US3714592A - Network for pulling a microwave generator to the frequency of its resonant load - Google Patents

Abstract

A resonant microwave load, such as a microwave linear accelerator section, is coupled to a magnetron oscillator via the intermediary of a circulator. Power reflected from the resonant load is reflected to the circulator and thence to a wave absorptive load. A composite wave reflector and variable phase shifter is provided between the absorptive load and the circulator for reflecting a portion of the power reflected from the resonant load to the magnetron oscillator for pulling the frequency of the oscillator to the frequency of the resonant load. The composite wave reflector and variable phase shifter includes a wave reflective member carried from a support rod passing through the wall of a rectangular waveguide. Rotation of the support rod causes the wave reflective member to move in a generally axial direction within the waveguide for adjusting the phase of the reflected wave such as to pull the frequency of the magnetron to the frequency of the resonant load.

Description

Emit/ed States Patent Jory Jan. 31 1973 2,883,651 5/l959 Brandon et al. ..33l/5 X Primary Examiner-Herman Karl Saalbach Assistant Examiner-Saxfield Chatmon, Jr.

[75] Inventor: Howard R. Jory, Menlo Park, Cahf. Attorney stanley Z. Cole et al' [73] Assignee: Varian Associates, Palo Alto, Calif. 22 Filed: Dec. 6, 1971 [57] ABSTRACT [211 App! 204 805 A resonant microwave load, such as a microwave linear accelerator section, is coupled to a magnetron Related U.S. A lication Data oscillator via the intermediar of a circulator. Power pp fl d f h l d fl d h re ecte romt e resonant 0a 15 re ecte tot e c1r- [63] gy g sgfigglg of culator and thence to a wave absorptive load. A composite wave reflector and variable phase shifter is pro- U.S Cl. vided between the absorptive load and tl'lfi circulator 33b5, 333/24 333/31 i for reflecting a portion of the power reflected from 511 1111. C1 .1101 j 23/00, 1 101 j 23/34 the resonant 109d the magnetron Oscillator for 581 Field of Search ..331/5, 6, 34; 333/24, 31 A; Pulling the frequency of the Oscillator I9 the frequency 315 35 541 395 32 23 of the resonant load. The composite wave reflector and variable phase shifter includes a wave reflective 56 References Ci member carried from a support rod passing through the wall of a rectangular waveguide. Rotation of the UNlTED STATES PATENTS support rod causes the wave reflective member to 3,178,652 4/1965 Scharfman ..331/5 move in a generally axial direction Within the 2,993,140 7/1961 Westbrook ..333 24 Waveguide for adjusting the Phase of the reflected 2,590,784 3/1952 Moulton ..331/5 X wave such as to pull the frequency of the magnetron 3,025,513 3/1962 Easy et a1. ..333/24 X to the frequency of the resonant load. 2,748,384 5/1956 Crane, Jr. et al. t ..33l/5 3,139,592 6/1964 Sisson ..331/5 14 Claims, 6 Drawing Figures LOAD 1 l REFLECTOR I &VARlABLE M'CROWAVE 6 SHIFTER I GENERNOR 4 1 I 8* I MAGNETRON 1 RESONANT LQAD PATENTEDJAN 30 m5 FIG.| LOAD 4/ EV /(k5 5i 'L MICROWAVE 0 SHIFTER 1 GENER/NOR 4X 8- I MAGNETRON s r 2 RESONANT' 7 LOADVJI -|/'2 b +|/'2 H MAGNETRON TUNER POSITION NETWORK FOR PULLING A MICROWAVE GENERATOR TO THE FREQUENCY OF ITS RESONANT LOAD This application is a continuation-in-part of copending patent application Ser. No. ll0,43l, filed Jan. 28, 1971 now abandoned, by the same inventor and assigned to the same assignee.

