US3066237A

US3066237A - Slow-wave structure

Info

Publication number: US3066237A
Application number: US781420A
Authority: US
Inventors: Jr John E Nevins
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1958-12-15
Filing date: 1958-12-15
Publication date: 1962-11-27
Anticipated expiration: 1979-11-27
Also published as: GB879997A; FR1237999A; DE1295705B

Description

Nov. 27, 1962 Filed Dec. 15, 1958 J. E. NEVXNS, JR

SLOW-WAVE STRUCTURE 4 Sheets-Sheet 1 imam.

Nov. 27, 1962 J. E. NEVINS, JR 3,066,237

SLOW-WAVE STRUCTURE Filed Dec. 15, 1958 4 Sheets-Sheet 2 Ava m4. I Jay/v E. Mar/Mr, (72.,

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' Ara/r Nov. 27, 1962 J. E. NEVINS, JR

SLOW-WAVE STRUCTURE 4 Sheets-Sheet 3 Filed Dec. 15, 1958 w w x95 v OwE uuwE MJOa Nov. 27, 1962 J. E. NEVINS, JR 3,066,237

SLOW-WAVE STRUCTURE Filed Dec. 15, 1958 4 Sheets-Sheet 4 ELECTRON GUN llilimlmhlm United States Patent 3,066,237 SLOW-WAVE STRUCTURE John E. Nevins, (In, Los Angeles, Calif, assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Dec. 15, 1958, Ser. No. 781,420 Claims. (Cl. 315-) This invention relates to traveling-wave tubes, and particularly to improved forms of slow-wave structures for traveling-wave tubes.

It is well known that in the operation of traveling-Wave tubes the interaction between the electron stream and the traveling wave causes a diminution in the axial velocity of the electron stream. The consequence of this fact is that the relative axial velocity of the traveling wave and the electron stream become appreciably different toward the collector end of the traveling-wave device. Accordingly, there is a reduction in the effective interaction between the stream and the traveling Wave, and a consequent reduction in the efliciency of the device.

A number of different techniques have been suggested for modifying the slow-wave structure so as to achieve improved interaction along the length of the travelingwave tube. Thus, with the helix type of slow-wave structure there has been provided a gradual decrease in the pitch of the helix along the path of the electron stream to cause a decrease in the axial velocity of the traveling wave to correspond to the similar decrease in the axial velocity of the electron stream. It is, however, difficult to fabricate helices of uniform pitch with the desired accuracy and rigidity, and even more difiicult to fabricate the desired varying pitch helix. It is also exceedingly troublesome and complex to design a tapered pitch slowwave structure which has proper correction and compensation along the structure for the parameters of the structure, for example, its impedance, which are unavoidably altered in the tapering scheme.

The advent of the folded waveguide and various modified folded waveguide slow-wave structures has resulted in the provision of a number of modifications intended to achieve the desired maximum interaction of the traveling wave and the electron stream. The modifications employed have usually been analogous to the change in the pitch of the helix. These modifications have in most instances involved considerable modification of the slowwave structure. In many instances, the predetermined change in the axial velocity of the traveling wave causes a number of problems such that although tapering of the tortuous path to be traversed by a traveling wave may provide increased interaction, it may also tend to decrease the stability and bandwidth of the traveling-wave device.

It is therefore an object of this invention to provide an improved traveling-wave device which has a maximum of interaction between the traveling Wave and the electron stream passing therealong.

It is another object of this invention to provide an improved slow-wave structure which has desirable varia tions in the phase relationship between the traveling wave and associated electron stream, but which is easy to fabricate and install.

A further object of this invention is to provide an improved slow-wave arrangement for the selective control of axial slow-wave velocity and which also can provide regularly and equally spaced interaction cells or cavities.

These and other objects of this invention are achieved, in one exemplification, by a slow-wave structure which employs a plurality of conductive discs which set off individual interaction elements defining cells or cavities or interaction cavity resonators spaced along and normal to the axis of an electron stream. The inner periphery of the discs, adjacent the stream, is terminated by indiice vidual ferrules which are supported by the discs and which are concentric with the beam axis. The remainder of the slow-wave structure is completed by conductive spacer rings between separate adjacent pairs of discs, by a highly conductive surface on the interior faces of the ferrules, discs and rings, and by coupling holes in the webbed portions of the discs between the ferrules and the radially separated spacer rings. A variation in the periodicity of the traveling wave with respect to the electron stream is achieved within this structure without affecting the relative positions of the successive interaction cells. In this arrangement, the axial spacings between successive ferrules is kept substantially unchanged. The position of the ferrule with respect to its associated disc, however, is successively shifted along the length of the tube, because the periodicity of the ferrules is different from that of the cells; that is, the length of the ferrule plus a gap is different from the length of a cell. Also, the ferrule length may vary slightly as a function of distance along the tube. Thus, as the traveling wave is propagated down this slow-wave structure, the point within each interaction cell where the interaction between the electron stream and the traveling wave occurs is successively shifted. The distance the traveling wave moves within the given inter action cell remains constant, so that the actual periodicity remains the same. The action of the slow-wave circuit is greatly benefited because, as seen by the traveling-wave energy, it is purely periodic even though it appears to be tapered to the decelerating electron stream.

