US3594606A

US3594606A - Velocity modulation tube employing cascaded harmonic prebunching

Info

Publication number: US3594606A
Application number: US28791A
Authority: US
Inventors: Erling L Lien
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1970-04-15
Filing date: 1970-04-15
Publication date: 1971-07-20
Anticipated expiration: 1988-07-20
Also published as: SE364804B; DE2117925A1; CA920271A; FR2089690A6; GB1351798A; DE2117925C3; DE2117925B2; JPS5612987B1

Abstract

A velocity modulation microwave tube is disclosed. The tube includes an input circuit, an output circuit and a penultimate resonator circuit disposed along the beam. A pair of second harmonic floating prebuncher resonators are disposed along the beam path intermediate the input circuit and the penultimate resonator. The second harmonic resonators improve the bunching of the beam by moving electrons from the interbunch region of the beam into the bunched region of the beam. A resonator tuned for a fundamental mode of resonance at the fundamental frequency of the tube is interposed between the pair of second harmonic resonators for rebunching the electron bunches of the beam downstream of the first second harmonic cavity. The second harmonic resonators with the fundamental resonator interposed therebetween substantially improve the conversion efficiency of the microwave tube.

Description

United States Patent [72] Inventor Erliug L. Lien 2.605,444 7/1952 Garbuny 315/543 [m Altos, Calif. 3,195,007 7/1965 Watson et a] 315/543 N 970 Primary Examiner-Herman Karl Saalbach gg 971 Assistant Examiner-Saxfield Chatmon, Jr.

3 s [73] Assignee v A Attorneys Stanley Z. Cole Palo Alto, Calif.

ABSTRACT: A velocity modulation microwave tube is dis- VELOCITY MODULATION TUBE m I" DYING closedd Thettube include: an nput rcumdanloutpi: (:ICUII CASCADE HARMONIC PREBUNCHING an a penu ima e resona or circuit ispose a ong t e earn. 7 cm 5 D" A pair of second harmonic floating prebuncher resonators are g disposed along the beam path intermediate the input circuit [52] US. Cl 315/543, and the penultimate resonate The econd h rmo ic resona- 31 /5-5 5/ 5 tors improve the bunching of the beam by moving electrons [51] Int. Cl H01j25/12 from the inter-bunch region of the beam into the bunched e- [50] Field 01 Search 315/539, gion of the beam, A resonator tuned for a fundamental mode 5.52 of resonance at the fundamental frequency of the tube is inter posed between the pair of second harmonic resonators for l56l References CM rebunching the electron bunches of the beam downstream of UNITED STATE PA E the first second harmonic cavity. The second harmonic 2,414,843 l/ 1947 Varian et a1. 315/552 X eso to s ith the fundamental resonator interposed 2,494,721 1/1950 Robertson et al... 3 15/543 therebetween substantially improve the conversion efficiency 2,579,480 12/1951 Feenberg 3 1 5/5 .43 of the microwave tube.

1 PRE BUNCHER7 J FINAL BUNCHER I *1 7 P STAGE Z STAGE /|5 A A i 1 3 f I I l 8 -I 6 9 H r r r T+ {1 PATENTED JUL20 I97! OUTPUT FIG. I

+ FINAL OUNCHER PRE BUNCHER 7 PRIOR ART R OUT PUT J FINAL BUNCHE PRE BUNCHER 7 FIG. 4

CASCADED HARMONIC PRE BUNCHERS INVENTOR. ERLING L. LIEN BY &Q. %Q2L ATTORNEY TIME BUNOHER VOLTAGE l.O -3O '20 I0 0 NORMALTZED VELOCITY DEVIATION FOR OWEST ELECTRON IN THE BUNCH 0.75 RAD. 3L

LOO RAD.

VELOCITY MODULATION TUBE EMPLO YING CASCADED I'IARMONIC PREBUNCI-IING DESCRIPTION OF THE PRIOR ART Heretofore, a pair of cascaded second harmonic resonators have been disposed along the beam path intermediate the input resonator and the penultimate resonator for improving the bunching of the beam by relocating electrons from the interbunch region. This improved bunching of the beam has substantially improved the conversion efficiency of velocity modulation microwave tubes. This feature is disclosed and claimed in copending US. application Ser. No. 767,774, filed Oct. 15, I968, and assigned to the same assignee as the present invention. While the use of the second harmonic resonators substantially improves the conversion efficiency of the tube by relocating electrons from the interbunch region, in doing so the second harmonic resonators have a tendency to debunch the electron bunches. Therefore, it would be desirable to provide a second harmonic prebuncher of such a character that it relocates electrons from the interbunch region without tending to debunch the electron bunches.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved velocity modulation tube employing harmonic prebunchers.

