US 3794879 A
A crossed field electromagnetic energy device of the magnetron type is provided with integral magnetic field producing means including permanently magnetized members of a ferromagnetic material diametrically disposed adjacent to opposing sides of the anode member sidewalls together with spaced return path members. The magnetic field leakage flux is reduced by tapering the return path member walls to as close as possible to the magnet member outline. Energy leakage is controlled by eliminating axial clearance passageways for the cathode assembly found in prior art axially mounted magnet packages. A circulating fluid medium is directed between cooling fins which partially surround each magnet member and are appended to the anode member walls to control magnet material temperatures during operation and improve magnetic field efficiencies. A lighter weight device which is simple to assemble results with lower manufacturing costs as well as reduced shipping and handling costs.
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Description (OCR text may contain errors)
4 United States Patent [191 [451 Feb. 26, 1974 Edwards MICROWAVE MAGNETRON  Inventor: Robert E. Edwards, Lexington,
Mass.  Assignee: Raytheon Company, Lexington,
 Filed: Oct. 24, 1972 211 Appl. No.: 300,066
 US. Cl 315/3971, 219/1055, 315/3951  Int. Cl. HOlj 25/50  Field of Search 315/3971, 39.51; 219/1055  References Cited UNITED STATES PATENTS 3,493,810 2/1970 Valles 315/3971 X 3,588,563 6/1971 Schmidt.. 315/3971 3,304,400 2/1967 Ojelid 315/3971 X 3,716,750 2/1973 Oguro et al. 315/3971 3,588,588 6/1971 Numata 315/3953 Primary ExaminerEli Lieberman Assistant Examiner-Saxfield Chatmon, Jr. Attorney, Agent, or Firm-Harold Murphy; Joseph D. Pannone Edgar O. Rost no n t A crossed field electromagnetic energy device of the ABSTRACT magnetron type is provided with integral magnetic field producing means including permanently magnetized members of a ferromagnetic material diametrically disposed adjacent to opposing sides of the anode member sidewalls together with spaced return path members. The magnetic field leakage flux is reduced by tapering the return path member walls to as close as possible to the magnet member outline. Energy leakage is controlled by eliminating axial clearance passageways for the cathode assembly found in prior art axially mounted magnet packages. A circulating fluid medium is directed between cooling fins which partially surround each magnet member and are appended to the anode member walls to control magnet material temperatures during operation and improve magnetic field efficiencies. A lighter weight device which is simple to assemble results with lower manufacturing costs as well as'reduced shipping and handling costs.
2 Claims, 7 Drawing Figures PATENTEDFmsmM 'SNEEI 1 BF 4 PATENTEDrmzsmm SHEET 2 0f 4 MAGNETRON ENERGY GENERATOR ASSEMBLY ELECTRICAL CIRCUITS PATENTEU FEB 2 6 I974 sum a or 4 PRIOR ART Pmmmrzazemm SHEEI 0F 4 PRIOR ART LEAKAGE FLUX LINES MICROWAVE MAGNETRON BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to crossed field electromagnetic energy devices of the magnetron type and, more particularly, to magnetic field producing means for such devices.
2. Descriptin of the Prior Art An electromagnetic energy generator which has received wide usage in the microwave field, particularly in heating apparatus, is the magnetron. Mutally perpendicular electric and magnetic fields extend within an interaction region defined between a central cathode emitter and an anode member having a plurality of circumferentially disposed cavity resonators between strapped vane elements. Such devices are energized by AC line voltages which are stepped up and rectified to DC voltages at values of, illustratively, 4-6 kilovolts. The electrical field extends transverse to the axis of the anode member and the magnetic field extends parallel to this axis. Electrons emitted from the heated cathode are accelerated toward the anode cavity resonators and rotate in a substantially circular orbital path to form a rotating spoke-like space charge. The electrons interact in energy-exchanging relationship with the electric fields to generate extremely high frequency energy oscillations.