DESCRIPTION OF THE PRIOR ART Heretofore, a resonant microwave load has been driven with microwave energy supplied from a magnetron via the intermediary of a circulator. Wave energy reflected from the resonant microwave load is passed back to the circulator and thence to a broadband load for absorption therein. A wave reflective iris and a variable phase shifter were provided between the absorptive load and the circulator. The reflective iris controlled the voltage standing wave ratio (VSWR) of the wave reflection, and the variable phase shifter varied the phase of that portion of the energy reflected to the magnetron for pulling the frequency of the magnetron to the frequency of the resonant load. See A Microwave Apparatus for Rapid Heating of Threadlines, Journal of Microwave Power, 4(4), 1969, pp. 268-293.

The problem with the use of the reflective iris and the variable phase shifter is that they are relatively waveguide such that by rotating the first leg of the support member the wave reflective member can be moved in a generally axial direction along the waveguide for varying the phase of the wave reflected therefrom.

As another feature of the present invention, the wave reflective member of the variable phase shifter is carried upon a dielectric support arm, said arm being pivotable so as to move the wave reflective member axially of the waveguide for varying the phase of the wave reflected from the wave reflective member.

As another feature of the present invention, a composite variable phase shifter and wave reflector includes a wave reflective member carried upon a pivotable arm which passes through the wall of a waveguide and is sealed thereto to allow a pressure differential to be maintained between the atmospheres within and outside the waveguide.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accombulky and expensive items. More particularly, the variable phase shifter required approximately 8 to 12 inches of waveguide at S-band; and in certain microwave applications insufficient space is available to accommodate such a relatively large phase shifter. Typically, the phase shifter consisted of a wedge of dielectric material variably insertable into the waveguide for varying the phase of the wave passing therethrough.

It is desired to replace the separate reflective iris and variable phase shifter with an inexpensive and relatively small device which will serve both as a reflector and as a phase shifter.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is to provide an improved network for pulling the frequency of a microwave generator to the frequency of its resonant load.

As one feature of the present invention, a composite wave reflector and variable phase shifter is provided by a single device comprising a wave reflective member which is axially translatable within a waveguide section for varying the phase of the wave reflected from the reflective element to an isolator and thence to a microwave generator for pulling the frequency of the microwave generator to the frequency of its resonant load. Thus, an inexpensive and relatively small composite wave reflector and variable phase shifter is obtained.

As another feature of the present invention, the wave reflective member is carried upon a generally L-shaped support member having one leg protruding out of a rectangular waveguide through one of the broad walls, said leg extending generally parallel to the narrowside walls of the waveguide and said wave reflective member being carried upon another leg of the L- shaped support member, said second leg being generally perpendicular to the narrow side wall of the panying drawings, wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic circuit diagram, partly in block diagram form, depicting a microwave apparatus incorporating features of the present invention,

FIG. 2 is a transverse sectional view, partly schematic, depicting a composite wave reflector and variable phase shifter incorporating features of the present invention,

FIG. 3 is a schematic sectional view of the structure of FIG. 2 taken along line 3-3 in the direction of the arrows,

FIG. 4 is an enlarged sectional view of a portion of thestructure of FIG. 2 delineated by line 4-4,

" FIG. 5 is a plot of tuning characteristics for particular configurations of the microwave system, showing the difference between the frequency of the magnetron and the resonant frequency of the load as a function of frequency tuner positions for the magnetron, and

FIG. 6 is a view similar to FIG. 3 showing another embodiment of the composite wave reflector and variable phase shifter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a microwave apparatus incorporating features of the present invention. The microwave apparatus 1 includes a resonant load 2, such as a linear accelerator, having a plurality of cavity resonators successively coupled together to form the microwave accelerator section 3. The accelerator section 3 accelerates a beam of electrons, as supplied from an electron gun 4, to a relatively high energy level as of several MeV. The high velocity electron beam 5 impinges upon an X-ray target 6 producing a high energy X-ray bean 7 which is projected from the target onto an object or person to be irradiated or treated.

In a typical example, accelerator section 3 has a Q of approximately 10,000 and operates at S-band with pulses of microwave power supplied to the microwave accelerator 3 from a microwave generator 8, such as a magnetron, via the intermediary of a suitable isolator 9, such as a 3 port circulator. The magnetron 8 supplies pulses of 2 MW peak power and 2KW average power to the resonant load 2.

