Patent Publication Number: US-7906912-B2

Title: Magnetron

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a magnetron for used in equipment using microwaves such as a microwave oven. 
     2. Description of the Related Art 
       FIG. 15  is a longitudinal section view of a general magnetron which is conventionally used in a microwave oven, and  FIG. 16  is an enlarged section view of the main portions of the magnetron shown in  FIG. 15 . In  FIGS. 15 and 16 , in the inside of a cylindrical-shaped anode barrel member  10 , there are radially disposed anode vanes  11 , while spaces respectively enclosed by the mutually adjoining anode vanes  11  and anode barrel member  10  constitute a cavity resonator. In the central portion of the anode barrel member  10 , there is disposed a cathode structure member  12 , while a space enclosed by the anode structure member  12  and anode vane  11  constitutes an action space  13 . On the upper end of the anode barrel member  10 , there is fixedly mounted a pole piece (which is hereinafter referred to as an output side pole piece)  14 , whereas, on the lower end thereof, there is fixedly mounted another pole piece (which is hereinafter referred to as an input side pole piece)  15 . 
     The output side pole piece  14  is formed in a funnel shape by drawing a magnetic plate member having small magnetic resistance such as an iron plate member. That is, the output side pole piece  14  provides a funnel shape which includes a small-diameter flat portion FL 1  having a penetration hole  14 A formed in the central portion thereof, a large-diameter flat portion FL 2  having a larger diameter than the small-diameter flat portion FL 1 , and a conical-shaped slanting portion SL which connects together the large-diameter and small-diameter flat portions FL 2  and FL 1 . In the output side pole piece  14 , besides the penetration hole  14 A formed in the central portion thereof, there is also formed another penetration hole  14 B through which an antenna  16  can be penetrated. 
     The input side pole piece  15 , similarly to the output side pole piece  14 , is formed in a funnel shape by drawing a magnetic plate member having small magnetic resistance such as an iron plate member. That is, the input side pole piece  15  provides a funnel shape which includes a small-diameter flat portion FL 1  having a penetration hole  14 A formed in the central portion thereof, a large-diameter flat portion FL 2  having a larger diameter than the small-diameter flat portion FL 1 , and a conical-shaped slanting portion SL which connects together the large-diameter and small-diameter flat portions FL 2  and FL 1 . Just above the output side pole piece  14 , there is disposed a metal ring  17  which covers the output side pole piece  14 , while, just below the input side pole piece  15 , there is disposed a metal ring  18  for covering the input side pole piece  15 . Just above the metal ring  17  and just below the metal ring  18 , there are respectively mounted ring-shaped magnets (not shown) in a close contact manner, the central portions of both of which are formed hollow. To the cathode structure member  12 , there is connected a lead  19  which is used to apply a direct current voltage to the cathode structure member  12 . 
     When using the conventional magnetron, after the inside of the magnetron is evacuated, a direct current high voltage is applied to between the anode vane  11  and cathode structure member  12 . In the action space  13 , there is formed a magnetic field due to the two magnets (not shown). When the direct current high voltage is applied to and between the anode vane  11  and cathode structure member  12 , electrons are drawn out from the cathode structure member  12  and thus they fly out toward the anode vane  11 . At the then time, the magnetic field due to the two magnets (not shown) concentrates in a gap existing between the output side pole piece  14  and input side pole piece  15 , and it acts on the action space  13  in a direction perpendicular to a direction where the cathode structure member  12  and anode barrel member  10  are opposed to each other. As a result of this, electrons flown out from the cathode structure member  12  are rotated and moved in a spiral by a force which is generated by the magnetic field due to the magnets (not shown), and the electrons finally arrive at the anode vane  11 . Energy generated due to the then time electrons movements is applied to the cavity resonator to contribute toward the oscillation of the magnetron. 
