US2957102A

US2957102A - Self-aligning traveling wave tube and method

Info

Publication number: US2957102A
Application number: US764886A
Authority: US
Inventors: Eugene J Flannery; Leonard Ted
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1958-10-02
Filing date: 1958-10-02
Publication date: 1960-10-18
Anticipated expiration: 1977-10-18
Also published as: USRE25329E; FR1231302A; US2956200A; GB911918A; US2985791A; US2985792A

Description

Oct. 18, 1960 E. J. FLANNERY ETAL 2,957,102

SELF-ALIGNING TRAVELING WAVE TUBE AND METHOD Filed Oct. 2, 1958 4 Sheets-Sheet 1 WWW R [wa /v5 Jam/#40;

Oct. 18, 1960 5. J. FLANNERY ETAL 7,

SELF-ALIGNING TRAVELING WAVE TUBE AND METHOD Filed Oct. 2. 1958 4 sheets-sheet 2 Era.- 2.

Oct. 18, 1960 E. J. FLANNERY ETAL 2,957,102

sELF-Auqnmc TRAVELING WAVE TUBE AND METHOD 4 Sheets-Sheet .5

Filed Oct. 2, 1958 $223 6 mm om om m m 053 M Oct. 18, 1960 E. J. FLANNERY ETAL 2,957,102

SELF-ALIGNING TRAVELING WAVE TUBE AND METHOD.

Filed Oqt. 2, 1958 4 Sheets-Sheet 4 ISOLATOR POLE PIECE ISOLATOR SPACER \40 [as l g l M ATTENUATORS uoo POLE PIECE wx zA razz idin Jaw/[1K 750450441! United States Patent O" SELF-ALIGNING TRAVELING WAVE TUBE AND METHOD Eugene J. Flannery, Northridge, and Ted Leonard, Los

Angeles, Calif., assiguors to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Oct. 2, 1958, Ser. No. 764,886

4 Claims. (CL 3153.5)

This invention relates to electron devices and particularly to methods of and means for providing slow wave structures for traveling wave tubes.

Recent developments in the construction of traveling wave tubes have seen the provision of a great variety of slow wave structures. These structures are of course intended to provide :an effectively tortuous path for radio frequency energy, about an electron stream, in order to slow the energy substantially below the velocity of light and thus [to make possible the desired interchange of energy between the electron stream and the radio frequency energy. Hitherto, slow wave structures have been provided in a number of configurations, the classical one of which is the helix. Later modifications of this structure have resulted in the provision of folded waveguide and equivalent structures. These structures, which utilize members protruding into the region of the electron stream, may be considerably more rugged than the helices and accordingly are less subject to manufacturing and use problems. Such structures are usually, however, extremely difiicult to manufacture whether they are machined from a single block of metal or fabricated in some other fashion.

Accordingly, a recent development in the provision of these structures has been the provision of a slow wave structure built up of a number of individual elements sequentially disposed along the path of the electron beam. By this means, the individual elements may be given quite complex shapes but still be much more simple to manufacture than the earlier structures. The great precision needed for any traveling wave tube becomes of even greater significance, when this type of structure is utilized in close proximity to an electron beam. It may be desired, for example, to utilize a focusing structure which is as close to the beam as possible. If this is done, the slightest deviation of electrons in the stream may resuit in the melting and distortion of a portion of the slow wave structure and consequent loss of the tube. It is highly desirable, therefore, to accurately align the elements of the slow wave structure and to maintain this operative alignment in subsequent operation of the tube. To minimize weight and expense and to increase the chiciency of the assembly it is highly desirable that this be accomplished with the active members of the slow wave structure. It is understood that external aligning devices may be employed, but these not only introduce added size and weight but in addition increase the complexity of. assembly in an undesirable fashion, in a degree which is usually directly dependent upon the accuracy of the alignment which is required.

Accordingly, it is an object of this invention to provide an improved slow wave structure which is more easily and more accurately aligned than the devices heretofore available.

Another object of this invention is to provide a method of very precisely aligning slow wave structures for traveling wave tubes.

A further object is to provide an improved traveling u 2,957,102 Patented Oct. 18, 1960 wave tube structure which may be assembled more readily than the arrangements heretofore available but which at the same time has great rigidity and precise alignment when assembled.

