US3668544A - High efficiency traveling wave tube employing harmonic bunching - Google Patents

Abstract

A slow wave tube is disclosed wherein the signal to be amplified and an harmonic thereof are applied concurrently over at least a portion of the slow wave circuit to increase the RF conversion efficiency of the tube. In a preferred embodiment, the harmonic wave energy is extracted from the output of the tube and fed back onto the slow wave circuit. The slow wave circuit portion, which provides the harmonic interaction with the beam, preferably has a dispersive characteristic such that the harmonic wave energy has a higher phase velocity than the fundamental wave energy for enhanced RF conversion efficiency.

Description

United States Patent Lien June 6, 1972 54] HIGH EFFICIENCY TRAVELING WAVE 3,243,735 3/1966 Gross ..315/3.5 x TUBE EIMPLOYING HARMONIC 3,335,314 8/1967 Espinosa et a1.. ...315/3.5 X CHI 3,366,897 1/1968 Konrad ..315/3.5 X BUN NG 3,448,330 6/1969 Farney ..3l5/3.6 [72] Inventor: Erling L. Lien, Los Altos, Calif.

Primary Examiner-Herman Karl Saalbach [73] Asslgnee' vanan Assoclates Palo Alto Cahf' Assistant Examiner-Saxfield Chatmon, Jr. [22] Filed; Sept 3, 7 Attorney-Stanley Z. Cole 1 1 pp 69,295 57 ABSTRACT A slow wave tube is disclosed wherein the signal to be am-' [52] U.S. Cl ..330/43, 315/35 plified and an harmonic thereof are applied concurrently over [51] Int. Cl. ..H0lj 25/34 at least a portion of the slow wave circuit to increase the RF [58] Field of Search ..315/3.5 X, 3.6, 393; 330/43 nversion fficien y f h tube- I a preferred im n the harmonic wave energy is extracted from the output of the 5 R f r Cited tube and fed back onto the slow wave circuit. The slow wave circuit portion, which provides the harmonic interaction with UNITED STATES PATENTS the beam, preferably has a dispersive characteristic such that the harmonic wave energy has a higher phase velocity than the 3,128,433 4/1964 Edson ..315/3-.5 X fundamental wave energy for enhanced RF conversion effb 2,829,252 4/1958 Bryant ..315/3.5 X ciency 2,681,951 6/1954 Warnecke et al .....315/3.5 X 2,811,664 10/1957 Kazan ..315/3.6 9Claims, 4Drawing Figures FATENTEDJUH 61972 3, 668 544 L L K 24 ELECTRON ELECTRON ELECTRON M 22 BUNCH 22 BUNCH 22 BUNCH INVENTOR.

ERLING L. LIEN Sw-EFW ATTORNEY HIGH EFFICIENCY TRAVELING WAVE TUBE EMPLOYING HARMONIC BUNCI-IING DESCRIPTION OF THE PRIOR ART Heretofore, it has been proposed that the efiiciency of a slow wave tube could be enhanced at the fundamental frequency by driving the slow wave circuit with the second harmonic of the output signal having a suitably adjusted amplitude and phase for counteracting undesired harmonic interaction in the tube. In the prior art, the coherent harmonic signal for driving the slow wave circuit was obtained from the output of a driver tube in a preceding stage which also served as a preamplifier for the tube which was to have the improved RF efficiency. Thus, it is known from the prior art to apply second harmonic energy simultaneously with the fundamental energy to be amplified onto the slow wave circuit of a traveling wave tube to counteract the normal second harmonic interaction in the beam to improve the efiiciency, particularly at the low frequency end of a broadband traveling wave tube. Such a broadband tube is described in an article titled, "Ultra- Broadband TWT Power Amplifiers Without Harmonic Capture," appearing in a paper delivered at the lntemational Electron Device meeting in Washington, D. C. in October, 1965.

It is also known from the prior art that the conversion efficiency of a klystron amplifier tube can be substantially increased by prebunching the beam with second harmonic ener gy. Such an improved klystron amplifier is disclosed and claimed in copending U. S. Pat. applications Ser. No. 767,774 filed Oct. 15, I968; Ser. No. 20,791 filed Apr. 15, 1970; and Ser. No. 28,792 filed Apr. 15, 1970, all assigned to the same assignee as the present invention.

- SUMMARY OF THE INVENTION The principal object of the present invention is the provision of an improved traveling wave tube amplifier having increased RF conversion efficiency.