     By the way, when discharging the air existing in the inside of the magnetron, the air on the input side, as shown in  FIG. 17 , passes not only through a penetration hole  15 A opened up in the central portion of the input side pole piece  15  but also through a penetration hole  21 A opened up in a lower end hat  21  which constitutes the cathode structure member  13 . Since the lower end hat  21  is situated in the penetration hole  15 A of the input side pole piece  15  and one end portion of a filament coil  22  is situated in the penetration hole  21 A of the lower end hat  21 , the portions of the penetration holes  15 A and  21 A, through which the air passes, are made narrow. This makes it impossible to provide a large air discharge conductance (an air exhaust efficiency), thereby taking much time to discharge the air. Owing to the fact that it takes much time for the air exhaust, there is a fear that there can occur a poor degree of vacuum. To solve this problem, there is proposed a structure in which an output side pole piece having a penetration hole  14 B, through which the antenna  16  is to be passed, is employed as an input side pole piece to thereby increase the air discharge conductance (for example, see Japanese Utility Model Publication Sho-63-18745). The air, which has passed through the input side pole piece  15  and flowed into the inside of the anode barrel member  10 , is discharged from an exhaust pipe  20  through the penetration hole  14 A opened up in the central portion of the output side pole piece  14  as well as through the penetration hole  14 B opened up for the passage of the antenna therethrough. 
     However, even when there is disposed a new opening in the input side pole piece  15  (there may also be the output side pole piece  14 ) in order to discharge the air on the input side with high efficiency, depending on the size of the opening, there is also a fear that the maximum magnetic field strength can be lowered or higher harmonic waves can leak. 
     SUMMARY OF THE INVENTION 
     The present invention is made in view of the above conventional circumstances. Thus, it is an object of the invention to provide a magnetron which can increase the air exhaust conductance without lowering the maximum magnetic field strength or causing the leakage of the higher harmonic waves. 
     The above object can be attained by the following structure and method. 
     (1) A magnetron, comprising: a cylindrical-shaped anode barrel member having two openings respectively formed in the two end portions thereof; a cathode structure member disposed on the center axis of the anode barrel member; more than one anode vane disposed radially through an action space in the periphery of the cathode structure member and fixedly mounted on the inner wall surface of the anode barrel member; and, a funnel-shaped input side pole piece disposed on the side of one of the two openings of the anode barrel member for supply of power to the cathode structure member, the input side pole piece including a small-diameter flat portion having a penetration hole formed in the central portion thereof, a large-diameter flat portion having a diameter larger than the diameter of the small-diameter flat portion, and a conical-shaped slanting portion for connecting the large-diameter flat portion and small-diameter flat portion to each other, wherein the input side pole piece further includes, besides the penetration hole formed in the central portion of the small-diameter flat portion, three or more penetration holes respectively formed in the slanting portion thereof. 
     (2) A pole piece manufacturing method for manufacturing a magnetron comprising: a cylindrical-shaped anode barrel member having two openings respectively formed in the two end portions thereof; a cathode structure member disposed on the center axis of the anode barrel member; more than one anode vane disposed radially through an action space in the periphery of the cathode structure member and fixedly mounted on the inner wall surface of the anode barrel member; and, a funnel-shaped input side pole piece disposed on the side of one of the two openings of the anode barrel member for supply of power to the cathode structure member, the input side pole piece including a small-diameter flat portion having a penetration hole formed in the central portion thereof, a large-diameter flat portion having a diameter larger than the diameter of the small-diameter flat portion, and a conical-shaped slanting portion for connecting the large-diameter flat portion and small-diameter flat portion to each other, wherein there is formed a penetration hole over the large-diameter flat portion and slanting portion of the input side pole piece so as to extend in the axial direction of the input side piece pole. 
     (3) In the pole piece manufacturing method as set forth in the above item (2), the area of the penetration hole is 16.6 mm 2  or smaller and three or more such penetration holes are formed at given intervals in the peripheral direction of the slanting portion of the input side pole piece. 
     According to the magnetron as set forth in the above item (1), since the input side pole piece has three or more penetration holes in the slanting portion thereof, a large air conductance can be provided, thereby being able to shorten the air exhaust time to discharge the air existing in the inside of the magnetron. Also, because the air of the inside of the magnetron can be discharged positively, the occurrence of a poor degree of vacuum within the magnetron can also be prevented. Further, since the area of each penetration hole is set for 16.6 mm 2  or smaller, the lowering of the maximum magnetic field strength and the leakage of higher harmonic waves can be prevented. 
