US3889149A

US3889149A - Liquid cooled attenuator

Info

Publication number: US3889149A
Application number: US409204A
Authority: US
Inventors: Lester M Winslow; Hazel E Brown
Original assignee: US Department of Navy
Current assignee: US Department of Navy
Priority date: 1973-10-24
Filing date: 1973-10-24
Publication date: 1975-06-10
Anticipated expiration: 1992-06-10

Abstract

A traveling-wave tube coupled interaction cavity slow-wave structure with longitudinally extending attenuating transmission lines having a distributed loss in a particular desired range. The degree of loss is varied to achieve desired electronic efficiency and stability. The variance is achieved by physically and electromagnetically coupling one or more transmission lines directly to the coupling holes between interaction cavities via irises places at various angles to the normal plane, by varying the width of the irises, and also by varying the placement of the transmission lines with respect to the interaction cavities.

Description

United States Patent Winslow et al.

LIQUID COOLED ATTENUATOR Inventors: Lester M. Winslow, Alexandria, Va.;

Hazel E. Brown, Washington, DC.

The United States of America as represented by the Secretary of the Navy, Washington, DC.

Filed: Oct. 24, 1973 Appl. No.1 409,204

Assignee:

US. Cl. 315/35; 315/36; 315/393; 333/31 A Int. Cl. H01 j 25/34 Field of Search 315/35, 3.6, 39.3; 333/31 A References Cited UNITED STATES PATENTS 6/1967 Winslow 315/35 11/1967 Hant 315/35 7/1 69 Cerko 315/35 1 June 10, 1975 3,602,766 8/1971 Grant ..3l5/3.6 3,771,010 11/1973 Winslow ..315/3.5

Primary ExaminerSaxfield Chatmon, Jr. Attorney, Agent, or FirmR. S. Sciascia; Arthur L. Branning [5 7] ABSTRACT A traveling-wave tube coupled interaction cavity slowwave structure with longitudinally extending attenuating transmission lines having a distributed loss in a particular desired range. The degree of loss is varied to achieve desired electronic efficiency and stability. The variance is achieved by physically and electromagnetically coupling one or more transmission lines directly to the coupling holes between interaction cavities via irises places at various angles to the normal plane, by varying the width of the irises, and also by varying the placement of the transmission lines with respect to the interaction cavities.

10 Claims, 6 Drawing Figures PATENTEDJUH 10 1915 :2, 889,149

SHEET 2 LIQUID COOLED ATTENUATOR BACKGROUND OF THE INVENTION This invention relates generally to microwave devices. and more particularly to traveling-wave tubes having means for substantially eliminating oscillation at frequencies at the edges of the frequency passband of the tube, as well as for providing a carefully shaped gain vs. frequency characteristic.

In travelling-wave tubes a stream of electrons is caused to interact with a propagating electromagnetic wave in a manner which amplifies the electromagnetic energy. In order to achieve such interaction, the electromagnetic wave is propagated along a slow-wave structure. This structure may be in the form of a conductive helix wound about the path of the electron stream or a folded waveguide type of structure in which a waveguide is effectively wound back and forth across the path of the electrons. The slow-wave structure provides a path of propagation for the electromagnetic wave which is considerably longer than the axial length of the structure. Through resort to this difference in path lengths, the traveling-wave may be made to effectively propagate at nearly the velocity of the electron stream. The interactions between the electrons in the stream and the traveling wave cause velocity modulations and bunching of the electrons in the stream. The net result will then be a transfer of energy from the electron beam to the wave traveling along the slowwave structure.

The present invention is primarily, although not necessarily, concerned with traveling-wave tubes utilizing slow-wave structures of the coupled cavity, or interconnected cell, type. In this type of slow-wave structure a series of interaction cavities, or cells, are disposed adjacent to each other sequentially along the axis of the tube.

