applied on the upper and lower areas Va and Vc by the jig pin  40  is smaller than the pressing force which the central area Vb receives therefrom. 
     Thus, when the pressing force exerted on the anode segments  52  is not uniform in the direction of the central axis, because of the above-mentioned reasons combined with the fact that the surface of the jig pin  40  is as smooth as the mirror finished surface, the anode segments  52  slide over the inner surface and are secured to the inner surface of the anode cylinder  51  with the one end surfaces of the anode segments  52  being inclined from the direction of the central axis. Consequently, in the conventional magnetron apparatus and the manufacturing method, the distance between two adjoining anode segments  52 , i.e. the pitch varies as shown at P 1 , P 2  and P 3  in FIG. 19, so that the plural anode segments  52  are not equally spaced inside the anode cylinder  51 . 
     As has been explained above, in the conventional magnetron apparatus and the manufacturing method, deformation of the anode segments  52  and the strap rings  54  and  55  and coming-off of brazed parts due to insufficient brazing are apt to occur, and the burrs  57  and the variation in pitch of the plural anode segments  52  result therefrom. Therefore, in the conventional magnetron apparatus and the manufacturing method, it has been impossible to define the desired resonant cavity inside the anode assembly  51 , so that it is impossible to oscillate microwaves of the fundamental frequency with stability. Further, the magnetron efficiency deteriorates and high-frequency noises are markedly generated. 
     Examples of a conventional magnetron apparatus intended for reducing the contact pressure between the jig pin  40  and the anode segments  52  include one disclosed in unexamined and published Japanese patent application TOKKAI Sho 64-52365. In the conventional magnetron apparatus, by forming the cylindrical portion of the jig pin  40  so as to have dimensions which are 50 to 70% of the inner end surface each of the anode segments  52 , the contact pressure is reduced which is caused when the jig pin  40  is press-fit or taken out. 
     However, in the conventional magnetron apparatus, when the anode segments  52  are pressed against the inner surface of the anode cylinder  51 , on the inner end surface each of the anode segments  52  there are produced one area which is pressed by being in contact with the cylindrical portion of the jig pin  40  and the other area which is not pressed because it does not come into contact with the cylindrical portion. Thereby, in the conventional magnetron apparatus, the pressing force which the anode segments  52  receive is unbalanced in the direction of the central axis, so that in addition to the problem that the anode segments are not equally spaced, a new problem arises that the diameter of an inscribed circle defined by the inner end surface each of the plural anode segments  52  varies in the direction of the central axis (the vertical direction). Because of these problems, the conventional magnetron apparatus is not realized and commercialized. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a magnetron apparatus and a manufacturing method for the same that can solve the aforementioned problems in the conventional apparatus and can be configured with less cost and has a long life. 
     In order to achieve the above-mentioned object, a magnetron apparatus comprises: 
     an anode cylinder, and a plurality of plate-shaped anode segments radially arranged around the central axis of the anode cylinder inside the anode cylinder, and pressed against an inner surface of the anode cylinder by a pin press-fit into the central portion of the anode cylinder, so that a far-end-side end surface each of the anode segments is secured to the inner surface, 
     wherein each of the anode segments has a concave at the central portion of an inner end surface which comes into contact with the pin. 
     According to this configuration, a conventionally-used existing assembly jig can be used without any modification. Further, the precision with which the magnetron apparatus is assembled can be easily improved, so that the magnetron apparatus can be operated with stability. 
     In the magnetron apparatus of another aspect of the present invention, a length of the concave in the direction of the central axis is 20 to 50% of a length of the inner end surface in the direction of the central axis. 
     According to this configuration, the deterioration of magnetron efficiency can be reduced. 
     In the magnetron apparatus of another aspect of the present invention, a chamfered portion is provided on at least one angular portion of the inner end surface in the direction of the central axis. 
     According to this configuration, a magnetron apparatus with higher assembly precision can be obtained. 
     A manufacturing method for a magnetron apparatus of the present invention comprises: 
     an anode cylinder; and a plurality of plate-form anode segments radially arranged around the central axis of the anode cylinder inside the anode cylinder, and pressed against an inner surface of the anode cylinder by a pin press-fit into the central portion of the anode cylinder, so that a far-end-side end surface each of the anode segments is secured to the inner surface, 
     said method includes: 
     a step in which a concave is provided in a central portion of an inner end surface each of the anode segments, which comes into contact with the pin; and 
     a step in which the pin is press-fit into the central portion of the anode cylinder and the far-end-side end surface is pressed against and secured to the inner surface of the anode cylinder. 