The conductive discs and ferrules may be nonmagnetic if the traveling-wave tube is focused by an external magnet. On the other hand, they may be of a magnetic material and actually be the individual magnets or magnet pole pieces if the tube is periodically focused with magnets which are integral with the slow-wave structure.

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

FIG. 1 is an overall view, partly in longitudinal section and partly broken away, of a traveling-wave tube which may be constructed with a tapered slow-wave structure in accordance with the present invention;

FIG. 2 is a detailed longitudinal sectional view of a portion of the tube illustrated in FIG. 1;

FIG. 3 is an exploded view of a set of typical elements included in the structure of an embodiment of the present mvention;

FIG. 4 is a simplified schematic type view showing a tapered slow-wave structure constructed in accordance with the present invention; and

FIG. 5 is a longitudinal sectional view of a practical embodiment of a conventionally focused high power traveling-wave tube utilizing a tapered slow-Wave structure constructed in accordance with the present invention.

Referring to the drawings and their description, 3. number of features are shown for completeness of description of an operable 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, for example: Periodically Focused Traveling-Wave Tube, by D. J. Bates, H. R. Johnson, and O. T. Purl, Serial No. 764,884, filed October 2, 1958, now Patent No. 2,985,792; Severed Periodically Focused Traveling-Wave Tube, by D. J. Bates and O. T. Purl, Serial No. 764,883, filed October 2, 1958, now Patent No. 2,985,791; Periodically Focused Traveling- Wave Tube With Tapered Phase Velocity, by D. J. Bates, Serial No. 764,885, filed October 2, 1958, now Patent f 3,066,237 j v 9 o No. 2,956,200; and Self-Aligning Traveling-Wave Tube and Method, by Eugene J. Flannery and Ted Leonard, Serial No. 764,886, filed October 2, 1958, now Patent No. 2,957,102.

Referring with more particularity to FIG. 1, there is shown a traveling-wave tube 12 utilizing a plurality of annular disc-shaped focusing magnets 14. In the example of this figure, these are permanent magnets and are diametrically split, as discussed later in connection with the description of FIG. 3, to permit their being easily slipped between assembled adjacent ones of a series of ferromagnetic pole pieces 16, which are shown in more detail in the later figures. The system of pole pieces 16 and magnets 14 form both a slow-wave structure and envelope 18.

Coupled to the right hand or input end of the slowwave structure 18 is an input waveguide transducer 20 which includes an impedance step transformer 22. A flange 24 is provided for coupling the assembled traveling-wave tube 12 to an external waveguide or other microwave transmission line (not shown). The construction of the flange 24 includes a microwave window (not shown) transparent to radio frequency energy but capable of maintaining a pressure differential for maintaining a vacuum within the traveling-wave tube 12. At the output end of the tube 12, shown in the drawing as the left-hand end, an output transducer 26 is provided which is substantially similar to the input impedance transducer 20.

An electron gun 28 is disposed at the right-hand end, as shown in the drawing, of the traveling-Wave tube 12 and comprises a cathode 30 which is heated by a filament 32. The cathode 30 has a small central opening 34 to aid in the axial alignment of the gun assembly with the remainder of the traveling-wave tube 12. The cathode 30 is secured about its periphery by a cylindrical shielding member 36 which is constructed in a manner to fold cylindrically, symmetrically back upon itself to form a double cylindrical shield and an extended thermal path from the cathode 30 to its outer supporting means. Such support and an electrical, highly conductive path to the cathode is thus achieved while providing considerable thermal insulation for the cathode and filament due to the extended or tortuous path for heat conduction, as well as because of the multiple cylindrical shielding against radiant heat which is provided by the cylinders shown. For additional details of this type of gun construction, see the patent to J. A. Dallons, No. 2,817,039, entitled Cathode Support, issued December 17, 1957, and assigned to the assignee of the present invention.

A focusing electrode 38 supports the cylindrical shielding member 36. The focusing electrode 38 is generally maintained at the same potential as that of the cathode 30 and is shaped to focus the electron stream emitted by the cathode in a well-collimated, high perveance beam of electrons which traverses the slow-wave structure 18 and electro-magnetically interacts with microwave energy being propagated therealong. The electron gun configuration is in accordance generally with the teachings in the Patent No. 2,811,667, by G. R. Brewer, which issued October 29, 1957, entitled Electron Gun, which is assigned to the assignee of the present invention, and to which reference may be made for a more detailed explanation. The focusing electrode 38 is in turn supported by a hollow cylindrical support 40 which extends from the periphery of the focusing electrode to the right-hand end of the traveling-wave tube 12. Its opening is hermetically sealed with a metal-to-ceramic seal 42 by means of a sealing flange 44 made of a material having a low coefficient of thermal expansion, such as Kovar. The right-hand extremity of the cylindrical support 40 is supported by an annular flange member 46, which also may be Kovar, and which is sealed in turn to a hollow ceramic supporting tube 48. The ceramic tube 48 further thermally insulates the inner intensively heated members of the electron gun 28 and also provides electrical insulation between the cathode-beam focusing assembly and the higher potential accelerating anode 52. Substantially encasing the electron gun 28 and secured to the central or radio frequency structure of the traveling-wave tube 12 is a hollow cylinder 50, which may be Kovar, to which is sealed the ceramic cylinder 48, thus completing the vacuum envelope about the right-hand end of the traveling-wave tube 12.