' In operation, electron stream passes through each interaction cell. Electromagnetic magnetic coupling is provided between each cell and the electron stream. Each interaction cell is also coupled to an adjacent cell by means ofa coupling hole at the end wall defining the cell. Generally, the coupling holes betwen adjacent cells are alternately disposed on opposite sides of the axis of the tube, although various other arrangements for staggering the coupling holes are possible and have been employed. When the coupling holes are so arranged, a folded waveguide type of energy propagation results, with the traveling-wave energy traversing the length of the tube by entering each interaction cell from one side, crossing the electron stream and then leaving the cell from the other side, thus traveling a sinuous, or serpentine, extended path.

One of the problems encountered in traveling-wave tubes of the coupled cavity variety, and especially high power tubes of this type, is a tendency for the tube to oscillate at frequencies near the edges of the tube passband. This problem arises from the fact that for wideband operation the phase velocity of the slow-wave circuit wave and the velocity of the electron beam should be essentially synchronized over as large a range of frequencies as possible; hence, these velocities are also close to synchronism near the upper and lower cutoff frequencies of the tube. Since the interaction impedance is high and the circuit impedance varies rapidly, usually creating high reflections, at and in the vicinity of the cutoff frequencies, the loop gain for the tube, or

even for a section of the tube, may be sufficiently large for oscillations to start.

One technique which has been used to solve this oscillation problem involves coupling to the slow-wave structure interaction cells specially designed cavities which are sharply resonant at a frequency in the vicinity ofa cutoff frequency of the slow-wave structure and providing lossy ceramic buttons in these special cavities in order to attenuate energy at the resonant frequency of the cavity. While this technique is useful for attenuating energy at those frequencies where the tube is most likely to oscillate without substantially affecting energy at frequencies throughout the remainder of the tube passband, a minimum reflection coefficient is not provided. A low reflection coefficient is highly desirable in preventing large fluctuations in gain as a function of frequency at the low frequency end of the tube passband.

Another technique has been developed wherein the resonant cavities and ceramic buttons are replaced by a longitudinally extending attenuating transmission line. This line typically may be a cylindrical or tubular lossy ceramic rod disposed coaxially about an electrically conductive rod.

Coupling irises in the side walls of at least certain ones of the slow-wave structure interaction cavities provide electromagnetic coupling between the slowwave structure and the transmission line. This technique is disclosed in US. Pat. No. 3,771,010, filed Nov. 22, 1972 by Winslow.

SUMMARY OF THE INVENTION The present invention includes means in a travelingwave tube for providing a stream of electrons along a predetermined path and a slow-wave structure having a plurality of intercoupled interaction cavities disposed sequentially along and about the electron stream path for propagating electromagnetic wave energy in such manner that it interacts with the stream of electrons. An attenuating transmission line is disposed proximate to and externally of the electron stream path, with the longitudinal axis of the lossy element being parallel to the electron stream path. Coupling irises are disposed so that at least certain ones of them couple the transmission lines directly into one of the coupling holes which lie between adjacent interaction cavities. The coupling irises may be angled into the coupling holes from both transmission lines to each coupling hole or the transmission lines may be coupled alternatively into the coupling holes by alternate coupling irises. The transmission lines may also extend laterally into the interaction cavities, be tangent thereto, or be wholely outside the interaction cavitiessCombinations of the above may be used to give the desired degree of loss in a particular range.

Accordingly, it is an object to the present invention to provide a traveling-wave tube in which any tendency for the tube to oscillate in the vicinity of the edges of the tube frequency passband in substantially eliminated, and at the same time, in which a minimum reflection coefficient is provided to minimize small signal gain variations at the low end of the frequency passband of the tube.

It is further object of the present invention to provide a coupled cavity traveling-wave tube having a readily controllable and carefully shaped gain vs. frequency characteristic.

It is still further object of the present invention to provide means for both suppressing oscillations and shaping the gain vs. frequency characteristic of a high power traveling-wave tube of the coupled cavity type, and which means is simpler in design and requires fewer parts than schemes heretofore employed.

Another object of the present invention is to provide means to vary the degree of loss in a simple manner to achieve high electronic efficiency over the bandwidth of the tube.

Other objects, advantages and novel features of the invention will become apparant from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein:

'BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an overall view partly in longitudinal section and partly broken away of a traveling-wave tube constructed in accordance with the present invention.

FIG. 2 is crosssectional view thken along line 2-2 of FIG. 1 showing the prior art arrangement.

