US3395314A - Coaxial magnetron having attenuator means for suppressing undesired modes - Google Patents

Description

y 3Q 1968 R. E. DECKER 3,395,314

COAXIAL MAGNETRON HAVING ATTENUATOR ANS FOR SUPPRESSING UNDESIRED MODE 2 Sheets-Sheet 1 Filed Nov. 24, 1964 INVENTOR Robert E. Decker M 14% ATTORNEY July 30. 1968 R. E. DECKER 3,395,314

COAXIAL MAGNETRON HAVING ATTENUATOR MEANS FOR SUPPRESSING UNDESIRBD MODES Filed Nov- 24, 1964 2 Sheets-Sheet 3 United States Patent COAXIAL MAGNETRON HAVING ATTENUATOR MEANS FOR SUPPRESSING UNDESIRED MODES Robert E. Decker, Sunnyvale, Calif., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Nov. 24, 1964, Ser. No. 413,586 3 Claims. (Cl. 31539.75)

ABSTRACT OF THE DISCLOSURE The invention relates to a coaxial magnetron having an inner resonator system and an outer cavity resonator which is adapted for maximum energy storage of the TE mode. The suppression of undesired modes in the outer cavity resonator is provided by an energy absorbing member within a groove provided in the lower surface of the end plate and situated within an annular cavity defined between a groove on the inner peripheral surface of the end wall and an undercut portion of the anode wall.

This invention relates to electron discharge devices of the magnetron type and more particularly to coaxial magnetron type.

In the R. J. Collier and J. Feinstein U.S. Patent 2,854,603 issued Sept. 30, 1958, there is described a coaxial magnetron structure which comprises an inner and outer resonant system. The inner resonant system includes a cylindrical anode wall together with a plurality of anode vanes radially extending inwardly therefrom. These vanes define a circumferential array of inner, or anode cavity resonators. An outer cavity resonator is defined between an outer wall and the cylindrical anode wall and forms the outer resonant system. The two systems, namely the inner resonant system and the outer cavity resonator, are coupled by a circumferential array of slots provided in the cylindrical anode wall which connects the outer resonant system with the inner resonator system. The inner resonant system is normally designed to oscillate in the pi-mode, while the outer system is dimensioned for maximum energy storage of the TE mode. The two systems are effectively locked together by the coupling slots provided between the inner resonant system and the outer cavity resonator.

The TE mode in cylindrical and coaxial cavities is often called the circular electric mode since the electric field exists only in a circular pattern and there is no radial or axial directed electric field. There are other modes that can exist in the right circular coaxial cavity as well as the TE such as TE TE and TE In addition TM modes also can exist within the cavity. These modes may be separated into competing and interfering modes. The TE modes are the most important since they couple into the vane structure of the inner resonant system. The TE mode coaxial cavity generates a pi-m ode. In a like manner the TE mode generates a pi-l mode field. As previously indicated, the current in the outer cavity in the TE mode is everywhere circumferentially and tends to be stronger at the central regions of the cylindrical an ode, the outer cylinder and the end covers therefor. The currents at the corners of the cavity are very small, and circumferential. In the TE mode, which is the most potential competing mode, currents move axially and radially and there are strong currents moving across the junctions of the cylindrical anode cylinder and the end covers. If one of these junctions is broken, it is possible to match a damping resistor or attenuator to the undesired TE mode with virtually no damping on the desired TE mode. The net effect is to greatly lower the TE 3,395,314 Patented July 30, 1968 mode Q to the point where it will not sustain oscillations.

Thus, in the previously mentioned Collier and Feinstein patent a quarter wave circular choke was provided Within an end cover adjacent to and coaxial with the cylindrical anode wall. The interior of the choke was lined with soft iron to provide magnetic losses.

Another problem associated with the coaxial magnetron is providing proper cooling of the tube. The vane tips which are subject to electron bombardment are the members primarily heated. The heat must flow through the vanes, the anode cylinder, the end covers and finally to the heat radiator which is positioned on the outer wall of the cavity resonator. It is therefore important that a continuous thermal path of good thermal conductive material be provided from the vane tips to the heat radiator. This heat problem must be considered in determining the proper suppression means used within the magnetron. The heat flow path normally is provided particularly in a tunable type cavity through the bottom of the anode structure. The top of the anode should be undisturbed to provide a top locating surface.

