US3475696A - Theta mode magnetron having means for suppressing the three halves mode - Google Patents

Description

Oct. 28, 1969 w. A. FRUTIGER 3,475,696

6 MODE MAGNETRON HAVING MEANS FOR SUPPRESSING THE THREE HALVES MODE Filed Jan. 4, 1968 ["6 MOE FIG.2'

rocuz I NVENTOR.

WILLIAM A. FRUTIGER BY v ORNEY United States Patent 3,475,696 0 MODE MAGNETRON HAVING MEANS FOR SUPPRESSING THE THREE HALVES MODE William A. Frutiger, Beverly, Mass., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed Jan. 4, 1968, Ser. No. 695,742 Int. Cl. H03b 9/10 US. Cl. 331-91 4 Claims ABSTRACT OF THE DISCLOSURE A 0 mode voltage tunable magnetron is disclosed. The magnetron includes a circular re-entrant periodic slow wave circuit coupled to a surrounding cavity resonator dimensioned for fundamental mode of resonance at the frequency of the 71' mode on the slow wave circuit. The slow wave circuit surrounds a non-emissive cathode sole electrode and an electron gun is disposed at one end of the cathode sole electrode structure for projecting a beam of electrons into the re-entrant stream magnetron interaction region defined between the cathode sole electrode and the surrounding slow wave circuit operated at anode potential. An output coupling means projects into the cavity resonator for coupling output wave energy from the magnetron to a suitable load. A conductive short circuit is positioned in the periodic slow wave circuit at a point approximately 90 around the cavity from the output coupling means for locking the position of the first 0 mode,

mode, in the resonator structure such that it is heavily coupled to the output coupling means and such that an interfering orthogonal first 0 mode is perturbed in frequency out of the operating band of the tube. The cavity is also capable of supporting an interfering mode of oscillation corresponding to the resonant frequency of the mode on the slow wave circuit. A lossy mode suppression post is disposed near the slow wave circuit in the cavity resonator at a position 180 around the circuit from the first 0 mode locking short member for heavily coupling the 3/2 mode of oscillation to the lossy post to suppress the 3/2 mode and prevent it from interfering with the desired first 6 mode. Suppression of the 3/2 mode reduces mode competition and facilitates starting of the magnetron. In addition, suppression of the 3/2 mode prevents this mode from interacting with the electron beam to produce a discontinuity in the power output characteristic of the tube over its electronically tunable band.

DESCRIPTION OF THE PRIOR ART Heretofore, 0 mode magnetrons have been constructed. Such a 0 magnetron is described in co-pending U.S. application 525,454 filed Feb. 7, 1966 and assigned to the same assignee as the present invention. In this prior 0 mode magnetron, the first 0 mode locking short was provided on the slow wave circuit at a position approximately 90 around the coupled slow wave circuit and surrounding cavit resonator from the output coupling means for locking the first 0 mode in position such that it was tightly coupled to the output load via the output coupling means. In addition, the shorting member on the slow wave structure served to perturb the frequency of an interfering orthogonal first 0 mode out of the operating band of the tube to prevent interference with this mode over the electronic tuning band of the tube. However, it was discovered in practice that the ALE 2 2 mode on the circuit had a resonant frequency falling within the electronic tuning band of the voltage tunable magnetron. As a result, this mode interfered with the desired operating mode within the upper frequency range of the tube and resulted in producing a substantial discontinuity in the power output versus frequency characteristic of the tube. In addition, the interfering 3/2 mode competed with the desired first 0 mode during starting making it difficult to start the magnetron in the proper mode of operation. Therefore, a need exists for means for suppressing or otherwise perturbing the 3/ 2 mode and preventing it from interfering or competing with the desired first 0 mode in the magnetron.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved 0 mode magnetron.

One feature of the present invention is the provision, in a 0 mode magnetron, of an attenuator structure coupled to the fields of the slow wave circuit at a position approximately around the circuit from a shorting memher which serves to lock the desired first 0 mode of the magnetron in a certain position on the circuit relative to the output coupling means, whereby the attenuator means attenuates the 3/ 2 mode without substantially attenuating the desired first 0 mode.