FIGS. 3A and 3B show cross-sectional views taken along line 2-2 of FIG. 1 showing one embodiment of the present invention.

FIGS. 4A and 4B show a cross-sectional view taken along lines 2-2 of FIG. 1 showing another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referrring now to FIG. 1, numeral designates generally a traveling-wave tube which includes an arragnement 12 of magnets, pole pieces and spacer elements, which are described in detail in US. Pat. No. 3,324,338, to Winslow. At this point it should suffice to state that the spacer elements and interior portions of the pole pieces function as a slow-wave structure, while the magnets and pole pieces constitute a periodic focusing device for the electron beam traversing the length of the slow-wave structure.

Coupled to the input end of the arrangement 12 is an input waveguide transducer 14 which includes an impedance step transformer 16. A flange 18 is provided for coupling the assembled traveling-wave tube 10 to an external waveguide or other microwave transmission line (not shown). The construction of the flange 18 may include a microwave window (not shown) transparent to microwave energy but capable of maintaining a vacuum within the traveling-wave tube 10. At the output end of the arrangement 12 an output transducer 20 is provided which is substantially similar to the input transducer 14 and which includes an impedance step transformer 22 and a coupling flange 24, which elements are similar to the elements 16 and 18, respectively of the input transducer 14. For vacuum pumping or outgassing the traveling-wave tube 10 during manufacture, a double-ended pumping tube 26 is connected to both the input and

output waveguide transducers

14 and 20.

An electron gun 28 is disposed at one end of the traveling-wave tube 10 which, although illustrated as the imput end in FIG. 1, may alternatively be the output end if a backward wave device is desired. The electron gun 28 functions to project a stream of electrons along the axis of the tube 10 and may be of any conventional construction well known in the art. For details as to the construction of the gun 28 reference is made to US.

Pat. No. 2,985,791, entitled, Periodically Focused Severed Traveling-Wave Tube," issued May 23, 1961 to D. J. Bates et al, and to US. Pat. No. 2,936,393, entitled, Low Noise Traveling-Wave Tube, issued May 10, I960, to M. R. Currie et al.

At the output end of the traveling-wave tube 10 there is provided a cooled collector structure 30 for collecting the electrons in the stream. The collector is conventional and may be of any form well known in the art. For details as to the construction of the collector, reference is made to the aforesaid US. Pat. No. 2,985.79] and to US. Pat. No. 2,860,277, entitled Traveling- Wave Tube Collector Electrode, issued Nov. 1 l. 1958, to A. H. Iversen.

Referring to FIG. 2, a view along 2-2 of FIG. 1 shows an essentially annular dish-shaped focusing magnet 32 interposed longitudinally along the tube between two ferromagnetic pole pieces 34, only one of which is illustrated. The ferromagnetic pole pieces 34 extend radially inwardly to the magnets 32 to approximately the perimeter of the region adapted to contain the axial electron stream. The individual pole pieces are constructed in such a manner that a short drift tube, or ferrule, 36 is provided at the inner extremity of each pole piece. The drift tube 36 is in the form ofa cylindrical extension, or lip, protruding axially along the path of the electron stream from both surfaces of pole piece 34 i.e. in both directions normal to the plane of the pole piece 34. The drift tubes 36 provided with central and axially aligned apertures 38 to provide a passage for the flow of the electron beam. Adjacent ones of the drift tubes 36 are separated by a gap which functions as a magnetic gap to provide a focusing lens for the electron beam and also as an interaction gap in which energy exchange between the electron beam and traveling-wave energy traversing the slow-wave structure occurs.

Disposed radially within each of the magnets 32 a slow-wave circuit spacer element 42 of a conductive nonmagnetic material such as copper. Each spacer element 42 has an annular portion of an outer diameter essentially equal to the inner diameter of the magnetic 32 and a pair of oppositely disposed

ear portions

43 and 44 projecting outwardly from the annular portion. Each spacer element also defines a central cylindrical aperture 45 to provide space for the microwave inter action cell, or cavity which is defined by the inner lat-.

eral surface of the spacer 42 and the walls of the two adjacent pole pieces 34 projecting inwardly of the spacer element 42. The inner diameter of the spacer 42 determines the radial extent of the interaction cell while the axial length of the spacer 42 determines the axial length of the cell.