At the left-hand end of the tube 12, as viewed in FIG. 1, there is shown a cooled collector electrode 60 which has a comically-shaped inner surface 62 for collecting the electrons from the high power electron stream and dissipating their kinetic energy over a large surface. The collector electrode is supported within the end of a water jacket cylinder 64 which is in turn supported by an end plate 66. A water chamber 68 is thus formed between the outer surface of the collector electrode 62 and the inner cylindrical surface of water jacket 64. A water input tube 70 supplies cool water to this chamber and a water output tube 72 exhausts the heated water out of the water chamber 68. Thus, considerable power may be dissipated without destruction of the collector electrode. Although water has been specified, obviously, other liquids or gases may be used as coolants.

The end plate 66 is sealed to a supporting cylinder 74, which may be 0E Kovar, and which is in turn sealed to a ceramic insulating cylinder 76. This ceramic insulating cylinder 76 is sealed at its opposite end to another Kovar supporting cylinder 78, which is in turn supported and sealed to the slow-wave structure end disc 80. The collector 62, the end plate 66, the supporting cylinders 74 and 78 and the ceramic insulating cylinder 76 are all coaxially supported in alignment with the axis of the traveling-wave tube 12.

For vacuum pumping or out-gassing the traveling-wave tube 12, a double-ended pumping tube 86 is connected to both of the input and

output waveguide transducers

20 and 26. Out-gassing during bake-out of the entire traveling-wave tube 12 is thus achieved as rapidly aspossible. After the out-gassing procedure, the tube 86 is separated from the vacuum pumping system by pinch-- ing off the tube at the tip 88.

The traveling-wave tube of the present invention may be severed into a number of amplifying sections 90, 92,. 94, 96 and 98. Each of the amplifying segments or sec* tions is isolated from the others by an isolator or

termination section

100, 102, 104 or 106. The structure of these isolating sections will be discussed in detail in connec-- tion with FIGS. 2 and 4. It sufiices at this point to dcscribe their function generally as providing a substantially complete radio frequency isolation between adjacent sections of the slow-wave structure 18 while at the sametime allowing the electron stream to pass straight through the entire length of the traveling-wave tube 12. Each amplifying section thus provides an optimum gain while; providing freedom from oscillations due to regeneration. The loss in gain due to each of these isolation sections is; of the order of a few decibels and is a low price to pay for the large overall gain and power handling capabilities; of a traveling-wave tube constructed in accordance withthe present invention. It should be noted that although. the isolation sections provide substantially complete: radio frequency isolation between adjacent amplifying, sections, the electron stream is modulated at the output of each amplifying section. The stream thus modulated, as it enters the subsequent amplifying section launches a; new wave therein which is further amplified by the interaction between the new traveling wave and the electron stream. Thus there is provided unidirectional cou-- pling through the electron stream between adjacent amplifying sections.

Referring with more particularity to FIG. 2, there is shown a detailed sectional view of a portion of the traveling-wave tube of FIG. 1. The ferromagnetic pole pieces 16 are shown to extend radially inwardly to approximately the perimeter of the axial electron stream. Disposed contiguously about the electron stream in each case is a short drift tube 110. The drift tube 110 is in the form of a cylinder or ferrule extending axially along the strea and supported by the pole piece 16.

Adjacent ones of the drift tubes 110 are separated by a gap 112 which functions as a magnetic gap to provide a focusing lens for the electron stream and also as an electromagnetic interaction gap to provide interaction between the electron stream and miscrowave energy traversing the slow-wave structure.

At a radial distance outwardly from the drift tubes 110 each of the pole pieces 16 has a short cylindrical extension 114 protruding from its surface. The extension 114 provides an annular shoulder concentric about the axis of the tube for aligning the assembly of the component elements of the slow-wave structure 18. Disposed radially within the extension 114 is a conductive, nonmagnetic circuit spacer 116 which has the form of an annular ring having an outer diameter substantially equal to the inner diameter of the cylindrical extension 114. The axial length of the spacer 116 determines the axial length of the microwave cavities 118 which are interconnected along the length of the slow-wave structure 18. It is thus seen that the slow-wave structure may be assembled and self-aligned by stacking alternately the pole pieces 16 and the spacers 116. Each spacer 116 has two annular channels 120 in which, during the stacking procedure, a sealing material, such as a brazing alloy, is placed. When the slow-wave structure 18 is as sembled, it may be placed in an oven within a protective non-oxidizing atmosphere and heated so that the brazing alloy in the channel 120 melts and fuses or brazes the adjacent members of the slow-wave structure 18 together to form a vacuiun-tight envelope. The spacers 116 are fabricated of a nonmagnetic material, such as copper, thus providing a highly conductive cavity wall while not magnetically shorting out the focusing gaps 112. The entire interior surfaces of the cavities are preferably plated with a highly conductive material, such as a thin silver or gold plating 121.