     According to the magnetron pole piece manufacturing method as set forth in the above item (2), since the penetration hole is formed in the axial direction (that is, in the vertical direction) over the large-diameter flat portion and slanting portion of the input side pole piece, the penetration hole can be formed simultaneously when the input side pole piece is manufactured by press working, which can minimize an increase in the cost for forming the penetration hole. 
     According to the magnetron pole piece manufacturing method as set forth in the above item (3), since three or more penetration holes are formed at given intervals in the peripheral direction of the slanting portion, a large air exhaust conductance can be secured when the magnetron is in operation, which makes it possible to shorten the air exhaust time to discharge the air existing in the inside of the magnetron. Also, because the air of the inside of the magnetron can be discharged positively, the occurrence of a poor degree of vacuum within the magnetron can also be prevented. Further, since the area of each penetration hole is set for 16.6 mm 2  or smaller, the lowering of the maximum magnetic field strength and the leakage of higher harmonic waves can be prevented. 
     Also, in the case of a microwave using apparatus according to the invention, since it includes the above-mentioned magnetron, the air exhaust time can be shortened as well as the stable operation of the apparatus can be realized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal section view of a magnetron according to an embodiment of the invention. 
         FIG. 2  is an enlarged section view of the main portions of the magnetron shown  FIG. 1 . 
         FIG. 3  is a view to show how the air passes in an input side pole piece employed in the magnetron shown in  FIG. 1 . 
         FIG. 4  is a view of an example of the experimental results of variations in the maximum magnetic field strength caused by the different number of penetration holes and the different diameters of penetration holes opened up in the input side pole piece shown in  FIG. 1 . 
         FIG. 5  is a graphical representation of the relationship between the surface area(s) of the hole(s) and the maximum magnetic field strength based on the experimental results shown in  FIG. 4 . 
         FIG. 6  is a graphical representation of the relationship between the number of holes and the maximum magnetic field strength based on the experimental results shown in  FIG. 4 . 
         FIG. 7  is an explanatory view of an experiment conducted (on a diameter-direction measuring portion) about the magnetic field distortion thereof. 
         FIG. 8  is an explanatory view of an experiment conducted (on an axial-direction measuring portion) about the magnetic field distortion thereof. 
         FIG. 9  is an explanatory view of an experiment conducted about the magnetic field distortion (magnetic field strength measured result values). 
         FIG. 10  is an explanatory view of an experiment conducted about the magnetic field distortion (a graph  1  showing the magnetic field strength measured results). 
         FIG. 11  is an explanatory view of an experiment conducted about the magnetic field distortion (a graph  2  showing the magnetic field strength measured results). 
         FIG. 12  is an explanatory view of an experiment conducted about the magnetic field distortion (a graph  3  showing the magnetic field strength measured results). 
         FIG. 13  is a graphical representation of results (the relationships between the hole area and damping quantity) obtained by an experiment conducted about the relationship between the hole diameters and higher harmonic waves. 
         FIG. 14  is a graphical representation of the measured results of the hole number and Efm when the area of a hole formed in the input side pole piece is 16.6 (mm 2 ). 
         FIG. 15  is a longitudinal section view of a conventional magnetron. 
         FIG. 16  is an enlarged section view of the main portions of the magnetron shown in  FIG. 15 . 
         FIG. 17  is a view to show how the air passes in an input side pole piece employed in the magnetron shown in  FIG. 15 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, description will be given below in detail of a preferred embodiment of a magnetron according to the invention with reference to the accompanying drawings. 
       FIG. 1  is a longitudinal section view of a magnetron according to an embodiment of the invention, while  FIG. 2  is an enlarged section view of the main portions of the magnetron shown in  FIG. 1 . In  FIG. 2 , the magnetron according to the present embodiment comprises: a cylindrical-shaped anode barrel member  10  having two openings respectively formed in two end portions thereof; a cathode structure member  12  disposed on the center axis of the anode barrel member  10 ; more than one anode vane  11  disposed radially through an action space  13  in the periphery of the cathode structure member  12  and fixedly mounted on the inner wall surface of the anode barrel member  10 ; and a pair of funnel-shaped pole pieces  14  and  30  respectively disposed in their associated ones of the two openings respectively formed in the two end portions of the anode barrel member  10 , each pole piece including a small-diameter flat portion FL 1  having a penetration hole formed in the central portion thereof, a large-diameter flat portion FL 2  having a diameter larger than the diameter of the small-diameter flat portion FL 1 , and a conical-shaped slanting portion SL for connecting the large-diameter flat portion FL 2  and small-diameter flat portion FL 1  to each other. Of the pair of pole pieces  14  and  30 , the output side pole piece  14 , which is disposed on the side where an antenna  16  is arranged, further includes, besides the penetration hole  14 A formed in the central portion thereof, a penetration hole  14 B through which the antenna  16  can be penetrated; and, the input side pole piece  30  disposed on the side for supply of power to the cathode structure member  12  includes, besides the penetration hole  30 A formed in the central portion thereof, three or more, preferably, four penetration holes  30 B formed in its slanting portion SL, each penetration hole  30 B having an area of 11.5 mm 2 . 