It is accordingly an object of this invention to provide an improved electron discharge device.

It is another object to provide an improved coaxial magnetron structure.

It is another object to provide an improved mode attenuator for competing modes in a coaxial magnetron.

It is another object to provide an improved mode suppressor for the TE mode within the cavity of a coaxial magnetron.

Briefly, the present invention provides a coaxial magnetron having an inner resonator system and an outer cavity resonator which is adapted for maximum energy storage of the TE mode. This suppresssion is accomplished by provision of an energy absorbing member within a groove that is below the surface of the end cover and extends below the surface of the cylindrical anode.

Further objects and advantages of the invention will become apparent as the following description proceeds, and features of novelty which characterize the invention will be pointed out in particularity in the claims annexed to and forming a part of the specification.

For a better understanding of the invention, reference may be had to the accompanying drawings, in which:

FIGURE 1 is a perspective view, partially in section and partially broken away of a coaxial magnetron illustrating one specific embodiment of the invention; and

FIG. 2 is a side view in elevation of the device in FIG. 1 to which appropriate magnets have been added.

Referring now to the drawings, there is shown a tunable coaxial magnetron. The coaxial magnetron is comprised of a body member 10 which is substantially cup-shaped and includes a cylindrical side wall or outer wall 12 and a bottom end plate or cover 14. The mamber 10 is of a suitable electrically and thermally conductive material such as copper. Heat radiating fins 13 are provided on the outer surface of the outer wall 12.

Centrally disposed with the body 10 is a cathode 16 which includes a sleeve member 18 of a suitable material such as molybdenum. The sleeve member 18 is provided with an electron emissive coating 20 on the outer surface. Suitable heating means (not shown) is provided with the cathode sleeve 18.

A cylindrical anode 22 of a suitable electrically and thermally conductive material such as copper surrounds the cathode 16. A plurality of radially extending vanes 24, also of suitable conductive material, are provided on the inner surface of the anode cylinder 22. The vanes 24 extend axially along a small portion of the length of the anode cylinder 22. The vanes 24 define an array of inner cavity resonators 26. Slots 28 extends along the anode cylinder 22 for a greater length than the vanes 24 and 3 are substantially parallel to the vanes 24. The slots 28 are located in alternate anode resonators 26 and provide communication between the inner cavity resonators 26 to an external or outer cavity resonator 30.

The cylindrical wall portion 12 of the body 10, the cylindrical anode 22, the bottom end plate 14 and an annular tuning member 44 define the cavity resonator 30. Extending through the wall 12 to communicate with the outer cavity resonator 30 is an output coupling opening 32. The opening 32 serves as means through which energy may be removed from the outer cavity resonator 30 and has been shown in its simplest form.

Positioned atop the cup-shaped body member is a substantially disk shaped cover means 34. The cover 34 is of a suitable non-magnetic material such as stainless steel and is vacuum sealed at its periphery to the cylindrical wall portion 12. The inner periphery of disk shaped end cover 34 is sealed to a pole piece 50.

Tuning is provided in the coaxial magnetron shown by axially moving the annular member 44 within the outer cavity resonator 30. In the specific embodiment shown, this conductive member 44 is an annular member of a suitable electrically conductive material such as copper. The member 44 defines one boundary surface of the cavity resonator 30. The tuning member 44 is substantially U-shaped in cross sectional area. The tuning member 44 is actuated by means of two rod members 64 which extend through suitable apertures 66 within the end cover 34. The rods 64 have one end fixed to the annular member 44 and the other end fixed to a cross bar member 62. The upper pole piece 50 is provided with an extended portion 68 which extends through an aperture 70 centrally located within the cross bar member 62 and into a centrally extending bore 72 within an actuating rod 60. The rod 60 is attached to the cross bar member 62 and is slidably mounted in a sleeve member 58 surrounding the rod 60. The sleeve member 58 is secured to a magnetic spacer member 52. The rod 60 may move within the sleeve 58 and provide movement of the annular member 44 to adjust dimensions of the cavity resonator 30. In this manner, tuning of the magnetron is accomplished.