For interconnecting adjacent interaction cells, a coupling hole 122 is provided in each of the ferromagnetic pole pieces 16, the more detailed shape and orientation of which will be described in connection with the description of FIG. 3 below. Also disposed between adjacent pole pieces 16 are the focusing magnets 14 which are annular in shape and fit angularly or azimuthally sym' metrically about the cylindrical shoulder extensions 114. The magnets 14 may be diametrically split to facilitate their being applied to the slow-wave structure 18 after it has been otherwise assembled. The axial length of the magnets 14- is substantially equal to the axial spacing between adjacent pole pieces 16, and their radial extent is approximately equal to or may be, as shown, greater than that of the pole pieces 16. To provide the focusing lenses in the gaps 112, adjacent ones of the magnets 14 are stacked with opposite polarity, thus causing a reversal of the magnetic field at each successive lens along the tube.

Referring to a typical isolator section 1120, there is shown a substantial continuity of the pole piece-magnetspacer assembly. However, thepole pieces 124 at either end of the isolator section and the spacer 126 are somewhat modified with respect to pole piece 16 and spacer 116, respectively, which will be shown with greater clarity in FIG. 3. It is sufficient here to point out that attenuating material, which may be in the form of lossy ceramic buttons 12% which extend from within a coupling hole 122 through the special spacer 126 and partially into the wall of the pole piece 124 opposite the coupling hole. The spacer 126 forms a pair of modified cavities 130 which lie opposite respective ones of the coupling holes 122 and which are substantially filled with the lossy attenuating material.

The two cavities 131! are substantially isolated from each other by a short circuiting vane, shown in a later figure, and are isolated from interaction with the electron stream by means of a central portion of the special spacer which has the form of a ring having substantially the same radial dimensions as the drift tubes and which extends between two of the drift tubes 110 as shown in a manner to substantially shield the electron stream from the slow-wave structure in the region of the isolator section 1%.

Along the length of slow-wave structure 18 the individual microwave cavities or interaction cells 118 are coupled to the electron stream by means of the gaps 112 between adjacent ones of the drift tubes 110'. In accordance with the present invention, the position of the individual coupling gaps 112 in each cell with respect to the axial center of the end walls of that cell may be varied in a manner to provide a taper of the slow-wave structure. For example, in each section of the traveling-wave tube 12, or particularly in the output section, it may be advantageous, as discussed above, to correct for the deceleration of the electron stream as it gives up energy to the traveling waves traversing the slow-wave structure. As also discussed above, this deceleration is normally inherent and results in a loss of synchronism between the modulated electron stream and the traveling waves which in turn results in a decrease in efficiency of the tube. One way to provide such tapering is to actually change the periodicity of the slow-wave structure so that, for example, the microwave cavities 118 are closer together so that the electron stream, though it is decelerating, will continue to interact with successive ones of the interaction cells with a constant periodicity. However, such changing of the geometric parameters of the interaction cells gives rise to a great many problems which makes the process extremely complicated since altering the geometric parameters of the cells affects the electric parameters, such as the impedance of the circuit. In accordance with the present invention, the geometric parameters of the individual interaction cells 118 which affect the electromagnetic properties thereof are not altered. The drift tubes 110 may be shortened slightly and are shifted upstream to the left progressively along the tube so that the electron stream may experience interaction through the gaps 112 at a substantially constant periodicity, even though the stream is decelerated. In other words, the axial placement of the coupling gap 112 with in each of the interaction cells 118 does not affect the electrical properties of the interaction cell, but the placement of the gaps does affect the point at which the electron stream interacts with the particular interaction cell.

in FIG. 2 it may be seen that the drift tubes 110 in the section of the slow-wave structure 18 disposed to the left of the isolator section 101} have been shifted to the left, that is, upstream so that in the last cell of that section, viz., that adjacent the isolator section 100, the coupling space 112 is all the way to the left in its respective interaction cell. It may also be seen that in the amplifier section to the right of the isolator section the drift tube 110 in the first interaction cell is disposed all the way to the right of that cell, and that in subsequent interaction cells it is shifted progressively to the left as shown. Other practical embodiments and a more schematic and simplified version of the invention is described in connection with FIGS. 4- and 5.

Referring to FIG. 3, one set of the plurality of pole pieces, magnets, spacers and drift tubes or ferrules is shown for purposes of describing more clearly how the individual elements of the slow-wave structure 18 are fabricated and assembled. A typical pole piece 16 is shown twice in the figure, once in plan and once in side elevation. A typical magnet 14 and a typical spacer 116 are shown in side elevation only.