Another feature of the present invention is the same as the preceding feature wherein the attenuator means comprises a lossy post structure disposed within the cavity resonator of the 0 mode magnetron near the outer circumference of the periodic slow wave circuit.

Another feature of the present invention is the same as any one or more of the preceding features wherein the lossy attenuator means comprises a lossy post structure extending across the cavity resonator parallel to the electric field lines of the mode.

Another feature of the present invention is the same as any one or more of the preceding features wherein the lossy attenuator means comprises a lossy post structure formed by a rod of carbon impregnated ceramic.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic transverse sectional view of a 0 mode magnetron depicting the desired first 0 mode and the interfering 3/2 mode together with the attenuator means for suppressing the undesired 3/2 mode of oscillation,

FIG. 2 is an w-fi diagram depicting the dispersion characteristic of the periodic slow wave circuit for the voltage tunable magnetron of the present invention,

FIG. 3 is a plot of power output versus frequency depicting the prior art power output characteristic and the effects of the interfering 3/2 mode, and

FIG. 4 is a longitudinal sectional view of the structure of FIG. 1 taken along line 44 in the direction of the arrows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 there is shown in schematic form a transverse sectional view of a voltage tunable magnetron of the present invention. The magnetron includes a cylindrical non-emmissive cathode sole electrode 2 coaxially surrounded by a re-entrant periodic slow wave circuit 3 to define a re-entrant stream annular magnetron interaction 4 in the space between the cathode sole electrode 2 and the anode circuit 3. The anode slow wave circuit 3 includes a conventional periodic slow wave circuit 5 such as an interdigital line surrounded by and coupled to a folded coaxial cavity resonator structure 6. An output coaxial line 7 is coupled to the resonator 6 for extracting output wave energy from the magnetron 1.

An annular stream of electrons is injected into the magnetron interaction region 4 for electromagnetic interaction between the electrons of the stream and wave energy on the re-entrant slow wave circuit 5 to produce the output microwave signal which is coupled to the load via the output coupling line 7.

The interdigital slow wave circuit 5 has a backward wave fundamental dispersion characteristic 8 as depicted in FIG. 2 having a high frequency cut-off of approximately 70 gHz. and a 1r mode cut-off of approximately 4.3 gHz. When the slow wave circuit 5 is formed in a re-entrant manner such that wave energy may continue to circulate around the slow wave circuit, the circuit 5 becomes resonant and operation may be had only for frequencies corresponding to an integral number of whole electrical wavelengths taken around the anode circuit 3. These discontinuous points of operation are indicated by the solid vertical lines on the dispersion curve 8. If there are N number of elements in the periodic circuit 5, there will be found to be a N/2 possible modes of operation for the circuit, such modes being equally spaced in ,6 as indicated in FIG. 2. Thus, assuming there are 24 periodic elements in the circuit 5, there will be N/ 2 or 12 possible modes of operation indicated as N/ 2,

(g -1), -2) etc.

From the relative steep dispersion curve 8 in the high phase shift per section range of the dispersion curve, i.e., between 1r/2 and 1r, as shown in FIG. 2, it

can be seen that a substantial amount of frequency separation is obtained between adjacent modes. In particular, it is seen that if operation is had on the mode that the operating bandwidth from m to w, will be substantial without interference from the two adjacent N/2 and modes. These possible modes of operaion, other than the conventional 1r mode, have become known in the art as 0 modes. The first 0 mode, i.e.,

is particularly attractive as its interaction impedance may be substantial.

