US3379926A

US3379926A - Coaxial magnetron having slot mode suppressing lossy material in anode resonators

Info

Publication number: US3379926A
Application number: US412059A
Authority: US
Inventors: Symes John
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1964-11-18
Filing date: 1964-11-18
Publication date: 1968-04-23
Anticipated expiration: 1985-04-23

Description

3,379,926 RESSING 5 J. SYM

April 23, 1968 ES SLOT 0 E ANODE SUPP ONATOR COAXIAL MAGNETRON HAVING LOSSY MATERIAL IN 2 Sheets-Sheet 1 Filed Nov. 18, 1964 I i: iiii. a s I INVENTOR John Sy mes obi. 1M ATTORNEY April 23, 1968 J. SYM 3,379,926

COAXIAL NETRON HAVING L T M SUPPRESSING LO MATERIAL IN ANODE RE ATORS Filed Nov. 18, 1964 2 Sheets-Sheet United States Patent 0 COAXIAL MAGNETRON HAVING SLOT MQDE SUPPRESSING LQSSY MATERIAL IN ANODE RESONATORS John Symes, Elmira, N.Y., assiguor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Nov. 18, 1954, Ser. No. 412,059 8 Claims. (Cl. 31539.75)

ABSTRACT OF THE DISCLOSURE The present invention is directed to a coaxial magnetron in whch a first group of inner anode resonators with coupling slots to provide coupling through a cavity resonator is provided and a second group of inner anode resonators is provided without coupling slots and a lossy material is provided within the second group of inner anode resonators to damp the slot modes.

This invention relates to electron discharge devices and more particularly to magnetrons.

In the R. J. Collier et al. Patent 2,854,603 issued Sept. 30, 1958, there is disclosed a coaxial magnetron structure which comprises an inner and outer resonant system. The inner resonant system, sometimes referred to as a vane and slot system, includes a cylindrical anode together with a plurality of anode vanes radially extending inwardly therefrom. These vanes define a circumferential array of inner, anode cavity resonators. The outer resonant system is a cavity resonator defined between an outer wall and the cylindrical anode. The inner and outer systems are coupled together by a circumferential array of spaced slots through the cylindrical anode which connect the outer resonant system with selected anode cavity resonators of the inner system. These slots are normally made to extend axially beyond the limits of the inner anode resonators for reasons including mode-dampening. The inner resonant system is designed to oscillate in the pi mode, while the outer system is designed to oscillate in the TE mode.

The above-mentioned patent discloses a device in which the cathode is positioned inside of the cylindrical anode. It is also possible to position the cathode external of the cylindrical anode and provide the cavity resonator within the cylindrical anode. This device is normally referred to as an inside-out configuration in comparison to the conventional coaxial magnetron structure in which the cathode is positioned within the anode. At frequencies removed from the resonant frequency of the T'E cavity resonator, there is little coupling between the cavity resonator and the inner system. The vane and slot system then behaves in a manner analogous to a slow wave structure or delay line as used in T.W.T.s or B.W.O.s, and energy can be propagated around the anode structure as traveling or propagating waves. Under these conditions, nearly all the energy of the waves is stored in the vane and slot system and there is little energy storage in the cavity resonator. The electrons will interact with these waves in the same manner as in a conventional crossed field amplifier. Since the structure is re-entrant, there will be certain frequencies at which oscillation can build up due to feedback around the anode structure. These frequencies will be determined by the propagation characteristics of the structure whch will in turn be determined by the vane and slot dimensions. These oscillations will in general be at lower frequencies than the desired operating mode and will start at lower values of applied voltage. When the magnetron is operated under pulsed condition, it follows that these oscillations may start on the leading edge of the pulse before the applied voltage has risen to the value required for the 3,379,926 Patented Apr. 23, 1968 ice desired operating mode. In extreme cases, the current drawn in these spurious modes may be sufiicient to prevent the applied voltage from reaching the required operating voltage. T-hese oscillations have been termed slot mode oscillations.

It has been suggested in these types of devices that certain of these slot modes may be removed or suppressed through the use of damping elements provided at the ends of the coupling slots in the cylindrical anode which absorb energy stored therein. It has also been suggested that the slOt mode interference may be reduced by placing certain non-uniformities in the slot pattern on the cylindrcal anode.

