Abstract:
A microwave tube collector assembly includes a metal cylinder and a chromium oxide film. The metal cylinder has at least a layer of a chromium alloy on its overall inner circumferential surface and is closed at one end thereof. The a chromium oxide film is formed on the basis of the chromium alloy to cover the inner circumferential surface of the metal cylinder.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a microwave tube collector assembly and, more particularly, to a microwave tube collector assembly having a conductive thin film on the inner circumferential surface of a metal cylinder. 
     A microwave tube collector assembly recovers an electron beam emitted from an electron gun. In the collector assembly, a metal cylinder is called a collector core and is closed at one end to have a bottom for directly recovering the electron beam. When the microwave tube is operated, the electron beam collides against the inner circumferential portion of the collector core. At this time, secondary electrons are emitted from the collector core. The secondary electron emission largely depends on the metal material of the collector core. In general, the average number of secondary electrons generated per electron of an incident electron beam is called a secondary electron emission ratio. 
     As the material of a collector core, oxygen-free copper having a high thermal conductivity is mainly used in consideration of heat generated by collision of the electron beam. However, the secondary electron emission from the oxygen-free copper causes a relatively large decrease in overall microwave tube efficiency. In addition, the secondary electrons emitted from the collector core are returned to a slow wave circuit, and the microwave tube may fail to amplify. As shown in FIGS. 5A and 5B, therefore, a coating having a low secondary electron emission ratio is formed on the inner circumferential surface of a conventional collector core to suppress secondary electron emission. In FIGS. 5A and 5B a conventional microwave tube collector assembly is shown, reference numeral 15 denotes an oxygen-free copper collector core having an outer diameter of 40 mm, a length of 120 mm, and a cylindrical recess portion having an inner diameter of 30 mm and a length of 100 mm, and reference numeral 16 denotes a carbon thin film coated on the inner circumferential surface of the oxygen-free copper collector core 15 and used for suppressing secondary electron emission. Reference numeral 17 denotes a large number of heat-radiating fins extending on the outer peripheral surface of the collector core 15 along the axial direction of the collector core 15. 
     In the above conventional microwave tube collector assembly, secondary electron emission from the collector core 15 is suppressed by coating a thin film consisting of carbon or the like which has a secondary electron emission ratio lower than that of an employed collector material on the inner circumferential surface of the collector core. However, since the inner circumferential portion of the collector core 15 generally has a small diameter and a large length, it is difficult to easily and uniformly coat the thin film consisting of carbon or the like on the inner circumferential surface of the collector core 15, and the large number of steps are undesirably required in this operation. In addition, when this operation is performed, the thin film 16 consisting of carbon or the like has a low adhesion strength on the inner circumferential surface of the collector core 15. For this reason, a part of the film 16 is removed during an operation of the microwave tube, thereby considerably degrading the performance of the microwave tube. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a microwave tube collector assembly capable of uniformly coating a thin film having a low secondary electron emission ratio on the inner circumferential surface of a collector core. 
     It is another object of the present invention to provide a microwave tube collector assembly including a thin film having a low secondary electron emission ratio and a high adhesion strength on the inner circumferential surface of a collector core. 
     It is still another object of the present invention to provide a microwave tube collector assembly having improved reliability. 