The magnetic field required for the operation of the magnetron generator is typically provided by externally mounted electromagnets or C-shaped permanent magents of a ferromagnetic material. The magnetron generator commonly generates frequencies in the microwave region of the electromagnetic energy spectrum having wavelengths in the order of from 1 meter to l millimeter and frequencies in excess of 300 MHz. The energy is coupled from the magnetron by means of an output member comprising a dome of a dielectric material housing an antenna and a waveguide launching section coupling the energy to an enclosure. Additional details relative to the magnetron energy generators may be obtained from the text Microwave Magnetrons, Radiation Laboratory Series, Vol. 6, by G. B. Collins,c McGraw-I-Iill Book Company, Inc., New York 19 48 In recent years microwave magentrons, particularly those employed for cooking applications, have utilized integral permanent magnet members to avoid the requirement of a separate voltage source for the operation of the prior art electromagnet structures. Magnets such as those employed in loudspeaker applications are commercially available at relatively low cost. One such magnetron incorporates a stack of barium ferrite magnets in either a cylindrical or a rectangular configuration axially positioned and surrounding the cathode external leads and supporting structure extending from an end wall of the anode envelope. The location of such magnetic field producing means requires axial clearanee passageways which introduces numerous problems involving suppression of any energy leakage over the cathode leads as well as magnetic field efficiencies. With the magnetic field producing means disposed at only one end of the device elongated and cumbersome magnetic field return path and shaping members must be provided. These structures include substantially U- shaped or tapered steel plate housings surrounding the energy generator and external cooling fins. Examples of such prior art permanent magnet type magnetrons are shown in US. Letters Patent Nos. 3,562,579 issued Feb. 9, 1971, and 3,588,588, issued June 28, 1971 The provision of the efficient magnetic field strengths involves consideration of the leakage flux paths which extend outside of the enclosed area and represent wasted energy. Field shaping means such as bucking magnets are positioned perpendicularly to the main magnet members with the magnetic polarities selected to develop a repulsive field to distort the main permanent magnet field and thereby assure proper direction within the anode-cathode interaction region.
Another problem inherent in the use of the stacked arrays of loudspeaker type magnets is the heat generated during the operation of the magnetron generator. The rise in the magnetic material temperature results in magnetic circuit inefficiency unless constant cooling is provided for the magnet members. Favorable magnetic field shaping through minimizing of the leakage flux paths, reduction of energy leakage over the external electrical leads and improved magnetic material cooling will result in an overall improvement in the power output efficiencies of the magnetron energy generators under consideration.
SUMMARY OF THE INVENTION In accordance with the invention, permanent magnet members of a ferromagnetic material are positioned as close to the anode member sidewalls as possible on diametrically opposing sides. Magnetic field return path plate members contact both ends of the magnet members. The spaced plate members are provided with a tapered wall configuration in lieu of intersecting corners tailored as closely as possible to the outer wall shape of the magnet walls. The reduced leakage flux paths provided with the tapered configuration results in high efficiency and the requirement for less ferromagnetic material for the permanent magnet members. The placement of the magnet members adjacent to the anode member sidewalls instead of surrounding the external cathode leads has also resulted in a higher magnetic circuit efficiency without the need for any bucking magnets for the shaping of the magnetic field. The removal of the permanent magnet members from the cathode end of the tube has obviated the need for axial passageways which previously led to required suppression of any energy leakage. The operating temperature of the magnet material is effectively controlled and stabilized by the direction of a stream of a coolant fluid through fins secured to the sidewalls of the anode member and partially surrounding the magnet members. The outermost walls of the permanent magnet members removed from the anode sidewalls are open to provide for exposure of a larger surface area to the cooling medium.
In an exemplary embodiment of a magnetron for generation of 700 watts output fewer components for cooling, magnetic field producing as well as energy suppression were required. Additionally, the total weight was less than 4 pounds whereas a prior art model having the permanent magnets extending coaxially around the cathode end weighs in excess of 8 pounds in one manu facturers version and approximately 7 pounds in another manufacturers version. The lighter weight tube structure results in not only lower manufacturing costs but simplified assembly and reduced shipping and handling costs. Magnetron generators of the type disclosed are also readily available at the l kilowatt level.
BRIEF DESCRIPTION OF THE DRAWINGS Details of the invention will be readily understood after consideration of the following description and reference to the accompanying drawings, wherein:
FIG. I is an isometric view of the illustrative embodiment of the invention;
FIG. 2 is an elevational view of the top magnetic field return path member;
FIG. 3 is an elevational view of the bottom magnetic field return path member;
FIG. 4 is a vertical cross-sectional view of a microwave heating apparatus embodying the magnetron of the invention;
FIG. 5 is a partial vertical cross-sectional and partial isometric view of a prior art permanent magnet type magnetron generator;
FIG. 6 is a vertical cross-sectional view of another prior art magnetic field producing structure as disclosed in US. Letters Patent No. 3,562,579; and
FIG. 7 is a diagrammatic view of the magnetic field leakage flux paths utilizing prior art structures shown in FIG. 6.
DESCRIPTION OF THE PREFERRED EMBODIMENT The illustrative embodiment of the invention comprising a permanent magnet type magnetron generator 10 is shown in FIG. 1. Before proceeding to the description of this embodiment, reference is directed to FIGS. 5-7 illustrating some prior art permanent magnet embodiments for such magnetron generators.
