US3886397A

US3886397A - Hybrid slow wave circuit

Info

Publication number: US3886397A
Application number: US432380A
Authority: US
Inventors: Yukio Hiramatsu
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1974-01-10
Filing date: 1974-01-10
Publication date: 1975-05-27
Anticipated expiration: 1992-05-27

Abstract

A slow wave circuit of a microwave tube includes a wide band helix-type slow wave circuit, such as a comb-supported ring bar (CSRB) circuit, followed by a coupled cavity slow wave output circuit in direct microwave coupling relationship with the CSRB circuit.

Description

United States Patent 1191* [111 3,886,397 Hiramatsu 1 May 27, 1975 [541 HYBRID SLOW WAVE CIRCUIT 3,181,024 4/1965 Seusiper 315/393 x 1751 Inventor: Yukio Los Altos, Calif- 31233332 311333 ii'fifl'fiiiiiiiiijiiiiiii:1111133.?15ii2 73} Assignee; varian Associates, p Aim. Calif 3,715,616 2/1973 Elfe, .lr. 315/35 [22] Filed: 1974 Primary Examiner-James W. Lawrence [21] Appl. No.: 432,380 Assistant ExaminerMarvin Nussbaum Attorney, Agent, or Firm-Stanley Z. Cole; D. R. 52 11.5. C1. 3158.6; sis/3.5; 315/393, Pressman;

333/31 A 511 1111. C1 1101 23/24; [-101 j 25/34 [571 ABSTRACT [58] Field of Search 315/35, 3.6, 39.3; A Slow wave r i f a i r a be includes a 333/31 A wide band helix-type slow wave circuit, such as a comb-supported ring bar (CSRB) circuit, followed by [56] References Cit d 21 coupled cavity slow wave output circuit in direct mi- UNITED STATES PATENTS crowave coupling relationship with the CSRB circuit. 2,927,832 3/1960 Marchese 315/36 1 Claim, 6 Drawing Figures (30MB SUPPORTED RING BAR IU- 4 9 +4 INPUT +3 12" OUTPUT 2 /& Q 26 1 -42 4s ---i I #a I l 52' /3 ELECTRON i, b 8752 GUN 1s 8 3 5a 4 $4 LLECTOR COUPLED CAVITY M CHING TRglfSFORMER PATENTEUEM 2 7 I975 CL m Um WE UM 0w f om A CL DI U W ME

nD mw FIG. I

6 i |.'2 NORMAUZED FREQUENCY HYBRID SLOW WAVE CIRCUIT FIELD OF THE INVENTION The present invention relates to slow wave structures of the type for interacting microwave energy with an electron beam. In its particular aspects, the invention relates to a slow wave structure comprising the combination of a helix-type slow wave circuit and a coupled cavity slow wave circuit.

BACKGROUND OF THE INVENTION Travelling wave tubes (TWTs) employing coupled cavity slow wave structures have been employed in high output power, relatively narrow bandwidth applications while wider bandwidth helix-type travelling wave tubes have been relegated to relatively low output power applications.

In response to demands for TWT's with both high output power and wide bandwidth, attempts have been made to increase the bandwidth of coupled cavity tubes. These attempts have been met by serious problems of tube band edge oscillations which have required extraordinary oscillation suppression techniques. Also, in response to such demands, efforts were made to increase the output power of helix-type TWTs. These efforts resulted in the development of the dielectric comb-supported ring bar (CSRB) circuit. Present travelling wave tubes employing CSRB helixtype slow wave structures have efficiencies and consequently output power significantly less than that of coupled cavity travelling wave tubes.

Heretofore, it has been known that the efficiency and output power of a TWT are strongly dependent on the circuit loss in the output part of the tube. For example, structures to provide losses within coupled cavity slow wave structures for band edge oscillation suppression purposes have not been coupled to and near the tube output part.

Because coupled cavity structures have lower loss than helix-type structures higher efficiencies and output powers are attained with the former than with the latter.

