US3825794A

US3825794A - Microwave tube having an improved output section

Info

Publication number: US3825794A
Application number: US00339444A
Authority: US
Inventors: R Butwell
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1973-03-08
Filing date: 1973-03-08
Publication date: 1974-07-23
Anticipated expiration: 1991-07-23

Abstract

A microwave beam tube is arranged such that an upstream driver portion of the microwave circuit is severed for radio frequency energy from a downstream output microwave circuit portion. The driver section is dimensioned and arranged and driven with microwave energy such that the signal component of beam current has a peak value at the output end of the driver section which is not less than 0.7 times the d.c. beam current. This modulated beam current then excites a signal wave on the output microwave circuit portion which is amplified thereon and coupled to a load. The output microwave slow wave circuit portion is a fundamental space harmonic forward wave circuit, such as a cloverleaf circuit, centipede circuit or long slot coupled cavity circuit. The output circuit portion has a small signal gain of not more than 13 db to obtain increased RF conversion efficiency and stability for the tube.

Description

= titre Butwell 1451 July 23, 1974 [75] Inventor:

l l MlCROWAVE TUBE HAVING AN EMPROVED OUTPUT SECTION Robert J. Butwell, Cupertino, Calif.

' [73] Assignee: Varian Associates, Palo Alto, Calif.

22 Filed:' Mans/1973 21 Appl. No.:339,444

[52] US. Cl 315/36, 315/35, 330/43 [51] Int. Cl. HOlj 25/34 [58] Field of Search 315/35 X, 3.6; 30/43 [56] References Cited UNITED STATES PATENTS 6/1959 l-Ieffner et al 315/36 10/1960 Currie et al..'..... 3l5/3.6

3,289,032 11 1966 Rubertet a1 11.. 3l5/3.6 3,292,033 12 1966 Kenmoku 315 36 3,532,926 10 1970 Winslow sis/3.5

A microwave beam tube is arranged such that an upstream driver portion of the microwave circuit is severed for radio frequency energy from a downstream output microwave circuit portion. The driver section is dimensioned and arranged and driven with microwave energy such that the signal component of beam current has a peak value at the output end of the driver section which is not less than 0.7 times the dc. beam current. This modulated beam current then excites a signal wave on the output microwave circuit portion which is amplified thereon and coupled to a load. The output microwave slow wave circuit portion is a fundamental space harmonic forwardwave circuit, such as a cloverleaf circuit, centipede circuit or long slot coupled cavity circuit. The output circuit portion has a small signal gain of not more than 13 db to obtain increased RF conversion efficiency and stability for the tube.

10 Claims, 2 Drawing Figures MICROWAVE TUBEI-IAVING AN IMPROVED OUTPUT SECTION BACKGROUND -OF THE INVENTION DESCRIPTION THE PRIOR ART Heretofore, high power microwave amplifier tubes have been proposed wherein a klystron driver section was disposed upstream of a traveling wave tube output section. The driver section velocityand current density modulated the beam with signal wave energy to be amplified. The current density modulated beam then passed from the output of the driver section into the slow wave output circuit section. The modulated beam excited a signal wave on the output circuit for exciting a growing signal wave thereon.v The signal wave cumulatively interacted with the modulated beam to produce an amplified output signal that was extracted from the downstream end of the slow wave output circuit. Such a microwave amplifier tube is disclosed and claimed in US. Pat. No. 3,289,032 issued Nov. 29, 1966 and assigned to the same assignee as the present invention.

It has been conventional in these prior art tubes that the signal component of beam current, at the output of the driver section with saturated drive, have a peak amplitude of approximately 0.4 times the dc. beam current. Also, the slow wave output circuit section typically had again of not less than 26 db in order to achieve reasonable RF conversion efficiency for the microwave tube. It has been shown in the prior art that for a severed traveling waveftube, i.e. a driver section severed for RF from an output slow wave circuit, that the gain of the output section shouldbe not less than 26. db to achieve reasonableRF conversion efficiencies. Such a conclusion is reached in an article titled Why a Circuit Sever Affects Traveling Wave Tube Efficiency, appearing in the IRE Transactions on Electron Devices in the January, 1962 issue, pages -40.

comprising 5 percent aluminum, 22 percent chromium,

0.5 percent cobalt and the balance iron. This lossy coating serves the purpose ofitheloading down certain undesired modes of oscillation to prevent resonance thereof. r

Inaddition, loops and other lossy members have been provided in certain of the coupled cavities of the output slow wave circuit portion for coupling to certain unde sired modesassociated with band edge oscillations.