The magnetic circuit in the present device shown in FIG. 2 is inclusive of two substantially identical horseshoe magnets 46, a bottom pole piece 48 and the upper pole piece 50. Also included in the magnetic circuit is the magnetic spacer 52 which is substantially a U-shaped member in which serves to connect the upper poles of the magnets 46 to the upper pole piece 50. The magnetic spacer 52 also serves as a support means for the tuning drive mechanism. Pole pieces 48 and 50 and spacer 52 may be of a suitable magnetic material such as soft iron.

The outer cavity resonator 30 is capable of sustaining a number of different modes of operation. In this particular application, the cavity resonator 30 is dismensioned to provide maximum storage of the TE mode at the operating frequency. The electric currents flow circumferentially on the outer surface of the anode wall 22 and along the inner surface of the outer wall 12 of cavity resonator 30. The inner resonant system defined by the resonators 26 will tend to oscillate in both the pi and various degenerate modes. Thus, the outer cavity resonator 30 and the inner anode resonators 26 can be considered as two distinct resonant systems; however, when the two systems are placed together they can be considered to be a single composite system. Current produced by the outer cavity resonator mode flows along the anode wall 22 in a direction perpendicular to the slots 28. The anode vanes 24 are approximately a quarter wave length in radial length at the operating frequency such that the high impedance termination at the free end of the anode vanes 26 is reflected back to the slots 28 as a low impedance and accordingly the electric current flows into the resonator 26 and down the adjacent vane 24. These high voltages appear across alternate anode vanes 24 at a given time because only alternate inner cavity resonators 26 are coupled by the slots 28 to the outer cavity resonator 30. Voltages at other alternate anode vanes are provided by mutual inductance resulting in such voltage being out of phase with the adjacent vanes. This is the proper condition for maintenance of the pi-mode oscillation with the inner resonant system. The electron beam of the inner resonant system due to the crossed electric and magnetic fields induces voltages in the tips of the vanes 24. These voltages in turn produce currents which fiow out through the slots 28 into the outer cavity resonator 30. The resonators 26 are normally constructed so as to support a frequency outside the frequency range of the outer cavity resonator 30 such that the natural resonant frequency of the inner structure has little effect on the tuning of the outer cavity or of the frequency of oscillation of the outer cavity.

The outer cavity resonator 30 is capable of sustaining a number of different modes of operation. In accordance with this invention, however, it is dimensioned as well known in the art, for maximum storage of the TE mode. In this type of mode, the magnetic field lines are radially along the upper and lower plates of the cavity resonator 30 and the electric field lines are entirely circumferential. Electric currents induced within the external cavity resonator 30 fiow circumferentially along the outer surface of the anode wall 22 along the inner surface of the outer wall 12 the inner surface of the tuning element 44 and the end plate 14. Currents induced by other modes such as the TE will have an axial component which may be damped by this invention. This is shown in FIG. 1. The anode cylinder 22 is reduced in its outer diameter at the lower end portion to provide an annular groove 43 at a point just below the slots 28. The thickness of the anode wall 22 at the groove 43 is about one-half the thickness of the remaining portion of the anode wall 22 which has a thickness of about .060 inch. The lower annular end plate 14 is to provide an aperture 45 for receiving the lower pole piece 48. The surface of the aperture 45 is provided with an annular groove 47. The groove 47 has a lower lip 49 and an upper lip 51. The diameter of the lip 49 is substantially the same as the inner diameter of the anode 22. The lower edge of the anode 22 is brazed to the upper surface of the lip 49. The upper lip 51 has a diameter greater than the outside diameter of the anode 22, and defines an annular gap 55.

