Abstract:
This invention is directed to crossed-field devices such as magnetrons to an inductive insert for a magnetron that compensates for the natural increase in cavity inductance with temperature which causes the output frequency to decline with increasing temperature.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to crossed-field devices such as magnetrons, and more particularly, to an inductive insert for a magnetron that compensates for the natural increase in cavity inductance with temperature which causes the output frequency to decline with increasing temperature. 
     2. Description of Related Art 
     Magnetrons are a type of crossed-field device that are commonly used to generate high power microwave energy for assorted applications, such as radar. A magnetron typically comprises a cylindrically shaped cathode that extends axially along a central axis of an anode structure comprising a plurality of anode vanes that extend radially from an annular anode ring. A space defined between the cathode surface and the anode structure provides an interaction region, and an electric potential is applied between the cathode and the anode forming a radial electric field in the interaction region. An axial magnetic field is provided in the interaction region in a direction perpendicular to the electric field by pole-pieces that focus magnetic flux from magnets disposed externally of the interaction region. The cathode may be provided with an internal heater disposed below the surface of the cathode to heat the cathode surface to a temperature sufficient to cause thermionic emission of electrons therefrom. The emitted electrons are caused to orbit around the cathode in the interaction region due to the resultant forces derived from the crossed electric and magnetic fields, during which they interact with an electro-magnetic wave that is caused to move on the anode structure. The orbiting electrons give up energy to the electromagnetic wave, thus resulting in a high-power microwave electromagnetic wave circulating around the anode structure. 
     The anode structure comprises a plurality of resonant cavities. Each cavity is defined as the space bounded by the sides of adjacent vanes, the corresponding inner surface of the anode ring and the exterior of the interaction region. In the desired mode of operation, alternate anode vanes are at the same RF potential. Accordingly, the anode structure of magnetrons of the “Vane and Strap” type further include straps that respectively couple alternating ones of the anode vanes to keep them at the same RF potential. Other types of magnetrons exist that do not use straps. 
     In order to put the high-power microwaves to use, an output circuit is provided to couple into the electric or magnetic (or both) fields that are supported on the anode structure and associated cavities. The output circuit can itself be coupled to a transmission system which takes the energy to the point of use. A typical transmission system includes waveguide. A typical output circuit includes a wire loop disposed in one of the cavities of the anode defined between adjacent anode vanes. 
     In the absence of any compensation, the natural increase in cavity inductance with temperature (in conjunction with several other less significant effects) causes the magnetron&#39;s output frequency to decline with increasing temperature: the rate of decline is referred to as the temperature coefficient. This coefficient is always negative for an uncompensated magnetron and is principally derived from the thermal expansion coefficients of the materials used in the cavity walls (typically the anode ring and vanes). 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, the rate of change of cavity inductance versus temperature (and by reference the temperature coefficient) can be selected at the design stage. Hence, this invention enables magnetrons to be designed with a wide range of rates of change of cavity inductance. Potentially, using this invention in any particular magnetron, the rate of change of cavity inductance can be made to both increase over that of the uncompensated magnetron or to decrease, even to the extent of becoming negative (i.e., where the cavity inductance actually reduces with increasing temperature). 
     The invention provides the capability of selecting the rate of change of inductance to be negative with increasing temperature which means that the temperature coefficient of frequency of the entire magnetron can be reduced below levels achieved by current design techniques to substantially zero. In other words by over-compensating the cavity inductance the change in cavity inductance and the other frequency-lowering effects can be entirely counteracted. One such other frequency-lowering effect would be a permanent magnet&#39;s temporary loss of magnetic field strength as temperature increases because a lower magnetic field strength often causes the output frequency of the magnetron to drop. 
     The invention comprises inductive inserts within the cavities of a magnetron oscillator. The metals from which the cavity and inserts are made, and their volumes, can be selected to cause the natural increase of cavity inductance with increasing temperature to be reduced, nullified and even reversed without any manual manipulation. The selection of the materials depends on the value of the coefficient of thermal expansion. 
     These and other objects of the present invention are achieved by a microwave source including a magnetron that has an anode ring concentrically disposed around and spaced from a cathode. The anode ring includes a plurality of anode vanes with cavities being defined between adjacent ones of the plurality of anode vanes. The cavities have inductive inserts selected such that the rate of change of cavity inductance is compensated as temperature varies. The inductive inserts are themselves thermally and mechanically attached to the anode structure. 