Although several schemes have been proposed for suppressing these so-called slot modes they all sufier from several disadvantages. There is difficulty in positioning the attenuator, especially at high frequencies, close to the ends of the coupling slot. A large volume of lossy material in the damping elements may become a troublesome source of gas. The desired attenuation of the slot modes usually results in a considerable loss in the desired mode.

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 in which undesirable oscillations are substantially suppressed.

It is another object to provide an improved method of suppression of the slot modes within a coaxial magnetron.

It is still another object to provide an improved method of suppression of slot modes in a coaxial magnetron with a minimum amount of damping of the desired mode.

Briefly, the present invention provides a coaxial magnetron in which a first group of inner anode resonators is provided with coupling slots to provide coupling to the TE cavity resonator and a second group of inner anode resonators is provided without coupling slots and a lossy material is provided within the second group of inner anode resonators to damp the slot modes.

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 embodying the present invention;

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

FIG. 3 is a sectional view taken along the lines III-III of FIG. 2 showing the anode construction; and

FIG. 4 is a view similar to FIG. 3 of an inside-out coaxial magnetron incorporating the teachings of the present invention.

With reference to FIGS. 1 and 2, there is shown a tunable coaxial magnetron embodying the present invention. The coaxial magnetron is comprised of a body member 10 which is substantially cup-shaped and includes a substantially cylindrical side or outer wall 12 and a bottom end plate 14 of a suitable electrically conductive material such as copper. The end plate 14 and the outer wall 12 define two boundaries of an annular cavity resonator 30. Centrally disposed within the body 10 is a cathode 16 which includes a sleeve member 18 of a suitable material such as molybdenum having thereon an electron emissive coating 20 of a suitable material such as barium oxide provided thereon. Surrounding the cathode 16 is a substantially cylindrical anode 22 of a suitable electrically conductive material such as copper having a plurality of vanes 24 of less axial length than the anode 22 and extending radially inwardly therefrom. The planes of the anode vanes 24 are inclusive of the axis of the cylindrical anode 22 and define in combination with the anode 22 an array of inner

anode cavity resonators

26 and 27. The vanes 24 may also be of copper.

In the specific embodiment shown, the inner anode cavity resonators 26 in section or group 23 of theranode are provided with coupling slots 28 through the anode wall 22 in alternate resonators. The coupling slots 28 provide coupling from the inner resonant systems to the outer cavity 30. The slots 28 may be of different shapes than shown in the drawing. The coupling slots 28 are centered between the vanes 24 defining the alternate resonators 26. The slots 28 extend beyond the end portions of the vanes 24.

The anode cavity resonators 27 in section 25 of the anode are not provided with coupling slots, but are instead provided with a suitable lossy material 29.

A lossy material is one which presents dielectric loss to electromagnetic waves. Microwaves may be attenuated by using a material with a lossy or poor electrically conductive metallic surface such as nickel. In the case of a lossy surface, the lossy portions of the resonator circuits would be provided as a coating over the inner surface of the inner anode resonators. Suitable materials for the lossy coating would be nickel, Invar or Nichrome.

Microwaves may also be attenuated by a lossy volume such as a lossy dielectric as illustrated in the drawings as item 29. It is also possible to use a material in which molecules become heated by internal polarization efiects under electric fields.

Examples of suitable lossy materials for the filling 29 damping used for magnetron spurious oscillations are: (1) carbon loaded alumina ceramic; (2) mixtures of silicon dioxide, glass, and alumina; and (3) a dental-type cement loaded with a few percent graphite. The latter two are especially suited for this invention as they can be placed in resonators 27 in slurry form and consequently hardened in place. The frequency of operation determines the exact composition of the mixture to be used.

In the case of lossy dielectrics, the electric field vector associated with the electromagnetic wave causes currents to flow in the lossy region according to the equation where J is the induced area current density, is the conductivity (l/resistivity) of the material, and E is the R.F. electric field strength. Therefore, this type of attenuation occurs throughout the volume of the dielectric; whenever E and 0' are not zero.

In the case of the lossy surface, the current is induced by the tangential R1 magnetic field at the surface of the wall.

These two types of attenuation are better for dilferent frequencies and different geometries. The choice of the two depends on the above as well as practical considerations of construction. In millimeter wave magnetrons, the lossy dielectric in the resonator in the form of a hardenable cement is the best.

The cylindrical wall portion 12 of the body 10, the cylindrical anode 22 along with the bottom portion 14 of the body define, in part, the outer cavity resonator 30. Extending through the wall 12 to communicate with the outer cavity resonator 30 is an output coupling slot 32. The coupling slot 32 serves as means through which energy may be removed from the outer cavity resonator 30 and has been shown in its simplest form for purposes of simplicity.