In FIG. 5 the magnetron generator 12 comprises a cylindrical conductive anode member 14 having a plurality of circumferentially disposed cavity resonators defined between vane elements 16. Opposed conductive straps l8 couple alternate vane elements 16 in the manner well known in the magnetron art. An axially disposed directly heated cathode 20 which may be of the thoriated tungsten coil type is provided with end shields 22. The emitted electrons are directed in a circular orbital path within interaction region 24 adjacent the ends of the vane elements 16. The cathode emitter assembly is connected and supported by electrical leads 26, 28 and 30 extending axially from one end of the anode member through a support structure including a conductive collar 32 and dielectric tubular member 34. The central lead 28 is connected to the upper end shield 22 and the outer leads 26 or 30 are connected to the lower end shield 22. Leads 26 or 30 and 28 provide the electrical voltages for operation of the generator. The electrical leads are coupled to the external circuitry through a shielded by-pass capacitor filter arrangement housed within a box-like conductive enclosure member 36. This member is permanently secured to magnetic return path plate member 38 and external leads 40 and 42 are provided with suitable clamps for connection to the high voltage DC rectified source. The electrical energy is coupled from the magnetron by means of a conductive antenna 44 secured to one of the vane elements 16. The antenna extends within a dielectric dome member 46 extending axially from the opposing side of the anode member. The magnetron generator is evacuated by means coupled to the dome member 46 which is then tipped off as at 418.
The magnetic field producing means includes conical-shaped inner pole piece members 50 and 52 enclosing the ends of the anode member for directing the magnetic field paths parallel to the anode axis within the interaction region. The electric fields extend transverse to the magnetic field lines between the anode and cathode. A stack of low-cost permanent magnets includes a pair of large rectangular magnets 54 and superimposed smaller rectangular magnets 56 all supported by return path plate member 38. The magnets are provided with axial passageways58 to accommodate the cathode leads and member 34. Typically, for the operation of magnetrons capable of generating 700 watts of energy a magnetic field of approximately 1800 Gauss is required. To shape the magnetic field paths within the magnetron generator interaction region rectangular opposing bucking" magnets 60 having opposite polar designations are supported by the U-shaped magnetic field return path member 62 which engages the plate member 38 and encompasses the anode member and main magnet stack.
Cooling fins 64 contact the sidewalls of the anode member 14 for directing a circulating fluid medium to effectively remove the heat generated by the high frequency oscillations in the interaction region. It will be noted that there is no direct exposure of the magnet members to the circulating medium forced through the fins to reduce their operating temperatures.
Referring next to FIGS. 6 and 7, another example of a prior art permanent magnet arrangement is illustrated exemplifying referenced U.S. Letters Patent No. 3,562,569. In this embodiment, parallel substantially rectangular magnetic field return path plate members 66 and 68 support spaced permanent magnetmembers 70 and 72. An approximation of the leakage flux paths 74 extending outside of the magnet package is shown to provide a background for the understanding of the present invention. These lines resemble a barrel around the intersecting corners and ends. It will be noted that a considerable amount of magnetic energy results out side of the central region 76 where the high magnetic field strengths are required.
Referring next to FIG. I, the illustrative embodiment it) comprises a similar cylindrical anode member 14 housing the components shown in FIG. 5 including antenna 44 and dielectric dome member 46. The cathode support assembly including the external electrical leads is enclosed by a perforated boxslike member 78 which effectively suppresses any energy traversing such electrical leads. The dimension of the holes is designed to permit circulation of a coolant as well as preventing radiation of stray energy. Within the enclosure 78 are the conventional by-pass capacitors as well as ferrite rings surrounding the cathode leads to absorb and shunt any high frequency energy. Terminals 80 and 82 are provided in the side of the enclosure 78 for connection of the appropriate external circuitry.
The magnetic field producing means include spaced parallel return path plate members 84 and 86 fabricated of a rectangular body of a ferrous material such as steel. The intersecting corners of the four-sided plate members have been removed to form substantially tapered sidewalls 88 tangential to the magnet outline. The tapering of walls 88, shown also in FIGS. 2 and 3, provides for a minimum of magnetic field leakage flux by the removal of the ferromagnetic material which leads to the orientation of the leakage flux paths as shown in FIG. 7. The upper plate member 84 is provided with an aperture 100 to accommodate the output components 44 and 46 and a braided gasket 90 substantially prevents any stray energy leakage at this end. Holes 92 are provided for mounting of the magnetron generator 12 to the waveguide transmission line walls for coupling the energy to the microwave heating apparatus to be hereinafter described. The enclosure 78 is secured to the'bottom plate member 86 by fastening means secured in the tapped holes 94.