The cavity resonator 6 (see FIG. 1) is dimensioned for a fundamental resonance at the operating frequency of the 11' mode on the coupled circuit 5. However, it is found that when the output coupling means 7 is inserted into the cavity 6 that it peiturbs the desired on the circuit and its associated cavity mode, TM mode, into two orthogonal components. These orthogonal components will adjust themselves within the cavity resonator 6 to minimize the coupling to the output means 7. Therefore a shorting member 9 is placed on the slow wave circuit 5 at a position approximately around the circumference of the cavity 6 from the output coupling means 7 to lock the position of the desired TM mode in the position as indicated in FIG. 1, namely, such as to have a maximum electric field opposite the output coupling means 7 such that one of the orthogonal modes is heavily coupled to the load. The shorting member 9 also serves to perturb the frequency of the uncoupled orthogonal mode out of the operating band of the tube such that it no longer interferes with the proper operation of the tube. The provision of the shorting member 9 on the slow wave circuit 5, for locking the desired first 0 mode in the position as shown in FIG. 1, is described and claimed in the aforecited US. application 525,454.

The provision of a shorting member 9 on the slow wave circuit 5 also produces a single reflection point thereon causing a set of additional 1/2 modes of operation to appear in the dispersion characteristic 8 of the tube between the integral 0 modes, namely, operation may now be obtained at One of these 1/2 modes of operation, namely, the

mode has a resonance near the upper end of the electronically tunable bandwidth of the tube.

The 3/2 mode has an unloaded Q of approximately 600 which is approximately the same as the unloaded Q of the desired first 0 mode. The loaded Q of the first 0 mode, as loaded by the output coupling means 7, is lowered to a Q of approximately 5 and the loaded Q of the 3/2 mode is loaded to a lesser extent by the output coupling means such that its loaded Q is approximately 100. The undesired 3/2 mode has a magnetic field configuration as indicated by the dotted lines 11 of FIG. 1. Although the 3/2 mode has a lower average interaction impedance with the electron stream, as compared to the desired first 0 mode, it represents a possible interfering mode of oscillation. As a result, the output characteristic for the prior art tube is as shown in FIG. 3 wherein it is seen that near the upper band edge of the electronic tunable bandwidth the power output characteristic has a sharp discontinuity wherein the power output jumps up to a higher level corresponding to operation on the 3/2 mode. In addition, it is found that the starting characteristic of the prior art tube is relatively poor because the 3/2 mode represents a competing mode of oscillation competing with the desired first 0 mode. Therefor it is difficult to start the magnetron in the proper first 0 mode.

It has been found that the undesired 3/2 mode may be suppressed by the provision of an attenuator structure 12 formed by a carbon impregnated ceramic rod disposed near the slow wave circuit 5 at a position approximately 180 around the cavity 6 from the position of the mode locking short 9. This position for the attenuator 12 positions the attenuator 12 at a point of maximum electric field intensity for the undesired 3/2 mode and at a null position for the desired first mode. Therefore, the attenuator post 12 does not substantially interfere with operation of the tube in the desired 0 mode while at the same time it effectively suppresses the undesired competing 3/2 mode. With the provision of the attenuator 12 in the position as indicated, the magnetron starting problems are eliminated and the power output characteristic is continuous over the electronic tunable band.

Referring now to FIG. 4, there is shown in longitudinal section, a voltage tunable magnetron 1 incorporating the features of the present invention. The magnetron 1 includes the non-emissive cathode sole electrode 2 axially disposed of the tube 1 and surround by the anode circuit 3. A filamentary emitter 13 is disposed at one end of the tube 1 in a position near one axial end of the cathode sole electrode 2 for emitting a stream of electrons. An injector electrode 14 coaxially surrounds the filamentary emitter l3 and serves to inject a stream of electrons axially of the tube into the magnetron interaction region 4 defined by the annular region between the cathode sole electrode 2 and the anode circuit 3.

The magnetron tube 1 is evacuated and the vacuum envelope is formed by metallized vacuum seals made between adjacent elements of the tube, and in particular, between the cathode sole 2, ceramic ring 16, anode circuit 3, ceramic ring 17 and end closing ceramic plate 18.

The anode circuit 3 includes the interdigital line portion 5 closely coupled to and surrounded by a folded section of radial cavity 6 which serves to support the interdigital line in what may be referred to as a crown-supported manner, i.e., the interdigitated fingers of the interdigital line 5 are supported at their ends from opposite axially spaced side walls of the cavity 6.