Positioned atop the cup-shaped body member 10 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 an annular member 44 within the outer cavity resonator 39. 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 the remaining boundary for 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 Wit-h an extended portion 68 which extends through an aperture 7 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 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 difierent modes of operation. In this particular application, the cavity resonator is dimensioned to provide maximum storage of the TE mode at the operating frequency. The electric currents of this mode flow circumferentially on the outer surface of the anode wall 22 and along the inner surface of the outer cavity resonator 30. The inner resonant system defined by the

resonators

26 and 27 will tend to oscillate in the pi and various degenerate modes. Thus, outer cavity resonator 30 and the

inner anode resonators

26 and 27 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 anodes 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 at the tips of the vanes 24. These voltages in turn produce currents which flow out through the slots 28 and into the outer cavity resonator 30. The resonators 26 are nor mally constructed so as to support a frequency outside the frequency range of the outer cavity resonator 30 such that tuning of the cavity resonator 30 by the member 44 has minimal efiects on the frequency of oscillation in the inner cavity resonator system. It is found in the system that spurious modes are produced in the manner already described by the propagation characteristics of the anode structure. 'For a magnetron to operate in one of these slot modes, the frequency of operation must be such that there is an integral number of half wave lengths around the anode so that resonance can be set up around the anode. To eliminate these modes, it is necessary to introduce sufiicient loss to prevent build up of these spurious oscillations. It is desirable that this loss should have little or no effect on the desired mode.

In this specific embodiment, this is accomplished by providing the section or sections wherein the anode resonators 27 are not provided with slots but are filled with a lossy material. In a typical case, this section could comprise up to about one half of the resonators. The absence of slots in section 25 assumes that no atenuation will be introduced on the desired mode by direct coupling between the cavity resonator 30 and thelossy material. The only desired mode energy which will be dissipated in the lossy material 29 is that which propagates into section '25 from the unmodified section 23 of the anode. lit the vanes in section '25 have an effective radial electrical length slightly greater than .a quarter wave-length at the operating frequency, then this section 2'5 will have an upper cut-off frequency lower than the operating frequency. Thus, at the operating frequency, this section Q5 will behave as a waveguide beyond cut-off which will not permit propagation of the operating frequency and will prevent energy at the operating frequency from being dissipated in the lossy material 29. At the slot mode frequencies however, this section Will not be cut-off and energy at these frequencies will be dissipated in the lossy material 29.

In FIG. 4, there is illustrated a partial view of an inside-out coaxial magnetron incorporating this invention. The remaining structure of the device may be of any suitable well known structure. A cylindrical cathode 70 is provided with an electron emissive material on the inner surface. A cylindrical anode 71 is provided within the cathode 70. Radially outwardly extending vanes 72 extend from the anode 71 toward the cathode -'72 and from cavity resonators '73, 74. The anode cavity resonators 74 are broken down into sections 76 and 78'. In section 78, alternate cavity resonators 74 are provided with slots 79. The slots 79 couple the anode cavity resonator 74 with a cavity resonator 80.

The section 76 is provided with a lossy material 82 between vanes 72 and no coupling slots are provided in the resonators 73 of the section 76. The function and operation of section 76 is similar to that described with respect to the conventional magnetron.

It is also possible that the attenuation could be applied either in one section as indicated in the drawings or in a number of small sections spaced around the anode to prevent resonances being set up due to any irregularities in the anode. The latter method of application is especially suitable for an anode with a large number of vanes. I

While there have been shown and described what are presently considered to be the preferred embodiments 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 arrangements shown and described and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

I claim as my invention:

1. A magnetron comprising a cathode, a plurality of anode resonators adjacent said cathode, and an output cavity resonator adjacent said anode resonators, said anode resonators and said output cavity resonator having a common wall portion, a first group of anode resonators having slots in said wall portion in alternate resonators for coupling said anode resonators to said cavity resonator and a second group of anode resonators isolated from said output cavity resonator through said common wall and having a lossy material provided within the anode resonators of said second group.

2. A magnetron comprising a cylindrical cathode for generating and projecting a stream of electrons, a cylindrical anode wall surrounding said cathode and coaxial therewith, an array of anode vanes extending radially inwardly from said anode wall and defining a plurality of anode resonators, means including said anode wall defining an outer cavity resonator, a first group of said anode resonators having coupling means in the form of slots extending through said cylindrical anode wall to selected anode resonators, a second group of anode resonators isolated from said outer cavity resonator and having a lossy material within said anode resonators of said second group.

