US2834915A - Traveling wave tube - Google Patents

Description

May 13, 1953 4 E. c. DENCH 2,834,915

TRAVELING WAVE. TUBE.

Original Filed Oct. 30, 1953 2 Sheets-Sheet 1 //v VEN TOI? EDWARD C. DEA/CH y 13, 1958 E. c. DENCH 2,834,915

IRAVELING WAVE TUBE Original Filed Oct. 50, 1953 2 Sheets-Sheet 2 FIG. /2

. INVENTOI? EDWARD C. DENCH ATTORNEY United States Patent Office 2,834,915 Patented May 13, 1958 TRAVELING WAVE TUBE Edward C. Dench, N eedham, Mass, assignor to Raytheon Manufacturing Company, Waltham, Mass., at corporation of Delaware Original application October 30, 1953, Serial No. 389,415. Divided and this application March 4, 1955, Serial No. 492,073

4 Claims. (Cl. 315-393) This application is a division of application, Serial No. 389,415, filed October 30, 1953, by Edward C. Dench.

This invention relates to improved techniques of constructing traveling wave electron discharge devices and the anode periodic structures used therein.

If a traveling wave tube is to function properly, the periodic anode delay line used therein must be mechanically accurate so that the proper phase relation between the electron beam and the traveling wave energy is maintained. As the operating frequency increases, the physical size of the elements of the periodic network, such as the fingers of an interdigital line, tends to decrease so that still greater accuracy of construction of the anode delay line must be attained at microwave frequency.

When the power requirements of the traveling wave tube are relatively small, a high order of precision in the construction of anode delay lines may be obtained by resorting to a printed circuit technique. Although several printed circuit techniques are available, perhaps the most accurate is the photoengraving process. An electri- Cally-insulating base is plated with a thin film of electri- Cally-conductive material, such as copper. A carefully constructed layout is made of the delay line and a picture of the line is taken with an appropriate camera. A light sensitive enamel which is also resistant to acids is applied to the metallized surface of the insulating base or support. The negative obtained by the aforesaid photographic means is positioned over the sensitized surface and light from a source of visible radiation is directed thereupon. The unexposed portion of the enameled surface is washed away and the metallized layer not coated by enamel is etched. After removing the remaining enamel resist, the metallized surface remaining on the insulating base forms an exact replica of the desired anode delay line. Once a negative has been made of the layout,

any number of anode delay lines may be manufactured,

all of the same high degree of accuracy.

Alternatively it is possible to apply an etching ink to the metallized layer through the apertures of a circuitdefining stencil to coat the ink with an acid resistant powder which is fused under heat, and to etch and clean as before.

Other methods of printing which may be used for forming the anode delay lines are painting or spraying a conductive film directly on an insulating base and then firing, an electrolytic process, or vacuum sputtering or evaporation techniques. 7

Another modification of a periodic anode delay line is a laminated structure. This is particularly adapted for use in an interdigital periodic delay line, such as described and illustrated in an'application for U. S. Letters Patent, SerialNo. 382,025, of E. C. Dench, filed September 24, 1953. These interdigital lines have been found very satisfactory for use in low power traveling wave amplifiers and oscillators and are characterized by good dispersion characteristics and a reasonably high impedance. Each half of the anode may be punched or stamped sepa i2. rately in the form of a comb and the two laminated portions positioned together and fastened to an appropriate base. The laminated periodic delay line, like that obtained by the printing techniques referred to above, is highly accurate, in addition to being relatively inexpensive to produce and readily reproducible.

Each of the techniques above described may be used in constructing other types of anode delay lines. For example, the photoengraving method previously described may be used to produce a helical conductor for helix-type traveling wave tubes.

The foregoing and other features of this invention will become more apparent from the detailed description of certain specific embodiments which follow. The description refers to the accompanying drawings wherein:

Fig. 1 is an isometric view showing a traveling wave tube incorporating an interdigital anode delay line obtained by printed circuit techniques;

Fig. 2 is a fragmentary cross-sectional view of the tube of Fig. 1 showing the anode delay structure;

Figs. 3 to 5 illustrate certain steps in the manufacture of the anode structure of Fig. 1;

Figure 6 illustrates an alternative method of manufacturing the aforesaid structure;

Figs. 7 to 10 are illustrative of a method of manufacturing a cylindrical anode delay structure by a printed circuit technique;

Fig. 11 is a view of a tubular member such as shown in Fig. 8 modified for producing a helix rather than an interdigital delay line; a

Fig. 12 is a .view showing a typical pair of laminations for use in an interdigital anode structure;

Fig. 13 is a plan view of the laminations of Fig. 12 as assembled to form an interdigital anode delay line; and

Fig. 14 is a cross-sectional view of the interdigital anode of Fig. 13.