One feature of the present invention is the provision, in a traveling wave tube amplifier, of harmonic feedback means coupled to the output of the slow wave circuit for coupling harmonic energy from the output of the tube back on to the slow wave circuit for additional electromagnetic interaction with the beam to increase the RF conversion efficiency of the tube.

Another feature of the present invention is the provision, in a traveling wave tube, of means for coupling onto the slow wave circuit harmonic wave energy for harmonic interaction with beam, such slow wave circuit having a dispersive characteristic such that the phase velocity of the harmonic wave energy exceeds the phase velocity of the fundamental wave energy, whereby the conversion efiiciency of the tube is increased at the fundamental frequency.

Another feature of the present invention is the same as any one or more of the preceding features including the provision of an input circuit disposed upstream of the second harmonic interaction circuit, such input slow wave circuit portion being terminated in a resistive load such as to be severed from the second harmonic interaction circuit disposed downstream of the input circuit.

Another feature of the present invention is the same as any one or more of the preceding features wherein the second harmonic interaction slow wave circuit includes a conductive sheath surrounding the interaction circuit and capacitive loading means, such as conductive vanes, extending from the sheath towards the slow wave circuit for producing a dispersive characteristic wherein the harmonic wave energy travels at a faster phase velocity than the fundamental wave energy.

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:

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic line diagram depicting a traveling wave tube incorporating features of the present invention,

FIG. 2 is a plot of axial electric field for the fundamental and second harmonic waves as a function of distance along the beam path and depicting the relative positions of the electron bunches and the electric fields for three different regions within the second harmonic interaction region of the tube of FIG. 1,

FIG. 3 is a dispersion diagram depicting the dispersive characteristics of the second harmonic interaction circuit of FIG. 1, and

FIG. 4 is a transverse sectional view of the structure of FIG. 1 taken along line 4-4 in the direction of the arrows and depicting, in schematic line diagram form, the slow wave circuit and surrounding caPacitively loaded sheath.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a traveling wave tube 1 incorporating features of the present invention. The traveling wave tube 1 includes an electron gun assembly 2 disposed at one end of the tube for formingv and projecting a beam of electrons 3 over an elongated beam path to a beam collector structure 4.

An input slow wave circuit portion 5 is disposed near the upstream end of the beam 3. The input slow wave circuit may comprise any one of a number of different kinds of slow wave circuits, such as a helix, a cross-wound helix, a ring and bar circuit, a succession of floating buncher cavities, a coupled cavity circuit, a cloverleaf circuit, or the like. The input end of the slow wave circuit 5 includes RF coupling means 6 for coupling wave energy to be amplified at a fundamental frequency onto the input of the slow wave circuit 5. The downstream end of the slow wave circuit 5 is terminated with a refiectionless wave attenuative element 7 such as a resistive card. Termination 7 serves as a circuit sever for severing the input circuit 5 from the remaining portion of the slow wave circuit of the tube.

A second harmonic interaction slow wave circuit 8 is disposed downstream from the input circuit 5. The second harmonic interaction circuit 8 may comprise any one of a number of different types of slow wave circuits, such as a helix, a cross-wound helix, a bifilar helix, a ring and bar circuit, a cloverleaf circuit, a coupled cavity circuit, etc. An output coupler 9 is disposed at the downstream end of the second harmonic circuit 8 for coupling output energy from slow wave circuit 8 to a utilization device, such as an antenna, not shown.

A power splitter l 1 is provided at the end of the output circuit 8, near the output coupler 9, for selectively coupling out of the output wave energy the energy which is harmonically related to the fundamental energy being amplified on the circuit. A feedback circuit, including a series connection of a variable attenuator l3 and a variable phase shifter 14, feeds the harmonic wave energy back to the input or upstream end of the second harmonic interaction circuit 8.

The second interaction circuit 8 is dimensioned for cumulative interaction with the beam at the fundamental frequency of wave energy to be amplified. The circuit is also capable of interacting with the beam at the second harmonic or at higher harmonics of the fundamental frequency. In a preferred embodiment, the second harmonic interaction circuit 8 has a negative dispersion characteristic as shown by the solid curve 15 of FIG. 3, such that the phase velocity of the second harmonic and higher harmonics is substantially higher than the phase velocity for the fundamental wave on the circuit 8. In the typical example of a helix slow wave circuit, the normal dispersive characteristic for the helix as enclosed in a sheath is as depicted by the dashed line 16 of FIG. 3, such curve 16 having a positive dispersive characteristic. The positive dispersive characteristic 16 is converted to a negative dispersive characteristic as depicted by curve 15 by providing capacitive loading of the helix to the sheath.