Another object of this invention is to provide a slow wave structure which includes integrally its own metal vacuum envelope and its own means for periodic focusing.

A further object of this invention is to provide an irnproved method of assembling a slow wave structure which does not require extensive assembly techniques having disadvantages of the prior art.

These and other objects of the present invention are achieved by an arrangement in accordance with the invention which utilizes a series of centrally apertured pole piece discs with interposed spacer rings to define a slow wave structure. The spacer rings may be spaced appreciably apart from the central apertures of the discs, but may form part of the slow wave structure assembly. The discs may each be provided with a shoulder or ridge concentric with the central aperture which defines the electronbeam path. The periphery of the spacing ring may then register with the associated shoulders, so as to provide proper radial alignment of the successive discs. The axial dimension of the rings may be selected so as to provide proper axial spacing. Grooves in the faces of the rings which register against the associated discs may be employed to retain brazing, material suitable for affixing the members together rigidly. The rings then also provide a metal vacuum envelopefor the assembly.

A method in accordance with the invention may utilize different coeflicients of expansion of the spacer rings and the pole piece discs to provide an assembly which is very easily put together but very precisely aligned when completed. Specifically, the rings may be made a loose fit within the associated shoulders of the associated discs at normal temperatures, but so selected as to be an exact fit at brazing temperature. With this arrangement, and with brazing materialinserted in the grooves provided in the spacing rings, an undersized rod may be entered through the center of the assembly to provide a simple means of holding the assembly together during brazing. In the brazing period this arrangement causes all the elements to move into the desired precise alignment, which alignment is retained after cooling, without need for any special alignment assembly or without any deleterious effects due to the heating.

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 reference numerals refer to like parts, and in which:

Fig. l is an overall view partly in longitudinal section and partly broken away of a traveling-wave tube constructed with spacer rings 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 invention;

Fig. 4 is a detailed exploded view of a typical isolator section of a traveling-wave tube of Fig. 1; and

Figs. 5 and 6 are separate perspective views of spacer rings which may be utilized in accordance with the 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:

Periodically Focused Traveling-wave Tube by D. I. Bates, H. R. Johnson, and O. T. Purl, Serial Number 764,884; 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 Figs. 2 and 3; and Periodically Focused T ravelingwave Tube With Tapered Phase Velocity by D. I. Bates, Serial Number 764,885.

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 shown in later figures, to permit their being easily slipped between assembled adjacent ones of a series of ferromagnetic pole pieces 16, which are also 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 slow- "wave structure 18 is an input waveguide transducer 20 which includes an impedance step transformer 22. A flange 24 is provided for coupling the assembled travelingwave 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 lefthand 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 electromagnetically interacts with microwave energy being propagated therealong. The electron gun configuration is in accordance generally with the teachings in the Patent No. 2,817,033, by G. R. Brewer, which issued December 17, 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 coeflicient 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 bev of 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 conically-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 of 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 travelingwave tube 12 is thus achieved as rapidly as possible. After the out-gassing procedure, the tube 86 is separated from the vacuum pumping system by pinching 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 sections is isolated from the others by an isolator or

termination section

100, 102, 184 or 106. The structure of these isolating sections will be disclssed in detail in connection with Figs. 2 and 4. It sufiices at this point to describe their function generally as providing a substantially complete radio frequency isolation between adjacent sections of the slow-wave structure 18 while at the same time 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 with the 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 coupling 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 cylindrical extension or lip protruding axially along the stream from the surface of 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 microwave energy traversing the slow-wave structure.

At a radial distance outwardly from the drift tubes 110 each of the pole pieces 16 has a second 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 assembled, it may be placed in an oven within a protective nonoxidizing atmosphere and heated so that the brazing alloy in the channels 120 melts and fuses or brazes the adjacent members of the slow-wave structure 18 together to form a vacuum 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 descriptoin 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 symmetrically 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 100, there is shown a substantial continuity of the pole piece-magnetspacer assembly. However, the pole pieces 124 at either end of the isolator section and the spacer 126 are some what modified, with respect to pole piece 16 and spacer 116 respectively, which will be shown with greater clarity in Fig. 4. It is sufficient here to point out that attenuating material, which may be in the form of lossy ceramic buttons 128 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 arpair of modified cavities 130 6 which lie opposite respective ones of thecoupling holes 122 and which are substantially filled with the lossy attenuating material.

