USRE25329E - Periodically focused traveling wave tube - Google Patents

Feb. 19, 1963 D. J. BATES R 25,329

PERIODICALLY FOCUSED TRAVELING WAVE TUBE WITH TAPERED PHASE VELOCITY Original Filed Oct. 2, 1958 5 Sheets-Sheet 1 K m/a1. 3 04140 67.294741,

Feb. 19, 1963 D. J. BATES Re. 25,329

PERIODICALLY FOCUSED TRAVELING WAVE TUBE WITH TAPERED PHASE VELOCITY Original Filed Oct. 2, 1958 5 Sheets-Sheet 2 Feb. 19, 1963 n. J. BATES mmonzcnu FOCUSED TRAVELING mwz TUBE 7 WITH TAPERED PHASE VELOCITY Original Filed Oct. 2, 1958 r 5 Sheets-Sheet 3 fixer/4K 04 /0 (7347:)",

\\\\\\\\ III Feb. 19, 1963 D. J. BATES PERIODICALLY FOCUSED TRAVELING WAVE TUBE WITH TAPERED PHASE VELOCITY Original Filed Oct. 2, 1958 5 Sheets-Sheet 4 Feb. 19, 1963 D. J. BATES PERIODICALLY FOCUSED TRAVELING WAVE TUBE WITH TAPERED PHASE VELOCITY Original Filed Oct; 2, 1958 5 Sheets-Sheet 5 I 1 a? 205: a: z as m a. w @2553: 2: 222cm; 5528 u m n I J 5 5 4 5:23 2 5528 2 E528 52285 2 EN m2 N2 5 02 i r 1 (I 1)! HOJJHOSl HOUHOSI NOLLZJHS HOlV'IOSl NOllOBS 301N105! NOLLQES NOIIZHS NDIDQS 801N109 NOIDBS HOLV'IOSI Unite tates ate Re. 25,329 Roissued Feb. 19, 1963 ice Matter enclosed in. heavy brackets E 1 appears in the original patent but forms no part of this reissuespecification; matter printedin italics indicates the additions made by reissue.

"This invention relates to traveling-wave tubes, and particularly to arrangements for tapering slow-Wave structures so as to achieve maximum interaction between an electron beam and radio frequency electromagnetic wave energy.

In traveling-wave tubes, as is well known, a wave of radio frequency electromagnetic energy is caused to interact with an electron stream. The wave is slowed to substantially below the velocity of light by being confined totraverse an eifectively tortuous path. The path extends at different points along the path of the electron stream so as to provide a periodic structure having the proper phase relationships with respect to the electron stream for the electron stream to interact with or electromagnetically push" and thus to amplify the radio frequency wave.

The position of the members which define the tortuous path relative to each other and to the electron stream is extremely critical. This slow-Wave structure, the classical form of which is the helix, accordingly has in prior structures been extremely difficult to manufacture with the required precision. When the desired precision has been achieved, it has usually resulted in a delicate structure which can easily be disarranged under extreme environmental conditions or through accident. A further serious complication is, therefore, added when it is desired to fabricate a slow-Wave structure so as to achieve most effective interaction between the radio frequency wave and the electrons projected along the entire length of the interaction volume. It is known that as an electron stream is caused to give up energy to the radio frequency wave, the electron stream is slowed down and the interaction becomes less effective. Accordingly, a number of techniques have been employed for achieving more efiicient interaction. These having included, as with the helix, a tapering of the space periodicity of the radio frequency structure toward the collector end of the tube. When such techniques have been employed, however, the requirements for fabricating the structures to the greater precision which is demanded have often been too severe to be practical.

Accordingly, it is an object of the present invention to provide an improved traveling-Wave tube which is simple to fabricate, yet which achieves maximum interaction between a radio frequency wave and an electron stream along the entire path of the stream.

Another object of this invention is to provide an improved slow-wave structure which is simple and economical to construct, but which may be readily tapered to extremely precise dimensions.

A further object of this invention is to provide an improved slow-wave structure tapering mechanism for traveling-wave tubes.