Referring to the side elevation view of the pole piece 16, it may be observed that the orientation thereof is concentric about the electron stream. Substantially immediately surrounding the electron stream and supported by the pole piece 16 is the short ferrule or drift tube 110 which extends axially along the electron stream. As indicated in FIGS. 2, 4 and 5, the axial position of the drift tube 110 with respect to the pole piece 16 may vary from pole piece to pole piece successively along the slow-wave structure 18. Again, additional discussion concerning the axial placement of the drift tubes 110 is deferred to the description of FIGS. 4 and below.

The pole piece apart from the drift tube 110 extends radially outwardly therefrom as shown. Positioned concentrically about the drift tube 110 and radially separated therefrom are the cylindrical shoulder extensions 114 which extend axially outwardly from either face of the pole piece 16.

The outer diameter of the cylindrical extension 114 supports the focusing magnet 14 coaxially about the electron stream while the inner diameter of the extension 114 rests against the outer periphery of the spacer 116. The inner diameter of the spacer 116 determines the outer radial dimension of the interaction cell which is formed between adjacent ones of the pole pieces 16. Before assembly, a sealing material is placed in the channels 120 which are continuous annular grooves in the end surfaces of the spacers 116. As indicated previously, in connection with the description of FIG. 1, the magnets 14 may be diametrically split into an upper half 14a and a lower half 14b to facilitate their insertion or replacement after the tube is otherwise assembled.

An off-center coupling hole 122 is provided through each of the pole pieces 16 to provide the transfer of radio frequency energy from cell to cell along the slow-wave structure 18.

The size, shape and orientation of the coupling hole 122 may be more clearly seen in the plan view thereof at the left hand end of FIG. 3. The drift tube 110 is shown as having an inner radius r slightly larger than the radius of the electron stream and having an outer radius r which substantially defines the inner radius of the interaction cell. The kidney-shaped coupling hole 122 may be formed by an end mill having a diameter extending from r to r The end mill is pressed through the thickness of the pole piece 16 centered upon the arc of a circle 132. The end mill, or preferably the work, may then be swung along this are keeping its center on the circle 132. The work is rotated through an arc of angle a where a may be any angle between zero degrees and, for example, approximately 60. Thus, the kidney-shaped coupling hole 122 lies between a radius r and 1' and has circular ends of diameter r r Disposed radially outwardly from the coupling hole 122 is a cylindrical shoulder extension 114, the inner radius of which is designated r and is substantially equal to the outer radius of the spacer 116. The inner radius r of the spacer 116 determines the outer dimension of the radio frequency interaction cell. The outer radius of the extension 114, designated as r is substantially equal to the inner radius of the magnet 14. The outer radius of the pole piece 16 is designated r and the outer radius of the magnet 14 is designated r For angular alignment purposes during assembly, one or more sets of holes 134 are provided through the pole pieces 16 to hold them in a predetermined angular position with respect to each other. A reference notch 136 may be provided on the periphery of each of the pole pieces 16 in order that one may always know from an observation of the outer surface of the assembled tube what the angular orientation of each pole piece is. In the example described here, the notch is always provided opposite the center of the kidney-shaped coupling hole 122.

The advantages of the slow-wave structure formed of which is the same direction successively alternating spacer rings 116, 117 or 126 and pole piece discs 16 joined together, as by brazing, include the fact that a combined slow-wave structure may be provided which is hermetically sealed and extremely rugged. At the same time, this structure does not require special aligning rods or other aligning devices. It is very precisely positioned, so that focusing of the electron stream may be accomplished with members which extend to the very edge of the electron stream, thereby increasing the efiiciency of the tube. The construction of the device from separate ceramic or metallic shapes of inherently strong configuration means that problems of tube deterioration or destruction due to heating or extreme environmental conditions are minimized. The shoulders 114 in the discs 16 are concentric with the desired electron beam path. Therefore, when the outer periphery of the

rings

116, 117 and 126 registers with the shoulders 114, all the members are accurately positioned and concentric. Furthermore, when the brazing material is fused, the result is a rugged air-tight envelope.

In the operation of the traveling wave tube 12, microwave energy traverses from right to left along the slowwave structure, being amplified first in section 98 due to its interaction with the electron stream. Near the output of this amplifying section, the traveling wave has grown and has caused considerable density modulation in the electron stream. At the first isolator section, section 106 in the drawing, the radio frequency energy in the slow-wave structure 18 is substantially completely absorbed. However, the modulated electron stream passes on into the next amplifier section, section 96, where it launches a new traveling wave in that section. The new traveling wave grows and is amplified by the electron stream until reaching its output end at the isolator section 104. The electron stream is further modulated and the rf energy in the slow-wave structure is again completely absorbed. This procedure is repeated until the highly modulated electron stream enters the output amplifier section through the isolator section 100 and launches a high energy traveling wave upon the output section 90 of the slow-wave structure 18. The output of this final section is fed into the output waveguide through the transducer 26;

The

isolator sections

100, 102, 104 and 106 each represent a loss of a few decibels of amplification. However, overall they vastly increase the amount of 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.