     Still other objects and advantages of the invention will become readily apparent to those skilled in the art from the following detailed description, wherein the preferred embodiments of the invention are shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description thereof are to be regarded as illustrative in nature, and not as restrictive. In particular the present invention is not limited by the particular shape of the insert, nor by the number per cavity, the present invention relates to the thermal coefficients of expansion of materials chosen and the volume of each cavity and the total volume of all inductive inserts per cavity. The accompanying figures and description of the preferred embodiment show two inserts per cavity that are supported at one axial end in a “vane and strap” style magnetron with a loop-style output coupling element; the present invention is not limited to inserts of the shape shown; the present invention is not limited to two inserts per cavity; the present invention is not limited by the method of supporting the insert; the present invention is not limited to “vane and strap” style magnetrons; the present invention is not limited by the type of output circuit or output coupling element; the present invention is not limited by the transmission system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example, and not by limitation, in the figures of the accompanying drawings, wherein elements having the same reference numeral designations represent like elements throughout and wherein: 
     FIG. 1A is a top perspective view of the magnetron system; 
     FIG. 1B is a side elevation view of FIG. 1A; 
     FIG. 2 is a sectional plan view of a segment of the anode structure comprising two adjacent cavities with inductive inserts in place, and showing the cathode and interaction region; and 
     FIG. 3 is a perspective view of inductive inserts and their support used in the magnetron of FIG. 1 according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For purposes of convenience, terms such as “upwardly”, “left”, “right”, etc., are used herein and should be construed in the relative sense. Referring first to FIGS. 1A and 1B, an exemplary magnetron  10  coupled to an output circuit  20  is illustrated. For clarity, the cathode and pole-pieces are not shown. The anode ring  12  has a plurality of vanes  14  extending radially inwardly. In the desired mode of operation, alternate anode vanes  14  are at the same RF potential. Accordingly, the magnetron  10  further includes straps that respectively couple alternating ones of the anode vanes  14  to keep the anode vanes  14  at the same RF potential and maintain separation between the frequencies of the π and π−1 modes of operation. One set of alternating vanes are connected to the inner strap  15  and the other set of alternating vanes are connected to the outer strap  16 . One of the vanes  21  is provided with a cut-out to facilitate the insertion of the output circuit&#39;s output coupling element  22  into the anode structure. Two supports  23  are provided for the thermal and mechanical connection of the inductive inserts  19  to the anode ring  12 . One support  23  is on each vertical side of anode ring  12 . Each support  23  carries one inductive insert  19  per cavity  26 . 
     Referring to FIG. 2, for clarity the line of sectioning has been set below the level of the straps but above the gap between the two inductive inserts that enter the cavities from opposite ends. The cylindrical cathode  17  lies at the axis of the magnetron. A cylindrical anode ring  12  made of molybdenum having an inner surface  13  and an outer surface  11  surrounds the cathode  17 . Vanes  14  made of molybdenum extend radially inward from the inner surface of the anode ring  13 . An interaction region  18  exists between the cathode  17  and the nearer ends of the vanes  14 . As known in the art, the spaces bounded by the sides of adjacent vanes  14  the corresponding inner surface of the anode ring  13  and the interaction region  18  define the cavities  26  of the magnetron. Inductive inserts  19  made of copper are positioned in the wider ends of the cavities  26  and are thermally and mechanically connected to the anode ring  12  by their supports  23 . 
     FIG. 3 illustrates an assembly of one of the supports and one set of inductive inserts. It comprises a broad copper annulus  23  from which the individual inductive inserts  19  extend axially. The individual inductive inserts are separated by gaps  24  that ultimately have the anode vanes passing through them. One pair of adjacent inductive inserts are reduced in size to provide a space  25  by which the output coupling element  22  can penetrate into the cavities from which it extracts power. 
     It should be appreciated by persons skilled in the art that an operational magnetron would further comprise additional elements, such as a magnet and magnetic pole-pieces arranged to couple magnetic flux to the interaction region  18 . These and other known aspects of the magnetron have been omitted herein for ease of illustration and description. It will be appreciated by those skilled in the art that the absence of any of the magnetic elements of the magnetron from the figures indicates that the present invention&#39;s effect on the rate of change of cavity inductance with temperature is entirely independent of the magnetic circuit chosen for the magnetron. An example of a conventional magnetron showing such known aspects is provided in U.S. Pat. No. 5,894,199, which is incorporated by reference herein. 
     It will also be appreciated by persons skilled in the art that the present invention&#39;s effect on the rate of change of cavity inductance with temperature is not dependant on the output circuit and output coupling element provided that account is taken of the volume of insert that may need to be omitted to make space for the output coupling element. A working magnetron has been made incorporating the present invention and has a measured temperature coefficient of −11 kHz/° C. The equivalent magnetron without the present invention has previously been built and had a measured temperature coefficient of −62 kHz/° C. The invention was solely responsible for the greater than five-fold reduction in the temperature coefficient. 
     It will be readily seen by one of ordinary skill in the art that the present invention fulfills all of the objects set forth above. After reading the foregoing specification, one of ordinary skill will be able to affect various changes, substitutions of equivalents and various other aspects of the invention as broadly disclosed herein. It is therefore intended that the protection granted hereon be limited only by the definition contained in the appended claims and equivalents thereof.