     In order to achieve the above objects, according to the present invention, there is provided a microwave tube collector assembly comprising a metal cylinder having at least a layer of a chromium alloy on an overall inner circumferential surface of the metal cylinder, the metal cylinder being closed at one end thereof, and a chromium oxide film formed on the basis of the chromium alloy to cover an inner circumferential surface of said metal cylinder. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a front view showing a microwave tube collector assembly according to an embodiment of the present invention, and FIG. 1B is a longitudinal sectional view showing the microwave tube collector assembly along line 1B--1B of FIG. 1A; 
     FIG. 2A is a front view showing a microwave tube collector assembly according to another embodiment of the present invention, and FIG. 2B is a longitudinal sectional view showing the microwave tube collector assembly taken along line 2B--2B of FIG. 2A; 
     FIG. 3A is a front view showing a microwave tube collector assembly according to still another embodiment of the present invention, and FIG. 3B is a longitudinal sectional view showing the microwave tube collector assembly taken along line 3B--3B of FIG. 3A; 
     FIG. 4A is a front view showing a microwave tube collector assembly according to still another embodiment of the present invention, and FIG. 4B is a longitudinal sectional view showing the microwave tube collector assembly taken along line 4B--4B of FIG. 4A; and 
     FIG. 5A is a front view showing a conventional microwave tube collector assembly and FIG. 5B is a longitudinal sectional view showing the microwave tube collector assembly taken along line 5B--5B of FIG. 5A. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described below with reference to the accompanying drawings. FIGS. 1A and 1B show a microwave tube collector assembly according to an embodiment of the present invention. Reference numeral 1 denotes a chromium-copper alloy collector core having an outer diameter of 40 mm, a length of 120 mm, and a cylindrical recess portion having an inner diameter of 30 mm and a length of 100 mm. The collector core 1 is closed at one end 1a as shown. The chromium-copper alloy contains chromium at a weight ratio of 1%. An Ni-plating layer having a thickness of 10 μm is formed on the outer peripheral surface of the chromium-copper alloy collector core 1. After the Ni-plating layer is formed, the chromium-copper alloy collector core 1 is annealed in a wet hydrogen atmosphere at 1,000° C. for 15 minutes. This annealing forms a chromium oxide thin film 2 having a thickness of about 300 Å on the inner circumferential surface of the chromium-copper alloy collector core 1. Since the chromium oxide has secondary electron emission ratio lower than that of copper or the like, it suppresses secondary electron emission. Reference numeral 3 denotes a copper heat-radiating fin having lengths 30 mm×120 mm and a thickness of 1 mm. The 80 copper heat-radiating fins uniformly extend on the outer peripheral portion of the chromium-copper alloy collector core 1 and are brazed with silver-copper alloy. 
     In this manner, the microwave tube collector assembly is finished. When the microwave tube collector assembly was continuously operated at 9 GHz to produce an output of 120 W, a body current was reduced to about one half that of a conventional collector assembly, and an effect to suppress secondary electron emission was confirmed. In addition, since any coating such as carbon need not be formed in the collector assembly, good operability could be obtained. Since the oxide film of the chromium-copper alloy was used, a uniform and strong film could be easily formed. Therefore, inconvenience such as removal of the film could be prevented even during an operation of the microwave tube. 
     FIGS. 2A and 2B show a microwave tube collector assembly according to another embodiment of the present invention. This embodiment has the same arrangement as that of the first embodiment except for the structure of a chromium-copper alloy collector core. In this embodiment, as shown in FIG. 2A, eight recess portions 4a each having a width of 5 mm and a length of 100 mm are uniformly formed on the inner circumferential surface of a cylindrical recess portion of a chromium-copper collector core 4 along the longitudinal direction of the recess portions 4a. The cylindrical recess portion has an inner diameter of 30 mm and a length of 100 mm. In this case, the chromium-copper alloy containing chromium at a weight ratio of 1% is used. An Ni-plated layer having a thickness of 10 μm is formed on the outer peripheral surface of the chromium-copper alloy collector core 4. After the Ni-plating layer is formed, the chromium-copper alloy collector core 4 is annealed in a wet hydrogen atmosphere at 1,000° C. for 15 minutes. This annealing forms a chromium oxide thin film 5 having a thickness of about 300 Å on the inner circumferential surface of the chromium-copper alloy collector core 4. 
     In this embodiment, since slits are formed in the inner circumferential portion of the chromium-copper collector core 4, a probability of secondary electrons emission into the inner circumferential portion of the collector core 4 is advantageously increased. The microwave tube collector assembly is finished as described above. When the microwave tube collector assembly was continuously operated at 9 GHz to produce an output of 120 W, a body current was reduced to about 2/5 that of a conventional collector assembly, and an effect to suppress secondary electron emission was confirmed. The heat radiating fins 6 are the same as the fins 3 in FIG. 1A. 