For interconnecting adjacent interaction cavities an off-center coupling hole 48 is provided through each of the pole pieces 34 to permit the transfer of electromagnetic wave energy from cell to cell. As is illustrated, the coupling holes 48 may be substantially kidney-shaped and may be alternately disposed apart with respect to the drift tubes 36. It should be pointed out, however, that the coupling holes 48 may be of other shapes and may be staggered in various other arrangements. In any event, it will be apparent that the spacer elements 42 and the portions of the pole pieces 34 projecting inwardly of the spacers 42 not only form an envelope for the tube, but also constitute a slow-wave structure for propagating travelingwave energy in a serpentine path along the axially traveling electron stream so as to support energy exchange between the electrons of the stream and the traveling-wave.

The axial length of the magnets 32, hence that of the spacers 42 is equal to the spacing between adjacent pole pieces 34, and the radial extent of the magnets 32 is approximately equal to or, as shown, slightly greater than that of the pole pieces 34. To provide focusing lenses in the gaps, the magnets'32 are stacked with alternating polarity along the axis of the tube, thus causing a reversal of the magnetic field at each magnetic lens and thereby providing a periodic focusing device. It should be pointed out, however, that although the lengths of the spacers 42 may be substantially constant, they may also be varied slightly with respect to each other so that the effective axial length of the cavities is varied as a function of distance along the tube to ensure that the desired interaction between the electron stream and the traveling waves will continue to a maximum degree even though the electrons are decelerated toward the collector end of the tube.

To provide the desired attenuation,

lossy transmission lines

56 and 60 are utilized. The first lossy transmission line 56 is disposed on one side of the slow wave circuit with its longitudinal axis parallel to the electron beam path. Second lossy transmission line 60 is similarly oriented on the other side of the slow wave circuit.

Transmission lines

56 and 60 are coupled into the interaction cavity by means of

coupling iries

63 and 64, respectively. As shown each transmission line may be hollow and with a center hole (50 and 52 respectively) though which a lossy fluid is passed to remove heat and provide a high rf power absorption capability.

The lossy transmission lines may, of course, be of other constructions and materials. Further details of other transmission lines and other prior art arrangements may be found in the afore-mentioned U.S. Pat. No. 3,324,338 and U.S. Pat. No. 3,771,010.

Referring to FIGS. 3A and 3B a particular embodiment of the present invention is shown. Coupling irises 65 and 66 in FIG. 3A couple the lossy transmission lines (56 and 60) directly to coupling hole 48. In FIG. 3B the next or alternate spacer element 42 has its coupling hole 49 directly coupled to the lossy transmission lines (56 and 60) by coupling

irises

67 and 68, respectively.

A second specific embodiment changing the angle of coupling is shown in FIGS. 4A and 48. FIG. 4A shows the first spacer element 42 with its coupling hole (48) directly coupled by iris 69 only to transmission line 60 and not directly coupled to transmission line 56. FIG. 4B shows alternate spacer element 42' with its coupling hole (49) directly coupled by coupling iris 72 only to transmission line 56. Other angles may be chosen than the particular set shown in FIG. 4A and FIG. 4B. The configuration of FIG. 3A and FIG. 38 represents a zero(normal)angle arrangement.

Two other aspects of the invention may be discussed either references to FIG. 3A. The first dimension of interest is the distance w between line y-y and line z-z. The line y-y' is a line tangent to the edge of the interaction cavity. Distance w may be varied by moving the perimeter of lossy transmission line 56 (line z-z') closer to line y-y or past line y-y (w) into the edge of the interaction cavity. The loss increases as w decreases to zero or goes negative. This position may of course be varied on either or both lines (56 and 60).

Loss may also be varied by varing the coupling iris (66) width.

To attain high electronic efficiency and stability the degree of loss per unit length must be easily varied. The embodiment and changes discussed above allow a particular desired degree of loss to be attained in a simple manner. As above discussed, a basic change over the prior art is coupling the transmission lines directly to the coupling holes via the coupling irises. Further changes in loss are attained by the position of the transmission lines with respect to the interaction cavity, width of the irises, and angel of the irises.