These and other objects of the invention are achieved by an arrangement which fabricates the traveling-wave tube slow-wave structure out of a plurality of centrally apertured ferromagnetic discs serially spaced along the path of an electron beam. Ferromagnetic drift tubes are disposed within the aperatures contiguous to thebeam and provide gaps between adjacent ones forfocusing the beam and for coupling between the beam and the traveling wave. Individual ones of a plurality of spacer rings are each interposed betwen a different adjacent pair of ferromagnetic discs and each define the outer periphery or cylindrical surface of an interaction cell or cavity making up the slow-wave structure. A highly conductive surfacing may be applied to the interior or the cavitysurfaces of the discs and spacer rings. 'With this arrangement, effective tapering may be achieved by a reduction in the axial dimensions of the ferromagnetic discs and drift tubes without otherwise affecting "the electromagnetic parameters of the interaction cells. .Another tapering mechanism is achieved by varying the axial thickness of the spacer rings in a manner to successively shorten the interaction cavities toward the collector end of the tube. By thus decreasing the distance between. points along the slow-wave structure where the electronstream is coupled to the interaction cells the electron stream travels a shorter distance between interaction. cells toward the output end of the tube and thereby maintains synchronism with the traveling-wave energy, even though the electrons of the stream are actually being decelerated. Thus it may be said that the relative phase velocity of the radio frequency energy is decreased. Another technique of tapering is to provide a means for shifting the location of the point or region Within each interaction cell where the cell is coupled to the electron stream without, as with the first method outlined above, otherwise altering the electromagnetic parameters of the interaction cell. The means for coupling between the interaction cells and the electron stream may be the gap between drift tubes extending from the ferromagnetic discs toward each other into the interaction cell. The gap between drift tubes maybe progressively shifted upstream with respect to the electron beam so that the electron stream interacts at progressively shorter distance with successive ones of the interaction cells along the length of the slow-wave structure. It is apparent that an inherent limitation exists in the magnitude of tapering which may be achieved with this latter mode of tapering, that is, the total shifting of the coupling gaps may not exceed the axial length of one interaction cell.

In accordance with one feature of the present invention, isolating, severing means which serve to prevent reflected energy along the slow-wave structure from causing undesired oscillations are utilized also for permitting combinations and repetitions of the above tapering mechanisms along the length of the tube. For example, the gap tapering technique may be utilized and instead of being limited to a total tapering of one drift tube length, there may be that amount of tapering per severed section.

In addition, discontinuities between adjacent sections of the slow-wave structure having different space periods or other characteristics due to utilizing any of the above tapering mechanisms is masked or minimized by the isolator means separating the two different sections.

The novel features of this invention, as well as the invention itself,- may be better understood from the following description, taken in conjunction with the accompanying drawings in which like reference numerals refer to like parts and in which:

FIG. 1 is a side view, partly broken away and partially in section, of a traveling-wave tube in accordance with the present invention;

FIG. 2 is an enlarged side sectionalview of a portion of the slow-wave structure of the traveling-wave tube of FIG. ,1;

FIG. 3 is an exploded view of the elements utilized in the slow-wave structure of PEG. 2;

FIG. 4 is an exploded perspective view ofa group of special elements employed in the slow-wave structure of FIG. 2;

FIG. 5 is a simplified, schematic view of two groups of successive elements employed in the slow-wave structure showing details of the tapering in accordance with the present invention; and

FIG. 6 is a simplified schematic view of a number of groups of elements employed in the slow-wave structure depicting further details of the tapering accomplished in accordance with the present invention.

Referring to the drawings and their description, a number of features are shown for purposes of completeness of discussion of a traveling-Wave tube according to the present invention, Which features are not claimed in the present application but are claimed and described more fully in applications assigned to the assignee of the present application and filed concurrently herewith: Self Aligning Traveling-Wave Tube and Method, by T. Leonard and T. J. Flannery, Serial Number 764,886; Severed Traveling-Wave Tube, by D. I. Bates and O. T. Purl, Serial Number 764,883, which discusses in greater detail and claims the structure illustrated in part in the present F168. 2 and 3; and Periodically Focused Traveling- Wave Tube, by D. J. Bates, H. R. Johnson and O. T. Purl, Serial Number 764,884.

.Thus there is provided unidirectional coupling through the electron stream betweenadjacent amplifying sections.