It would be desirable to provide an improved'output circuit section which eliminates the requirement for gss -asv k-fl stance,- and qne'whicjh 'is. shorter; in overall-length:su'c'h coating a preponderanceof its length with,

as to reduce the size of the beam focusing magnet structure.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved microwave tube.

In one feature of the present invention, the microwave interaction circuit of the tube includes an upstream driver portion severed for propagation of microwave energy therealong from a downstream output slow wave circuit portion, such output slow wave circuit portion having an overall interaction length sufficiently short such as to produce not more than 13 db of small signal gain therein, whereby increased RF conversion efficiency and stability for the microwave tube are obtained.

In another feature of the present invention, the upstream severed driver portion of the microwave circuit modulates the beam prior to passing into the output slow wave circuit portion with signal wave energy such that the peak beam current at the microwave signal frequency is not less than 0.7 times the value of do beam current when the tube is driven to saturation.

In another feature of the present invention, an output coupled cavity slow wave circuit portion of the microwave interaction circuit is severed from the upstream portion of the circuit and wherein at least 75 percent of the coupled cavities in the output section are free of lossy coating material ,over their interior surfaces, whereby fabrication of the tube is facilitated.

' 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 longitudinal schematic sectional view of a prior art microwave tube, and

FIG. 2 is a detail view of that portion of the circuit of FIG. 1 delineated by line 2.2 and depicting the improved output circuit portion of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown the prior art microwave amplifier tube 11. Microwave tube 11 is of the type disclosed andv claimed in the aforecited US. Pat. No. 3,289,032 which is hereby incorporated by reference for a complete description thereof. Briefly, tube 11 includes anelectron gun assembly 12 for forming and projecting a beam of electrons 13 over an elongated beam path to a beam collector structure 14. A microwave circuit 15 is disposed along the beam path 13 for electromagnetic interaction with the beam to produce an'amplified output signal'which is extracted via a conventional output coupler 16, such as a waveguide, disposedat the downstream end of the microwave circuit 15.

The electron gun 12-includes a thermionic cathode emitter button 17 having a spherically concave cathode emitting surface 18 axially aligned with a central'aper ture in an anode 19 for drawing a convergentflow of:

electrons from the emitter surface 1,8'through the anode 1-9. The cathode emitter 17- is heated to thermifoniccathode emitting temperature via a filamentary cathode heater 2lsupplied with power from a' power supply 22. An anode supply 23 is connected between the cathode l7 and the anode 19 for applying beam voltage to the anode 19 relative to the cathode 17. A beam focus solenoid 24 coaxially surrounds the microwave circuit for producing an axially directed magnetic field for focusing the beam 13 through the interaction circuit 15. An evacuated envelope encloses the electron gun 12, microwave circuit 15, and beam collector 14.

The microwave circuit 15 includes an upstream driver section 27 axially separated along the beam path from a downstream microwave slow wave output circuit portion or output circuit 28. The output circuit 28 is severed from the upstream circuit 27 such that, in the absence of the electron beam 13, no microwave signal energy will propagate along the microwave circuit from the upstream circuit portion 27 into the downstream or output circuit portion 28.

The upstream or driver circuit portion 27 comprises a plurality of cavity resonators 29 sequentially disposed along the beam path 13 and separated by RF field free drift regions 31 wherein the velocity modulation imparted to the beam by the succeeding upstream resonator or resonators is converted into current density modulation by the drifting of the electrons in the field free regions between successive resonators.

Signal wave energy to be amplified is fed into the upstream resonator 29' and the electromagnetic fields of the excited input resonator 29' serve to velocity modulate the beam. The velocity modulation on the beam is converted into current density modulation in the first drift space; this in turn excites the second resonator 29" which is a floating resonator, i.e. there is no external RF drive for the resonator 29". The succeeding floating resonators are excited in the same manner and serve to further velocity modulate the beam such that at the output of the driver section 27'the beam is current density modulated with signal wave energy.