     The penetration hole  30 A, which is formed in the central portion of the input side pole piece  30 , is similar in size to one formed in the conventional magnetron. 
     The four penetration holes  30 B of the slanting portion SL are formed at 90° intervals in the peripheral direction of the slanting portion SL and extend in the axial direction (that is, in the vertical direction) over the large-diameter flat portion FL 2  and slanting portion SL. Thanks to such formation of the penetration holes  30 B, when producing the input side pole piece  30  by press working, the four penetration holes  30 B together with the penetration hole  30 A formed in the central portion can be formed simultaneously, which can minimize an increase in the cost for forming the four penetration holes  30 B. By the way, when trying to form a penetration hole perpendicularly to the surface of the slanting portion SL, generally, there is necessary press working which uses a cam die. Especially, in the case of a progressive metal mold, there is necessary a metal mold installation space for each hole, which requires a large space and thus increases the cost for formation of holes. 
     Thanks to new formation of the four penetration holes  30 B in the input side pole piece  30 , the air existing on the input side can be discharged with high efficiency and thus a large air exhaust conductance can be secured. Also, owing to the fact that each of the penetration holes  30 B is formed to have a size of 11.5 mm 2 , it has been found by an experiment that the magnetic field distribution cannot be distorted and the magnetic field strength cannot be lowered. 
     When discharging the air existing in the inside of the magnetron, the air on the input side, as shown in  FIG. 3 , passes through the penetration hole  30 A formed in the central portion of the input side pole piece  30 , the four penetration holes  30 B formed in the slanting portion SL, and a penetration hole  21 A opened up in a lower end hat  21  which constitutes the cathode structure member  13 , respectively. Especially, since a large amount of air passes through the newly formed four penetration holes  30 B, there can be provided a large air exhaust conductance (air exhaust efficiency). This can shorten the time necessary for the air exhaust and also can prevent occurrence of a poor degree of vacuum. 
     Next, description will be given of the results of the experiment conducted by the inventors. 
       FIG. 4  shows the experimentally obtained results of the relationships between the hole diameter/hole number and the magnetic strength. In this case, the number of holes is up to four, while the diameters of the holes are respectively set for 3.3 mm, 3.8 mm, 4.2 mm, 4.6 mm, and 6.5 mm. In  FIG. 4 , for example, when the hole diameter is 6.5 mm and the hole number is 1, the area of the hole provides 33.2 mm 2  and the maximum magnetic field strength provides 181.8 mT; and, when the hole diameter is 6.5 mm and the hole number is three, the hole area provides 99.5 mm 2  and the maximum magnetic field strength provides 181.4 mT. Also, when the hole diameter is 4.2 mm and the hole number is 1, the hole area provides 13.9 mm 2  and the maximum magnetic field strength provides 182.4 mT, and, when the hole diameter is 4.2 mm and the hole number is 3, the hole area provides 41.6 mm 2  and the maximum magnetic field strength provides 182.4 mT By the way, although not shown in  FIG. 4 , for no hole, the maximum magnetic field strength provides 182.4 mT. 
     Now,  FIGS. 5 and 6  are respectively graphical representations of the results that have been obtained in the above experiment. Specifically,  FIG. 5  shows the relationship between the hole area (mm 2 ) and the maximum magnetic field strength (mT), and  FIG. 6  shows the relationship between the hole number (piece) and the maximum magnetic field strength (mT). As can be seen from  FIG. 5 , when the hole diameter is equal to or smaller than 4.2 mm, the maximum magnetic field strength (mT) shows a good value. Also, as can be seen from  FIG. 6 , for the hole diameter equal to or smaller than 4.2 mm, even when the hole number (piece) is set for four, the maximum magnetic field strength (mT) shows a good value. 