Having considered the nature of the construction of this novel slow-wave structure and traveling-wave tube, the arrangement by which maximum beam and traveling wave interaction is provided may be explained with reference to FIG. 4. FIG. 4 is a simplified schematic representation of a slow-wave structure which utilizes tapering in accordance with the present invention. The general form of intercoupled cavity slow-wave structures is illustrated by a number of conductive walls or fins 142 equally spaced to form interconnected cavities 143 along the path 144 of an electron stream. The electron stream is assumed to travel from left to right in the drawing, 146 as the axial movement The electron stream moves be tween an electron gun 148 at the left-hand end and a collector structure 150 at the right-hand end.

A separate ferrule or drift tube 152 lies within and may be supported by each of the cavity walls or fins 142 and concentrically encompasses the electron stream. Each of the cavity walls 142 is apertured at some point 154 spaced radially apart from the respective drift tube 152 to provide energy coupling to the adjacent cavity. With this arrangement, it may be seen that the geometrical dimensions of the separate cavities 143 are the same. That of the traveling wave.

is, the characteristic length of each cavity 143 as defined by the separation between adjacent walls is constant. The distance between adjacent ferrules 152 which defines the gap 156 of each cavity is maintained constant also. The length of each of the drift tubes 152 may, however, vary slightly without electromagnetically affecting its respective cavity in order to facilitate the shifting of the drift tubes.

The distance which is varied in this structure, however, is a distance 158 between the centers of the adjacent gaps. This may also be visualized as a shifting of each gap with respect to the cavity. Note that the shifting is in the direction toward the electron gun 148 so that in effect the ferrules or drift tubes are shifted upstream with respect to the electron stream. The maximum amount of tapering of this type which can be done is determined by the length of the drift tubes and by the gap spacings with respect to the cavities, as well as by the degree of taper or the amount of incremental shifting which is desired between cavities.

The operation of the general intercoupled cavity structure 140 illustrated in FIG. 4 provides maximum interaction between the electron stream and the traveling wave. The traveling wave is isolated, in a sense, frorri the electron stream over an appreciable portion of its travel through each of the cavities 143. Interaction occurs between the traveling wave and the stream only at each of the gaps 156 in the cavities. The interaction results in the charged particle stream giving up some of its energy to the traveling wave because of the slightly greater average axial velocity of the electron stream. This electromagnetic coupling in turn slows down the electron beam. It is important to note that if the traveling wave slow-wave structure is periodic throughout, it will have greatest stability and be easiest to control. This constant periodicity is maintained, in the present invention, even though the interaction point of the gaps 156 is adjusted along the length of the slow-wave structure to compensate for the decrease in the axial velocity of the electron stream.

As the beam gives up energy, the time of passage from gap to gap increases, if the gaps remain equally spaced. The tapering or incremental shifting here of the gaps 156 relative to the cavities 143, however, compensates for this slowing down. In effect, the electrical length of each cavity as seen by the stream is kept equal to the phase shift per cavity.

Whereas, the arrangement of FIGS. 1 through 3 illustrates the operation of the invention with periodic permanent magnet focusing, non-periodic and electromagnet focusing may also be employed, as is illustrated in FIG. 5. As shown therein, a traveling-wave tube 160 having an electron gun end 162, a collector end 164 and an intermediate slow-wave structure 166 may have an electromagnet 168 encompassing the slow-wave structure 166. The slow-wave structure 166 is again of the intercoupled cavity type, and an input 170 and output 172 for the traveling wave are coupled to the extremities of the slowwave structure 166. As described in conjunction with previous arrangements, the intercoupled cavities 174 include drift tubes 176 having fixed gap spacings 178 relative to each other and concentric with the electron stream.

The necessary focusing of the electron stream for this arrangement is provided because the permeability of the intercoupled cavity structure is not sufiicient to provide shunting of the magnetic focusing field created by the electromagnet. The gaps 178 between the drift tubes 176, are, however, successively shifted upstream, in accordance with the previous description, to keep the periodicity of the traveling wave structure constant while utilizing the optimum, substantially equally time spaced, interaction points in or along the electron stream.

Thus there has been described an improved slow-wave structure for traveling-Wave tubes which is extremely It) easy to manufacture and which has important operative advantages but which does not increase the complexity of the slow-wave structure. Maximum interaction between the traveling wave and an electron stream can be achieved by creating an independence between the desired constant periodicity of the slow-wave circuit and the tapered periodicity of the stream interaction gaps without the introduction of phase instabilities or traveling wave degradation.

What is claimed is:

l. A slow-wave structure for providing interaction between an electromagnetic wave being propagated thereby and a stream of charged particles being projected along a predetermined path comprising: a series of electromagnetic elements each defining an interaction cavity disposed in sequence along said path, each of said interaction cavities being electromagnetically exposed to said stream at an axial position within said interaction cavity, said axial position with respect to the axial center of each of said interaction cavities being shifted progressively upstream along the length of said slow-wave structure in a manner to effectively taper the interaction along said slow-wave structure while not otherwise affecting the physical parameters of said interaction cavities.