The two cavities 130 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 100.

Along the length of the slow-wave structure 18, in dividual ones of the pole pieces 16 are spaced by axial distances as represented by a, b, c, and d. In a preferred arrangement of the traveling-Wave tube of the present invention, these distances and the associated length of the spacers 116 may be slightly varied with respect to each other so that the effective axial length of the interaction cavities is successively increased toward the output or collector end. This is done in order to decrease the axial phase velocity of the traveling waves so that the desired interaction between the electron stream and the traveling waves will continue to a maximum degree even though the electrons are slowed down toward the collector end.

Referring to Fig. 3, one set of the plurality of pole pieces, magnets and spacers 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, the orientation of the pole piece 16 concentrically about the electron stream is shown. Substantially immediately surrounding the electron stream is the short drift tube 111) which extends axially in both directions normal to the plane of the pole piece 16. The remainder of the pole piece extends radially outwardly from the drift tube 116 as shown. Positioned radially in between these two extremes are the cylindrical shoulder extensions 114 which extend axially outwardly from both faces 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 spacer116.

The inner diameter of the spacer 116 determines the outer 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.

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 sli htly 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 r.;, 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 1' 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 throu h 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.

Referring to Fig. 4, there is shown an exploded view of a typical one of the isolator sections shown in dotted lines in Fig. l, for example, the isolator section 100. The isolator pole pieces 124 are shown in perspective to point out the manner in which they are modified from the typical circuit pole pieces 16. A pair of overlapping circular recessions 136 are provided in the face of each of the isolator pole pieces 124 toward the middle of the isolator section 100. The circular recessions 136 extend approximately half-way through the pole piece 124 and retain the enlarged head portions 138 of the attenuator buttons 128. The attenuator buttons 12% may be formed of a porous ceramic impregnated with carbon. This may be done by soaking the ceramic in a carbohydrate solution, such as sugar, and then baking the soaked piece in an oxygen-free atmosphere to leave a residue of carbon distributed uniformly throughout the volume of the ceramic.

The focusing magnet 14 is typical of the remainder of the focusing magnets and need not be specially modified for the isolator section. The special isolator spacer 126 fits radially within the cylindrical shoulder extensions 114 and has a pair of cavities 130 one each associated with a coupling hole 122. A web end portion 140 closes the end of each of the cavities 130 except for a pair of overlapped openings 142 which are oriented respectively concentric with the circular recessions 136, but have a lesser diameter. The attenuator buttons 128 extend then from the depth of the recessions 136 through the openings 14 2 in the Web end portion 140 through a cavity 130 to approximately half-way through the opposite coupling hole 122.

A circular shoulder 146 is provided on each side of the spacer 126 to receive the end of the drift tube 110 from each of the pole pieces. It is thus seen that the two cavities 130 are isolated from each other by a con ductive mid-portion or vane 150'. The microwave energy in the slow-wave structure 18 to the left in the drawing of the isolator spacer 126 may enter coupling hole 122 of the left hand isolator pole piece shown in Fig. 4 and will intercept the ends of two of the attenuator buttons 128 approximately half-way through the coupling hole 122 Whatever fraction of the microwave energy is not absorbed and dissipated in that portion of the lossy ceramic may pass on into the associated cavity 130 where it will eventually be completely absorbed.

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.

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 1% 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 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 described the general details of the slow wave structure and traveling wave tube with which the present invention may be employed, attention may now be turned to the construction and technique for achieving alignment and spacing. It should first be noted that a number of different spacers may be used. Thus, in Figs. 3 and 6 the spacer 116 may be seen to be a symmetric ring, while as in Fig. 5 the ring 117 may be U-shaped and as in Figs. 2 and 4 the special spacer 126 may be a webbed disc having specially shaped apertures. The