     According to this configuration, a conventionally-used existing assembly jig can be used as it is without any modification. Further, the assembly precision of the magnetron apparatus can be easily improved, so that the magnetron apparatus can be operated with stability. 
     In the manufacturing method for the magnetron apparatus of another aspect of the present invention, further comprises a step in which a length of the concave in the direction of the central axis is formed so as to be 20 to 50% of a length of the inner end surface in the direction of the central axis. 
     According to this configuration, the pressure exerted on the anode segments by an assembly member can be sufficiently reduced, so that a magnetron apparatus with high assembly precision can be obtained. 
     In the manufacturing method for the magnetron apparatus of another aspect of the present invention, further comprises a step in which a chamfered portion is provided on at least one angular portion of the inner end surface in the direction of the central axis. 
     According to this configuration, the insertion pressure of the assembly member exerted on the central portion of the anode cylinder can be further reduced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view showing a configuration of a magnetron apparatus of a first embodiment of the present invention. 
     FIG. 2 is a partially cutaway perspective view showing a configuration of a principal part of an anode assembly in the magnetron apparatus shown in FIG. 1 before a brazing filler metal is melted. 
     FIG. 3 is a cross sectional view showing the configuration of the principal part of the anode assembly in the magnetron apparatus shown in FIG. 1 after the brazing filler metal is melted. 
     FIG. 4 is a graph showing a relationship between magnetron efficiency and the ratio of a length Hb to a length Ha. 
     FIG. 5 is a view showing a configuration of a modified version of the anode segment shown in FIG.  3 . 
     FIG. 6 is a view showing a configuration of another modified version of the anode segment shown in FIG.  3 . 
     FIG. 7 is a cross sectional view showing a configuration of a principal part of an anode assembly of a magnetron apparatus in a second embodiment of the present invention. 
     FIG. 8 is a view showing a configuration of a modified version of the anode assembly shown in FIG.  7 . 
     FIG. 9 is a view showing a configuration of another modified version of the anode assembly shown in FIG.  7 . 
     FIG. 10 is a view showing a configuration of another modified version of the anode assembly shown in FIG.  7 . 
     FIG. 11 is a view showing a configuration of another modified version of the anode assembly shown in FIG.  7 . 
     FIG. 12 is a graph showing measurement results of the noise level at the fifth harmonic. 
     FIG. 13 is a measurement result showing noise characteristics in the vicinity of the fifth harmonic in the conventional magnetron apparatus shown in FIG.  16 . 
     FIG. 14 is a measurement result showing noise characteristics in the vicinity of the fifth harmonic in the magnetron apparatus of the first embodiment. 
     FIG. 15 is a measurement result showing noise characteristics in the vicinity of the fifth harmonic in the magnetron apparatus of the second embodiment. 
     FIG. 16 is a partially cutaway perspective view showing a configuration of a principal part of an anode assembly in a conventional magnetron apparatus before a brazing filler metal is melted. 
     FIG. 17 is a cross sectional view showing the configuration of the principal part of the anode assembly in the conventional magnetron apparatus after the brazing filler metal is melted. 
     FIG. 18 is an explanatory view showing the generation of burrs in the conventional magnetron apparatus. 
     FIG. 19 is an explanatory view showing the variation in pitch of the anode segments in the conventional magnetron apparatus. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, preferred embodiments of a magnetron apparatus and that of a manufacturing method in accordance with the present invention will be described with reference to the accompanying drawings. 
     &lt;&lt;First Embodiment&gt;&gt; 
     FIG. 1 is a cross sectional view showing a configuration of a magnetron apparatus of a first embodiment of the present invention. FIG. 2 is a partially cutaway perspective view showing a configuration of a principal part of an anode assembly in the magnetron apparatus shown in FIG. 1 before a brazing filler metal is melted. FIG. 3 is a cross sectional view showing the configuration of the principal part of the anode assembly in the magnetron apparatus shown in FIG. 1 after the brazing filler metal is melted. It should be appreciated that common reference numerals are used within the figures herein to represent the same or similar structures within the various embodiments. 