Referring now to FIG. 4, there is shown a capacitively loaded helix slow wave circuit for producing the negative dispersive characteristic, as depicted by curve 15 of FIG. 3.

More specifically, the helix 8 is surrounded by conductive sheath 19, as of copper, to which a plurality of conductive fins 21 are affixed and which project from the surrounding sheath 19 toward the helix 8 to provide capacitive loading between the sheath and the helix. The conductive fins 21 extend longitudinally of the sheath as well as projecting inwardly therefrom.

Referring now to FIG. 2, the bunching action produced by the second harmonic interaction with the electron beam is depicted. More specifically, at the upstream end of the second harmonic interaction circuit 8, in the region identified as A, the electron bunches are phased relative to the fundamental electric field of the fundamental wave on the circuit such that the electron bunches are located substantially at the node of the fundamental electric field, as depicted in that portion of FIG. 2 identified as A. The second harmonic wave which is fed onto the interaction circuit 8 is preferably phased, in the upstream region A, such that the second harmonic is in phase with the fundamental. This produces a composite wave as shown by the sawtooth wave 24. This improves the effectiveness of the bunching by removing undesired electrons from the interbunch region.

Due to the dispersive characteristics of the circuit 8, the second harmonic wave advances in phase relative to the phase of the fundamental and the electron bunches move slightly ahead of the fundamental as shown in the region identified by region B of FIG. 2. Region B corresponds to that portion of the circuit identified as B in FIG. 1. In region B, the electron bunches are delivering energy to both the fundamental and second harmonic components of the electric field on the circuit. The fundamental component of energy delivered from the beam to the circuit creates the growing fundamental wave on the circuit 8 and the energy delivered at the second harmonic from the electron bunches to the circuit provides a second harmonic wave which will be extracted from the circuit to provide the second harmonic drive in region A.

Referring now to that portion of FIG. 2 identified by C, which corresponds to the region of circuit 8 identified by c, in this region the electron bunch continues to deliver energy to the fundamental wave 22, thereby further contributing to the growing fundamental wave on the circuit. In addition, the second harmonic wave 23 has new advanced to a position tending to rebunch the electrons into the electron bunch to compensate for space charge forces tending to spread the electron bunch.

Thus, it is seen that the second harmonic energy which is fed back onto the second harmonic interaction circuit 8 substantially improves the electron bunching and significantly contributes to increased RF conversion efi'iciency.

As an alternative use of the severed circuit, the slow wave circuit may be continuous with the feedback being applied at the input end of the tube. In addition, it is possible to derive the coherent second harmonic or higher harmonic wave from the preamplifier or driver tube and to feed the second harmonic wave energy onto the circuit, at the upstream end of circuit 8 or at the upstream end of the slow wave circuit.

The phase shifter 14 and attenuator 13 are adjusted to obtain the proper phasing of the harmonic wave energy with the fundamental to obtain optimum RF conversion efficiency.

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:

1. In a slow wave tube, means for projecting a beam of electrons over an elongated beam path, slow wave circuit means disposed along the beam path for electromagnetic interaction with the beam to produce output wave energy of a fundamental frequency and of harmonics of the fundamental frequency, said harmonics being integral multiples of said fundamental frequency, output coupling means for extracting the output wave energy from the tube, the improvement comprising,

harmonic feedback means coupled to said output coupling means for coupling out of the output wave energy at least a portion of the harmonic energy and for feeding the harmonic wave energy back onto said slow wave circuit means at a feedback point upstream of said output coupling means for electromagnetic interaction with the beam to increase the RF conversion efficiency of the tube at the fundamental frequency. 2. The apparatus of claim 1 wherein said harmonic feedback means includes, variable phase shifting means for variably shifting the phase of the harmonic energy fed back to said circuit means.

3. The apparatus of claim 1 wherein said harmonic feedback means includes, variable attenuator means for attenuating the harmonic energy fed back to said circuit means.

4. The apparatus of claim 1 wherein said slow wave circuit means includes a portion disposed intermediate said feedback point and said output coupling means which has a dispersive characteristic wherein the phase velocity of the harmonic wave energy substantially exceeds .the phase velocity of the fundamental wave energy for enhancing the RF conversion efficiency of the tube at the fundamental frequency.

5. The apparatus of claim 4 wherein said slow wave circuit portion disposed intermediate said feedback point and said output coupling means comprises, conductive sheath means surrounding said slow wave circuit means in spaced relation therefrom, and capacitive loading means extending from said surrounding conductive sheath means toward said slow wave circuit for producing the aforementioned dispersive characteristic of said slow wave circuit portion.

