Abstract:
A novel magnetron achieves increased power output at high frequencies by replacing a typical resonant cavity with a slow-wave waveguide structure. Waveguides built into the anode body sustain oscillations having phase change coefficients of 2*pi*n radians per section, where n is a positive integer. The magnetron is capable of supporting RF oscillations at frequency harmonics of the fundamental frequency, permitting it to operate at frequencies double or quadruple that of a similarly sized conventional magnetron.

Description:
RELATED APPLICATIONS DATA 
       [0001]    This application claims priority pursuant to 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 61/609,154, filed Mar. 9, 2012, the subject matter of which is incorporated by reference herein in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to magnetrons, and more particularly, to specially designed anodes that enable higher frequencies to be generated by using frequency harmonics. 
         [0004]    2. Description of Related Art 
         [0005]    Magnetrons are known in the art and operate to convert DC electrical current to Radio Frequency (RF) power.  FIG. 1  illustrates the basic operation of a class of magnetron known in the art as a slot-and-hole anode type magnetron. A heated cathode  102  acts as a source of electrons. The cathode passes through a central cylindrical cavity  106  passing through the anode  112 . The anode  112  is formed from a conductive metal such as copper. A DC electric field is created by applying a DC voltage (not shown) between the anode  112  and the cathode  102 ; in the absence of a magnetic field this would cause electrons to travel in the radial direction  104  from the negatively charged cathode toward the anode. In the slot-and-hole type anode depicted, an even number of resonant cavities are formed by a rectangular opening (“slot”)  116  connected to a circular opening (“hole”)  114  with each slot forming a connection between the central cylindrical cavity  106  and the hole. A magnetic field  122  is applied in a direction perpendicular to the electric field (pointing out of the page in the figure). The magnetic field acts on the radially accelerating electrons causing the electron&#39;s movement path to curve. As the electrons pass through the magnetic field, electrons passing by a slot opening give up some energy, and the resonant cavities begin to oscillate at a natural resonant frequency determined by the geometry of the cavities. The metal walls of each resonant cavity facing the central cavity  120  (“vanes”), interact with the passing electrons and build up localized charge distributions that change with the resonant cavity oscillations. 
         [0006]      FIG. 2  shows an instantaneous view of the RF electric field of a magnetron operating in the Π mode. The term Π mode refers to the phase difference in radians of the RF electric field between adjacent vanes in the magnetron anode. Arrowed lines  202  show the direction of the RF electric field vectors at a particular half cycle maximum of the magnetron operation. Two shorting rings  207  connect to alternating vanes at contact points  209 , the inner ring connects to “even” vanes while the outer ring connects to “odd” vanes. The alternating circumferential field maximums  204  interact with the electrons curling through the magnetic field, and allow a sustained oscillation at the operating frequency of the device. 
         [0007]    Because the operating frequency of magnetron operation depends on the dimensions of the anode and resonant cavities, magnetrons typical of the prior art operating at higher frequencies decrease in size as operating frequencies increase. The smaller vanes associated with smaller cavities are unable to remove heat as quickly. Smaller central cavities also require a greater magnetic field strength to properly divert the electrons emitted from the cathode over a shorter traveling distance between anode and cathode. Since output power also depends on the DC electric field established between the cathode and anode, the corresponding reduced central cavity dimensions increase the likelihood of breakdown voltage gradients. Additionally, cathode loading becomes a limiting factor as frequency increases. Due to these limitations, a larger magnetron anode capable of operating at higher frequencies is desirable. 