     In FIG. 1, the magnetron apparatus of the present invention comprises an anode cylinder  1 , first and second magnetic pole pieces  2  and  3  attached to upper and lower open ends of the anode cylinder  1 , respectively, and first and second grommetted metal cylinders  4  and  5  attached to the first and second magnetic pole pieces  2  and  3 , respectively. The outer end surface of the first magnetic pole piece  2  is covered with a flange  4   a  provided at one end of the first metal cylinder  4 , and a rim of the flange  4   a  is secured to the upper open end of the anode cylinder  1 . To the other end of the first metal cylinder  4 , a microwave output terminal  7  is sealed through an insulating ring  6 . Likewise, the outer end surface of the second magnetic pole piece  3  is covered with a flange  5   a  provided at one end of the second metal cylinder  5 , and a rim of the flange  5   a  is secured to the lower open end of the anode cylinder  1 . To the other end of the second metal cylinder  5 , a cathode terminal lead stem  8  is sealed. 
     On the periphery of the anode cylinder  1 , a plurality of fins  9  are provided in a multiplicity of stages in order to discharge heat generated inside the anode cylinder  1 . On the peripheral end surface of the first magnetic pole piece  2 , a first annular permanent magnet  10  is placed coaxially with on the flange  4   a , and one magnetic pole surface  10   a  and the first magnetic pole piece  2  are magnetically associated with each other. Similarly, on the peripheral end surface of the second magnetic piece  3 , a second annular permanent magnet  11  is placed coaxially with on the flange  5   a , and one magnetic pole surface  11   a  and the second magnetic pole piece  3  are magnetically associated with each other. The other magnetic pole surfaces  10   b  and  11   b  of the first and second permanent magnets  10  and  11  are magnetically interconnected by a pot-shaped yoke  12  surrounding the fins  9 . In order to prevent leakage of high-frequency noises, a metallic shield case  13  incorporating the above-mentioned stem  8  and a known LC filter circuit member (not shown) is attached to the bottom of the pot-shaped yoke  12 . 
     Inside the anode cylinder  1 , a coiled cathode  14  disposed along the central axis of the anode cylinder  1  and plural anode segments  15  coaxially and radially arranged around the cathode  14  for defining a resonant cavity are provided. The cathode  14  is connected to a pair of cathode terminals  14   a  and  14   b  inside the anode cylinder  1 . The pair of cathode terminals  14   a  and  14   b  are led out of the anode cylinder  1  through the stem  8 , and connected to a non-illustrated high-frequency power source. Inside the anode cylinder  1 , an antenna  16  with one end connected to the microwave output terminal  7  is connected to one of the anode segments  15 . Thereby, the magnetron apparatus discharges a microwave having a fundamental frequency of, for example, 2,450 MHz from the microwave output terminal  7 . 
     Here, an anode assembly of the magnetron apparatus of this embodiment will be described in more detail with reference to FIG. 1 to FIG.  3 . 
     In FIG. 1 to FIG. 3, the anode assembly is one of the assembly units at the time of manufacture of the magnetron apparatus, and is an integral assembly of the anode cylinder  1 , the first and second magnetic pole pieces  2  and  3 , the plural anode segments  15 , the antenna  16  and two pairs of strap rings  17  ( 17   a  and  17   b ) and  18  ( 18   a  and  18   b ) for interconnecting the plural anode segments  15  inside the anode cylinder  1  as seen in FIGS. 2,  3 . Such an anode assembly enables improvement of the assembly precision of the magnetron apparatus. The anode cylinder  1 , the anode segments  15  and the strap rings  17  and  18  are made of the same metal material, for example, oxygen free copper, and secured by the brazing and soldering method using a brazing filler material made of an alloy of silver and copper. The antenna  16  is made of, for example, oxygen free copper, and the first and second magnetic pole pieces  2  and  3  are made of a magnetic material such as iron. 
     Inside the anode cylinder  1 , the plural, for example, ten anode segments  15  ( 15   a ,  15   b ,  15   c ,  15   d , as seen in FIG. 2) are equally spaced. Each of the anode segments  15  is formed into a plate shape having a longitudinal size of 9.5 mm, a lateral size of 13 mm, and a thickness size of 2 mm, for example. These anode segments  15  are pressed against the inner surface of the anode cylinder  1  by a jig pin  40 , which is temporarily used assembling pin, shown by the dash and dotted line of FIGS. 2,  3 , and one end surface on the shorter side is secured to the inner surface of the anode cylinder  1  by melting a wire-form brazing filler metal  19  (FIG.  2 ). At opposite end surfaces (i.e., upper surface and lower surface) on the longer side of each of the anode segments  15 , strap ring grooves  20   a  and  20   b  are provided for brazing the two pairs of the strap rings  17  ( 17   a  and  17   b ) and  18  ( 18   a  and  18   b ). At the upper end surface of each of the anode segments  15  where the strap ring groove  20   a  is provided, a terminal groove  20   c  is provided for connecting one end of the antenna  16 . The strap rings  17   b  and  18   a  are brazed to every two anode segments  15   a ,  15   c , - - - , and the strap rings  17   a  and  18   b  are brazed to the remaining anode segments  15   b ,  15   d , - - - . A plating layer (not shown) of the brazing filler metal  19  is formed on the surface of each of the strap rings  17  and  18 , and when the brazing filler metal  19  is melted to secure the one end surface each of the anode segments  15  to the inner surface of the anode cylinder  1 , the plating layer is also melted, so that the strap rings  17  and  18  are secured to the corresponding anode segments  15 . 