Permanent magnet members 96 and 98 of a ferromagnetic material, such as Alnico, are disposed between the spaced magnetic return path members. The magnets are shaped as a single cylindrical body or a stack of individual magnets which can be secured together by an adhesive. To optimize the magnetic field paths and shape the field within the central passageways 100 and 102 in the desired direction parallel to the axis of the anode member 14, the magnets are placed close to the anode member sidewalls. To minimize the leakage flux the tapered walls 88 are formed as close as possible to the outline of the magnet members 96 and 98 (shown as dashed lines in FIGS. 2 and 3) and have a length substantially longer than the re maining narrow straight sides of plates 84 and 86 adjacent to the open ends of notches 108 in fins 104. This results in the use of less ferromagnetic material to achieve the required magnetic field strengths due to the higher efficiency of the overall magnet and return path circuit.
The magnet members as well as anode member 14 operate at reduced temperatures by the provision of cooling fins 104 stacked in an array with each of the fins only partially surrounding the magnet members and thereby exposing the outermost sidewalls of the magnet members to the full thrust of a transverse flow of the circulating air indicated by arrows 106. The partial enveloping of the magnet members results from a substantially U-shaped notch 108 in diametrically opposite sides of the coolingfins 104. The stream of air may be provided by such means as a motor driven fan positioned adjacent to the magnetron energy generator.
The microwave heating apparatus 110 is shown in FIG. 4. Parallel conductive walls 112 define a heating enclosure 114 having an access opening (not shown) which is closed by means of a door to permit. introduction and removal of the articles to be heated. The electrical circuitry including timers, lights, interlock switches, as well as the high voltage supply has not been specifically illustrated since it is believed to be well known in the art and is indicated generally by the box 116. The magnetron energy generator energized by the electric circuits 116 has output antenna 44 disposed within a launching rectangular waveguide section 118 adapted to couple the microwave energy for distribution within the enclosure 114. The waveguide section 118 is short circuited at one end by wall member 120 and is open at the inner end 122. The energy is efficiently distributed by any of the means well known in the art such as, illustratively, a mode stirrer 124 having a plurality of paddles 126 driven by motor 128. The articles to be heated are supported on a lossy dielectric plate member 130 within enclosure 114.
The magnetron energy generator of the invention typically operates at an anode voltage of from 4.1 4.2 kilovolts with an average anode current of 300 milliamperes for the 700 watt output and 420 milliamperes for the 1,000 watt power output. The overall weight of an exemplary embodiment was 3 pounds and 9 ounces compared to a prior art model having the same performance characteristics of 8 pounds and 3 ounces. The new magnetron energy generator has a considerably fewer number of parts compared to the prior art embodiments which results in lower costs and simplified assembly. The magnetic field producing means have a minimum of leakage flux by reason of the tapering of the magnetic return path plate members. The efficient cooling of the permanent magnets by the fins partially enveloping them and the full exposure of their outermost sidewalls results in more stable temperature characteristics. Stray radiation problems have also been reduced by the movement of the magnets closer to the interaction region away from the cathode external leads. It is intended that the foregoing illustrative embodiment and detailed description be considered broadly since numerous variations, modifications and alterations will be readily apparent to those skilled in the art.
I claim: 1. A microwave magnetron comprising: a cylindrical anode member defining a plurality of circumferentially disposed cavity resonators; a cathode having external electrical leads centrally disposed within said anode member; means for producing a magnetic field parallel to the longitudinal axis of the anode member including permanently magnetized members of a ferromagnetic material disposed parallel to diametrically opposed sidewalls of said anode member and contacting spaced rectangular return path plate members;
said plate members having tapered corner walls substantially longer than the remaining straight narrow sides; and
a plurality of cooling fin members contacting the sidewalls of said anode member and partially surrounding said magnet members.
2. A microwave heating apparatus comprising:
conductive wall structure defining an enclosure;
a source of microwave electromagnetic energy;
means for coupling said energy from said source to energize said enclosure;
said microwave source comprising an anode member, central cathode with external electrical leads, permanent magnet members disposed parallel to diametrically opposing sidewalls of said anode member and spaced rectangular return path plate members contacting opposing ends of said magnet members;
said plate members having tapered corner walls substantially longer than the remaining straight narrow sides; and
means for cooling said microwave source comprising a plurality of spaced conductive fin members appended to the anode member sidewalls and partially surrounding said magnet members for directing the flow of a fluid medium. l