Adjacent ones of the drift tubesllltl-areseparated by agap 112 which functions as a magnetic gap toprovide a focusing lens for the electron stream and also as an electromagnetic interaction gap to provideinteractionbetween. the electron stream and microwave energy traversing the slow-wave structure.

' cells is decreased.

appears to pass the cells at the same rate, even though it is actually slowing down, thus the relativephase velocity of the traveling waves, it may be considered, is decreased and'the desiredsynchronous interaction may continue to a maximum degree along the entire length of the travelingwave tube.

Additionaldiscussion, on the tapering is given below in connection with the description of FIGS. 3, 5. and 6, subsequent to a completion of the presentation of the other aspects of the figures.

Referring to FIG. 4, there is shown an exploded view of a typical one of the isolator sections shown in dotted lines in FIG. 1, for example, the isolator section ltlll.

In exactly the same manner, microwave energy in the slow-wave structure to the right of the isolator section and traveling toward the isolator section will be substantially completely absorbed by the other termination.

The isolator sections 11. 6, 192, like and 166 each rcpresent a loss of a few decibels of amplification. However, overall they vastly increase the amount or" power amplification or gain which may be achieved in a single traveling-wave tube. The isolation sections isolate adjacent amplifying sections, thereby to preclude instability and oscillations due to reflections and to too great an amplification in a single traveling-wave tube section.

Referring to FIG. 5, a simplified schematic type of drawing is used in order to illustrate clearly another ern- 'bodiment of a tapering traveling-wave tube constructed in accordance with the present invention.

Referring to FIG. 6, there is shown a composite length of slow-wave structure which illustrates, with an example 'of five different amplifying sections'22tl22i, the versatility of the present invention and its subcombinations.

bandwidth characteristics of the tube.

In the fifth of gap tapering, both' web .are utilized. Accordingly,

stream in that-region.

A further advantage, in addition to, thoseof improved be gained in taperrng'a slowwave structure as taught herein is. that of improvmgthe The tapering permits the circuit velocity to be variedso that, with .a given electron bearnvelocity and a dispersive structure, the gain may be made constant over a greater portion of the circuit passband. The eifect 'of this is analogous to the efiect'produced by stagger or oiiset tuning of cascaded, tuned amplifier stages in conventional vacuum tube circuitry.

There has thus been described a novel periodically fo cused, tapered slow-wave structure traveling-wave tube which'combines in one typical element thereof a radlo frequency slow-Wave structure interaction. cell and a periwave tubes of this character, for example: impedance matching devices for further broadening the band of operation and improving the coupling efficiency between the slow-wave structure and an external transmission line; means providing greater stability with regard to undesired oscillations such as those caused from excessive interaction between the electron beam'and higher order perturoed cavity modes or wave-guide modes associtated with the nearly periodic, tapered filter type circuit hereinabo-ve discussed; and means for providing automatic selfalignment in theassernbly of this type of slow-wave structure.

I claim:

1. A slow-wave structure :for a traveling-wave tube having an axis and an electron gunfor projecting an electron stream along said axis comprising: a plurality of radio frequency isolated groups of substantially space periodic interaction cells disposed sequentially along said axis of the traveling-Wave tube, individual ones of said interaction cells being coupled at a coupling point therein to said electron stream, the axial spacing of said coupling points being varied to provide a space periodicity different from that of said interaction cells, said spacing of said coupling points being varied throughout {more than one] at least two of said plurality of groups, and the space 'periodicitz'es of the cells of said two groups being difierent from one another.

2. A slow-wave structure for a traveling-wave tube having an axis and an electron gun for projecting an electron stream along said axis comprising: aplurality of radio frequency isolated group of substantially space iperiodic" interaction 'cells disposed. sequentially along: said cases axis of the traveling-wave tube, individual ones of said interaction cells being coupled at a coupling point therein to said electron stream, the axial spacing of said coupling points being varied to provide a space periodicity different from that of said interaction cells, said spacing of said coupling points being varied throughout more than one of said plurality of groups, said interaction cells of each of said groups of cells having a predetermined Espacedfi space period Ethroughout each of said groups of cells, the said predetermined space periodicitie of] different {ones of said groups being at a selected predetermined relation to one another? from that of the cells of each of the other groups.