     With reference to FIGS. 2 and 3, at an end surface of each of the anode segments  15  on the central side in the direction of the arrangement, i.e. an inner end surface  21  opposed to one end surface on the shorter side and in contact with the jig pin  40 , a concave  22  having a rectangular opening configuration is provided in the central portion in the direction of the central axis (shown by the arrow F of FIGS. 1-3) of the anode cylinder  1 . Here, the opening configuration is the configuration of the concave  22  sighted in a thickness direction each of the anode segments  15 . As illustrated in FIG. 3, the concave  22  is formed by cutting the inner end surface  21  so as to have a length Hb in the direction of the central axis and a depth D in the direction of the radius of the anode cylinder  1 . The length Hb of the concave  22  is selected so as to be 20 to 50% of a length Ha of the inner end surface  21  in the direction of the central axis. At the inner end surface  21 , a chamfered portion may be provided in which at least one of the angular portions  21   a  and  21   b  in the direction of the central axis is chamfered. 
     With this configuration, in the magnetron apparatus of this embodiment, the area of contact between the anode segments  15  and the jig pin  40  can be reduced, so that the pressure exerted on the anode segments  15  by the jig pin  40  can be reduced. Consequently, in the magnetron apparatus of this embodiment, the problems can be solved such as the deformation of the anode segments and the detachment of brazed parts due to insufficient brazing in the conventional magnetron apparatus described previously and the generation of burrs shown in FIG. 18, so that microwaves of the fundamental frequency can be oscillated with stability without any faulty oscillation. Further, in the magnetron apparatus of this embodiment, a conventionally used converntaional ordinary assembly jig such as the jig pin  40  can be used without any modification, so that the manufacture cost can be reduced due to a modification of manufacture equipment. 
     The jig pin  40  is made of an expensive ceramic member containing silicon nitride (Si 3 N 4 ), and the surface of a cylindrical portion which is in contact with the inner end surface  21  is formed so as to be as smooth as a mirror finished surface. The outer diameter of the cylindrical portion is set so that the diameter of an inscribed circle defined by a plurality of coaxially radially arranged anode segments  15  is a value which is decided based on the theory of operation for the magnetron apparatus. 
     Next, technical advantages of the concave  22  will be explained concretely. In the below-mentioned description, each anode segment  15  is divided into three areas, i.e. a central area Vy having the concave  22  and upper and lower areas Vx and Vz situated above and below the central area Vy as depicted in FIG.  3 . 
     In the anode segments  15  of this embodiment, except for the portion of the concave  22 , two portions, i.e. the inner end surface  21  in the upper area Vx and the inner end surface  21  in the lower area Vz are in contact with the jig pin  40 . Therefore, the pressure from the jig pin  40  is exerted only on the upper and lower areas Vx and Vz and the area of contact with the jig pin  40  can be reduced. Consequently, in the magnetron apparatus of this embodiment, the anode segments  15  can be supported in a well balanced manner at the upper and lower two portions divided in the direction of the central axis with respect to the jig pin  40 , so that the assembly precision of the magnetron apparatus can be easily improved. Moreover, since the area of contact with the jig pin  40  is reduced, the flatness of the contact surface which comes into contact with the jig pin  40  can be also easily improved, so that the insertion pressure of the jig pin  40  exerted on the central portion in the direction of the arrangement of the anode segments  15  can be reduced. 
     Further, the strap ring grooves  20   a  and  20   b  are provided at the end surface on the longer side of each anode segment  15 . Therefore, the pressure exerted on the upper and lower areas Vx and Vz by the jig pin  40  is reduced, so that the insertion pressure of the jig pin  40  exerted on the central portion in the direction of the arrangement of the anode segments  15  can be further reduced. Even if unbalance occurs in the pressure from the jig pin  40  in the upper and lower areas Vx and Vz, the unbalance can be absorbed by the portions of the strap ring grooves  20   a  and  20   b.    