     As the hole diameter increases, even when the area is the same, the maximum magnetic field strength decreases. That is, the maximum magnetic field strength decreases when the hole area per hole is equal to or larger than 16.6 (mm 2 ). Also, for the same hole area, when the area per hole decreases and the hole number increases, the maximum magnetic field strength is hard to decrease. 
     Now,  FIGS. 7 to 12  respectively show the results as to the magnetic field distortion that have been obtained by experiments.  FIG. 7A  shows an input side pole piece having no other penetration hole than a penetration hole formed in the central portion thereof and a diameter-direction measuring portion Ph 1  corresponding to the penetration hole. This input side pole piece is similar to the conventional input side pole piece and, therefore, a reference numeral  15  is given to it.  FIG. 10  is a graphical representation which shows the results obtained by measuring the magnetic field strength, at the position of the diameter-direction measuring portion Ph 1 , in the respective axial-direction measuring portions Pv- 8 ˜pv 8  respectively shown in  FIG. 8 . 
     Also,  FIG. 7B  shows an input side pole piece having a penetration hole in addition to a penetration hole formed in the central portion thereof and two diameter-direction measuring portions Ph 1  and Ph 2  respectively corresponding to the two penetration holes. This input side pole piece is similar to the input side pole piece  30  according to the present embodiment and, therefore, reference numerals  30  and  30 B are given to them, respectively. The diameter-direction measuring portion Ph 1  is a portion in which no hole is formed, whereas the diameter-direction measuring portion Ph 2  is a portion in which a hole is formed.  FIG. 11  shows the results obtained by measuring the magnetic field strength, at their respective positions, in the respective axial-direction measuring portions Pv- 8 ˜pv 8  respectively shown in  FIG. 8 . 
     Also,  FIG. 7C  shows an input side pole piece having four penetration holes in addition to a penetration hole formed in the central portion thereof and two diameter-direction measuring portions Ph 1  and Ph 2  respectively corresponding to these penetration holes. This input side pole piece is also similar to the input side pole piece  30  according to the present embodiment and, therefore, reference numerals  30  and  30 B are given to them, respectively. The diameter-direction measuring portion Ph 1  is a portion in which no hole is formed, whereas the diameter-direction measuring portion Ph 2  is a portion in which a hole is formed.  FIG. 11  shows the results obtained by measuring the magnetic field strength, at their respective positions, in the respective axial-direction measuring portions Pv- 8 ˜pv 8  respectively shown in  FIG. 8 . 
     Now,  FIG. 9  shows the measured results of the magnetic field strength in the respective cases shown in  FIGS. 7A to 7C . In  FIG. 9 , in the case shown in  7 A, the magnetic field strength in the axial-direction measuring portion Pv- 6  is 127.3 mT, the magnetic field strength in the axial-direction measuring portion Pv- 5  is 147.7 mT, the magnetic field strength in the axial-direction measuring portion Pv- 4  is 166.3 mT, the magnetic field strength in the axial-direction measuring portion Pv- 3  is 174.9 mT, the magnetic field strength in the axial-direction measuring portion Pv- 2  is 180 mT, the magnetic field strength in the axial-direction measuring portion Pv- 1  is 182.2 mT, the magnetic field strength in the axial-direction measuring portion Pv 0  is 182.4 mT, the magnetic field strength in the axial-direction measuring portion Pv 1  is 181.2 mT, the magnetic field strength in the axial-direction measuring portion Pv 2  is 177.4 mT, the magnetic field strength in the axial-direction measuring portion Pv 3  is 169.8 mT, the magnetic field strength in the axial-direction measuring portion Pv 4  is 158.2 mT, the magnetic field strength in the axial-direction measuring portion Pv 5  is 140 mT and the magnetic field strength in the axial-direction measuring portion Pv 6  is 113.4 mT. 