A wave absorptive load 11 is coupled to the third port of the circulator 9 via the intermediary of a composite wave reflector and variable phase shifter 12. The load 11 absorbs power reflected from the resonant load 2 back to the circulator 9. More particularly, an impedance mismatch, as encountered at the beginning and end of each pulse of microwave energy supplied to the resonant load 2, is reflected from the load 2 back to the circulator 9 and thence to the wave absorptive load 11.

The wave reflector and variable phase shifter 12 is provided for reflecting a certain small fraction of the reflected power back to the magnetron. This small fraction of the reflected power is larger than any reflection obtained from slight mismatches in the circulator 9, or other elements in the microwave circuit. This intentionally introduce reflection controls the pulling effect on the operating frequency of the magnetron 8 such that the resonant frequency of the magnetron is pulled to or locked to the resonant frequency of the load 2. With a proper setting of the phase of the intentionally introduced wave reflection produced by the reflector and variable phase shifter 12, the magnetron is intentionally pulled toward the resonant frequency of the load 2 for maximum power transfer to the resonant load 2.

Referring now to FIGS. 2, 3 and 4, the composite wave reflector and variable phase shifter 12 is shown in greater detail. Briefly, the reflector and variable phase shifter 12 includes a length of rectangular waveguide 13 having a pair of parallel broad walls 14 interconnected by a pair of narrow side walls 15. At S-band, standard waveguide has a width of 2.84 inches and a height of 1.34 inches.

A wave reflective member 16 is centrally disposed of the waveguide 13. In a typical example, the wave reflective member 16 comprises a hollow cylinder of dielectric or metal. In the case of a dielectric cylinder 16, the cylinder is made of alumina or beryllia ceramic as of 1.13 centimeters in diameter and 1.57 centimeters in length having a longitudinal threaded bore therein of a diameter of 0.38 centimeters.

A generally L-shaped support member 17, as of dielectric or metal, has the wave reflective member 16 carried from the inner end thereof, as by being screwed onto the threaded inner end of the support member 17. The L-shaped support member 17, in the neutral position, has one leg extending generally parallel to the broad walls of the waveguide and generally perpendicular to the narrow side walls of the waveguide 13 and the other leg 18 extends in the vertical direction generally parallel to the side walls and perpendicular to the top wall through a hole 19 in the top wall 14 of the waveguide 13.

A knob 21 is affixed to the vertical leg 18 of the support member 17 external of the waveguide 13 for rotating the support member 17 about the axis of the vertical leg 18 thereof. The vertical leg 18 of the support member 17 is rotatable through an angle of approximately 90 for varying the axial position of the wave reflective member 16 from a neutral position to i 45 from the neutral position. The center of the wave reflective member 16, in the neutral position, is slightly off the center plane 20 of the waveguide 13 on the side remote from the axis of the vertical leg 18, such that as the wave reflective member 16 is rotated from +45 to 45 relative to the neutral plane 10, the mean position of the center of the wave reflective member 16 falls approximately on the center plane of the waveguide 13 as indicated by the center line 20 of FIG. 3.

In a typical example, the reflective member 16 is dimensioned to produce a VSWR of between 1.23 and 1.29 over its entire range of axial movement within the waveguide 13. This movement is sufficient to produce a relative phase shift of approximately between the 45 and +45 deflections from the neutral plane 10 perpendicular to the narrow side walls 15.

Maximum power transfer from magnetron 8 to the resonant load 2 occurs when the resonant operating frequency of the magnetron is at the same frequency as the resonant frequency of the load 2. Thus, in order to deliver maximum power to the accelerator 3 and thereby produce a maximum output X-ray beam 7, the composite wave reflector and variable phase shifter 12 must be adjusted to a proper setting. This proper setting is determined by the physical and electrical characteristics of the various components of the microwave system.