     Thus, in the magnetron apparatus of this embodiment, by providing the concave  22  in the central portion in the direction of the central axis of the inner end surface  21 , the pressure exerted on the anode segments  15  by the jig pin  40  can be reduced and made uniform. Consequently, in the magnetron apparatus of this embodiment, the problems of the conventional magnetron apparatus can be solved such as the deformation of the anode segments and the strap rings caused at the time of assembly, the detachment of brazed parts due to insufficient brazing, the generation of burrs shown in FIG.  18  and the variation in pitch shown in FIG.  19 . Thus, in accordance with this embodiment of the invention, the production of undesired oscillations can be considerably reduced using a conventional assembly jig. 
     On the contrary, in the conventional magnetron apparatus, as described previously with reference to FIG. 17, when the jig pin  40  is press-fit in the central portion in the direction of the arrangement of the anode segments, the pressing force exerted on the central area Vb is greater than that exerted on the upper and lower areas Va and Vc in the direction of the central axis of the anode segments. Therefore, in the conventional magnetron apparatus, variation in the pitch of the anode segments is caused as illustrated in FIG. 19, so that the plural anode segments are not equally spaced. 
     Next, the depth D and the length Hb of the concave  22  will be explained in detail. 
     The depth D of the concave  22  defines the distance from the inner end surface  21  each of the anode segments  15  in a direction toward the inner surface of the anode cylinder  1  (the distance in the direction of the radius) when the anode segments  15  are secured to the anode cylinder  1 . The effects of reducing and making uniform the pressure exerted on the anode segments  15  by the jig pin  40  can be always obtained by providing the concave  22  so that the portion of the concave  22  is kept from contact with the jig pin  40 . Therefore, the depth D of the concave  22  may be any depth as long as the portion of the concave  22  can be always kept from contact with the jig pin  40 . 
     Therefore, in view of the deformation of the anode segments  15  at the time of expansion, it is necessary that the depth D of the concave  22  be not less than approximately 0.1 mm. For mass production, in view of the dimensional tolerance of the anode segments  15  and variation due to the press manufacturing method, it is necessary that the depth D be not less than 0.2 mm. 
     The length Hb of the concave  22  defines the length in the direction of the central axis when the anode segments  15  are secured to the anode cylinder  1 . The inventors have found through an examination that it is necessary that the ratio of the length Hb to a length of the anode segments  15  in the direction of the central axis, i.e. the length Ha of the inner end surfaces  21  be not less than 20% in order to improve the assembly precision of the anode assembly by reducing and uniformizing the pressure exerted on the anode segments  15  by the jig pin  40 . 
     Further, in view of the fact that the pressure from the jig pin  40  is absorbed by the anode segments  15 , it is most desirable to provide the concave  22  to all the central portions in the direction of the central axis of the anode segments  15  which central portions are not opposed to the strap ring grooves  20   a  and  20   b . That is, as shown in FIG. 3, when the length of the strap ring grooves  20   a  and  20   c  is Hc, it is most desirable to form the concave  22  so that a relationship Hb=Ha−2×Hc holds. In the anode segments  15  of a typical magnetron apparatus, since the length Hc is 10 to 30% of the length Ha, the ratio of the length Hb to the length Ha is approximately 40 to 80%. 
     On the other hand, when the concave  22  is provided at the inner end surface  21  each of the anode segments  15  in a magnetron apparatus, the distance from the cathode  14  disposed in the central portion in the direction of the arrangement increases at the portion of the concave  22  during operation of the magnetron apparatus. Thereby, there is a possibility that the magnetron efficiency is reduced. Accordingly, in view of the magnetron efficiency, it is desirable that the length Hb of the concave  22  be as small as possible. 
     Here, a relationship between magnetron efficiency and the depth D and the length Hb of the concave  22  obtained through an experiment by the inventors will be described with reference to FIG.  4 . 
     FIG. 4 is a graph showing a relationship between magnetron efficiency and the ratio of the length Hb to the length Ha. Graphs  31 ,  32  and  33  shown in FIG. 4 are results of the experiment when the depth D of the concave  22  is 0.2 mm, 0.3 mm and 0.4 mm, respectively. 
     As is apparent from the graphs  31 ,  32  and  33  of FIG. 4, as the ratio of the length Hb of the concave  22  to the length Ha of the inner end surface  21  is greater, the magnetron efficiency is lower, and as the depth D of the concave  22  is greater, the deterioration of the magnetron efficiency is greater. In the magnetron apparatus, magnetron efficiency of not less than approximately 70% is required in practical use as well known. Therefore, when the depth D of the concave  22  is set to 0.2 mm in view of the dimensional tolerance at the time of mass production, it is desirable that the length Hb of the concave  22  be set to less than 50% of the length Ha of the inner end surface  21 . 