In the prior art, the driver section 27 was normally arranged such that at the output end of the driver section, the beam was current density modulated with signal wave energy to an extent such that the signal current component of the dc. beam current I,,, was approximately 0.4 I,,,. Y

The current density modulated beam passes into the severed output slow wave circuit portion 28 wherein the signal energy current component of the beam excites a wave on the slow wave circuit 28 at the signal frequency. The signal wave on the circuit cumulatively interacts with the beam, as it travels along the slow wave circuit 28, to produce a greatly amplified output signal at the terminal end of the slow wave circuit 28. The output wave energy is coupled off of the circuit 28 via RF coupler 16 to the load.

Output section 28, in the prior art, typically has a gain in excess of 26 db for the reasons previously cited in the aforecited article in the IRE Transactions on Electron Devices. In order to prevent unwanted RF oscillation, the upstream two thirds of the coupled cavities of the cloverleaf circuit 28 was coated with a lossy material, namely, Kanthal alloy A, as indicated in the drawing by the letter K in several of the coupled resocommunicating between adjacent resonators of the slow wave circuit, and the 5H mode of resonance at 2 fl, corresponding to an undesired harmonic of the coupled resonators. It was generally considered, in order for the tube to have reasonable efficiency, that the output section 28 had to have a gain of at least 26 db. This is taught in the aforecited IRE Transactions article.

Referring now to FIG. 2, there is shown the output circuit portion 28 of the present invention. In the tube of the present invention, the output circuit 28' of FIG. 2 is substituted for the longer output section 28 of FIG. 1. Output section 28 is dimensioned and arranged such that the small signal gain of the output slow wave circuit portion 28 is not greater than 13 db and preferably less than 10 db but greater than 3 db. Typically, this means that the output circuit, when it comprises a coupled cavity forward wave fundamental space harmonic circuit, includes less than 13 and more than four coupled cavity resonators 35.

In addition, the driver section 27 is arranged and driven so that the signal current component I of the beam current at the exit of the drive section 27 has a peak amplitude equal to or greater than 0.7 times the dc. beam current I and preferably equal to or greater than 0.8 I This condition is determined when the entire tube is driven to saturation. It is the short (in time and distance), intense interaction between the .beam and the field provided by this low gain output circuit 28' that provides a significant improvement in efficiency, i.e. 10 percent or more.

Due to the lower gain of the output section 28, only approximately l/ 10th of the amount of Kanthal or lossy coating material is required for stable operation of the output section 28. Accordingly, the interior surfaces of one of the intermediate resonators 35 is coated with Kanthal. The benefits of the improved output circuit section 28' include, increased RF conversion efficiency, greaterstability due to the relatively low gain of the output section, and less costly manufacturing due mainly to the significant reduction or complete elimination of the custom application of lossy material to the interior surfaces of the output circuit 28 and because the overall length of the tube can be reduced, thereby decreasing the size and current requirements of the beam focusing solenoid 24.

Although the output section 28 has been described as a cloverleaf negative mutual inductively coupled cavity slow wave circuit to achieve a fundamental forward wave space harmonic circuit, other slow wave circuits are also suitable. More particularly, one such suitable circuit includes the centipede circuit, which is another version of a negative mutual inductively coupled cavity circuit having a forward fundamental space harmonic mode and is disclosed in U.S. Pat. No. 3,532,924 issued Oct. 6, 1970 and assigned to the same assignee as the present invention. The output circuit 28' may also comprise a long slot circuit described in Stanford University Microwave Laboratory, W. W. Hansen Laboratory of Physics, First and Second Annual Report for the period July, 1958 to June, 1960 titled Development of High Power Broad Band Tubes and Related Studies under Air Force Contract AF 30l(602)-1844 published January, 1961 at pages 93l24. Although a preferred embodiment of the present'invention'uses a klystron type driver section 27, other driver sections may be employed'which are capable of'achieving'the same degree of signal modulation of the beam, i.e. greater than 0.7 l

What is claimed is:

1. In a-microwave tube:

electron gun means for forming and projecting a stream of electrons over a predetermined beam path; microwave circuit means disposed along said beam path for electromagnetic interaction with the stream of electrons to develop output signal wave energy on said microwave circuit means, said microwave circuit means including, an output circuit portion, a driver portion disposed upstream of said output portion and a circuit sever portion disposed intermediate said driver and output portions for attenuating flow of wave energy therebetween;

output coupler means for coupling said output signal wave energy from said output circuit portion for use externally of the microwave tube; and

said output circuit portion being comprised of cavity resonators, and having an interaction length with said stream of electrons which is less than that length which will yield 13 db of small signal gain in said output circuit portion.