Referring now to FIG. 5, line 24 shows the pulling effect on the magnetron 8 when the composite wave reflector and variable phase shifter 12 is set to the proper position for a given condition of the entire system, such as along line 10 as shown in FIG. 3, and the conventional frequency tuner on the magnetron is varied. The Af 0 value on the ordinate of FIG. 5 represents zero deviation of the output frequency of the magnetron from the resonant frequency of the load at a given instant of time. Any deviation, positive or negative, of the output frequency of the magnetron from the resonant frequency of the load at a given instant of time would be represented bysome value on the ordinate above or below Af 0. TheO value on the abscissa of FIG. 5 represents the particular dial setting of the magnetron frequency tuner that will cause the magnetron to put out a signal whose frequency is identical with the resonant frequency of the load at any given instant of time, assuming there is no reflection causing the magnetron to operate at frequency different from that selected by its tuner. For a stable load, with no reflection from the load'to affect the magnetron, it would be expected :that varying the dial setting of the magnetron frequency tuner wouldcause the output frequency of the magnetron to deviate from the resonant frequency of the load according to some linear'relation such as that shown by straight line 24. However, in a typical system it is found that reflections from the load, combined'with imperfections in the circulator or in the absorptive load, upset the expected linear relationship between the magnetron oscillation frequency and the tuner dial setting. Unless the phase of the wave reflected to the magnetron can be controlled so as to cause the reflected wave to interact with the magnetron in a way that will pull the magnetron output frequency to that of the resonant load, the reflected wave will cause the magnetron to become unstable about the frequency of the resonant load. This instability is indicated by the discontinuous curve 25 in FIG. 5. Where the phase of the reflected wave is uncontrolled, it may not be possible to achieve Af 0 when the magnetron frequency tuner dial setting is varied about the frequency of the resonant load. However, where the phase of the reflected wave is controlled by the technique of the present invention, it is seen that over the range of magnetron frequency tuner dial settings which would effectively tune the magnetron in the absence of any reflection from the load, the frequency of the magnetron will be pulled to the resonant frequency of the load. As indicated by the substantially horizontal portion of line 23 in FIG. 5, the frequency of the magnetron is pulled to the resonant frequency of the load for dial settings within the range of frequency tuner dial settings arbitrarily indicated by i k. Outside this il range, the frequency of the magnetron is pulled or pushed off the resonant frequency of the load. However, it is significant that within this relatively broad range of magnetron frequency tuner dial settings, the output frequency of the magnetron will follow the resonant frequency of the load without appreciable deviation therefrom. In other words, the magnetron frequency tuner dial setting is not critical over a relatively broad range.

To obtain the proper setting for the composite wave reflector and variable phase shifter 12, the magnetron frequency tuner is initially set at a frequency approximating t he estimated resonantfrequency of the load. The tuning characteristic of the microwave system is then examined. Unless the composite wave reflector and variable phase shifter 12 has already fortuitously been set at its proper setting, the tuning characteristic of the system will be illustrated by a curve such as curve 25 shown in FIG. 5. In general, there will be an instability in the output frequency response of the magnetron at the particular dial setting of the magnetron frequency tuner which corresponds to the resonant frequency of the load. In such case the composite wave reflector and variable phase shifter 12 is manually adjusted by moving reflective member 16 to a new position; the magnetron frequency tuner is varied about its zero turn point, and the tuning characteristic of the system is again examined. This procedure is repeated as necessary until the tuning characteristic of the system exhibits two unstable regions, each located an equal distance -one above and one below -the resonant frequency of the load, with a broad range of stability about the resonant frequency of the load as shown by curve 23 in FIG. 5.

The composite wave reflector and variable phase shifter 12 is normally pressurized with a gas having a relatively strong dielectric constant, such as Freon to a suitable pressure, as of 40 psi, relative to atmospheric pressure. When the waveguide 13 is to be pressurized, a rotatable hermetic seal must be provided between the vertical shaft 18 and the hole 19 in the broad wall 14 of the waveguide 13 as shown in detail in FIG. 4. More particularly, a ring 25, as of stainless steel in brazed to the wall 14 in axial alignment with the hole 19 and shaft 18. A concave seat 26 is formed in the outer end of the ring 25 to accomodate a rubber O-ring 27. A retaining collar 28 is threaded over the ring 25 to press the 0- ring 27 against the seat 26 and to form a seal between the O-ring 27 and the shaft 18, The control knob 21 is affixed to the shaft 18.