     From the above-described examination results, it is apparent that the ratio of the length Hb of the concave  22  to the length Ha of the inner end surface  21  is desirably selected and set so as to be 20 to 50%. 
     Further, according to an experiment by the inventors, for example, a magnetron apparatus for a microwave oven with an output of 500 to 1000 W was produced. Therein, the magnetron apparatus (hereinafter, referred to as experimental product 1) had the anode segments  15  in which the length Ha of the inner end surface  21  is 9.5 mm, the depth Hc of the strap ring grooves  20   a  and  20   b  is 2.6 mm, the depth D of the concave  22  is 0.2 mm and the length Hb of the concave  22  is 4.0 mm (Hb/Ha=42%). In the experimental product 1, results which are sufficient for practical use were obtained such that the assembly precision is sufficient and the magnetron efficiency is approximately 71%. 
     In the above-mentioned description, the opening configuration of the concave  22  of each anode segments  15  is rectangular. However, the opening configuration may have any configuration as long as there is a predetermined spatial distance in the central portion in the direction of the central axis each of the anode segments  15 , and concaves  23 ,  24  may have a tapered opening configuration  23  or a circular opening configuration  24  as shown in FIG.  5  and FIG. 6, respectively. At this time, the depth D is a distance from a point in the concaves  23 ,  24  which are farthest from the inner end surface  21 , and the length Hb is the size of the widest part of the concaves  23 ,  24 , i.e. the size of the concaves  23 ,  24  at the inner end surface  21  each of the anode segments  15 . 
     In the above-mentioned description, the anode segments  15  are pressed against the inner surface of the anode cylinder  1  by use of the jig pin  40  having the cylindrical portion which comes into contact with a plurality of the inner end surfaces  21 . However, the jig pin  40  is not limited to the one having the cylindrical portion, but any assembly member may be used that is designed so as to come into contact with the inner end surface  21  each of the anode segments  15 . 
     [Manufacturing Method] 
     In the manufacturing method for the magnetron apparatus of this embodiment, first, the plural anode segments  15  and the strap rings  17  and  18  are placed in the respective predetermined positions inside the anode cylinder  1  by use of a non-illustrated temporary assembling jig. Then, the jig pin  40  is moved along the central axis of the anode cylinder  1  and press-fit from below into the central portion in the direction of the arrangement of the anode segments  15  (the central portion of the anode cylinder  1 ) as shown by the arrow Y of FIG.  2 . So that the jig pin  40  contacts with the inner end surface  21  each of the anode segments  15 . Thereby, the anode assembly is maintained in a preassembled condition where the one end surface each of the anode segments  15  is pressed against the inner surface of the anode cylinder  1  by the jig pin  40 . Then, only the temporary assembling jig is detached, and the brazing filler metal  19  is put on the end surface on the longer side each of the anode segments  15  so as to be in contact with the inner surface of the anode cylinder  1  as shown in FIG.  2 . After the magnetic pole piece  2  is attached to the upper open end of the anode cylinder  1 , one end of the antenna  16  is mounted to one of the anode segments  15  (see FIG.  1 ). Then, the anode assembly in the preassembled condition is heated to a predetermined temperature (for example, 800 to 900° C.) in a non-illustrated furnace. Thereby, the brazing filler metal  19  is melted and flows into a clearance between the inner surface of the anode cylinder  1  and the one end surface each of the anode segments  15  caused by expansion. At this time, the plating layers on the strap rings  17  and  18  and the antenna  16  are also melted. Then, by taking the anode assembly out of the furnace while maintaining the preassembled condition, and cooling it, the inner surface of the anode cylinder  1  and the one end surface each of the anode segments  15 , the strap ring grooves  20   a  and  20   b  and the strap rings  17  and  18 , and the antenna  16  and the one of the anode segments  15  are secured. Then, after the jig pin  40  is downwardly pulled out, the magnetic pole piece  3  is attached to the lower open end of the anode cylinder  1  (see FIG.  1 ), so that the assembly of the anode assembly is finished. 
     In the manufacturing method for the magnetron apparatus of this embodiment, because of the provision of the concave  22  in the central portion of the inner end surface  21  each of the anode segments  15 , the area of contact between the inner end surface  21  and the jig pin  40  is smaller than in the conventional apparatus, so that the pressure exerted on the anode segments  15  by the jig pin  40  is reduced. Consequently, the pressure exerted on the two pairs of the strap rings  17  and  18  situated at the upper and lower ends in the direction of the central axis each of the anode segments  15  is smaller than in the conventional apparatus, so that the brazing precision improves and the deformation of the strap rings  17  and  18  and the coming-off of brazed parts due to insufficient brazing can be prevented during the while the jig pin  40  being press-fit and taken out. 