. Referring to Fig. 1, a traveling wave oscillator tube 20 is shown incorporating a linear periodic anode structure 22. This anode delay line, which is formed in a manner to be described later, is carried by an electrically insulating base plate 24 which, in turn, is securely attached by any appropriate means to a continuous electrically-conductive backing member 25. Backing member 25 forms one of the walls of an evacuated envelope which further includes an oppositely disposed wall 26, side walls 27 and 28 and a pair of end walls, one of which, namely wall 30, is shown in Fig. 1.

The interdigital anode network 22, like that described in the aforesaid copending application, comprises a pair of electrically-conductive interleaved assemblies or combs 31 and 32, each including a longitudinal portion and a plurality of spaced fingers which connect to said longitudinal portion and extend transverselyto the latter and almost to the opposite assembly.

Attenuation may be introduced into the interdigital anode network 22 by means of a coating of lossy material, such as iron, which may be plated on the fingers over the region of the network in which attenuation is desired.

The inner conductor 34 of a coaxial coupling device 35 extends through an aperture 36 in wall 25 and base plate 2-4 and is attached, as by soldering, to an end finger of the interdigital line, as clearly shown in Fig. 1.

Although the device as shown in Fig. 1 has only one energy coupling means, and therefore is adapted to operate as the traveling wave oscillator, it should be understood that the invention is not so limited, but may be applied equally well to a traveling wave amplifier, provided, of course, that the electron beam is synchronized with the proper space harmonic of the electro-magnetic field.

The traveling wave tube 20 further includes a cathode structure 38 having an electron emissive surface 39; this cathode is positioned adjacent the output end of the structure. Cathode 38 is supported by a hollow supporting cylinder 40 extending through an aperture in wall 26 of the tube envelope. A central conductor 41 is located inside cylinder 40 of a coaxial support assembly 47 and is connected to one end of a heater coil, also not shown, positioned in thermal proximity to the cathode emissive surface. Cylinder 40 is insulatedly supported with respect to wall 26 by means of a metallic member 44 sealed, in turn, to insulating seals 45. The latter are connected to an electrically-conductive cylinder 46 which surrounds cylinder 44) and is, in turn, attached to recesses in wall 26 surrounding the apertures through which cylinder 40 passes. The construction of this cathode is set forth in considerable detail in an application for U. S. Letters Patent, Serial No. 255,499 of E. C. Dench, filed November 8, 1951, now Patent No. 2,809,328, dated October 8, 1957.

An auxiliary electrode or sole 42 is disposed substantially parallel to the anode network and spaced therefrom. Sole 42 is a U-shaped member which is supported with respect to the tube envelope by means of a pair of supporting rods 43 rigidly attached to the sole and forming part of a coaxial support assembly 47 similar in construction to that of assembly 47 used for support of the cathode.

Positioned at the opposite end of the sole from the cathode and in substantial alignment with the sole is a collector electrode 48 rigidly supported by means of a rod 49 extending through an aperture in wall 26 and spaced from said wall. Rod 49 is supported relative to wall 26 by means of a terminal support assembly 50 similar to that used for sole 42. In some applications, the collector electrode 48 may be omitted.

An electric field may be established between the anode and sole by connecting a source 52 of voltage therebetween. The necessary electrical connection between anode 22 and wall may be made by means of electrically-conductive screws or rivets (not shown) used for attaching the anode assembly, including base 24, to the tube envelope. The cathode 38 is negative with respect to anode 22 and may or may not be at the same potential as sole 42.

A transverse magnetic field is produced in the space between the periodic anode structure and the sole in a direction perpendicular to the electric field. The pole pieces 54 and 55 of the magnetic field producing means are shown in Fig. 2. By proper adjustment of the magnetic field, the electrons emitted from the cathode 38 will be directed along a path adjacent the fingers of the periodic structure 22. Interaction of the electron beam.

with a backward wave traversing the periodic anode structure will result in the generation of oscillations in the tube. See the prior comments as to the adaptability of the periodic anode structure in a traveling wave amplifier.

One method of producting the interdigital anode delay line is shown in Figs. 3 to 5. For purposes of clarity the thickness of certain layers of the material and circuit defining objects is exaggerated.

A layout of the particular interdigital anode desired is made, as, for example, by India ink on white drawing paper. A picture is taken of this layout by means of a camera having a reasonably high quality lens. A printing negative 60 is then produced by the usual photographic techniques so that the light portions thereof correspond in configuration to that of the interdigital line, while the dark portions correspond to the portion of the original layout not covered by drawing ink.

The insulating base plate 62, which may be of ceramic, glass or other acid resistant electrically-insulating materials, is coated, as by any well-known plating process, with a thin layer or film 63 of metal, such as copper.