One feature of the present invention is the provision, in a velocity modulation microwave tube, of a pair of cascaded harmonic prebunchers, each of such prebunchers including a fundamental mode resonator followed by a harmonic prebuncher resonator such that the fundamental resonator of the downstream second harmonic prebuncher stage rebunches the bunched electrons in the beam passing into the second hannonic prebuncher resonator of the second stage of harmonic prebunching, whereby the conversion efficiency of the microwave tube is substantially increased.

Another feature of the present invention is the same as the preceding feature wherein the pair of harmonic prebuncher resonators are tuned for the second harmonic of the tube.

Another feature of the present invention is the same as any one or more of the preceding features wherein the fundamental resonators and harmonic resonators of the cascaded harmonic prebunchers are cavity resonators.

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

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic line diagram of a multicavity klystron amplifier employing the prior art second harmonic prebuncher scheme,

FIG. 2 is a plot of buncher voltage versus time depicting the mode of operation of the second harmonic buncher voltage for removing electrons from the interbunch region,

FIG.'3 is a schematic line diagram of a velocity modulation electron tube incorporating the cascaded harmonic prebuncher stage of the present invention,

FIG. 4 is a plot of normalized radio frequency beam current versus normalized velocity deviation for slowest electron in the bunches and showing the characteristics of the prior art and of the present invention, and

FIG. is a plot of conversion efficiency versus normalized load conductance for the output resonator and showing the efficiency for three values of output interaction gap length.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. I, there is shown the prior art microwave klystron tube 1 incorporating cascaded second harmonic prebunchers as disclosed in the aforecited copending US. application Ser. No. 767,774. Briefly, the electron tube 1 includes an electron gun 2 for forming and projecting a beam of electrons 3 over an elongated beam path to a collector structure 4 disposed at the terminal end of the beam path. An input cavity resonator 5 is disposed at the upstream end of the electron beam 3 for velocity modulating the beam with microwave energy supplied to the cavity 5 via input coupler 6. An output cavity resonator 7 is disposed adjacent the collector 4 for interaction with the bunched electron beam to extract amplified output microwave energy from the beam. Output energy is extracted from the output resonator 7 via output coupling loop 8, and transmitted to a suitable utilization device, such as an antenna, not shown.

A pair of floating second harmonic cavity resonators 9 and 11 are disposed in the prebuncher section of the electron tube. The second harmonic cavities 9 and 11 are tuned for a fundamental mode of resonance at a frequency which is substantially twice the center passband frequency of the amplifier tube 1. As used herein, floating" is defined to mean a resonator which does not have any substantial source of energy external to the microwave tube 1 and which is not coupled to a load utilizing the output of the resonator; however, a circuit may be coupled to the floating resonator solely for effecting some electrical characteristic of the floating resonator such as its 0 or frequency.

The floating harmonic resonator 9 and the cavity resonator 5 function to produce a bunching voltage V having a periodic sawtooth waveform as shown in Fig. 2. More specifically, the input cavity resonator 5 produces a fundamental frequency component of bunching RF voltage V,. When a second harmonic voltage V is superimposed upon V the total bunching voltage V, has a periodic sawtooth waveform. Utilizing pure electron ballistics theory, and neglecting space charge forces, such a bunching waveform produces tight bunches in the output resonator. However, such bunching produces an undesired velocity spread in the electrons as they pass through the gap in the output resonator 7. In addition, in relatively high-perveance beam tubes, i.e., perveance greater than 10', space charge forces cannot be neglected and as a result the tight bunching which is desired may not be obtained in practice. The floating harmonic resonator ll cancels the second harmonic velocity modulation produced by resonator 9. By providing a final buncher section between the prebuncher section and the output, the electrons are grouped by the harmonic prebuncher for a more efficient modulation by the fundamental

frequency buncher cavities

12 and 13 as used in the final buncher section. In other words, the second harmonic prebuncher cavities 9 and 11 provide a beneficial phase grouping of the electrons at the entrance of the final buncher stage.

Final buncher cavities

12 and 13 are tuned for a fundamental mode of resonance near and above the passband of the tube and such resonators are of the floating type. Prior art tubes of the type shown in Fig. 1 (without resonator 11 present) provide a calculated R.F. conversion efficiency of approximately percent and have yielded 70 percent conversion efficiency in practice.