     The pressure which the anode segments  15  from the jig pin  40  is dispersed and uniformized into the upper and lower areas Vx and Vz in the direction of the central axis because the concave  22  is provided in the central portion in the direction of the central axis. Further, since the strap ring grooves  20   a  and  20   b  are provided in the upper and lower areas Vx and Vz, even if the anode segments  15  expand due to temperature increase at the time of brazing, the expanded portions are absorbed by the strap ring grooves  20   a  and  20   b , so that the pressure is equally exerted. 
     Particularly, since the central area Vy each of the anode segments  15  includes a spatial distance defined by the depth D of the concave  22  from the jig pin  40 , even if outer dimension variation or expansion of the anode segments  15  is caused, no pressure is exerted on the central area Vy by the jig pin  40 . Therefore, even if the anode segments  15  expand when heated, the pressures exerted on the upper and lower areas Vx and Vz are similar. Consequently, the anode segments  15  can be pressed against the jig pin  40  always in a stable condition at the two portions of the upper and lower areas Vx and Vz, so that even if the jig pin  40  has a surface which is as smooth as a mirror finished surface, the variation in pitch as illustrated in FIG. 19 is never caused. That is, in the manufacturing method for the magnetron apparatus of this embodiment, the plural anode segments  15  can be equally spaced in the anode cylinder  1 , so that the magnetron apparatus which operates with stability can be obtained. 
     As has been explained in the above, according to the manufacturing method for the magnetron apparatus of the present invention, the precision with which the anode assembly is assembled can be easily improved without modifying the process from the preassembly to the brazing by use of the conventional ordinary assembly jig as it is without any modification. Particularly, as the jig pin  40  which is expensive because high heat resistance and high wear resistance are required therefor, a conventional temporary assembling pin can be used as it is without any modification, so that the manufacture cost is prevented from greatly increasing. 
     &lt;&lt;Second Embodiment&gt;&gt; 
     FIG. 7 is a cross sectional view showing a configuration of a principal part of an anode assembly of a magnetron apparatus in a second embodiment of the present invention. In this embodiment, in the configuration of the magnetron apparatus, a chamfered portion is provided in which at least one angular portion of the inner end surface each of the anode segments is chamfered. The other elements and portions are similar to those of the first embodiment, and therefore overlapping descriptions on the similar points are omitted from the description of this figure. 
     As shown in FIG. 7, in the magnetron apparatus of this embodiment, a tapered chamfered portion  26  is provided at one angular portion of the inner end surface  21  each of anode segments  25  and  25 ′, and the anode segments  25  and  25 ′ are secured to the inner surface of the anode cylinder  1  so that the chamfered portions  26  are situated at the upper side in the direction of the central axis. That is, in the anode segment  25 , the chamfered portion  26  is formed by chamfering an angular portion at which the inner end surface  21  intersects the end surface where the strap ring groove  20   a  is provided. In the anode segment  25 ′, the chamfered portion  26  is formed by chamfering an angular portion at which the inner end surface  21  intersects the end surface where the strap ring groove  20   b  is provided. By providing such a chamfered portion  26 , in the magnetron apparatus of this embodiment, the area of contact between the jig pin  40  and the anode segments  25  and  25 ′ is smaller than in the first embodiment, so that the pressure exerted on the anode segments  25  and  25 ′ by the jig pin  40  can be reduced. 
     Moreover, as shown in FIG. 8, the anode segments  25  and  25 ′ may be secured to the inner surface of the anode cylinder  1  so that the chamfered portions  26  are situated at the lower side in the direction of the central axis. 
     Further, as shown in FIG.  9  and FIG. 10, the anode segments which are secured to the inner surface of the anode cylinder  1  may be only one kind of the two anode segments  25  and  25 ′ (see FIG.  9 ). 
     Moreover, as shown in FIG. 11, an anode segment  27  in which the chamfered portion  26  is provided at the angular portion at each of the upper and lower ends of the inner end surface  21  in the direction of the central axis may be secured to the inner surface of the anode cylinder  1 . 
     In the anode assemblies shown in FIG. 7 to FIG. 10, the contact area can be reduced by substantially the same extent. In the anode assemblies shown in FIG.  8  and FIG. 11, since the chamfered portion  26  is situated at the side where the jig pin  40  is inserted, the jig pin  40  is more easily inserted than in the other anode assemblies. 