The copper-clad plate is next coated with a light-sensitive enamel 64 which is impervious to acid, such as a dichromate solution. After the enamel has dried sufficiently, the negative 60 is placed over the sensitized base plate 62-64 and the masked plate is exposed to a high intensity source 65 of light for a proper time, dependent upon the type of enamel and the type of light source used, as shown in Fig. 3.

The light passing through the transparent regions of negative 60 causes the portion of the enamel layer 64 corresponding to the transparent regions to harden. After removing the negative, the unhardened portion of the enamel surface is washed away by means of an appropriate enamel solvent, such as alcohol or water, leaving a structure as shown in Fig. 4. The plate is next exposed to heat until the enameled surface still extant is completely dried and hardened. This surface 64 now serves as a resist which is impervious to the action of acids.

The plate is next etched in an acid solution which is chosen in accordance with the material of which the anode delay line 22 is to be made. If the metallized layer 63 is made of copper, a solution of ferric chloride may be used for etching purposes. As soon as the areas of the copper layer 63 on base plate 62 not protected by the enamel resist 64 are completely removed by the chemical action of the etching solution, the plate is rinsed, thereby leaving the enamel covered interdigital anode line shown in Fig. 5. Finally the enamel resist 64 is removed by an appropriate enamel remover, such as a caustic potash solution. The copper-clad base plate bearing a desired circuit design is now ready for mounting by any desired means to the wall 25 of traveling wave tube 20.

In Fig. 6 an alternate approach for producing the interdigital line of Fig. 1 is shown. An electrically-insulating base plate 62 is coated with a thin layer 63 of copper, silver or other conductive material, as before. A stencil 66 having apertures 67 therein defining the desired anode line is placed over the metal-clad base plate 62, 63 and an etching ink applied, as by a brush 68, to the surface of the metal-clad plate through the stencil apertures. The inked image on the plate is protected from the action of acid by means of an acid resisting powder, such as com'minuted resin or shellac, which, when poured over the greasy etching ink, adheres thereto. The plate is next inserted in an etching bath suited to the particular metal of which the metallic layer 63 is constituted. For example, a copper layer may be etched by the ferric chloride solution previously referred to or by an aqueous solution of nitric acid. Finally the resist (combination of ink and resin) is removed from the plate by means of a strong solution of caustic potash.

It is, of course, possible to form the interdigital line 22 by applying conductive paint, such as a lacquer containing a relatively large proportion of metal powder, directly to the base plate 62 through a stencil 66 of Fig. 6. This paint may be applied either by brushing or spraying.

In Figs. 7 to 10, a means for producing a cylindrical anode interdigital delay line suitable for use in a cylindrical traveling wave tube is shown. An electrically-insulating tube 70 made of glass, quartz or ceramic is coated on the inside with a layer 63 of copper or other electrica'lly-conductive material approximately .002 inch thick. The thickness, however, may vary with the material being used and the power requirements of the traveling wave tube. A photosensitive layer 64, such as already described, is then applied over the metallic surface 63, as shown in Fig. 7. An optically transparent plastic tube 74, whose outside diameter is slightly less than the inner diameter of tube 70, when coated, is painted with a material which is opaque to light, as shown in Fig. 8.

The pattern on tube 74 may also be formed by wrapping a photographic negative around the outside of the tube. This tube which corresponds to the negative 60 of Fig. 3 is preferably, though not necessarily, split so that it may be resiliently adapted to fit snugly Within the tube 70. *In preparing tube 74 it is necessary to leave the tube uncoated over the areas corresponding to the various fingers of the desired interdigital anode line.

Tube 74, of course, may be made of any resilient material which is transparent to light. As shown in Fig. 7,

the partially transparent tube 74 resulting from the application of the coating is shown in Fig. 8. Tube 74, having alternate regions which are transparent and opaque to light, is placed within tube 70 of Fig. 7 and a light source, such as a tubular are lamp 75, is next inserted within tube 74. After the necessary exposure has been obtained, the light source 75 and transparent tube 74 are removed and the unh-ardened portion-of the photosensitive layer 64 is washed away. After etching and removal of the hardened portion of the photosensitive layer, a periodic structure 22 having arcuate fingers 77 connected to and extending from a longitudinally disposed base element 76 is obtained, as shown in Figs. 9 and 10. A coaxial input and output coupling means 78 and 79, respectively, are shown in Fig. 9. An electron beam produced by a standard type electron gun is directed longitudinally adjacent the various fingers 77 of the line as indicated by the arrow in Fig. 9. The inner conductors 80 and 81 of respective coaxial means 78 and 79 may be attached to the end fingers of the line, as by soldering. The use of two energy coupling means at opposite ends of the anode delay line presupposes operation of the associated traveling wave tube as an amplifier. If a traveling wave oscillator is desired, it is obvious that only a single energy coupling means would be used.