       SUMMARY OF THE INVENTION 
       [0008]    An embodiment of the present invention achieves a magnetron with greater power output at high frequencies by replacing a typical resonant cavity with a slow-wave waveguide structure. In one embodiment, a harmonic mode magnetron for converting DC electrical current to RF power includes an anode body formed from a conductive material such as copper and including a central cavity. A cathode is fixed within the central cavity of the anode body. The anode body includes at least two slow-wave waveguide structures each having a channel with a rectangular cross section cutting through the anode body in a continuous path. Each slow-wave waveguide structure has two openings into the central cavity such that the two openings and the channel define a central column of anode body material between them. The central column forms one broadwall of the slow-wave waveguide structure and another broadwall of the slow-wave waveguide structure is formed from the anode body material. Each of the slow-wave waveguide structures also includes one narrow wall formed by the anode body. The other narrow wall may be formed from a cap material joined to the anode body or may be formed from the anode body itself. 
         [0009]    In some embodiments, the harmonic mode magnetron may include mode-isolating structures for suppressing certain harmonic oscillation modes. For example, the mode-isolating structures may comprise irises formed within the slow-wave waveguide structures. These irises may be transmitting or non-transmitting irises. In other embodiments, the mode-isolating structures may be discrete high-Q cavities coupled to each slow-wave waveguide structure. In other embodiments, the mode-isolating structure may be a coaxial cavity coupled to all of the slow-wave waveguide structures. 
         [0010]    In some embodiments of a harmonic mode magnetron, the anode may include an odd number of slow-wave waveguide structures. Because the harmonic mode magnetron is designed to operate in a 2Π*n resonant mode, an odd number of resonators is advantageous in that the  7  mode is effortlessly suppressed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  depicts a slot-and-hole magnetron structure typical of the prior art; 
           [0012]      FIG. 2  depicts a magnetron typical of the prior art operating in the Π mode; 
           [0013]      FIG. 3A  depicts a top view of a slow-wave waveguide harmonic mode magnetron structure; 
           [0014]      FIG. 3B  depicts a cross sectional view through the slow-wave waveguide harmonic mode magnetron structure depicted in  FIG. 3A ; 
           [0015]      FIG. 4  depicts a slow-wave waveguide harmonic mode magnetron structure operating in the 2Π mode; 
           [0016]      FIG. 5  depicts a slow-wave waveguide harmonic mode magnetron structure operating in the 4Π mode; 
           [0017]      FIG. 6  depicts a variety of structures for providing mode selection; and 
           [0018]      FIG. 7  depicts a slow-wave waveguide harmonic mode magnetron structure with an odd number of resonant waveguides. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0019]    An embodiment of the present invention includes a method and apparatus for operating a magnetron with greater power output at high frequencies by replacing a typical resonant cavity with a slow-wave waveguide structure. 
         [0020]      FIGS. 3A and 3B  illustrates two views of one embodiment of a harmonic mode magnetron in accordance with the present invention. The magnetron anode  312  has a central cavity  106 , with cathode element  102  passing through the center. Rather than having resonant cavities forming the anode, the anode comprises a number of slow-wave waveguide structures. A portion of the anode material,  316 , remains forming one of the broadwalls of a waveguide where the channel  314  forms the waveguide structure. 
         [0021]      FIG. 3B  shows the cross-section of one of the waveguide structures. In this embodiment the waveguide cavity is constructed within one solid piece of the anode  312 . The central column  316  forms one broadwall of the waveguide, while the rest of the anode bulk  312  forms the other broadwall of the waveguide. In addition, one narrow wall of the waveguide is formed by the end of the waveguide channel  314  along the anode bulk  312 . In this embodiment, an additional piece of cap material  320  forms the second narrow wall when joined with the rest of the anode  312 . Heat caused by interaction of electrons and the waveguide openings is conducted along the bulk material forming the broad walls and the central columns toward the anode bulk and cap areas where it can be dissipated. The large amount of conducting material near the interaction area allows for significant heat dissipation. This heat dissipation is important because heat is a limiting factor for magnetron power output. 
         [0022]    Although the anode construction is described in terms of a bulk piece of material and a cap piece of material, one skilled in the art will recognize that other anode fabrication techniques, which form wave guides acting as the resonating element in a magnetron, are within the scope and spirit of the present invention. For example, a similar anode could be constructing using a mold and casting technique and thus being constructing from one solid piece of metal. 