The upper surface of the lip 51 is in the same plane as the remaining inner surface of the end cover 14 and is above the groove 43 in the anode wall 22. The annular groove 43 opposed the annular groove 47 and defines a region or cavity within the junction of the anode 22 and the end cover 14 wherein an annular ring 57 suitable lossy material such as carbon impregnated alumina, silicon carbide, or resistive film coated ceramic may be positioned. It is found that by making the thickness of the lip oprtion 51 of a thickness of about .010 inch and the annular gap 55 between the lip 51 and the outer diameter of the larger portion of the anode 22 of about .010 inch the resulting TE mode unloaded Q is very low with little effect on the unloaded pi-mode Q. The heat flow path from the vanes 24, through anode cylinder 22 is still provided by means of the restricted portion of the anode 22 at the groove 43 and through the end plate 14 to the outer cylindrical wall 12.

The radial currents of the TE anode cannot jump the gap 55 and are attenuated as 1 R losses as they pass around the walls of the grooves 43 and 47 holding the attenuator ring 57. The cutback in the diameter of the anode 22 and the long thin lip 51 increases the discontinuity of the current path. This utilizes nearly the entire surface of the attenuator ring 57 which fits into the copper cavity formed by grooves 43 and 47. Even though there is little pi-mode current in area, narrowing the gap 55 increases the pi-mode unloaded Q by making the cavity appear more perfect at the anode cavity junction.

The long narrow lip 51 or the cutback anode alone will aid in attenuating the TE mode but the combination thereof achieves the most desirable result.

While there have been shown and described what are presently considered to be the preferred embodiment of the invention, modifications thereto will readily occur to those skilled in the art. It is not desired, therefore, that the invention be limited to the specific arrangement shown and described and is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A coaxial magnetron comprising a cylindrical anode wall member, a plurality of vanes extending inwardly from said anode wall member and defining a plurality of anode cavity resonators within said anode wall, a cathode positioned centrally of said anode wall member, the outer surface of said anode wall member defining one Wall of an annular output cavity resonator, said output cavity resonator coupled through openings in said anode wall to said anode cavity resonators, said outer cavity resonator including a lower annular end wall member having a portion of its inner periphery secured to the lower end of said anode wall member, said anode wall member having a reduced outer diameter portion about one half the thickness of the remaining portion of said anode wall member and below the upper surface of said end wall member, the upper surface of said end wall member provided with a first annular opening therein to an annular chamber, said annular chamber defined by said reduced diameter portion and a groove provided in the inner periphery surface of said annular end wall, the wall between said chamber and said output cavity resonator within said end wall having a thickness of about .010 inch and an attenuator means positioned within said chamber.

2. A magnetron comprising a cylindrical wall member, a plurality of vanes extending inwardly from said wall member and defining inner cavity resonators, a cathode positioned centrally of said wall member, means including said Wall member defining an outer cavity resonator encompassing said inner cavity resonators, means for coupling said inner cavity resonators to said outer cavity resonator, said outer cavity resonator including a lower end plate member secured to the lower end of said Wall member, said wall member having a reduced outer diameter portion to provide a wall thickness of about one half the thickness of the remaining portion of said wall member, at the lower end thereof, said reduced outer portion below the plane of the inner surface of said lowerend plate and an attenuator ring positioned about the reduced diameter portion of said wall member and below the plane of the inner surface of said lower end plate member.

3. A magnetron comprising a cylindrical. anode member, a plurality of vanes extending inwardly from said anode member and defining anode cavity resonators, a cathode positioned centrally of said anode member, means including said anode defining an outer cavity resonator encompassing said anode cavity resonators, means for coupling said anode cavity resonators to said outer cavity resonator, said outer cavity resonator including a lower end plate member electrically and thermally connected to the lower end of said anode member and forming a junction therewith, said anode having a reduced outer diameter surface below the upper surface of said lower end plate member, an annular slot provided in the upper surface said end plate member, said annular slot having a diameter greater than said outer surface of said anode wall member, said annular slot communicating with an annular cavity within the junction of said anode wall and said end plate member beneath the upper surface of said end plate member, the walls of said chamber defined by the reduced diameter surface portion of said anode wall and a groove in the surface of said end plate member facing said reduced outer diameter portion of said anode member and an attenuator ring positioned within said annular cavity.

References Cited UNITED STATES PATENTS PAUL L. GENSLER, Primary Examiner.

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