OBJECTS OF THE INVENTION It is an object of the present invention to provide a new and improved slow wave structure for achieving combinations of efficiency and bandwidth generally not heretofore possible.

It is a further object of the present invention to provide a new and improved travelling wave tube with both high bandwidth and high output power capability.

It is yet another object of the invention to provide a new and improved slow wave structure which can be easily optimized in terms of bandwidth and efficiency for given applications.

It is still another object of the present invention to provide a wide band travelling wave tube with a rugged slow wave structure.

SUMMARY OF THE INVENTION According to the teachings of the present invention, a single microwave tube employs a slow wave structure comprising a hybrid combination of slow wave circuit types. In particular, a main part of the slow wave structure is a helix-type circuit. This main part is followed by an aligned short coupled cavity output section.

Since the output power and efficiency of a TWT are primarily determined by the characteristics of the slow wave structure at the output portion of the tube, the resultant hybrid tube has a bandwidth approaching that of a helix-type tube and efficiency and output power capabilities approaching those of coupled cavity tubes.

To achieve these results the helix-type and coupled cavity slow wave circuits are directly coupled in microwave energy exchange relationship, with each being designed according to known criteria to have comparable phase velocities and interaction impedance to achieve a broadband, high output power hybrid TWT. It is desirable in some applications to provide a more narrow band coupled cavity output section characterized by higher interaction impedance than the helix-type circuit. In such applications, the output power capability of the hybrid tube is further increased at the expense of bandwidth. The combined, diverse TWT slow wave structures permit certain trade-offs enabling the design of microwave tubes which are optimally tailored for various specific applications. For example, the diverse slow wave circuits can be combined to achieve a bandwidth intermediate the capabilities of helix-type and coupled cavity-type tubes at the highest efficiency presently possible.

The hybrid microwave tube of the invention is more rugged than conventional helix-type TWTs since the coupled cavity output section is considerably more resistant to burn out failures caused by high power microwave propagation than is a helix-type circuit.

In general it is necessary to match the characteristic wave impedance of the helix-type and coupled cavity circuits. In particular, when the helix-type circuit is a CSRB circuit, matching of the characteristic impedances of the two circuits is provided by an impedance transformer section comprising a first generally open cavity of the coupled cavity circuit. The ring bar circuit protrudes into the open cavity and is aligned with the drift tubes of the coupled cavity circuit.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic, top view of one embodiment of a travelling wave tube of the invention, with a portion broken away in cross-section;

FIGS. 2, 3 and 4 are cross-sectional views respectively taken along the lines 2-2, 3-3 and 4-4 of FIG.

FIG. 5 is a plot of phase velocity versus normalized frequency; and

FIG. 6 is a plot of interaction impedance versus normalized frequency.

DETAILED DESCRIPTION OF THE DRAWING Referring to FIG. 1, a composite or hybrid slow wave structure comprising a helix-type slow wave circuit 8, preferably a comb-supported ring bar circuit, and a coupled cavity circuit 9 are incorporated into a single microwave travelling wave tube 10. Electrically conducting, axially aligned joined barrel portions I2 and 12" house the respective slow wave circuits 8 and 9 that are generally directed along the barrel longitudinal axis 14. A conventional electron gun 16 at one end of tube provides means for forming and launching or projecting a beam of electrons 18 over an elongated beam path comprising the barrel longitudinal axis 14. Beam 18 propagates through aligned circuits 8 and 9 to a beam collector 20 which dissipates and collects the energy of the electron beam. The helix-type and coupled cavity slow wave circuits 8 and 9 provide progressive microwave interaction with electron beam 18. The coupled cavity slow wave circuit 9 forms an output or high power section which further amplifies microwave power received from the helix-type circuit 8. As is usual, there is provided around the exterior of tube 10 a focusing magnet means (not shown) for producing an axial magnetic field along the beam path to confine the beam to a small cross-section.