The advantage of the composite reflector and variable phase shifter 12, as contrasted with the prior art, is that the composite wave reflector and variable phase shifter 12 are combined into a unitary and relatively inexpensive structure which has an overall length which is very short. For example, the entire wave reflector and variable phase shifter 12 may be incorporated into a single waveguide flange in such a manner that the wave reflective member 16 may be pivoted into the adjacent waveguide sections. In this manner, the entire added length of the waveguide between the circulator 9 and the load 1 1 is merely the thickness of the flange.

The wave reflective member can have shapes other than the cylindrical shape shown in FIGS. 2 and 3. For example, FIG. 6 shows a composite wave reflective member shifter 16 shaped as a sphere. As in the case of member 16, the member 16' can be metal or dielectric. Member 16' functions the same as member 16, and the spherical shape has been found to be a particularly desirable configuration for the wave reflective member.

Since many changes could be made in the above construction, and many apparently widely different embodiments of this invention could 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:

l. A microwave network for pulling the frequency of a microwave generator to the frequency of its resonant load, said network comprising isolator means having at least three ports connected between the microwave generator and the resonant load for channeling microwave output energy from the microwave generator to the resonant load through first and second ports of said isolator means and for channeling substantially all power reflected from the resonant load to a third port of said isolator means, power absorbing means connected to said third port of said isolator means for absorbing the power reflected from said resonant load, microwave reflector means connected in circuit between said power absorbing means and said isolator means for reflecting a portion only of the energy reflected from the resonant load to the microwave generator, substantially all of said portion of said reflected energy being channeled through said isolator means to said microwave generator only, said microwave reflector means comprising a section of hollow waveguide, a movable support member, a microwave reflective element disposed within said waveguide and carried on said movable support member, means for moving said support member relative to said waveguide to cause movement of said microwave reflective element within said waveguide and thereby to produce a variable phase shift in the microwave energy reflected to the microwave generator.

2. The apparatus of claim 1 wherein said support member is dielectric.

3. The apparatus of claim 1 wherein said support member is metal.

4. The apparatus of claim 1 wherein said microwave reflective element is dielectric.

5. The apparatus of claim 1 wherein said microwave reflective element is metal.

6. The apparatus of claim 1 wherein said microwave reflective element is cylindrical in shape.

7. The apparatus of claim 1 wherein said microwave reflective element is spherical in shape.

8. The apparatus of claim 1 wherein said hollow waveguide is rectangular having a pair of parallel broad walls interconnected by a pair of parallel narrow side walls, and wherein said support member is pivotable and generally L-shaped with one leg thereof extending through one of said broad walls generally perpendicular thereto and with the other leg thereof extending generally perpendicular to the plane of said narrow side walls, whereby rotation of said support member about the axis of said one leg extending through said broad wall causes translation of said microwave reflective element carried from said other leg, such translation of said microwave reflective element being generally axial of the waveguide.

9. The apparatus of claim 1 additionally comprising seal means for rotational sealing said support member to said waveguide to allow a pressure differential to be maintained between the atmospheres inside and outside said waveguide.

10. The apparatus of claim 1 wherein the resonant load comprises a linear accelerator section for accelerating charge particles.

11. The apparatus of claim 1 wherein the microwave generator comprises a magnetron oscillator.

12. The apparatus of claim 1 wherein the isolator means comprises a microwave circulator.

13. A composite microwave reflector and variable phase shifter comprising a section of hollow waveguide, said hollow waveguide having a pair of parallel broad walls interconnected by a pair of parallel narrow side walls, a movable support member, said support member being pivotable and generally L-shaped with one leg thereof extending through one of said broad walls generally perpendicular thereto and with the other leg thereof extending generally perpendicular to the plane of said narrow side walls, a microwave reflective element disposed within said waveguide and carried on said movable support member, whereby rotation of said support member about the axis of said one leg extending through said broad wall causes translation of said microwave reflective element carried from said other leg, said translation of said microwave reflective element being generally axial of said hollow waveguide, said translation of said microwave reflective element thereby producing a variable phase shift of microwave energy reflected thereby to the waveguide.