6. The apparatus of claim 1 wherein said slow wave circuit means includes an input slow wave circuit portion disposed along the beam path upstream of said feedback point, and means for applying wave energy at the fundamental frequency to said input circuit portion.

7. The apparatus of claim 6 including, circuit sever means at the downstream end of said input circuit means for terminating said input slow wave circuit portion.

8. In a slow wave tube, means for projecting a beam of electrons over an elongated beam path, slow wave circuit means disposed along the beam path for electromagnetic interaction with the beam to produce output wave energy of a fundamental frequency and of harmonic frequencies of the fundamental frequency, output coupling means for extracting the output wave energy of the tube, the improvement comprising,

means for applying wave energy which is an integral multiple of the fundamental frequency to said slow wave circuit means over a harmonic interaction region for harmonic electromagnetic interaction with the beam, and

wherein said harmonic interaction region of said slow wave circuit has a dispersive characteristic such that the phase velocity of the harmonic wave energy exceeds the phase velocity of the fundamental wave energy to increase the RF conversion energy of the tube at the fundamental frequency.

9. The apparatus of claim 8 including, conductive sheath means surrounding said slow wave circuit means in spaced relation therefrom, and capacitive loading means extending from said surrounding conductive sheath means toward said slow wave circuit for producing the aforementioned dispersive characteristic of said slow wave circuit portion.

Claims (9)

1. In a slow wave tube, means for projecting a beam of electrons over an elongated beam path, slow wave circuit means disposed along the beam path for electromagnetic interaction with the beam to produce output wave energy of a fundamental frequency and of harmonics of the fundamental frequency, said harmonics being integral multiples of said fundamental frequency, output coupling means for extracting the output wave energy from the tube, the improvement comprising, harmonic feedback means coupled to said output coupling means for coupling out of the output wave energy at least a portion of the harmonic energy and for feeding the harmonic wave energy back onto said slow wave circuit means at a feedback point upstream of said output coupling means for electromagnetic interaction with the beam to increase the RF conversion efficiency of the tube at the fundamental frequency.

2. The apparatus of claim 1 wherein said harmonic feedback means includes, variable phase shifting means for variably shifting the phase of the harmonic energy fed back to said circuit means.

3. The apparatus of claim 1 wherein said harmonic feedback means includes, variable attenuator means for attenuating the harmonic energy fed back to said circuit means.

4. The apparatus of claim 1 wherein said slow wave circuit means includes a portion disposed intermediate said feedback point and said output coupling means which has a dispersive characteristic wherein the phase velocity of the harmonic wave energy substantially exceeds the phase velocity of the fundamental wave energy for enhancing the RF conversion efficiency of the tube at the fundamental frequency.

5. The apparatus of claim 4 wherein said slow wave circuit portion disposed intermediate said feedback point and said output coupling means comprises, conductive sheath means surrounding said slow wave circuit means in spaced relation therefrom, and capacitive loading means extending from said surrounding conductive sheath means toward said slow wave circuit for producing the aforementioned dispersive characteristic of said slow wave circuit portion.

6. The apparatus of claim 1 wherein said slow wave circuit means includes an input slow wave circuit portion disposed along the beam path upstream of said feedback point, and means for applying wave energy at the fundamental frequency to said input circuit portion.

7. The apparatus of claim 6 including, circuit sever means at the downstream end of said input circuit means for terminating said input slow wave circuit portion.

8. In a slow wave tube, means for projecting a beam of electrons over an elongated beam path, slow wave circuit means disposed along the beam path for electromagnetic interaction with the beam to produce output wave energy of a fundamental frequency and of harmoniC frequencies of the fundamental frequency, output coupling means for extracting the output wave energy of the tube, the improvement comprising, means for applying wave energy which is an integral multiple of the fundamental frequency to said slow wave circuit means over a harmonic interaction region for harmonic electromagnetic interaction with the beam, and wherein said harmonic interaction region of said slow wave circuit has a dispersive characteristic such that the phase velocity of the harmonic wave energy exceeds the phase velocity of the fundamental wave energy to increase the RF conversion energy of the tube at the fundamental frequency.

9. The apparatus of claim 8 including, conductive sheath means surrounding said slow wave circuit means in spaced relation therefrom, and capacitive loading means extending from said surrounding conductive sheath means toward said slow wave circuit for producing the aforementioned dispersive characteristic of said slow wave circuit portion.

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