The microwave tube 10 further comprises an input waveguide 22 for coupling microwave energy to be amplified into a first portion 24 of the helix-type circuit 8 and an output waveguide 26 for extracting amplified microwave energy from the coupled cavity circuit. The input and

output waveguides

22 and 26 are coupled to the respective slow wave circuits 8 and 9 in the usual manner employed in helix-type TWTs on the one hand and coupled cavity TWTs on the other. The helix-type circuit 8 also includes a conventional sever 28 between a first helix-type portion 24 and a second helix-type portion 30. As is well known, sever 28 allows only electron beam coupling between

portions

24 and 30 to obviate backward wave oscillations.

In the first portion 24 electron beam 18 is current density modulated to induce microwave energy in secend portion 30. By progressive interaction with the electron beam 18, a growing wave of microwave energy is formed in the second portion 30. The growing wave advances longitudinally and directly impinges upon the coupled cavity slow wave circuit 9 in a matching transformer section 31, forming a part of both circuits 8 and 9. The wave continues to grow in amplitude in coupled cavity circuit 9 until it is extracted at output waveguide 26 at the end of the circuit 9.

The helix-type slow wave circuit 8 may for the purposes of the invention advantageously be any helixderived circuit, but is preferably for high bandwidth, high output power applications, a comb-supported ring bar (CSRB) circuit. Because of the relatively high power handling capabilities of a CSRB circuit in comparison with a simple helix circuit, the coupled cavity circuit can comprise fewer cavities when preceded by a CSRB circuit because less additional power gain is required of the coupled cavity circuit for a given application. With only a few cavities, such as two or three, in the coupled cavity circuit 9, the possibility of tube band edge oscillations is minimized.

The CSRB slow wave circuit is generally of the type disclosed in US. Pat. Nos. 3,505,730 and 3,508,108, assigned to the same assignee as the present invention, which patents were respectively issued on Apr. 14 and 17, 1970. Referring to FIGS. 1 and 2 herein, the CSRB circuit 8 is a topologically equivalent ring and bar helixderived circuit including a pair of opposed meander line portions 32 disposed in transverse registration upon concave cylindrical trough surfaces 34 of a pair of serpentine-shaped ceramic insulator support structures 36, as of alumina or beryllia ceramic.

The opposed meander line portions 32, formed of electrically conductive sheet metal, are adhesively secured to trough surfaces 34 of support structures 36 to form rings 32' that alternate with longitudinally directed bars 32". Barrel portion 12, surrounding CSRB circuit 8, has a rectangular cross-section and is longitudinally split in half to form two registered

opposed halves

12a and 12b. The support structures 36 are secured to the barrel halves 12a and 12b by brazing the support structures to a pair of flat intermediate slab members 38 having a coefficient of expansion matching that of the support structures; one material suitable for members 38 is a copper impregnated tungsten matrix. The intermediate members 38 are then brazed to

barrel halves

12a and 12b, and the barrel halves 12a and 12b are thereafter brazed together.

With reference to FIGS. 1, 3 and 4, the coupled cavity circuit 9 comprises a plurality of axially aligned tubular cavity interior defining tubes 40 interleaved with axially aligned, cylindrical, centrally apertured cavity end wall discs 42. Axially aligned drift tubes 41 within which the electron beam 18 passes, are brazed in the center of end wall discs 42. Interleaved tubes 40 and discs 42, which are copper, are brazed together whereby the outer diameter of the tubes and discs forms the circular barrel portion 12" which houses the coupled cavity circuit 9. The electron beam 18 passes through the aligned drift tubes 41 of the end wall discs 42 to interact with microwave energy in each cavity 45. Kidney-shaped coupling slots 46 are provided in the end wall discs 42 for coupling microwave energy between successive cavities 45. Slots 46 and cavities 45 have resonant respective frequencies that are chosen according to well known techniques for broadbanding the coupled cavity circuit. Discs 42 are preferably oriented so that successive coupling slots 46 are staggered, alternating between radially opposite sides of the axis 14.