3. In a traveling-wave tube of the character having an axis along which an electron stream is projected, a slow- Wave structure comprising: a plurality of groups of substantially space periodic interaction cells for radio frequency energy positioned successively along said axis of the traveling-wave tube in proximity to said electron stream; radio frequency attenuator means positioned between the different adjacent ones of said interaction cells to define the groups thereof, the cells of at least one of said groups having a successively varying periodicity of axial length within the group, and the periodicity of the cells of at least one of the groups being varied with respect to the periodicity of the cells of at least one other of the groups.

4. In a traveling-wave tube having an axis and an electron gun for projecting an electron stream along said axis, a slow-wave structure comprising: a plurality of radio frequency isolated groups of ubstantially space periodic interaction cells disposed sequentially along said axis of the traveling-wave tube, individual ones of said interaction cells being coupled at a coupling point therein to said electron stream, the axial spacing of said coupling points being varied to provide a space periodicity different from that of said interaction cells, said spacing of said coupling points being varied throughout more than one of said plurality of groups, said interaction cells of each of said groups of cells having a predetermined space period Ethroughout each of said groups of cells, the said predetermined space periodicities of] different {ones of said group being at a selected predetermined relation to one another] from that of the cells of each of the other groups, and isolator means disposed between individual ones of said groups for radio frequency terminating the ends of said groups.

5. A slow-wave structure having an elongated axis for providing interaction of radio frequency energy with an electron stream passing along said axis comprising: means defining a plurality of interaction cells spaced sequentially along said axis, said means including a plurality of drift tubes, means electromagnetically isolating selected ones of said cells from the adjacent cells to provide groups of interaction cells, the drift tubes within each of said groups being successively axially displaced in sequential fashion with respect to the position of the associated interaction cells, the interaction cells of successive ones of said groups having a successively closer spacing.

6. A severed slow-Wave structure having a longitudinal axis for providing interaction of radio frequency energy with an electron stream projected along said longitudinal axis comprising: isolator means defining a pinrality of groups of radio frequency isolated interaction cells spaced in ncninterfering relation with said stream sequcntially along said axis: each of said cells being defined by a pair of transverse walls, a plurality of drift tubes, individual ones of which being associated with individual ones of said Kinteraction cells] walls, said drift tubes {associatedfi within Eeach] at least two of said groups being progressively successively axially displaced {in substantially continuous fashionIE with respect to the axial position of its respective associated Einteraction cell] wall, the interaction cells within at least one of said two groups having {thereby an apparently} a closer spac- 12 ing as seen by said electron stream than that of EsaidE the cells within the other of said two groups.

7. A traveling-wave tube slow-wave structure of the character having an elongated axis along with an electron stream is projected comprising: a plurality of radio frequency interaction cells disposed successively along the path of the electron stream of the traveling-wave tube, each of said interaction cells having the general form of a pair of axially separated drift tubes encompassing the electron stream, a pair of supporting discs extending radially outward from each of the drift tubes and a spacer ring between the adjacent discs and substantially concentric with the drift tubes; radio frequency attenuating means positioned at and between selected individual interaction cells, thereby to define individual groups of interaction cells along the slow-wave structure, said groups of interaction cells being arranged to have different, ap parent and actual periodicities in a selected pattern, at least one of said groups having a successive variation in the axial position of the drift tubes therein with respect to the supporting discs, the spacing between the drift tubes remaining the same while the position of the spacing axially with respect to the discs is successively shifted within the cell from a point adjacent one disc to a point adjacent the relatively opposite disc, and the cells of at least one of the groups having discs of different axial dimensions than the cells of at least one other of the groups.

8. A traveling-wave tube comprising electron gun means providing an electron stream, collector means spaced apart from said electron gun means and defining therewith an axis for the traveling-wave tube, and a slow-wave structure positioned along and about said axis for providing interaction of said radio frequency energy with the electron stream, said slow-wave structure comprising: a plurality of pole piece discs positioned successively along and about said axis, said discs having central apertures for the passage of the electron stream therethrough; a plurality of drift tube-defining ferrules, each positioned within the central aperture of a different pole piece disc and concentric with said axis; and a plurality of individual spacer rings concentric with the axis and encompassing said electron stream at a radial distance greater than said ferules, each of said spacer rings being positioned between a different adjacent pair of said pole piece discs, adjacent ones of said ferrules and the adjacent sides of the associated pole piece discs forming together with said spacer rings an interaction cell of said slow-wave structure; attenuating means positioned within individual selected ones of said interaction cells at selected points along said slow-wave structure thereby to define radio frequency isolated groups of interaction cells, the axial position of the ferrules within each group with respect to the associated pole piece discs being sequentially varied, the axial spacing between adjacent ferrules remaining the same, thus to provide an apparent change in the periodicity of the group of interaction cells, the axial thickness of the pole piece discs in the adjacent groups being altered in a succesive fashion, thereby to provide an actual change in the periodicity of one group of cells with respect to another.