3. A coaxial magnetron comprising a circular cathode, a circular anode surrounding said cathode said anode having a plurality of radially extending vanes to provide a first arcuate group and a second arcuate group of spaced anode resonators surrounding said cathode, an output cavity resonator surrounding said anode and defining an annular cavity, said first arcuate group of anode resonators having coupling means for coupling substantially all of the energy between said anode resonators and said output cavity resonator and said second arcuate group of anode resonators having a lossy material provided between said vanes defining the anode resonators.

4. A coaxial magnetron comprising a circular cathode, a circular anode coaxial with respect to said cathode and including a plurality of vane members extending radially toward said cathode and defining a plurality of anode cavity resonators, said anode cavity resonators including a first arcuate group and a second arcuate group, an output cavity resonator positioned on the opposite side of said anode with respect to said cathode, said first arcuate group of anode resonators provided with means for coupling energy between said anode resonators and said output cavity resonator, said second arcuate group of anode resonators provided with lossy material between said vane members and isolated from said output cavity resonator to prevent energy exchange between said output cavity resonator and said second group of anode resonators, the length of said vane members within said second arcuate group of anode resonators is such that the upper cut-off frequency of said second arcuate group of anode resonators is below the operating frequency of the coaxial magnetron to prevent absorption of energy of the operating mode in the lossy material and permit absorption of spurious energy of frequencies below the operating frequency of said coaxial magnetron.

5. A coaxial magnetron comprising a circular anode surrounding said cathode, said anode including a cylindrical wall with vane members radially extending from said cylindrical wall toward said cathode to define a plurality of circularly spaced anode resonators about said cathode and including at least a first arcuate group and a second arcuate group of anode resonators, an output cavity resonator positioned about said anode and designed to operate in the TE mode, said first group of anode resonators provided with coupling slots in said cylindrical wall of said anode for coupling energy between said anode resonators and said output cavity resonator, said second arcuate group of anode resonators having a lossy material positioned between said vane members and the cylindrical wall, said second group of anode resonators isolated from energy transfer with said output cavity to prevent loss of energy of the operating mode within the output cavity into said lossy material, the length of the vane members within said second group of anode resonators having an effective electrical length greater than a quarter wavelength of the operating frequency of the coaxial magnetron so that said second group of anode resonators is beyond cutoff at said operating frequency to refiect energy at said operating frequency from said lossy material and absorb energy of spurious modes of oscillation below said operating frequency.

6. A coaxial magnetron comprising a circular cathode, a circular anode surrounding said cathode having a first arcuate group and a second arcuate group of spaced anode resonators, an output cavity resonator surrounding said anode and defining an annular cavity, said first arcuate group of anode resonators having coupling means for coupling energy between said anode resonators and said output cavity resonators and said second arcuate group of anode resonators having a lossy material provided between said vanes defining the anode resonators for absorption of spurious energy.

7. A coaxial magnetron comprising a circular cathode, a circular anode coaxial with respect to said cathode, said anode including a first arcuate portion comprising a plurality of anode resonators, an output cavity resonator positioned on the opposite side of said anode with respect to said cathode, said first anode portion provided with means for coupling energy between said anode resonators and said output cavity resonator, a second arcuate anode portion provided with lossy material and means to provide an upper cutoff frequency below the operating frequency of the coaxial magnetron to prevent absorption of energy of the operating mode in the lossy material and permit absorption of energy of frequencies below the operating frequency of said coaxial magnetron.

8. A coaxial magnetron comprising a circular anode surrounding a cathode, said anode including a cylindrical wall with vane members radially extending from said cylindrical wall toward said cathode to define a plurality of circularly spaced anode resonators about said cathode, an output cavity resonator positioned about said anode and designed to operate in the TE mode, a first arcuate group of said anode resonators provided with coupling slots in said cylindrical wall of said anode for coupling energy between said first group of anode resonators and said output cavity resonator, a second arcuate group of anode resonators isolated from said output cavity resonator and provided with a lossy material between said vane members and the cylindrical wall to absorb energy of spurious modes of oscillation below the operating frequency.

References Cited UNITED STATES PATENTS 1/1958 Feinstein 315-39.77 7/1966 Peasley et a1. 31539.6l X