spacing ring

116, 117 or 126, in other words, may serve one or more purposes in addition to its spacing and aligning function, and these additional purposes need not always be the same. The symmetrically shaped ring 116 (Figs. 3 and 6) is the spacer principally used in the slow wave struc ture of Fig. 1. The U-shaped ring 117 (Fig. 5) is employed at the input and output sections of the structure of Fig. 1. The special spacer 126 (Figs. 2 and 4) is used in the isolator sections of the traveling wave tube. With each of these configurations, however, the

spacing ring

116, 117 or 126 may employ a groove or recessed channel 120 in its fiat faces which are to be normal to the path of the electron stream and associated with the adjacent pole piece disc 16 surfaces. A ring of fusible brazing or solder material may be interposed within the grooves 120 when the structure is being assembled. One significant feature in the construction of these spacer rings 116, 117 or 126 is that their size with respect to the shoulders 114 (see Figs. 2 and 3 particularly) in the pole piece discs 16 against which they abut may be carefully selected with regard to the different thermal coefiicients of expansion of the spacer rings 116, 117 or 126 and the pole piece discs 16. Brazing or solder material may be employed within the grooves or channels 120 to provide a unitary structure of the succession of pole piece discs 16 and spacer rings 116, 117 or 126. The brazing may reqnire a predetermined heat level, so that the relative sizes of the spacer rings 116, 117 or 126 and the abutting shoulders 114 of the pole piece discs 16 may change during the heating process. This fact is used to material advantage, as described in the method steps outlined below.

The advantages of the slow wave structure formed of 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 electronstream may be accomplished with members which extend to the very edge of the electron stream, thereby'increasing the efliciency 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 an air tight envelope.

A method employed in assemblying this slow wave structure utilizes the spacer rings 116, 117 or 126 to achieve material advantages in the assembly process. As stated above, the spacer rings 116, 117 or 126 have a different thermal coefficient of expansion than the pole piece discs 16. The spacer rings may be of copper, and the pole pieces 16 of an iron or other ferromagnetic material to provide the magnetic field configurations described above. Because the copper spacer rings 116, 117 or 126 have the higher thermal coeflicient of expansion, they may be fabricated so as to have at normal room temperature a loose fit with respect to the shoulders 114 of the pole piece disc 16 against which they will abut. In assembling this structure, the successive pole piece discs 16 and spacer rings 116, 117 or 126 may be placed on a threaded rod (not shown), sufficiently small in diameter so as to not contact the inner diameters of the supported elements at brazing temperature. The undersized rod extends through the central aperture contained in each of the elements. The special shapes of spacer rings 116 and the pole piece discs 16 may be inserted in the series at the desired points. Brazing material may be inserted in the grooves 120 in the spacer rings 116, 117 or 126. With the assembly thus placed together and all the appropriate sizes in registry, holding members, such as adjustable nuts (not shown), may be threaded onto each end of the rod to hold the members of the assembly in contact. The rod need not, however, serve any purpose as far as alignment is concerned. No separate alignment or jigging assembly is needed with this arrangement, nor need the configuration of the separate elements be altered to provide for alignment techniques. Where the

spacer ring

116, 117 or 126 is not a symmetrical cross section, as with the open-ended shape 117, the relative position of this element may be established accurately by abutment against associated members.

The assembly thus formed may be placed in a vertical position, with respect to the longitudinal axis of the traveling wave tube, in a brazing furnace having the appropriate temperature for the brazing materials used. The brazing temperature should only be such as to insure fusion, bus should not be as high as to destroy the magnetism of the magnets. As the structure is heated, the spacer rings 116, 117 or 126 expand into forcible registry and contact with the associated shoulders 114. As this happens, the spacer rings 116, 117 or 126 and the pole piece discs 16 shift relative to each other and to the cen tral axis of the tube so that each becomes concentric with the central axis of the tube. Concurrently, the brazing material is melted and distributed evenly within the groove 120 and between the spacer rings 116, 117, 126 and the associated pole piece discs 16. When the assembly is cooled, the brazing material provides hermetic sealing at all desired points to provide, in combination, a hermetically sealed envelope. Because the brazing material solidifies before cooling of the spacer rings 116, 117 or 126 disturbs the alignment, the precise alignment achieved is maintained in the rigid structure.