One of the problems with the prior art tube of Fig. l is that the second harmonic prebuncher cavities 9 and I1 tend to debunch the electron bunches. This adversely affects the conversion efficiency of the tube.

Referring now to Fig. 3, there is shown a multicavity klystron amplifier tube 15 incorporating features of the present invention. The tube 15 is substantially identical to the klystron tube 1 of Fig. l with the exception that an additional fundamental frequency resonator 16 has been disposed along the beam path intermediate the pair of second hannonic prebuncher resonators 9 and 11. The fundamental resonator 16 is tuned for a fundamental mode of resonance slightly above (no greater than 10 percent of the center frequency of the passband) the passband of the tube and serves to rebunch the electron bunches before such bunches see" the second hannonic prebuncher voltage produced by the second stage harmonic prebuncher cavity 11. More particularly, the first prebuncher stage includes a fundamental frequency input cavity followed by the first second harmonic cavity 9. The second harmonic cavity 9 serves to relocate the interbunch electrons but in doing so adversely affects the desired electron bunches tending to debunch same. However, fundamental frequency resonator 16 of the second prebuncher stage serves to rcbunch the electrons prior to their entering the downstream second harmonic resonator 11. The net result of utilizing two stages of second harmonic prebunching with a fundamental resonator between the pair of second harmonic resonators is to substantially increase the fundamental component of RP. beam current at the location of the output gap, to obtain a more uniform velocity of the electrons within the bunches and to obtain a more uniform electron distribution in the bunches. The increased fundamental component of the R.F. beam current versus improved velocity distribution within the bunches can be seen by reference to Fig. 4 wherein curve 18 shows a plot of normalized fundamental frequency RF. beam current versus normalized velocity deviation for the slowest electron in the bunches for various gap voltages across the gap of the penultimate resonator 12 in the tube of Fig. 3. Curve 19 shows a similar plot employing only a single stage of second harmonic prebunching, i.e., that is without the provision of a fundamental mode resonator between a pair of second harmonic prebunching resonators.

Referring now to Fig. 5, the calculated conversion efficiency for the tube 15 of Fig. 3 is shown as a function of the normalized load conductance of the output resonator. The plots of Fig. 5 represent an improvement in conversion efficiency of approximately 5 percent as compared to the conversion efficiency of the prior art tube as exemplified by tube 1 of Fig. 1.

In a typical example of the klystron tube 15 incorporating features of the present invention, the resonators are tuned as follows: the input resonator 5 is preferably tuned essentially to the center of the passband of the tube namely 8f/f=0 where f is the center frequency of the passband of the tube. The first second harmonic resonator 9 is preferably tuned to a frequency slightly less than twice the centerband frequency, namely, 6fl2f=0.0045; the second prebuncher fundamental frequency resonator 16 is preferably tuned to a frequency slightly above the passband frequency, namely, 8f/f=l-0.04; the second harmonic resonator 11 is preferably tuned to the same frequency as the first second harmonic resonator 9, namely 8f/2f=0.0045; the final buncher resonators l2 and 13 are preferably tuned to substantially the same frequency slightly above the centerband frequency, namely, 8flf=l-0.022, and the output resonator 7 is preferably tuned substantially to the frequency of the center of the passband, namely, 8f/f=0. As used herein, passband" is defined as that band of frequencies between 3 db. points in the power output versus frequency plot for the tube.

Although the embodiment of the present invention, as depicted in Fig. 3, has employed only a single-fundamental frequency resonator 5 upstream of the first second harmonic resonator 9, this is not a requirement. Additional fundamental frequency resonators may be provided along the beam path between the input resonator 5 and the first second harmonic resonator 9 for providing increased gain for the tube. In addition, it is not a requirement that the harmonic resonators 9 and 11 be tuned to the second harmonic of the center of passband of the tube. Such harmonic resonators may be tuned to higher frequency harmonic as the third, fourth, etc. However, it is definitely preferred that the harmonic cavities be tuned to the lowest frequency harmonic higher than the first (namely the second), since with harmonics higher than the second and especially at frequencies above S-band and at high-power levels, an inordinate proportion of the volume of the cavity is occupied by the beam.