14. The apparatus of claim 13 wherein said microwave reflective element is spherical.

Claims (14)

1. A microwave network for pulling the frequency of a microwave generator to the frequency of its resonant load, said network comprising isolator means having at least three ports connected between the microwave generator and the resonant load for Channeling microwave output energy from the microwave generator to the resonant load through first and second ports of said isolator means and for channeling substantially all power reflected from the resonant load to a third port of said isolator means, power absorbing means connected to said third port of said isolator means for absorbing the power reflected from said resonant load, microwave reflector means connected in circuit between said power absorbing means and said isolator means for reflecting a portion only of the energy reflected from the resonant load to the microwave generator, substantially all of said portion of said reflected energy being channeled through said isolator means to said microwave generator only, said microwave reflector means comprising a section of hollow waveguide, a movable support member, a microwave reflective element disposed within said waveguide and carried on said movable support member, means for moving said support member relative to said waveguide to cause movement of said microwave reflective element within said waveguide and thereby to produce a variable phase shift in the microwave energy reflected to the microwave generator.

1. A microwave network for pulling the frequency of a microwave generator to the frequency of its resonant load, said network comprising isolator means having at least three ports connected between the microwave generator and the resonant load for Channeling microwave output energy from the microwave generator to the resonant load through first and second ports of said isolator means and for channeling substantially all power reflected from the resonant load to a third port of said isolator means, power absorbing means connected to said third port of said isolator means for absorbing the power reflected from said resonant load, microwave reflector means connected in circuit between said power absorbing means and said isolator means for reflecting a portion only of the energy reflected from the resonant load to the microwave generator, substantially all of said portion of said reflected energy being channeled through said isolator means to said microwave generator only, said microwave reflector means comprising a section of hollow waveguide, a movable support member, a microwave reflective element disposed within said waveguide and carried on said movable support member, means for moving said support member relative to said waveguide to cause movement of said microwave reflective element within said waveguide and thereby to produce a variable phase shift in the microwave energy reflected to the microwave generator.

2. The apparatus of claim 1 wherein said support member is dielectric.

3. The apparatus of claim 1 wherein said support member is metal.

4. The apparatus of claim 1 wherein said microwave reflective element is dielectric.

5. The apparatus of claim 1 wherein said microwave reflective element is metal.

6. The apparatus of claim 1 wherein said microwave reflective element is cylindrical in shape.

7. The apparatus of claim 1 wherein said microwave reflective element is spherical in shape.

8. The apparatus of claim 1 wherein said hollow waveguide is rectangular having a pair of parallel broad walls interconnected by a pair of parallel narrow side walls, and wherein said support member is pivotable and generally L-shaped with one leg thereof extending through one of said broad walls generally perpendicular thereto and with the other leg thereof extending generally perpendicular to the plane of said narrow side walls, whereby rotation of said support member about the axis of said one leg extending through said broad wall causes translation of said microwave reflective element carried from said other leg, such translation of said microwave reflective element being generally axial of the waveguide.

9. The apparatus of claim 1 additionally comprising seal means for rotational sealing said support member to said waveguide to allow a pressure differential to be maintained between the atmospheres inside and outside said waveguide.

10. The apparatus of claim 1 wherein the resonant load comprises a linear accelerator section for accelerating charge particles.

11. The apparatus of claim 1 wherein the microwave generator comprises a magnetron oscillator.

12. The apparatus of claim 1 wherein the isolator means comprises a microwave circulator.

13. A composite microwave reflector and variable phase shifter comprising a section of hollow waveguide, said hollow waveguide having a pair of parallel broad walls interconnected by a pair of parallel narrow side walls, a movable support member, said support member being pivotable and generally L-shaped with one leg thereof extending through one of said broad walls generally perpendicular thereto and with the other leg thereof extending generally perpendicular to the plane of said narrow side walls, a microwave reflective element disposed within said waveguide and carried on said movable support member, whereby rotation of said support member about the axis of said one leg extending through said broad wall causes translation of said microwave reflective element carried from said other leg, said translation of said microwave reflective element being generally axial of said hollow waveguide, said translation of said microwave reflective element thereby producing a variable phase shift of microwave energy reflected thereby to the waveguide.

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