This method has the advantages that all of the succmsive elements may be very quickly and readily placed on the assembly structure without a need for a jig assembly of any kind. The alignment of the various parts to extreme precise dimensions is achieved in a fashion which may be considered to be automatic. Because no external or internal jigs or fixtures need be used, there are none of the recurrent problems of uneven thermal stresses in the alignment structure itself. Consequently, there are no problems in withdrawal of an additional alignment structure or of changes in jig dimensions due to repeated heat cycling. The traveling Wave tubes may be made in various sizes, lengths and configurations without the need for some device for specially aligning each configuration.

Thus there has been described an approved arrangement and method for the construction of slow wave structures having extremely precise alignment in traveling wave tubes. The assembly which is provided may not only be considered to be self-aligning and self-adjusting, but provides an all metal structure which is fully hermetically sealed without requiring a separate and fragile glass envelope and which is extremely rugged and capable of wide structural variation.

What is claimed is:

1. In a traveling wave tube, a slow wave structure consisting of a series of apertured discs which are serially arranged along and normal to the path of an electron stream and which include axially protruding surfaces at selected radial spacings therefrom, a plurality of individual spacer elements each interposed between a different adjacent pair of said discs and each having a central aperture for the passage of said electron stream, each of said spacer elements having opposite flat faces registering with the associated faces of the adjacent discs, an annulus of fusible sealing materials disposed between each of said registering faces, and said spacer elements also each having an outer periphery shaped to conform to and fit within the associated axially protruding surfaces of the adjacent discs, said spacer elements having a higher thermal coeflicient of expansion than said discs whereby heating said structure to a temperature less than the melting point of said sealing materials will cause said spacer elements to forcibly expand against the associated protruding surfaces to concentrically align said spacer elements and said discs.

2. A slow wave structure comprising: a plurality of substantially flat, centrally apertured discs arranged along a given axis and each substantially normal to the given axis, each of said discs having shoulders on each face thereof spaced apart from said central axis a selected distance; a plurality of nonmagnetic conductive spacer rings, each of said rings having a peripheral portion of a size to register radially with the shoulder of said discs, each of said spacer rings being interposed between a different adjacent pair of said discs and hermetically sealed thereto to thereby form a rigid airtight structure, the periphery of said spacer rings being less than the periphery of said discs to form a space therebetween, permanent magnets disposed in each of said spaces to thereby create flux fields in the regions of said apertures.

3. A slow Wave structure comprising: a plurality of substantially flat, centrally apertured ferromagnetic discs arranged along a given axis and each substantially normal to said given axis, each of said discs having a circular ridge on each of the faces thereof and concentric with said given axis, said ridges being spaced apart from said central axis a selected distance; a plurality of nonmagnetic conductive spacer rings, each of said rings having a central aperture, a circular outer periphery and flat side faces, and each being positioned between a different adjacent pair of said discs, the outer periphery of said rings being of slightly smaller diameter than the diameter of the ridges in said discs, the thermal coeflicient of expansion of said rings being greater than the thermal coefficient of expansion of said discs, such that on heating said rings and said ridges abut forcefully to concentrically align said discs and said rings with each other and with said given axis; and a conductive surfacing dis- 11' posed on the interior portions of said discs and said rings to define a tortuous path for a slow wave structure therealong.

4. A slow-Wave structure for a traveling-wave tube, said structure comprising a series of apertured discs which are serially arranged along and normal to the path of an electron stream and which include axially protruding surfaces at selected radial spacings therefrom, a plurality of individual spacer elements, each interposed between a difierent adjacent pair of said discs and each having a central aperture for the passage of said electron stream, each of said spacer elements having opposite flat faces registering with the associated faces of the adjacent discs, each of said spacer elements having an outer pheriphery shaped to conform to and fit 15 within said associated axially protruding surfaces of the adjacent discs, said spacer elements having a higher thermal coeificient of expansion than said discs whereby heating said structure will cause said spacer elements to forcibly expand radially outwardly against the associated protruding surfaces to concentrically align said spacer elements and said discs.

References Cited in the file of this patent UNITED STATES PATENTS 2,637,001 Pierce Apr. 28, 1953 2,741,718 Wang Apr. 10, 1956 2,847,607 Pierce Aug. 12, 1958