     In the case shown in  FIG. 7B , in the diameter-direction measuring portion P 1  in which no hole is formed, the magnetic field strength in the axial-direction measuring portion Pv- 6  is 115.1 mT, the magnetic field strength in the axial-direction measuring portion Pv- 5  is 140.3 mT, the magnetic field strength in the axial-direction measuring portion Pv 4  is 161.3 mT the magnetic field strength in the axial-direction measuring portion Pv- 3  is 172.4 mT, the magnetic field strength in the axial-direction measuring portion Pv- 2  is 178.9 mT, the magnetic field strength in the axial-direction measuring portion Pv- 1  is 181.5 mT, the magnetic field strength in the axial-direction measuring portion Pv 0  is 182.3 mT the magnetic field strength in the axial-direction measuring portion Pv 1  is 180.9 mT, the magnetic field strength in the axial-direction measuring portion Pv 2  is 177.3 mT the magnetic field strength in the axial-direction measuring portion Pv 3  is 172.6 mT the magnetic field strength in the axial-direction measuring portion Pv 4  is 160.4 mT the magnetic field strength in the axial-direction measuring portion Pv 5  is 143.2 mT and the magnetic field strength in the axial-direction measuring portion Pv 6  is 116.1 mT. 
     In the case shown in  FIG. 7B , in the diameter-direction measuring portion P 2  in which a hole is formed, the magnetic field strength in the axial-direction measuring portion Pv- 6  is 140 mT, the magnetic field strength in the axial-direction measuring portion Pv- 5  is 160 mT, the magnetic field strength in the axial-direction measuring portion Pv 4  is 173 mT, the magnetic field strength in the axial-direction measuring portion Pv- 3  is 179.2 mT, the magnetic field strength in the axial-direction measuring portion Pv- 2  is 181.3 mT, the magnetic field strength in the axial-direction measuring portion Pv- 1  is 181.8 mT, the magnetic field strength in the axial-direction measuring portion Pv 0  is 180.5 mT, the magnetic field strength in the axial-direction measuring portion Pv 1  is 176.8 mT, the magnetic field strength in the axial-direction measuring portion Pv 2  is 171.8 mT, the magnetic field strength in the axial-direction measuring portion Pv 3  is 159.2 mT, the magnetic field strength in the axial-direction measuring portion Pv 4  is 139.7 mT, the magnetic field strength in the axial-direction measuring portion Pv 5  is 117.2 mT, and the magnetic field strength in the axial-direction measuring portion Pv 6  is 91 mT. 
     In the case shown in  FIG. 7C , in the diameter-direction measuring portion P 1  in which no hole is formed, the magnetic field strength in the axial-direction measuring portion Pv- 6  is 115.8 mT, the magnetic field strength in the axial-direction measuring portion Pv- 5  is 140.9 mT, the magnetic field strength in the axial-direction measuring portion Pv- 4  is 161.2 mT, the magnetic field strength in the axial-direction measuring portion Pv- 3  is 170.3 mT, the magnetic field strength in the axial-direction measuring portion Pv- 2  is 176.3 mT, the magnetic field strength in the axial-direction measuring portion Pv- 1  is 180.1 mT, the magnetic field strength in the axial-direction measuring portion Pv 0  is 180.9 mT, the magnetic field strength in the axial-direction measuring portion Pv 1  is 180.9 mT, the magnetic field strength in the axial-direction measuring portion Pv 2  is 177.6 mT the magnetic field strength in the axial-direction measuring portion Pv 3  is 172.1 mT the magnetic field strength in the axial-direction measuring portion Pv 4  is 161.6 mT the magnetic field strength in the axial-direction measuring portion Pv 5  is 144.9 mT, and the magnetic field strength in the axial-direction measuring portion Pv 6  is 118.1 mT. 