     In addition, since any coating such as carbon need not be formed in the collector assembly, good operability could be obtained. Since the oxide film of the chromium-copper alloy was used, a uniform and strong film could be easily formed regardless of the shape of the inner circumferential surface of the collector core. Therefore, inconvenience such as removal of the film could be prevented even during an operation of the microwave tube. 
     FIGS. 3A and 3B show a microwave tube collector assembly according to still another embodiment of the present invention. Reference numeral 7 denotes an oxide-free copper collector core having an outer diameter of 40 mm, a length of 120 mm, and a cylindrical recess portion having an inner diameter of 30 mm and a length of 100 mm. Reference numeral 10 denotes a stainless steel (SUS304) cylinder which has an inner diameter of 28.8 mm, an outer diameter of 29.0 mm, and a length of 100 mm and inserted under pressure in the inner circumferential portion of the collector core 7. After an Ni-plating layer having a thickness of 50 μm is formed on the outer peripheral surface of the stainless steel cylinder 10, an Ag-plating layer having a thickness of 50 μm is formed on the Ni-plating layer. The oxygen-free copper collector core 7 and the stainless steel cylinder 10 are diffused and brazed in a hydrogen atmosphere using the Ag-plating layer. After the brazing is performed, this integral body is annealed in a wet hydrogen atmosphere at 650° C. for 10 minutes. This annealing forms an chromium oxide thin film 8 having a thickness of about 500 Å on the inner circumferential surface of the stainless steel cylinder 10. The heat-radiating fins are the same as fins 3 in FIG. 1A. 
     When the microwave tube collector assembly was continuously operated at 9 GHz to produce an output of 120 W, a body current was reduced to about one half that of a conventional collector assembly, and an effect to suppress secondary electron emission was confirmed. In addition, since any coating such as carbon need not be formed in the collector assembly, good operability could be obtained. Since the oxide film of the stainless steel (SUS304) was used, a uniform and strong film could be easily formed. Therefore, inconvenience such as removal of the film could be prevented even during an operation of the microwave tube. 
     FIGS. 4A and 4B show a microwave tube collector assembly according to still another embodiment of the present invention. This embodiment has the same arrangement as that of the embodiment of FIGS. 3A and 3B except for the structure of a stainless steel cylinder. Seven slits 13a (see FIG. 3A) each having a width of 2 mm and a length of 95 mm are uniformly formed in a stainless steel cylinder 13 having an inner diameter of 28.8 mm, an outer diameter of 29.0 mm, and a length of 100 mm. After an Ni-plating layer having a thickness of 50 μm is formed on the outer peripheral surface of the stainless steel cylinder 13, an Ag-plating layer having a thickness of 50 μm is formed on the Ni-plating layer. This stainless steel cylinder 13 is annealed in a wet hydrogen atmosphere at 1,100° C. for 15 minutes. The annealing forms an chromium oxide thin film 14 having a thickness of about 500 Å on the inner circumferential surface of the stainless steel cylinder 13 except for the portions of slits 13a. The microwave tube collector assembly is finished as described above. The heat radiating fins 12 are the same as the fins 3 in FIG. 1A. The collector core 11 is the same as the collector core 7. The chromium oxide thin film 14 is the same as film 8. 
     In this embodiment, since the slits 13a are formed in the stainless steel cylinder 13, the collector core metal is exposed by the slits 13a. For this reason, although secondary electrons are generated from the collector core metal, a heat-radiating effect is improved by the slits 13a because stainless steel has a thermal conductivity 20 times that of oxygen-free copper, thereby increasing the overall microwave tube efficiency. In addition, the microwave tube collector assembly was continuously operated at 9 GHz to produce an output of 120 W, a body current was reduced to about one half that of a conventional collector assembly, and an effect to suppress secondary electron emission was confirmed. 
     As described above, according to the present invention, a chromium oxide thin film having a low secondary electron emission ratio can be formed on the inner circumferential surface of a chromium-copper alloy collector core or on the inner circumferential surface of a stainless cylinder, thereby preventing return secondary electrons. In addition, a uniform thin film having a high adhesion strength is obtained by using an oxide film consisting of a material of the collector core or the cylinder, and the thin film is prevented from removing during an operation of the microwave tube, thereby obtaining a highly reliable microwave tube collector assembly.