         [0023]      FIG. 4  shows the local field maxima during one half cycle of the harmonic mode magnetron operating in the 2Π mode. The arrow lines  402  represent circumferential electric field vectors during one half cycle maximum of the RF frequency. The field line  404  shows that half way through the waveguide, the RF signal has an inverse phase relationship with the two waveguide openings in the 2Π mode. The RF circumferential field maximums  406  are also in anti-phase at the circumference, which allows the harmonic mode magnetron to sustain oscillations having phase change coefficients of 2Π radians per section. Similarly,  FIG. 5  shows the local field maximums during one half cycle of the harmonic mode magnetron operating in the 4Π mode. The arrow lines  502  represent circumferential electric field vectors during one half cycle maximum of the RF frequency. The RF circumferential field maximums  506  are also in anti-phase at the circumference, which allows the harmonic mode magnetron to sustain oscillations having phase change coefficients of 4Π radians per section. It is evident that the principle of using waveguides in the anode would sustain oscillations having phase change coefficients of 2Π*n radians per section (where n is a positive integer). 
         [0024]    Using the waveguide in place of a traditional resonant cavity allows for a larger magnetron and magnetron anode operating at the same high frequency when compared with a magnetron typical of the prior art. The waveguide structure allows a harmonic mode magnetron operating in the 2Π mode to have the same circumferential 
         [0025]    RF field components as a magnetron typical of the prior art when operating in the Π mode. In addition, the circumferential RF field components also remain the same for all 2Π*n modes where n is a positive integer. In other words, the harmonic mode magnetron is capable of supporting RF oscillation at frequency harmonics of the fundamental frequency. This could permit a harmonic mode magnetron to operate at frequencies that are double or four times those that a similarly sized magnetron of standard construction would achieve. Also, a harmonic mode magnetron operating at the same frequency as a magnetron of standard construction is capable of producing more output power. 
         [0026]    Because the harmonic mode magnetron is capable of supporting operating modes at a number of different frequency harmonics of the frequency corresponding to the normal Π mode, it is anticipated that the slow-wave structure proposed will require techniques for mode isolation.  FIG. 6  illustrates some of the proposed structures to achieve the goal of mode isolation. Each waveguide section is shown in conjunction with one of the proposed methods. The waveguides in the top and right positions,  602  and  604 , illustrate the use of periodically placed irises providing mode selection. The three periodically placed irises in  602  would support a 4Π mode of oscillation while the single iris shown in  604  would support a 2Π mode of oscillation. The irises can be either transmitting or non-transmitting irises, which will be determined by the design parameters of the magnetron. In this form utilizing irises to provide mode selection, the invention acts as a “coupled cavity magnetron.” 
         [0027]    The waveguides in the bottom and left positions,  606  and  608 , illustrate the use of a resonant high Q cavity in proximity to the magnetron anode to effectuate mode selection. In this type of topology, the preferred mode of operation is achieved by selecting or designing a resonant high Q cavity with a resonant frequency corresponding to the desired operating frequency of the magnetron. Element  606  illustrates the use of discrete resonant high Q cavities coupled with slow-wave waveguide structures within the anode. Element  608  illustrates the use of a coaxial high Q cavity coupled with the slow-wave waveguide structures within the anode. Although both of these designs are shown with a coupling slot, it is not strictly necessary that each waveguide within the structure is coupled to a high Q cavity to achieve the preferred operating mode. One skilled in the art will understand that any topology that supports a particular operating frequency consistent with a desired 2Π*n mode fall within the scope and spirit of this invention. 
         [0028]      FIG. 7  demonstrates a magnetron anode comprising an odd number of resonators. The distinction between this invention and prior magnetron art is that a conventional magnetron having an odd number of resonators cannot work. Since this invention is specifically designed to operate in 2Π*n mode, an odd number of resonators is expected to be an advantage in that the Π mode is effortlessly suppressed. The invention is solely defined by the following claims.