2. A slow-wave structure having a changing apparent periodicity to the electron stream and an actually constant periodicity, said slow-wave structure comprising: a plurality of elements each defining an interaction cavity positioned along the axis of the electron stream, said interaction cavities being defined by a plurality of regularly spaced planar Web members exending radially outwardly from a point adjacent the electron stream and ring member disposed between and interconnecting adjacent web members, said ring members being radially spaced apart from said electron stream; and a plurality of drift tube segments, each encompassing the electron stream and concentric therewith, and each coupled to a different web member, the relative axial position of said rift tubes with respect to said regularly spaced web members being shifted along the length of the slow-wave structure.

3. A slow-wave structure for providing interaction between an electromagnetic Wave being propagated therealong and a stream of charged particles traversing a predetermined path, said structure comprising: a series of planar conductive disc members disposed transversely to and concentrically about said stream, said members being substantially equally spaced and being the axial termini of adjacent electromagnetic elements each defining an interaction cavity, and a like series of conductive ferrules having an axial length greater than the axial thickness of said planar disc members, individual ones of Which are supported in a predetermined axial position by and with respect to respective and individual ones of said disc members, said ferrules being axially spaced to provide interaction coupling between said stream and respective ones of said interaction cavities, said conductive ferrules along said path being progressively shifted upstream with respect to its respective cavity to provide a slow-wave structure which appears to the electron stream to be tapered whereby the time between interaction with successive ones of said interaction cavities as experienced by the decelerating stream of particles remain substantially constant throughout the length of said structure.

4. A high power periodically focused traveling-wave tube having a tapered slow-Wave structure comprising: means for producing an axial electron stream along the length of said tube; a plurality of radio frequency elements each defining an interaction cavity intercoupled along the length of said tube, each comprising a ferromagnetic drift tube disposed contiguously about said electron stream; a ferromagnetic pole piece forming an end wall of each cavity and a nonmagnetic conductive short hollow cylindrical spacer disposed concentrically about said electron stream between adjacent ones of said pole pieces; an

annular focusing magnet having an axial length substantially equal to that of said spacer and disposed concentrically thereabout, said pole piece extending radially from said drift tube to approximately the radial extremity of said magnet and being relieved forming a coupling hole therethrough between said drift tube and said spacer, said spacer being hermetically bonded along its end to said pole pieces whereby along the length of said slowwave structure a vacuum envelope is provided, said drift tube having an axial length substantially greater than the axial thickness of said pole piece, the drift tubes associated with said cavities toward the output end of said tube being axially shifted with respect to said pole piece toward the input end of said tube whereby said electron stream traverses a progressively shorter distance between interaction cavities, thus effectively tapering the traveling-wave tube in a manner whereby said electron stream, while giving up a portion of its energy thereby slowing down, continues to deliver energy to the radio frequency traveling waves traversing said slow-wave structure.

5. A slow-wave structure for providing electromagnetic energy being propagated by said slow-wave structure and a stream of charged particles traveling in a given direction along a given path, said slow-wave structure comprising: a series of conductive, substantially planar members disposed sequentially along said path perpendicularly thereto, and a series of conductive ferrules supported in axial registry contiguously about said path by respective ones of said conductive planar members, adjacent ones of said planar conductive members determining axially a series of interaction cavities, adjacent ones of said conductive ferrules being spaced from each other to provide interaction coupling between said stream and said interaction cavities, said conductive ferrules being shifted upstream with respect to their respective supporting planar conductive members in a manner such that the coupling spacing between successive adjacent ones of said ferrules is progressively shifted upstream, thereby effectively to taper said slow-wave structure while not otherwise affecting the geometric parameters of said interaction cavities.

6. A slow-wave structure for propagating an electromagnetic wave in energy exchange relation with a stream of charged particles projected along an axial path comprising: a series of electromagnetic elements each defining an interaction cavity disposed in sequence along said path, each of said cavities being axially terminated by a conductive Wall disposed transversely to said path at opposite axial ends of said cavity, and each of said cavities being radially determined on its outer extremity by a conductive wall extending between said axial end walls substantially orthogonal thereto and being terminated on its inner extremity by a surface comprising a conductive ferrule supported by one of said end walls and extending into said cavity in a manner whereby a series of ferrules corresponding one to one with said conductive walls are disposed contiguously about said path with a gap between adjacent ones of said ferrules to provide electromagnetic coupling between said stream of charged particles and said cavities, the relative axial position of successive ones of said ferrules with respect to its respective supporting conductive wall being shifted toward the upstream direction of said stream of charged particles to thereby provide a tapering for said slow-wave structure.

7. A slow-wave structure for traveling-wave tubes of the character having a longitudinal axis along which an electron stream is projected, said slow-wave structure comprising: a plurality of planar ferromagnetic discs, said discs being positioned at successive individual points along the electron stream axis of said traveling-wave tube, and substantially normal thereto and concentric therewith, each of said discs having a central apertured portion about the electron stream; ferromagnetic ferrule members, each of said ferrules defining the inner aperture of a different ferromagnetic disc and each being concentric with the electron stream, said ferrules being of like length and having like axial spacing, the axial relationship along the electron stream of successive ferrules with respect to the associated magnetic disc being successively shifted along the length of the traveling-wave tube; a plurality of conductive spacer elements, each positioned between a different adjacent pair of ferromagnetic discs and spaced radially apart from the ferrules thereof; and highly conductive surfacing disposed in the interior portions of said spacer rings and on said ferrules and the interior portions of said ferromagnetic discs.