The particular degress of loss desired is in the range of 0.3 d 2.0. Here d is the normalized Pierce loss parameter (i.e. normalized dbs/wavelength). Different loss levels are required by different tube structures and different points in a particular tube may also require different loss levels. Since the loss (in db) from the coupled line has the greatest affect on the d parameter, it is necessary to obtain the particular d level desired.

For example, in a particular structure at l 1 Ghz and angle zero (FIGS. 3A and 3B) doubling the slot width gave a change in loss from 0.08 to 1.21 (.db/cavity). Another test of a FIG. 3A structure at l 1 Ghz showed loss equal to 1.37 (db/cavity) with w equal to zero (tangent) and loss equal to 4.14 (db/cavity) with w equal to .05 inch (or 0.05 into the interaction cavity). Thus it is seen that coupling the transmission line or lines directly to the coupling holes in the various arrangements discussed above allows the loss and thus d to be easily and widely varied as desired.

Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of the United States is:

1. In a traveling-wave tube having means for providing a stream of electrons along a predetermined path and means propagating electromagnetic energy comprising means defining a plurality of intercoupled interaction cavities in a manner so that interaction takes place with the stream of electrons further comprising:

a plurality of support pole pieces, each having at least one coupling hole means providing intercoupling of electromagnetic energy between said interaction cavities;

at least one lossy transmission line means disposed proximate and external to said predetermined path;

and

means for preventing bondedge oscillation comprising a plurality of coupling irises located in said support pole pieces coupling directly at least one of said lossy transmission line means and at least one of said coupling means.

2. The traveling-wave tube of claim 1, wherein:

said transmission line means is tangent to said interaction cavities.

3. The traveling-wave tube of claim 1, wherein:

said transmission line means is external of said interaction cavities.

4. The traveling-wave tube of claim 1, wherein:

said transmission line means is at least partially disposed in said interaction cavities.

5. The traveling-wave tube of claim 1, wherein:

' said transmission lines are hollow tubes adapted for lossy fluid flow therein. 9} The traveling-wave tube of claim 6, wherein: said transmission line means comprises two transmission lines; and said coupling irises couple both of said transmission lines to each alternate coupling hole means. 10. The traveling-wave tube of claim 9, wherein: said transmission lines are hollow tubes adapted for

Claims

1. In a traveling-wave tube having means for providing a stream of electrons along a predetermined path and means propagating electromagnetic energy comprising means defining a plurality of intercoupled interaction cavities in a manner so that interaction takes place with the stream of electrons further comprising: a plurality of support pole pieces, each having at least one coupling hole means providing intercoupling of electromagnetic energy between said interaction cavities; at least one lossy transmission line means disposed proximate and external to said predetermined path; and means for preventing bondedge oscillation comprising a plurality of coupling irises located in said support pole pieces coupling directly at least one of said lossy transmission line means and at least one of said coupling means.

2. The traveling-wave tube of claim 1, wherein: said transmission line means is tangent to said interaction cavities.

3. The traveling-wave tube of claim 1, wherein: said transmission line means is external of said interaction cavities.

4. The traveling-wave tube of claim 1, wherein: said transmission line means is at least partially disposed in said interaction cavities.

5. The traveling-wave tube of claim 1, wherein: the width of said coupling irises is in the range of one half to equal the outside diameter of said transmission line means.

6. The traveling-wave tube of claim 1, wherein: said transmission line means comprises two transmission lines; and said coupling irises couple said transmission lines alternatively to alternative coupling hole means.

7. The traveling-wave tube of claim 6, wherein: said coupling irises are disposed between 0* and 90* of the normal plane.

8. The traveling-wave tube of claim 6, wherein: said transmission lines are hollow tubes adapted for lossy fluid flow therein.

9. The traveling-wave tube of claim 6, wherein: said transmission line means comprises two transmission lines; and said coupling irises couple both of said transmission lines to each alternate coupling hole means.

10. The traveling-wave tube of claim 9, wherein: said transmission lines are hollow tubes adapted for lossy fluid flow therein.