Moreover, it is not a requirement that the resonator circuits employed in the tube of the present invention be reentrant cavity resonators. it is contemplated that other types of resonant circuits may be employed such as, distributed field helical resonators (either the single helix or cross-wound helix type may be emplo ed). Moreover, the output circuit may comprise, for examp e, a slow wave circuit or an extended interaction circuit formed by a plurality of coupled cavity resonators. In other words, the present invention is applicable not only to klystron amplifiers but to velocity modulation, microwave tubes in general which employ resonant structures disposed along the beam path for interaction with the beam.

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.

lclaim:

1. in a velocity modulation microwave tube apparatus having a certain operating passband of frequencies; means for projecting a stream of electrons over an elongated beam path; wave supportive means arranged successively along the beam path for successive electromagnetic interaction with the electron stream to produce output microwave energy; said wave supportive means including an input circuit, an output circuit, and a penultimate resonator circuit; said penultimate resonator circuit being tuned for a fundamental mode of resonance at a frequency above the passband of the tube, the frequency of said fundamental mode of resonance being less than twice the center passband frequency; said penultimate resonator circuit comprising at least one floating resonator for bunching the beam passing into said output circuit; a pair of hannonic floating resonator means spaced apart along the beam path intermediate said input circuit and said penultimate resonator circuit; said pair of harmonic resonator means each having a fundamental mode of resonance approximately at a frequency corresponding to a harmonic which is an integral multiple of the center frequency of the passband of the tube and a floating resonator means interposed along the beam path between said pair of harmonic resonator means and being tuned for a fundamental mode of resonance at a frequency approximately at the fundamental frequency of the passband of the tube, whereby the conversion efficiency of the tube is enhanced.

2. The apparatus of claim 1 wherein said pair of harmonic resonator means are tuned to frequencies approximately at the second harmonic of the center frequency of the passband of the tube, and wherein said resonator interposed between said pair of harmonic resonator means is tuned to a frequency above the passband of the tube.

3. The apparatus of claim 1 wherein said resonators are reentrant cavity resonators.

4. The apparatus of claim 1 wherein said pair of hannonic resonator means are each tuned for a fundamental mode of resonance approximately at the second harmonic of the center frequency of the passband of the tube.

5. The apparatus of claim 1 wherein said pair of harmonic resonator means are each tuned to a frequency which is twice a frequency within the passband of the tube but below the center frequency of the passband of the tube 6. The apparatus according to claim 5 wherein said resonator interposed between said pair of harmonic resonator means is tuned to a frequency above the passband of the tube.

7. The apparatus of claim 6 wherein said penultimate resonator is tuned to a frequency within the passband of the tube but above the center frequency of the passband of the tube.

Disclaimer 3,594,606.-Erlin L. Lien, Los Altos, Calif. VELOCITY MODULATION EMPLOYING OASCADED HARMONIC PREBUNCH- ING. Patent dated July 20, 1971. Disclaimer filed June 1, 1973, by the assignee, Vam'a n Associates. Hereby enters this disclaimer to claim 7 of said patent.

[Oflieial Gazette November 6, 1973.]

Claims

1. In a velocity modulation microwave tube apparatus having a certain operating passband of frequencies; means for projecting a stream of electrons over an elongated beam path; wave supportive means arranged successively along the beam path for successive electromagnetic interaction with the electron stream to produce output microwave energy; said wave supportive means including an input circuit, an output circuit, and a penultimate resonator circuit; said penultimate resonator circuit being tuned for a fundamental mode of resonance at a frequency above the passband of the tube, the frequency of said fundamental mode of resonance being less than twice the center passband frequency; said penultimate resonator circuit comprising at least one floating resonator for bunching the beam passing into said output circuit; a pair of harmonic floating resonator means spaced apart along the beam path intermediate said input circuit and said penultimate resonator circuit; said pair of harmonic resonator means each having a fundamental mode of resonance approximately at a frequency corresponding to a harmonic which is an integral multiple of the center frequency of the passband of the tube and a floating resonator means interposed along the beam path between said pair of harmonic resonator means and bEing tuned for a fundamental mode of resonance at a frequency approximately at the fundamental frequency of the passband of the tube, whereby the conversion efficiency of the tube is enhanced.

4. The apparatus of claim 1 wherein said pair of harmonic resonator means are each tuned for a fundamental mode of resonance approximately at the second harmonic of the center frequency of the passband of the tube.

5. The apparatus of claim 1 wherein said pair of harmonic resonator means are each tuned to a frequency which is twice a frequency within the passband of the tube but below the center frequency of the passband of the tube

6. The apparatus according to claim 5 wherein said resonator interposed between said pair of harmonic resonator means is tuned to a frequency above the passband of the tube.