2. The apparatus of claim 1 wherein said driver portion of said microwave circuit has sufficient interaction length and gain to bunch the beam of electrons sufficiently such that theRF signal current in the beam when the tube is driven to saturation has a peak value which is at least 0.7 times the dc. beam current.

3. The apparatus of claim 1 wherein said driver portion of said microwave circuit includes a plurality of cavity resonators spaced apart along the beam path, and a plurality of microwave field free drift spaces spaced apart along the beam path and disposed intermediate the adjacent cavity resonators.

4. The apparatus of claim 2 wherein said output microwave circuit portion comprises a slow wave circuit.

5. The apparatus of claim 4 wherein said output microwave slow wave circuit portion comprises a coupled cavity forward wave fundamental space harmonic circuit.

6. The apparatus of claim 4 wherein said output microwave slow wave circuit portion comprises a circuit selected from the class consisting of, cloverleaf, centipede, and long slot coupled cavity circuits.

7. The apparatus of claim 5 wherein said output circuit portion includes less than 13 and more than 4 coupled cavity resonators.

8. The apparatus of claim 7 wherein at least percent of said coupled cavity resonators are free of a coating of lossy material over their interior surfaces for attenuating wave energy therein.

9. in a method of operating a microwave beam tube of the type wherein a stream of electrons is projected through a microwave circuit to produce an output microwave signal the steps of:

interacting the electron stream wih electromagnetic fields of signal wave energy of sufficient amplitude over a sufficient interaction length to velocity and current density modulate the beam such as to produce at a first position intermediate the length of the beam path a peak beam current modulation component at the signal frequency of at least 0.7 times the dc. beam current when the tube is driven to saturation,

interacting the signal modulated beam downstream of said first position with an output microwave slow wave circuit having a small signal gain of less than 13 db to produce output signal wave energy on said output slow wave circuit; and

coupling said output signal wave energy off of said output slow wave circuit. 7

10. The method of claim 9 including the step of:

terminating the upstream end of said output slow wave circuit in a lossy attenuator for attenuating backward traveling waves on said output slow wave circuit.

Claims

1. In a microwave tube: electron gun means for forming and projecting a stream of electrons over a predetermined beam path; microwave circuit means disposed along said beam path for electromagnetic interaction with the stream of electrons to develop output signal wave energy on said microwave circuit means, said microwave circuit means including, an output circuit portion, a driver portion disposed upstream of said output portion and a circuit sever portion disposed intermediate said driver and output portions for attenuating flow of wave energy therebetween; output coupler means for coupling said output signal wave enerGy from said output circuit portion for use externally of the microwave tube; and said output circuit portion being comprised of cavity resonators, and having an interaction length with said stream of electrons which is less than that length which will yield 13 db of small signal gain in said output circuit portion.

2. The apparatus of claim 1 wherein said driver portion of said microwave circuit has sufficient interaction length and gain to bunch the beam of electrons sufficiently such that the RF signal current in the beam when the tube is driven to saturation has a peak value which is at least 0.7 times the d.c. beam current.

8. The apparatus of claim 7 wherein at least 75 percent of said coupled cavity resonators are free of a coating of lossy material over their interior surfaces for attenuating wave energy therein.

9. In a method of operating a microwave beam tube of the type wherein a stream of electrons is projected through a microwave circuit to produce an output microwave signal the steps of: interacting the electron stream wih electromagnetic fields of signal wave energy of sufficient amplitude over a sufficient interaction length to velocity and current density modulate the beam such as to produce at a first position intermediate the length of the beam path a peak beam current modulation component at the signal frequency of at least 0.7 times the d.c. beam current when the tube is driven to saturation, interacting the signal modulated beam downstream of said first position with an output microwave slow wave circuit having a small signal gain of less than 13 db to produce output signal wave energy on said output slow wave circuit; and coupling said output signal wave energy off of said output slow wave circuit.

10. The method of claim 9 including the step of: terminating the upstream end of said output slow wave circuit in a lossy attenuator for attenuating backward traveling waves on said output slow wave circuit.