8. A traveling-wave tube slow-wave structure comprising: a plurality of radio frequency interaction cavities disposed successively along the path of the electron stream of the traveling-wave tube, each of said interaction cavities having the general form of a pair of axially gapped, separated drift tubes and contiguously encompassing the electron stream; a pair of conductive supporting discs extending radially outwardly from each of the drift tubes and a spacer ring between the adjacent discs and disposed substantially concentric with the drift tubes, said interaction cavities being arranged to have different apparent, as seen ,by the electron stream, and actual, as seen by the traveling wave, periodicities in a selected pattern, at least some of said cavities 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 cavity from a point adjacent one disc to a point adjacent the relatively opposite disc.

9. A traveling-wave tube comprising: an electron gun positioned at one end thereof; means for projecting an electron stream of predetermined average diameter along the longitudinal axis of said traveling-wave tube; input means including an input microwave transmission means for impressing upon the input end of said tube a microwave signal to be amplified; output means including an output microwave transmission means for extracting an amplified signal from said traveling-wave tube; a plurality of electrically conductive magnetic disc members axially positioned along the electron stream path and being spaced apart by a substantially constant axial distance, each of said discs comprising an inner axially extended drift tube portion defining a central aperture for passage of the electron stream, adjacent ones of said drift tubes being axially separated at a predetermined point between adjacent ones of said discs to permit interaction between said microwave signals and said electron stream, said predetermined point being progressively varied with respect to its axial position between said discs along said traveling-wave tube to provide a tapered slow-Wave structure whereby as the electron stream is decelerated it continues to interact to a maximum degree with the microwave energy traversing the length of said traveling-wave tube, each of said disc members also including an inner web portion extending radially outwardly from said drift tube and having an axial intercoupling' aperture therethrough; a plurality of conductive nonmagnetic annular cylindrical spacers, each concentric with said longitudinal axis and hermetically sealed between adjacent ones of said disc members at a radial position such that the inner surfaces of said spacer define the outer cylindrical surfaces of the radio frequency interaction cells between adjacent ones of said disc members; highly electrically conductive surfacing disposed over the outer surfaces of said drift tubes and over the inner web portion of said disc members for providing a continuing surface of high electrical conductivity along the surfaces defining the slow-wave structure of said traveling-wave tube; a plurality of split annular permanent magnets concentric with the longitudinal axis of said tube and at least coextensive with the outer portions of said disc members radially beyond the outer surfaces of said spacers, said split annular magnets having an axial length aong said tube substantially equal to that of the spacer between respective ones of said disc members and substantially registering with the outer extremity, each of split annular magnets being positioned between a different successive pair of said disc members and being magnetized substantially parallel to said longitudinal axis to form magnetic poles of opposite polarity on the axial extremities of adjacent respective ones of said drift tubes, thus providing periodic focusing lenses for the electron stream passing con-tiguously therewithin.

10. A periodically focused traveling-wave tube ineluding a plurality of annular focusing magnets and disc-like ferromagnetic pole pieces arranged alternately in sequence along the length of said tube, each said pole piece extending radially outwardly to approximately the radial extremities of said magnets and extending inwardly to a point radially contiguous to the electron stream of said tube and having a short axially extending cylindrical extension protruding from the plane of said disc-like pole piece at the inner region thereof about said electron stream to form a drift tube therefor, said traveling-wave tube also including nonmagnetic conductive annular spacer ring members having inner and outer diameters between that of said drift tube and the outer diameters of said pole piece individually disposed between a pair of adjacent pole pieces for maintaining critical axial spacing therebetween, each said spacer ring member having an inner diameter equal to a predetermined diameter of a desired radio frequency interaction cavity, the pole pieces having coupling holes therethrough between said drift tube and said second cylindrical extension, whereby there is formed an interaction cavity determined by a pair of said pole pieces separated by said spacer, the inner surface thereof defining the outer cylindrical surface of said cavity, and said drift tubes extending axially from each of said pole pieces defining the inner cylindrical surface of said cavity, said drift tubes being separated by a gap disposed at a predetermined position Within said cavities for coupling radio frequency energy in said interaction cavities with said electron stream, as well as a magnetic focusing lens disposed contiguously about said electron stream, said predetermined position of said gaps being shifted progressively upstream along the length of said travelingwave tube toward its output end to provide an effectively tapered slow-Wave structure while maintaining the actual periodicity of said interaction cells as well as their electrical parameters substantially constant along the length of said traveling-Wave tube.

References Cited in the file of this patent UNITED STATES PATENTS 2,636,948 Pierce Apr. 28, 1953 2,841,738 Pierce July 1, 1958 2,847,607 Pierce Aug. 12, 1958 2,922,920 Convert Jan. 26, 1960 2,956,200 Bates Oct. 11, 1960 FOREIGN PATENTS 753,999 Great Britain Aug. 1, 1956