9. A traveling-Wave tube slow-wave structure of the character having an elongated central axis along which an electron stream is projected comprising: a plurality of radio frequency interaction cells disposed successively along the path of said electron stream, each of said interaction cells having the general form of a pair of axially gapped, separated drift tubes and contiguously encompassing the electron stream; a pair of ferromagnetic supporting discs extending radially outwardly from each of the drift tubes and a spacer ring between the adjacent discs and substantially concentric with the drift tubes; radio frequency isolating means positioned at and between selected individual interaction cells, thereby to define electromagnetically isolated groups of interaction cells along the slow-wave structure, said groups of interaction cells being arranged to have different apparent and actual periodicities in a selected pattern, at least one of said groups having a succesive variation in the axial position of the drift tubes therein with respect to the supporting discs, the gap between the drift tubes remaining the same while the position of the spacing axially with respect to the discs is successively shifted within the cell from a point adjacent one disc to a point adjacent the relatively opposite disc and the cells of at least one of the groups having spacer rings of a different axial dimension than those of the cells of at least one other of the groups.

10. A Web tapered and cavity tapered, severed traveling-wave tube slow-Wave structure having a longitudinal axis and comprising: means providing an electron stream along said longitudinal axis of said tube; a plurality of magnetic, electrically conductive elements having predetermined axial dimensions, each positioned individually at a different point with predetermined spacing along the length of the longitudinal axis of said tube and each extending into close relation with the electron stream which is provided; means including a highly conductive surface disposed upon the portions of said magnetic conductive elements which are in close relation to the electron stream, thereby to provide a slow-wave structure; and means including a plurality of annular magnet means, each positioned between a different adjacent pair of said magnetic conductive elements and encompassing the electron stream for providing a magnetic field employing said elements to complete a portion of the flux path therefrom, said predetermined axial dimensions of at least some of said magnetic, conductive elements and said predetermined spacing thereof being progressively lessened toward the output end of said traveling-wave tube structure.

11. A traveling-wave tube slow-wave structure of the character having an axis along which an electron stream is projected comprising: a plurality of radio frequency interaction cells disposed successively along the path of said electron stream, each of said interaction cells having the general form of a pair of axially gapped, separated drift tubes and contiguously encompassing the electron stream; a pair of ferromagnetic supporting discs extending radially outwardly from each of the drift tubes and a spacer ring between the adjacent discs and substantially concentric with the drift tubes; radio frequency isolating means positioned at and between selected individual interaction cells, thereby to define electromagnetically isolated groups of interaction cells along the slow-wave structure, said groups of interaction cells being arranged to have different apparent and actual periodicities in a selected pattern, at least one of said groups having a successive variation in the axial position of the drift tubes therein With respect to the supporting discs, the gap between the drift tubes remaining the same while the position of the spacing axially with respect to the discs is successively shifted within the cell from a point adjacent one disc to a point adjacent the relatively opposite disc, the cells of at least one of the groups having spacer rings of a diiierent axial dimension than those of the cells of at least one other of "the groups, and the cells of at least one of the groups having discs of a different axial dimension than those of the cells of at least one other of the groups.

References Cited in the file of this patent or the original patent UNITED STATES PATENTS 2,543,082 Webster Feb. 27, 1951 2,636,948 Pierce Apr. 28, 1953 2,637,001 Pierce Apr. 28, 1953 2,741,718 Wang Apr. 10, 1956 2,810,854 Cutler Oct. 22, 1957 2,813,996 Chodorow Nov. 19, 1957 2,847,607 Pierce Aug. 12, 1958