The output coupling means 7 is coupled to the fields of the resonator 6 for extracting wave energy from the circuit 3 for propagation to a suitable load, not shown, via the intermediary of a coaxial line 24. The coupling means 7 includes an extension of the center conductor 25 of the coaxial line 24. The extension of the center conductor 25 reaches across the cavity 6 from one side wall to the other making contact to the opposed wall and passing through an aperture in the outer wall of the cavity 6. A cylindrical dielectric wave permeable window member 26 coaxially surrounds the extension of the center conductor 25 forming a gas tight seal thereto at one end and a gas tight seal to the outer conductor of the coaxial line at its other end.

The 3/2 mode attenuator structure 12 includes a carbon impregnated alumina ceramic rod positioned near the outer circumference of the slow wave circuit 5 within the cavity resonator 6. The post 12 is disposed across the resonator from one broad wall thereof to an opposed broad wall and extends across the cavity parallel to the electric field lines of the undesired 3/ 2 mode. The attenuator post 12 is disposed within axially directed bore in the bottom wall of the cavity 6 and is bonded into place by a suitable adhesive. In a typical example for an X-band voltage tunable magnetron 1, the lossy ceramic post 12 is approximately 0.040 in diameter with its center line being disposed approximately 0.175" from the axial center line of the tube. This positions the center line of the attenuator post 12 approximately 0.050 from the outside circumference of the interdigital slow wave circuit 5'.

In an X-band tube of the present invention, the first (1/ 2) mode has a resonant frequency of approximately 8.7 gHz. and the next highest (3/2) mode resonates at a frequency of about 10.2 gHz. With the anode circuit 3 heavily loaded to the output via the output coupling 7,

the anode circuit exhibited a Q of approximately 5. An electronically tunable bandwidth of approximately 1000 mHz. was obtained between synchronous voltages of V and V The slow wave circuit 5 had 24 periodic elements and the tube yielded a power output of approximately 2.5 watts over the tunable band.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention can be made without departing from the scope thereof it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is;

1. In a 0 mode magnetron oscillator, means forming a cathode electrode structure, means forming an anode electrode structure surrounding said cathode electrode structure to define a re-entrant stream annular magnetron interaction region between said anode and cathode electrode structures, means forming a re-entrant periodic slow wave circuit forming a portion of said anode electrode structure disposed adjacent said magnetron interaction region, means for producing a stream of electrons in said magnetron interaction region for electromagnetic interaction between wave energy on said slow wave circuit and the stream of electrons, means forming a cavity resonator structure surrounding said slow wave circuit and being coupled thereto such that said slow wave circuit forms a coupled portion of said cavity resonator, said cavity resonator being dimensioned for a fundamental mode of resonance therein at the frequency of the 1r mode on said slow wave circuit, said cavity and slow wave circuit being capable of supporting a desired resonant operating mode at the frequency of the mode on said slow wave circuit and an interfering mode of resonance on said slow wave circuit at the resonant frequency of the mode on said slow wave circuit, means for coupling to the mode of said cavity for extracting the output signal energy from said circuit, means in said coupled slow wave and cavity resonator circuit and disposed approximately around said cavity from said output coupling means for locking the mode of said cavity to said output coupling means, the improvement comprising, attenuator means in said coupled slow wave and cavity resonator circuit at a position approximately around said cavity resonator from said mode locking means for suppressing the interfering on said slow wave circuit.

2. The apparatus of claim 1 wherein said attenuator means includes a lossy post structure disposed Within said cavity resonator near the outer circumference of said periodic slow wave circuit for coupling to the mode on said slow wave circuit.

7 8 3. The apparatus of claim 2 wherein said lossy post References Cited structure extends across said cavity resonator parallel UNITED STATES PATENTS 1 fi 1d 1' to the 6 ms f; 3,418,522 12/1968 Cook 331 91 2? 5 JOHN KOMINSKI, Primary Examiner mode and from one wall of said cavity to an opposed wall U S C1 X R thereof.

4. The apparatus of claim 2 wherein said lossy post SIS-39.63, 39.73, 39.77 structure comprises a rod of carbon impregnated ceramic.

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