     In a conventional anode assembly for the magnetron apparatus, typically, anode segments of the same configuration are arranged so that every two anode segments are vertically inverted. However, when the anode segments  25  and  25 ′ shown in FIG.  7  and FIG. 8 are used, it is necessary to select those anode segments  25  and  25 ′ and arranged them alternately. On the other hand, when the anode segments  27  shown in FIG. 11 are used, since the chamfered portion  26  is provided at the angular portion at each of the upper and lower ends of the inner end surface  21 , the selection of anode segments is unnecessary, so that the time necessary for assembling the anode assembly can be reduced the most. Further, the contact area can be reduced the most and the insertion of the jig pin  40  is facilitated. Thus, the anode segments  27  are most suitable for practical use. 
     According to an experiment by the inventors, in the anode segments  27  for use in the magnetron apparatus for the microwave oven with an output of 500 to 1000 W, the most desirable result where the magnetron efficiency is the highest was obtained when the chamfered portion  26  of C=0.2 to 0.6 mm was provided at each of the upper and lower ends of the inner end surface  21 , where C is the length of one edge of the chamfer. 
     As described above, in the magnetron apparatus of this embodiment, the chamfered portion  26  is provided on at least one angular portion of the inner end surface  21 . Thereby, the area of contact between the anode segments and the jig pin  40  is smaller than in the first embodiment, so that the aforementioned deformation of the anode segments and the strap rings  17  and  18 , the detachment of brazed parts due to insufficient brazing and the generation of burrs due to nonuniformity of components can be further reduced. 
     In the above description, the tapered chamfered portion  26  is provided at the inner end surface  21  which faces the jig pin  40 . However, the configuration of the chamfered portion is not limited to the tapered configuration as long as the dimension in the direction of the central axis of the inner end surface  21  which faces the jig pin  40  can be reduced. For example, a circular chamfered portion may be provided. 
     Further, in the above description, the chamfered portion  26  is provided on at least  21  one of the upper and lower ends of the inner end surface in the direction of the central axis. However, the chamfered portion may be provided at an angular portion which faces the concave  22  of the inner end surface  21 . 
     Test results on noise characteristics of the magnetron apparatus of the present invention will be explained with reference to FIG. 12 to FIG.  15 . 
     FIG. 12 is a graph showing measurement results of the noise level at a fifth harmonic. FIG. 13 is a measurement result showing noise characteristics in the vicinity of the fifth harmonic in the conventional magnetron apparatus shown in FIG.  16 . FIG. 14 is a measurement result showing noise characteristics in the vicinity of the fifth harmonic in the magnetron apparatus of the first embodiment. FIG. 15 is a measurement result showing noise characteristics in the vicinity of the fifth harmonic in the magnetron apparatus of the second embodiment. 
     In this test, three kinds of magnetron apparatuses, i.e. the aforementioned experimental product 1 of the first embodiment, an experimental product 2 of the second embodiment in which the chamfered portion  26  of C=0.5 mm is provided in each anode segment of the experimental product 1, and the conventional apparatus shown in FIG. 16 were operated at a fundamental frequency of 2,450 MHz, and noise levels at the fifth harmonic 12.25 GHz and at frequencies in the vicinity thereof were measured. This is because the fifth harmonic of such magnetron apparatuses falls within the frequency range (11.7 to 12.7 GHz) of the satellite broadcasting band on which strict regulation has been imposed in recent years. In this test, it was examined whether the standard of CISPR (International Special Committee on Radio Interference) was satisfied or not. Specifically, the effective radiated power of electromagnetic waves within the frequency range of 11.7 to 12.7 GHz was measured with a half-wave dipole antenna as the reference, and it was examined whether or not the measurement results were not more than 57 dB which is the permissible electric power of the radio frequency radiation jamming wave defined by the standard. 
     As a result, in the experimental products 1 and 2, the measurement results of the noise level at the fifth harmonic were 47 to 51 dB and 43 to 48 dB, respectively, as shown at B and E of FIG. 12, and both were below the permissive value 57 dB and satisfied the CISPR standard. Moreover, it was found that the experimental product 2 having the anode segments  27  provided with the chamfered portion  26  was more effective for reducing the noise level at the fifth harmonic than the experimental product 1. On the contrary, in the conventional apparatus, the measurement results were 55 to 58 dB as shown at A of FIG.  12  and the CISPR standard was not satisfied. 
     Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that such disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains, after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the true spirit and scope of the invention.