     In the case shown in  FIG. 7C , in the diameter-direction measuring portion P 2  in which a hole is formed, the magnetic field strength in the axial-direction measuring portion Pv- 6  is 116 mT, the magnetic field strength in the axial-direction measuring portion Pv- 5  is 141.8 mT the magnetic field strength in the axial-direction measuring portion Pv- 4  is 160.6 mT, the magnetic field strength in the axial-direction measuring portion Pv- 3  is 171.3 mT, the magnetic field strength in the axial-direction measuring portion Pv- 2  is 177.8 mT, the magnetic field strength in the axial-direction measuring portion Pv- 1  is 180.4 mT the magnetic field strength in the axial-direction measuring portion Pv 0  is 181.3 mT the magnetic field strength in the axial-direction measuring portion Pv 1  is 180.4 mT, the magnetic field strength in the axial-direction measuring portion Pv 2  is 177.1 mT the magnetic field strength in the axial-direction measuring portion Pv 3  is 171.5 mT, the magnetic field strength in the axial-direction measuring portion Pv 4  is 161.2 mT the magnetic field strength in the axial-direction measuring portion Pv 5  is 144.6 mT, and the magnetic field strength in the axial-direction measuring portion Pv 6  is 117.2 mT. 
     The results of  FIG. 7B  shown in  FIG. 11  shows that, in the case where the number of the penetration hole  30 B is one, the distribution of the magnetic field strength differs between the portion having a hole and the portion having no hole. On the other hand, the results of  FIG. 7C  shown in  FIG. 12  shows that, in the case where the number of the penetration hole  30 B is four, the distribution of the magnetic field strength differs little between the portion having the holes and the portion having no hole. Therefore, it can be judged that, preferably, there may be formed four penetration holes  30 B. 
     Now,  FIG. 13  is a graphical representation of the relationship of the damping quantity (dB) of higher harmonic waves with respect to the area of a hole when the plate thickness of an input side pole piece is 1.6 (mm). Generally, when the damping quantity is equal to or more than 30 (dB), it can be expected that the higher harmonic wave noise is hardly influenced. When the area of each penetration hole is taken into account, if the area of the hole is smaller than 27 (mm 2 ), the leakage of the higher harmonic wave noise has little influence on the worsening of the higher harmonic wave noise; but, if the area of the hole is equal to or larger than 27 (mm 2 ), there is a possibility that the higher harmonic wave noise can be worsened. 
     From the above-mentioned experimental results, it can be judged that the optimum value of the area of the penetration hole  30 B to be able to provide a large air exhaust conductance without generating any distortion in the magnetic field distribution nor lowering the magnetic field strength is 16.6 (mm 2 ) or smaller. 
       FIG. 14  shows the measured results of the hole number and Efm when the area of the hole of the input side pole piece is set 16.6 (mm 2 ). The Efm is one of the characteristics of the magnetron and is also a parameter which can tell whether the vacuum degree is good or not. As the vacuum degree is worsened, the Efm is increased. While the Efm of the conventional magnetron is 1.4 V, the Efm of a magnetron including two holes is 1.1 V and the Efm of a magnetron including three or more holes is 1.0 V, that is, it is stable.  FIG. 14  shows that, when the number of holes is large, the vacuum degree of a magnetron is good. Execution of the exhaust of the air in a portion where the Efm is stable can prevent the occurrence of a poor vacuum degree. 
     As described above, according to the magnetron of the present embodiment, since, in the input side pole piece  30  disposed on the side where power is supplied to the cathode structure member  12 , there are formed four penetration holes  30 B each having an area of 16.6 mm 2  or smaller in the slanting portion SL in addition to the penetration hole  30 A formed in the central portion of the input side pole piece  30 , it is possible to provide a large air exhaust conductance, thereby being able to reduce the exhaust time necessary to discharge the air existing in the inside of the magnetron. And, because the air existing in the inside of the magnetron can be exhausted positively, the occurrence of the poor vacuum degree within the magnetron can be prevented. Also, by setting the area of each penetration hole  30 B for 16.6 mm 2  or smaller, the lowering of the maximum magnetic field strength as well as the leakage of the higher harmonic waves can be prevented. 
     Also, since the respective penetration holes  30 B are formed in the vertical direction (that is, in the axial direction of the input side pole piece) over the large-diameter flat portion FL 2  and slanting portion SL, the penetration holes  30 B can be produced simultaneously when the input side pole piece  30  is produced by press working. This can minimize an increase in the cost necessary for forming the respective penetration holes  30 B. 
     The present invention provides an effect that the air exhaust conductance can be increased without lowering the maximum magnetic field strength or causing the leakage of the higher harmonic waves, and thus the invention can be used effectively as a microwave oscillation device for use in a microwave oven and the like.