The axially aligned barrel portions 12' and 12" are joined via a metal adaptor plate 50. Plate 50 has a circular periphery matching and brazed to the outside of barrel portion 12'', as well as a centrally located rectangular aperture receiving and having edges brazed to the barrel portion 12'. By bonding barrel portions 12' and 12" to the edges of the aperture and outer periphery of plate 50 a radially stepped, barrel cross-section is provided.

Adaptor plate 50 forms an end wall of the first cavity section 45' of coupled cavity circuit 45. The last ring 32 of the ring bar circuit, approximately of the same transverse dimensions as the drift tubes 41, protrudes past adaptor plate 50 into the first cavity 45' a sufficient distance d to form an impedancae transformer section in the first cavity 45' for matching the characteristics wave impedances of the slow wave circuits 8 and 9. The radial and axial dimensions defining the interior of the first cavity 45 may also be adjusted for matching the two circuits 8 and 9.

When it is desired to provide increased output power or efficiency with respect to conventional helix-type TWTs while paying a minimum penalty in bandwidth, the coupled cavity output section 9 is designed according to well known techniques to have interaction impedance and phase velocity characteristics comparable to those of helix-type circuit 8. FIGS. 5 and 6. respectively indicate the general form of the phase velocity and interaction impedance characteristics of the CSRB and wide band coupled cavity slow wave structures 8 and 9. With reference to FIG. 4, the CSRB phase velocity curve 51 is a generally decreasing function of frequency while the coupled cavity phase velocity curve 52 is a generally increasing function of frequency. By choosing the various dimensional parameters of the slow wave circuits according to well known techniques, the curves S1 and 52 can be made to cross at a point 54 which is approximately at the center frequency, f,,, of tube operation to provide comparable phase velocities throughout the tube bandwidth, as of from 0.8 f,, to 1.2 f Similarly, in FIG. 5 the CSRB and coupled cavity structures 8 and 9 respectively have interaction impedance curves 56 and 58 that can, by appropriate choice of slow wave circuit parameters, simultaneously be made to cross at a point 60 approximately at the center frequency of tube operation, f,,, thereby providing comparable interaction impedances for the two different slow wave circuits. it is estimated that in such a design, the output power of the overall tube may be increased by at least a factor of two with only a slight decrease in overall bandwidth.

When it is desired to trade excess bandwidth in the helix-type CSRB for even more increased overall output power or efficiency, the coupled cavity circuit may be provided with significantly higher interaction impedance than curve 58 over a narrower range of frequency.

Having described one preferred embodiment of the invention, it should be apparent that numerous modifications are possible within its spirit and scope. Thus it is intended that this embodiment be interpreted as illustrative and not in a limiting scope,

What is claimed is:

l. A device for progressively interacting microwave energy with an electron beam comprising:

a. means for projecting a beam of electrons over an elongted beam path;

b. a ring and bar slow wave circuit disposed along said beam path in enegy exchange relationship with said beam;

c. a coupled cavity slow wave circuit disposed along said beam path downstream of said ring and bar circuit and in energy exchange relationship with said beam;

d. said ring and bar circuit being in direct microwave energy exchange relationship with said coupled cavity circuit by protruding into a first cavity of said coupled cavity circuit.

Claims

1. A device for progressively interacting micrOwave energy with an electron beam comprising: a. means for projecting a beam of electrons over an elongted beam path; b. a ring and bar slow wave circuit disposed along said beam path in enegy exchange relationship with said beam; c. a coupled cavity slow wave circuit disposed along said beam path downstream of said ring and bar circuit and in energy exchange relationship with said beam; d. said ring and bar circuit being in direct microwave energy exchange relationship with said coupled cavity circuit by protruding into a first cavity of said coupled cavity circuit.