The construction of the printed anode line and the connection of the energy coupling means to the line is more clearly shown in Fig. 10 in which the line has been removed from the inner surface of the tube 70.

In Fig. 11, plastic tube 74 is coated with a helical layer of material 83 which is opaque to light. The tube 74 is positioned within tube 70 and the process previously described in connection with Figs. 7 to 9 is repeated. In i this manner a helix is formed on the inner surface of tube 70 instead of an interdigital line. Although Figs. 7 to 11 have disclosed a helix and inter-digital line, it is possible to form anode delay lines of other configurations by means of the invention.

In Figs. 12 to 14, an additional approach to the problem of constructing interdigital anode delay lines for traveling wave tubes is illustrated. In Fig. 12 the two opposing halves 85 and 86 of the interdigital line 22 are stamped or otherwise formed from sheets of electricallyconductive material, such as copper. These laminations may contain circularapertures 87 for mounting purposes. The thickness of the sheets will be determined, to a large extent, on the power handling requirements of the traveling wave tube incorporating said anode structure. For example, for relatively low power tubes, the laminations 85 and 86 may be made from metal foil. When higher power tubes are desired, several identical sheet metal laminations may be stacked together as shown in Fig. 14. The selective arrangement of the laminations 85 and 86 of Fig. 12 is shown in Fig. 13. The laminations may be fastened directly to the wall 25 of traveling wave tube 20, as by rivets 90.

In Fig. 14, the stacked laminations 85 and 86 are shown fastened to base 88 as by screws 89 inserted through aligned apertures in the laminations and base. Base 88 may be fiat, or U-shaped, as shown in Fig. 14.

It should be understood that this invention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. For example, the traveling wave tubes need not be linear, as shown in the drawings, but may have other configurations such as circular or arcuate. In addition, the modifications shown 6 in the drawings may be applicable either to traveling wave amplifiers or oscillators. Therefore, it is desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

1. A traveling wave tube comprising an evacuated envelope containing a continuous slow wave energy propagating structure for producing along a path adjacent thereto fields of electromagnetic wave energy being transmitted, a source of electrons, means for directing a beam of said electrons along the interaction space between said structure and said electrode and in energy interacting relation with said wave fields, said means for directing including means for producing a magnetic field transverse to said electron beam, 'an electrically-insulating base supported by a portion of said envelope, said structure including an electrically-conductive element printed upon said base,and energy coupling means connected through an aperture in said base to said conductive element.

2. A traveling wave tube comprising an evacuated envelope containing a continuous slow wave energy propagating structure for producing along a path adjacent thereto fields of electromagnetic wave energy being transmitted, a source of electrons, means for directing a beam of said electrons along the interaction space between said structure and said electrode and in energy interacting relation with said wave fields,.said means for directing including means for producing a magnetic field transverse to said electron beam, said structure including an electrically-insulating base supported by a portion of said envel0pe,,said structure including a pair of interleaved electrically-conductive elements printed upon said base, and energy coupling means connected through an aperture in said base to one of said conductive elements.

3. A traveling wave tube comprising an evacuated envelope containing a continuous slow wave energy propagating structure for producing along a path adjacent thereto fields of electromagnetic wave energy being transmitted, an electrode spaced from and substantially coextensive with said translation structure and maintained at a potential negative with respect thereto, a source of elec trons, means for directing a beam of said electrons along the interaction space between said structure and said electrode and in energy interacting relation with said Wave fields, an electrically-insulating base supported by a portion of said envelope, said structure including a pair of interleaved comb-shaped electrically-conductive elements printed upon said base, and energy coupling means connected through said base to at least one of said conductive elements.

4. A traveling wave electron discharge device comprising an electrically conductive envelope, an electrically insulating member mounted on a portion of said envelope, a pair of thin interleaved electrically conductive coatings attached to said insulating member, said coatings forming an interdigital periodic slow wave propagating network, and energy coupling means having a portion thereof connected to said network through an aperture in said insulating member.

References Cited in the file of this patent UNITED STATES PATENTS 2,611,101 Wallauschek Sept. 16, 1952 2,622,158 Ludi Dec. 16, 1952 2,654,842 Engelmann Oct. 6, 1953 2,768,322 Fletcher ,Oct. 23, 1956 OTHER REFERENCES Article entitled, Manufacture of Microstrip, Eleotrical Communications, vol. 29, pages 251-259, December 1952.

U. S. DEPARTMENT OF COMMERCE PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,834,915 Edward C, Dench I May 13, 1958 It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Let ters Patent should read as corrected below.

Column 3 line 59, for "producting" read a-producing-q column 6,; lines 31 32, strike out "said structure includin 1 Signed and sealed this 1st day of July 1958,.

(SEAL) Attest: KARL H. LINE ROBERT C WATSON Attesting Officer Conmissioner of Patents

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